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Lyotropic ion effects in guar gum adsorption on various minerals Ma, Xiaodong 2007

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Lyotropic Ion Effects in Guar Gum Adsorption on Various Minerals by XIAODONG MA B.Sc, Northeastern University, China, 1996 M.A.Sc. University of British Columbia, Canada, 2005 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Mining Engineering) THE UNIVERSITY OF BRITISH COLUMBIA September 2007 © Xiaodong Ma, 2007 ABSTRACT Adsorption of guar gum was studied on a number of oxide and clay minerals. The tests were performed as a function of salt concentration and pH from solutions of lithium, sodium, potassium, and cesium chlorides. The four salts allowed an assessment to be made of the significance of lyotropic ion phenomena in controlling the adsorption of the polysaccharide. The results showed that the adsorption of the polymer was independent of pH and acidity/basicity of the minerals. Therefore, no evidence of acid-base type of chemical interactions was observed and hydrogen bonding was suggested as the adsorption mechanism. It was also observed that the adsorption of guar gum on quartz, kaolinite and illite proceeded differently from lithium and sodium chloride solutions in comparison to potassium and cesium chlorides. In contrast, no significant effect of salt type and concentration was observed in guar gum adsorption on titania, hematite, and alumina. It was postulated that the presence or absence of an extensive hydration layer at the mineral-solution interface was the dominant factor and that the interfacial water created a barrier against guar gum adsorption. Therefore, the role of ions of a given lyotropic series is to destabilize the interfacial water structure and promote guar gum adsorption. On the other hand, the absence of an extensive hydration layer on titania, alumina, and hematite allowed guar gum to freely interact with the surfaces and thus no lyotropic ion effect was observed. Therefore, lyotropic ion effects are very strong only in the case of strongly hydrated quartz and, apparently, clay minerals. 11 It was also shown that guar gum undergoes extensive aggregation in concentrated solutions of kosmotropic salts (LiCl and NaCl) while chaotropic potassium and cesium chlorides turned out to be very powerful solvents for the polymer. As a result, it can be postulated that individual guar gum molecules adsorb on minerals from concentrated KCI solutions, while entire guar gum aggregates adsorb from concentrated NaCl solutions. Such findings are of particular interest to potash flotation which is carried out in saturated salt solutions. It was stressed that any analysis of the adsorption results using traditional approaches and models must be performed with extreme caution since guar gum solutions are inherently heterogeneous due to the presence of undissolved, colloidal polymer aggregates. iii TABLE OF CONTENTS Abstract • b Table of Contents iv List of Tables • vi List of Figures vii Acknowledgement x Co-authorship Statement xi CHAPTER 1 Introduction 1 1.1 General Literature Review 1 1.2 References 6 1.3 Research Objectives 8 1.4 Thesis Overview 9 CHAPTER 2 Adsorption of Guar Gum onto Quartz from Dilute Mixed Electrolyte Solutions 10 2.1 Introduction 10 2.2 Materials and Methods 12 2.3 Results and Discussion 14 2.4 Conclusions 25 2.5 References 26 CHAPTER 3 Intrinsic Viscosities and Huggins Constants of Guar Gum in Alkali Metal Chloride Solutions 28 3.1 Introduction 28 3.2 Materials and Methods... 30 3.3 Results and Discussion 34 3.4 Conclusions 53 3.5 References 54 CHAPTER 4 Role of Background Ions in Guar Gum Adsorption on Oxide Minerals and Kaolinite 57 4.1 Introduction 57 4.2 Materials and Methods 58 4.3 Results 62 iv 4.4 Discussion 70 . 4.5 Conclusions 82 4.6 References 83 CHAPTER 5 Adsorption of Guar Gum on Potash Slimes 88 5.1 Introduction.... 88 5.2 Materials and Methods .: 90 ' 5.3 Results and Discussion 93 5.4 Conclusions 104 5.5 References 106 CHAPTER 6 General Conclusions 108 CHAPTER 7 Recommendations for Future Studies I l l APPENDICES •••• 113 Appendix 1 Calibration Curves for Guar Gum 113 Appendix 2 Determination of i.e.p. of Tested Minerals 114 Appendix 3 Electrokinetic Sonic Amplitutde (ESA) 117 v List of Tables Table 3.1 Huggins constants and intrinsic viscosities of guar gum in concentrated urea and saturated NaCl, LiCl, and CsCl solutions 43 Table 4.1 The volume-average particle sizes and BET specific surfaces areas of the tested samples 61 Table 5.1 Mineralogical composition of the tested minerals 91 vi List of Figures Figure 2.1 A structural formula of guar gum 10 Figure 2.2 Adsorption isotherms of guar gum on quartz in the presence of 0.01 mol/L electrolytes at pH 5.2 (pH 2.3 for HCI) 15 Figure 2.3 Adsorption isotherms of guar gum on quartz obtained in solutions containing various molar proportions of potassium and sodium chlorides: pH 5.2 17 Figure 2.4 Adsorption isotherms of guar gum on quartz obtained in solutions containing various molar proportions of potassium chloride and hydrochloric acid: pH 5.2 (KCI only) to 2.3 (HCI only) 17 Figure 2.5 Adsorption isotherms of guar gum on quartz obtained in solutions containing various molar proportions of potassium and calcium chlorides: pH 5.2 18 Figure 2.6 Adsorption isotherms of guar gum on quartz obtained in solutions containing various molar proportions of potassium and magnesium chlorides: pH 5.2 18 Figure 2.7 Effect of ionic strength on the adsorption of guar gum on quartz in simple chloride solutions: pH 5.2 except HCI (pH 5.2-1.4). Initial guar gum concentration 150 mg/L 19 Figure 2.8 Effect of pH on the adsorption of guar gum from 0.01 mol/L CaC^ and MgCh solutions. Initial guar gum concentration 150 mg/L 21 Figure 2.9 A species distribution diagram (Ca 2 + and M g 2 + cations omitted for clarity) for 0.01 mol/L solutions of CaCl 2 and MgCl 2 at 298 K 23 Figure 3.1 FTIR spectra of guar gum films prepared at pH 5.5 and pH 3. Figure lb (right) shows the effect of pH on the IR absorbance of the carboxylic groups of carboxymethyl cellulose 35 Figure 3.2 Reduced viscosity of guar gum solutions in distilled water at various temperatures 37 Figure 3.3 Effect of lithium chloride on reduced viscosity of guar gum solutions at24°C 38 Figure 3.4 Effect of sodium chloride on reduced viscosity of guar gum solutions at 24 °C 38 vii Figure 3.5 Effect of potassium chloride on reduced viscosity of guar gum solutions at 24 °C. A set of data for saturated potash brine is also shown 39 Figure 3.6 Effect of cesium chloride on reduced viscosity of guar gum solutions at 24 °C 39 Figure 3.7 Effect of urea on reduced viscosity of guar gum solutions at 24 °C 42 Figure 3.8 Effect of electrolyte/urea concentration on Huggins constants of guar gum (T = 24 °C) 44 Figure 4.1 Adsorption of guar gum on kaolinite in the presence of 0.01 mol/L alkali metal chlorides... 63 Figure 4.2 Adsorption of guar gum on hematite and alumina in the presence of 0.01 mol/L alkali metal chlorides, pH 11 63 Figure 4.3 Adsorption of guar gum on titania in the presence of 0.01 mol/L alkali metal chlorides, pH 11 64 Figure 4.4 Effect of pH on guar gum adsorption on kaolinite in 0.01 mol/L alkali metal chlorides. Initial guar gum concentration 270 mg/L 65 Figure 4.5 Effect of pH on guar gum adsorption on hematite and alumina in 0.01 mol/L alkali metal chlorides. Initial guar gum concentration 490 mg/L for alumina, and 545 mg/L for hematite 66 Figure 4.6 Effect of pH on guar gum adsorption on titania in 0.01 mol/L alkali metal chlorides. Initial guar gum concentration 390 mg/L 66 Figure 4.7 Effect of ionic strength on guar gum adsorption on quartz, pH 5.2-5.5. Initial guar gum concentration 110 mg/L 68 Figure 4.8 Effect of ionic strength on guar gum adsorption on kaolinite, pH 5.2-5.5. Initial guar gum concentration 270 mg/L 68 Figure 4.9 Effect of ionic strength on guar gum adsorption on alumina, hematite, and titania, at pH 11. Initial guar gum concentration 490 mg/L for alumina, 390 mg/L for titania, and 550 mg/L for hematite 69 Figure 5.1 The reduced viscosity of guar gum solutions, T = 23 °C 94 Figure 5.2 Effect of NaCl and KC1 concentration on adsorption of guar gum on kaolinite. Initial concentration of guar gum 270 mg/L 96 viii Figure 5.3 Effect of NaCl and KCI concentration on adsorption of guar gum on illite. Initial concentration of guar gum 440 mg/L 96 Figure 5.4 Effect of NaCl and KCI concentration on adsorption of guar gum on dolomite. Initial concentration of guar gum 150 mg/L 97 Figure 5.5 Effect of electrolyte type and concentration on adsorption of guar gum onto potash slimes. 98 Figure 5.6 Turbidity of slime suspensions (10% wt.) in saturated salt solutions in the presence of guar gum 98 Figure A . l Calibration curve of guar gum in distilled water 113 Figure A.2 Calibration curve of guar gum in 50% saturated salt solutions 113 Figure A.3 ESA of hematite in 0.01M NaCl solution 114 Figure A.4 ESA of titania in 0.01M NaCl solution .....114 Figure A.5 ESA of alumina in 0.01M NaCl solution 115 Figure A.6 ESA of kaolinite in salt solutions 115 Figure A.7 ESA of illite in salt solutions 116 ix Acknowledgement I would like to express my deepest gratitude to Dr. Marek Pawlik for supervising this research and my gratitude is also extended to my supervising committee members: Dr. John Meech, Dr. Bern Klein and Dr. Elod Gyenge. Their extensive experience provided valuable information for this research. Ms Sally Finora is also gratefully acknowledged for her assistance at various stages of this work. Finally, I would like to express my gratitude to my parents, Mr. Shucheng Ma and Ms Peifang Du, and my wife, Hellen Zhao, for their encouragements arid sacrifice throughout the long years of my graduate studies. x Co-authorship Statement The main body of this manuscript-based thesis consists of four refereed journal publications (Chapters 2, 3, 4, and 5), which were co-authored by my research supervisor, Dr. Marek Pawlik. My role in these joint publications was to perform the experimental work, compile the results into presentable figures, propose discussion and conclusions, and prepare/draft individual manuscripts for submission. x i CHAPTER 1 Introduction 1.1 General Literature Review The froth flotation process is the most important commercial technology used world-wide to extract valuable minerals from their ores. Polysaccharides have been utilized in froth flotation for about 80 years. Although many different types of polysaccharides exist in nature, only a small number of them have been used in flotation and the number of successful industrial applications of these environmentally friendly reagents is quite limited. In iron ore processing, hematite can be separated from quartz by a reverse flotation process in which starch selectively depresses hematite allowing quartz to be floated using a cationic collector. Certain sulfide ore operations use carboxymethyl cellulose to depress naturally hydrophobic gangue minerals such as talc or graphite. In the flotation of platinum group metals-bearing ores guar gum is used to depress talc. In potash flotation, a process that is carried out in saturated KCl/NaCl brine, guar gum is routinely used to "blind" water insoluble slimes such as clays, carbonates and quartz. The role of guar gum is to adsorb on these particles and prevent a cationic amine collector from adsorbing onto these unwanted particles. Polysaccharides are used in froth flotation under a wide range of physicochemical conditions: from low ionic strength systems such as the depression of talc by guar gum in platinum group metal ore flotation or the reverse flotation separation of quartz from hematite using starch, to high ionic strength processes such as the "blinding" of water-insoluble slimes in potash flotation carried out in a saturated KCl/NaCl brine. All these 1 applications rely on selective adsorption of polysaccharides on specific minerals, and the reasons for the selectivity of adsorption are still poorly understood. The general lack of understanding of the mechanisms involved in polysaccharide adsorption and the common view that polysaccharides are non-selective (i.e., that they indiscriminately adsorb on all minerals) strongly hinder their wider application. All the applications of polysaccharides in mineral processing rely on (selective) adsorption of the polymers on mineral surfaces. From this point of view, the adsorption behavior of polysaccharides has been studied quite extensively, and a number of different adsorption mechanisms were proposed even for the same mineral-polysaccharide system. The term lyotropic effects refer to subtle phenomena that take place as a result of differences in ionic sizes. In aqueous solutions, ions of the same charge but of varying crystallographic radii (e.g., alkali metal cations: L i + , Na+, K + , Rb+, Cs+) differently interact with the surrounding water molecules. As first proposed by Gurney [1] and Frank and Wen [2], each ion is surrounded by three distinct regions of water structure. In the first layer, water molecules are tightly bound to the ion. The second region extends farther away from the ion and is referred to as the region of structure breaking. Only at larger distances, where the ionic field is weak, water molecules form the "normal" hydrogen-bonded ice-like structure. Small (in terms of crystallographic radii), strongly hydrated ions (e.g., L i + and Na+) reinforce the "normal" structure of water and the region of structure breaking disappears. Such ions are called water-structure making ions. In contrast, large less-hydrated ions disturb the ice-like structure and generate an extensive region of structure breaking. Such structure-breaking ions (e.g., K + and Cs+) are also referred to as chaotropes whereas structure-makers are known as kosmotropes. 2 The concepts of chaotropes and kosmotropes are widely used in interpreting interfacial and transport phenomena in biological systems. Ionic pumps and channels (Na+/K+), enzymatic catalysis, osmosis, partitioning of L and D enantiomers by chromatography, and even bone growth were shown to depend on the concentrations of kosmotropes and chaotropes [3,4]. At the same time, however, the role of structure-breaking and structure making ions in mineral processing systems has not been thoroughly considered and the number of publications on the subject is rather limited. Ma and Pawlik [5] investigated the adsorption of guar gum onto the negatively-charged quartz surface from dilute solutions of alkali metal chlorides and demonstrated that the chaotropic potassium and cesium cations were capable of enhancing guar gum adsorption in comparison to the levels observed in distilled water and in kosmotropic lithium and sodium chlorides. In a recent study on the flotation of water-soluble salt-type minerals from saturated brines, Hancer et al. [6] showed that the selectivity of primary amine collectors towards the mineral sylvite (KC1) in potash flotation seemed to be related to the different structures of the interfacial water layer present on sylvite and halite crystals. Apparently, the chaotically oriented water molecules on the KC1 surfaces facilitated the adsorption of amine-type collectors, while a much more oriented and ordered water structure on the halite (NaCl) surfaces prevented the collector from attaching to the mineral. The difference in flotation response of Na2CC>3 and NaHC03 with cationic and anionic collectors was also attributed to differences in the interfacial water layer in the presence of chaotropic (HCO3") and kosmotropic (CO3 ") ions at the surfaces of these two minerals 3 [7]. It was also shown that the presence of a small amount of CO3 " ions in the saturated NaHC03 solution completely destroyed the floatability of NaHC03 crystals [7]. In relation to flotation from high-salinity process waters, Quinn et al. studied foam stability, gas hold-up and bubble sizes in various electrolytes and observed large differences in the foaming "power" of salt solutions [8J. Although no detailed explanation was provided, it was noted that the studied electrolytes, and cations in particular, were of different kosmotropic and chaotropic strengths. Studies on the flotation of naturally-hydrophobic minerals such as talc and molybdenite showed that the floatability of these minerals was not affected by polyvalent cations (Fe3+, Cr 3 + , Al 3 + ) at low pH, or by hydrolyzable metal-hydroxy complexes in more alkaline solutions [9, 10, 11]. However, almost complete depression of both minerals was observed at very high pH in the absence of any background ions [9, 10]. Sulfate anions also produced a measurable level of molybdenite depression, but no effect of chloride or nitrate ions was observed (all anions were introduced as sodium salts) [11]. In the context of this review it is noteworthy that these metal-hydroxy cations may be treated as chaotropes compared to the corresponding, strongly kosmotropic cations (e.g., Fe(OH)2+ vs. Fe3 +) due to differences in their sizes and degrees of hydration. Also, sulfate anions are very strong kosmotropes while CI" and N03~are chaotropes [4]. It would thus appear that cations in general do not affect the wettability of talc and molybdenite and that the lyotropic properties of anions are more important for these naturally-hydrophobic minerals. As noted, both talc and molybdenite can be depressed by high pH alone when the concentrations of OH" ions are relatively high. Although the depression of talc and other naturally hydrophobic solids at high pH has traditionally been explained by 4 increasing contributions of electrostatic interactions to the total work of adhesion between water and the mineral surface [12], it is very interesting to observe that the OH" anion is one of very few monovalent kosmotropic anions [4]. Another process whose performance is very likely related to lyotropic effects of background ions is the "salt" flotation of fine coal [13]. Although the process never found any large scale applications, it could potentially be used for recovering fine coal from tailing ponds should the exact mechanism behind it be better understood. According to Laskowski's data, the salt flotation of various coals proceeds differently depending on the salt type with anions apparently producing a stronger effect compared to cations [13]. Considering that certain coals are naturally hydrophobic, these trends are in line with the observations made above on the role of anions in molybdenite and talc flotation. Ionic affinities towards mineral surfaces can be explained, at least for some model hydrophilic oxide systems, using a thermodynamic model of ion adsorption developed by James and Healy [14]. The model demonstrated that a low-dielectric oxide surface, such as quartz (dielectric constant 4.6), will preferentially interact with less hydrated cations while strongly hydrated ions will not as easily adsorb on such a surface. The opposite was predicted for titania, an oxide characterized by a dielectric constant of 120. These differences were attributed to the ease or difficulty for a metal ion to exchange its secondary hydration sheath for the interfacial water upon approach to the surface. Along the same lines, Dumont et al. formulated the "like adsorb like" concept in which it is simply postulated that kosmotropic ions will show preference towards kosmotropic surfaces [15]. The concept is based on the observation that the hydration of oxide-type surfaces is related to the heat of immersion and to the point of zero charge (pzc) of the 5 surfaces [16]. Accordingly, high-pzc oxide minerals should preferentially adsorb well hydrated (kosmotropic) ions, while low-pzc oxides preferentially adsorb poorly-hydrated (chaotropic) ions. Overall, however, there is no model or theory to adequately predict whether a mineral surface is of kosmotropic or chaotropic nature, and the frequently-observed differences in the specific adsorption of ions of a given lyotropic series remain poorly understood [17]. In addition, no systematic data exist on the interactions of flotation reagents with mineral surfaces as a function of the lyotropic properties of background ions, and on any potentially-significant implications of these phenomena to the froth flotation process itself. Since this thesis is based on a number of published and submitted papers, more detailed reviews of the pertinent literature are presented in the introductory sections of each chapter. 1.2 References [1] Gurney, R.W., Ionic Processes in Solution, McGraw Hill Pres, New York, (1953). [2] Frank, H.S. and Wen, W.Y., Ion-solvent interaction - staictural aspects of ion-solvent interaction in aqueous solutions: a suggested picture of water structure, Discussions of the Faraday Society, Vol. 24, pp. 133-140, (1957). [3] Wiggins, P.M., High and low density intracellular water - a review, Cellular and Molecular Biology, Vol. 47, pp. 735-744, (2001). [4] Wiggins, P.M., Enzyme reactions and two-state water, Journal of Biological Physics and Chemistry, Vol. 2, pp. 25-37, (2002). [5] Ma, X. and Pawlik, M. , Effect of alkali metal cations on adsorption of guar gum onto quartz, Journal of Colloid and Interface Science, Vol. 289, pp. 48-55, (2005). 6 [6] Hancer, M. , Celik, M.S., and Miller, J.D., The significance of interfacial water structure in soluble salt flotation systems , Journal of Colloid and Interface Science, Vol. 235, pp. 150-161,(2001). [71 Ozcan, O. and Miller, J.D., Flotation of sodium carbonate and sodium bicarbonate salts from their saturated brines, Minerals Engineering, Vol. 15, pp. 577-584, (2002). [8] Quinn, J.J., Gomez, CO. , and Finch, J.A., Effect of salt type and concentration on gas dispersion properties. In: Xu, Z. and Liu, Q. (Ed.), Interfacial Phenomena in Fine Particle Technology, Metallurgical Society of CIM, Montreal, pp. 417-428, (2006). . [91 Chander, S. and Fuerstenau, D.W., On the natural Floatability of Molybdenite, Transactions of AIME, Vol. 252, pp. 62-68, (1972). 110] Fuerstenau, M.C., Lopez-Valdivieso, A. and Fuerstenau, D.W., Role of hydrolyzed cations in the natural hydrophobicity of talc, International Journal of Mineral Processing, Vol. 23, pp. 161-170, (1988). [I l l Castro, S. and Bobadilla, C , The depressant effect of some inorganic ions on the flotation of molybdenite. In: Laskowski, J.S. and Poling, G.W. (Ed.), Processing of Hydrophobic Minerals and Fine Coal, Metallurgical Society of CIM, Montreal, pp. 95-103, (1995). [12] Laskowski, J. and Kitchener, J.A., The hydrophilic—hydrophobic transition on silica, Journal of Colloid Interface Science, Vol. 29, pp. 670-679, (1969). [13] Laskowski, J.S., Coal Flotation and Fine Coal Utilization, Elsevier, Amsterdam, (2001). [14] James, R.O. and Healy, T.W., Adsorption of hydrolyzable metal ions at the oxide-water interface. III. A thermodynamic model of adsorption, Journal of Colloid and Interface Science, Vol. 40, pp. 65-81, (1972). [15] Dumont, F., Warlus, J. and Watillon, A., Influence of the point of zero charge of titanium dioxide hydrosols on the ionic adsorption sequences, Journal of Colloid and Interface Science, Vol. 138, pp. 543-554, (1990). [16] Healy, T.W. and Fuerstenau, D.W., The oxide-water interface-interrelation of the zero point of charge and the heat of immersion, Journal of Colloid and Interface Science, Vol. 20, pp. 376-386, (1965): [17] Lyklema, J., Lyotropic sequences in colloid stability revisited, Advances in Colloid and Interface Science, Vol. 100-102, pp. 1-12, (2003). 7 1.3 Research Objectives 1. To demonstrate the importance of lyotropic ion effects in the adsorption of guar gum onto various mineral surfaces. 2. To propose a general mechanism of guar gum-mineral interactions. 3. To analyze the effect of salt type and concentration on the conformation of guar gum in aqueous solutions through intrinsic viscosity measurements. 4. To determine the adsorption behavior of guar gum in concentrated salt solutions in relation to the blinding of slimes in potash flotation. 5. To assess the significance of solvency effects in controlling guar gum adsorption from concentrated electrolyte solutions. 8 1.4 Thesis Overview This manuscript-based thesis consists of four main chapters (Chapter 2,3,4 and 5) that are based on four separate papers, either published (Chapters 2 and 3) or submitted for publication (Chapters 4 and 5) in a refereed journal. Chapter 2 analyzes the adsorption of guar gum onto quartz - a model mineral -from dilute, single and mixed electrolytes and demonstrates that guar gum adsorption on quartz strongly depends on the chaotropic and kosmotropic properties of the background ions. In Chapter 3, a viscometric investigation of the behavior of guar gum in alkali metal chloride solutions is presented. The effect of salt type and concentration on the solution properties of guar gum was investigated over a wide range of ionic strengths -from distilled water to saturated salt solutions. It was shown that the polymer itself was affected by more concentrated electrolytes, and the action of kosmotropic ions was very different from the effect of chaotropes. Chapter 4 presents a systematic study of guar gum adsorption on several oxide minerals (hematite, alumina, titania, and quartz) and kaolinite, over a wide range of ionic strengths and pH values. The main findings of this thesis are discussed in this chapter. Chapter 5 investigates the adsorption of the polysaccharide on actual potash ore slimes - a mixed mineral system where guar gum is used on an industrial scale as a "blinder". In contrast to the presumed role of the polysaccharide as a slime blinder, the results actually show that certain components of the mixture are not fully coated by the adsorbing polymer. The thesis ends with general concluding remarks (Chapter 6) and recommendations for future studies (Chapter 7). 9 C H A P T E R 2 A d s o r p t i o n o f G u a r G u m o n t o Q u a r t z f r o m D i l u t e M i x e d E l e c t r o l y t e S o l u t i o n s * 2.1 I n t r o d u c t i o n Guar gum is a natural nonionic polysaccharide. Guaran, the functional polysaccharide in guar gum, is a chain of (1—>4)-linked /?-D-mannopyranose units with a-D-galactopyranose units connected to the mannose backbone through (1—>6) glycosidic linkages. The degree of substitution of the poly-mannose chain varies from 1.8 to 1.0 [1], while the average molecular weight of the polysaccharide is on the order of 1-2 million [2, 3]. Figure 2.1 shows a schematic depiction of the structure of a guar gum monomer. polymannose chain: H H C H 2 H O H I galactose side group: C H 2 O H Figure 2.1 A structural formula of guar gum. In mineral processing guar gum is most frequently used as a depressant of naturally hydrophobic gangue minerals, e.g., talc, or as a blinder of water-insoluble slimes in potash flotation — a process that is carried out in saturated KCl/NaCl brine. Adsorption of guar gum, and polysaccharides in general, on various mineral * A version of this chapter was published in Journal of Colloid and Interface Science. [Reference: Ma, X. and Pawlik, M. , Adsorption of Guar Gum onto Quartz from Dilute Mixed Electrolyte Solutions, Journal of Colloid and Interface Science, vol. 298, no. 2, pp. 609-614, (2006).] 10 surfaces has been studied quite extensively. Liu et al. have recently reviewed the different mechanisms of polysaccharide adsorption and concluded that polysaccharides adsorb at mineral surfaces through complexation with metal-hydroxyl surface sites, and the nature of the interaction, whether hydrogen bonding or chemical, is of acid-base type and strongly depends on the acidity of the surface metal-hydroxyl groups [4]. Hydrophobic interactions were also proposed for guar gum adsorption onto the naturally hydrophobic basal cleavage planes of talc [5, 6], but a recent study by Wang et al. [7], with the use of urea as a hydrogen-bond breaker, shows that hydrogen bonding between guar gum and talc cannot be ruled out. In a previous contribution by Ma and Pawlik [8], guar gum adsorption onto quartz was investigated in 0.01M LiCl, NaCl, KC1 and CsCl solutions. The results showed that the adsorption density of the polysaccharide was significantly higher in the presence of KC1 and CsCl compared to the levels observed in LiCl and NaCl solutions. The adsorption data fell on two curves irrespective of pH: those in the presence of KC1 and CsCl, and those in the presence of LiCl and NaCl. More importantly, L i and Na cations produced the same guar gum adsorption densities as those obtained in distilled water. The enhanced guar gum adsorption in the presence of Cs and K cations was attributed to the water-structure-breaking properties of these two cations. Assuming hydrogen bonding to be the adsorption mechanism, it was proposed that Cs and K ions are capable of disrupting the interfacial water layer on the quartz surface, thus allowing the polymer to adsorb more densely onto the exposed/dehydrated silanol surface sites. The guar gum adsorption process should thus be viewed as competition between the polysaccharide and water molecules for the polar surface sites. For the first time, the water-structure-breaking 11 and -making capabilities of the background ions were considered in analyzing the adsorption data of a polymer. Although the concepts of chaotropes (structure breakers) and kosmotropes (structure makers) are commonly applied to explain various biochemical interfacial phenomena [9], their role in mineral processing systems has received limited attention. Since all relevant bonds in the quartz-solution-guar gum system are hydrogen bonds, i.e., water-water, water-quartz, water-guar gum, and quartz-guar gum, it can be expected that the presence of kosmotropes and chaotropes will affect the overall behavior and equilibrium of the system. Kosmotropes reinforce the "order" of the hydrogen-bonded water structure (hence the term kosmotrope), while chaotropes break the normal water structure and are surrounded by an extensive region of disordered water molecules (hence the term chaotrope). Since the quartz surface is negatively charged over a wide pH range, it is postulated that it is the hydrated quartz-solution interface that is primarily affected by the presence of various cations. In this work, magnesium, calcium, and hydrogen cations are included in the investigation, and guar gum adsorption onto quartz is studied in dilute mixed electrolyte solutions containing both structure-breaking (K+) and structure-making cations (Na+, Ca 2 + , Mg 2 + , H +). 2.2 Materials and methods A quartz sample for this study was obtained from Sigma-Aldrich—the same material as in an earlier study by Ma and Pawlik [8]. The BET (Bamauer-Emmett-Teller) specific surface area for the sample, determined from nitrogen adsorption after 0 2 outgassing at 200 C under vacuum, was found to be 6.0 m /g. The sample was also 12 characterized by high porosity, with 4.3 mVg of the total BET specific surface area contained within pores smaller than 62 A. Since only the external surface area is accessible to guar gum, a value of 1.7 m/g was used for calculating the adsorption densities of the polymer. The external BET specific surface area was obtained by subtracting the surface area contained in pores (through a simultaneous porosity measurement) from the total BET surface areas obtained from nitrogen adsorption. The agreement between the total BET values and those obtained from the particle size distribution (assuming that particles were spherical and non-porous) was very poor, which was indicative of highly non-uniform surface topography and non-spherical particle shapes. The particle size distribution of the quartz sample, as determined with the use of a Malvern Mastersizer 2000, fell entirely in the range from 0.3 to 13.8 pm with a volume-average size of 2.3 pm. The sample was used "as received". Guar gum was the Rantec KP4000. The average molecular weight of the KP4000 was found from intrinsic viscosity measurement to be 1.39xl06 [10], which agrees well with the value provided by the manufacturer (1.5xl06). The KP4000 contained about 12% of water-insoluble residue which was removed from all stock solutions by prolonged centrifuging at 10,000 g. Fresh guar gum solutions were prepared daily. Potassium, sodium, calcium, and magnesium chlorides were all ACS-certified chemicals from Fisher. ACS-certified hydrochloric acid (37.0% solution) was also obtained from Fisher. Magnesium and calcium oxides (ACS-certified, Fisher) were used in some tests for pH adjustment. Adsorption tests were carried out as a function of guar gum concentration, ionic strength, and pH (for MgCL and CaCbJ. Five-gram samples of quartz were first mixed 13 with 12.5 mL of a background electrolyte solution and conditioned for 20 min in a shaker. Then, a further 12.5 mL of a guar gum solution in distilled water were added and the entire mixture was conditioned for 30 min. Finally, the pH of the suspension was measured, the solids were separated from solution by centrifuging, and the equilibrium guar gum concentration was measured using the phenol-sulfuric acid method [11]. From the large number of guar gum assays'performed in this thesis, it can also be concluded that guar gum solutions with concentrations lower than 1 mg/L are very difficult to be reproducibly measured using the phenol-sulfuric acid method. For higher guar gum concentrations results obtained using this method are reproducible within 2% stand deviation and can thus be concluded reliable. Since the phenol-sulfuric acid method is widely used in polysaccharide adsorption research, the results of this research are readily comparable with other works despite the analytical limitation of the phenol-sulfuric acid. Guar gum solutions of known concentrations were used for calibration (see Appendix 1). Blank tests were also performed for each background salt by subjecting a quartz suspension, without any guar gum added, to the conditioning/centrifuging procedure and measuring the absorbance of the resulting supernatant. 2.3 Results and discussion Figure 2.2 illustrates adsorption isotherms of guar gum on quartz in the presence of 0.01 mol/L electrolytes. For comparison, a set of data obtained in distilled water is also shown. It can be seen that the adsorption results for distilled water, NaCl, HCI, and CaCL, fall on the same curve. The adsorption density in the presence of KCI is much higher than 14 for the other electrolytes, while there is a measurable decrease in guar gum adsorption in 0.01 M MgCb relative to adsorption in distilled water. These data demonstrate that water structure-breaking cations (chaotropes), K + ions in this case, are capable of increasing guar gum adsorption onto quartz by disturbing the interfacial water structure at the quartz-solution interface [8]. The remaining cations are all water-structure makers (kosmotropes) and their effect, except for Mg ions, on guar gum adsorption is negligible compared to the adsorption density obtained in distilled water. -o .0 < 3 o E < 0 20 40 60 80 Guar Gum Equilibrium Concentration [mg/L] /Figure 2.2 Adsorption isotherms of guar gum on quartz in the presence of 0.01 M electrolytes at pH 5.2 (pH 2.3 for HCI). Figures 2.3 through 2.6 show the guar gum adsorption density measured in mixed electrolyte solutions of varying composition. In each case, one of the background cations is potassium, a water structure breaker, while the other cation is a structure maker: H + , Na+, Ca 2 + or Mg 2 + . 15 In Figures 2.3-2.5, the isotherms corresponding to single electrolytes are taken from Figure 2.2. It is quite clear that the adsorption data for the K + /Na + , K + / H + and K + / Ca 2 +ion combinations (Figures 2.3, 2.4, and 2.5, respectively) are essentially identical. In all three cases, the effect of 0.01 M potassium chloride on guar gum adsorption is not countered/affected by the addition of a small amount (0.001 M), or even an equal amount 0.01 M KC1 + 0.001 M H/Na/Ca chloride, or even 0.01 M KC1 + 0.01 M Na/H/Ca chloride fall onto the same isotherm. However, the same small addition of KC1 to a 10-fold higher concentration of the other chloride markedly increases guar gum adsorption for any of the combinations with NaCl, HCI and CaC^. In contrast to these three electrolytes, increasing concentrations of Mg ions (0.001 and 0.01 M) gradually decrease guar gum adsorption from the levels observed in 0.01 M KC1 only. Moreover, the presence of 0.001 M KC1 does not significantly affect guar gum adsorption relative to the adsorption density in 0.01 M MgCl 2 alone. In other words, the effect of a small KC1 concentration is not so pronounced when a high concentration of MgCl2 is present in solution. Interestingly, for a mixture of equal concentrations of M g 2 + and K + ions the adsorption data fall exactly between the adsorption isotherms for KC1 and MgCl 2 alone. Figure 2.7 shows the adsorption of guar gum onto quartz as a function of ionic strength for the simple chlorides. It can be seen that KC1 gives the highest adsorption densities regardless of ionic strength, while the adsorption values in the presence of the other electrolytes do not increase so significantly. In the case of MgCl 2 , the results appear to go through a shallow minimum at 0.01 M and an upward trend can be seen in all 0.1 M solutions. 16 0.35 0.30H 6 0 CD -a < c o a • 0.0IM KCI • O.OlMKCl+O.OOIMNaCl • O.OlMKCl+O.OlMNaCl 0 O.OOlMKCl+O.OIMNaCl • 0.0IM NaCl 20 40 60 80 Guar Gum Equilibrium Concentration [mg/L] 100 Figure 2.3 Adsorption isotherms of guar gum on quartz obtained in solutions containing various molar proportions of potassium and sodium chlorides: pH 5.2. -o • a < o S < 0.35 0.30H 0.25H 0.20H 0.15H 0.10H 0.05 0.00 , , , , 20 40 60 80 Guar Gum Equilibrium Concentration [mg/L] Figure 2.4 Adsorption isotherms of guar gum on quartz obtained in solutions containing various molar proportions of potassium chloride and hydrochloric acid: pH 5.2 (KCI only) to 2.3 (HCI only). 17 0.30H on ^ 0.2(H < £3 3 O • 0.01MKC1 • 0.01MKCl+0.001MCaCl2 + 0.0IMKCl+0.01MCaCl 2 O 0.001MKCl+0.01MCaCl2 • 0.0!MCaCI 2 r 1 20 40 60 80 Guar Gum Equilibrium Concentration [mg/L] 100 Figure 2.5 Adsorption isotherms of guar gum on quartz obtained in solutions containing various molar proportions of potassium and calcium chlorides: pH 5.2. 0.30H a o 0.20H a T 3 < a o S < • 0.01MKCl+0.001MMgCI2 + 0.01MKCl+0.01MMgCI2 O 0.001MKCl+0.01MMgCl2 • 0.01MMgCI2 20 40 60 80 Guar Gum Equilibrium Concentration [mg/L] 100 Figure 2.6 Adsorption isotherms of guar gum on quartz obtained in solutions, containing various molar proportions of potassium and magnesium chlorides: pH 5.2. 18 0.35 • o.io47A T—I I I I I T— I I I I I T—r—r T T Dist. Water Electrolyte Concentration [mol/L] 0.001 0.01 0.1 Figure 2.7 Effect of ionic strength on the adsorption of guar gum on quartz in simple chloride solutions: pH 5.2 except HCI (pH 5.2-1.4). Initial guar gum concentration 150 mg/L. The traditional view is that K + and Na + are indifferent ions for quartz. Ca 2 + and Mg ions are known to act as specifically adsorbing ions, while H cations are invariably treated as potential-determining ions not only for quartz but also for all other oxide-type minerals. Although these are very different types of ions from the surface chemistry point of view, the only property that correlates well with the observed adsorption results at natural pH (Figure 2.7) is the hydration of the cations. Although the exact hydration numbers for these ions are not necessarily known, their free energies of hydration increase in the order K + < Na + < H + < Ca 2 + < M g 2 + [12, 13]. Small, strongly hydrated magnesium ions are therefore the strongest kosmotropes in the series, while large (in terms of the crystallographic radii), poorly hydrated K + ions are the strongest chaotropes. In this approach, hydrogen cations are treated as "simple" ions, i.e., adsorbing on 19 quartz through electrostatic forces, since there is really no special relationship between the potential determining role of H + and the adsorption behavior of guar gum on quartz, even when the surface charge of quartz changes (Figures 2.4 and 2.7) as the concentration of FT cations increases. Assuming the point of zero charge of quartz to be around pH 2, the quartz surface was very likely positively charged when the concentration of HCI was 0.1 M (Figure 2.7). An apparent effect of increasing pH on guar gum adsorption on quartz was observed in an earlier study by Ma and Pawlik [8] but the results were attributed to increasing concentrations of metal cations (K + and Cs+) introduced with the corresponding bases during pH adjustments. The fact that the adsorption of guar gum is independent of pH (surface charge) points toward hydrogen bonding as the adsorption mechanism. The adsorption of calcium and magnesium ions onto quartz is a function of pH. High uptakes of Mg and Ca are observed only under alkaline pH values when higher and higher amounts of calcium- and magnesium-hydroxy species (CaOH+ and MgOH+) appear in solution [14]. The MgOH + ion is actually thermodynamically unstable and tends to undergo a condensation reaction to form the Mg4(OH)4+ 4 complex [15]. Nevertheless, at pH 5.2 Ca 2 + and M g 2 + must by far be the dominant Ca and Mg species in solution, so any effect of the hydroxy species on guar gum adsorption can be neglected. In the context of this work it is noteworthy that these hydroxy complexes are less hydrated than the corresponding cations. As Figure 2.8 shows, the adsorption of guar gum onto quartz increases at more alkaline pH values when CaC^ and MgCh are used as background electrolytes. In the case of CaCL, the pH values were adjusted using HCI (below pH 5.2) or a 20 saturated CaO solution to avoid introducing a foreign cation. For MgCl 2 , achieving pH values higher than 9 was very difficult with the use of a saturated MgO solution alone, so higher pH levels were reached using NaOH. A previous study by Ma and Pawlik [8] showed that NaOH and Na + ions do not affect guar gum adsorption, .so any trends seen in Figure 2.8 can be attributed to the speciation of Ca and Mg in solution. • i i i i i • i • i • 3 4 5 6 7 8 9 10 11 12 pH Figure 2.8 Effect of pH on the adsorption of guar gum from 0.01 M CaCl 2 and MgCl 2 solutions. Initial guar gum concentration 150 mg/L. Using reference thermodynamic data (the free energies of formation, Go) for calcium and magnesium species [15, 16], and applying the well-known expression AGo = -2.303RTX \og(K), the equilibrium constants, K, for the reactions Ca(OH)+ + H + —• Ca 2 + + H 2 0 and Mg 4(OH) 4 + 4 + 4H + — 4Mg 2 + + 4H 2 0 were calculated to be 4.72 x 1012 and 2.85 x 10' , respectively. These values were then used to construct a species distribution 21 diagram, shown in Figure 2.9, for a total Mg and Ca species concentration of 0.01 mol/L. As the diagram shows, the concentration of Ca(OFf)+ ions is higher than the concentration of the tetra-magnesium hydroxyl ions up to a pH value of about 10. In that pH range the adsorption of guar gum is measurably higher in the presence of CaCl 2 than in MgCl2. If the MgOH + species were involved in the guar gum adsorption process, the effect of pH on guar gum adsorption from MgCl 2 solution would be detectable at much lower pH than actually seen from Figure 2.8. However, there is an excellent correlation between the concentration of Mg 4(OH) 4 + 4 ions (Figure 2.9) and the magnitude of guar gum adsorption on quartz in 0.01 M MgCl 2 (Figure 2.8). The amount of the tetra-magnesium complex in solution becomes higher, at pH about 10, than the concentration of Ca(OH)+' ions and this trend correlates very well with the adsorption results in Figure 2.8. These observations are a very good indication that it is the Mg 4(OH) 4 + 4 complex that is responsible for enhanced guar gum adsorption on quartz. It thus appears that the observed increase in guar gum adsorption is due to the presence of calcium and magnesium hydroxyl complexes at higher pH values. These complex ions may be treated as water structure breakers compared to the corresponding cations but, because of their specific nonelectrostatic adsorption onto quartz, it is also plausible that the Ca/Mg hydroxyl complexes on the quartz surface serve as adsorption sites for guar gum. In such a case, the overall adsorption mechanism of the polysaccharide would be a combination of hydrogen bonding with silanol groups and complexation with Ca- and Mg-hydroxyl sites. From this point of view, it should be noted that magnesium sites are believed to take part in guar gum adsorption on talc [17]. The results in Figures 2.3 through 2.6 strongly indicate that there is a transition 22 value of the free energy of hydration that divides the structure-making ions into, "strong" and "weak" ions in terms of their "countereffect" to the effect of K + (the free energy ofhydration -338.2 kJ/mol) on guar gum adsorption. Although Na+, H + , and Ca 2 + cations Figure 2.9 A species distribution diagram (Ca and Mg" cations omitted for . clarity) for 0.01 M solutions of CaCl 2 and MgCl 2 at 298 K. are characterized by different free energies of hydration (-411.5, -1090.5, and -1594.0 kJ/mol, respectively), and all three are structure makers, there are only very small. differences between their effects (alone and in mixtures with K + ) on guar gum adsorption. However, when a sufficiently strong structure-making ion (Mg 2 + , free energy of hydration -1906.7 kJ/mol) is mixed with K + , their combined influence on guar gum adsorption becomes additive and the adsorption density of guar gum on quartz decreases in proportion to the amount of the structure-maker ion added to solution. 23 It should also be pointed out that although ionic strength, defined as l/l^cz"), was not constant in a series of tests for a given combination of ions (Figures 2.3-2.6), it is clear that the limiting adsorption isotherms of guar gum were obtained in pure chaotrope or kosmotrope solutions—solutions of the lowest ionic strength. Any intermediate responses were related to the relative structure-making or -breaking strength of the cations and not to the total ionic strength. An increase in polymer adsorption at higher ionic strengths, such as seen in the results obtained for KC1 in Figure 2.7, is usually associated with the adsorption of ionic polymers. A combined effect of the coiling of charged macromolecules and the screening of surface charges are typically given as the causes for such enhanced adsorption. In the case of the quartz-guar gum system the observed effect of KC1 concentration and the lack of any effect of the other cations/salts can again be satisfactorily explained by the structure-breaking and -making properties of the background ions. The data for M g 2 + ions suggest that strongly hydrated ions (strong structure makers) can measurably decrease the adsorption density of guar gum on quartz and this conclusion is also consistent with the postulate that water-structure-making ions are capable of reinforcing the interfacial water structure at the quartz-solution interface, thus preventing the polymer from adsorbing onto the hydrated mineral surface. The results for mixed electrolytes indicate that the concept of ionic strength is rather insufficient to explain the observed trends. The adsorption behavior of guar gum on quartz from dilute mixed electrolytes is completely controlled by the presence and relative strength of chaotropes and kosmotropes, and not by the total electrolyte 24 concentration, provided that ion interactions with the quartz surface are of a physical/electrostatic nature. In Chapter 4, the above phenomena will be investigated for a number of different minerals. - . ' 2.4. Conclusions In dilute electrolyte solutions, guar gum adsorption onto quartz is governed by the water-structure-making or -breaking properties of the background counterions, under the condition that these ions do not chemically interact with the quartz surface. Small, strongly hydrated cations interfere with the adsorption of the polymer by preventing it from interacting with the surface silanol groups. For mixed electrolyte systems, the total ionic strength of the solution does not play a significant role in the adsorption of the polymer and the results depend only on the relative concentrations of kosmotropes and chaotropes. In mixed electrolytes, the structure-breaking K + ions control the adsorption behavior of the polysaccharide regardless of the concentration of weak structure-making ions. Only when a very strong kosmotropic ion is present along with a chaotrope does the overall effect on guar gum adsorption become additive. The results are consistent with the proposed mechanism in which water-structure-breaking ions disturb the interfacial water layer and allow the polysaccharide to interact with the exposed surface silanol groups through hydrogen bonding. The adsorption process is treated as competition between water and polymer molecules for the polar surface sites. The hydration of the quartz surface inhibits guar gum adsorption but the 25 competition equilibrium, and hence adsorption, can be affected by the presence of chaotropes or kosmotropes. Mixed electrolyte systems are very common in mineral processing and the results highlight the importance of identifying water-structure-making and -breaking ions in the system in order to fully understand the data. 2.5 References [1] Painter, T.J., Gonzalez, J.J. and Hemmer, P.C., The distribution of D-galactosyl groups in guaran and locust-bean gum: new evidence from periodate oxidation, Carbohydrate Research, Vol. 69, pp. 217-226, (1979). [2] Cheng, Y., Brown, K. M . and Prud'homme, R. K., Characterization and intermolecular interactions of hydroxypropyl guar gum solutions, Biomacromolecules, Vol. 3, pp. 456-461, (2002). [3] Robinson, G., Ross-Murphy, S. B. and Morris, E. R., Viscosity-molecular weight relationships, intrinsic chain flexibility, and dynamic solution properties of guar galactomannan, Carbohydrate Research, Vol. 107, pp. 17-32, (1982). [4] Liu, Q., Zhang, Y. and Laskowski, J. S., The adsorption of polysaccharides onto mineral surfaces: an acid/base interaction, International Journal of Mineral Processing, Vol. 60, pp. 229-245, (2000). [5] Steenberg, E. and Harris, P. J., Adsorption of carboxymethyl cellulose, guar gum and starch onto talc, sulphides, oxides and salt-type minerals, South African Journal of Chemistry, Vol. 37, pp. 85-90, (1984). [6] Jenkins, P. and Ralston, J., The adsorption of a polysaccharide at the talc-aqueous solution interface, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 139, pp. 27-40, (1998). [7] Wang, J., Somasundaran, P. and Nagaraj, D. R., Adsorption mechanism of guar gum at solid-liquid interfaces, Minerals Engineering, Vol. 18, pp. 77-81, (2005). [8] Ma, X. and Pawlik, M. , Effect of alkali metal cations on adsorption of guar gum onto quartz, Journal of Colloid and Interface Science, Vol. 289, pp. 48-55, (2005). [9] Wiggins, P. M. , High and low-density intracellular water, Cellular and Molecular Biology, Vol. 47, pp. 735-744, (2001). 26 [10] Pawlik, M . and Laskowski, J. S., Effect of ionic strength on stabilization of mineral suspensions by carboxymethyl cellulose and guar'gum, 2004 Society for Mining Metallurgy and Exploration (SME) Annual Meeting & Exhibit, Feb. 23-25, Denver, Preprint # 04-059. [11] Dubois, M., Gilles, K.A., Hamilton, J.K., Rebers, P.A. and Smith, F., Colorimetric method for determination of sugars and related substances, Analytical Chemistry, Vol. 28, pp. 350-356, (1956). [12] Franks, F., Water: A Comprehensive Treatise, Plenum, New York, (1972). [13] Tissandier, M . D., Cowen, K. A., Feng, W. Y., Gundlach, E., Cohen, M . H. Earhart, A. D., Coe, J. V. and Tuttle, T. R., The proton's absolute aqueous enthalpy and gibbs free energy of solvation from cluster-ion solvation data, Journal of Physical Chemistry A, Vol. 102, pp. 7787-7794, (1998). [14] Clark, S.W. and Cooke, S.R.B, Adsorption of calcium, magnesium, and sodium ion by quartz, Transactions of the American Institute of Mining, Metallurgical and Petroleum Engineers, Vol. 241, pp. 334-341, (1968). [15] Perrault, G.G., Magnesium. In: Bard, A.J., Parsons, R. and Jordan, J. (Eds.), Standard Potentials in Aqueous Solution, Dekker, New York/Basel, pp. 687-699, (1985). [16] Toshima, S., Calcium, strontium, barium, and radium. In: Bard, A.J., Parsons, R. and Jordan, J., Standard Potentials in Aqueous Solution, Dekker, New York/Basel, pp.700-725, (1985). [17] Rath, R. K., Subramanian, S. and Laskowski, J. S., Adsorption of dextrin and guar gum onto talc- a comparitive study, Langmuir, Vol. 13, pp. 6260-6266, (1997). 27 C H A P T E R 3 Intrinsic Viscosities and Huggins Constants of Guar Gum in Alkal i Metal Chloride Solutions* 3.1 Introduction Guar gum is a natural nonionic polysaccharide produced from the seeds of two annual leguminous plants, Cyamopsis tetragonalobus and psoraloides. The guar gum macromolecule is a chain of (l—>4)-linked (3-D-mannopyranose units with a-D-galactopyranose units connected to the mannose backbone through (1—>6) glycosidic linkages. The poly-mannose chain is randomly substituted with galactose units at a mannose-to-galactose ratio of 1.8-1.0 [1, 2]. Because of the random nature of the substitution, the least substituted sections of the guar gum' molecules show the greatest tendency to associate, while the more densely substituted regions serve to solubilize the polymer chain [3]. Guar gum does not form "truly molecular" solutions. Under normal conditions, aqueous solutions of guar gum contain a small fraction of undissolved colloidal aggregates and only a combination of high temperatures and pressures during the solution preparation stage results in a complete (or nearly complete) dissolution and hydration of the polymer [4]. The size of such guar gum aggregates was found to be on the order of 10 - 100 microns at guar concentrations as low as 0.038% (wt.) [5]. The heterogeneous nature of guar gum solutions makes their characterization quite difficult especially when the testing methods rely on light scattering. Guar gum has a number of applications in the mining and mineral processing * A version of this chapter was published in Carbohydrate Polymers. [Reference: Ma, X. and Pawlik, M., Intrinsic Viscosities and Huggins Constants of Guar Gum in Alkali Metal Chloride Solutions, Carbohydrate Polymers, vol. 70, no. 1, pp. 15-24, (2007).] 28 industry. In the froth flotation of base metal and platinum group metal ores, guar gum is used as a depressant of naturally-hydrophobic waste minerals, such as talc. The role of the polysaccharide is to adsorb on the talc surface, render it hydrophilic, and prevent its flotation. A wide variety of modified guar gums are available for this application [6]. In another important flotation application, natural guar gum is employed as a "blinder" of water-insoluble, ultra-fine minerals in the froth flotation of potash ores. Canadian potash ores typically contain over 90% of the minerals sylvite (KC1) and halite (NaCl) with only a few percent of the so-called water-insoluble slimes, represented mostly by various clay and carbonate minerals. The objective of the process is to selectively separate sylvite from halite while completely rejecting the slimes. Since both sylvite and halite are highly soluble in water, the entire potash flotation process is carried out in a saturated KCl-NaCl brine. Sylvite is floated from halite with the use of a primary amine-based collector that also shows a high affinity towards the unwanted slimes. Therefore, the "blinding" action of guar gum in this high-ionic strength system relies on the adsorption of the polymer on the ultrafine minerals, thus forming a protective coating around these particles, and allowing the collector to selectively interact with the sylvite (KC1) crystals. In the absence of guar gum, the amine consumption would be very high and the selectivity of the amine-type collector towards KC1 would drastically be reduced. In relation to mineral processing applications, the adsorption of guar gum, and polysaccharides in general, on various minerals has been studied quite extensively [7, 8, 9]. Ma and Pawlik [10] have recently shown that adsorption of the polysaccharide on quartz proceeds differently from KC1 than from NaCl solutions, and the observed differences were attributed to the chaotropic and kosmotropic properties of K + and Na + 29 cations, respectively. It should also be pointed out that very few studies have ever been published on guar gum adsorption from high ionic strength brines [11, 12] and the solution chemistry of the polymer in concentrated electrolytes is still poorly understood. The main objective of this study is to gain insight into the behavior of guar gum in different electrolyte solutions of varying ionic strength through intrinsic viscosity measurements. 3.2 Materials and Methods 3.2.1 Materials A sample of guar gum, Rantec KP4000, was obtained from Rantec Corporation (Ranchester, WY, USA). The KP4000 is widely used by the Canadian potash industry as a slime blinder. First, a stock guar gum solution was prepared at a concentration of 1.8 g/L by mixing 0.9 grams of raw guar powder with 400 mL of distilled water at room temperature. The powder was slowly added into a vortex formed under thorough mixing with a magnetic stirrer. The guar-water suspension was then mixed for 4 hours at room temperature. Finally, the volume was made up to 500 mL while mixing. In order to remove any insoluble residues, all stock guar gum solutions were centrifuged at 10,000g (Heraeus, Biofuge-primo centrifuge). Using the phenol-sulfuric acid method developed by [13] to measure the absorbance of guar gum solutions before and after centrifuging, it was found that the absorbance values did not decrease further after a centrifuging time of 30 minutes. Although the initial raw guar gum concentration (1.8 g/L) was arbitrarily chosen, it also greatly sped up and enhanced the efficiency of the centrifuging step due to the rather low viscosity of the centrifuged solution. The preparation method thus also showed that the KP4000 contained 12% of water-insoluble parts, a number that agrees 30 very well with previously published data for a range of natural guar [14]. Stock guar gum solutions were prepared daily to minimize biochemical degradation, and the actual guar concentration was calculated from the phenol-sulfuric acid assay. The ionic strength of guar gum solutions was adjusted with lithium, sodium, potassium and cesium chlorides, and with the salts obtained after evaporating a saturated potash brine. The brine was prepared from a potash ore (Lanigan mine, Saskatchewan) by dissolving the ore in warm distilled water. The resulting solution contained a large amount of suspended slimes which were allowed to settle over a period of 3 weeks. The clear brine was siphoned out and evaporated. A chemical analysis of the brine revealed the following concentrations of the main cations: Na + 4.4 mol/L, K + 1.7 mol/L, Ca 2 + 0.040 mol/L and M g 2 + 0.015 mol/L. The brine can thus be treated as a 6.1-molar solution of KCl-NaCl. Cesium chloride (crystalline, 99.9% pure) was supplied by Aldrich. Lithium, sodium and potassium chlorides were ACS-certified chemicals from Fisher. Viscosity measurements were also made using urea (ACS) provided by Alfa Aesar. 3.2.2 Viscosity measurements Dilute guar gum solutions were made by mixing a stock solution (after centrifuging) in distilled water with concentrated salt solutions, or by adding crystalline salts to a guar solution in distilled water and taking a concentration correction for a change in the solution volume (for saturated salt solutions). The viscosities of guar gum solutions were measured using Cannon-Fenske capillary viscometers (Schott Gerate GmbH, Germany). After preparation, the solutions of desired polymer and salt concentration were left overnight (10-12 hrs). Then, 7-ml 31 aliquots of the tested solutions were transferred to a capillary viscometer, the viscometer was then placed in a water bath (set at 24 °C) for 30 minutes. Tests at higher temperatures (45°C, 70°C, and 90°C ) were performed only in distilled water. For high-temperature experiments, guar gum solutions were first tested at 24°C, then the temperature of the bath was raised to the desired value without removing the capillary (still containing the sample), and finally the capillary was allowed to equilibrate at the final temperature for 30 minutes before measurement. Afterwards, the bath was left to cool to 24 °C and the sample was tested again to investigate the reversibility of the effect of temperature on measured viscosities. Only guar gum solutions up to a concentration of less than 1 g/L were tested since more concentrated solutions (>1.5 g/L) gave a measurably non-Newtonian response, as determined with the use of a Haake rotational viscometer. The tested polymer concentration range also falls well below the limit c ~ 4/[n\ ([n] - intrinsic viscosity) sometimes given as the onset of entanglement formation for polysaccharides [15]. The kinematic viscosity was measured by allowing each solution to flow under gravity through the capillary. A Lauda PVS1 photo-timing and processing system, interfaced with a computer, was used to automatically measure the flow times from which the kinematic viscosities were calculated. Capillaries of different diameters (from 0.44 mm to 1.26 mm) were used so that the flow times were between 100 and 200 seconds. All the measurements were done as triplicates and the average values are reported. In order to find the true viscosities, the densities of all the solutions were also determined at 24 °C as the true viscosity is the product of the kinematic viscosity and density. 32 One of the simplest ways of assessing interactions between background salts and guar gum is through intrinsic viscosity measurements. The intrinsic viscosity [n] is a measure of the inherent ability of a polymer to increase solution viscosity. The most general relationship between intrinsic viscosity and the viscosity of dilute polymer solutions is a power series in concentration and can be given as: nsp/c = [nl + kl[n\2c + k2[nfc2 + k3[nfc3 + .... [3.1] where [n] is the intrinsic viscosity, rjsp is the specific viscosity (nret -• 1, and rjrei is the relative viscosity - the viscosity of solution divided by the viscosity of the solvent), and k\ are dimensionless constants. Since nsp /c is the reduced viscosity, nreci, which at c —> 0 becomes the intrinsic viscosity [n], the above power series is often truncated to a linear approximation which is best known as the Huggins equation [16]: rjred=[n] + k[f]fc [3.2] where nrej is the reduced viscosity, [n] is the intrinsic viscosity [dL/g], k is the Huggins coefficient (k ~ ki), and c is the concentration of the poiymer [g/dL]. It can be seen that a plot of rjred -f(c) should be a straight line with the intercept equal to [n] and the slope equal to k[rj]2. Thus, k and [rj] can graphically be calculated. Based on experimental observations, the physical meaning of the dimensionless Huggins coefficient k can be summarized as follows [17]: (a) a polymer exhibits a higher value of k in a poor solvent than in a good solvent, i.e., when the polymer-polymer interactions become favorable over polymer-solvent interactions, (b) it has a value of 0.5-0.7 in a theta solvent (the transition from a good solvent to poor solvent); and (c) k is very sensitive to the formation of molecular aggregates. 33 In other words, the Huggins coefficient k can provide additional information about the state of guar gum macromolecules in various electrolyte solutions, and be a measure of the goodness of the solvent (electrolyte solution). It should be noted that there are several other empirical equations that can be used to approximately obtain the intrinsic viscosity, or the constant k\ in Eq.l, and certain aspects of such alternative treatments of experimental data will be discussed later. Al l the intrinsic viscosity values given in this study are calculated from the Huggins equation. 3.2.3 Infrared spectroscopy In order to characterize the main functional groups on the guar gum macromolecule, Fourier Transform Infrared Spectroscopy (FTIR) was employed and FTIR spectra were recorded with the use of a Perkin-Elmer System 2000 infrared spectrophotometer. A droplet of 1-g/L guar gum solution was first frozen on an AgCl infrared window, then evaporated under vacuum (freeze-dried) and the resulting film of guar gum on the AgCl window was used for infrared analysis. The film preparation procedure was basically that of Rogers and Poling [18] for polyacryl amides. 3.3 Results and Discussion Figure 3.1a shows FTIR spectra of guar gum films prepared at pH 3 and pH 5.5. The spectra exhibit all typical bands and peaks characteristic of polysaccharides. The 2800-3000 cm'1 wavenumber range is associated with the stretching modes of the C-H bonds of methyl groups (-CH3), the broad band at 3400 cm'1 results from the presence of -OH groups, and the 900-1200 cm-1 range represents various vibrations of C-O-C glycosidic and C-O-H bonds. 34 4000 3500 3000 2500 2000 1500 1000 500 1800 1700 1600 1500 Wavenumber [cm1] Figure 3.1 FTIR spectra of guar gum films, prepared at pH 5.5 and pH 3. Figure 3.1b (right) shows the effect of pH on the IR absorbance of the carboxylic groups of carboxymethyl cellulose. Of special interest to this investigation is the range between 1500 cm -1 and 1800 cm 1 , typically used to detect the presence of carboxylic groups. It can be seen that there is a small peak on both guar gum spectra at 1640 cm"1 (Figure 3.1a) which is often assigned to the dissociated carboxylate group (-COO"). However, the peak does not change its relative position or intensity when the pH is lowered to 3, and no new peak appears near a wavenumber of 1740 cm"1 where the protonated COOH group should produce a strong band. The expected shift of the peak at 1640 cm"1 towards the 1730-1760 cm"' range can best be illustrated by the data shown in Figure 3.1b for two films of carboxymethyl cellulose (MW=250,000, degree of substitution 0.9, supplied by Polysciences) which were prepared following the same procedure as those of guar gum. Although this effect can be expected to be much more dramatic for carboxymethyl 35 cellulose, due to a high content of carboxylic groups in the cellulose ether, it appears that the KP4000 is free from any residual carboxylic derivatives and can thus be treated as totally nonionic. Natural gums are known to contain a fraction of uronic derivatives which would impart a weakly anionic character to the guar gum macromolecule [19]. It is also worth pointing out that a peak near 1640 cm"1 may originate from in-plane deformations of water molecules hydrogen-bonded to the polysaccharide molecule [20]. Since the KP4000 appears to be nonionic, any changes in the viscosity of its solutions in the presence of electrolytes cannot originate from changes in the conformation of the macromolecules (coiled vs. extended) resulting from the screening of anionic functional groups by dissolved ions. Figure 3.2 presents the effect of temperature on the viscosity of guar gum solutions in distilled water. The solid lines in Figure 3.2 show the Huggins fits to the individual sets of data, except the top line which was averaged over all the points obtained at 24 °C. It can be seen that there is a marked decrease in the viscosity values when the temperature is raised to 45°C from 24°C, and further changes take place when the temperature is raised to 70°C and 90°C. More importantly, the effect of temperature is completely reversible. When the same solutions are cooled down to 24 °C, the reduced viscosity values fully "recover" and the data fall on the same line at 24 °C. All individual sets of results follow the Huggins equation very well. It is also quite clear from Figure 3.2 that the intrinsic viscosity of guar gum (the intercept of the linear fits at zero guar gum concentration) does not significantly change with temperature in the studied range - it remains practically the same at 24 °C and 45 °C, it measurably decreases at 70 °C, 36 and only a minor further decrease can be observed at 90 °C. The average intrinsic viscosity for all the distilled water data, excluding the results at 70 °C and 90 °C, is 11.0 ± 0.25 dL/g (standard deviation) which is also a good indication of experimental errors involved. The intrinsic viscosity of guar gum is 10.6 dL/g (± 0.1 dL/g) at 70 °C, and 10.3 dL/g at 90 °C. ' 30-25 H -a . ^ 2 0 H o o O D Distilled Water O 2 4 ° C O Cooled to 24°C from 4 5 ° C • Cooled to 24°C from 7 0 ° C O Cooled to 24°C from 9 0 ° C V 4 5 ° C A 7 0 ° C 9 0 ° C 15H 0.02 0.04 0.06 0.08 Guar Gum Concentration [g/dL] Figure 3.2 Reduced viscosity of guar gum solutions in distilled water at various temperatures. Figures 3.3 through 3.6 show the reduced viscosities of guar gum in alkali metal chloride solutions of varying ionic strength; from that of distilled water to saturation - all at 24 °C. Potassium chloride is the least soluble salt among the tested electrolytes. At 24 °C the concentration of saturated KC1 is about 4.1 mol/L. This concentration was therefore the highest concentration at which the effect of the different salts could directly 37 0 0.02 0.04 0.06 0.08 0.1 Guar Gum Concentration [g/dL] Figure 3.4 Effect of sodium chloride on reduced viscosity of guar gum solutions at 24 C. 38 i—1—i—1—i—r—r 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 Guar Gum Concentration [g/dL] Figure 3 . 5 Effect of potassium chloride on reduced viscosity of guar gum solutions at 2 4 °C. A set of data for saturated potash brine is also shown. 3 0 --a o o > (D O -a a> Pi 2 5 H 2 0 H 1 5 H 1 0 ^ _L Cesium Chloride (mol/L) T T T 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 Guar Gum Concentration [g/dL] 0 . 1 Figure 3 . 6 Effect of cesium chloride on reduced viscosity of guar gum solutions at 2 4 C. 3 9 be compared. The concentrations of saturated LiCl, NaCl, and CsCl solutions are about 13.3, 5.3, and 7.5 mol/L, respectively. As Figure 3.3 illustrates, lithium chloride solutions do not affect the intrinsic viscosity of guar gum up to a salt concentration of 4.1 mol/L (average intrinsic viscosity: 11.7 ± 0.25 dL/g). Only a saturated solution of LiCl gives an intrinsic viscosity value of 21.7 dL/g - the highest value among the tested electrolytes (and urea). As seen from Figure 3.4, the effect of sodium chloride is similar and only a saturated NaCl solution gives a significantly higher intrinsic viscosity (13.4 dL/g), but not nearly as high as that obtained in saturated LiCl. The other NaCl concentrations produce very similar intrinsic viscosity values with an average of 11.6 ± 0.35 dL/g. Figures 3.5 and 3.6 present the reduced viscosities of guar gum in potassium and cesium chloride solutions, respectively. Both salts gave qualitatively similar results. Even for highly concentrated KC1 and CsCl solutions, the intrinsic viscosities are almost the same as those obtained in distilled water. The average intrinsic viscosity of guar, gum in both potassium and cesium chlorides is 11.2 dL/g (± 0.25 dL/g in KC1, and ± 0.3 dL/g in CsCl). One set of data seems to fall outside the statistically significant range; that for saturated CsCl solution for which the intrinsic viscosity is 10.6 dL/g. Although there appears to be a trend in the average values of intrinsic viscosity in the different electrolytes: 11.7 dL/g for LiCl, 11.6 dL/g for NaCl, 11.2 dL/g for KC1 and CsCl, and 11.0 dL/g for distilled water at 24°C (which may be quite interesting in the context of the subsequent discussion), considering the experimental errors involved these small differences are not quite statistically significant. Therefore, the results for all the salt concentrations (< 4.1 mol/L) and distilled water suggest that the intrinsic viscosity of 40 guar gum is essentially constant over a very wide range of ionic strengths, and only saturated solutions of LiCl, NaCl and CsCl give a significant effect. Excluding the three saturated solutions, the average intrinsic viscosity of guar gum for all the electrolyte data is 11.3±0.35dL/g(3%). Urea gave rather unexpected results, as shown in Figure 3.7. Up to a urea concentration of 1 mol/L, the intrinsic viscosity of guar gum remained largely unaffected compared to the value obtained in the salt solutions and distilled water. A 4-molar urea solution produced an intrinsic viscosity of 14.5 dL/g, while a saturated solution of urea gave a value of 12.5 dL/g. Interestingly, all guar gum solutions regardless of urea concentration generally yielded viscosities higher than the viscosity of guar gum solutions in distilled water. As far as other methods of evaluating the raw viscosity data are concerned, several linear approximations were also tested and compared against the Huggins equation, i.e., the Schulz-Blaschke equation (l/nred = l/[n] - ksec), the Kraemer equation (\n(nre/)/c = [n] + k^r/Yc), and the Martin equation (\n(nsp/c) = ln([>/]) + ku[yf\c), where the k\ coefficients denote the corresponding dimensionless constants. Since the main focus of this study is on reporting relative changes in [//] and k rather than on obtaining the true absolute values, only a qualitative summary of the different approaches is provided. In general, all three equations gave higher intrinsic viscosities compared to those obtained from the Huggins equation. In the worst case - the Huggins and Schulz-Blaschke fits to the 0.1 mol/L LiCl data (Figure 3.3) - the difference between the extrapolated intrinsic viscosities was 12% in the studied polymer concentration range. There were also sets of data that gave an excellent agreement (within 2%) between 41 intrinsic viscosities regardless of the method used, e.g., the data obtained in saturated salt solutions (even for saturated LiCl). In fact, it can mathematically be demonstrated that for 0.02 0.04 0.06 0.08 0.1 Guar Gum Concentration [g/dL] Figure 3.7 Effect of urea on reduced viscosity of guar gum solutions at 24 °C. a set of results, that follow the Huggins equation, as all the data in this study do, the three alternative equations will consistently overestimate intrinsic viscosity compared to the value obtained with the Huggins equation, and the difference will depend on the Huggins constant [21, 22, 23]. For data characterized by a high Huggins coefficient (>0.6-0.7), the plotted relationships between llnred (Schulz-Blaschke), \n(rjrei)/c (Kraemer), and \n(tjsp/c) (Martin) and polymer concentration, c, actually give quite non-linear results even when relative viscosity, rjrei, is less than 2 (c < -0.05 g/dL in this work). On the other hand, when k values are lower than 0.5 the plotted quantities become linear functions of c, and the agreement between the different methods of data treatment is very good. The data in 42 this research show exactly such trends but because of the nature of these results (i.e., conversion on the same intrinsic viscosity value) other functions are not shown for clarity. Since the intrinsic viscosity of guar gum is essentially constant (except saturated NaCl, KC1, CsCl and more concentrated urea), the observed differences in the reduced viscosity values in alkali metal chloride solutions, and in more dilute urea, are primarily due to changes in the Huggins coefficient, k (calculated using the slopes and intrinsic viscosities from Equation 2). The "anomalous" data for 4-molar urea, saturated urea, and saturated LiCl, NaCl, and CsCl are summarized in Table 3.1. Huggins constants calculated with the use of Equation 2 are plotted in Figure 3.8 as a function of electrolyte concentration. It is important to note that the results shown in Figure 3.8 reflect changes in the Huggins coefficient under the conditions of nearly constant intrinsic viscosity. The intrinsic viscosity can be treated as a measure of molecular size in solution, and is often Table 3.1. Huggins constants and intrinsic viscosities of guar gum in concentrated urea and saturated NaCl, LiCl, and CsCl solutions. Solution Intrinsic Viscosity, [n] (dL/g) Huggins constant, k Urea, 4 mol/L 14.5 0.53 Urea, saturated 12.5 0.95 NaCl, saturated 13.4 0.63 L i C l , saturated 21.7 0.52 CsCl, saturated 10.6 0.50 43 1.5 O NaCl • KCI A CsCl saturated potash brine . + 0 I I I 11II I I I I III I I I 11II I I I I III Dist. Water 0.01 0.1 1 Salt/Urea Concentration [mol/L] 10 Figure 3.8 Effect of electrolyte/urea concentration on Huggins constants of guar gum (T related to the molecular weight of guar gum through a Mark-Houwink-type of equation [15]. Based on the exponent value of the Mark-Houwink equation and the calculated intrinsic chain flexibility, Robinson et al. [15] concluded that guar gum behaves as a "slightly stiffened" random coil in aqueous solutions - a conclusion that was reinforced by a recent analysis of Picout et al. [24]. It should also be recognized that the solution preparation procedure employed in this study does not guarantee that the resulting solutions are truly molecular. Most likely, and based on the study of Picout et al. [4], the tested guar gum solutions contained a = 24 UC). 44 small amount of undissolved guar gum aggregates. The k values in distilled water (Figure 3.8) at 24°C are very high and are certainly consistent with the presence of aggregates [22]. According to Sakai's calculations [17] k values in theta solvents can be as high as 1.86 depending on the shape and volume factors of aggregates, with the highest values corresponding to ellipsoid or capsule-like shapes. The Huggins constant for guar gum in distilled water decreases from 1.07 at 24 °C to 0.73 (± 0.02) at both 45 °C and 70 °C, and to only 0.07 at 90 °C. In this context, it is interesting to observe that the Huggins constant of the KP4000 guar gum at 45 °C and 70 °C (0.73) appears to be very close (as far as visual analysis of published data permits) to the Huggins constant obtained by Picout et al. [4] for a guar solution prepared under conditions described by those authors as "best dissolution conditions", i.e., producing a truly molecular solution. This particular sample (treated for 10 minutes at 130 °C under 12-bar pressure) was also characterized by an intrinsic viscosity of -11.3 dL/g (Huggins fit) which is very close to the intrinsic viscosity of the KP4000 in distilled water (11.0 dL/g). It should also be noted that although high pressures. and temperatures were employed by those authors in preparing molecular solutions, the actual viscosity measurements were performed at 25 °C. Therefore, it should be pointed out that truly molecular solutions of guar gum do not necessarily give low k values (<0.75). It is worthwhile stressing that the usually accepted values of k for polymers in theta solvents (0.5-0.8) are applicable to flexible chain polymers, and there is both experimental and modeling evidence to suggest that galactomannan chains with a mannose-to-galactose ratio of 2:1 are rather stiff [15, 4, 24, 25]. In other words, it is highly unlikely that guar gum chains in dilute solutions form significant intramolecular 45 bonds and change their overall conformation as the solvent power changes. Such a collapse of a chain would inevitably result in a large decrease of intrinsic viscosity values but as the data show the intrinsic viscosity of the KP4000 is not strongly affected, and only a small decrease can be detected at temperatures higher than 70 °C. At the same time, however, a dramatic change in the Huggins constant can be observed and this decrease can be attributed to the disappearance/dissolution of the colloidal aggregates of guar gum at higher temperatures. The fact that the intrinsic viscosity of guar gum is quite constant over the studied temperature range does not exclude the possibility of the presence of colloidal guar particles since the contribution of such aggregates to the overall intrinsic viscosity value is very small [26], and thus the intrinsic viscosity alone is not an accurate measure of the extent of guar gum dissolution [15]. The idea that the observed trends in the Huggins constant are due to the enhanced solubility of guar gum aggregates is indirectly supported by the data obtained at 70 °C and 90 °C. As seen from Figure 3.2, the intrinsic viscosity of the KP4000 in distilled water is slightly lowered at high temperatures compared to 24 °C. As discussed by Picout et al. [4], this relatively small decrease of the intrinsic viscosity could be interpreted as "the ideal result" in the preparation of truly molecular solutions. Since polymer aggregates can be treated as having very high molecular weights (and light scattering methods certainly sense this effect), in such an "ideal" case, the complete dissolution of the aggregates should lead to a small* measurable decrease in the intrinsic viscosity which in a sense would be equivalent to the disappearance of these apparently high molecular weight fractions. 46 Since the effect of temperature on the viscosity of guar gum in distilled water is reversible it would be very hard to argue that the observed decrease in the viscosity of guar gum solutions at higher temperature could result from de-polymerization of the polysaccharide. The effect of increasing salt concentration is remarkably similar to the effect of increasing temperature. Assuming that high temperatures promote the dissolution of aggregates, the effect of salts can also be analyzed in terms of their ability to either promote or inhibit the dissolution of colloidal guar gum aggregates by following changes in the Huggins.constant, k. One of the most interesting features of the data presented in Figure 3.8 is the maximum on the curves at 0.1 mol/L salts. Dilute salts (< 0.1 mol/L) have a tendency to promote the formation of guar aggregates but no detectable differences can be seen between individual electrolytes. The maximum on the curve at an electrolyte concentration of 0.1 mol/L can qualitatively be predicted by a model of the viscosity of polymer solution proposed by Peterson and Fixman [27]. The important assumptions of the Peterson-Fixman model that lead to the appearance of a maximum value of k are that polymer molecules are allowed to form ellipsoid-shaped aggregates, and that aggregates move as rigid bodies in a shear field. In order to reconcile the data in Figure 3.8 with the model one would also need to assume that distilled water at room temperature (24 °C) is a theta-solvent (or a nearly theta-solvent) for guar gum, and that the solvent power (goodness) increases with salt concentration. The results in Figure 3.8 certainly suggest that warm distilled water is indeed a better solvent than cold distilled water, and that concentrated salt solutions generally reduce the Huggins constant consistent with 47 increasing solvent power. Therefore, the k value in distilled water at 24 °C (1.07) seems to be in-line with the earlier mentioned calculations of k for differently shaped aggregates in theta solvents [17] and the initial increase of k with salt concentration could also result from changes in the shape of the aggregates under the flow conditions. Interestingly, the model of Peterson and Fixman was criticized for the predicted maximum value of k, which was in contrast to a broad range of known experimental data, and the results presented in this study are the first to show such a maximum. In the low salt concentration range, guar gum remains in an aggregated state regardless of the solute type. However, as the electrolyte concentration increases above 0.1 mol/L, alkali metal chlorides not only start decreasing the Huggins coefficient but also clear differences between the salts can be observed. Interestingly, LiCl and NaCl appear to approach a limiting k value (0.98 for LiCl, and 0.86 for NaCl), while KCI and CsCl continuously reduce the Huggins coefficient to such values that the polymer is effectively under much better solvent conditions in these salts in comparison to distilled water or NaCl and LiCl solutions. A 4.1-molar CsCl solution gives a k value of only 0.35 implying that 4.1-molar CsCl is a very powerful solvent for guar gum. Recalling that all these changes in the Huggins coefficient take place under constant intrinsic viscosity, it seems that the apparent transition from a poor solvent (distilled water, LiCl and NaCl solutions) to a good solvent (warm distilled water, 4.1-molar KCI and CsCl) is associated with the dissolution of colloidal guar gum aggregates. Only saturated CsCl slightly reduces the intrinsic viscosity of guar gum which is again consistent with the "ideal dissolution conditions", as described by Picout et al. [4]. Comparing the final k values in 4.1-molar solutions of all the salts, the following 48 clear trend can be noted; LiCl gives the highest k value, then NaCl, KC1, and finally CsCl. This observation suggests that the dissolution of guar gum aggregates is much more favorable in KC1 or CsCl than in LiCl and NaCl. The order of the salts in their ability to enhance the solubility of guar gum is actually very significant. Sodium and lithium cations are known to be strongly kosmotropic ions, while potassium and cesium are very strong chaotropes. The terms "kosmotropes" and "chaotropes" refer to ions that are capable of either "reinforcing" or breaking the hydrogen-bonded network of water molecules. Kosmotropes are usually small (in terms of crystallographic radii), strongly hydrated ions, while chaotropes are relatively large and poorly hydrated. Therefore, kosmotropic ions are also called water structure making ions, and chaotropes are referred to as water-structure breaking ions. It is noteworthy that one of the simplest ways of assessing whether an electrolyte is of chaotropic or kosmotropic nature is through viscosity measurements. As the salt concentration in solution gradually increases, the viscosity of the solution will increase for kosmotropic ions, and it will slightly decrease (compared to the viscosity of pure water) for chaotropes. Chloride anions are usually classified as weakly chaotropic [28], and in metal chloride solutions it is usually the cation that determines the "net" behavior of the salt. Thus NaCl and LiCl continuously increase the viscosity of aqueous solutions (LiCl more than NaCl), while the viscosities of KC1 and CsCl solutions go through a shallow minimum (corresponding roughly to 90% of the viscosity of pure water) before increasing again near saturation. At electrolyte (and urea) concentrations higher than 4.1 mol/L, the intrinsic viscosity changes as well, so these highly concentrated solutions should be treated 49 separately. The significantly higher values of the intrinsic viscosity of guar gum (Table 3.1) indicate that a different type of guar gum structure forms in these systems in comparison to the "simple" aggregates. It should be stressed that such a direct comparison of the saturated solutions is not entirely correct since their molar concentrations are quite different. Nevertheless, it seems that only saturated solutions of the kosmotropic cations (Na and Li) can result in an increase of the intrinsic viscosity above the average value obtained for more dilute electrolytes (< 4.1 mol/L). The presence of large amounts of these strongly hydrated ions in guar gum solutions must lead to competitive hydration effects between the ions and the polysaccharide. Hence, it is reasonable to postulate that Li and Na cations extensively bind water molecules into their hydration sheaths leaving no free water molecules to hydrate/solubilize the polymer chain in saturated salt solutions. Under these conditions, strong polymer-polymer interactions are much more favorable than polymer-solvent (water) interactions and the strongly "dehydrated" guar gum undergoes an even further, more extensive networking/ aggregation than in more dilute salt solutions. The dehydrated polysaccharide chains must aggregate to adjust to these new solvent conditions, i.e., to the lack of free solvent molecules. The resulting new aggregated structures are characterized by higher intrinsic viscosities than those measured in distilled water or in more dilute salt solutions. It should also be noted that this transition to a more aggregated state at very high salt concentrations is accompanied by a decrease of the Huggins constant for both LiCl and NaCl (Figure 3.8 and Table 3.1). In the case of saturated salt solutions, however, it would probably be more appropriate to refer to these solutions as good solvents for those new types of structures, appearing only in saturated solutions, rather than for the hydrated 50 "simple" guar gum aggregates. In contrast to Na+ and L i + cations, poorly hydrated K + and Cs + ions do not strongly interfere with the hydration process of the polymer and allow guar gum to freely interact with water molecules. Therefore, concentrated KC1 and CsCl solutions are more powerful solvents for guar gum than NaCl and LiCl. Considering the above, the effect of urea on the viscosity of guar gum is much more similar to the action of kosmotropic NaCl and LiCl than to the effect of KC1 and CsCl. As the urea concentration increases above 1 mole/L, the Huggins constant appears to go through a minimum at 4 mol/L (Table 3.1 and Figure 3.8). At the same time, however, the intrinsic viscosity of guar gum in 4-molar urea - the concentration that is often used to destroy extensive polysaccharide structures in more concentrated polymer solutions - is the highest of all urea solutions studied, and is much.higher than the intrinsic viscosity in 4.1-molar salts. This trend again suggests that, as in the case of highly concentrated LiCl and NaCl, guar gum still remains in an aggregated state in 4-molar urea although the "goodness" of this type of solvent is quite different from the goodness of distilled water. In some adsorption studies, 4-molar urea is also used as a "breaker" of hydrogen bonds between guar gum and mineral surfaces which provides useful information about the mechanism of interactions between the polymer and various minerals [7]. It is plausible that these aggregate-breaking properties of urea do not manifest themselves in the low guar gum concentration range studied in this work, where the formation of extensive guar gum structures is not as pronounced as in more concentrated polysaccharide solutions. The well-documented fact that more concentrated guar gum solutions are strongly viscoelastic is an excellent macroscopic evidence of the 51 presence of highly interconnected structures whose solution properties could not be probed by viscosity measurements made on very dilute guar gum solutions. For the KP4000 guar gum, this onset of viscoelasticity appears to be near a polysaccharide concentration of 1.5 g/L, as mentioned in the experimental section. Finally, the saturated potash brine behaves more like 4.4-molar NaCl than 1.7-molar KCI (note the composition of the brine given in the experimental section). The brine produces an intrinsic viscosity value of 11.2 dL/g (Figure 3.5) and a k value of 0.82. For comparison, the k values are 0.86 and about 0.65 in 4.1-molar NaCl and 1.7-molar KCI, respectively. Following the above discussion, guar gum appears to be in an aggregated state suggesting that the dehydration of the polymer by the high concentration of Na + ions dominates over the structure-breaking capabilities of K + in this saturated mixed-electrolyte system. The similarity between the effect of high temperature and high salt concentrations deserves a couple of additional comments. The main property that is affected in both systems is the relative degree of water-water vs. water-guar (polymer hydration) hydrogen bonding which in turn is fundamentally related to the way guar gum dissolves in aqueous solutions. Both high temperature and chaotropic ions reduce the viscosity of water which is a macroscopic manifestation of the breaking of hydrogen bonds between water molecules. Gittings et al. [5] analyzed the behavior of guar gum in heavy water (D20) and concluded that D 2 0 is a worse solvent for guar gum compared to H 2 0, as suggested by larger guar aggregate sizes measured in D 2 0. It should be mentioned that the viscosity of D 2 0 is about 25% higher than that of Ff20 at 25 °C [29], and D 2 0 should form stronger hydrogen bonds than H 2 0 [30]. 52 Although direct light scattering measurements would be needed to confirm the dissolution of aggregates in chaotropic salt solutions (KC1 and CsCl), The results highlight the importance of background ions in controlling polymer hydration phenomena and defining the behavior of guar gum in dilute and concentrated electrolyte solutions. A recent molecular dynamics simulation (MDS) study of alkali metal chloride solutions by Du et al. [31] generally confirms the above comments on the binding of water molecules around alkali metal cations and on the resulting structure of the surrounding solution. From the adsorption point of view, the presence or dissolution of colloidal guar gum aggregates, as influenced by various background salts, should lead to different modes of polymer adsorption on mineral surfaces. Traditionally, it is assumed that individual guar gum molecules participate in the adsorption process from dilute guar gum solutions, but as the results suggest it is probably more accurate to consider the adsorption of entire guar gum aggregates as opposed to single polymer chains. 3.4 Conclusions The intrinsic viscosity of the KP4000 guar gum is remarkably constant over a wide range of alkali metal chloride concentrations at 24°C. No effect of the salts on the intrinsic viscosity can be detected up to an electrolyte concentration of 4.1 mol/L. The average intrinsic viscosity under these conditions is 11.3 dL/g (± 3.0%). A simple analysis of changes in the Huggins coefficient indicates that guar gum chains are in an aggregated state in distilled water and in dilute salt solutions. At higher electrolyte concentrations (> 0.1 mol/L), the state of the aggregation of the polysaccharide depends on the kosmotropic or chaotropic properties of the background electrolytes. The presence of strongly chaotropic ions, such as K + and Cs+, (water-structure breaking ions) seems to 53 induce the dissolution of guar gum aggregates into individual molecules. Kosmotropic ions, such as L i + and Na+, affect guar solutions differently and these strongly hydrated ions effectively decrease the amount of free water molecules in solution that would otherwise be available for polymer chain solubilization. As a result, guar gum chains undergo an even more extensive aggregation in saturated solutions of LiCl and NaCl to form larger structures characterized by significantly higher intrinsic viscosities. Overall, concentrated chaotropic electrolyte solutions are very powerful solvents for guar gum. The effect of urea on the properties of dilute solutions of the KP4000 guar gum is comparable to the effect of NaCl. Therefore, the "dispersing" capabilities of urea towards guar gum aggregates appear to be very poor in the studied polymer concentration range. 3.5 References [1] Whistler, R. L., and Hymowitz, T., Guar: agronomy, production, industrial use, and nutrition, Purdue University Press, Indiana, (1979). [2] Painter, T.J., Gonzalez, J.J., and Heramer, P.C., The distribution of D-galactosyl groups in guaran and locust-bean gum: new evidence from periodate oxidation. Carbohydrate Research, Vol.69, pp. 217-226, (1979). [3] Dea, I .CM., Conformational origins of polysaccharide solution and gel properties. In Whistler, R.L. and BeMiller, J.N. (Ed), Industrial gums — polysaccharides and their derivatives, Academic Press, (1993). 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[8] Liu, Q., Zhang, Y., and Laskowski, J.S., The adsorption of polysaccharides onto mineral surfaces: an acid/base interaction, International Journal of Mineral Processing, Vol. 60, pp. 229-245, (2000). [9] Shortridge, P.G., Harris, P.J., Bradshaw, D.J. and Koopal, L.K., The effect of chemical composition and molecular weight of polysaccharide depressants on the flotation of talc, International Journal of Mineral Processing, Vol. 59, pp. 215-224, (2000). [10] Ma, X. and Pawlik, M., Effect of alkali metal cations on adsorption of guar gum onto quartz, Journal of Colloid and Interface Science, Vol. 289, pp. 48-55, (2005). [11] Pawlik, M . and Laskowski, J.S., Stabilization of mineral suspensions by guar gum in potash ore flotation systems, Canadian Journal of Chemical Engineering, Vol. 84, pp. 532-538,(2006). [12] Ma, X. and Pawlik, M. , Adsorption of guar gum on potash slimes, In Z. Xu, & Q. Liu. Interfacial Phenomena in Fine Particle Technology, pp. 509-524, Montreal, QC: Metallurgical Society of CIM, (2006). [13] Dubois, M. , Gilles, A., Hamilton, J.K., Rebers, P.A., and Smith, F., Colorimetric method for determination of sugars and related substances, Analytical Chemistry, Vol. 28, pp. 350-356, (1956). [14] Chatterji, J. and Borchardt, J.K., Applications of water-soluble polymers in the oil field, Journal of Petroleum Technology, Vol. 33, pp. 2042-2056, (1981). [15] Robinson, G., Ross-Murphy, S. B., and Morris, E. R., Viscosity-molecular weight relationships, intrinsic chain flexibility, and dynamic solution properties of guar galactomannan, Carbohydrate Research, Vol. 107, pp. 17-32, (1982). [16] Huggins, M. L., The viscosity of dilute solutions of long-chain molecules. IV. dependence on concentration, Journal of the American Chemical Society, Vol. 64, pp. 2716-2718, (1942). [17] Sakai, T., Huggins constant k' for flexible chain polymers, Journal of Polymer Science A-2, Vol. 6, pp. 1535-1549, (1968a). [18] Rogers, D. and Poling, G., Compositions and performance characteristics of some commercial polyacrylamide flocculants, CIM Bulletin, Vol. 71, pp. 152-155, (1978). 55 [19] Wang, Q., Ellis, P.R. and Ross-Murphy, S.B., Dissolution kinetics of guar gum powders-II. Effects of concentration and molecular weight, Carbohydrate Polymers, Vol. 53, pp. 75-83, (2003). [20] Synytsya, A., Copikova, J., Matejka, P. and Machovic, V., Fourier transform Raman and infrared spectroscopy of pectins, Carbohydrate Polymers, Vol. 54, pp. 97-106, (2003). [21] Sakai, T., Extrapolation procedures for intrinsic viscosity and for Huggins constant k', Journal of Polymer Science A-2, Vol. 6, pp. 1659-1672, (1968b). [22] Bohdanecky, M. , and Kovaf, J., Viscosity of Polymer Solutions, Elsevier, (1982). [23] Lovell, P.A., Dilute solution viscometry. In Price, C. and Booth, C. (Ed), Comprehensive Polymer Science: Polymer Characterization, Vol. 1, pp. 173-197, Pergamon Press, (1989). [24] Picout, D.R., Ross-Murphy, S.B., Jumel, K., and Harding, S.E., Pressure cell assisted solution characterization of polysaccharides - 2. Locust bean gum and tara gum, Biomacromolecules, Vol. 3, pp. 761-767, (2002). [25] Petkowicz, C.L.O., Reicher, F., and Mazeau, K., Conformational analysis of galactomannans: from oligomeric segments to polymeric chains, Carbohydrate Polymers, Vol. 37, pp. 25-39, (1998). [26] Kratochvil, P., Particle scattering functions. In M . Huglin (Eds), Light Scattering from Polymer Solutions, pp. 333-384, Academic Press, (1972). [27] Peterson, J.M. and Fixman, M. , Viscosity of polymer solutions, Journal of Chemical Physics, Vol. 39, pp. 2516-2523, (1963). [28] Wiggins, P.M., Enzyme reactions and two-state water, Journal of Biological Physics and Chemistry, Vol. 2, pp. 25-37, (2002). [29] Horita, J., and Cole, D.R., Stable isotope partitioning in aqueous and hydrothermal systems to elevated temperatures. In Palmer, D.A., Fernandez-Prini, R. and Harvey, A.H. (Ed), Aqueous systems at elevated temperatures and pressures: Physical chemistry in water, steam and hydrothermal solutions, pp. 277-319, Elsevier, (2004). [30] Chen, B., Ivanov, I., Klein, M.L. and Parrinello, M. , Hydrogen bonding in water, Physical Review Letters, Vol. 91, letter 215503, pp. 1-4, (2003). [31] Du, H., Rasaiah, J.C. and Miller, J.D., Structural and Dynamic Properties of Concentrated Alkali Halide Solutions: A Molecular Dynamics Simulation Study, Journal of Physical Chemistry B, vol. I l l , pp. 209-217, (2007). 56 C h a p t e r 4 R o l e o f B a c k g r o u n d I o n s i n G u a r G u m A d s o r p t i o n o n O x i d e M i n e r a l s a n d K a o l i n i t e * 4 . 1 . I n t r o d u c t i o n Guar gum is a natural nonionic polysaccharide produced from the seeds of two annual leguminous plants, Cyamopsis tetragonalobus and psoraloides. The guar gum macromolecule is a chain of (1—>4)-linked p-D-mannopyranose units with a-D-galactopyranose units connected to the mannose backbone through (1—>6) glycosidic linkages. The poly-mannose chain is randomly substituted with galactose units at a mannose-to-galactose ratio of 1.8-1.0 [1, 2]. Because of the random nature of the substitution, the least substituted sections of the guar gum molecules are believed to show the greatest tendency to associate, while the more densely substituted regions serve to solubilize the polymer chain [3]. In relation to froth flotation applications, the adsorption of guar gum and polysaccharides in general on various minerals has been studied quite extensively and more detailed summaries of the published data can be found in other recent papers [4-6]. As demonstrated in Chapter 2 and in a previous.study by Ma and Pawlik [7] on guar gum adsorption on quartz from dilute alkali metal chloride solutions, a number of previously undescribed trends were observed. The adsorption density of the polymer doubled in the presence of potassium and cesium chlorides (0.01 mol/L), compared to the adsorption density in sodium and lithium chlorides and in distilled water. When measured as a function of ionic strength (from 10"3 to 10"' mol/L KCI, NaCl, HCI, MgCL and CaCb), the adsorption density increased significantly only in the presence of * A version of this chapter was published in Journal of Colloid and Interface Science. [Reference: Ma, X. and Pawlik, M. , Role of Background Ions in Guar Gum Adsorption on Oxide Minerals and Kaolinite , Journal of Colloid and Interface Science, vol. 313, no. 2, pp. 440-448, (2007).] 57 KCI with MgCL giving a measurable decrease in guar adsorption. Interestingly, H + ions (from HCI) behaved as "simple" background ions rather than as potential determining ions towards quartz (see Chapter 2). This enhanced adsorption from dilute, cesium and potassium chloride solutions was attributed to the water-structure breaking properties of poorly-hydrated K + and Cs + cations. Even the observed increase in guar gum adsorption at high pH on quartz was correlated with higher and higher concentrations of K + and Cs + ions introduced with the corresponding bases (KOH and CsOH) for pH adjustments. Assuming hydrogen bonding as the adsorption mechanism, it was suggested that the polysaccharide adsorption process should be treated as competition between water molecules and polysaccharide chains for the surface hydroxyl groups, and that water-structure breaking cations (chaotropes) were capable of disrupting the interfacial water layer on the quartz surface and allowing the polymer to interact with the surface silanol sites. In order to assess the significance of lyotropic ion effects in controlling the adsorption of guar gum on oxide minerals, quartz, alumina, titania (rutile), hematite and kaolinite were compared as model adsorbents in this study. 4.2. Materials and Methods 4.2.1 Materials A sample of guar gum, Rantec KP4000, was obtained from Rantec Corporation ^ (Ranchester, WY, USA). The polymer can be characterized by an intrinsic viscosity of 11.3 dL/g over a wide range of ionic strengths (up to 4.1 mol/L LiCl, NaCl, KCI, and CsCl) [see Chapter 3] which gives a weight-average molecular weight of 1.54 million [8]. Since the preparation method of a guar gum solution has a significant impact on 58 its final properties, as discussed in later sections, the following paragraph provides more detail about the procedure adopted in this study. First, a stock guar gum solution was prepared at a concentration of 1.8 g/L by mixing 0.9 grams of raw guar powder with 400 mL of distilled water at room temperature. The powder was slowly added into a vortex formed under thorough mixing with a magnetic stirrer. The guar-water suspension was then mixed at high speed for 4 hours at room temperature. Finally, the volume was made up to 500 mL while mixing. In order to remove any water-insoluble residues, all stock guar gum solutions were centrifuged at 10,000g (Heraeus, Biofuge-primo centrifuge). Using the phenol-sulfuric acid method [9] to measure the absorbance of guar gum solutions before and after centrifuging, it was found that the absorbance values did not decrease further after a centrifuging time of 30 minutes. Although the initial raw guar gum concentration (1.8 g/L) was arbitrarily chosen, it also greatly sped up and enhanced the efficiency of the centrifuging step due to the rather low viscosity of the centrifuged solution. The preparation method also showed that the KP4000 contained 12% of water-insoluble parts, a number that agrees very well with previously published data for a range of natural guar gums [10]. Stock guar gum solutions were prepared daily to minimize biochemical degradation. Quartz was obtained from Sigma-Aldrich. Its detailed characterization is given elsewhere [7]. Samples of iron oxide, titania (99.5% rutile), and alumina (99.99% a-AI2O3) were supplied by Alfa-Aesar. A quantitative x-ray diffraction analysis of the iron oxide sample showed that the material actually contained 94%.hematite (a-FeiO?,) and 6% goethite. Kaolinite was provided by Ward's Natural Science Establishment. The 59 sample was dry-ground in a porcelain ball mill to 100% passing 38 microns. The sample was found to contain (x-ray diffraction analysis) 97% kaolinite and 3% K-feldspar. The isoelectric point (i.e.p.) of each mineral was determined with the use of a ZetaProbe (Colloidal Dynamics, Warwick, RI) by potentiometric titration of a 10% (wt) suspension in 0.01 mol/L NaCl using 2 mol/L NaOH and HCI for pH adjustments. The i.e.p. values of alumina and iron oxide (hematite) were found to be 8.8 and 9.0, respectively (see Appendix 2 and 3). The quartz sample was negatively charged in the entire pH range tested (3-11) [7]. The apparent i.e.p. of kaolinite was found to be at pH 3.0. This value is referred to as "apparent" due to the anisotropic nature of the kaolinite particles. The raw rutile powder gave an i.e.p. value of 3.9 indicative of surface contamination. After several washing steps with concentrated NaOH (in a Nalgene polyethylene pipette jar), and by measuring the i.e.p. after each step, it was found that the final i.e.p. value increased to 5.1 - still below a value of 5.5 given for silica-free rutile [11]. It is, therefore, possible that the cleaned rutile surface was still covered by a small amount of a silica-like coating. The average particle sizes (Malvern Mastersizer 2000) and BET (Brunauer, Emmet, Teller) specific surface areas of the samples measured from nitrogen adsorption are summarized in Table 4.1. It should be noted that the BET values are all corrected for the microporosity of the samples, as determined with the use of an Autosorb-IMP BET analyzer (Quantachrome). For calculating the adsoiption densities of guar gum, only the external specific surface area was used since only these external surfaces are accessible to guar gum. These external BET specific surface areas were obtained by subtracting the surface area contained in pores (through a simultaneous porosity measurement) from the 60 total BET surface areas obtained from nitrogen adsorption. The agreement between the total BET values and those obtained from the particle size distribution (assuming that particles were spherical and non-porous) was very poor, which was indicative of highly non-uniform surface topography and non-spherical particle shapes. For quartz, a BET specific surface area of 1.7 m7g was used [7]. Cesium chloride (crystalline, 99.9%) cesium hydroxide (50 wt% solution 99% pure) and lithium hydroxide (crystalline, .98% pure) were supplied by Aldrich. Lithium, sodium, and potassium chlorides, as well as sodium and potassium hydroxides, were ACS-certified chemicals from Fisher. Hydrochloric acid (ACS-Fisher) was also used for pH adjustments. All chemicals were used "as received". Table 4.1 The volume-average particle sizes and BET specific surfaces areas of the tested samples. Hematite Rutile Alumina Kaolinite BET specific surface area [m2/g] 4.6 3.2 12.8 2.7 volume-average particle size [micron] 2.8 0.11 3.1 5.2 4.2.2 Methods In the adsorption tests, 10 grams of a mineral were mixed with 50 ml of a background solution in a 125-ml Nalgene polyethylene bottle, and conditioned in a thermostated shaker for 20 minutes. Then, 50 ml of guar gum solution of known polymer concentration and ionic strength (prepared by dilution of a centrifuged stock solution) were added and the entire mixture was shaken for a further 30 minutes. In the case of saturated salt solutions, crystalline salts were added to a guar gum solution in distilled water while mixing until a small amount of crystals was still left in solution after a mixing time of 60 minutes. For each saturated solution, a correction for a change in the solution volume was taken to obtain the exact guar gum concentration. After 61 conditioning, the pH of the suspension was measured, the adsorption mixture was centrifuged to remove the solids, and the supernatant was assayed for the presence of guar gum using the spectrophotometric phenol-sulfuric acid method (1-cm optical path length quartz cell, Cary 50 spectrophotometer, Varian) [9]. For samples in saturated salt • solutions, the supernatants were diluted 1:1 with distilled water prior to the addition of phenol and sulfuric acid. This dilution step was a safety precaution since saturated chloride solutions boiled quite violently when mixed with concentrated sulfuric acid releasing large quantities of HCI fumes. Blank absorbance corrections were taken for each background salt solution. The residual (equilibrium) guar gum concentration was read from a calibration curve prepared separately for each electrolyte. 4.3 Results Figure 4.1 shows the effect of alkali metal chlorides (0.01 mol/L) on the adsorption of guar gum on kaolinite. The results are essentially identical to those obtained in the earlier study on quartz [7]. The adsorption densities in LiCl, NaCl and distilled water are practically equal, while KCI and CsCl significantly enhance the adsorption of the polysaccharide. Figure 4.2 presents analogous data for hematite and alumina at pH 11 - or above the i.e.p. values of the oxides when the surfaces are negatively charged - in order to demonstrate the effect of alkali metal counter-ions on guar gum adsorption. The high pH of the adsorption mixtures was adjusted using the corresponding hydroxides. It should also be noted that these two oxides were found to possess very similar i.e.p. values; pH 8.8 and 9.0 for alumina and hematite, respectively. 62 -o -o < c =J o S < 1.20-1.00-0.80-0.60-0.40-0.20-0 0.00-_ _ i I Kaolinite pH 5.2-5.5 20 • Distilled Water O 0.01 mol/L LiCl [• O 0.01 mol/L NaCl \_ • 0.01 mol/L KC1 A 0.01 mol/L CsCl L 40 60 80 100 120 Guar Gum Equilibrium Concentration [mg/L] Figure 4.1 Adsorption of guar gum on kaolinite in the presence of 0.01 mol/L alkali metal chlorides. S -a .o 1.20-1.00-0.80-o 0.60H -a < § 0.40H o.oo-0 O 0.01 mol/L L i C l O 0.01 mol/L NaCl • 0.01 mol/L KC1 A 0.01 mol/L CsCl 71 Hematite pH 10.9-11.1 [ ^X-OA-O—€ty Aluinina pH 10.9-11.1 I-T I n 1 1 1 r 20 40 60 80 Guar Gum Equilibrium Concentration [mg/L] 100 Figure 4.2 Adsorption of guar gum on hematite and alumina in the presence of 0.01 mol/L alkali metal chlorides, pH 11. 63 Hematite and alumina give two very different adsorption densities but in neither case is the adsorption density of guar gum significantly affected by the salt type. This adsorption response is quite different from the behavior of kaolinite seen in Figure 4.1, or quartz, as discussed elsewhere |7|. Figure 4.3 illustrates the adsorption of guar gum on titania in 0.01 mol/L alkali metal chlorides at pH 11. These results were separated from the other oxides for clarity since the adsorption densities on titania are almost the same as those on hematite (Figure 4.2). As in the cases of hematite and alumina, guar gum adsorption on titania is not affected by the salt type - certainly not as strongly as on quartz and kaolinite - and it is possible that the presence of residual silica on the surface (as judged from the i.e.p. value of the washed titania sample) causes some preferential adsorption from KCI and CsCl. For the tests performed as a function of pH, the initial guar gum concentrations were such < - O 0.01 mol/L LiCl § 0.40- O 0.01 mol/L NaCl | .: • 0.01 mol/L KCI m n J A 0.01 mol/L CsCl 0.00-| 1 1 1 1 1 1 1 1 1 f-• 0 20 40 60 80 100 Guar Gum Equilibrium Concentration [mg/L] Figure 4.3 Adsorption of guar gum on titania in the presence of 0.01 mol/L alkali metal chlorides, pH 11. 64 that a small amount of guar gum, about 20-25 mg/L, was always present in equilibrium with the solids. The effect of pH on guar gum adsorption on kaolinite is shown in Figure 4.4. The results are again remarkably similar to the earlier data for quartz [7]. Two distinct sets of data can clearly be seen, those in LiCl and NaCl and those in KC1 and CsCl. pH values higher than 5.5 were adjusted with the corresponding hydroxides in order to avoid introducing foreign cations. Regardless of the background electrolyte type, the adsorption of guar gum is independent of pH in the range from 3 to 11, but CsCl and KC1 give higher adsorption densities compared to NaCl and LiCl. These results are in agreement with the trends seen in Figure 4.1. 1.00-0.80-S -o -o <• 0.60-0.40H 0.20-0.00-—a-_L ^ ^k-Kaolinite -$ O 0.01 mol/L LiCl O 0.01 mol/L NaCl • 0.01 mol/L KC1 A 0.01 mol/L CsCl 10 pH 12 Figure 4.4 Effect of pH on guar gum adsorption on kaolinite in 0.01 mol/L alkali metal chlorides. Initial guar gum concentration 270 mg/L. In contrast, the effect of pH on guar gum adsorption on alumina and hematite presented in Figure 4.5 is very weak. 65 E 00 -a ' <u -o < o E < 1.20-I.OOH 0.80H 0.60-0.40-0.20H 0.00-Hematite 0 0.01 mol/L LiCl 0 0.01 mol/L NaCl • 0.01 mol/L KCI , A 0.01 mol/L CsCl -oOZr Alumina 10 12 pH Figure 4.5 Effect of pH on guar gum adsorption on hematite and alumina in 0.01 mol/L alkali metal chlorides. Initial guar gum concentration 490 mg/L for alumina, and 545 mg/L for hematite. E oo E < o E < 1.20-1.00-0.80-0.60-0.40-0.20H 0.00-Titania O 0.01 mol/L LiCl O 0.01 mol/L NaCl • 0.01 mol/L KCI A 0.01 mol/L CsCl pH 10 12 Figure 4.6 Effect of pH on guar gum adsorption on titania in 0.01 mol/L alkali metal chlorides. Initial guar gum concentration 390 mg/L. 66 As seen in Figure 4.6, guar gum adsorption on titania does not depend on pH and salt type - the results basically fall on the same line consistent with the data shown in Figure 4.3, and the differences between individual electrolytes are not quite significant (within 5%). In contrast to kaolinite and quartz, these two oxides give the same adsorption densities in the presence of the different salts. The adsorption densities on titania are very similar to the adsorption densities on hematite. Since KC1 and NaCl are capable of producing significantly different adsorption densities of the polymer, the effect of ionic strength was investigated in more detail for these two salts. Cesium and lithium chlorides are expected to produce qualitatively similar results, as could be deduced from the experiments with quartz carried out in 0.1 mol/L alkali metal chlorides [7|. The adsorption results for quartz and kaolinite over the full ionic strength range are presented in Figure 4.7 and 4.8, respectively. Both kaolinite and quartz gave the same type of adsorption results. Low concentrations of KC1 significantly increase the adsorption of guar gum in comparison to adsorption from dilute NaCl. Guar gum adsorption appears to reach a maximum at KC1 concentrations on the order of 0.1-1 mol/L and then measurably decreases as the salt concentration approaches saturation. In contrast, adsorption from dilute NaCl (0.01 mol/L) is not very different from adsorption from distilled water, but it steadily increases at higher concentrations of NaCl. Interestingly, both saturated NaCl and KC1 - the adsorption points at the highest salt concentrations - produce almost equal adsorption values. 67 J I I I I I 111 I I I I 1 I 111 I I I I I 1 111 1 1—I I I I 1 1 0.00—I 1 1 — i i i 11 rj 1 1—i i i i 111 1 1—i i i i 111 1 1—i i • n 11 j — Distilled 0.01 0.1 1 10 Water Salt Concentration [mol/L] Figure 4.7 Effect of ionic strength on guar gum adsorption on quartz, pH 5.2-5.5. Initial guar gum concentration 110 mg/L. 1.20-1.00H j ? 0.80--o 1 0.60H < -w I 0.40-0.20H _l I i i i i i i I I | i * i i i i I I i I J I J . 1.1.1 I | I I , 1111 0.00 Kaolinite, pH 5.2-5.5 O NaCl • KCI —I 1 1 I 111 Vj 1 1 1 - T - T T T T | 1 1 r I I I I 11 1 1 V I M i l Distilled 0.01 0.1 1 10 Water Salt Concentration [mol/L] Figure 4.8 Effect of ionic strength on guar gum adsorption on kaolinite, pH 5.2-5.5. Initial guar gum concentration 270 mg/L. 68 Figure 4.9 shows the effect of KC1 and NaCl concentration on guar gum adsorption on hematite, titania, and alumina. No differences between NaCl and KC1 could be detected for alumina, and only a small change was seen on hematite and titania. Although the possible contamination of titania by residual silica cannot be ruled out, the differential effect of KC1 over NaCl for these three oxides is rather weak (if any). ' ' " I i i • ' ' • II I a 1 0.80-- o -a < S3 3 o a < 0.60H 0.40d 0.20H 0 NaCl - Hematite • KC1-Hematite • NaCl-Titania • KC1-Titania A KC1 - Alumina 0 NaCl-Alumina 6 o.oo-Distilled Water - i — i — I I I I I I I i—i—i 1 1 1 1 1 1 1—i—i 1 1 1 1 1 1 1—i—i 1 1 1 1 1 0.01 0.1 1 Salt Concentration [mol/L] 10 Figure 4.9 Effect of ionic strength on guar gum adsorption on alumina, hematite, and titania, at pH 11.0. Initial guar gum concentration 490 mg/L for alumina, 390 mg/L for titania, and 550 mg/L for hematite. The adsorption of guar gum on hematite, alumina, and titania decreases as the salt concentration reaches saturation for both NaCl and KC1, which makes the behavior of these adsorbents quite different from that of quartz and kaolinite. Overall, the adsorption density of the polymer is quite constant on hematite, alumina and titania over a wide ionic strength range. 69 4.4. Discussion 4.4.1 General comments and observations. It is clear that all the tested minerals give "very high affinity"-type of adsorption data. In fact, for all the minerals, there was a range of guar gum concentrations in which the adsorption of the polymer was complete and no polymer could be detected in equilibrium with the suspended solids. This observation suggests that the KP4000 interacts with all the minerals in a similar way regardless of the surface properties of the oxides. Another important observation is that there is no effect of pH on the adsorption density of the polymer on any of the minerals, and the same observation was made in a earlier work on quartz [7]. A very significant conclusion from this trend is that guar gum adsorption is not related to the amount of hydroxyl groups on the mineral surfaces. There seems to be a common agreement in the literature that the OH groups of various polysaccharides interact with surface Me-OH sites on mineral surfaces to form acid-base complexes. As the data show, however, guar gum does not exhibit any enhanced affinity towards oxide surfaces (and kaolinite) at the corresponding i.e.p. values of the minerals. At the i.e.p, all oxide surfaces could be expected to have the highest concentration bf hydroxyl groups but this does not seem to be a factor in guar gum adsorption, at least not in the studied pH range. It is also quite clear that guar gum adsorption does not depend on the sign and density of the surface charges of the oxide surfaces. Therefore, the results for guar gum drastically differ from the extensive data of Liu and Laskowski on the adsorption of dextrin on oxide surfaces [12-14]. For example, the adsorption density of low molecular weight dextrin on titania, hematite and litharge (PbO) was shown to reach a well-defined maximum near the corresponding i.e.p. of the 70 oxides and then decreased to zero away from the i.e.p., particularly on the alkaline side. Simultaneously, the same dextrin sample gave very low adsorption on quartz regardless of pH [12, 14]. A similar strong pH-dependence of dextrin adsorption on other oxides (y-alumina and magnetite) was reported by Raju et al. [15]. The results for a different' dextrin sample confirm the low affinity of dextrin towards quartz in comparison to the high affinity of guar gum, and in contrast to guar gum no enhancing effect of KCI over NaCl on dextrin adsorption was observed [7]. In the case of the KP4000 guar gum, it is also very difficult to correlate the i.e.p. of the minerals with the measured adsorption densities, but such a relationship was successfully determined for dextrin adsorption on various minerals including metal oxides [4, 14, 15]. All these observations are a very strong indication that the adsorption behavior of guar gum on oxide minerals is fundamentally different from the behavior of dextrin. It is also important to recognize that even nominally non-ionic, natural guar gums may in fact contain a fraction of anionic uronic units along the macromolecule chain which would widen the range of possible interaction mechanisms with mineral surfaces. Fourier-transform infrared analysis of the KP4000 guar gum supports the view that the KP4000 guar gum is actually totally non-ionic (see Chapter 3). Overall then, the lack of any effect of pH and ionic strength - especially on hematite, alumina, and titania - on guar gum adsorption should not be surprising for an uncharged polymer adsorbing through hydrogen bonding, and hence no indications of chemical interactions of acid-base type between the KP4000 guar gum and the tested minerals could be seen. It should be remembered that hydrogen bonding, due to its nature, may form between any polar surface sites, including charged -OPt2 + or -O" surface groups, and the polar OH groups of 71 the adsorbing polymer. One of the most neglected and least researched aspects of polysaccharide adsorption is the simple fact that polysaccharides do not easily form truly molecular solutions. Prolonged intense stirring, heating or boiling of a polymer powder with water are not sufficient to ensure complete dissolution and hydration of the polymer macromolecules, and only a combination of high temperature and pressure produces what could be called a truly molecular system. A detailed regime of such a drastic solution preparation method for a number of galactomannans (including guar gum) has recently been described by Picout et al. [16, 17] and their studies are based on a similar high-temperature-and-pressure treatment required to produce truly molecular solutions of starches [18]. Otherwise, polysaccharide solutions prepared under "usual" conditions inevitably contain a fraction of undissolved colloidal polysaccharide aggregates. There is strong experimental evidence that the size of such guar gum aggregates in aqueous solutions can be on the order of tens of microns [19, 20] and thus it is not surprising that the presence of colloidal aggregates dramatically complicates the determination of the true molecular weight of polysaccharides through light scattering, chromatography, or intrinsic viscosity measurements [8, 16, 17, 21]. This "problem" of aggregates was also described for a range of cellulose derivatives [22]. It can only be imagined that even for the same polysaccharide, from the same source/supplier, the many different solution preparation procedures employed by various researchers will result in as many different "polymer solutions" with often different proportions of aggregates to individual macromolecules. 72 What follows is that any fundamental discussion of polysaccharide adsorption mechanisms that is based on comparing adsorption densities, or on calculations of the free energy of adsorption from the Langmuir equation and the resulting adsorbed layer thickness ("monolayer" coverage), or on computer simulations of the adsorbed molecule conformation, must be carried out with extreme caution since there are presently no systematic data to allow a distinction to be made between the contribution of colloidal aggregate "adsorption" and that of molecular adsorption to the overall adsorption density of polysaccharides on various mineral surfaces. There are basically two well established types of adsorption isotherm: the Langmuir adsorption isotherm and the Freundlich adsorption isotherm. According to Hunter [23], to draw meaningful conclusions from the shape of the adsorption isotherms, the Langmuir form is expected to hold for nonporous surfaces and the Freundlich isotherm works for adsorbents with different type of surfaces showing significantly different affinities for the adsorbate. In this work, all the adsorbents were characterized as porous, without surfaces of different affinities to the adsorbate. Therefore, the Langmuir adsorption isotherm and the Freundlich adsorption isotherm were not used to model the adsorption data in this work. Since the nature of the adsorbed species, whether it is molecular or aggregate, cannot readily be identified one can also only speculate how macromolecule aggregation or dissolution affect spectroscopic characterization of adsorbed polysaccharides, especially the frequencies of OH groups and ring deformations which are very likely involved in the formation of polymer aggregates. 73 Since the solution preparation procedure used in this study was nowhere near the conditions required for forming truly molecular solutions, and viscometric evidence of the presence of such aggregates in the solutions was shown in Chapter 3, the very high affinity type of adsorption is mainly, a result of adsorption of entire colloidal guar gum aggregates. For the reasons described above, the present discussion does not make a significant note of the obvious differences in the adsorption densities of guar gum on, for example, hematite and quartz. In this context, it should also be noted that the adsorption density on quartz can readily be doubled regardless of pH if KC1 is used as a background electrolyte, and under the same conditions (presence of KC1) the adsorption density on kaolinite can be as high as that on hematite. At the same time, the adsorption density on alumina is almost the same as that on quartz (and even lower than adsorption on kaolinite) but alumina, in contrast to quartz and kaolinite, does not exhibit any effect of the salt type on guar gum adsorption. The very weak influence of pH on guar gum adsorption appears to be an inherent property of guar gum but what seems to be highly unusual about guar gum is the sensitivity of its adsorption on certain types of surfaces to the presence of chaotropic (K +, Cs+) or kosmotropic (Na+, Li + ) counter ions. In this respect, it is very interesting to observe that the adsorption of guar gum on alumina, hematite, and titania is actually very similar to guar gum adsorption on talc, which is also known to be independent of pH and ionic strength up to 1.5 mol/L KC1 [24, 25]. Admittedly, however, no other study attempted to compare a sodium salt with a potassium salt as background electrolytes so it is hard to deduce how common such effects are for other polymer systems. 74 Therefore, and for the sake of further discussion, it seems appropriate to divide the tested minerals into two groups: those that give different adsorption densities depending on the type of background ions (quartz and kaolinite), and those that show no effect of electrolyte type (alumina, titania, hematite). 4.4.2 N o n - D L V O i n t e r a c t i o n s b e t w e e n o x i d e s u r f a c e s i n r e l a t i o n t o g u a r g u m a d s o r p t i o n o n o x i d e s According to the Derjaguin-Landau-Vervey-Overbeek (DLVO) theory, the stability of particles towards aggregation is determined by a balance between attractive van der Waals and repulsive electrostatic forces. As noted by Overbeek [26], the theory does not account for subtle surface effects that result from differences in the structure of interfacial and bulk solvent (water), and the presence of water molecules at the interface may introduce additional forces between approaching particles. With the development of experimental tools, such as the surface force apparatus (SFA) or the atomic force microscope (AFM), many studies focused on the direct measurement of forces acting between mineral surfaces. The following paragraphs are by no means exhaustive and only the key findings will be brought out. Peschel et al. [27] measured disjoining forces between fused silica plates and found that a short-range repulsive force could be detected in distilled water and over a wide range of ionic strengths (up to 3 mol/L LiCl, NaCl, KCI). Interestingly, the magnitude of this repulsive interaction decreased at higher electrolyte concentrations compared to distilled water. Similar observations were made in several other studies on quartz/silica [28-32]. The main theme of those works is that a short range repulsive force, commonly referred to as "the hydration force", is inherently present on quartz/silica surfaces and cannot easily be eliminated at higher electrolyte concentrations. The 75 hydration force between quartz spheres could also be detected over the pH range from 2 to 10 (in 0.01 mol/L NaCl) although its magnitude measurably diminished near pH 2 [32]. Yotsumoto and Yoon estimated that the non-DLVO repulsive force rapidly becomes insignificant above an NaCl concentration of 1.5 mol/L at pH 2.0 (i.e.p. of quartz) [33]. According to Rabinovich and Derjaguin [29] and Derjaguin and Churaev [30], the hydration force between quartz filaments decreases when the KC1 concentration increases to 0.01 mol/L, and in more concentrated KC1 solutions the quartz surface actually becomes weakly hydrophobic with a measurable contact angle of about 16 deg in 1 mol/L KC1. In saturated KC1 and NaCl solutions, the contact angle on quartz was reported to increase to about 23 deg (for both NaCl and KC1) [34]. In contrast to quartz, direct measurements of surface forces (using AFM) between a rutile crystal and rutile sphere did not give any indication of repulsive "hydration" forces in distilled water or dilute salt solutions (up to 0.01 mol/L KNO3), and over a range of pH values (6-9.2) [35]. Based on turbidity measurements on colloidal rutile suspensions (i.e.p. at pH 6.2), Yotsumoto and Yoon postulated that a repulsive hydration force gradually appears only at ionic strengths higher than about 0.02 mol/L (NaCl) [36] with extensive redispersion observed at salt concentrations higher than 1 mol/L. In this case, however, an argument could also be made that the unusual stability of rutile at very high ionic strengths is a result of the specific adsorption of Na + ions that shifts the i.e.p. of rutile to a higher pH value effectively introducing additional electrostatic repulsion at pH 6.2 rather than a non-DLVO hydration force [37]. Using the SFA, Horn et al. did not detect any short range non-DLVO repulsion between sapphire (AI2O3) platelets in distilled water and dilute NaCl (up to 10"3 mol/L) in 76 the pH range from 6.7 to 11 [38]. Ducker et al. found that a hydration-type repulsive force between sapphire surfaces appeared at pH 3 and higher salt concentrations (0.1 NaBr) but the force data followed the "conventional" DLVO theory at pH 7.2 in distilled water or in more dilute electrolyte [39]. These results agree with the observations of Beattie et al. who noted an exceptional stability of y-alumina (i.e.p. at pH 9-9.5) against coagulation at pH 4.5 above KCI and NaCl concentrations of 0.1-0.4 mol/L but the behavior was more DLVO-like at pH 7.5 [40]. Neither the stability of rutile nor the stability of y-alumina showed any deviations from the DLVO theory near their corresponding i.e.p. values in dilute KNO3 solutions [41]. Although no direct force measurements have so far been reported for hematite, it is interesting to note that the coagulation behavior of the mineral can readily be modeled using the DLVO theory over a wide range of ionic strengths and pH values [42-44]. In relation to the presented adsorption results, the arguments here can most strongly be developed for the quartz-guar gum system since quartz/silica were perhaps the most extensively studied minerals through AFM/SFA and related techniques. It seems that minerals (titania, alumina, and presumably hematite) for which a repulsive hydration force appears only at certain pH and ionic strength, the adsorption of guar gum does not depend on the salt type. On the other hand, the adsorption density on quartz (and kaolinite) strongly depends on the salt type and, in this case, correlates with the inherently-present hydration force for quartz/silica systems over a wide range of pH values and ionic strengths. It is intriguing to observe that the increase •in guar gum adsorption on quartz at higher NaCl concentrations (Figure 4.7) correlates well with the 77 observations of Yotsumoto and Yoon that a hydration force on silica disappears at NaCl concentrations higher than 1.5 mol/L [33]. Also, the adsorption densities of guar gum on quartz in saturated NaCl and KC1 are practically the same and seem to coincide with the earlier-mentioned mild hydrophobicity of the quartz surface with same contact angle value both in saturated NaCl and KC1 [34], indicative of weakened adhesion of water to the quartz surface at high ionic strengths regardless of the salt type. The fact that KC1 seems to be more effective (i.e., at lower concentrations) in reducing the hydration force on quartz [29] compared to NaCl also correlates with the marked increase in guar gum adsorption at low KC1 concentrations, where almost no effect of dilute NaCl on guar adsorption can be found. These observations generally show that the idea of the disruption of the interfacial water layer on quartz by poorly hydrated staicture breaking ions (K +, Cs+) can quite successfully be correlated with the presence or disappearance of a non-DLVO "hydration" force whose origin is often seen in the presence of an extended water layer, stabilized or destabilized by ions, on particle surfaces. Under conditions where such a force is present on the quartz surface (i.e., in distilled water and dilute electrolyte solutions regardless of pH), the adsorption of the polymer on quartz is rather low but can significantly be increased if the hydration force is eliminated by high concentrations of structure making salts (NaCl), or by the presence of low concentrations of chaotropic electrolytes such as KC1. The similarity between the results obtained for quartz and kaolinite, especially the strong effect of KC1 concentration (Figure 4.8), simply indicate that the adsorption of guar gum on kaolinite is dominated by adsorption onto the silica-like "faces". 78 In contrast, the lack of a hydration force on titania under normal pH-ionic strength conditions suggests that the interfacial water layer on this oxide does not pose any barrier for guar gum adsorption and the role of background ions in promoting the adsorption process by disrupting the interfacial water is very limited, if any. A similar argument can be made for guar gum adsorption on alumina under alkaline conditions, where the stability of the oxide is described by the DLVO theory (and the lack of a repulsive hydration force). In the case of alumina, the adsorption results show a small decrease at lower pH values which could perhaps be due to the appearance of a hydration force (regardless of its true origin) in more acidic solutions, as reported in some studies. Guar gum adsorption on hematite clearly shows the same trends as those seen on alumina and titania. Although the insufficient amount of interparticle force data does not allow a similar analysis to be made for this oxide, it should be mentioned that Hiemstra and Van Riemsdijk describe the "iron oxide-dilute electrolyte solution" interface as characterized by an interfacial water layer of a relatively high dielectric constant, indicative of a chaotic orientation of water molecules within the interface resembling the structure.of bulk water [45]. A similar qualitative description was given for the interfacial water layer on titania [46]. Alumina, titania and hematite also show similar adsorption response in saturated NaCl and KCI solutions. In all three cases, the adsorption density of guar gum decreases at very high salt concentrations. Also, these minerals were postulated to produce a hydration force at high electrolyte concentrations. However, in the cases of quartz and kaolinite concentrated NaCl and KCI produce opposing effects on guar gum adsorption. Two additional phenomena have to be considered in this very high-ionic strength 79 environment: specific adsorption of background ions, and "salting-in/salting-out" of guar gum. Electroacoustic measurements at high electrolyte concentrations clearly show that alkali metal cations (but not so much chloride anions) at sufficiently high concentrations specifically adsorb on titania, quartz, and alumina [37, 47, 48] thus interfering with the ionization of surface OH groups. The specific affinity of background cations towards the oxide surfaces follows the well-known lyotropic series of ions. In the case of quartz, alkali metal cations show increased specificity in the order Li<Na<K<Cs, while for alumina and titania the trend is reverse. For example, at 1 mol/L of alkali metal chlorides only Li ions, but not K or Cs, show strong specific adsorption on alumina [48], while Cs shifts the i.e.p. of quartz more than Li does at 0.1 mol/L [47]. The high specific adsorption of ions could be expected to block the adsorption sites from guar gum and should overall lead to a decrease in the adsorption density. While this seems to be the case with alumina, titania, and hematite, such an explanation does not hold for guar gum adsorption on quartz and kaolinite especially in highly concentrated NaCl - although it should be noted that sodium ions show a lower affinity towards quartz compared to potassium ions so the surface density of the adsorbed K and Na ions should be different. The intrinsic viscosity measurements on the KP4000 guar gum in LiCl, NaCl, KC1 and CsCl over a wide range of ionic strengths - from distilled water to saturated salt solutions - indicate that guar gum undergoes more extensive aggregation in saturated NaCl consistent with the salting-out of the polymer by the hydrated kosmotropic Na + ions. A saturated KC1 solution, however, gave results consistent with enhanced dissolution (or "salting-in") of guar gum aggregates (see Chapter 3). Thus it could be argued that adsorption of extensive large guar gum aggregates from saturated NaCl (which could 80 simply be viewed as a poor solvent for guar gum) should increase the adsorption density of the polymer while adsorption of individual macromolecules from saturated KCI should cause a decrease in the adsorption density. This dominant influence of solvency seems to hold true only for quartz and kaolinite but fails to explain the adsoiption results on hematite, alumina, and titania. This change in the quality of salt solutions as solvents for guar gum actually becomes detectable at concentrations of at least 0.1 mol/L [9], so the maximum adsorption from KCI solutions at 0.1 mol/L on quartz and kaolinite can also be viewed as an onset of strong solvency effects. Since more dilute salt solutions do not measurably affect the behavior of guar gum [9], adsorption of the polymer from more dilute electrolyte solutions is only controlled by the stability of interfacial water layer on the mineral surfaces (quartz and kaolinite) as affected by background ions. The fact that no such differences between more concentrated NaCl and KCI could be seen on titania, alumina, and hematite suggests that both aggregates and individual polymer molecules can equally easily "break through" the weak hydration layer and adsorb on these oxides producing a constant adsorption density. It is also plausible that a combination of specific ion adsorption and solvency effects is responsible for the trends obtained in highly concentrated electrolytes, both of which are closely related to the relative hydration of ions, polymer, and mineral surfaces. Miller and his colleagues explicitly state that the froth flotation of water-soluble salt-type minerals from saturated brines with the use of colloidal surfactant-based collectors (e.g., n-alkyl amines) is possible only when the surface of the floating mineral is characterized by a distorted, weak water layer which allows a selective direct 81 interaction with the collector to take place [49-51]. From this point of view, highly relevant results were also reported by Jucker et al. [52]. These authors considered a colloidal nature of bacterial polysaccharides and pure dextrans in aqueous solutions and viewed the adsorption of these polysaccharides on quartz, alumina, and titania as a coagulation process. Interestingly, the adsorption density of dextrans.of relatively low molecular weights (MW from 1,000 to 80,000) was found to be the highest on titania, significantly lower on alumina, and practically immeasurable on quartz (all in 0.1N KCI, pH ~5). These differences in adsorption densities were attributed to the differences in the net forces (electrostatic, van der Waals, and Lewis acid-base type) acting between colloidal polysaccharides and oxide particles. Most importantly, however, Jucker et al. demonstrated through attenuated total reflectance infrared spectroscopy that the actual attachment of polysaccharides to the oxide surfaces proceeded through hydrogen bonding despite the large differences in adsorption densities. As discussed above, very similar phenomena strongly affect the adsorption behavior of guar gum, and the particulate nature of guar gum solutions certainly lends itself to a "colloidal" interpretation of adsorption results. It is our belief that the adsorption of guar gum, whether molecular or aggregate, on the tested oxides is also due to hydrogen bonding. 4.5 Conclusions Interactions between guar gum and oxide minerals do not depend on pH indicating that acid-base type chemical interactions are not significant in the adsorption process. This pH-independent adsorption behavior of guar gum actually seems to be an inherent property of the polysaccharide. It is therefore concluded that hydrogen bonding 82 is the main adsorption mechanism of guar gum on the tested minerals regardless of the nature of the adsorbing species (aggregate or molecular). However, two distinct types of adsorption responses of the polymer on oxide minerals were clearly shown depending on the type and concentration of the background electrolyte. The adsorption of the polymer can significantly be enhanced in the presence of chaotropic counter-ions only on the surfaces of quartz (and kaolinite). This adsorption-promoting effect of low concentrations of chaotropic K and Cs ions can satisfactorily be explained by the breaking of the extended interfacial water layer known to exist on quartz and allowing the polymer to more densely adsorb on the polar hydroxyl sites. In contrast, oxide minerals (titania, hematite, alumina) characterized by the absence of a non-DLVO hydration force, which is an indication of a distorted/weak interfacial water layer, readily adsorb. guar gum regardless of electrolyte type and concentration. The interfacial water layer on such oxides does not prevent the polymer from reaching the surface sites. Therefore, the role of background ions over a wide range of ionic strengths is to control the stability/structure of the interfacial water layer and to facilitate/inhibit access by the polymer to the adsorption sites. In saturated salt solutions, the adsorption of guar gum is affected by a combination of solvency and specific ion adsorption effects, but the exact mechanism of adsorption under such extreme conditions cannot easily be deduced. 4.6 References [1] Whistler, R.L. and Hymowitz, T., Guar: Agronomy, Production, Industrial Use, and Nutrition, Purdue University Press, West Lafayette, (1979). [2] Painter, T.J., Gonzalez, J.J. and Hemmer, P.C., The distribution of D-galactosyl groups in guaran and locust-bean gum: new evidence from periodate oxidation, Carbohydrate Research, Vol. 69, pp. 217-226, (1979). 83 [3] Dea, I.C.M., Conformational origins of polysaccharide solution and gel properties. In: Whistler, R.L. and BeMiller, J.N. (Ed.), Industrial Gums - Polysaccharides and Their Derivatives, Academic Press, (1993). 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[8] Robinson, G., Ross-Murphy, S.B. and Morris, E.R., Viscosity-molecular weight relationships, intrinsic chain flexibility,1 and dynamic solution properties of guar galactomannan, Carbohydrate Research, Vol. 107, pp. 17-32, (1982). [9] Dubois, M., Gilles, K.A., Hamilton, J.K., Rebers, P.A. and Smith, F., Colorimetric method for determination of sugars and related substances, Analytical Chemistry, Vol. 28, pp. 350-356, (1956). [10] Chatterji, J. and Borchardt, J.K., Applications of water-soluble polymers in the oil field, Journal of Petroleum Technology, Vol. 33, pp. 2042-2056, (1981). [11] Furlong, D.N., Sing, K.S.W. and Parfitt, G.D., The precipitation of silica on titanium dioxide surfaces: I. Preparation of coated surfaces and examination by electrophoresis, Journal of Colloid and Interface Science, Vol. 69, pp. 409-419, (1979). [12] Liu, Q. and Laskowski, J.S., The role of metal hydroxides at mineral surfaces in dextrin adsorption, I. Studies on modified quartz samples, International Journal of Mineral Processing, Vol. 26, pp. 297-316, (1989). [13] Laskowski, J.S., Liu, Q. and Bolin, N.J., Polysaccharides in flotation of sulphides. Part I. Adsorption of polysaccharides onto mineral surfaces, International Journal of Mineral Processing, Vol. 33, pp. 223-234, (1991). [14] Liu, Q., Laskowski, J.S., L i , Y. and Wang, D., Synergistic effect of mineral surface constituents in dextrin adsorption, International Journal of Mineral Processing, Vol. 42, pp. 251-266, (1994); 84 [15] Raju, G.B., Holmgren, A. and Forsling, W., Adsorption of dextrin at mineral/water interface, Journal of Colloid and Interface Science, Vol. 193, pp. 215-222, (1997). [16] Picout, D.R., Ross-Murphy, S.B., Errington, N. and Harding, S.E., Pressure cell assisted solution characterization of polysaccharides. 1. guar gum, Biomacromolecules, Vol. 2, pp. 1301-1309, (2001). [17] Picout, D.R., Ross-Murphy, Jumel, K. and Harding, S.E., Pressure cell assisted solution characterization of polysaccharides. 2. Locust bean gum and tara gum, Biomacromolecules, Vol. 3, pp. 761-767, (2002). [18] Aberle, Th., Burchard, W., Vorwerg, W. and Radosta, S., Conformational contributions of amylose and amylopectin to the structural properties of starches from various sources, Starch, Vol. 46, pp. 329-335, (1994). [19] Gittings, M. R., Cipelletti, L., Trappe, V., Weitz, D. A., In, M . and Marques, C , Structure of guar in solutions of H 2 0 and D 2 0: an ultra-small-angle light-scattering study, Journal of Physical Chemistry B, Vol.104, pp. 4381-4386, (2000). [20] Gittings, M. R., Cipelletti, L., Trappe, V., Weitz, D. A., In, M . and Lai, J., The effect of solvent and ions on the structure and rheological properties of guar solutions, Journal of Physical Chemistry A, Vol. 105, pp. 9310-9315, (2001). [21] Kratochvil, P., Particle scattering functions. In Huglin, M. , (Eds), Light Scattering from Polymer Solutions, pp. 333-384, Academic Press, (1972). [22] Kraemer, E.O., Molecular weights of celluloses and cellulose Derivates, Industrial and Engineering Chemistry, Vol. 30, pp. 1200-1203, (1938). [23] Hunter, R.J., Foundations of colloid science, Oxford University Press, 2000. [24] Jenkins, P. and Ralston, J., The adsorption of a polysaccharide at the talc-aqueous solution interface, Colloids and Surfaces. A, Vol.139, pp. 27-40, (1998). [25] Wang, J., Somasundaran, P. and Nagaraj, D.R., Adsorption mechanism of guar gum at solid-liquid interfaces, Minerals Engineering, Vol. 18, pp. 77-81, (2005) [26] Overbeek, J.Th.G., Colloid stability in aqueous and non-aqueous media-introductory paper, Discussions of the Faraday Society, Vol. 42, pp. 7-13, (1966). [27] Peschel, G., Belouschek, P., Muller, M.M., Muller, M.R. and Kbnig, R., The interaction of solid surfaces in aqueous systems, Colloid and Polymer Science, Vol. 260, pp. 444-451, (1982). 85 [28] Rabinovich, Y.I., Derjaguin, .B.V. and Churaev, N.V., Direct measurements of long-range surface forces in gas and liquid media, Advances in Colloid and Interface Science, Vol. 16, pp. 63-78, (1982). [29] Rabinovich, Y.I. and Derjaguin, B.V., Calculation of the surface charge and potential and of the structural component of disjoining pressure in an interlayer in potassium chloride solutions between glass samples,Langmuir,Vol.3, pp.625-628, (1987). [30] Derjaguin, B.V. and Churaev, N.V., Structure of water in thin layers, Langmuir, Vol. 3, pp. 607-612, (1987). [31] Horn, R.G., Smith, D.T. and Haller, W., Surface forces and viscosity of water measured between silica sheets, Chemical physics letters, Vol. 162, pp. 404-408, (1989). [32] Ducker, W.A., Senden, T.J. and Pashley, R.M., Measurement of forces in liquids using a force microscope, Langmuir, Vol. 8, pp. 1831-1836, (1992). [33] Yotsumoto, H. and Yoon, R.H., Application of extended DLVO theory: II. stability of silica suspensions, Journal of Colloid and Interface Science, Vol. 157, pp. 434-441, (1993). [34] Veeramasuneni, S., Hu, Y.,. Yalamanchili, M.R. and Miller, J.D., Interaction forces at high ionic strengths: the role of polar interfacial interactions, Journal of Colloid and Interface Science, Vol. 188, pp. 473-480, (1997). [35] Larson, I., Drummond, C.J., Chan, D.Y.C. and Grieser, F., Direct force measurements between titanium dioxide surfaces, Journal of the American Chemical Society, Vol. 115, pp. 11885-11890,(1993). [36] Yotsumoto, H. and Yoon, R.H., Application of extended DLVO theory: I. stability of rutile suspensions, Journal of Colloid and Interface Science, Vol. 157, pp. 426-433, (1993). [37] Kosmulski, M. and Rosenholm, J.B., Electroacoustic study of adsorption of ions on anatase and zirconia from very concentrated electrolytes, Journal of Physical Chemistry, Vol. 100, pp. 11681-11687,(1996). [38] Horn, R.G., Clarke, D.R. and Clarkson, M.T., Direct measurement of surface forces between sapphire crystals in aqueous solutions, Journal of materials research, Vol. 3, pp. 413-416, (1988). [39] Ducker, W.A., Xu, Z., Clarke, D.R. and Israelachvili, J.N., Forces between alumina surfaces in salt solutions: non-DLVO Forces and the implications for colloidal processing, Journal of the American Ceramic Society, Vol. 77, pp. 437-443, (1994). [40] Beattie, J.K., Cleaver, J.K. and Waite, T.D., Anomalous aggregation behaviour of 86 aluminium oxyhydroxides, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. I l l , pp. 131-138,(1996). [41] Wiese, G.R. and Healy, T.W., Coagulation and electrokinetic behavior of Ti02 and AI2O3 colloidal dispersions, Journal of Colloid and Interface Science, Vol. 51, pp. 427-433, (1975). [42] Amal, R., Coury, J.R., Raper, J.A., Walsh, W.P. and Waite, T.D., Structure and kinetics of aggregating colloidal hematite, Colloids and Surfaces,Vol.46, pp.1-19, (1990). [43] Amal, R., Raper, J.A. and Waite, T.D., Fractal structure of hematite aggregates, Journal of Colloid and Interface Science, Vol. 140, pp. 158-168, (1990). [44] Schudel, M. , Behrens, S.H., Holthoff, H., Kretzschmar, R., and Borkovec, M. , Absolute aggregation rate constants of hematite particles in aqueous suspensions: a comparison of two different surface morphologies, Journal of Colloid and Interface Science, Vol. 196, pp. 241-253, (1997). [45] Hiemstra, T. and Van Riemsdijk, W.H., On the relationship between charge distribution, surface hydration, and the structure of the interface of metal hydroxides, Journal of Colloid and Interface Science, Vol. 301, pp. 1-18, (2006). [46] Bourikas, K., Hiemstra, T. and Van Riemsdijk, W.H., Ion pair formation and primary charging behavior of titanium oxide (anatase and aitile), Langmuir, Vol. 17, pp. 749-756,(2001). [47] Kosmulski, M. , Positive electrokinetic charge of silica in the presence of chlorides, Journal of Colloid and Interface Science, Vol. 208, pp. 543-545, (1998). [48] Johnson, S.B., Scales, P.J. and Healy, T.W., The binding of monovalent electrolyte ions on cc-alumina. I. electroacoustic studies at high electrolyte concentrations, Langmuir, Vol. 15, pp. 2836-2843, (1999). [49] Hancer, M. and Miller, J.D., The flotation chemistry of potassium double salts: Schoenite, kainite, and carnallite, Minerals Engineering, Vol. 13, pp. 1483-1493, (2000). [50] Hancer, M. , Celik, M.S., and Miller, J.D., The significance of interfacial water structure in soluble salt flotation systems, Journal of Colloid and Interface Science, Vol. 235, pp. 150-161, (2001). [51] Ozcan, O. and Miller, J.D., Flotation of sodium carbonate and sodium bicarbonate salts from their saturated brines, Minerals Engineering, Vol. 15, pp. 577-584, (2002). [52] Jucker, B.A., Harms, H., Hug, S.J. and Zehnder, A.J.B., Adsorption of bacterial surface polysaccharides on mineral oxides is mediated by hydrogen bonds, Colloids and Surfaces. B, Vol. 9, pp. 331-343, (1997). 87 C H A P T E R 5 Adsorption of Guar Gum on Potash Slimes* 5.1 Introduction In the potash flotation process, sylvite (KC1) is separated from halite (NaCl) with the use of long-chain primary amines as sylvite collectors. A typical potash ore also contains a few percent of very fine water-insoluble minerals (clays, carbonates). Because of the large specific surface area of these slime particles, the primary amine collector tends to adsorb on the fines rather than on the sylvite surfaces which leads to high reagent consumption and poor selectivity of the flotation process. The water-insoluble slimes can be removed from the flotation feed by classification using cyclones, or by flocculation followed by pre-flotation of the floes. The deslimed ore going to the sylvite flotation stage still contains a small amount of water-insoluble fines. In order to depress the remaining slimes, two different polysaccharides are usually employed; carboxymethyl cellulose or guar gum. Carboxymethyl cellulose is strongly anionic and of relatively low molecular weight (MW = 300,000). In contrast, guar gum is nonionic with a molecular weight of about 1.5 million. Mixtures of these two blinders can also be used. By adsorbing on the unwanted mineral particles, the polymers form a protective coating that prevents the amine collector from interacting with the slimes. The adsorption of polysaccharides on various minerals has been studied quite extensively. Liu et al. [1] have recently analyzed the different adsorption mechanisms of polysaccharides and proposed a generalized model of polysaccharide adsorption in which the attachment to the mineral surface takes place through complexation with metal-* A version of this chapter was published in Canadian Metallurgical Quarterly. [Reference: Ma, X. and Pawlik, M. , "Adsorption of Guar Gum on Potash Slimes", Canadian Metallurgical Quarterly, vol. 46, no. 3, pp. 321-328, (2007).] 88 hydroxy surface sites and the nature of the interaction, whether hydrogen bonding or chemical, is of acid-base type and depends on the acidity of the surface groups. Ma and Pawlik [2] investigated adsorption of guar gum on quartz from dilute (0.01-0.1 mol-L"1) alkali metal chloride solutions (LiCl, NaCl, KCI, and CsCl), and described a phenomenon of enhanced guar gum adsorption on quartz in the presence of potassium and cesium chlorides. In comparison to adsorption measured in distilled water, the adsorption levels almost doubled in the presence of potassium and cesium chlorides. At the same time, no significant effect of sodium and lithium chlorides on guar gum adsorption was observed. Assuming hydrogen bonding to be the adsorption mechanism, it was suggested that the adsorption of guar gum on quartz depended on the water-structure breaking or making properties of the background cations. Water-structure breaking ions (chaotropes) such as K + and Cs + were apparently capable of disrupting the interfacial water layer thus allowing the polymer to adsorb on surface silanol groups. The adsorption process was viewed as a competition between water and polymer molecules for the surface sites. There is very little information on interactions of polysaccharides with slime minerals in concentrated salt solutions. The vast majority of adsorption studies on guar gum were performed either in distilled water or in dilute electrolytes (typically sodium or potassium salts). Pawlik and Laskowski reported adsorption densities of guar gum on illite and dolomite from solutions of the ionic strength ranging from distilled water to 50%-saturated Lanigan ore brine (~3 mol-L"1 NaCl-KCl) [3]. No significant effect of ionic strength was observed on guar gum adsorption on illite, while the adsorption density of the polysaccharide on dolomite significantly decreased at high ionic strengths and no satisfactory explanation for this effect was proposed. 89 Since guar gum and background ions are hydrated in aqueous solutions, it is very likely that the polymer and background ions compete for free water molecules especially at high ionic strengths. Competitive hydration phenomena may lead to a change in the quality of the solvent which in turn could affect the adsorption of a polymer. For example, the adsorption of polystyrene sulfonate sharply increased around the theta-point (a transition solvent between good solvent and poor solvent) which for this polymer in NaCl solutions was identified at 4.2 mol-L'1 NaCl [4]. Intrinsic viscosity measurements are a convenient way of assessing solvency effects. Koral et al. [5] found that the adsorption of polyvinyl acetate was generally much higher from a poor solvent than from a good one, and that the amount of polymer adsorbed was inversely proportional to the intrinsic viscosity. Considering the potential significance of sodium and potassium ions in controlling the adsorption behavior of guar gum [2], the objective of this study was to investigate not only the effect of the total ionic strength but also the function of kosmotropes and chaotropes in the adsorption of guar gum on slime minerals. 5.2 Materials and Methods Samples of illite, dolomite and kaolinite were obtained from Ward Natural Science Establishment. The minerals were dry-ground to 100% passing through a 38-micron sieve. The mineralogical composition of the samples, as analyzed through x-ray diffraction, is shown in Table 5.1. The BET (Brunauer, Emmett, Teller) specific surface areas were determined from nitrogen adsorption and were found to be 1.52 m2-g~', 0.724 m2-g"', and 2.73 m2-g'' for the illite, dolomite and kaolinite samples, respectively. These values were obtained after correction for the microporosity of the samples, as measured 90 with the use of a Quantachrome IMP BET analyzer equipped with a 1-mm Hg pressure transducer. For Calculating the adsorption densities of guar gum, only the external specific surface area was used since only these external surfaces are accessible to guar gum. These external BET specific surface areas were obtained by subtracting the surface area contained in pores (through a simultaneous porosity measurement) from the total BET surface areas obtained from nitrogen adsorption. The agreement between the total BET values and those obtained from the particle size distribution (assuming that particles were spherical and non-porous) was very poor, which was indicative of highly non-uniform surface topography and non-spherical particle shapes. Table 5.1 Mineralogical composition of the tested minerals. I l l i t e D o l o m i t e K a o l i n i t e Muscovite/Illite 72.2% Dolomite 97.4% Kaolinite 96.8% Quartz 19.5% Calcite 2.0% K-feldspar 3.2% Clinochlore 3.5% Quartz 0.6% K-feldspar 2.3% Ankerite/Dolomite 2.1% For the illite sample, the presence of a significant fraction of quartz should be noted. The ionic strength of background solutions was adjusted with potassium and sodium chlorides (Fisher), and with the salts obtained after evaporating a saturated potash brine. The brine was prepared from a potash ore from a mine in Saskatchewan by dissolving the ore in warm distilled water. The resulting solution contained a large amount of suspended slimes which were allowed to settle over a period of 3 weeks. The clear brine was siphoned out and evaporated. The settled fines were washed several times 91 with distilled water until the conductivity of the equilibrium solution was approximately equal to the conductivity of lab tap water (30 pS-cm"1). The slimes were then dried at 110°C and used in adsorption studies. The BET specific surface area of the slimes was 15.1 m2-g"'. An additional x-ray diffraction analysis performed on the slime minerals gave the following composition: dolomite 32.8%, muscovite/illite 27.1%, clinochlore 16.5%, quartz 16.2%, K-feldspar 5.0%, hematite 1.7%, and gypsum 0.7%. A chemical analysis of the brine revealed the following concentrations of the main cations: Na + 4.4 mol-L"1, K + 1.7 mol-L"1, Ca 2 + 0.040 mol-L"1, and M g 2 + 0.015 mol-L"1. The brine can thus be treated as a 6.1-molar solution of KCl-NaCl. Guar gum was the Rantec KP4000. The sample contained about 12% of water insoluble organic residue which was removed from all stock solutions by prolonged centrifuging. Adsorption tests were carried out by conditioning 10 grams of a mineral with 50 ml of a background solution for 20 minutes, and then adding a further 50 ml of a guar gum solution. The mixtures were conditioned for an additional 30 minutes after which the solids were separated from solution by centrifuging at 10,000g. The resulting supernatants were assayed for the presence of guar gum using the phenol-sulfuric acid method [6]. Calibration curves of guar gum in 50% saturated salt solutions were used to calculate the concentration of guar gum in saturated salt solutions (see Appendix 1). In order to prepare guar gum solutions in a saturated brine, a stock guar gum solution was first prepared in distilled water and the evaporated salts from the brine were added to the solution until saturation. A correction for a change in volume due to salt addition was taken to obtain the exact guar gum concentration. The same salt addition procedure was 92 followed for tests performed in saturated KCI and NaCl solutions. In the case of. actual potash slimes, a series of turbidity measurements were performed on supernatant suspensions obtained after a settling time of 20 minutes. The same suspensions were then assayed (after centrifuging) for residual guar gum as mentioned above, so a direct correlation could be obtained between polymer adsorption and the colloid stability of the slime minerals. Turbidity values are expressed in nephelometric turbidity units (NTU). The viscosity of guar gum solutions was measured using Cannon-Fenske capillary viscometers (Schott Gerate GmbH, Germany) within the Newtonian range of guar gum concentrations. After preparation, 7-ml aliquots of the tested solutions were transferred to a capillary viscometer and the viscometer was placed in a water bath (set at 24 °C) for 30 minutes. The kinematic viscosity was measured by allowing each solution to flow under gravity through the capillary. A Lauda PVS1 photo-timing and processing system was used to automatically measure the flow times from which the kinematic viscosities were calculated. All measurements were done as triplicates and the average values are reported. 5.3 Results and Discussion The relative viscosities, rjrei, of guar gum solutions (the viscosity of solution divided by the viscosity of the solvent) were measured in distilled water, 0.1-molar, 1-molar, saturated KCI and NaCl solutions, and in a saturated brine, as also shown in Chapter 3. The relative viscosities were then recalculated to reduced viscosities which were plotted as a function of guar gum concentration. According to the Huggins equation [7]: 93 rired=m + k\iifc where 7]red is the reduced viscosity ((rjrerl)/c), \rj\ is the intrinsic viscosity, c is the polymer concentration, and k is the Huggins coefficient - a plot of r/reci vs. c should give a straight line with the intercept equal to [77]. Consequently, the Huggins coefficients can also be obtained from the slope values. For clarity, Figure 5.1 illustrates only the results obtained in distilled water, saturated brine, and saturated NaCl and KC1 solutions with the corresponding Huggins fits. Except saturated NaCl, it was found that the intrinsic viscosity of guar gum (intercept of the linear fits to zero guar gum concentration) was basically constant in the studied polymer concentration range regardless of the electrolyte type and concentration. The average intrinsic viscosity value for all the data (except saturated NaCl) was found to be 11.3 dL-g"1 (± 3%). A saturated NaCl solution gave an intrinsic viscosity value of 13.4 dL-g'. i o - | — ' — 1 — • — 1 1 — 1 — « — 1 — 1 — r 0 0.02 0.04 0.06 0.08 0.1 Guar Gum Concentration [g*dL_1] Figure 5.1 The reduced viscosity of guar gum solutions, T = 24 °C. 94 The intrinsic viscosity of a polymer is a measure of the effective (hydrodynamic) size and conformation of macromolecules in solution, and. its values are often used to calculate the weight-average molecular weight of a polymer. Since the intrinsic viscosity of guar gum is essentially constant, the observed differences in the slopes of the linear fits to individual sets of data in Figure 5.1 are entirely due to changes in .the Huggins constant, k, as can be deduced from Equation 1. According to Sakai [8], the dimensionless Huggins constant, k, is very sensitive to the presence of intermolecular aggregates. Values of k may vary between 0.5 and 0.7 for a polymer under theta-conditions, i.e., when polymer-polymer interactions become favorable over polymer-solvent interactions. This onset of polymer-polymer associations at the expense of polymer-solvent interactions is related to.transition from a good solvent to a poor solvent. Generally, a polymer exhibits higher values of A: in a poor solvent than in a good solvent. Figures 5.2, 5.3 and 5.4 show the effect of electrolyte type and concentration on the adsorption density of the KP4000 on the model minerals. The initial guar gum concentrations were chosen such that there was always some amount of the polymer left in equilibrium with the minerals after adsorption. In the cases of kaolinite and illite, large differences in the adsorption behavior of the polymer become evident when the results are plotted as a function of ionic strength, as shown in Figures 5.2 and 5.3, respectively. Interestingly, when the electrolyte concentration approaches saturation, these differences tend to disappear and saturated NaCl, KCI and potash brine give practically the same adsorption level. As the NaCl concentration increases, the amount of the polysaccharide adsorbed on kaolinite and illite does not markedly change compared to adsorption in distilled water 95 I Kaolinite O NaCl • KCI O4AV I 1 1 1 • • | — i — i — 1 1 1 1 1 1 1 — i — i — 1 1 1 1 1 1 1 — i — i — 1 1 1 1 1 Dist. 0.01 0.1 1 10 Water Eiectrolyte Concentration [mol*L~'] Figure 5.2 Effect of NaCl and KCI concentration on adsorption of guar gum on kaolinite. Initial concentration of guar gum 270 mg/L. 2 . 8 - r i \ \ 1 • i 1 1 1 nl i "I i Muscovite/Illite O NaCl • KCI 0 . 8 — I i 1 1 1 1 1 1 — i — i i 1 1 1 • 11—i—i i 1 1 1 1 i j — i — i 1 1 1 1 i i Dist. 0.01 0.1 1 10 Water Electrolyte Concentration [mol*/L_1] Figure 5.3 Effect of NaCl and KCI concentration on adsorption of guar gum on illite. Initial concentration of guar gum 440 mg/L. 96 Dolomite O NaCl • KC1 o 3 0.4-Saturated Brine T — I I I I 11 TTT I I I I I I I I I I 11 Dist. Water Electrolyte Concentration [mol*L/'] 0.01 0.1 1 10 Figure 5.4 Effect of NaCl and KC1 concentration on adsorption of guar gum on dolomite. Initial concentration of guar gum 150 mg/L. until the NaCl concentration increases to about 2-3 mol/L. Only above this limiting concentration, does the adsorption level dramatically increase. In contrast to NaCl, even relatively low amounts of KC1 markedly increase the adsorption of the polysaccharide, decrease and become essentially the same as that in concentrated NaCl. As seen in Figure 5.4, the adsorption behavior of guar gum on dolomite, as a function of ionic strength, is very different from that on illite and kaolinite. As the concentration of salts increases guar gum adsorption steadily decreases. Figure 5.5 shows adsorption isotherms of guar gum on potash slimes. The results illustrate the same qualitative trends that were also observed on the clay minerals: low concentrations of KC1 increase guar gum adsorption, while NaCl gives a measurable and only near the saturation limit of KC1 does the adsorption density of guar gum 97 0.4—1 L—1 ' 1 L 0 20 40 60 80 100 Guar Gum Equilibrium Concentration [mg*L'] Figure 5.5 Effect of electrolyte type and concentration on adsorption of guar gum onto potash slimes. Initial Guar Gum Concentration [mg*L-'] Figure 5.6 Turbidity of slime suspensions (10% wt., solid symbols) in saturated salt solutions in the presence of guar gum. The corresponding equilibrium guar concentrations taken from Fig 5.5 are also shown (open symbols). 98 increase only near the saturation limit. Differences gradually disappear as the electrolyte concentrations increase to saturation. Figure 5.6 shows the corresponding turbidity of slime suspensions (for the same data as in Figure 5.5) in saturated salt solutions and brine. The results are characteristic of typical flocculants. Low dosages result in gradual flocculation and an excess of the polysaccharide results in redispersion of the slime minerals. Low turbidity values are indicative of strong flocculation by guar gum. It is noteworthy that maximum flocculation (minimum turbidity) took place at an initial guar gum concentration of 150 mg/L regardless of the electrolyte type. At this initial guar gum concentration, the adsorption of the polymer on the slimes was complete, and no residual guar gum could be detected in equilibrium with the fines. The.adsorption density of the polymer (-0.08 mg/m2) at maximum flocculation also corresponds with only 30% of the complete surface coverage (Figure 5.5). It should also be noted that the redispersion of the slimes is more difficult in saturated KC1, as seen from lower turbidities, than in saturated NaCl. In Chapter 3, it was found that k values (Huggins constants) for the KP4000 ranged from 1.07 in distilled water, reached a maximum value in 0.1-molar salts (1.29 in NaCl and 1.17 in KC1) and decreased continuously with increasing salt concentration. The effect of more concentrated NaCl was quite different from KC1. A 4.1-molar NaCl gave a k value of 0.87, while the same concentration of KC1 (corresponding to the saturation limit of KC1) produced a k value of 0.59. Saturated NaCl (5.3-molar) gave a k value of 0.63. For comparison, saturated potash brine resulted in a k value of 0.83 (Figure 5.1). These trends suggest that concentrated salt solutions are actually better solvents for 99 guar gum compared to distilled water or more dilute salts. It should also be remembered, and this aspect is rarely discussed in guar gum adsorption studies, that guar gum does not form truly molecular solutions in water unless a combination of high temperatures and pressures is used during the solution preparation stage [9, 10]. Otherwise, guar gum solutions prepared under ambient conditions inevitably contain a small fraction of undissolved colloidal aggregates. This simple observation confirms the notion that distilled water and dilute salt solutions are poor solvents for guar gum. The solution preparation procedure in this work was nowhere near the conditions required for the preparation of truly molecular solutions and it must be assumed that all the guar gum solutions tested in this study contained intermolecular aggregates. In fact, the high values of k obtained in distilled water and more dilute electrolytes are consistent with the presence of such aggregates [11]. Interestingly, the intrinsic viscosity of guar gum, which is also related to the effective molecular size in solution, should not be expected to change much in the presence of aggregates. Since the shape of siich aggregates is almost spherical, their hydrodynamic contribution to the overall intrinsic viscosity value is very small compared to dissolved extended polymer chains [12]. The adsorption results for illite and kaolinite illustrate that poorly hydrated cations, such as K + , are capable of enhancing guar gum adsorption on certain mineral surfaces by disrupting the interfacial water layer and allowing the polymer to more closely approach the surface and interact with surface groups [2]. Strongly hydrated sodium cations cannot produce the same effect and the adsorption density in distilled water is basically the same as in dilute NaCl solutions. The guar gum adsorption process is treated as a competition between polymer and water molecules for polar surface 100 groups. Three important assumptions in this approach should be recalled: a) the main polymer adsorption mechanism is through hydrogen bonding, b) the adsorption of background ions on the mineral surface is of electrostatic/physical nature, c) background counter-ions show different affinities towards the surface. In this respect, the adsorbent behavior of the clay minerals towards guar gum is qualitatively the same as that of quartz. Based on the study in Chapter 2, the effect of sodium ions shown in Figures 5.2 and 5.3 could, to some extent, be predicted. As the salt concentration increased from 10" to 10"' mol-L"1, only potassium enhanced the adsorption of guar gum on quartz at low concentrations (103 mol/L), but all the tested electrolytes (LiCl, NaCl, HCI, MgCl 2 , CaCb) markedly increased the adsorption of the polymer at 0.1 mol-L"1 with sodium - the weakest kosmotrope (i.e., least hydrated ion) in the series - producing relatively the largest change in adsorption. Therefore, it could be deduced that increasing concentrations of all the cations, regardless of their chaotropic or kosmotropic character, would eventually result in a high adsorption density of guar gum on quartz. However, much higher concentrations of more hydrated cations (e.g., Na+) would be required to generate the same increase in guar adsorption as that caused by low concentrations of less hydrated ions (K+). This trend can be seen in Figures 5.2 and 5.3, and in the adsorption results for potash slimes (Figure 5.5). In general, at low salt concentrations, the water-structure-breaking and making properties of the background cations determine the adsorption behavior of guar gum on the clay minerals. It should be noted that the net surface charge on illite and kaolinite is negative and, as electrophoretic results indicate (see Appendix 2), the affinity of alkali metal cations towards both minerals increases in the order L i + < Na + < K + < Cs + - the 101 same sequence as in the case of quartz. In dilute solutions, the concentration of cations is relatively higher at the negatively charged mineral-solution interface than in the bulk solution, so it is the mineral-solution interface that is primarily affected by the presence of Na + and K + ions. At the same time, the Huggins constants show that dilute NaCl and KC1 solutions are of the same "goodness" towards guar gum so solvency effects are not a major factor in dilute electrolytes although the adsorption of entire aggregates must also be considered. At least one additional phenomenon can be proposed as influencing guar gum adsorption from more concentrated salt solutions. With increasing ionic strength, higher and higher concentrations of background electrolytes become available in the bulk solution and start affecting guar gum macromolecules.; As competition between the polymer and background ions for hydration water becomes significant, guar gum chains seem to interact with water much more readily in KC1 than in NaCl. It is reasonable to expect that high concentrations of strongly hydrated Na + ions bind large amounts of water molecules leaving no free water for guar gum dissolution and hydration. In contrast, poorly hydrated K cations do not really compete with the polymer for free water molecules. This process could perhaps be viewed as the dissolution of guar gum aggregates into individual molecules. Although direct light scattering measurements would be needed to confirm the disappearance of aggregates in concentrated KC1 solutions, the adsorption of individual polysaccharide molecules should bring about a lower adsorption density compared to the adsorption of entire aggregates. On the other hand, guar gum appears to undergo an even more extensive aggregation/gelling in saturated NaCl leading to a measurable increase in the intrinsic viscosity. This worsening in the quality of saturated NaCl as a solvent for guar gum - which is in a sense equivalent 102 to "precipitation" of the polymer - correlates very well with the observed increase in guar gum adsorption from more concentrated NaCl solutions. Since potassium ions show a higher affinity towards the aluminosilicate (and quartz) surfaces than sodium ions do, it is also entirely plausible that sufficiently high concentrations of K + ions simply block adsorption sites on the mineral surfaces thus contributing to the observed decrease in the adsorption density of the polymer at very high KCI concentrations. On the other hand, the main roles of sodium ions at high electrolyte concentrations are to weaken the water layer around the clay particles and promote the formation of guar aggregates thus overall enhancing guar gum adsorption. Dolomite, however, should be. treated as a separate case from the clay minerals. The mineral is positively charged in 10"4 mol/L NaCl at a natural pH value of 9.5 [13]. Adsorption data in dilute electrolytes indicate that guar gum adsorption on dolomite is related to the cvs-configured OH groups of the mannose units of guar gum, and to the concentration of Mg and Ca species on the dolomite surface. It turns out that Ca and Mg ions can be leached off the dolomite surface by high salt concentrations which correlates with the observed decrease in guar gum adsorption [14]. In other words, a mechanism other than hydrogen bonding may be involved in the adsoiption process on dolomite. A spectroscopic analysis of functional groups on the dolomite surface revealed the presence of -Me-OH 2 + and -Me-OH groups (Me = Ca/Mg), especially on a net-positively charged dolomite surface [15]. These groups form as a result of surface hydration with water molecules coordinated to Ca/Mg sites through oxygen atoms. It could thus be envisaged that the oxygen atoms of the hydroxyl groups of guar gum form similar complexes with 103 Ca and Mg sites at ionic strengths below 1 mol/L, but additional phenomena are also involved in adsorption from highly concentrated salt solutions. It is noteworthy that the adsorption of guar gum on dolomite from concentrated NaCl solutions is measurably higher than the adsorption density of the polymer observed in KC1 solutions of the same concentration. This difference again points towards the presence and adsorption of guar aggregates in NaCl solutions regardless of the actual adsorption mechanism. The low adsorption of guar gum on dolomite in concentrated salt solutions strongly contrasts with its presumed role as a slime blinder. Even though dolomite is the main component of the tested slimes, the adsorption data for slimes are much more similar to the adsorption results obtained for pure kaolinite and illite than to the results for pure dolomite. This observation suggests that it is actually the clay and quartz components of the slimes that control the overall adsorption behavior of the mixture. This conclusion in turn indicates that the dolomitic component of the slimes is not fully "blinded" by guar gum in a saturated brine. The stability/turbidity data also suggest that the guar gum concentrations required to reach maximum adsorption on the slime surfaces, which would correspond with the complete blinding of the slimes, should also result in re-dispersion of the fine particles and potentially adversely affect the recycling of the brine from flotation tailings. 5.4 Conclusions In dilute electrolyte solutions, the water-structure breaking or making properties of the background ions govern the adsorption of guar gum on illite, kaolinite, and potash slime minerals. Thus, large differences between the effect of NaCl and KC1 concentration 104 can be observed. Water-structure making Na + ions significantly influence the guar adsoiption density only at very high concentrations. In contrast, much lower concentrations of water-structure breaking K + ions strongly enhance the adsorption of the polysaccharide on the clay minerals. The adsorption properties of illite and kaolinite are therefore quite similar to quartz, so the mechanism of interfacial water disruption by chaotropic counter-ions also seems to be applicable to these two minerals. As the viscosity results indicate (a nearly-constant intrinsic viscosity), concentrated salt solutions primarily affect the degree of intermolecular association of guar gum rather than the conformation of individual macromolecules (coiled vs. stretched). As a result, it can be postulated that individual guar gum molecules adsorb on the minerals from concentrated KCI solutions, while entire guar gum aggregates adsorb from concentrated NaCl solutions. Guar gum adsorption on dolomite proceeds differently and a chemical complexation process is very likely involved in the adsorption mechanism. The adsorption process on potash slime minerals in concentrated salts is also a function of the chaotropic/kosmotropic properties of the background electrolytes. The data, however, strongly suggest that the dolomite component of the slimes is not completely protected by guar gum in saturated potash brine. Guar gum acts as a flocculant of the slimes regardless of the type and concentration of the background electrolytes but higher doses of the polymer redisperse the fines which may impede the "debrining" process of the tailings suspensions. 105 Overall, the results demonstrate that the adsorption behavior of guar gum in concentrated electrolytes cannot easily be inferred from its action in distilled water or in dilute salt solutions. 5.5 References [1] Liu, Q., Zhang, Y. and Laskowski, J.S., The adsorption of polysaccharides on mineral surfaces: an acid /base interaction, International Journal of Mineral Processing, Vol. 60, pp. 229-245, (2000). [2] Ma, X. and. Pawlik, M , Effect of alkali metal cations on adsorption of guar gum onto quartz, Journal of Colloid and Interface Science, Vol. 289, pp. 48-55, (2005). [3] Pawlik, M. and Laskowski, J.S., Effect of ionic strength on stabilization of mineral suspensions by carboxymethyl cellulose and guar gum, 2004 Society for Mining Metallurgy and Exploration (SME) Annual Meeting & Exhibit, Feb 23-25, Denver, CO, Preprint #04-059. [4] Marra, J., Van Der Schee, H.A., Fleer, G.J. and Lyklema, J., Polyelectrolyte adsorption from saline solutions. In: Ottewill, R.H., Rochester, C H . and Smith, A.L. (Eds.), Adsorption from solution, Academic Press, New York, pp. 245-258, (1983). [5] Koral, J., Ullman, R. and Eirich, F.R., The adsorption of polyvinyl acetate, Journal of Physical Chemistry, Vol. 62, pp. 541-550, (1958). [6] Dubois, M., Gilles, K.A., Hamilton, J.K., Rebers, P.A. and Smith, F., Colorimetric method for determination of sugars and related substances, Analytical Chemistry, Vol.'28, pp. 350-356, (1956). [7] Huggins, M.L., The viscosity of dilute solutions of long-chain molecules. IV. dependence on concentration, Journal of the American Chemical Society, Vol. 64, pp. 2716-2718,(1942). [8] Sakai, T., Huggins constant k for flexible chain polymers, Journal of Polymer Science, Vol. 6, pp. 1535-1549, (1968). [9] Picout, D.R., Ross-Murphy, S.B., Errington, N. and Harding, S.E., Pressure cell assisted solution characterization of polysaccharides. 1. guar gum, Biomacromolecules, Vol. 2, pp. 1301-1309, (2001). 106 [10] Picout, D.R., Ross-Murphy, S.B., Jumel, K. and Harding, S.E., Pressure cell assisted solution characterization of polysaccharides. 2. locust bean gum and tara gum, Biomacromolecules, Vol. 3, pp. 761-767, (2002). [11] Bohdanecky, M. and Kovaf, J., Viscosity of polymer solutions, Elsevier, Amsterdam, (1982). [12] Robinson, G., Ross-Murphy, S.B. and Morris, E.R., Viscosity-molecular weight relationships, intrinsic chain flexibility and dynamic solution properties of guar galactomannan, Carbohydrate Research, Vol. 107, pp. 17-32, (1982). [13] Alonso, E.A., Laskowski, J.S., and Zuleta, M. , Aggregation of dolomitic slimes by polyelectrolytes in water and brine. In: Laskowski, J.S.(Ed.), Polymers in Mineral Processing, Metallurgical Society of the Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Canada, pp. 463-477, (1999). [14] Ma, X. and Pawlik, M. , Adsorption of guar gum on potash slimes, In Z. Xu, & Q. Liu. Interfacial Phenomena in Fine Particle Technology, pp. 509-524, Montreal, QC: Metallurgical Society of CIM, (2006). [15] Pokrovsky, O.S., Mielczarski, J.A., Barres, O. and Schott, J., Surface speciation of calcite and dolomite/aqueous solution interfaces and their spectroscopic evaluation, Langmuir , Vol. 16, pp. 2677-2688, (2000). 107 CHAPTER 6 General Conclusions This thesis analyzed several phenomena that have largely been neglected in all previous adsorption studies on polysaccharides. It was demonstrated that the acid-base (chemical) interaction model of polysaccharide adsorption could not be generalized for all polysaccharides, and guar gum was a very important exception. The adsorption of guar gum on all the tested oxide and clay minerals was independent of pH and consistent with hydrogen bonding. This conclusion was further supported by the lack of correlation between the i.e.p. of the tested minerals and the adsorption density of the polymer. The high affinity-type of adsorption isotherms of guar gum could be explained by the aggregate nature of guar gum solutions, and the resulting adsorption of entire guar gum aggregates, even though the actual attachment to the surface was through hydrogen bonding. The results also offer the first experimental piece of evidence that - in contrast to the common belief - polysaccharide adsorption through hydrogen bonding can in fact be selective. The ease or difficulty of guar gum adsorption on a mineral surface is determined by the presence of the interfacial water layer which promotes or inhibits guar gum adsorption. Adsorption proceeds more easily onto surfaces characterized by a weak, distorted interfacial water layer (e.g., titania, hematite, alumina). On mineral surfaces possessing an extensive hydration layer (silica-like surfaces), this interfacial water must first be destabilized in order to facilitate the adsorption of the polymer. The adsorption process should be viewed as competition between the polymer and water for the polar surface metal-hydroxyl sites. The main role of ions in guar gum adsorption is to affect the 108 relative hydration of the adsorbent surface and the polymer, and to control the stability of the interfacial water layer. Oxide and clay minerals can be divided into two groups depending on the effect of chaotropic and kosmotropic ions on guar gum adsorption. Chaotropic counter-ions, electrostatically attracted to the charged surface, destabilize the interfacial water layer present on silica-like surfaces and allow the polymer to more densely adsorb on the surface sites. The overall effect is also enhanced by the higher affinity of potassium and cesium ions, in comparison to lithium and sodium, towards silica/quartz and clay minerals. Therefore, much higher concentrations of sodium ions than potassium ions are needed to induce a similar effect on quartz, kaolinite and illite. Although strongly hydrated cations are known to show a higher affinity towards hematite, alumina, and titania, sodium and lithium ions do not cause any enhanced adsorption on these oxides compared to cesium or potassium. In these cases, the interfacial water layer does not prevent the polymer from adsorbing onto the surface sites. The role of background ions in disrupting the interfacial water layer on such solids is very limited and guar gum adsorption is largely independent of salt type. For the first time, it was shown through intrinsic viscosity measurements that concentrated electrolytes differently affect the behavior of guar gum in aqueous solutions depending again on the chaotropic and kosmotropic properties of the electrolytes. Competition for free water molecules between the ions and the polymer leads to enhanced aggregation of the polysaccharide by strongly hydrated kosmotropic ions. On the other hand, concentrated chaotropic salt solutions are very powerful solvents for guar gum and their effect on guar viscosity is consistent with the "salting-in" of the polymer. It 109 is, however, noteworthy that the intrinsic viscosity of the polysaccharide is remarkably constant over a wide range of ionic strengths and temperatures. The two groups of minerals also give two characteristic adsorption responses in highly concentrated electrolytes. The decrease in guar gum adsorption on hematite, titania, and alumina in saturated potassium and sodium chlorides indicates that solvency effects do not significantly contribute to adsorption on these oxides. In contrast, adsorption on quartz and kaolinite sharply increases under conditions of enhanced guar gum aggregation (saturated NaCl) and decreases when the polymer finds itself in.a good solvent (saturated KC1). For both groups of minerals, specific adsorption of background ions at such high ionic strengths must also be a significant inhibiting factor although its exact contribution could not readily be determined at this stage. Understanding the role of chaotropes and kosmotropes in controlling the adsorption of guar gum from concentrated electrolytes leads to the conclusion that the dolomitic component of sylvinite ore slimes is not "blinded" by the polymer at low dosage during potash flotation carried out in a saturated brine. A different type bf blinder is probably needed for high-dolomite potash ores to ensure the complete coverage of slime particles. This thesis is the first systematic study in which adsorption of a nonionic polymer was shown to depend on the concentration and type of the background salts. The presented results also demonstrated that certain trends in guar gum adsorption, e.g., the effect of potassium ions on guar gum adsorption on quartz, can easily be misinterpreted if the chaotropic properties of ions are not recognized and taken into account. 110 CHAPTER 7 Recommendations for Future Studies Aggregate adsorption deserves further detailed investigations since macromolecule aggregation in aqueous solutions is very common not only for guar gum but also for other important polysaccharides such as dextrin, starch, and cellulose derivatives. There is a clear need to identify the contribution of aggregate versus molecular adsorption to the total adsorption density since any straight application of a polymer adsorption theory/model, all of which invariably assume adsorption of individual macromolecules, is not appropriate in the case of aggregate systems. In fact, the described preferential adsorption of guar gum on quartz from dilute KCI solutions cannot easily be predicted by any of polymer adsorption models. Systematic adsorption studies with the use of "truly molecular" solutions of polysaccharides, whose preparation has only recently been described in the literature, are strongly recommended before a definitive model of "aggregate vs. molecular adsorption" is developed. The bridging mechanism of the flocculation of fine particles by high molecular weight polysaccharides (starch, guar gum) may also need to be revisited since the presumed bridging of fine mineral particles by a stretched single macromolecule may in fact result from "bridging" by large aggregates. In addition to mineral processing, polymers have been widely used in various industries as flocculants or dispersants. For example, in environmental engineering, polymers are employed as flocculants for pollutants; in nanotechnology, polymers are used to disperse nanomaterials. It would be highly desirable to assess the importance of such lyotropic ion phenomena for these polymer systems whose performance is totally based on the adsorption behavior of polymers. Ultrasmall-angle light scattering (USALS) technique can provide information on the structure of polymer aggregates on length scale from a few angstroms to several tens 111 of microns. The interfacial water layer on hydrophobic and hydrophilic surfaces can be expected to possess different degrees of structuring, as judged from the density of hydrogen bonding with the surfaces, and such changes in the density of hydrogen bonding can be probed using Fourier-Transform Infrared Internal Reflection Spectroscopy (FTIR/IRS). Theoretical support for such findings can be provided by molecular dynamics simulation (MDS) studies similar to that of Du et al. (ref. 31 in Chapter 3) 112 APPENDICES APPENDIX 1 Calibration curves of guar gum 0 40 80 120 160 Guar Gum Concentration [mg/L] Fig. A . l Calibration curve of guar gum in distilled water. 1.6 0 40 80 120 160 Guar gum concentration [mg/L] Fig. A.2 Calibration curve of guar gum in 50% saturated salt solutions. Fig. A.6 ESA of kaolinite in salt solutions. 2 4 6 8 10 12 pH Fig. A.7 ESA of illite in salt solutions. 116 APPENDIX 3 Electrokinetic Sonic Amplitutde (ESA) The Electrokinetic Sonic Amplitutde (ESA) effect occurs when an alternating electric field, of known amplitude and frequency, is applied to a suspension of fine charged particles. The electric field causes the particles to move back and forth (oscillate) due to their surface charges, and in so doing each particle generates a tiny pressure,wave of some amplitude and frequency. The velocity of this oscillatory motion is proportional to the surface charge of the particles. All the pressure waves then add up to a macroscopic sound wave that travels through the suspension to the measuring electrodes coupled with pressure transducers. The raw ESA signal measured is given by: P ESA = — E where P is the measured magnitude of the pressure wave (in Pascals) while E is the applied electric field strength (in V/m). As the frequency of the applied field is increased, the inertial forces acting on the particles increase causing both a decrease in particle mobility magnitude and an increase in phase lag. This simply means that small particles easily keep up with the changes in the electric field, while the ESA signal from large particles is significantly delayed with respect to the applied field, particularly at higher frequencies. At low frequencies, when particles are able to quickly respond to any changes in the electric field, the inertia effects can be neglected. ZetaProbe operates in the frequency range 0.3 - 11.6 MHz and an internal algorithm of the ZetaProbe allows it to automatically calculate particle sizes in the range from 1 nm to 10 microns, but this type of data is not available to the user. 117 It should be noted that the magnitude of the dynamic mobility decreases with particle size, while the phase lag (delay) increases with particle size. This observation immediately suggests that there is a particle size for which the magnitude of the dynamic mobility is zero (the particle does not move at all) and the zeta potential cannot be determined. For suspensions containing very coarse particles, zeta potential calculations make physical sense only if the particles produce a measurable mobility at the lowest field frequency. The instrument can actually be "told " the size distribution of the sample by inputting dso and (50% and 85% passing) sizes, and assuming log-normal size distribution, but one must always be aware of the existence of this upper size limit. If the particle size distribution of the sample is not specified, the ZetaProbe will automatically assign the lowest measured mobility to a size of 10 microns and perform zeta potential calculations based on that assumption. If the coarsest particles are much larger than 10 microns then this "automatic size assignment" will drastically underestimate the magnitude of the zeta potential. According to some literature sources, particles as coarse as 150 microns actually produce a measurable signal, but this limit certainly varies for different materials. Since the tested suspensions contained very coarse particles, for which inertia effects were significant, only the raw ESA signal rather than some "apparent" zeta potential was reported. 118 

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