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Effect of alkali metal cations on adsorption of guar gum onto quartz Ma, Xiaodong 2005

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EFFECT OF A L K A L I METAL CATIONS ON ADSORPTION OF GUAR G U M ONTO QUARTZ by X I A O D O N G M A B . S c , Northeastern University, 1996 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F M A S T E R O F A P P L I E D S C I E N C E in T H E F A C U L T Y O F G R A D U A T E S T U D I E S M I N I N G E N G I N E E R I N G T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A A p r i l 2005 © Xiaodong M a , 2005 Abstract The effect o f a lka l i metal chlorides ( L i C l , N a C l , K C 1 , C s C l ) o n the adsorption o f -guar gum, a natural ly occur r ing polysaccharide, onto quartz f rom dilute aqueous solutions o f the electrolytes was investigated. The adsorption o f guar g u m onto quartz and the resul t ing c o l l o i d a l stabili ty o f the system were determined through adsorption, zeta potential and turbidi ty measurements. T h i s thesis analyzes a prev ious ly undescribed phenomenon o f enhanced polysacchar ide adsorption i n the presence o f ces ium and potassium cations. A t the same t ime, l i t h i u m and sod ium cations show no effect o n the polysaccharide adsorption density compared to that observed i n d is t i l led water. These differences i n the behavior o f the s imple a lka l i chlorides are attributed to their water structure breaking or water structure m a k i n g capabil i t ies . In this approach, since hydrogen bond ing is the m a i n adsorpt ion mechan ism, the p o l y m e r adsorption process is treated as a compet i t ion between p o l y m e r and water molecules for polar surface sites on the quartz surface. W h i l e water structure-breaking cations disturb the interfacial water layer a l l o w i n g guar gum to more densely adsorb o n the exposed surface s i lanol groups, structure-making cations better fit into the interfacial water layer and do not affect the guar gum-water compet i t ion for the polar surface sites. The results strongly suggest that s imple a lka l i metal chlorides are not total ly indifferent, and even i n dilute solutions their water-structure making/break ing capabil i t ies should be taken into account to better understand the behavior o f a m o d e l system such as quartz-guar gum. The study is relevant to several industrial flotation processes where polysaccharides are used as selective depressants. Th i s thesis should be a valuable cont r ibut ion to our understanding o f the mechanisms o f polysaccharide adsorption o n minera l surfaces. TABLE OF CONTENTS Abstrac t i i Table o f Contents i i i L i s t o f Figures v L i s t o f tables v i i A c k n o w l e d g e m e n t s v i i i Object ives i x C H A P T E R 1 Introduction , 1 1.1 Water Structure and Aqueous So lu t ion 3 1.1.1 Ion-Water Interactions 3 1.1.2 Interfacial Water 8 1.2 Surface Chemis t ry o f Quartz 10 1.2.1 Genera l Information 10 1.2.2. Quar tz-Water Interface 12 1.2.3 Surface Charge o f Quartz 14 1.2.4 E lec t r i ca l D o u b l e L a y e r A r o u n d a Charged Part icle 16 1.3 G u a r G u m 22 1.3.1 Genera l Information 22 1.3.2 H y d r a t i o n o f G u a r G u m 24 1.3.3 A d s o r p t i o n o f Po lymers 26 C H A P T E R 2 Exper imenta l Procedures 30 2.1 Mate r ia l s 30 2.2 M e t h o d s 35 2.2 .1 . A d s o r p t i o n tests 35 2.2.2. Ze ta Potent ia l Measurements 36 2.2.3 Turb id i ty 38 C H A P T E R 3 Resul ts 39 3.1 Ze ta Potent ia l Measurements 39 3.2 A d s o r p t i o n Measurements 44 3.3 Turb id i ty Measurements 50 C H A P T E R 4 D i s c u s s i o n 53 i i i C H A P T E R 5 Conc lus ions 60 R E C O M M E N D A T I O N S 61 R E F E R E N C E S 62 A P P E N D I C E S 74 A p p e n d i x 1. Ca l ib ra t ion Curve for Guar G u m 74 A p p e n d i x 2. Ca l ib ra t ion Curve for D e x t r i n 74 A p p e n d i x 3. Ca l ib ra t ion Curve for Dex t ran 75 A p p e n d i x 4. A d s o r p t i o n Isotherms for Dex t ran on Quar tz 75 A p p e n d i x 5. Da ta o f A d s o r p t i o n Measurements 76 i v List of Figures Figure 1.1. A s imple m o d e l for the water structure modif icat ions produced by a smal l i o n 4 F igure 1.2 . V i s c o s i t y o f a lka l i halide solutions as a function o f concentrat ion 6 F igure 1.3. Func t iona l groups that occur on the surfaces o f quartz 10 F igure 1.4. Surface densities o f the functional groups o n quartz as a funct ion o f p H 14 F igure 1.5. A modern v i e w o f electrical double layer 16 F igure 1.6. A structural formula o f guar gum 22 F igure 1.7. S w e l l i n g ratio o f P V P gels i n aqueous solutions o f Chlor ides 25 F igure 1.8. Conf igura t ion o f an adsorbed po lymer molecule o n a s o l i d surface 26 F igure 2 .1 . A n X - r a y diffract ion pattern for the quartz sample 31 F igure 2.2. The pore size dis tr ibut ion o f the quartz sample 32 F igure 2.3. Par t ic le size dis tr ibut ion o f the quartz sample 33 F igure 3.1. Ze ta potential o f quartz i n 0.01 M solutions o f salts 39 F igure 3.2 Ze ta potential o f quartz i n 0.1 M solutions o f salts 40 F igure 3.3 Ze ta potential o f quartz i n 0 . 0 1 M C s C l , condi t ioned for 30 minutes and 24 hours 41 F igure 3.4 Ze ta potential o f quartz i n 0 . 0 1 M K C 1 , condi t ioned for 30 minutes and 24 hours 41 F igure 3.5 Ze ta potential o f quartz i n 0.01 M N a C l , condi t ioned for 30 minutes and 24 hours 42 F igure 3.6 Ze ta potential o f quartz i n 0 . 0 1 M L i C l , condi t ioned for 30 minutes and 24 hours 42 v Figure 3.6 Ze ta potential o f quartz i n 0 . 0 1 M L i C l , condi t ioned for 30 minutes and 24 hours 42 F igure 3.7 The effect o f guar gum addi t ion o n the zeta potential o f quartz particles. Gua r gum concentration 120 m g / L 43 F igure 3.8 A d s o r p t i o n isotherms o f guar gum o n quartz at p H 5.2 and a lka l i metal chloride concentration o f 0.01 M . A set o f adsorption results for a dextr in sample is also inc luded 44 F igure 3.9 Effect o f ion ic strength on the adsorption o f guar g u m a t p H 5 . 2 45 F igure 3.10 Effect o f p H o n guar gum adsorption on quartz. p H adjustments were made w i t h various hydroxides or w i t h hydrochlor ic acid. Ini t ial guar gum concentration 90 m g / L 46 F igure 3.11 A d s o r p t i o n o f guar gum as a function o f p H i n the presence o f 0.01 M background electrolytes. Ini t ia l guar gum concentration 90 m g / L 47 F igure 3.12 Co-effect o f background electrolyte and added base o n the adsorption o f guar gum. Ini t ial guar gum concentrat ion 90 m g / L 48 F igure 3.13 Co-effect o f K C 1 and N a C l o n the adsorption o f guar gum at natural p H (5.2). Ionic strength 0.01 M . Ini t ia l guar gum concentration 120 m g / L 49 F igure 3.14 Effect o f a lka l i metal cations on the stabili ty o f • quartz 50 F igure 3.15 Turb id i ty o f the guar guar-quartz system i n the presence o f 0 . 0 1 N electrolytes at natural p H 51 F igure 3.16 Turb id i ty o f the guar gum-quartz system i n the presence o f 0 . 0 1 M electrolytes at p H 10 52 F igure 3.17 Turb id i ty o f the guar gum-quartz system i n the presence o f 0 . 1 M electrolytes at natural p H 52 v i List of Tables Table 1 Free energies of hydration of hydrogen cation and alkali cations at 25 °C 7 Table 2 Crystallographic and hydrated radii for alkali metal cations 18 vii Acknowledgements The w o r k presented here was funded by: Na tura l Sciences and Engineer ing Research C o u n c i l o f Canada. The w o r k was directed and supervised by: D r . M a r e k P a w l i k The advice and support o f a l l is greatly acknowledged. Objectives 1. T o demonstrate the role o f s imple a lka l i metal cations i n the adsorption o f guar g u m onto quartz 2. T o show the effect o f a lka l i metal ions on the f locculat ion/dispers ion o f quartz by guar gum. 3. T o propose a mechan i sm o f quartz-guar gum interactions i n the presence o f a l k a l i metal cations. ix CHAPTER 1 Introduction The mode o f ac t ion o f natural and synthetic polysaccharides i n the selective f lotat ion o f minerals has been a subject o f very extensive research over the past 20 years. Starches, dextrins, various gums and synthetic derivatives o f cel lulose (e.g. ca rboxymethy l cel lulose) have been tested as s l ime blinders, selective depressants o f natural ly hydrophobic minerals , and selective depressants o f base metal sulfides. There is o n l y a l imi t ed number o f successful industr ial applications o f these very envi ronmenta l ly fr iendly reagents. In potash flotation, a process that is carr ied out i n a saturated K C l / N a C l brine, guar gum is routinely used to " b l i n d " water insoluble s l imes such as c lays , carbonates and quartz. The role o f guar gum is to adsorb o n these sub-m i c r o n particles and prevent a cationic amine col lector f rom adsorbing onto these unwanted particles. In i ron ore processing, hematite can be separated from quartz by a reverse f lotat ion process i n w h i c h starch select ively flocculates and bl inds hematite a l l o w i n g quartz to be floated us ing a cat ionic col lector . Cer ta in sulfide ore operations use carboxymethyl cel lulose to depress the f loatabi l i ty o f natural ly hydrophobic gangue minerals such as talc or graphite. In the f lotat ion o f p la t inum group metals-bearing ores guar gum is used to depress talc. In the current industr ial practice o f sulfide ore flotation, the selective separation o f bu lk flotation concentrates is potential ly a very important area o f polysacchar ide appl ica t ion since tox ic depressants such as sod ium bisulf ide ( N a H S ) or cyanides are s t i l l standard reagents. In a l l these processes, the use o f polysaccharides is based on a "tr ial-and-error" approach rather than o n a sound scientific evidence. 1 The c o m m o n v i e w that polysaccharides are non-selective (i.e. that they ind isc r imina te ly adsorb o n a l l minerals) , the general lack o f understanding o f the mechanisms i n v o l v e d i n polysaccharide adsorption, and the fact that a w ide chemica l var iety o f these reagents exists strongly hinder their wider appl icat ion. A s w i l l be discussed, a number o f different adsorption mechanisms were proposed i n the literature even for the same mineral-polysaccharide system. A s this thesis w i l l show, a ve ry important, p rev ious ly undescribed phenomenon has systematical ly been over looked i n a l l the studies o n polysaccharide adsorption. S imple a lka l i metal chlor ides , ve ry c o m m o n salts i n minera l processing systems, are rout inely assumed to be indifferent, so-cal led background electrolytes towards minera l surfaces. The presented analysis o f the results strongly indicates that adsorption data for a g iven minera l -polysacchar ide system can be misinterpreted i f the water structure-making or breaking properties o f the a lka l i metal chlorides are not taken into account. Th i s dissertation opens a who le n e w area for further research o f polysacchar ide-mineral interactions. 2 1.1 Water Structure and Aqueous Solution P h y s i c o c h e m i c a l methods o f fine particle processing, e.g. f lotat ion, f loccula t ion , o i l agglomerat ion, and even fi l tration, re ly o n interfacial phenomena occur r ing at the minera l - so lu t ion interface. Reagent adsorption, changes i n the resul t ing wet tabi l i ty o f minerals , or the state o f fine particle aggregation/dispersion are just some o f the under ly ing processes that have been researched i n quite a detail . H o w e v e r , the m a i n focus o f a l l the past studies was o n "the minera l surface", or "the reagent". A l t h o u g h the presence o f water molecules at the minera l -solut ion interface is taken for granted, the role o f the interfacial water and its structure have never been adequately correlated w i t h mineral-reagent interactions. 1.1.1 Ion-Water Interactions L i q u i d water has long been k n o w n to possess dist inct ive structural features, i.e. it retains a certain degree o f s imi lar i ty to ice. The amount o f this " ice- l ikeness" m a y be altered by changes i n temperature and pressure, and may also be affected by the presence o f solutes (Frank and W e n , 1957). In particular, charged species strongly distort the structure o f surrounding water, as a consequence o f the change o f mic roscop ic balance o f in termolecular forces, f rom that o f water-water interactions i n the pure solvent to i o n -water interactions i n the result ing solut ion ( C h i a l v o et a l . , 1999). Because ion-water interactions are determining factors i n the behavior o f aqueous solut ions, w i t h w ide appl ica t ion i n b io log ica l , geochemical , environmental and industr ia l processes, the structure o f water and the effect o f ions upon it have been a subject o f numerous investigations. These studies were frequently based o n the scheme in t roduced by F rank and W e n (1957), and Gurney (1953) w h o analyzed the behavior o f ions i n terms o f their water-structure breaking and water-structure making capabili t ies. T h i s m o d e l is schemat ica l ly i l lustrated i n F igure 1.1. F igure 1.1 shows that an i o n i n aqueous solut ion is surrounded by three concentr ic regions. R e g i o n A is composed o f i m m o b i l i z e d water molecules , w h i c h are h i g h l y 3 ordered by the ion ic f ie ld . In region B , the water molecules are less ordered. These water molecules are more random i n organizat ion than the ones i n the water networks or water clusters i n bu lk water (note that bu lk water is composed o f free water molecules and water networks or water clusters whose component water molecules cannot m o v e as freely as free water molecules) . R e g i o n C represents bu lk water. Figure 1.1. A s imple mode l for the water structure modif icat ions produced by a s m a l l i o n (Frank and W e n , 1957). R e g i o n A : i m m o b i l i z e d water molecules (hydration sheath); R e g i o n B : less ordered water molecules (region o f structure breaking); R e g i o n C : bu lk water. 4 In reg ion A , water molecules are t ight ly bound to the ion . R e g i o n B extends farther away f rom the i o n and is referred to as the region o f structure breaking. O n l y at larger distances, where the ion ic f ie ld is weak, water molecules form the " n o r m a l " ice -l i ke structure. S m a l l ( in terms o f crystal lographic radii) , strongly hydrated ions reinforce the " n o r m a l " structure o f water and the region o f structure breaking (region B ) disappears. In contrast, large poor ly hydrated ions disturb the ice - l ike structure and generate an extensive reg ion o f structure breaking so that region A disappears. Structure-breaking ions are also referred to as chaotropes whereas structure-makers are k n o w n as kosmotropes (De X a m m a r Oro , 2001). Exper imen ta l ly , these two effects can readi ly be dis t inguished through v i scos i ty measurements. Increasing concentrations o f structure-breaking ions result i n a decrease o f the v i scos i ty o f aqueous solutions. O n the other hand, increasing amounts o f structure-makers s ignif icant ly increase the v i scos i ty o f water (Stokes and M i l l s , 1965). In general, the var ia t ion o f the relative v iscos i ty nr w i t h the molar i ty c can be represented by Jones and D o l e equation: r\r=nlno= 1 +A„cl/2 + Bnc (1) where n and no are the viscosi t ies o f the solut ion and solvent respectively, An is a constant depending o n the long-range cou lombic forces, and Bn is an adjustable parameter w h i c h is related to the size o f the ions and to different ion-solvent interactions. The Jones and D o l e equation is l imi ted to fair ly l o w concentrations (< 0.1 M ) . F o r higher concentrations, addi t ional parameters are required. In most cases, it is sufficient to add a second order term, c2 ( K a m i n s k y , 1957): nr=n/no=l+A„cl/2+ Bnc + Dnc2 (2) The v i scos i ty Bn coefficient provides a very useful quantitative measure o f ions ' effects o n water structure (Gurney, 1953). F o r structure breaking ions, the v i scos i ty Bn coefficients are negative, w h i l e structure m a k i n g ions give posi t ive Bn coefficients. 5 Figure 1.2 shows the relative v iscos i ty versus concentration for aqueous solutions o f hal ide salts. A l l the curves f o l l o w the parabol ic equation 2. It is clear f rom Figure 1.2 that increasing concentrations o f L i C l and N a C l result i n relative viscosi t ies greater than 1, w h i l e the remain ing a lka l i halides ( K C 1 and C s C l ) cont inuously decrease relat ive viscosi t ies . O n this basis, L i + and N a + are classif ied as structure m a k i n g ions w h i l e K + and C s + are c lass i f ied as structure breaking ions. 1.3 In 1-25 w I 1.2 rz E 2. 1.15 H & 8 1.1 w > 105 -t—» m <D or • LiCl • NaCl A KCI A CsCl o _ • 1 ^ A A A A A A A A A A A ^ A A A A A A A A A A A A A 0.95 0.9 0.5 1 Concentration [mol/L] 1.5 Figure 1.2. V i s c o s i t y o f a lka l i halide solutions as a function o f concentration. (Weast , 1971). It is also important to point out that the free energies o f hydra t ion o f a l k a l i metal cations vary greatly f rom L i + to C s + . A selection o f so-cal led "best" s ing le - ion data is g iven i n Table 1 (Franks, 1972). The free energies o f hydrat ion also show that the 6 hydra t ion o f L i + is more thermodynamica l ly favorable than the hydra t ion o f C s + . T h i s order ing o f ions f rom the least to the most hydrated is k n o w n as the Hofmeis te r series (Napper, 1970). Table 1 Free energies o f hydrat ion o f hydrogen cat ion and a lka l i metal cations at 25 C Ions H + L i + N a + K + C s + Free energies o f hydra t ion [kcal /mol] - 260.5 - 123.5 - 9 8 . 3 ' - 80.8 - 7 1 7 1.1.2 Interfacial Water Water molecules tend to adsorb on a l l surfaces, especial ly on polar surface sites. M o s t insoluble metal oxides ( i f not all) adsorb water by chemisorpt ion to form surface h y d r o x y l groups o n top o f w h i c h water molecules are phys i ca l ly adsorbed. The two species, surface h y d r o x y l groups and adsorbed water molecules , can readi ly be ident i f ied by infrared spectroscopy (Li t t le , 1966). There is considerable evidence that l i q u i d water becomes h igh ly structured as some types o f surfaces are approached. I f the so l id surface is crystalline and the surface atoms are i n an order ly arrangement, e.g. quartz, one can imagine an order ly arrangement o f adsorbed water molecules w h i c h , through hydrogen bonding , cou ld cause an extension o f such an order ( ice- l ike structure) for some distance into the solut ion, p roduc ing a network o f water molecules (Drost-Hansen, 1977). H o w e v e r , such ordered structuring o f water is h igh ly u n l i k e l y to occur o n the surface o f amorphous s i l i ca , o n w h i c h the S i O H groups have no regulari ty beyond two or three s i lanol sites (Her, 1979). The interfacial water structure is fundamentally different i n nature f rom the no rma l structure o f the clusters i n bu lk water: 1. The interfacial structure is more stable w i t h respect to temperature and hydrostatic pressure than the bu lk structure ( H o m e et a l , 1968). F o r example, interfacial water remains " l i q u i d " d o w n to about - 40 °C (Schufle and Venugopa lan , 1967). 2. C o m p a r e d w i t h bu lk water, the interfacial water has a higher v i scos i ty (Henniker , 1949). 3. Ion transport processes such as diffusion and electr ical conduct iv i ty are roughly 10 t imes s lower i n interfacial water than i n bu lk water (Spiegler and C o r y e l l , 1953; R i c h m a n and Thomas , 1956). 4. The interfacial water has a lower vapor pressure (Franks, 1972). 5. The interfacial water has a higher specific gravity (Franks, 1972). 8 It is quite certain that the properties o f the interfacial water ment ioned above are determined by interactions w i t h the surface. The S i O H groups are stationary and are often ordered into patterns w h i c h might fit into the ice structure. W i t h i n a th in f i l m or pore, the d imens ion o f the f i l m or pore may affect water structure by not a l l o w i n g sufficient space for the development o f bu lk water structure (Franks, 1972). 9 1.2 Surface Chemistry of Quartz 1.2.1 General Information Quar tz is the m a i n gangue minera l i n prac t ica l ly every f lotat ion separation. A significant propor t ion o f the available literature concerning po lymer adsorpt ion at the so l id-aqueous solut ion interface concerns quartz (Trompette et al . , 1994; B o h m e r and K o o p a l , 1992). Consequent ly , the chemistry associated w i t h quartz surfaces has been w i d e l y studied, as summar ized i n comprehensive reviews by B o l t (1957), I le r (1979) and B e r g n a (1994). It should be noted that some features described here for quartz are der ived f rom s i l i ca . A l t h o u g h the differences between amorphous s i l i ca and crystal quartz are obv ious , they do have numerous c o m m o n surface properties and hence are considered s imi l a r by some researchers (K i s i e l ev , 1970; Staszczuk, 1985). The surface properties o f quartz are determined by the presence o f functional groups o n the surfaces o f quartz. In general, several different functional groups can be found o n the surface o f quartz, depending o n the preparation or o r ig in o f the minera l and, i f i n solut ion, the nature o f that solution. Funct iona l groups c o m m o n l y associated w i t h the quartz surface are depicted schematical ly i n F igure 1.3. H H H / N / X / ? ? ? HO 0 H ^ 0 ^ H ^ - 0 ^ H ^ 0 - ^ H Si Si Si . Si Si Si Si ! 'o I o i o I o; :o o ' J> P J o | V i c i n a l S i lanols S i l aned io l V i c i n a l S i lanols \ / ° \ / \ / ° \ / Si Si Si Si | / \ / \ / \ \ / \ Siloxanes Figure 1.3. Func t iona l groups that occur on the surfaces o f quartz (Bergna et a l . , 1994). 10 W h e n surface chemica l groups are ma in ly s i loxane S i - O - S i groups, the surfaces o f quartz are hydrophobic ; when the surfaces expose s i lanol S i - O H groups, they are hyd roph i l i c (Her, 1979). H y d r o p h o b i c quartz can revers ibly be rendered hyd roph i l i c by hydroxy la t ing the s i loxane S i - O - S i groups into s i lanol S i - O H groups ( L a m b and F u r l o n g , 1982). 11 1.2.2. Quartz-Water Interface W h e n quartz is exposed to water, it reacts to form surface s i l ano l groups ( S i y O H ) (Iler, 1979): S i O S i , + H 2 0 = 2 S L O H The s i l ano l groups are c o m m o n l y be l ieved to be the sites for adsorption o f water molecules (Iler, 1979). In a detailed study o f the behavior o f water o n the surfaces o f quartz, K l i e r and Zet t lemoyer (1977) showed that water molecules sit " o x y g e n d o w n " o n the S i O H groups. M i c h e l l e et a l . (1990) observed that decreasing the number o f surface s i l ano l groups by dehydroxyla t ion led to reduced adsorption o f water o n the surfaces o f quartz. The water f i l m o n the surfaces o f quartz is strongly bonded to quartz and hence possesses properties different from those o f bu lk water (Staszczuk, 1985). There is evidence f rom v i scos i ty measurements that a monolayer o f water molecules is i m m o b i l i z e d on the S i O H surface by hydrogen bonding (Iler, 1979). Furthermore, Y a l a m a n c h i l i et a l . (1995) detected an ordered ice- l ike interfacial water structure o n hydroph i l i c s i l i con surface by in-s i tu Four ie r transform infrared/internal ref lect ion spectroscopy, and such an ordered ice- l ike interfacial water structure is be l ieved to exist o n a l l the surfaces o f hydrophi l i c minerals . Iler (1979) also reported that the first two layers o f water molecules o n the surfaces o f quartz possess quite a h igh v i scos i ty and further deduced that the v i scos i ty is probably shear-sensitive because o f breaking o f the hydrogen bonds between the first layer o f water molecules and s i lanol groups. A s the temperature is raised, the hydrated water is r emoved and the h y d r o x y l groups condense to fo rm si loxane bonds, evo lv ing water (Iler, 1979): 2 S L O H = S i O S L , + H 2 0 12 M i c h e l l e et a l . (1990) reported that on dry quartz surface's s i lanol groups start to condense and evolve water extensively above 170 ° C . A t 500 ° C , approximate ly h a l f o f the s i l ano l groups are removed. Bu t , i n this stage, most o f the remain ing h y d r o x y l groups are s t i l l i n the neighborhood o f another h y d r o x y l group. A b o v e 750 ° C , on ly isolated s i l ano l groups remain. A t 1000 °C , on ly 0.4 O H g roups /A 2 r emain o n the surfaces o f quartz, compared to 4.6 O H groups /A 2 for the fu l ly hydroxyla ted quartz surface (Bergna et a l . , 1994). H o w e v e r , when dehydroxylated quartz is treated w i t h water, surface O H groups can form again as a result o f rehydroxyla t ion (Zhuravlev , 2000). 13 1.2.3 Surface Charge of Quartz Just l ike other minera l ox ide surfaces, quartz has a surface charge character that is defined by the relative concentrations o f H + and O H " (the potential determining ions) i n solut ion, as g iven by the f o l l o w i n g equations. S i O H + H + <=> S i O H 2 + (3) K, S i O H + O H - <=> S i O - + H 2 0 (4) K 2 W h i l e S i O " groups are responsible for the negative charge o f quartz, S i O H 2 + groups render quartz pos i t ive ly charged. Therefore, it is the relative magnitude o f the equ i l i b r i um constants Kj and K~2 i n equations (1) and (2) that determine the net charge o n the quartz surface. The pK values proposed i n the literature (Hiemstra et a l . , 1989) are i n the f o l l o w i n g ranges: pKi = 3.0 ± 1.0 and pK2 = 7.0 ± 1.0. D u v a l et a l . (2002) reported the densities o f the m a i n functional groups o n quartz as a funct ion o f p H (Figure 1.4). 0 2 4 6 Solution pH Figure 1.4. Surface densities o f the functional groups on quartz as a funct ion o f p H . ( D u v a l et a l . 2002). 14 It is generally accepted that the point o f zero charge (pzc) for quartz occurs at approximate ly p H 2 (Bol t , 1957), and is somewhat dependent o n the exact nature o f the surface. F i g . 1.4 demonstrates that, at p H 2, the density o f S i O " groups is equal to that o f SiOH2 + groups, thus def ining the point o f zero charge (pzc) o f quartz. A t p H values higher than the pzc , the density o f S i C T groups is higher than the density o f SiOH2 + groups, rendering quartz negatively charged. In contrast, at p H values l ower than the pzc , the density o f S i O - groups is lower than the density o f SiOH2 + groups, resul t ing i n pos i t ive ly charged quartz surfaces. Prac t ica l ly , a l l industr ial f lotat ion separations are carr ied out at neutral or a lkal ine p H values. In these systems, the quartz surface therefore is negat ively charged. W h e n ana lyz ing the quartz surface charge, the structure o f the oxide layer is never a negl ig ib le factor. H y d r o x y l a t e d quartz has a h igh density o f h y d r o x y l groups that are i n close p r o x i m i t y to one another (Bol t , 1957). It is also generally be l ieved that adjacent groups (v ic ina l ) can interact w i t h each other through hydrogen bonding . Consequent ly , the h y d r o x y l hydrogen atoms are strongly bound at normal p H levels , resul t ing i n the l o w surface charge o f hydroxyla ted quartz. 15 1.2.4 Electrical Double Layer Around a Charged Particle The ion concentration profile that develops around a charged particle in an electrolyte solution is commonly referred to as the electrical double layer (EDL). The layer of counter-ions close to the particle surface is usually strongly bound by electrostatic forces and as such is called "the inner layer" or "the compact layer". Ions farther away from the surface are weakly attracted to the particle surface and these loosely attached ions form "the diffuse layer" (Figure 1.5). Mineral Surface Plane (IHP) (OHP) Inner Outer Helmholtz Helmholtz Plane Solvated cations Specifically adsorbed anion in IHP Solvated anion Normal water structure e « 78.5 == dielectric constant Primary water layer e ~ 6 Secondary water layer e « 3 2 Figure 1.5. A modern view of the electrical double layer (Bockris and Reddy, 1998). 16 The compact (inner) layer is d iv ided further into the Inner H e l m h o l t z Plane ( I H P ) and the Outer H e l m h o l t z Plane ( O H P ) . The I H P runs through the centers o f spec i f ica l ly adsorbed ions, w h i l e the O H P runs through the centers o f hydrated counter-ions at the distance o f closest possible approach to the surface. The diffuse layer lies beyond the O H P . W h e n two charged particles randomly move i n an electrolyte solut ion, the diffuse layer o f E D L consis t ing o f weak ly attached ions is sheared o f f and the compact layer becomes exposed to other approaching or c o l l i d i n g particles. D u e to this shearing effect, the compact layer is said to end at the shear plane, essentially where the O H P ends. B y def in i t ion , the electr ical w o r k needed to br ing a unit charge from inf in i ty to the shear plane is referred to as the zeta potential. A s such, the zeta potential can be v i e w e d as the effective energy barrier that a charged particle senses as it approaches another charged particle (Hunter, 1993). A d s o r p t i o n o f metal ions at minera l -water interfaces is a complex process i n v o l v i n g many possible variables. Cer ta in types o f ions affect the surface charge by reacting w i t h surface functional groups. Such ions are referred to as potential determining ions (PDI ) . The surfaces o f oxide minerals possess h y d r o x y l groups due to strong hydrat ion. These groups react w i t h acids or bases to produce posi t ive or negative sites (e.g. react ion 3 and 4 for quartz). Therefore, for oxide minerals , H + and O H ~ ions are potential determining ions. Ions that are not potential determining but are capable o f strongly interfering the adsorpt ion o f P D I are ca l l ed specifically adsorbing ions. These ions often show a strong chemica l affinity towards the minera l surface. H o w e v e r , there is no clear rule as to w h i c h k i n d o f metal ions are permit ted to m o v e into the speci f ica l ly adsorbed layer. Po lyva len t and divalent ions have been reported to be capable o f specific adsorption. A l k a l i metal salts (chlorides, nitrates, perchlorates) are normal ly treated as indifferent electrolytes towards minera l surfaces. 17 T h i s means that the adsorption o f a lka l i metal cations (counterions) o n negat ively charged minera l surfaces takes place on ly through electrostatic attraction wi thout fo rming any permanent chemica l bonds w i t h the negative surface sites. Thus , the negat ively charged surface sites are not neutralized by the cations, but are o n l y par t ia l ly screened. The extent o f screening, w h i c h is also a function o f electrolyte concentration, affects the measured zeta potentials. The higher the concentration o f the indifferent electrolyte, the more efficient the screening o f the surface charges and the lower the zeta potentials. T h i s effect is also k n o w n as the compress ion o f the electrical double layer (Hunter, 1993). Iler (1979) and B o c k r i s and R e d d y (1998) summar ized the development o f the theories o n the adsorpt ion o f metal ions onto quartz: It was first recognized that metal ions, attracted by the negative charge o f quartz, are kept i n m o t i o n near the surface o f quartz by thermal energy. T h e n it was rea l ized that large ions cannot approach the surface o f quartz as c lose ly as smal l ions can. N o t e that after hydra t ion , large ions , such as potassium and ces ium, become smaller , w h i l e sma l l ions, such as sod ium and l i t h ium ions, become larger (Table 2). Table 2 Crys ta l lographic and hydrated radi i for a lka l i metal cations (Robinson , 1959; Shannon and Prewit t , 1969) L i + N a + K + C s + Crys ta l lographic rad i i (A) 0.74 1.02 1.38 1.70 Hydra t ed rad i i (A) 2.37 1.83 1.38 1.70 18 A l t h o u g h the exact hydrat ion number for ces ium and potass ium ions is not k n o w n , the hydrated and crystal lographic radi i o f the ions are assumed to be equal since the hydra t ion o f these ions is very poor. Other models o f the electr ical double layer have also been developed to m o d e l the adsorpt ion o f ions onto oxides , for example the s i te-binding mode l (Yates et a l . , 1974) and the tr ipple layer m o d e l (Davis et al . ,1978; Hayes et al . ,1987, 1988). Based o n the tr ipple layer mode l , Sver iensky (2001) predicted the distances o f adsorbed a l k a l i ions f rom the surface" o f quartz.' Sver iensky (2001) argued that for a lka l i ions to d i rec t ly adsorb o n the surfaces o f minerals i.e. to enter the inner layer, they must lose their hydra t ion water. Thus the calculated distance o f closest approach correlated w e l l w i t h the hydrated rad i i o f a lka l i metal ions. The affinity o f ions w i t h different degrees o f hydra t ion towards ox ide surfaces can be expla ined by a thermodynamic mode l o f i o n adsorption at the ox ide-so lu t ion interface developed by James and H e a l y (1972). These authors made a clear d is t inc t ion between the adsorpt ion behavior o f oxides characterized by a l o w dielectr ic constant and those characterized by a h igh dielectr ic constant. The mode l demonstrated that a low-die lec t r ic ox ide surface, such as quartz (s = 4.6), w i l l preferentially interact w i t h less hydrated cations w h i l e strongly hydrated cations w i l l not as easi ly adsorb o n such a surface. The opposite was predicted for titania, an oxide characterized by a dielectr ic constant o f 120. These differences were attributed to the ease or dif f icul ty for a metal i o n to exchange its secondary hydra t ion sheath for the interfacial water molecules upon approaching to the surface (adsorption was assumed to take place i n the Inner H e l m h o l t z Plane o f the double layer). The structure o f the interfacial water, as judged f rom the calculated values o f the dielectr ic constant for these i m m o b i l i z e d water molecules , was also an important factor. Genera l ly , for a series o f ions w i t h f ixed charge, this solvat ion energy barrier for adsorpt ion decreases as the crystal lographic ionic size increases. The crystal lographic 19 rad i i o f a lka l i m e t a l cations, and thus their affinity towards the quartz surfaces, increase i n the order L i + < N a + < K + < C s + , but their hydrated radi i increase i n the opposite di rect ion. The i o n adsorption sequences can also be expla ined by the " l i k e adsorb l i k e " concept (Dumon t et a l . , 1990). A c c o r d i n g to this approach, surfaces w i t h a h i g h heat o f i m m e r s i o n preferentially adsorb wel l -hydrated ions wh i l e surfaces w i t h a l o w heat o f i m m e r s i o n preferentially adsorb poor ly hydrated ions. The degree o f hydra t ion o f mine ra l surfaces is related to its heat o f immers ion and thus to the pzc o f the minera l (Hea ly and Fuerstenau, 1965). A c c o r d i n g l y , h igh-pzc minerals preferentially adsorb wel l -hydra ted ions and l o w - p z c minerals (e.g. quartz, pzc at p H ~ 2 ) preferentially adsorb p o o r l y hydrated ions. Th i s concept satisfactorily explains the observed adsorption sequences o n quartz and a number o f other oxides (Dumont et a l . , 1990). B o c k r i s and R e d d y (1998) also stated that for a lka l i metal cations to enter the inner layer, the metal cations should be dehydrated, but on ly part ial dehydrat ion is required. T h e y argued that, since smal l a lka l i metal cations t ight ly adsorb a large number o f water molecules , these metal cations cannot easi ly get r i d o f their hydra t ion sheath and thus w i l l not enter the inner layer. O n the contrary, large metal cations can on ly loose ly adsorb m u c h fewer water molecules . Such metal cations can easi ly th row away their hydra t ion sheath and hence tend to enter the inner layer. F o r the a lka l i metal cations to enter the inner layer, some o f the water molecules i m m o b i l i z e d o n minera l surfaces shou ld be removed to make room for the a lka l i metal cations to adsorb. T h i s modern v i e w o f the electrical double layer c lear ly recognizes the presence o f differently oriented water molecules at the interface. B o c k r i s and R e d d y (1998) and B o c k r i s and K h a n (1993) concluded that the water molecules i n the i nne r -He lmho l t z plane ( I H P ) are comple te ly i m m o b i l i z e d and strongly oriented, whereas water molecules i n the outer H e l m h o l t z plane ( O H P ) are on ly weak ly oriented. A c c o r d i n g to, Dros t -Hansen (1977) the i m m o b i l i z e d water molecules on so l id surfaces are capable o f 20 producing a network of water molecules or water clusters through hydrogen bonding some distance into the solution. 1.3 Guar Gum 1.3.1 General Information Guar g u m is a natural nonion ic polysaccharide, obtained f rom the seeds o f two annual leguminous plants, Cyamopsis tetragonalobus and psoraloides. S ince its c o m m e r c i a l in t roduct ion i n 1953, the consumpt ion o f guar g u m has g r o w n rapid ly . The rapid g rowth is largely because o f guar gum's function not on ly i n the t radi t ional role o f a v i scos i ty bui lder for water systems, but also as a hydrogen-bonding, reagent-type chemica l for such industries as m i n i n g and paper m a k i n g ( M a i e r et a l . , 1993). F igure 1.6 shows a schematic depic t ion o f the structure o f a guar g u m monomer . po lymannose chain : H H C H 2 H O H I galactose side group: C H 2 O H Figure 1.6. A structural formula o f guar gum. Guaran , the functional polysaccharide i n guar g u m is a cha in o f (1—»4)- l inked p1-D-mannopyranose units w i t h a-D-galactopyranose units connected to the mannose backbone through (1—>6) g lycos id ic l inkages. The poly-mannose cha in is r andomly substituted w i t h galactose units at a mannose-to-galactose ratio o f 1.8-1.0 (Whis t l e r and H y m o w i t z , 1979; Painter et a l . , 1979). The least substituted sections o f the guar g u m molecules show the greatest tendency to associate and precipitate, facil i tated by the cis-22 configurat ion o f the mannose O H groups, wh i l e the more densely substituted regions serve to so lub i l i ze the precipitated gel- l ike structure (Dea, 1993). G u a r gum's unusual ly h igh abi l i ty to increase the v i scos i ty o f aqueous solutions is due to its large hydrodynamic vo lume and the nature o f its specific intermolecular interactions (entanglements) (Ma ie r et a l . , 1993). L i k e most h igh-molecular -weight po lymers , guar g u m shows pseudoplastic or shear th inning behavior i n solut ion. The degree o f pseudoplast ici ty o f a guar gum solut ion increases w i t h concentrat ion and molecu la r weight . Solut ions o f guar g u m do not exhibi t y i e l d stress properties ( B i l g e n , 1972). G u a r g u m is stable over a wide p H range. In solutions o f p H 3 or less, guar g u m is h y d r o l y z e d by acids, result ing i n rapid loss o f v iscos i ty . A b o v e p H 11, hydra t ion o f guar g u m is depressed and l o w viscos i ty results. In industries, guar gum is s t i l l used where h i g h p H condit ions c o u l d hydro lyze synthetic po lyamides , such as i n hydrometa l lurg ica l leaching o f a l u m i n u m from bauxite, soda from trona, and uran ium from its ores. T h e r m a l degradation occurs when guar gum solut ion is heated to 80 to 95 °C for extended periods o f t ime, result ing i n loss o f v iscos i ty (Golds te in and A l t e r , 1959; M a i e r et a l . , 1993). 23 1.3.2 Hydration of G u a r G u m A factor that has received little attention is the effect o f hydrat ion o f a p o l y m e r o n its adsorption properties. Ishihara et a l . (1998) found that protein adsorption is reduced o n h i g h l y hydrated substrates, wh i l e H a i g h et a l . (2000) showed a clear trend o f increasing adsorption o f I g G ( i m m u n o g l o b u l i n - G , an important b l o o d protein) onto hydroge l surfaces w i t h decreasing hydrat ion. These phenomena have been attributed to the layers o f water s terical ly inh ib i t ing the adsorption o f protein or I g G onto bio-substrates, w h i c h should also be an important aspect o f interactions between polymers and minera l surfaces. M o s t po lymers influence the m o b i l i t y and structuring o f water beyond the immediate interface to a thickness o f several molecular diameters (Blanshard , 1970; B a r f o d , 1988; R i c k a y z e n , 1989). Y a k u b u et a l . (1990) identif ied four forms o f water o n corn starch: t ight ly bound water, w e a k l y bound water, surface trapped water and bu lk water. O k a z a k i et a l . (1996) proposed the f o l l o w i n g mechanism for hydra t ion o f po lymers : 1. H y d r a t i o n o f po lymers is dominated by hydrat ion to the h y d r o x y l group. 2. The hydra t ion to the h y d r o x y l group is characterized as hydrogen bond ing between the lone-pair electron o n oxygen o f the h y d r o x y l group and the proton o f water. 3. The stabil i ty o f hydrogen-bonding hydrat ion is dependent on the electron-pair donat ion ( E P D ) and acceptance ( E P A ) abil i t ies o f water molecules . 4. These properties are enhanced or reduced v i a hydrat ion to ions. Takano et a l (1998) measured the swe l l ing ratios o f P V P gels (poly j V - v i n y l - 2 -pyrro l idone) , w i t h a molecular weight o f 6.3 x 10 5 , i n aqueous solutions o f chlor ides (Figure 1.7). 24 1 . 4 1.2 o 0.8 t 9 LiCl @ • NaCl O KCt ® CsCI 0.6 - <• MgCl2 • CaClj 0 10'3 10"2 10"! 10° 10 C, (mol/1) Figure 1.7. Swelling ratio of PVP gels in aqueous solutions of chlorides. (Takano et al. The swelling ratios were estimated as (d/d0)3, where d 0 is the diameter of the water-swollen gel and d is that of the salt solution-swollen gel. Figure 1.7 clearly demonstrates the effect of structure makers and breakers on the hydration of PVP gels. In the presence of L i + , a structure maker, the diameters of PVP gels increase because of a thicker water layer on the gel surfaces; while in the presence of K + and Cs + , structure breakers, the diameters of PVP gels decrease due to a thinner water layer on the surfaces of PVP gels. Goldstein and Alter (1959) studied the effect of A1C13, CaCl 2 and NaCl on the hydration of guar gum. The structure making capabilities of the cations increase according to the sequence A l 3 + > C a 2 + > Na + . It was found that at electrolyte concentrations higher than 10% (by weight), the viscosities of guar gum solutions increased dramatically following the same sequence, with AICI3 producing the highest viscosity. This trend coincides well with the observations of Takano et al (1998) (Figure 1.7), i.e. when the concentration of an electrolyte is high enough, the effect of metal ions on the hydration of polymers becomes significant and depends on the structure making/breaking capabilities of the ions. 1998). 25 1.3.3 Adsorption of Polymers The adsorption behavior o f long chain polymers (flocculants) is quite complex and not w e l l understood. There are many theories o f po lymer adsorption, as rev iewed by Fleer and Scheutjens (1993) but, generally speaking, it is accepted that a p o l y m e r molecu le adsorbs s imultaneously onto many sites on a so l id surface assuming conformations referred to as trains, loops and tails as shown i n F igure 1.8. Figure 1.8. Conf igura t ion o f an adsorbed po lymer molecule o n a so l id surface. The actual attachment to the so l id surface can take place through many different interactions (Gregory 1987): ionic (electrostatic) interaction, when an adsorbate adsorbs on a surface bear ing opposi te ly charged ion ic groups, e.g. anionic caboxymethyl -ce l lu lose o n pos i t ive ly charged oxide surfaces. hydrophobic bonding, responsible for adsorption o f nonpolar segments o f polymers and surfactants o n hydrophobic surfaces, e.g. al iphatic side chains o f ce l lu lose ethers o n hydrophobic coals. - hydrogen bonding takes place when the surface and the po lymer have suitable H -bond ing sites e.g. h y d r o x y l groups o f polysaccharides and oxide minerals . ion binding, sometimes it is found that a certain amount o f divalent metal ions (such as C a 2 + or M g 2 + ) is required to promote adsorption o f anionic polyelectrolytes onto negat ively charged surfaces. These metal ions are k n o w n to b i n d strongly to 26 carboxylate groups and serve as l inks between these groups and negative sites o n the surface. Th i s mechan i sm is i nvo lved i n the adsorption o f anionic po lymers (polyacrylates) on calc ium-act ivated quartz surface. dipole-crystal-field effects, this mechanism is c o m m o n i n adsorption o f po lymers onto a crystal l ine salt-type surface, e.g. f loccula t ion o f minerals such as apatite and fluorite by po lyac ry lamide . Prac t ica l ly a l l o f the above mechanisms were proposed to exp la in interactions between polysaccharides and minera l surfaces. W h i l e hydrogen bonding was considered as the p r imary adsorption mechanism for starch (Bala j i and Iwasaki , 1969; Hanna , 1973; A f e n y a , 1982), hydrophobic bonding was proposed for the adsorption o f dextr in onto inherently hydrophobic minerals , such as talc, molybdeni te and bi tuminous coa l . ( W i e and Fuerstenau, 1974; H a u n g et al . ,1978; M i l l e r et a l . , 1984). Th i s was later apparently conf i rmed by experiments w i t h o x i d i z e d coal ( M i l l e r et a l . , 1983). In the case o f o x i d i z e d , hydroph i l i c coals, dextr in adsorption was substantially lower than o n hydrophobic , deminera l ized ones. Ev idence for chemical interaction o f starch w i t h calcite and hematite was also reported (Somasundaran, 1969; K h o s l a et a l . , 1984). The f indings o f Subramanian and L a s k o w s k i (1993) also indicate that dext r in adsorption o n hydrophobic graphite, another hydrophobic mode l minera l , is associated w i t h chemica l interactions o f dextr in w i t h metal impuri t ies rather than w i t h hydrophobic bond ing since the adsorpt ion density decreased substantially after leaching trace metals o f f the natural ly hydrophobic graphite surface. The importance o f metallic sites was demonstrated quite c o n v i n c i n g l y by L i u and L a s k o w s k i (1989). It was reported that w h i l e dext r in d i d not adsorb onto either hydroph i l i c quartz or methylated hydrophobic quartz, it adsorbed strongly onto both these samples w h e n the quartz surface was activated w i t h lead ions. Based o n such results, L i u et a l . (2000) general ized that a l l polysaccharides adsorb on minera l surfaces through complexa t ion w i t h meta l -hydroxyl surface sites, and the nature o f the interaction, whether hydrogen bond ing or chemica l , is o f acid-base type and strongly depends o n the ac id i ty o f the surface meta l -hydroxyl groups. Th i s postulate was consistent w i t h the 27 observat ion that the adsorption o f guar gum onto talc was strongly dependent o n the surface concentrat ion o f magnesium sites (Rath et a l . , 1995; R a t h et a l . , 1997). In the case o f guar gum, Whis t l e r (1973) reported that the straight cha in conf igurat ion o f the molecu le w i t h its regularly occurr ing D -galactopyranosyl branches makes it w e l l suited to form hydrogen bonds w i t h minera l surfaces. W a n g et a l (2005) observed a substantial decrease i n guar gum adsorption onto talc i n the presence o f urea, a hydrogen bond breaker. Based on this observation, and due to the fact that guar g u m adsorpt ion was not affected by ionic strength (adjusted w i t h K C 1 ) , W a n g et a l conc luded that hydrogen bonding was the m a i n d r iv ing force behind guar gum adsorption onto talc. In a comparat ive study o f guar gum and dextrin, R a t h et a l . (1997) reported higher adsorpt ion density o f guar gum onto talc than that o f dextr in. Th i s phenomenon was attributed to the more favorable cis-configurat ion o f the h y d r o x y l groups i n mannose, as opposed to the trans-configuration i n dextrin, apart from the differences i n their molecu la r weights . In contrast, Steenberg and Harr is (1984) and Jenkins and Ra l s t on (1998) proposed that the adsorption o f guar g u m on talc occurs m a i n l y at the basal planes v i a hydrophobic force. R a t h and Subramanian (1997) proposed that the adsorption mechan i sm o f guar g u m o n m i c a is governed by hydrogen bonding and chemical interaction. T h e y also reported that adsorption densities o f guar gum onto m i c a increase w i t h p H (adjusted w i t h K O H ) . P a w l i k et a l . (2003) reported that the amount o f guar gum adsorbed o n i l l i t e , a t yp ica l potash ore component, was independent o f ion ic strength over a w i d e range o f salt concentrations. The adsorption density o f the polysaccharide on i l l i te d i d not measurably change w i t h ion ic strength from that o f d i s t i l l ed water to about 50%-saturated K C l / N a C l potash brine ( ~ 3 M K C l / N a C l ) . Howeve r , guar g u m adsorption on dolomite dramat ica l ly decreased over the same range o f ion ic strengths, and no rel iable explanat ion o f that phenomenon was proposed. 28 A d s o r p t i o n o f polymers affects the stabili ty o f fine particles towards aggregation. H i g h molecu la r weight po lymers can cause particle aggregation, the so-cal led "b r idg ing f loccu la t ion" , by s imultaneously adsorbing o n several particles. W h e n two particles coated w i t h po lymers are brought together it is possible for the loops and tails (Figure 1.8) o f one p o l y m e r molecu le to adsorb onto bare patches on the other particle and fo rm "bridges". H o w e v e r , w h e n adsorption densities are so h igh that the adsorbed po lymers cannot f ind avai lable adsorption sites on another particle, the effectiveness o f f locculants drops dramat ica l ly resul t ing i n steric s tabi l izat ion (Napper, 1970, 1977). The densely adsorbed p o l y m e r molecules provide a mechanica l barrier that prevents particles f rom aggregating. Th i s interparticle " repu ls ion" can be considered to arise f rom a s w e l l i n g pressure at the point o f sufficiently close approach (Kitchener , 1972). Therefore, depending on the dosage, even the same po lymer can induce two opposite effects: f loccu la t ion or dispers ion (steric stabil ization). 29 CHAPTER 2 Experimental Procedures 2.1 Materials Natura l guar g u m , Rantec K P 4 0 0 0 , was obtained from Rantec Corpora t ion . In Canada , Rantec K P 4 0 0 0 is w i d e l y used by the potash industry as a s l ime bl inder . G u a r g u m has one o f the highest molecular weights o f a l l naturally occur r ing water-soluble po lymers . C h e n g et a l . (2002) found that the molecular weight o f natural guar g u m is 1.935 x 10 6 , w h i l e R o b i n s o n et a l . (1982) reported that the molecula r weight o f f ive c o m m e r c i a l guar gums ranged from 4.4 x 10 5 to 16.5 x 10 5 . The molecula r weight o f K P 4 0 0 0 was calculated to be 1.39 x 10 6 by P a w l i k and L a s k o w s k i (2004) based on their v i scos i ty data us ing the f o l l o w i n g formula (Rob inson et a l . , 1982): [rj] = 3.8 x 1 0 - 4 M 0 7 2 3 Where [rj] is the intr insic v iscos i ty and M is the molecular weight. A s natural guar gum contains some insoluble residue, the f o l l o w i n g method was used to pur i fy guar gum stock solutions and determine the percentage o f the insoluble organics. Firs t , a 100 p p m solut ion o f raw guar gum was centrifuged at lOOOOg over var ious periods o f t ime. T h e n the phenol-sulfuric ac id method developed by D u b o i s et a l (1956) was used to measure the absorbance o f the guar gum solutions before and after centr i fuging (see section 2.2.1 for the analyt ical details). It was found that the absorbance values d i d not change further after a centrifuging t ime o f 30 minutes. The absorbance o f the " r a w " solut ion was 1.4711 and it decreased to 1.2957 after centrifuging. Therefore, a correc t ion factor o f 0.8808 (1.2957 /1.4711 = 0.8808) was appl ied to obtain the actual guar g u m concentrations. The correct ion factor indicates that the K P 4 0 0 0 sample contains about 12% o f water-insoluble residue, w h i c h agrees w e l l w i t h p rev ious ly publ i shed data for natural guar gums (Chatterji and Borchardt , 1981). A stock solu t ion o f guar gum ( l g / L ) was prepared every other day by q u i c k l y adding 0.5 g o f guar g u m powder into 500 m l o f v igorous ly stirred water and m i x i n g for 4 hours to ensure complete hydrat ion and dissolut ion o f guar gum. S u c h a p ro longed 30 disso lu t ion procedure is also practiced by various operations u t i l i z i n g guar gum. The stock so lu t ion was then centrifuged at lOOOOg for 30 minutes to remove any undisso lved impuri t ies . A l im i t ed number o f adsorption tests was performed w i t h the use o f dext r in (Tap ioca D e x t r i n 12, f rom A . E . Staley, M W = 5 6 , 0 0 0 ) . D e x t r i n adsorption experiments were carr ied out for reference i n d is t i l led water, i n 0 . 0 1 M K C 1 and N a C l , and i n 0 . 1 M K C 1 and N a C l solutions. D e x t r i n was completely soluble i n water and was used "as rece ived" . A quartz sample for this study was obtained from S i g m a - A l d r i c h . The sample was described by the supplier as " s i l i c o n d iox ide , f inely ground, natural ly occur r ing microcrys ta l l ine s i l i ca , h is tor ica l ly ca l led amorphous". A n x-ray analysis o f this mater ial conf i rmed that the sample is actually quartz and the posit ions o f the observed diffract ion ^ 20000 ^ 15000 10 20 30 40 50 2 theta (degrees) Figure 2.1. A n X - r a y diffraction pattern for the quartz sample. 31 peaks match perfectly the diffraction data for a standard quartz sample ("Selected P o w d e r Di f f r ac t ion D a t a for M i n e r a l s " , 1974). A n x-ray diffractogram for the S i g m a - A l d r i c h quartz is shown i n F igure 2 .1 . The B E T (Brunauer Emmet t Tel ler) specific surface area for the sample, determined f rom ni t rogen adsorption after outgassing at 2 0 0 ° C under v a c u u m , was found to be 6.0 m 2 / g . The microporos i ty o f the sample was measured us ing an A u t o s o r b - I M P B E T analyzer (Quantachrome) equipped w i t h a 1-mm H g pressure transducer. The density funct ional theory ( D F T ) method was used to obtain the pore size dis t r ibut ion, w h i c h is presented i n F igure 2.2 as the differential specific surface area contained w i t h i n the pores. 0.3 T 1 O ) 0 10 20 30 40 50 60 70 Pore Width [A] Figure 2.2. The pore size dis tr ibut ion o f the quartz sample. 32 0.1 1 10 Particle Size [micron] 100 Figure 2.3. Par t ic le size distr ibution o f the quartz sample The particle size dis t r ibut ion o f the quartz sample, as determined w i t h the use o f a M a l v e r n Mas te rs izer 2000, fe l l entirely i n the range f rom 0.3 to 13.8 microns w i t h a volume-average size o f 2.3 microns (Figure 2.3). The specific surface area calculated f rom the size dis t r ibut ion, assuming perfectly spherical and smooth particles, was 1.5 m 2 / g . The cumulat ive fraction o f the specific surface area contained i n the pores smaller than 62 A was 4.3 m 2 / g . The difference between the total B E T area and the internal pore surface area is 1.7 m 2 / g and agrees reasonably w e l l w i t h the value calculated f rom the size dis t r ibut ion. Since the radius o f gyrat ion o f guar gum i n aqueous solutions is about 980 Angs t roms ( F r o l l i n i et a l . , 1995), the po lymer cannot enter any o f the pores shown i n F igure 2.2. Thus the value o f 1.7 m 2 / g was used for calculat ing the p o l y m e r adsorption densities since that is the external surface area accessible to guar gum. 33 C e s i u m chlor ide (crystall ine, 99 .9% pure), ces ium hydroxide (50 w t % aqueous solut ion, 9 9 % pure) and l i t h ium hydroxide (crystall ine, 98%+ pure), were suppl ied by A l d r i c h . L i t h i u m , sod ium and potassium chlorides, as w e l l as sod ium and potass ium hydroxides , were a l l A C S - c e r t i f i e d chemicals f rom Fisher . In some tests, hydroch lo r ic ac id (Fisher) was also used for p H adjustments. A l l the experiments were carried out i n the p H range from 3 to 11, i.e. o n the a lka l ine side o f the point o f zero charge when the quartz surfaces were negat ively charged. 34 2.2 Methods 2.2.1. Adsorption tests In the adsorption tests, 5 g of quartz was first conditioned with 12.5 m l of salt solution or distilled water in a thermostated shaker for 20 minutes. Before conditioning, desired p H values were approximately set by adding concentrated solutions of HC1, L i O H , N a O H , K O H or C s O H to the samples. Then 12.5 ml of a guar gum solution of known concentration were added and the entire mixture was conditioned for 1 hour. Afterwards, the samples were centrifuged to remove quartz and the supernatant was assayed for guar gum following the phenol-sulfuric acid method developed by Dubois et al (1956) for polysaccharides. At this point, the p H of the supernatants was measured and reported. Dextrin assays were performed with the same procedure. The calibration curve for dextrin is shown in Appendix 2. The analytical procedure for the phenol-sulfuric acid method involved the following steps. First, a 4-ml aliquot of a supernatant was transferred to a beaker. Second, 1.5ml of a 60g/dm 3 phenol solution was added to the sample. Then, 10 ml of concentrated sulfuric acid was carefully introduced while thoroughly stirring to avoid excessive boiling and hence spilling of the mixture. First 2-3 ml of the acid were added drop by drop, and the remaining amount was allowed to flow freely. The reacting mixture was stirred again and left to cool at room temperature for 30 minutes. Depending on the concentration of guar gum in solution, the reacting mixture acquires a more or less intense yellow color with the maximum absorbance at 488 nm. The absorbance of each sample was measured at 488 nm using a Cary 50 U V - V I S spectrophotometer (Varian). The guar gum concentration was read from a calibration curve (Appendix 1). In all cases, blank samples were prepared by conditioning 5 g quartz with 25 m l of an appropriate salt solution or distilled water. The amount of guar gum adsorbed was calculated from the difference between the initial (known) and equilibrium (measured) concentrations. The amount adsorbed was calculated from the following formula: 35 A _ ( C m . - C j x 0 .025Z 5gxSBET Where A is amount o f guar gum adsorbed, C i n is the in i t i a l guar g u m concentrat ion (known) , Ceq. is the equ i l ib r ium guar g u m concentrat ion (measured), and S B E T is the specific surface area (1.7m 2 /g ) . The standard devia t ion i n these experiments was 2 -5%. 2.2.2. Zeta Potential Measurements In order to correlate the adsorption o f a lka l i metal cations o n quartz w i t h the c o l l o i d a l stabil i ty o f the quartz suspensions, a series o f zeta potential measurements was carr ied out. In addi t ion, since i o n diffusion at the quartz-solution interface can affect the measured zeta potentials, a number o f tests were performed for l ong and short cond i t ion ing t imes w i t h the background electrolytes. The zeta potential studies were performed through electroacoustic measurements us ing a C o l l o i d a l D y n a m i c s ZetaProbe. The zeta potential was calculated us ing O ' B r i e n ' s m o d e l ( O ' B r i e n , 1988; O ' B r i e n , 1990; O ' B r i e n et a l . , 1995). The tests were carr ied out at 10% (wt) sol ids . Elec t rokine t ic Sonic A m p l i t u d e ( E S A ) is an electroacoustic effect that occurs w h e n an alternating electric f ie ld o f k n o w n ampli tude and frequency is appl ied to a suspension o f fine charged particles. The electric f ie ld causes the particles to m o v e back and forth (oscillate) due to their surface charges, and i n so do ing each particle generates a t iny pressure wave o f some ampli tude and frequency. The ve loc i ty o f this osc i l la tory m o t i o n is propor t ional to the surface charge o f the particles. A l l the pressure waves then add up to a macroscopic sound wave that travels through the suspension to the measur ing electrodes coupled w i t h pressure transducers (Hunter, 1993). 36 A s the frequency o f the appl ied f ie ld is increased, the inert ial forces act ing o n the particles increase causing both a decrease i n the magnitude o f part icle m o b i l i t y and an increase i n the phase lag between the appl ied field and measured s ignal . Th i s s i m p l y means that smal l particles easi ly keep up w i t h changes i n the electric field, w h i l e the E S A signal f rom large particles is s ignif icant ly delayed w i t h respect to the appl ied field, par t icular ly at higher field frequencies. A t l o w frequencies, w h e n particles are able to q u i c k l y respond to any changes i n the electric field, the inertia effects can be neglected (Hunter, 1993). The p H o f the quartz suspensions was adjusted to 11 w i t h the corresponding base to a v o i d in t roducing foreign cations, and then the zeta potential measurements were performed as automatic titrations from h igh to l o w p H us ing 2.0 M H C 1 as the titrant. Af te r p H adjustment the suspension was a l l owed to equilibrate w h i l e stirred for 5 minutes before the zeta potential measurement was performed. The zeta potential o f quartz was measured i n 0.01 and 0.1 M a lka l i metal chloride solutions. The background electrolyte ions can also produce an E S A signal , usua l ly signif icant at salt concentrations above about 0.01 - 0.1 M concentrations. F o r this reason the background electrolyte s ignal was measured and subtracted f rom the total s ignal on ly for 0 . 1 M salts. F o r each suspension measured, a salt solut ion o f the same salt type and concentrat ion as i n the continuous phase o f the suspension was prepared. The background s ignal o f these solutions was measured at p H 5.8 and subtracted from the total s ignal measured for the corresponding suspension at a l l p H values investigated. The difference between two titration runs under the same condit ions was less than 3 m V . 37 2.2.3 Turbidity Turb id i ty measurements are a s imple and convenient too l to study the f loccu la t ion process. A s the f locculated particles g row i n size, their settling rates increase. Af te r a g iven pe r iod o f t ime, the amount o f solids left i n suspension is a measure o f the effectiveness o f f loccula t ion. Strongly f locculated suspensions produce ve ry clear supernatants o f ve ry l o w turbidities, wh i l e poor ly f locculated or dispersed systems remains turbid over a long per iod o f t ime. Changes i n the turbidities o f the quartz-guar g u m suspensions were measured w i t h a H a c h M o d e l 2 1 0 0 A N Laboratory Turbidimeter . The instrument is a nephelometer, compr i sed o f a 90 deg detector to moni tor scattered light, a forward-scatter l ight detector, a transmitted-light detector, and a back-scatter l ight detector. The experiments were performed by first condi t ion ing 0.5g quartz w i t h 50 m l o f salt solutions or d i s t i l l ed water i n a thermostated shaker for 20 minutes, then adding 50 m l guar g u m solut ion and condi t ion ing for 1 hour. F i n a l l y , the samples were a l l o w e d to settle for 10 minutes, the supernatants o f w h i c h (30 ml ) were col lected as representative samples to measure turbidities. W h e n ful ly dispersed by h igh p H i n d is t i l l ed water, the turbidi ty o f a reference quartz suspension was just be low the upper measur ing l i m i t o f the instrument. The experimental error (standard deviation) i n these tests was about 200 N T U at a turbidi ty o f 6500 N T U . 38 CHAPTER 3 Results 3.1 Zeta Potential Measurements A s Figures 3.1 and 3.2 demonstrate, the magnitude o f the zeta potential o f quartz particles at any p H value fo l lows the Hofmeis ter series. The zeta potentials are most negative i n the presence o f l i t h ium and least negative i n the presence o f ces ium. The results indicate that ces ium cations adsorb at the quartz-solut ion interface i n higher quantities than l i t h i u m , thus more efficiently screening the negative surface charges around the quartz particles, and hence decreasing the magnitude o f the zeta potential . 39 The zeta potential results show that increasing concentrations o f the electrolytes compress the electr ical double layer around quartz particles. The absolute values o f the zeta potentials are more negative i n 0.01 M salt solutions than i n 0.1 M salt solutions. In compar i son to the trends i n 0 . 0 1 M electrolytes, the same qualitative differences between i n d i v i d u a l salts can be seen i n 0 . 1 M solutions. F igures 3.3 to 3.6 indicate that no significant effect o f pro longed cond i t ion ing w i t h the electrolytes, over a per iod o f 24 hours, on the measured zeta potentials was observed. Figure 3.2 Ze ta potential o f quartz i n 0.1 M solutions o f salts. 40 0 -60 -I— 1 —1— 1 —I— 1 —I— 1 —I— 1 —I—'—i—'—I— 1 —I— 1— I 3 4 5 6 7 8 9 10 11 12 PH Figure 3.3 Ze ta potential o f quartz i n 0 . 0 1 M C s C l , condi t ioned for 30 minutes and 24 hours. -10 -A-0.01 M KCI24h -O-0.01M KCI30 min -20 > "ro § -30 o I -40 N -50 -60 - I — 1 — I — 1 — I — ' — I — ' — I — ' — I — ' — i — ' — i — ' — i — ' — I 3 4 5 6 7 8 9 10 11 12 PH Figure 3.4 Ze ta potential o f quartz i n 0 . 0 1 M K C 1 , condi t ioned for 30 minutes and 24 hours. 41 0 -10 -20 > E "co £ -30 o Q. & _40 N -50 -60 -A-0.01MNaCI24h - • - 0.01 M NaCl 30min 3 4 5 6 7 8 9 10 11 PH 12 Figure 3.5 Zeta potential of quartz in 0.01M NaCl, conditioned for 30 minutes and 24 hours. | co & CL CO <D N 0 -10 -20 -30 -40 -50 -60 -70 - A — 0.01 M LiCl 24h -o- 0.01 M LiCl 30min V ******** • 3 4 5 6 7 8 9 10 11 12 pH Figure 3.6 Zeta potential of quartz in 0.01M LiCl, conditioned for 30 minutes and 24 hours. 42 0 -60 - I — 1 — I — 1 — I — 1 — I — 1 — I — 1 — I — 1 — I — 1 — I — 1 — I — 1 — I 3 4 5 6 7 8 9 10 11 12 PH Figure 3.7 The effect o f guar gum addi t ion o n the zeta potential o f the quartz particles. G u a r g u m concentrat ion 120 m g / L . F igure 3.7 illustrates the effect o f guar gum addi t ion (at 120 m g / L ) o n the zeta potential o f the quartz particles. The guar gum concentration was such that the adsorpt ion density o f the p o l y m e r was at its plateau so the surfaces o f the particles were saturated w i t h the po lymer . E v e n though there is a clear decrease i n the magnitude o f the zeta potential values, compared to the curves from Figure 3.1, there are s t i l l the same relative differences between the results for different background electrolytes. 43 3.2 Adsorption Measurements Figures 3.8 and 3.9 present adsorption isotherms o f guar g u m onto quartz at a p H value o f 5.2 (natural p H for the quartz suspensions) i n d is t i l led water, i n the presence o f 0 . 0 1 M , and 0 . 1 M a lka l i metal chlorides. In 0 . 0 1 M electrolytes, the data for guar g u m fa l l o n two curves. The adsorption densities o f guar g u m i n d is t i l led water, 0 . 0 1 M sod ium chlor ide , and 0 . 0 1 M l i t h i u m chloride do not differ s ignif icant ly, wh i l e the amount o f guar g u m adsorbed o n quartz almost doubles i n the presence o f 0 . 0 1 M ces ium and potass ium chlor ides . There is a marked increase i n the adsorption density for a l l 0 . 1 M salts (Figure 3.9) compared to the results i n 0 . 0 1 M electrolytes, but s t i l l ces ium and potass ium chlor ides give consistently higher adsorption densities than sod ium and l i t h i u m . + Distilled Water • 0.01M LiCI • 0.01M NaCl AU.01M KCI AO.OIMCsCI X Dextrin Distilled Water o Dextrin 0.01 M KCI • Dextrin 0.01 M NaCl O Dextrin 0.1 M KCI • Dextrin 0.1 M NaCl 0 20 40 60 80 Guar Gum/Dextrin Equilibrium Concentration [mg/L] Figure 3.8 A d s o r p t i o n isotherms o f guar gum on quartz at p H 5.2 and a l k a l i metal ch lor ide concentrat ion o f 0 . 0 1 M . A set o f adsorption results for a dextr in sample is also inc luded . 44 In contrast, the adsorption densities o f dextr in i n d is t i l l ed water are ve ry l o w - l e s s than 0.05 m g / m 2 at an equ i l ib r ium concentration o f 95 m g / L - and the adsorpt ion isotherms are linear. E v e n though the affinity o f dextr in towards quartz is l o w , F igure 3.8 indicates that there is a smal l but measurable increase i n dextr in adsorption i n 0.1 M K C 1 . 0.5 E 0.4 0.3 E CD -Q i_ O w < 0.2 c o E 0.1 0 • A • • • 0.1MKCI A0.1MUCI • • A AO.IMNaCI • O.IMCsCI 0 20 40 60 80 Guar Gum Equilibrium Concentration [mg/L] Figure 3.9 Effect o f ion ic strength o n the adsorption o f guar gum at p H 5.2. The apparent effect o f p H on guar gum adsorption i n d is t i l led water is i l lustrated i n F igure 3.10. It should be pointed out that since the natural p H o f the quar tz-dis t i l led water system was 5.2, on ly values higher than 5.2 required hydrox ide addi t ion. A l l p H values b e l o w 5.2 were set us ing hydrochlor ic ac id so no metal cations were in t roduced into the system b e l o w p H 5.2. Seemingly , additions o f potassium and ces ium hydroxides b r ing about an increase i n the adsorption o f guar gum, wh i l e sod ium and l i t h ium hydroxides have prac t ica l ly no effect. 45 0.3 E cn j=, 0.25 T3 CD _Q i— O CO < O E < 0.2 0.15 • KOH A NaOH g • LiOH • i—i • CsOH • u - +.HCI • • • • • • - • • A A A A A A A A 4 • + 1 1 ! 1 1 1 — 1 — i — 1 — i — 1 — i — L — H 1 7 pH 8 9 10 11 Figure 3.10 Effect o f p H on guar gum adsorption on quartz. p H adjustments were made w i t h var ious hydroxides or w i t h hydrochlor ic acid. Ini t ial guar gum concentrat ion 90 m g / L . A s F igure 3.11 shows, this apparent effect o f p H i n F igure 3.10 is not rea l ly due to changes i n the p H values, but results f rom increasing concentrations o f metal cations in t roduced w i t h the strong bases at higher p H . p H 11 is equivalent to a ca t ion concentrat ion o f 0.001 M . It is noteworthy that once a background electrolyte is present i n the system, the "effect" o f p H ( K O H and C s O H addit ion) disappears. The adsorpt ion data fa l l o n two curves i n a w a y s imi lar to the data i n Figures 3.8 and 3.9. These results conf i rm the no t ion that it is the concentration o f the monovalent cations that determines the magnitude o f guar g u m adsorption. 46 0.3 0.25 CO -a CD _Q o 0.2 to T3 < | 0.15 < 0.1 A A • 0.01 M KCI+KOH, or HCI A 0.01 M NaCI+NaOH, or HCI • 0.01 M LiCI+LiOH, or HCI • 0.01 M CsCI+CsOH, or HCI 7 pH 10 11 Figure 3.11 A d s o r p t i o n o f guar gum as a funct ion o f p H i n the presence o f 0.01 M background electrolytes. Ini t ial guar gum concentration 90 m g / L . Some o f the most significant results are i l lustrated i n F igure 3.12 w h i c h shows the adsorpt ion density as a function o f p H when two different cations are present i n the guar gum-quartz system - one from the background salt and one from the base. Interestingly, w h e n N a C l is used to provide constant ion ic strength, additions o f K O H produce the same effect as i n F igure 3.10 and guar gum adsorption increases at higher p H values. The same observat ion can be made for the L i C l - K O H combina t ion - p H adjustments w i t h the use o f K O H clear ly increase the adsorption density o f guar gum. The results for the N a C l - K O H and L i C l - K O H combinat ions are very s imi la r to the potass ium and ces ium curves i n F igure 3.10. H o w e v e r , w h e n K C I is used as the background electrolyte, N a O H addit ions do not affect the adsorption o f the polymer . It m a y thus be conc luded that it is the presence o f potassium ions (and very l i k e l y ces ium ions) that determines the response o f the m i x e d electrolytes. 47 0.30 CD M. 0.25 T3 0 -Q k— O </> < c 0.20 o E < 0.15 • c A 0.01 M NaCI+KOH • 0.01M KCI+NaOH A 0.01 M LiCI+KOH A ± A A A • • A A 8 9 pH 10 11 12 Figure 3.12 Co-effect o f background electrolyte and added base on the adsorption o f guar gum. Ini t ia l guar g u m concentration 90 m g / L . F igure 3.13 further demonstrates the co-effect o f K and N a o n guar g u m adsorption. W h i l e the total ion ic strength was kept constant at 0 . 0 1 M , adsorption o f guar g u m increased w i t h increasing amounts o f K C 1 , w h i c h confirms that potass ium ions determine the response o f m i x e d electrolytes. The data therefore strongly suggest that N a + and L i + ions are t ruly indifferent i n dilute solutions w h i l e potassium and ces ium are capable o f enhancing guar gum-quartz interactions even at concentrations as l o w as 10 - 5 m o l / L ( p H 9 adjusted w i t h K O H or C s O H ) . S o d i u m and l i t h ium cations start affecting guar gum adsorption at concentrations o f the order o f 0.1 M . 48 0.40 -i Fraction of KCI in mixture with NaCl Figure 3.13 Co-effect of KCI and NaCl on the adsorption of guar gum at natural pH (5.2). Ionic strength 0.01 M. Initial guar gum concentration 120 mg/L. 49 3.3 Turbidity Measurements Effect o f a lka l i metal ions on the stabili ty o f quartz particles towards aggregation is demonstrated i n F igure 3.14. Turb id i ty values were expressed i n nephelometr ic turbidi ty units ( N T U ) . The results indicate that at a g iven electrolyte concentrat ion turbidit ies increase accord ing to the sequence C s + < K + < N a + < L i + , w i t h L i + p roduc ing the highest turbidit ies. In other words , aggregation o f quartz is stronger i n the presence o f C s + t h a n i n the presence o f L i + . In addi t ion, turbidities decrease w i t h increasing electrolyte concentrations. 7000 6000 -| „ 5000 z> I-2. 4000 1 3000 2000 1000 0 A • LiCl • NaCl AKCI A CsCl • Distilled Water A A 4 0.001 0.01 Electrolyte Concentration [mol/L] 0.1 Figure 3.14 Effect o f a lka l i ions o n the stabil i ty o f quartz ( p H 5.2). F igure 3.15 illustrates the effect o f guar gum on the c o l l o i d stabil i ty o f the guar gum-quartz suspensions i n the presence o f 0 . 0 1 M a lka l i metal chlor ides at p H 5.2. G u a r g u m acts as a f locculant towards quartz particles. The f loccula t ion o f quartz by guar g u m i n d i s t i l l ed water is rather weak. In the presence o f a l l the salts even traces o f the polysacchar ide produce supernatants w i t h the clari ty o f pure water. A s the guar g u m concentrat ion increases, the results again tend to fa l l on two curves; one for L i + and N a + , and one for C s + and K + . In 0 . 0 1 M sod ium and l i t h i u m chlorides, the turbidi ty o f the system can easi ly be brought back to that o f the quartz-dist i l led water system, w h i l e 50 m u c h higher concentrations o f guar gum are needed to sterically redisperse the quartz particles i n the presence o f ces ium and potassium. 7000 6000 + Distilled Water • 0.01 M NaCl A 0 . 0 1 M U C I • 0.01 M KCI • O.OIMCsCI 0 10 20 30 40 50 60 Initial Concentration of Guar Gum [mg/L] Figure 3.15 Turb id i ty o f the guar-quartz system i n the presence o f 0 . 0 1 N electrolytes at natural p H . F l o c c u l a t i o n o f quartz by guar gum is s t i l l possible even at p H 10 (Figure 3.16). The data again fa l l o n two curves but m a x i m u m f loccula t ion (lowest turbidi ty) i n the presence o f sod ium and l i th ium is s ignif icantly weaker compared to ces ium and potass ium. A t p H 10 increasing guar gum concentrations more readi ly redisperse the system than at p H 5.2 but fu l l redispersion again appears to be ve ry diff icul t i n the presence o f potassium and ces ium. The restabil izat ion o f the system at p H 10 seem to be aided b y stronger electrostatic repuls ion between the particles. A s shown i n Figure 3.17, the system is destabi l ized towards aggregation even further i n 0 . 1 M electrolytes, and fu l l redispersion o f quartz is imposs ib le i n the studied guar g u m concentrat ion range. The turbidi ty values for C s and K are very l o w and it is di f f icul t to conclude whether the data fa l l on the same curve, but there are some measurable differences between sodium and l i th ium cations. 51 8000 j 7000 T 6000 " 1- 5000 -z 4000 \a \— ZJ 3000 -1-2000 1000 -0 -A 0.01 M LiCl A 0.01 M NaCl • 0.01 MKCI • O.OIMCsCI 0 10 20 30 40 50 60 Initial Concentration of Guar Gum [mg/L] Figure 3.16 Turbidity of the guar gum-quartz system in the presence of 0.01M electrolytes at pH 10. 7000 Z5 I-z TJ 3 • 0.1M LiCl • 0.1M NaCl A0.1M KCI AU .1M CsCl + Distilled Water 0 10 20 30 40 50 Initial Concetration of Guar gum [mg/L] Figure 3.17 Turbidity of the guar gum-quartz system in the presence of 0.1 M electrolytes at natural pH. 52 CHAPTER 4 Discussion The affinity o f a lka l i metal cations towards s i l i c a surfaces generally fo l lows the Hofmeis ter series ( L i + , N a + , K + , C s + ) w i t h ces ium cations adsorbing i n greater quantities than l i t h i u m (Tadros and L y k l e m a , 1968; Sonnefeld et a l . , 1995). Th i s trend also manifests i t se l f i n the zeta potential results for the quartz sample (Figure 3.1 and 3.2) w i t h ces ium produc ing the least negative zeta potentials. Such enhanced adsorpt ion for a series o f ions o f the same charge but o f a different size is typ ica l ly interpreted i n terms o f some sort o f poor ly defined, non-electrostatic "specif ic interact ion" (Tadros and L y k l e m a , 1968; James and H e a l y , 1972; L y k l e m a , 2003) between the surface and the ions. Tadros and L y k l e m a (1968) and L y k l e m a (1968) postulated that the observed differences i n the adsorption densities o f a lka l i metal cations at the s i l i ca -so lu t ion interface were largely determined by the abi l i ty o f the ions to penetrate a porous, ge l - l ike surface layer present around the s i l i ca particles. Thus , less hydrated, effect ively smal ler ces ium and potass ium cations exhibi t higher affinities towards such a surface compared to larger strongly hydrated l i t h ium and sod ium ions. V e r m e u l e n and C a n t w e l l (1995) reported s imi la r diffusion effects for sod ium ions adsorbing o n a fused si l ica-type o f surface. The s low diffusion o f sod ium manifested i t se l f by a drift o f the zeta potential towards an " e q u i l i b r i u m " value. Interestingly, Tadros and L y k l e m a stated that condi t ion ing times between 30 and 90 minutes were necessary to achieve constant e lectromotive force readings dur ing potentiometric titrations o f s i l i c a suspensions i n the presence o f a lka l i metal chlorides (Tadros and L y k l e m a , 1968). L o n g cond i t ion ing t imes were also required i n electroosmotic studies o f the s i l i ca -so lu t ion interface (Huang et a l . , 1993). E v e n though the quartz sample used i n this study was microporous (Figure 2.2), no signif icant effect o f pro longed condi t ion ing w i t h the electrolytes, over a per iod o f 24 hrs, o n the measured zeta potentials was observed suggesting that i o n di f fus ion effects 53 were not pronounced i n the studied system. It should also be pointed out that crystal l ine minerals are less soluble than amorphous ones so the format ion o f a "ge l layer" is less l i k e l y to occur o n quartz than o n amorphous s i l i ca . It is interesting to note that even though the four cations adsorb i n different quantities o n quartz, as seen from the zeta potential results, on ly two sets o f data are consistently observed i n guar gum adsorption; those i n the presence o f C s and K , and those for L i and N a . The same can be said about the f loccula t ion o f quartz. A s expected, i n the absence o f guar gum the coagulat ing power o f the background cations towards quartz increases i n the order L i + < N a + < K + < C s + and this trend correlates w e l l w i t h the decreasing magnitude o f the zeta potentials (Figure 3.1). The zeta potentials o f guar-coated quartz particles are also different i n the presence o f the four cations, even under the condi t ions o f surface saturation by the polymer , and yet the stabil i ty results do not f o l l o w four different curves. A l t h o u g h hydrogen bonding is weaker than electrostatic repuls ion , it seems that, regardless o f the type o f the background electrolyte, hydrogen bonds between the quartz surface and the polysaccharide are sufficiently numerous to overcome electrostatic repulsive forces between the negat ively charged quartz particles and to cause b r idg ing f loccula t ion. Therefore, the stabili ty o f the quartz-guar g u m system correlates m u c h better w i t h the adsorption density o f the po lymer rather than w i t h the magnitude o f electrostatic repulsion. The fact that the adsorption data fa l l on two curves suggests that there is a threshold "va lue" o f the structure-breaking capabil i t ies o f the a lka l i metal cations for disrupting the interfacial water and enhancing guar g u m adsorption. F o l l o w i n g the arguments developed by James and H e a l y (1972), this c r i t i ca l t ransi t ion should be related to the hydrat ion energy o f the cations. The results also suggest that guar gum adsorption does not interfere w i t h the charging mechan i sm o f the quartz surface and the adsorbed macromolecules s i m p l y m o v e the shear plane farther away from the minera l surface thus decreasing the magnitude o f the zeta potential . 54 Flocculants are most efficient at adsorption densities corresponding o n l y to a fraction o f the complete surface coverage (Kitchener , 1972). T h i s agrees w e l l w i t h the turbidi ty results that show that quartz f loccula t ion is most pronounced at very l o w in i t i a l guar g u m concentrations. A t higher po lymer adsorption densities, steric s tabi l iza t ion takes place and brings about redispersion o f the quartz particles. A s the guar g u m concentrat ion increases, the condit ions for the onset o f steric s tabi l izat ion, i.e. surface saturation by the po lymer , are met first i n the presence o f sod ium and l i t h i u m . S ign i f i can t ly higher concentrations o f the polysaccharide are necessary to reach the saturation leve l for ces ium and potassium and to induce steric effects. A s the in i t i a l concentrat ion o f guar g u m gradually increases, there is a cr i t ica l guar g u m dosage at w h i c h the quartz surface becomes saturated by the po lymer i n the presence o f L i and N a , but the adsorption density i n the presence o f K and C s at this c r i t ica l in i t i a l dosage is s t i l l far f rom saturation. In other words , at such a transit ion concentration, steric effects start redispersing the quartz suspensions i n the presence o f L i and N a , but f loccula t ion w i l l s t i l l proceed i n the presence o f K and C s unt i l the saturation leve l is reached at higher guar g u m dosages. It is not the absolute adsorption density that determines the onset o f steric s tabi l izat ion, but rather a relative/fractional coverage o f the surface. These observations general ly exp la in w e l l the fact that the stabil i ty o f the guar gum-quartz system at a g i v e n guar g u m concentration is always better i n the presence o f sod ium and l i t h i u m than i n the presence o f ces ium and potassium. B a s e d o n these results, the f o l l o w i n g mechanism o f guar-quartz interactions is suggested. Guar g u m adsorption o n quartz should be v i e w e d as compet ing w i t h the adsorpt ion o f water molecules since the same adsorption sites are most l i k e l y i n v o l v e d i n both processes i f hydrogen bonding is assumed to be the d r iv ing mechanism. W h e n a water-structure-making cat ion approaches the negatively charged quartz surface its presence does not change the interfacial water structure and therefore does not affect the "compet i t ion e q u i l i b r i u m " - the same amount o f the polysaccharide adsorbs i n water as i n a structure maker solut ion. In this context, it is noteworthy that the hydrogen cat ion is also a water-structure maker. 55 In contrast, structure-breaking regions around potassium and ces ium ions, combined w i t h higher adsorption o f these ions, produce areas o f disturbed water layers on the quartz surface thus a l l o w i n g guar gum to attach i t se l f to the n o w more accessible surface s i lanol groups. B o t h the h y d r o x y l groups o f the polysacchar ide and the surface s i l ano l groups o n quartz are hydrated i n aqueous solutions. F o r a hydrogen bond to fo rm between the p o l y m e r and the surface, the hydrat ion water must somehow be removed . The postulate that a structure breaking cat ion can cause such an effect and facilitate adsorpt ion seems to be in- l ine w i t h this type o f reasoning. The proposed interpretation agrees w e l l w i t h the recent observat ion that structure-breaking ions i n concentrated brines can displace interfacial water molecules f rom a hydroph i l i c so l id surface to such an extent that the surface becomes s l ight ly hydrophob ic (Hancer , 2001) . It is noteworthy that none o f the studies referenced i n the literature r ev i ew sect ion provide any d iscuss ion or explanat ion as to w h i c h polysaccharides are " a l l o w e d " to break through the tight layer o f water molecules on the minera l surface and form a l l the different types o f direct bonds w i t h the surface. In other words , the presence o f the interfacial water and the effect o f structure-making or breaking capabil i t ies o f background electrolytes (usually sod ium or potassium salts) on polysacchar ide adsorpt ion have so far been comple te ly ignored. There are no systematic data available o n interactions between guar g u m and a l k a l i metal cations i n dilute electrolyte solutions. Recent v i scos i ty measurements for a number o f mod i f i ed guar gums indicate that guar gum molecules are not affected by potass ium nitrate concentrations up to 0 . 1 M (Shortridge et a l , 2000). Some l im i t ed results obtained i n concentrated electrolytes suggest that guar gum is extremely stable at h i g h ion ic strengths ( ~ 3 M salts) i n terms o f its so lubi l i ty and abi l i ty to main ta in h i g h v i scos i ty . (Pawl ik and L a s k o w s k i , 2004; M a i e r e t a l . , 1993) indica t ing that the conformat ion o f the po lymer i n solut ion does not change w i t h electrolyte addit ions. Gi t t ings et a l . (2001) found that the fractal d imens ion o f guar gum changed very litt le i n 56 1 M N a C l compared to that i n d is t i l led water. Such observations typ i ca l ly mean that po lymer -ca t ion interactions are not strong even i n concentrated salt solutions. O n the other hand, Nappe r found that aggregation o f a s terical ly s tabi l ized dispers ion ( p o l y v i n y l acetate s tabi l ized by polyethylene oxide) cou ld be induced i n 2-molar a l k a l i metal chlor ide solutions at higher temperatures (50-90 °C) (Napper, 1970). T h i s type o f coagula t ion is caused by changes i n the "goodness" o f the solvent resul t ing f rom increasing po lymer -po lymer interactions and d imin i sh ing polymer-solvent interactions i n the presence o f electrolytes. The coagulat ing power o f a lka l i metal cations i n des tab i l iz ing such systems was opposite to what one w o u l d expect i f the s imple "sa l t ing-out" mechan i sm was assumed, i.e. l i th ium cation, the most hydrated i o n i n the series, was the weakest coagulant (Napper, 1970). Napper attributed the observed behavior o f the ions to their water structure-making or breaking capabil i t ies, but also indicated that such a response is probably very system-specific. In this work , it was assumed that po lymer -a lka l i metal i o n interactions were negl ig ib le i n the overa l l phenomenon, at least at the electrolyte concentrat ion levels and temperatures studied. In this respect it should also be pointed out that, due to electrostatic attraction between the negatively charged quartz surface and cations i n solut ion, the concentrat ion o f the cations at the negatively charged quartz surface is m u c h higher than i n the bu lk solut ion. Therefore, the effect o f the a lka l i metal cations o n the interfacial water is m u c h more significant than any effects related to the hydrat ion o f guar gum. The "indifferent character" o f a lka l i metal cations should perhaps be revis i ted. W h i l e l o w concentrations o f sod ium and l i t h ium affect the quartz-guar g u m system o n l y by compress ing the electr ical double layer around the quartz particles, potass ium and ces ium addi t iona l ly disturb the structure o f the interfacial water and thus affect the adsorpt ion o f the polymer . F r o m this point o f v i e w , the apparent effect o f p H is noteworthy as it may lead to erroneous conclusions i f the effect o f the ca t ion is not "decoup led" f rom the true effect o f the surface charge. Th i s type o f p H and ion ic strength-dependent adsorption is typ ica l ly associated w i t h adsorption o f ion ic po lymers . 57 In solutions containing both structure-making and structure-breaking ions, it is the structure breaker that controls the response o f the system (Figures 3.12 and 3.13). S u c h m i x e d solutions are very c o m m o n i n minera l processing. The usual assumption that the role o f the electrolytes is just to provide constant ion ic strength m a y not be sufficient to fu l ly understand the under ly ing mechanisms. A s the data show, a sod ium chlor ide concentrat ion o f 0 . 0 1 M does not produce any effect o f ion ic strength on guar g u m adsorpt ion (wi th respect to d is t i l led water), wh i l e the same concentration o f K C I does (Figure 3.8). In more concentrated electrolyte solutions (-0.1 M ) addi t ional factors should also be considered, e.g. competi t ive hydrat ion between the ions and the po lymer , so lvency effects, and the conformat ion o f guar gum molecules . C lea r ly , addi t ional w o r k is needed to elucidate the role o f such effects i n guar gum adsorption. A s reported i n many studies, guar gum exhibits high-aff ini ty type o f adsorpt ion o n var ious mine ra l surfaces w h i c h is quite inconsistent w i t h the assumed weak hydrogen bond ing interactions between the po lymer and the minera l surfaces. A s demonstrated i n this study, ve ry h i g h affinity adsorption isotherms were also obtained for the quartz sample. A c c o r d i n g to the acid-base interaction mode l o f polysacchar ide adsorption proposed by L i u et a l . (2000), guar gum should not adsorb on the ac idic quartz surface i n such h i g h quantities. The relat ively h igh molecular weight o f the polysacchar ide , and hence increased chemica l affinity compared to shorter cha in polymers (e.g. dextrin) , can account to some extent for such adsorption behavior. The randomness o f substitution o f the polymannose m a i n cha in by galactose is never perfect. The least substituted sections o f the guar molecules show the greatest tendency to associate and precipitate, w h i l e the more densely substituted regions serve to so lubi l ize the po lymer cha in (Dea , 1993). It is reasonable to assume that i n a natural guar gum solut ion a fraction o f the macromolecules fo rm intermolecular entanglements w h i c h cou ld adsorb on the so l id surface. T h i s "aggregate" adsorption o f entire entanglements w o u l d w e l l exp la in the h i g h adsorpt ion densities. Unfortunately , this type o f particulate effects i n polysacchar ide adsorpt ion have never been adequately studied. 58 A s indicated earlier, the adsorption o f dextr in o n quartz is very l o w and i n contrast to guar g u m the effect o f the background electrolyte is not so obvious . Thus the described effect o f a lka l i metal cations o n guar gum adsorption is also related to the molecula r weight o f adsorbing polysaccharides. A s imi la r effect o f a lka l i metal chlor ides o n the adsorption o f a h i g h molecular weight ( M W = 5 m i l l i o n ) dextran o n quartz was also observed (see A p p e n d i x 4). The results for dextr in also strongly suggest that the quartz surface was free f rom metal impuri t ies w h i c h w o u l d otherwise "act ivate" the surface resul t ing i n m u c h higher dextr in adsorption densities than the observed ones. 59 CHAPTER 5 Conclusions In dilute electrolyte solutions, guar gum adsorption o n quartz strongly depends o n the water structure-making or breaking properties o f the background cations. The adsorpt ion density o f the po lymer is independent o f the p H o f the quartz suspensions, but mis l ead ing trends m a y be observed i f the effect o f the base cat ion is not separated f rom the true effect o f the surface charge. Since the adsorption density o f guar gum does not change w i t h the surface charge (pH) o n the quartz surface, and assuming that the quartz surface is free f rom impuri t ies , it is postulated that hydrogen bonding is the m a i n adsorption mechanism. A n addi t ional phenomenon i n guar gum-quartz interactions is suggested i n w h i c h water structure-breaking cations disturb the interfacial water layer a l l o w i n g guar g u m to more densely adsorb on the exposed/dehydrated surface s i lanol groups. Structure-making cations better fit into the interfacial water layer and do not affect the guar gum-water compet i t ion for the polar surface sites. The results strongly suggest that s imple a lka l i metal chlorides are not total ly indifferent, and their water-structure making/breaking capabil i t ies should be taken into account to better understand the behavior o f a mode l system such as quartz-guar gum. Differences i n the adsorption densities o f guar g u m correlate w i t h the stabil i ty o f quartz suspensions. The f loccula t ion o f quartz is m u c h more efficient i n solutions o f structure breaking ions. A t the same t ime, steric s tabi l izat ion is m u c h more dif f icul t i n the presence o f such electrolytes. 60 RECOMMENDATIONS: The results o f this w o r k show that al though N a C l and L i C l d i d not affect adsorpt ion o f guar g u m at l o w concentration ( 0 . 0 1 M ) , they d i d increase adsorpt ion o f guar g u m w h e n the salt concentration was h igh enough ( 0 . 1 M ) . H o w e v e r , the mechan i sm behind this phenomenon is not ful ly understood. It is possible that there is a t ransi t ion electrolyte concentrat ion at w h i c h solvency effects start to p lay a dominant role. In concentrated salt solutions, the conformat ion and hydrat ion o f guar g u m can be affected s ignif icant ly , w h i c h w o u l d influence the adsorption behavior o f guar gum. Therefore, addi t ional adsorption studies i n concentrated salt solutions need to be carr ied out to investigate the role o f so lvency effects i n greater detail . The effect o f a lka l i metal cations also seems to be related to the molecu la r weight o f po lymers . F o r a h igh molecular weight po lymer , i.e. guar gum, its adsorption o n quartz increased s ignif icant ly i n the presence o f K C 1 and C s C l , wh i l e for a l o w molecu la r weight po lymer , i.e. dextr in, its adsorption o n quartz was not affected by K C 1 so s ignif icant ly . Thus it seems that the higher the molecular weight , the more signif icant the effect o f a lka l i metal cations. Ideally, a series o f guar gums o f different molecu la r weights should be used to research this effect. Other po lymers adsorbing through hydrogen bond ing c o u l d also be investigated. The sol ids concentration i n the reported tests was re la t ively h igh . The vast majori ty o f adsorption studies reported i n technical literature were performed at m u c h lower sol ids contents. It is possible that some vo lume exc lus ion effects p lay a role i n the adsorpt ion process - guar gum always exhibits a dis t r ibut ion o f molecula r weights and poss ib ly o n l y certain molecular weight fractions actually adsorb o n the minera l surfaces at h igher sol ids contents. P o l y m e r diffusion effects ( m i x i n g condit ions) are also k n o w n to be signif icant i n more concentrated suspensions. A systematic study o n the influence o f the sol ids content o n the adsorption process is therefore recommended. 61 REFERENCES A f e n y a , P . 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Calibration Curve for Dextrin 0 20 40 60 80 100 120 Concentration of dextrin [mg/L] Appendix 3 . Calibration Curve for Dextran. 2.5 -, 0 20 40 60 80 100 120 Concentration of Dextran [mg/L] Appendix 4. Adsorption Isotherms for Dextran on Quartz. Appendix 5. Data of Adsorption Measurements Table 1 Numerical data for Figure 3.8 titled "Adsorption isotherms of guar gum on quartz at pH 5.2 and alkali metal chloride concentration of 0.01M. A set of adsorption results for a dextrin sample is also included". Adsorption data for dextrin are in Table la. 0.01MKC1 O.OlMCsCl Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) 91.1 0.0 0.2681 91.1 0.0 0.2681 109.1 5.8 0.3039 109.1 3.7 0.3100 128.3 18.0 0.3246 128.4 17.1 0.3274 152.4 35.1 0.3449 149.3 31.9 0.3452 162.5 41.9 0.3545 163.0 44.8 0.3475 179.9 52.9 0.3735 179.4 59.2 0.3536 200.0 71.6 0.3776 193.7 69.7 0.3647 0.01M LiCl Distilled Water Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) 38.4 0.0 0.1129 40.9 0.0 0.1202 57.8 5.0 0.1554 54.2 2.2 0.1527 75.3 18.1 0.1682 74.2 16.0 0.1711 93.4 34.6 0.1728 89.4 27.1 0.1833 111.5 49.6 0.1821 118.3 52.5 0.1935 130.8 66.9 0.1878 132.1 63.8 0.2007 146.5 81.3 0.1918 144.6 75.5 0.2029 O.OlMNaCl Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) 53.7 0.5 0.1565 74.0 13.6 0.1776 91.4 29.0 0.1835 114.3 45.2 0.2034 129.6 58.2 0.2101 76 Table l a Adsorption data for dextrin. Distilled Water 0.01MKC1 Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) 4.6 4.6 0.0000 5.1 3.8 0.0036 11.7 7.2 0.0131 10.3 8.0 0.0068 20.6 15.2 0.0157 21.4 16.1 0.0158 43.2 34.4 0.0259 41.1 32.6 0.0251 62.3 53.3 0.0266 64.1 54.7 0.0276 82.7 73.1 0.0284 81.3 71.2 0.0298 109.9 95.9 0.0412 105.5 91.2 0.0419 125.8 111.5 0.0421 121.4 107.5 0.0410 O . l M N a C l 0.1MKC1 Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) 21.4 16.2 0.0154 21.4 13.3 0.0239 61.5 56.8 0.0140 61.5 49.7 0.0349 81.7 75.3 0.0188 81.7 67.1 0.0428 105.5 89.0 0.0484 105.5 85.5 0.0587 0.01M NaCl Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) 7.5 7.5 0.0000 22.1 16.5 0.0166 61.0 52.1 0.0262 103.2 89.6 0.0401 122.0 107.4 0.0430 77 Table 2 Numerical data for Figure 3.9 titled "Effect of ionic strength on the adsorption of guar gum atpH 5.2". 0.1 M C s C l 0.1 M L i C l Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) 92.1 0.0 0.2708 58.7 20.0 0.1139 111.8 1.2 0.3250 79.0 24.0 0.1616 131.2 11.8 0.3510 87.6 25.9 0.1813 148.6 23.7 0.3674 94.6 27.1 0.1985 165.2 36.1 0.3798 99.0 28.1 0.2086 182.7 46.9 0.3994 103.3 29.5 0.2170 203.5 67.2 0.4009 106.7 29.6 0.2268 0.1 M KCI 0.1 M NaCl Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) 90.5 0.0 0.2663 48.6 0.0 0.1430 109.6 0.1 0.3221 67.1 0.9 0.1947 126.7 1.8 0.3673 85.3 11.1 0.2182 144.7 9.8 0.3967 109.6 27.1 0.2426 161.5 20.7 0.4139 126.6 41.7 0.2498 181.7 35.2 0.4308 144.7 55.8 0.2614 202.2 52.8 0.4395 217.2 65.7 0.4457 78 Table 3 Numerical data for Figure 3.10 titled "Effect of pH on guar gum adsorption on quartz. pH adjustments were made with various hydroxides or with hydrochloric acid. Initial guar gum concentration 90 mg/L". K O H NaOH Initial Concentration Equilibrium Concentration Adsorbed Amount Initial Concentration Equilibrium Concentration Adsorbed Amount pH (mg/L) (mg/L) (mg/m2) P H (mg/L) (mg/L) (mg/m2) 5.7 90.1 26.8 0.1863 5.5 90.1 27.5 0.1842 6.4 90.1 23.1 0.1972 7.0 90.1 26.5 0.1872 7.3 90.1 19.8 0.2069 7.7 90.1 25.8 0.1890 8.1 90.1 16.8 0.2157 8.5 90.1 25.4 0.1902 8.7 90.1 6.8 0.2451 9.3 90.1 26.7 0.1866 9.5 90.1 0.9 0.2622 10.2 90.1 22.5 0.1988 -10.6 90.1 0.0 0.2650 10.5 90.1 22.6 0.1985 L i O H CsOH Initial Concentration Equilibrium Concentration Adsorbed Amount Initial Concentration Equilibrium Concentration Adsorbed Amount (mg/L) (mg/L) (mg/m2) pH (mg/L) (mg/L) (mg/m2) 6.4 90.1 25.5 0.1900 6.6 90.1 21.2 0.2027 6.8 90.1 25.1 0.1911 7.7 90.1 16.4 0.2169 7.7 90.1 26.3 0.1876 8.2 90.1 11.9 0.2301 8.3 90.1 26.9 0.1860 8.4 90.1 6.6 0.2457 9.4 90.1 27.3 0.1846 9.3 90.1 0.0 0.2680 10.5 90.1 26.6 0.1867 10.8 90.1 0.0 0.2756 HC1 Initial Concentration Equilibrium Concentration Adsorbed Amount pH (mg/L) (mg/L) (mg/m2) 3.3 90.1 32.9 0.1683 4.2 90.1 30.3 0.1758 4.9 90.1 28.4 0.1814 5.0 90.1 28.8 0.1803 79 Table 4 Numerical data for Figure 3.11 titled "Adsorption of guar gum as a function of pH in the presence of 0.01 M background electrolytes. Initial guar gum concentration 90 mg/L". 0.01MKC1+KOH or HCI O.OlMCsCl+CsOH or HCI PH Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) pH Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) 3.79 90.1 4.4 0.2520 3.32 90.1 1.1 0.2618 5.45 90.1 0.9 0.2622 4.46 90.1 0.0 0.2688 6.16 90.1 • 1.1 0.2619 5.19 90.1 0.0 0.2686 7.1 90.1 0.6 0.2633 6.5 90.1 0.0 0.2683 7.62 90.1 0.6 0.2633 7.32 90.1 0.0 0.2686 8.44 90.1 0.6 0.2633 8.01 90.1 0.0 0.2684 9.45 90.1 0.6 0.2633 9.2 90.1 0.0 0.2681 10.27 90.1 . 0.6 0.2633 10.91 90.1 0.0 0.2685 0.01M NaCl+NaOH or HCI O.OlMLiCl+LiOHor HCI PH Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) PH Initial Concentration (mg/L) Equilibrium Concentration (mg/L) Adsorbed Amount (mg/m2) 3.63 90.1 33.2 0.1674 3.43 90.1 33.5 0.1664 5.55 90.1 28.7 0.1806 4.69 90.1 31.1 0.1736 6.69 90.1 26.5 0.1869 5.63 90.1 27.2 • 0.1850 7.2 90.1 26.6 0.1866 6.42 90.1 29.2 0.1792 8.09 90.1 32.9 0.1682 7.24 90.1 31.7 0.1716 8.11 90.1 33.1 0.1676 8.19 90.1 33.9 0.1653 8.96 90.1 33.0 0.1678 9.32 90.1 36.4 0.1579 9.71 90.1 33.3 0.1672 10.19 90.1 34.8 0.1625 10.35 90.1 28.3 0.1816 10.71 90.1 33.8 0.1655 80 Table 5 Numerical data for Figure 3.12 titled "Co-effect of background electrolyte and added base on the adsorption of guar gum. Initial guar gum concentration 90 mg/L". O.OlMNaCl+KOH O.OlMLiCl+KOH Initial Equilibrium Adsorbed Initial Equilibrium Adsorbed Concentration Concentration Amount Concentration Concentration Amount PH (mg/L) (mg/L) (mg/m2) pH (mg/L) (mg/L) (mg/m2) 5.4 90.0 30.4 0.1753 5.4 90.0 33.4 0.1668 6.5 90.0 27.4 0.1842 6.7 90.0 30.7 0.1747 6.9 90.0 26.5 0.1866 7.3 90.0 30.2 0.1761 7.3 90.0 25.6 0.1893 8.3 90.0 30.2 0.1761 8.6 90.0 19.0 0.2088 9.1 90.0 21.8 0.2009 9.5 90.0 10.1 0.2351 9.5 90.0 17.2 0.2145 10.5 90.0 1.8 0.2593 10.5 90.0 1.5 0.2607 O.OlMKCl+NaOH Initial Equilibrium Adsorbed Concentration Concentration Amount Ph (mg/L) (mg/L) (mg/m2) 6.9 90.0 1.4 0.2608 7.7 90.0 1.4 0.2608 8.9 90.0 1.4 0.2608 9.7 90.0 1.4 0.2608 10.5 90.0 1.4 0.2609 10.8 90.0 1.4 0.2608 81 Table 6 N u m e r i c a l data for F igure 3 . 1 3 t i t led "Co-effect o f K C 1 and N a C l o n the adsorpt ion o f guar g u m at natural p H (5.2). Ionic strength 0 . 0 1 M . Ini t ial guar g u m concentrat ion 120 m g / L " . Fraction of KC1 in mixture with NaCl Equilibrium Concentration (mg/L) Amount Adsorbed (mg/m2) 0.0 50.9 0.2036 0.3 23.0 0.2855 0.5 15.1 0.3089 0.8 7.5 0.3311 1.0 0.0 0.3752 82 

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