British Columbia Mine Reclamation Symposium

Phosphorus fixation characteristics of Valley Mine tailings Hackinen, Coleen Marie, 1957- 1985-12-18

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th  Proceedings of the 9 Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1985. The Technical and Research Committee on Reclamation  PHOSPHORUS FIXATION CHARACTERISTICS OF VALLEY MINE TAILINGS  by Coleen Hackinen ABSTRACT  Phosphorus is often a limiting plant nutrient and, when applied as fertilizer, may be "fixed" in forms unavailable to plants. Thus an understanding of phosphorus behaviour in mine tailings is important to any reclamation program. Phosphorus adsorption isotherms were constructed for three copper mine tailing materials, one of which had been subjected to artificial weathering in a Soxhlet apparatus. Equilibrating conditions were: 23°C, 0.01 M CaCl2 and distilled water, for 72 hours. There was no difference in adsorption as a result of the ionic concentration of the equilibrating solution. This was probably a reflection of the extremely low cation exchange capacity of the tailing material. Adsorption behaviour in all samples was best described by the Langmuir equation. Maximum adsorption of the untreated material occurred at 42-48 µ/g P/g tailings, whereas the artificially weathered tailing adsorption maximum was reached at 105 µ/g P/g tailings. At equilibrium concentrations exceeding approximately 60 ppm P, precipitation reactions were indicated. To obtain an equilibrium concentration of 0.5 ppm P in solution, the equivalent of 41 kg P2O5/ha must be supplied. INTRODUCTION  Successful revegetation of mill tailings derived from Cominco's Valley Operation cannot be achieved without nutrient supplementation. Previous field trails failed to produce normal growth of grasses. Visual symptoms of the plants suggested a phosphorous deficiency even though high rates of phosphorus fertilizer had been applied. It was suspected that the applied fertilizer phosphorus was "fixed" into forms unavailable to the plants, resulting in solution phosphorus levels which may have been growth limiting. This study, therefore, was designed to quantify the proportion of phosphorus removed from solution under given conditions. The goal being to determine the required phosphorus application rate to achieve adequate solution phosphorus levels. Phosphate adsorption isotherms have been used by a number of workers for this purpose (Fox and Kamprath, 1970, Beckwith, 1964, Woodruff and Kamprath, 1965). Fredericks (1981) found relatively close correlation between field plot data and fertilizer recommendations based on sorption isotherms for several mine waste materials. Soil phosphorus may exist as either solution or solid phase forms in continual flux as the system moves toward an equilibrium condition (Larsen, 1967). The solid phase forms include; (a) organic phosphorus, This study was funded by Cominco Copper Division, Valley Operations and an NSERC Scholarship. 113  th  Proceedings of the 9 Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1985. The Technical and Research Committee on Reclamation  (b) phosphorus adsorbed onto soil particles, and (c) low solubility phosphorus minerals. At present, the tailings are devoid of organic matter. Consequently, solution phosphorus (plant available P) will only be influenced by adsorbed and precipitated phosphorus forms. Reaction time, in response to equilibrium disruption, is relatively rapid between solution and adsorbed P. Equilibria reactions controlling the dissolution of sparingly soluble phosphorus minerals in response to declining solution phosphorus levels, however, are very slow. As a result, phosphorus combined in these forms is not useful in supplying plant available phosphorus in the short term (i.e. over a single growing season). Mechanisms of Phosphorus Adsorption Losses in plant available phosphorus are rarely a result of leaching, since this element normally has a high affinity for soil constituents. Phosphorus is believed to be specifically adsorbed through a process of ligand exchange, where the phosphate anion enters into six-fold coordination with Al^+ or Fe^+ ions on a hydrous oxide surface (Bonn et al, 1979). Phosphorus anions may also be nonspecifically adsorbed through electrostatic attraction to positively charged soil colloids. Adsorption of phosphorus onto layer silicates has been found to occur in two stages. Initially, phosphorus is rapidly adsorbed by nonspecific processes and ligand exchange on mineral edges. Over time, reorganization of the initially adsorbed phosphorus occurs through mineral dissolution and precipitation, involving exchangeable and mineral lattice cations. The nature of the precipitated phosphorus compounds and conditions controlling their formation are important in determining solution P levels, and consequently plant available phosphorus. Adsorption Isotherms Adsorption isotherms describe the relationship between the amount of solute adsorbed by an adsorbent as a function of equilibrium concentration of the adsorbate. The Langmuir adsorption equation, which was used in this study, assumes three conditions; (1) the adsorbing surface is homogeneous, (2) there is no interaction between adsorbate ions, and (3) maximum adsorption occurs when a complete monomolecular layer exists on all reactive adsorbent surfaces. The equation is as follows:  114  th  Proceedings of the 9 Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1985. The Technical and Research Committee on Reclamation  MATERIALS AND METHODS  Surface tailing samples were collected from two sites at Cominco Copper Division Valley Operation mine in B.C.'s southern interior. The tailings are dominantly sand-sized particles composed mainly of quartz, muscovite, K-feldspars and plagioclases. Organic matter content, available NÛ3-N, and available P are negligible. Levels of available K, Ca, and Mg appear adequate for plant growth. One site is currently serving as a field test plot area. The second site was located on the main tailing pond in an area of recent discharge. A subsample of this material was previously subjected to Soxhlet artificial weathering treatment and served as the third sample in this study. Artificial weathering was carried out by leaching a tailing sample continuously for seven weeks in hot 0.3 N acetic acid in a Soxhlet apparatus. This technique was designed to mimic mineral degradation and recombination processes which might occur in time in a natural setting. Comparison of corresponding fresh and artificially weathered samples will allow some degree of quantification of the chemical and mineralogical changes which might be anticipated. Duplicate 3 g samples were equilibrated for 24, 48 and 72 hours in 30 mL of 100 ppm P solution (as KH2PO4) in two matrices; 0.01 M CaCl2 and distilled water. Equilibration was carried out in 50 mL plastic centrifuge tubes to which two drops of toluene had been added. The tubes were stoppered and shaken laterally for one half hour each 24 hour period. Phosphorus determination was carried out on the supernatant solution after centrifuging 15 minutes at 2400 r.p.m. Equilibrium was assumed to be achieved when the solution phosphorus level stabilized. This occurred at some time between 48 and 72 hours. Adsorption isotherm data were obtained by equilibrating 3 g samples of tailings with 30 mL aliquots of O, 10, 20, 30, 40, 50, 60, 70, 80, 90, or 100 ppm P solutions in distilled water at 23°C for 72 hours. The 0.01 M CaCl2 matrix was not used in this portion of the study since preliminary equilibration results showed no difference between matrices. The tubes were centrifuged and a 1 mL aliquot of supernatant solution was removed for colorimetric treatment using the method of Watanabe and Olsen (1965). Measurement of phosphorus in solution was carried out immediately on a Guilford spectrophotometer. Absorbance readings were made at a wavelength of 700 nm. Acid ammonium oxalate extractable Fe and Al values were obtained using the technique of McKeague and Day (1966). This extractant was used to remove free Al and Fe, elements which easily form complexes with phosphate anions. Levels in solution were determined by atomic adsorption spectrophotometry. Mineralogical determinations were based on random powder mounts scanned from 4° to 56° using a Philips X-ray diffractometer.  115  th  Proceedings of the 9 Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1985. The Technical and Research Committee on Reclamation  X-ray fluorescence analysis was used to determine total levels of Ca, Fe and Al in each sample. pH was measured in the supernatant after 72 hours of equilibration with the phosphorus solution. RESULTS AND DISCUSSION Fixation Mechanisms  As shown in Figure 1, it would appear that two fixation mechanisms are in operation in this system. Initially, added phosphorus is probably adsorbed, either through specific proceses (anion exchange) or electrostatic attraction to positively charged sites, or both. Once the adsorption complex is saturated, phosphate anions stay in solution until a critical concentration is reached, at which point precipitation reactions are indicated. This phenomenon is represented by a rapid increase in apparent adsorption with only a small change in equilibrium solution concentration (Bohn et al, 1979). As shown in Figure 1, the critical equilibrium phosphorus concentration occurs near 60 ppm. Adsorption Isotherms Data from the adsorption area of Figure 1 (from C = O to 60 ppm P), for all three samples, were best described by the Langmuir equation. The isotherms were constructed by plotting C/X/m versus C, where C = concentration of phosphorus in the equilibrium solution and X/m = µg phosphorus adsorbed/g tailings (See Figure 2). Maximum adsorption was achieved at a lower level in the unweathered tailings as compared to the artificially weathered material (See Table 1). It is presumed that this difference is a result of the corresponding pH drop apparent after artificial weathering (See Table 2). The weathered tailings, at pH 3.8, would exhibit a greater degree of protonation of hydroxylated surfaces resulting in more positively charged sites available for anion adsorption (Dixon and Weed, 1977). Specific adsorption would also be favoured at greater acidity. Phosphorus Fertilizer Rates Fertilizer application rates were based on a target equilibrium concentration of 0.5 ppm P. Suggested plant requirements for maximum yield vary from 0.03 ppm P to 0.3 ppm P (Asher and Loneragan, 1967, Ozanne and Shaw, 1968), therefore, 0.5 ppm P should prove adequate to sustain plant growth. Fertilizer application rates calculated using the Langmuir equation are similar for all three samples (Table 1). Intuitively, one would expect the artificially weathered material to have a higher P requirement, due to a higher adsorption capacity. This is not the case, however, since the  116  th  Proceedings of the 9 Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1985. The Technical and Research Committee on Reclamation  117  th  Proceedings of the 9 Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1985. The Technical and Research Committee on Reclamation  FIGURE 1 Phosphorus fixed versus equilibrium concentration.  118  th  Proceedings of the 9 Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1985. The Technical and Research Committee on Reclamation  FIGURE 2 Langmuir adsorption isotherms.  119  th  Proceedings of the 9 Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1985. The Technical and Research Committee on Reclamation  adsorption isotherm lines, at very low equilibrium P levels, are very close. Differences would be more obvious at higher equilibrium levels as the lines exhibit greater diversion (See Figure 2). Phosphorus fertilizer rates required to supply adequate available P, based on experimental adsorption data, are much lower than is generally accepted for reclamation or even agricultural situations. The magnitude of this discrepancy warrants the implementation of field trials, prior to any large scale fertilization program. Phosphorus Precipitation Products Adsorption isotherms do not take into account precipitation reactions, which may drastically alter the nature of the equilibrium solution. Theoretically, a variety of sparingly soluble phosphate compounds will form, even at very low concentrations of phosphorus in solution. The nature of the precipitation complex will vary with pH, pC02, temperature, ionic species present and their activity in solution (Lindsay and Moreno, 1960). Artificial weathering changes the chemical properties of the tailings (See Table 2), therefore, the forms of the precipitated phosphorus will differ between the weathered and unweathered materials. Utilizing a solubility diagram for phosphate compounds developed by Lindsay and Moreno (1960), it is possible to predict the nature of the precipitates which will likely occur on addition of phosphorus to the tailing materials studies (See Figure 3). It is assumed that the levels of associated elements (Fe, Al and Ca) are adequate for the formation of these highly insoluble compounds (See Table 2). Phosphorus added to the unweathered tailings at pH 6 should form fluorapatite (Ca10(P04)6F2) at pH2PU4 6.8 or about 9 ppb total P in solution. At greater concentrations of P, strengite (FePO4-2H20), variscite (AlPO4-2H2O) and hydroxyapatite (Ca10(P04)6(OH)2) may exist. The formation of dicalcium phosphate dihydrate is unlikely since the P equilibrium concentration must exceed about 60 ppm. This would require the application of approximately 1500 kg P/ha furrow slice.  The addition of phosphorus to the Soxhlet weathered tailings, at pH 3.8, will likely cause the precipitation of strengite and variscite. The minimum solution P concentration required for their formation is about 2 ppb and 8 ppb, respectively. It is therefore, unavoidable to apply fertilizer phosphorus at rates insufficient for the formation of strengite, variscite, or fluorapatite and still provide adequate phosphorus for plant growth. The magnitude of the discrepancy between observed phosphorus concentrations at equilibrium (about 60 ppm) and theoretical values (ppb range), suggests that the precipitates actually formed did not include strengite, variscite, fluorapatite or hydroxyapatite. It is likely that the equilibration time of three days was not adequate to allow the effect of the production of these minerals to be observed. The precipitation products actually formed 120  th  Proceedings of the 9 Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1985. The Technical and Research Committee on Reclamation  FIGURE 3  Solubility diagram for phosphate compounds in soils at 250C and 0.005 M Ca concentrations.  From W.L. Lindsay and E.G. Moreno. 1960. Phosphate phase equilibria in soils. Soil Sci. Soc. Amer. Proc. 24:177-182.  121  th  Proceedings of the 9 Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1985. The Technical and Research Committee on Reclamation  were probably disordered compounds which in time would revert to more insoluble forms, since solution P levels were far in excess of the saturation requirements. The synthesis of sparingly soluble phosphorus compounds is significant in that phosphorus combined in these forms is essentially unavailable to plants. These minerals are highly insoluble and can only be considered as a phosphorus pool over the long term. That is, over a single growing season, plant available phosphorus, released through dissolution of these compounds, is negligible. Phosphorus, as such is not to be considered wasted, however, since it represents a long term storage pool which will be critical at some future date when maintenance fertilization practices are discontinued. Maximum phosphorus availability can be achieved in slightly acid to neutral soil where the solubilities of Al, Fe and Ca phosphates are highest, simultaneously. Phosphorus Leaching The coarse texture and relatively low phosphorus adsorption capacity of the tailings, in conjunction with irrigation practices, may promote considerable losses of phosphorus through leaching - a situation which rarely occurs in agricultural soils. This possibility was suggested by a laboratory experiment in which 50 g of tailings (sample T - 1) were leached with 100 ml of 50 ppm P solution and then rinsed twice with 100 ml of distilled water. Eighty-seven percent of the added phosphorus was recovered in the leachate. The actual losses on site may not be this severe since parameters such as soil:solution ratio, amount of applied P and rate of water movement will probably be lower in a field situation. The experiment does illustrate, however, that potential phosphorus losses through leaching cannot be overlooked. Recommendations Implementation of the experimentally determined phosphorus application rates, should first be carried out in a small scale field trial. These calculations were based on a short term equilibration period and may not be relevant over a growing season, in view of theoretical solubility characteristics of insoluble phosphorus compounds. The low adsorption capacity and high leaching potential of the tailings, suggest that large single applications of readily soluble phosphorus fertilizer (e.g. ammonium phosphate, 11-55-0) are contraindicated. Much of this phosphorus would become unavailable, either through leaching or the formation of insoluble phosphorus precipitates. Multiple applications of small amounts of high analyses P fertilizer would minimize wastage through these avenues. Alternatively, less soluble phosphate fertilizers, such as rock phosphate or bone meal, in a single application, would achieve the same result (Tisdale and Nelson, 1975). Higher application rates would be required, however, since these forms are of a much lower grade. 122  th  Proceedings of the 9 Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1985. The Technical and Research Committee on Reclamation  BIBLIOGRAPHY  Asher, C.J. and J.F. Loneragan. 1967. Response of plants to phosphate concentration in solution culture. I. Growth and phosphorus content. Soil Sci. 103:225-233. Beckwith, R.S. 1964. Sorbed phosphate at standard supernatant concentration as an estimate of the phosphate needs of soils. Aust. J. Exp. Agr. and An. Hus. 5:52-58. Bonn, H.L., B.L. McNeal and G.A. O'Connor. 1979. Soil Chemistry. John Wiley and Sons, Toronto. 329 pp. Dixon, J.B. and S.B. Weed (eds.). 1977. Minerals in soil environments. Soil Sci. Soc. of Amer., Madison, Wise. 948 pp. Fox, R.L. and E.J. Kamprath. 1970. Phosphate sorption isotherms for evaluating the phosphate requirements of soils. Soil Sci. Amer. Proc. 34:902-907. Fredericks, J.H. 1981. Phosphorus adsorption of selected mine waste materials as a method of determining phosphorus requirements. B.Sc. thesis. University of British Columbia. Unpublished. Larsen, Sigurd. 1967. Soil phosphorus. Advances in Agronomy. 19:151-206. Lindsay, W.L. and E.G. Moreno. 1960. Phosphate phase equilibria in soils. Soil Sci. Soc. of Amer. Proc. 24:177-182. McKeague, J.A. and J.H. Day. 1966. Dithionite and Oxalate extractable iron and aluminum as aids in differentiating various classes of soils. Can. J. Soil Sci. 46:13-22. Ozanne, P.G. and T.C. Shaw. 1968. Advantages of the recently developed phosphate sorption test over the older extractant methods for soil phosphate. Trans. 9th Int. Cong. Soil Sci. 2:273-280. Tisdale, S.L. and W.L. Nelson. 1975. Soil fertility and fertilizers. 3rd ed. McMillan Pub. Co. Inc., New York. 694 pp. Watanabe, F.S. and S.R. Olsen. 1965. Test of an ascorbic acid method for determining phosphorus in water and NaHCO3 extracts from soil. Soil Sci. Soc. Amer. Proc. 29:677-678. Woodruff, J.R. and E.J. Kamprath. 1965. Phosphorus adsorption maximum as measured by the Langmuir isotherm and its relationship to phosphorus availability. Soil Sci. Soc. Amer. Proc. 29:148-150.  123  


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