Phosphate sorption

Pyrite is not only one of the key compounds in Wachtershauser s theory, but could also have fulfilled an important function for phosphate chemistry in prebiotic syntheses. A group in Rio de Janeiro studied the conditions for phosphate sorption and desorption under conditions which may have been present in the primeval ocean. In particular, the question arises as to the enrichment of free, soluble inorganic phosphate (Pi), which was probably present in low concentrations similar to those of today (10 7-10 8M) (Miller and Keffe, 1995). Experiments show that acid conditions favour sorption at FeS2, while a weakly alkaline milieu works in an opposite manner. Sorption of Pi can be favoured by various factors, such as hydrophobic coating of pyrite with molecules such as acetate, which could have been formed in the vicinity of hydrothermal systems, or the neutralisation of mineral surface charges by Na+ and K+. 

As an example, Eq. (4.32) appears to describe phosphate sorption by soils quite well (see Fig. 4.7). 

Phosphate sorption has also been shown to be described by the rate law... 

Fig. 6.10 shows idealized isotherms (at constant pH) for cation binding to an oxide surface. In the case of cation binding, onto a solid hydrous oxide, a metal hydroxide may precipitate and may form at the surface prior to their formation in bulk solution and thus contribute to the total apparent "sorption". The contribution of surface precipitation to the overall sorption increases as the sorbate/sorbent ratio is increased. At very high ratios, surface precipitation may become the dominant "apparent" sorption mechanism. Isotherms showing reversals as shown by e have been observed in studies of phosphate sorption by calcite (Freeman and Rowell, 1981). 

Another widely used sorption model is the Langmuir equation. It was developed by Irving Langmuir [140] to describe the adsorption of gas molecules on a planar surface. It was first applied to soils by Fried and Shapiro [ 141 ] and Olsen and Watanabe [142] to describe phosphate sorption on soils. Since that time, it has been heavily employed in many environmental fields to describe sorption on various solid surfaces [19,65]. The general Langmuir model is... 

Polysopoulos NA, Keramidas VZ, Pavlatou A (1986) On the limitation of the simplified Elovich equation in describing the kinetics of phosphate sorption and release from soils. J Soil Sci... 

Colombo, C. Buondonno, A. Violante, A. Torrent, J. (1991) The contrasting effect of goethite and hematite on phosphate sorption and desorption by Terre Rosse. Z. Pflanzener-nahr. Bodenk. 154 301-305... 

Earl, K. Syers, J. McLaughlin, R. (1979) Origin of the effect of citrate, tartrate and acetate on phosphate sorption by soils and synthetic gels. Soil Sci Soc. Am. J. 43 674-678... 

Karim, M.J. Adams, W.A. (1984) Relationships between sesquioxides, kaolinite and phosphate sorption in a catena of Oxisols in Malawi. Soil Sci. Soc. Am. J. 48 406-409... 

Conformations and aggregate structures of sorbed natural organic matter on muscovite and hematite. Geochim. Cosmo-chim. Ada 65 1047-1057 Nanzyo, M. Watanabe,Y. (1981) Material balance associated with phosphate sorption by amorphous clays, silica-alumina gel and synthetic goethite. Soil Sd. Plant Nutr. 27 329-337... 

Mechanisms of phosphate sorption by soils and hydrous ferric oxide gel. J. Soil Sd. 

Scheinost, A. Schwertmann, U. (1995) Prediction of phosphate sorption curves in a soils-cape on molasse sediments. Soil Sci. Soc. 

Torrent, J. Schwertmann, U. (1987) Influence of hematite on the color of red beds. J. Sediment. Petrology 57 682-686 Torrent, J. (1987) Rapid and slow phosphate sorption by Mediterranean soils effect of iron oxides. Soil Sci. Soc. Am. J. 51 78-82 Torrent, J. (1991) Activation energy of the slow reaction between phosphate and goethites of different morphology. Aust. J. Soil Res. 29 69-74... 

Fast and slow phosphate sorption by goethite-rich natural materials. Clays Clay Min. 40 14-21... 

Phosphate sorption by natural hematites. Eur. J. Soil Sd. 45 45—51 Torrent, J. Schwertmann, U. Schulze, D.G. (1980) Iron oxide mineralogy of some soils of two river terrace sequences in Spain. Geoderma 23 191-208... 

Figure 7.20. An Elovich equation plot representing phosphate sorption on two soils where Q is the initial phosphorus concentration added at time 0 and C is the phosphorus concentration in the soil solution at time t. The quantity (Cq-C) equals q, the amount sorbed at any time (from Chien and Clayton, 1980, with permission).

The solubility of Fe(0H)2 is greater than that of Fe(0H)3 (30) and its surface properties may differ from those of Fe(0H)3. TRiTs, differences in amount and speciation of iron may have accounted for differences in phosphate sorption. 

Eilho M. V., de Mesquita, and Torrent J. (1993) Phosphate sorption as related to mineralogy of a hydrosequence of soils from the Cerrado region (Brazil). Geoderma 58, 107-123. 

Ryden, J. C., Syers, J. K., and Tillman, R. W., 1987, Inorganic anion sorption and interactions with phosphate sorption by hydrous ferric oxide gel Journal of Soil Science, v. 38, p. 211 -217. 

Soils may also already contain some of the substance being studied. That is, one must study say phosphate sorption in the presence of the phosphate already there. This has caused some problems in the interpretations of phosphate sorption and desorption experiments [59]. This is one aspect of soil chemistry in which Western Australian soil scientists have an advantage. The very low content of phosphate in most soils simplifies such studies. Farmers, however, do not regard this as an advantage ... 

AH+ and complex A1 ions. In their studies, soil scientists have tended to use electrolyte solutions which they see as relevant. Often the solution used is a dilute solution of calcium chloride. This contrasts with those who have studied oxides. Their choice of background electrolyte has been motivated by a desire to use electrolytes which are close to indifferent . Ideally, this means that neither the cation nor the anion has any affinity for the surface. In practice, it means that the affinity of both ions is small and similar. These differences in the choice of background electrolyte mean that the observed effects of pH differ. Fig. 8 shows that the decline in phosphate sorption by a soil with increasing pH is steeper when the background electrolyte is sodium chloride than when it is calcium chloride. This is because the presence of the divalent cation compresses the distribution of ions near the surface and the change with pH in the electric potential is less steep. 

In terms of equation (1), this would mean a normal distribution of log (Kj). However, it is also possible that there would be heterogeneity in the term of equation(l). Evidence for this is obtained from a study of the effect of salt concentration on adsorption of anions. Fig. 10 shows that there are effects of salt concentration on phosphate sorption by a soil. These effects are analogous to those on phosphate sorption by goethite at low pH, increasing salt concentration decreases sorption at higher pH it increases sorption. There is thus an intermediate pH at which salt concentration has no effect. The value of this point of zero salt effect decreases with increasing sorption as would be expected. However, it occurs at a higher pH than the point of zero salt effect on pH (Fig.ll). 

Figure 11. Effect of level of addition of phosphate on the point of zero salt effect on phosphate sorption and on the point of zero salt effect for pH [38].

Butkus, M.A. and Grasso, D., Impact of phosphate sorption on water-treatment residual surface characteristics Prelude to reuse, Environ. Eng. Sci., 16, 117, 1999. 

Figure 4.8. Time-dependence of phosphate sorption by a soil, attributed to the difference in chemisorption and precipitation kinetics. Total sorption is taken to be the sum of chemisorption and precipitation. (Adapted from S.E.A.T.M. Van der Zee and W. H. Van Riemsdijk. 1991. Model for the reaction kinetics of phosphate with oxides and soil. In G. H. Bolt et al. (eds.), Interactions at the Soil Colloid-Soil Solution Interface. Dordrecht Kluwer.)...

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