Origin of the Effects of Citrate, Tartrate, and Acetate on Phosphate Sorption by Soils and Synthetic Gels

Abstract

AbstractThe ability of two contrasting soils and synthetic Fe and Al gels to sorb inorganic phosphate (P) in the presence of three commonly‐occurring organic anions was evaluated using sorption isotherms. At 10−3M, citrate was markedly effective in reducing P sorption (50% reduction), tartrate was only moderately effective (20% reduction), whereas acetate had little or no effect (1% reduction) at a final solution P concentration of 0.20 mmole/liter for the four sorbents. A close linear relationship was established between the reduction in P sorption and the amounts of (Fe + Al), Fe, or Al removed from the Fe‐rich soil, Fe gel, or Al gel, respectively, by varying concentrations of citrate and tartrate. At 10−1 M, citrate and tartrate removed 91% of the Fe and 88% of the Al from the Fe and Al gel, respectively, reducing P sorption by 89 and 88%. At equivalent anion concentrations, however, tartrate was sorbed to a greater extent than citrate, resulting in a larger reduction in P sorption by this anion for a given amount of (Fe + Al) or Fe removed from the two soils and Fe gel. The curvilinear relationship obtained between the reduction in P sorption and the amount of (Fe + Al) removed from an Al‐rich soil resulted from the removal, at low organic anion concentrations, of a readily‐extractable form of Al which had little effect on P sorption. A similar finding was obtained for Al gel. The reduction in P sorption observed in the presence of citrate resulted primarily from the elimination of a significant proportion of P‐sorbing sites rather than from the competitive specific sorption of citrate. In contrast, tartrate reduced P sorption through a combined dissolution‐competitive sorption effect. Although 10−1M acetate had little effect on P sorption, a high proportion was sorbed by the sorbents, indicating that the sorption mechanism or site of binding of acetate differs from that of P.