ORIGIN OF THE EFFECTS OF CITRATE, TARTRATE, AND ACETATE ON PHOSPHATE SORPTION BY SOILS AND SYNTHETIC GELS

The ability of 2 contrasting soils and synthetic Fe and Al gels to sorb inorganic phosphate (P) in the presence of 3 commonly-occurring organic anions was elevated using sorption isotherms. At 10-3 M, 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 4 sorbents. A close linear relationship was established between the reduction in P sorption and the amounts of (Fe + Al), Fe, or Al removed for 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 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 2 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.