The presence of oxyanions such as phosphate (P) in soils is believed to have effects on lead (Pb) interaction with minerals and is often considered as a remediation method for Pb immobilization in soil. In this study, the effect of P on Pb sorption on kaolinite was investigated using batch equilibrium studies and an equilibrium sorption ion-exchange model. Batch equilibrium studies indicated that sorption on pristine and P-pretreated kaolinite surfaces was highly pH dependent. Sorption equilibrium was attained within 10 min for both surfaces. The amount of Pb sorbed significantly increased at low pH (pH range, 4-6) when presorbed P was present on the surface. Preferential sorption of Pb onto pristine kaolinite is likely when Pb is present in ionic Pb hydroxy forms in solution, apparently due to the stronger affinity of PbOH+ species to the surface relative to Pb2+. At different pH values, solution speciation of Pb as well as a Pb sorption model suggest solid precipitation of Pb(OH)2(s) and/or adsorption of various aqueous Pb(OH)x species to the surface at pH greater than 6. Modeling studies also suggest the possibility of formation of a surface complex similar to pyromorphite at low pH (pH4)when P is on the surface. Lead sorption on pristine kaolinite was endothermic, whereas sorption on P-presorbed surface was exothermic. Lead sorption onto P-presorbed kaolinite resulted in a lower entropy (Symbol) of the system compared with the pristine kaolinite, which is indicative of a stronger binding of Pb to a P-presorbed surface.
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