Competitive sorption-desorption kinetics of arsenate and phosphate in soils

The competition between arsenate (AsO4-3) and phosphate (PO4-3) on mineral surfaces has the potential of increasing arsenic mobility and bioavailability in the soil and water environment. In this study, kinetics of competitive sorption of ASO(4)(-3) and PO4-3 in three soils was investigated in batch systems by simultaneously introducing the ligands at different molar ratios. Adsorption was carried out at different retention times, and release was investigated using successive dilutions after adsorption. Nonlinear sorption isotherms of ASO(4)(-3) and PO4-3 were observed for all soils. Rates and amounts of AsO4-3 adsorption were significantly reduced when PO4-3 concentrations in the soil solution increased. In addition, the relative sorption preference of AsO4-3 and PO4-3 did not exhibit changes with reaction time. Desorption and sequential extractions results indicated that a significant amount of AsO4-3 was irreversibly retained by all soils. Kinetic retention data of AsO4-3 and PO4-3 were successfully described using a mechanistic multireaction model that accounted for competitive retention. This study indicates that competition of AsO4-3 and PO4-3 for adsorption sites should be considered in models predicting arsenic release from soils receiving high phosphorus inputs.