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Adsorption-Desorption Kinetics of Zn in Soils: Influence of Phosphate

The reactivities of heavy metals such as zinc in the soil environment are affected by several rate-limiting processes, including kinetic sorption and release reactions. In this study, kinetic batch experiments were carried out to evaluate sorption and desorption of Zn for soils having different properties. Sorption isotherms exhibited strong nonlinearity as well as kinetic behavior. Distinct differences in the amounts of Zn sorbed among the different soils were observed where lowest sorption was associated with acidic soils (Windsor and Olivier) soil. In contrast, Webster (neutral) soil exhibited highest sorption capacity. Discrepancies between adsorption and successive desorption isotherms indicate considerable hysteresis for Zn release, the extent of which varied among the three soils. For the two acidic soils, Zn release as a percentage of that total sorbed was 47% to 51% and 42% to 49% for the Windsor and Olivier soils, respectively; whereas for the neutral soil, only 9% to 11% of sorbed Zn was released over time. A second objective was to test the hypothesis that phosphate addition to soils increases Zn adsorption. The influence of the presence of P on increased Zn sorption was clearly manifested by the isotherms where similar trends were observed for all three soils. Moreover, P additions reduced Zn desorption, which indicates that the presence of P increased the affinity or specific sorption sites for Zn. A multireaction model that accounts for nonlinear equilibrium and kinetic retention reactions was successfully used to simulate the time-dependent behavior of Zn sorption-desorption for all three soils and at different P levels. Moreover, the addition of different levels of P did not affect the kinetic sorption-desorption behavior of Zn in all soils.