The Influence of Phosphate on the Adsorption-Desorption Kinetics of Vanadium in an Acidic Soil

Quantitative understanding of the mechanisms controlling the competitive retention and transport of V and phosphate on soils is essential for accurately evaluating the environmental risks of contaminants in the environment. Batch and stir-flow chamber experiments were performed to quantify the extent of kinetics of V and phosphate competitive retention in an acidic soil (Sharkey clay). In this study, a stir-flow model was used to describe tracer and competitive reactive solute adsorption, and desorption processes in soils. Based on optimized and predictive modeling results, a fully reversible-irreversible multi-reaction model successfully described the time-dependent competitive V and phosphate retention and transport process in Sharkey soil. Adsorption for V and phosphate were highly nonlinear and time dependent, where V binding affinities were stronger than those for phosphate. Results from batch experiments indicated that that the rate and extent (amount) of V released increased significantly in the presence of phosphate. Breakthrough curves for V, from stir-flow experiments, were asymmetrical and exhibited slow release or tailing, indicating that nonequilibrium retention on the surface of soil was the dominant mechanism of the time-dependent adsorption of V. Results of stir-flow experiments indicated that increased mobility of V was observed in the presence of phosphate caused by direct competition for available retention sites. In conclusion, increased addition of phosphate causes decreasing sorption capacity and increasing mobility of V and needs to be considered in modeling the fate and transport of V in soil.