Modeling arsenic desorption from herbicide-contaminated soils.

The application of arsenical herbicides has created legacy environmental problems by contaminating soil in some agricultural areas and at various industrial sites. Numerous previous studies have suggested that the adsorption of arsenic by common soil components is largely controlled by kinetic factors. Four arsenic-contaminated soil samples collected from industrial sites were characterized and subjected to sequential leaching using a synthetic acid rain solution in order to study the release of arsenic. A dual-site numerical sorption-desorption model was constructed that describes arsenic desorption from these soils in terms of two different release mechanisms: Release from type I (equilibrium) and type II (kinetic) sorption sites. Arsenic held on both type I and II sorption sites is accessible through extensive acid rain leaching. Arsenic desorption from these sites follows a linear Kd model; the manner of approaching the Kd model, however, differs. Arsenic desorption from type I sites reached equilibrium with the aqueous phase under the physical environment provided by the experiment (shaking for 24 h at 25 degrees C), while desorption from type II sites followed a first-order kinetic pattern when approaching equilibrium. During synthetic acid rain sequential leaching of the soils, type I sites released their sorbed arsenic rapidly and subsequent desorption was dominated by the kinetic release of arsenic from type II sites. This shift in desorption mechanism dominance generated data corresponding to two intersecting straight lines in the n-logC dimension for all four soils. The dual-site desorption model was solved analytically and proven to be successful in simulating sorption processes where two different mechanisms are simultaneously controlling the aqueous concentration of a trace element.