THE ENVIRONMENTAL FATE OF ARSENIC IN SURFACE SOIL CONTAMINATED BY ARSENICAL HERBICIDE

Soils from many industrial and agricultural sites are contaminated with arsenic because of the historical application of herbicide containing arsenic trioxide. The strong affinity of aqueous arsenic species for soil components has led to the retention of significant amounts of arsenic in surface soils decades after the original source application. In this study, soil collected from a site which received a one-time surface application of arsenical herbicide in the 1950s was investigated to understand the fate of arsenic after five decades of natural leaching. The occurrence of arsenic in this soil was characterized by multiple techniques. No crystalline arsenic phases were detected in the soil sample by XRD analysis. Although SEM/EDS analysis indicated possible residual herbicide particles in the soil sample, sequential chemical extraction of the contaminated soil revealed that the majority of the arsenic is in secondary form (i.e. associated with amorphous and crystalline iron oxides). Natural leaching of the soil sample was simulated by sequentially leaching the contaminated soil with a synthetic acid rain (SAR) solution in batch experiments.

Leaching of arsenic from the weathered soil can be divided into two distinct stages. During the first stage, the leachate arsenic concentration underwent a rapid decline which suggests an equilibrium-controlled release event. The second leaching stage was marked by a slow, steady release of arsenic, a characteristic of a kinetically controlled process. A mathematical model was developed to describe arsenic release from the contaminated soil. This model considers both equilibrium desorption and kinetic desorption processes simultaneously and produces leachate arsenic concentrations in good agreement with the experimental data. According to the modeling results, a significant amount of residual arsenic still exists in the surface soil five decades after the initial herbicide application event. Twenty percent of the arsenic remaining in the soil resides in the herbicide source material after five decades of natural leaching; 25 percent exists on reversible adsorption sites and 55 percent is present on irreversible adsorption sites.