Arsenic Mobility, Distribution and Speciation in Soil Contaminated by Surface Application of Arsenolite

Arsenic contamination of soil and groundwater resulted from the application of arsenolite (As₂O₃) to soils as an herbicide between 1950-1970. The mechanisms by which arsenic was leached from the herbicide, transported through the underlying soil and partially immobilized in secondary solid phases are poorly understood. Column experiments were conducted using background soil collected near one of the contaminated sites and spiked with a surface application of arsenolite, with and without carbonate gravel cover, to simulate the contamination process and better understand the transformations of arsenic during weathering of the arsenolite. Synthetic acid rain was applied at the tops of the soil columns to simulate natural leaching. Effluent samples were taken at the bottoms of the columns and analyzed by ICP-OES and ion chromatography. At the end of the experiments, the packed soil columns were extruded, sectioned and analyzed by a variety of techniques.

Observed differences in column effluent arsenic concentrations and temporal trends during long-term leaching can be explained by the presence/absence of gravel cover. Lower total arsenic leached from the graveled column is likely due to precipitation of calcium arsenate in the soil, which was identified by XAS analysis of similar soil contaminated with arsenolite more than 50 years ago. Early in the leaching process, aqueous arsenic was dominated by As(III); however, after 180 pore volumes of acid rain leaching, As(V) began to dominate the effluent arsenic chemistry for all of the spiked columns. Speciation studies indicate that approximately one third of the As(V) was produced at the top of the column, and the remaining two thirds was oxidized in contact with soil grains. XPS analysis of the column soil sections shows oxidation of As(III) to As(V) at the arsenolite surface, decreasing soil arsenic concentrations from column top to bottom, and that arsenic sorbed to soil particles is As(V).