Experimental simulation of soil contamination by arsenolite

Column experiments were conducted to experimentally simulate the initial conditions at a field site where legacy As contamination was caused by application of arsenolite as a herbicide to the soil. The experiments were designed to investigate the influence of herbicide loading and carbonate gravel cover on the release of As to, and the As adsorption capacity of, the background soil. The results showed that the As release from the doubly spiked column was three times higher than for the singly spiked columns. The addition of gravel above the herbicide layer halved the leaching time needed for peak As release from the soil. Extrapolation of the experimental data predicted that the columns with carbonate gravel should retain more As (15.6 g for column FWAsG and 12.3 g for column F2AsG) than the column without gravel (0.13 g for column FWAs). The results also indicated the thickness of the arsenolite layer was not important in terms of the As retention capacity of the soil columns. Arsenate was detected and quantified in the effluent solutions by ion chromatography. The data indicated As(III) dominated in the effluents for the first 180 pore volumes (PVs). After 180 PVs, As(III) almost disappeared and As(V) dominated in the effluents. The unexpected high As peak release from the doubly spiked column FW2AsG indicated that solutions having higher As(III) concentrations had a higher capability to dissolve arsenolite, possibly through solute–solute interactions or polymerization. Effluent solution Eh–pH values indicated that As(V) was the thermodynamically stable form of As throughout the experiments. Therefore, As(III) dominance in the initial column effluent solutions was kinetically controlled.