Arsenic fractionation in a fine soil fraction and influence of various anions on its mobility in the subsurface environment

The degree to which anions can affect As mobility in subsurface environments depends on As speciation and fractionation in the soil. In this study, the fine soil fraction of an acidic soil was deliberately contaminated with arsenate (As(V)) and adequately aged for more than 220 days. A selective sequential extraction (SSE) scheme was developed to fractionate As in the soil into 6 fractions, while the influence of various anions on As mobility was assessed by evaluating its extractability by various salts such as Na3PO4, Na2CO3, Na2SO4 and NaCl. The variables of the extraction experiment were type of salt, salt concentration and reaction time. Fractionations of As in soil samples after extraction with the salts were also examined and the results compared against the initial As fractionation. The SSE results showed that the majority of the As was strongly adsorbed via surface complexation in the fine soil. A considerable amount of As was also associated with the exchangeable fraction, amorphous Fe oxyhydroxides, and the residual fraction. Abilities of the anions in mobilizing As bound in the fine soil followed the order of PO43-≫CO32->SO42-≈Cl-. Arsenic mobilization by PO43- was believed to be due primarily to a ligand exchange mechanism, through which the strongly bound As was replaced by the PO43- anion. Anion exchange might also contribute to the mobilization of As in this study. Increase in the concentration of the anions tested did not further mobilize As from the soil, with the exception of PO43-. The kinetics of As release in the PO43- solution could be best described by the Elovich model, which represents an exponential decrease in As desorption rate as more As was extracted from the soil.