Arsenic transport in contaminated mine tailings following liming

The practice of liming to remediate contaminated soils and mine tailings has the potential to mobilize arsenic (As), due to the pH dependence of As sorption reactions on oxide minerals and layer silicates. The objectives of this study were to determine the effects of liming on As mobility in mine tailings and identify possible mechanisms controlling As mobilization with increased pH. Six mine tailing samples obtained from an abandoned copper smelter near Anaconda, Montana were analyzed for total As and soluble constituents using saturated paste extractions. Concentrations of soluble As among the six samples did not correlate with total As, but were more closely related to pH. Saturation indices with respect to known metal arsenate solid phases suggested that metal arsenate solid phases were not controlling soluble As in these samples. Two low pH samples (pond tailings and reprocessed tailings) were chosen for more detailed chemical characterization and unsaturated column transport experiments before and after liming. Soluble As concentrations measured in column effluent increased by factors of 10 (reprocessed tailings, RT) to 400 (pond tailings, Pr) following liming. Sequential extractions of these tailing samples showed that the PT contained significantly higher ''labile'' As relative to the RT, consistent with amounts of As mobilized after liming. Further characterization of these samples using scanning electron microscopy (SEM) and energy dispersive analysis of x-rays (EDAX) suggested that these samples do not contain discrete metal arsenate solid phases. Based on this suite of experimental data, increased mobility of As with liming appears to be consistent with the pH dependence of sorption reactions of As on Fe oxide minerals rather than dissolution-precipitation reactions involving As.