EVALUATION OF REACTIVE MIXTURES FOR REMOVAL OF ARSENIC FROM GROUNDWATER

The Long Lake Mine near Sudbury, Ontario, was the site of intermittent Au production from 1909-1939, during which tailings containing on average 5 wt. % As were deposited without containment on the mine site. Dissolved As concentrations up to 600 mg L^-1 are present in the tailings pore water, and acidic conditions prevail. A series of laboratory column experiments was conducted to evaluate the effectiveness of a mixture containing natural organic material and granular zero-valent iron for As removal. The reactive mixtures varied from between 20-40 wt. % organic carbon and 10-30 wt. % zero-valent iron. Arsenic-rich groundwater collected from the tailings area was used as influent for these experiments. Groundwater was pumped through the columns at a velocity three times greater than the site groundwater velocity; the extent of As removal at the accelerated velocity can be used to predict long-term treatment capacity. In addition, a reaction cell, consisting of a 30 cm ID by 98 cm long PVC cylinder was installed on site, and As-bearing groundwater from a piezometer downgradient from the tailings area was pumped through the cell. The reaction cell was operated for several months during the fall of 2018 to evaluate temperature-dependence and seasonal performance of the reactive media. The experimental results indicate extensive removal of As in both the laboratory and field experiments, with removal of As observed up-gradient from the first sampling port along the flow path. Average influent concentrations of As were between 5-10 mg L^-1 while the concentrations of As in all column and reaction cell effluent water were below 10 ug L^-1. Arsenic removal was accompanied by an increase in pH from acidic to circumneutral values and a decline in the concentrations of dissolved metals, including Al, Cu, and Zn. Removal of As and dissolved metals is attributed to precipitation of low-solubility sulfide phases due to microbially-mediated sulfate reduction; these observations are consistent with the presence of aqueous sulfide in the effluent water and the results of microbiological analyses.