- © 2014 Mineralogical Society of America
Arsenic contamination of mine and metallurgical waters has long been recognized as a global problem. More stringent guidelines, based on demonstration of potential toxicity to humans and ecological receptors, have motivated regulators and operators to address both legacy sites and existing or future operational discharges to mitigate potential impacts. The safe disposal of material considered to be hazardous is a natural part of good housekeeping for any industrial development. This is particularly so for the mining industry, which historically was not always well managed in this aspect and as a result, has a high-political profile today.
Arsenic can occur in several oxidation states in natural waters although the trivalent arsenite (As(III)) or pentavalent arsenate (As(V)) are the most common (Smedley and Kinniburgh 2002). The most thermodynamically stable species over the natural range of groundwater redox conditions (150–500 mV, Bass Becking et al. 1960) and pH (4–7, Baas Becking et al. 1960) are H2AsO4−, HAsO4−, and in acid rock drainage waters (pH below 5) H2AsO4−. In more reduced waters, As(OH)3 is the most common species. Thioarsenic species may also be present but in general are not observed in natural waters. The kinetics of arsenic reduction-oxidation (redox) reactions is not rapid, so the predicted proportions of arsenic species based on thermodynamic calculations do not always correspond to analytical results (O’Neil 1990). An Eh-pH diagram showing the thermodynamically stable regions for arsenic species is shown in Figure 1.
Because of arsenic toxicity, the World Health Organization placed a guideline maximum allowable concentration of arsenic in drinking water of 10 μg L−1 (WHO 1998). The USEPA reduced the drinking water standard from 50 to 10 μg L−1 in 2002 (USEPA 2001). Arsenite is considered to be more …