Arsenic and zinc biogeochemistry in pyrite mine waste from the Aznalcóllar environmental disaster

This laboratory experiment systematically examines arsenic, iron, zinc, and sulfate solubility and fractionation in pyrite mine waste suspensions as affected by redox potential (Eh) at pH 7.0. Under aerobic conditions and pH 7.0, As solubility was low, however, under moderately reducing conditions (0–100 mV), As solubility significantly increased due to dissolution of iron oxy-hydroxides. Upon reduction to −250 mV, As solubility was controlled by the formation of insoluble sulfides, and as a result, soluble As contents dramatically decreased. Soluble Fe concentration increased with time under anaerobic conditions, however, it decreased under aerobic conditions likely due to formation of insoluble oxy-hydroxides. Under aerobic conditions and pH 7.0, soluble Zn significantly increased with incubation time and reached concentrations as high as 800 mg kg−1 waste. Perhaps zinc was initially present as insoluble zinc sulfides, however, after further oxidation, sulfide was transformed to sulfate and Zn2+ was then released into the waste solution. Selective extraction of incubated wastes maintained at pH 7.0 illustrated that arsenic biogeochemistry was mainly controlled by As bound to: (a) amorphous Fe oxy-hydroxides and (b) insoluble organics and sulfides. Remediation of a site polluted by both arsenic and zinc is quite complicated because the redox conditions favoring insolubility of arsenic, however, favors maximum solubility of zinc, and vice versa. Therefore, the best way for the remediation of an arsenic- and zinc-polluted environment, in our opinion, is: (a) phyto-remediation with plants accumulating large amounts of Zn in their tissues, and (b) simultaneous addition of: (1) amorphous iron oxy-hydroxides (aerobic Eh) or (2) organic matter rich in S compounds (anaerobic Eh).