Abstract

The aqueous geochemistry of Zn, Cu, Cd, Fe, Mn and As is discussed within the context of an anaerobic treatment wetland in Butte, Montana. The water being treated had a circum-neutral pH with high concentrations of trace metals and sulfate. Reducing conditions in the wetland substrate promoted bacterial sulfate reduction (BSR) and precipitation of dissolved metal as sulfide minerals. ZnS was the most common sulfide phase found, and consisted of framboidal clusters of individual spheres with diameters in the submicron range. Some of the ZnS particles passed through the subsurface flow, anaerobic cells in suspended form. The concentration of “dissolved” trace metals (passing through a 0.45 µm filter) was monitored as a function of H₂S concentration, and compared to predicted solubilities based on experimental studies of aqueous metal complexation with dissolved sulfide. Whereas the theoretical predictions produce “U-shaped” solubility curves as a function of H₂S, the field data show a flat dependence of metal concentration on H₂S. Observed metal concentrations for Zn, Cu and Cd were greater than the predicted values, particularly at low H₂S concentration, whereas Mn and As were undersaturated with their respective metal sulfides. Results from this study show that water treatment facilities employing BSR have the potential to mobilize arsenic out of mineral substrates at levels that may exceed regulatory criteria. Dissolved iron was close to equilibrium saturation with amorphous FeS at the higher range of sulfide concentrations observed (>0.1 mmol H₂S), but was more likely constrained by goethite at lower H₂S levels. Inconsistencies between our field results and theoretical predictions may be due to several problems, including: (i) a lack of understanding of the form, valence, and thermodynamic stability of poorly crystalline metal sulfide precipitates; (ii) the possible influence of metal sulfide colloids imparting an erroneously high “dissolved” metal concentration; (iii) inaccurate or incomplete thermodynamic data for aqueous metal complexes at the conditions of the treatment facility; and (iv) difficulties in accurately measuring low concentrations of dissolved sulfide in the field.