Effects of pH on metals precipitation and sorption: field bioremediation and geochemical modeling approaches

At the Sanders Lead car-battery recycling plant, near Troy, AL, groundwater is highly acidic (pH varies from 3 to 3.5) and carries high concentrations of Pb, Cd, Zn, Cu, and Fe. Pilot field experiments conducted at the site show that in situ metabolism of sulfate reducing bacteria (SRB) can produce desired geochemical effects to remove heavy metal Pb, Cd, Zn, and Cu from groundwater. A reaction path model of sulfate reduction shows the redox potential (Eh) effects on mineral precipitation and pH controls on the sorption of different metals. Lead strongly adsorbs to hydrous ferric oxide (HFO) present in the aquifer over a wide pH range. Both sorption (due to pH increase) and solid sulfide formation are important for removing Pb. Although theoretical modeling shows that the sorption of most cations is promoted as pH increases, HFO can only scavenge Zn, Cd, Co, and Ni at relatively neutral pH conditions. Thus concentrations of our primary contaminants Zn and Cd attenuate in acidic conditions primarily via precipitation or coprecipitation of solid sulfide phase as Eh drops. The modeling result explains why the Pb plume is retarded in migration with respect to the Cd plume under the low-pH conditions at the site. For As, arsenate sorbs strongly onto the protonated weak sites of ferric hydroxide for the pH range of calculation. Arsenite sorption is also favored by increasing pH, however, arsenite desorbs and becomes mobilized at very low oxidation state as it reacts with dissolved sulfide to form AsS complexes. In addition, intermittent rainfall events could cause short-term Eh increases, potentially leading to oxidation of sulfide solids and subsequent pH decrease, and the remobilization of metals. This study argues for the importance of accounting for pH changes when evaluating the fate, transport, and long-term stability of metals at shallow contaminated sites.