Effects of microbial iron oxide reduction on pH and alkalinity in anaerobic bicarbonate-buffered media: implications for metal mobility

The iron cycle in aquatic environments involves the reduction of ferric iron (Fe(III)) followed by the oxidation of ferrous iron (Fe(II)). Under natural conditions these reactions are microbially catalyzed. The reduction of Fe(III) is thought to be one of the most important microbially mediated processes in aquatic systems. Fe(II) tends to be soluble and mobile in water, while Fe(III) tends to form insoluble oxyhydroxides and remain immobile. Iron oxides have high surface areas and adsorption capacities and are therefore effective scavengers of trace metals. However, the adsorbed trace metals can be released upon reduction of Fe(III) phases (Chapelle, 1993), which may be catalyzed due to the action of dissimilatory Fe(III)-reducing bacteria and dissimilatory-sulphate reducing bacteria present in anaerobic sediments (Lovley, 1991). Much of the Fe(II) and trace metals that are released upon reduction do not enter the aqueous phase, but are instead sequestered as metal oxides, carbonates, and iron bearing sulphides, Another interesting aspect of microbially mediated Fe(III) reduction is its possible affect in the carbonate cycle. The reduction of Fe(III) by microorganisms via organic matter oxidation can potentially increase the alkalinity of the system through the production of HCO-3 and OH. The increase in alkalinity can cause authigenic carbonate mineral precipitation by increasing the degree of water saturation with respect to carbonate phases (Coleman et al., 1993). Roden and Lovley (1993) showed the precipitation of FeCO3 during growth of Fe(III)-reducing bacteria in a bicarbonate-buffered medium. Recent studies have shown that precipitation of carbonate mineral phases can directly contribute to the immobilization of dissolved trace metals and radionuclides present in subsurface and groundwater systems (Thompson and Ferris, 1990). The potential exploitation of this immobilization concept has relevance to development of new clean-up technologies to be used at contaminated sites where Fe(III)oxide phases are abundant. To better understand geochemical processes operating in such an environment, the exact nature of pH, alkalinity, and dissolved inorganic carbon (DIC) changes due to Fe(III) reduction must be thoroughly quantified. The objectives of this study were to (1) determine how quickly and to what extent microbial reduction of Fe(III)-oxides change pH, alkalinity, and DIC in both natural and synthetic Fe(III)-oxide media, and (2) to compare the effects of carbonate precipitation induced by microbial Fe(III)-oxide reduction on the distribution and speciation of two metals (Ca 2+, St2+).