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Impact of Natural Organic Matter on Bioreduction of Fe(III) and U(VI)

Bill Burgos
Department of Civil & Environmental Engineering
Penn State

A developing technology for the in situ treatment of metal and radionuclides is the stimulation of dissimilatory metal-reducing bacteria (DMRB) to create an Fe(II) enriched reactive zone. Conceptually, a non-fermentable carbon source/electron donor (e.g., Na-acetate) would be added to a contaminated zone to promote the activity of native DMRB over other species in the subsurface microbial community. The biological reduction of ferric oxides would produce Fe(II) for the "indirect" chemical reduction of contaminants [e.g., soluble U(VI) reduced to insoluble U^IVO₂(s)], or DMRB could "directly" reduce contaminants capable of serving as terminal electron acceptors [e.g., Cr(VI), Tc(VII), U(VI)]. Natural organic matter (NOM) can stimulate the biological reduction of solid-phase iron oxides by serving as an electron shuttle and by complexing biogenic Fe(II). The addition of NOM, along with Na-acetate, to contaminated zones has been proposed to further stimulate iron reduction and the fortuitous reduction and immobilization of contaminants. However, little research has been conducted on ternary systems that contain ferric oxides, NOM, and metals or radionuclides. In our laboratory we have conducted experiments with: hematite and NOM, hematite and zinc, hematite and NOM and zinc, and uranyl acetate and NOM. All of these experiments have been conducted with the DMRB Shewanella putrefaciens strain CN32. The current presentation will attempt to summarize results from all of these studies for general discussion. As noted above, NOM, in the absence of zinc or uranium, significantly enhanced the rate and extent of hematite bioreduction. Zinc was shown to inhibit both hematite and nitrate bioreduction. It was assumed that the addition of NOM would decrease zinc inhibition due the ability of NOM to complex Me(II) cations and lower the "free" dissolved zinc concentrations. Indeed this occurred with nitrate bioreduction, however, NOM significantly increased the inhibitory effect of zinc during hematite bioreduction. In addition, non-toxic Mn(II) became inhibitory in the presence of NOM. These results suggest that Me(II)-NOM complexes may be specifically inhibitory to solid-phase bioreduction. It was assumed that the addition of NOM would stimulate the rate of uranium bioreduction because of its electron shuttling abilities. However, uranium bioreduction was completely inhibited by NOM and by citrate, suggesting that U(VI)-NOM complexes may be biologically "unavailable" as terminal electron acceptors. Overall these results demonstrate that important side reactions between NOM and metal/radionuclide contaminants may control the effectiveness of this biostimulation strategy.