PNNL

Abstract

Abstract-The potential for reduction of 9~CO- (aq) to poorly soluble 9~COZ .nHzO(s) by biogenic sediment-associated Fe(ll) was investigated with three Fe(IlI)-oxide containing subsurface materials and the dissimilatory metal-reducing subsurface bacterium Shewanella putrefaciens CN32. Two of the subsurface materials from the U.S. Department of Energy's Hanford and Oak Ridge sites contained significant amounts of Mn(IlI,IV) oxides and net bioreduction of Fe(llI) to Fe(ll) was not observed until essentially all of the 1 hydroxylamine HCl-extractable Mn was reduced. In anoxic, unreduced sediment or where Mn oxide bioreduction was incomplete, exogenous biogenic TcOz .nHzO(s) was slowly oxidized over a period of weeks. Subsurface materials that were bioreduced to varying degrees and then pasteurized to eliminate biological activity, reduced TcO-: (aq) at rates that generally increased with increasing concentrations of 0.5 N HC1F~(Il~o of the sediments showed a common relationship between th~~8I~e~oncentra-= e ~/.!I80 20 'rY'\rI) --' t~?_~ (~~1 and the first-order reduction rate (in h-1, whereas the third demonstrated a markedly different trend. A combination of chemical extractions and s7Fe Mossbauer spectroscopy were used to characterize the Fe(llI) and Fe(ll) phases. There was little evidence of the formation of secondary Fe(ll) biominerals as a result of bioreduction, suggesting that the reactive forms of Fe(ll) were predominantly surface complexes of different forms. The reduction rates of Tc(VIl)O-; were slowest in the sediment that contained plentiful layer silicates (illite, vermiculite, and smectite), suggesting that Fe(ll) sorption complexes on these phases were least reactive toward pertechnetate. These results suggest that the in situ microbial reduction of sediment-associated Fe(IlI), either naturally or via redox manipulation, may be effective at immobiJizing TcO-: (aq) associated with groundwater contaminant plumes.