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Green rust compounds are used to reduce Cr(VI) and Cr(III) at different concentrations and pH levels. The disappearance of Cr(VI) and Cr(III) with carbonate green rust is graphed over time and with different pH levels.

Microbial respiration of Fe(III) oxides has been shown to produce reduced Fe phases that are capable of transforming a variety of oxidized contaminants. Little data, however, are available on how these Fe phases evolve over time and how this evolution may affect their ability to reduce contaminants. Here, the evolution and reactivity of biologically reduced ferrihydrite were monitored over a period of 14 months. Solids were collected from a culture of Geobacter metallireducens (GS-15) that was incubated with ferrihydrite (as the electron acceptor) for 0, 1, 10, 20, 75, and 400 days. Mineralogical composition and surface area of the biologically reduced solids were characterized using Mössbauer spectroscopy, X-ray diffraction, and BET with N2 adsorption. By day 10, ferrihydrite began to transform, and a nanoparticle magnetite/maghemite phase, as well as two ferrous phases, was observed. One of the ferrous phases was identified as siderite, whereas the other could not be positively identified. Likely candidates, however, include Fe(OH)2(s) or an adsorbed Fe(II) species. Over the next few months, ferrihydrite was completely reduced and evolved into a mixture containing about 70% magnetite/ maghemite, 19% siderite, and 11% of the second Fe(II) phase. The effect of incubation time on the reactivity of the biologically reduced solids was evaluated by measuring the kinetics of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) transformation. The only products observed were the three reduced nitroso products. Rate coefficients (k) for RDX transformation were dramatically influenced by incubation time with half-lives of about 1 month observed in the presence of solids incubated for 10 and 20 days, 3 months with solids incubated for 75 days, and negligible removal with solids incubated for 400 days. The loss of reactivity was not directly correlated to any one mineralogical variable but may be due to particle size or surface chemistry changes in the reactive Fe phase or to cell die-off and the accumulation of cell lysis products after consumption of the electron acceptor. The dramatic effect of incubation time on the rate of RDX removal highlights a potential limitation of studying complex systems, as we have here, in batch reactors and suggests that incubation time is an important variable to consider when measuring and comparing rates of contaminant reduction. [ABSTRACT FROM AUTHOR]

Using the isotope specificity of 57Fe Mössbauer spectroscopy, we report spectroscopic observations of Fe(ll) reacted with oxide surfaces under conditions typical of natural environments (i.e., wet, anoxic, circumneutral pH, and about 1% Fe(ll)). Mössbauer spectra of Fe(lI) adsorbed to rutile (TiO2) and aluminum oxide (A12O3) show only Fe(II) species, whereas spectra of Fe(ll) reacted with goethite (α- FeOOH), hematite (α-Fe2O3), and ferrihydrite (Fe5HO9) demonstrate electron transfer between the adsorbed Fe(ll) and the underlying iron(III) oxide. Electron-transfer induces growth of an Fe(lll) layer on the oxide surface that is similar to the bulk oxide. The resulting oxide is capable of reducing nitrobenzene (as expected based on previous studies), but interestingly, the oxide is only reactive when aqueous Fe(ll) is present This finding suggests a novel pathway for the biogeochemical cycling of Fe and also raises important questions regarding the mechanism of contaminant reduction by Fe(ll) in the presence of oxide surfaces. [ABSTRACT FROM AUTHOR]

Perchlorate was reduced by a mixed bacterial culture over a pH range of 7.0-8.9. Similar rates of perchlorate reduction were observed between pH 7.0 and 8.5, whereas significantly slower reduction occurred at pH 8.9. Addition of iron metal, Fe(0), to the mixed bacterial culture resulted in slower rates of perchlorate reduction. Negligible perchlorate reduction was observed under abiotic conditions with Fe(0) alone in a reduced anaerobic medium. The inhibition of perchlorate reduction observed in the presence of Fe(0) is in contrast to previous studies that have shown faster rates of contaminant reduction when bacteria and Fe(0) were combined compared to bacteria alone. The addition of Fe(0) resulted in a rise in pH, as well as precipitation of Fe minerals that appeared to encapsulate the bacterial cells. In experiments where pH was kept constant, the addition of Fe(0) still resulted in slower rates of perchlorate reduction suggesting that encapsulation of bacteria by Fe precipitates contributed to the inhibition of the bacterial activity independent of the effect of pH on bacteria. These results provide the first evidence linking accumulation of iron precipitates at the cell surface to inhibition of environmental contaminant degradation. Fe(0) was not a suitable amendment to stimulate perchlorate-degrading bacteria and the bacterial inhibition caused by precipitation of reduced Fe species may be important in other combined anaerobic bacterial-Fe(0) systems. Furthermore, the inhibition of bacterial activity by iron precipitation may have significant implications for the design of in situ bioremediation technologies for treatment of perchlorate plumes.