Microbial Reduction of Hexavalent Chromium
Berkeley Lab scientists have used the new Fourier-transform infrared (FTIR) spectromicroscopy beamline (Beamline 1.4.3) at the ALS to obtain spatially resolved, time-dependent evidence for biogeochemical transformation of hexavalent chromium (Cr6+), a widespread industrial contaminant. Previously, two mechanisms for reducing these compounds to significantly less harmful trivalent chromium (Cr3+) compounds were postulated. The biogeochemical mechanism requires the presence of microorganisms to aerobically reduce the Cr6+, whereas the chemical mechanism relies on metal oxides, such as Fe(II) compounds, to catalyze the Cr6+-reduction reaction.
The researchers conducted FTIR experiments with synchrotron radiation (SR-FTIR) to distinguish the relative significance of these two mechanisms. In addition, they evaluated effects of common organic co-contaminants, such as toluene vapor, on the biotic reduction process. Experiments with magnetite surfaces exposed to Cr6+ compounds showed evidence of reduction only when living organisms were present. The reduction was significantly enhanced when the samples were incubated in toluene vapor.
The brightness of the infrared radiation at the ALS makes spatially resolved spectroscopy (spectromicroscopy) possible. Imaging of the surface at characteristic absorption bands showed a strong correlation between peak depletion of Cr6+ and toluene and peak concentration of biological molecules. To determine if this microbial reduction process could occur in real geologic samples, composite mineral surfaces of basalt rock chips containing resident communities of microbes were exposed to solutions of hexavalent chromium and toluene vapor. At the end of four months, SR-FTIR spectromicroscopy showed that Cr6+-tolerant and Cr6+-reducing natural microorganisms (left) were thriving in association with Cr3+ (right).
Research Conducted by H.-Y. N. Holman, D. L. Perry, G. M. Lamble, and
J. C. Hunter-Cevera (Berkeley Lab); and M. C. Martin and W. R. McKinney
(ALS), using Beamlines 1.4.3 and 10.3.2.
Funding: U. S. Department of Energy, Office of Health and Environmental
Sciences.
Publication about this experiment: H.-Y. N. Holman et al., Geomicrobiology J. 16(4), 307 (1999).
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