Real-time characterization of biogeochemical reduction of Cr(VI) on basaltsurfaces by SR-FTIR imaging

Synchrotron radiation-based (SR) Fouwrier-transform infrared (FTIR) spectro microscopy in the mid-infrared region is a surface analytical technique tha t can provide direct insights into the localization and real-time mechanism s for the reduction of the (CrO4)(2)-chromate [Cr(VI)] species on surfaces of geologic materials. Time-resolved SR-FTIR spectra indicate that, in the presence of endoliths (mineral-inhabiting microorganisms), microbial reduct ion of Cr(VI) to Cr(III) compounds on basaltic mineral surfaces is the key mechanism of Cr(VI) transformation. It proceeds in at least a two-step reac tion with Cr(V) compounds as possible intermediate products, with the reduc tion of Cr(VI) increasing during the concomitant biodegradation of a dilute organic vapor (toluene). Analyses of spatially resolved SR-FTIR spectra sh ow that the maximum reduction of Cr(VI) to Cr(III) compounds occurs on surf aces densely populated by microorganisms. The oxidation state of Cr(III) co mpounds was confirmed by micro-x-ray absorption fine-structure spectroscopy . Both the time- and space-resolved SR-FTIR spectra show that in the absenc e of endoliths, Cr(VI) reduction is insignificant. With this effort, the po tential use of SR-FTIR spectromicroscopy in providing mechanistic informati on of reduction of Cr(VI) has been demonstrated. This method can now be exp anded to examine other IR-amenable microbial/chemical contaminant systems.