Bioremediation of chromate in alkaline sediment-water systems

The poorly controlled disposal of chromium ore processing residue (COPR) is a globally widespread problem due to its potential to form chromium contaminated hyperalkaline (pH > 12) leachates. These highly oxidising leachates typically contain chromium in the Cr(VI) oxidation state as its chromate anion (CrO42-). This anion is highly mobile, toxic, carcinogenic, and exhibits a high degree of bioavailability. Under reducing conditions chromium exists in the non-toxic and poorly soluble Cr(III) oxidation state. Thus, the reduction of Cr(VI) to Cr(III) is often the goal of remediative strategies. In anaerobic subsurface environments where reducing conditions are established by the indigenous microbial population, chromium reduction can occur naturally. The microbial transformation of Cr(VI) to Cr(III) can be both a result of its direct use in microbial metabolism, or through its indirect reaction with microbially produced reduced species, e.g. Fe(II). This study has used a multidisciplinary approach to investigate the biogeochemical influences on the fate and stability of Cr(VI) leaching from a site of COPR in the north of England. Reducing sediments encountered directly beneath the COPR waste were found contain elevated concentrations of chromium. These sediments were shown to be able to remove aqueous Cr(VI) from solution when incubated with contaminated site groundwater in microcosm incubation experiments. This removal is likely a result of the abiotic reduction by soil associated microbially produced Fe(II), followed by precipitation as insoluble Cr(III) hydroxides. X-ray absorption spectroscopy (XAS) and electron microscopy confirms the association of chromium as Cr(III) with iron in these soils, hosted as a mixed Cr(III)-Fe(III) oxyhydroxide phase. Upon air oxidation, only minor amounts of chromium was remobilised from these sediments as Cr(VI). A diverse population of alkaliphilic microorganisms are indigenous to this horizon, capable of successful metabolism despite elevated pH values. This population was found to contain a consortium of microorganisms capable of iron reduction when incubated at pH 9 to 9.5. Microbial community analysis found taxonomic similarity to several known metal reducing alkaliphiles from the phylum Firmicutes. These results suggest that the novel action of iron reducing alkaliphiles indigenous to reducing sediments beneath COPR sites may provide zones of natural chromium attenuation via microbially mediated mechanisms of Cr(VI) transformation.