Effect of Microbially Induced Anoxia on Cr(VI) Mobility at a Site Contaminated with Hyperalkaline Residue from Chromite Ore Processing

This paper reports an investigation of microbially mediated Cr(VI) reduction in a hyper alkaline, chromium contaminated soil-water system representative of the conditions at a chromite ore processing residue (COPR) disposal site. Soil from the former surface layer that has been buried beneath a COPR tip for over 100 years was shown to have an active microbial population despite the pH value of 10.5. This microbial population was able to reduce nitrate using an electron donor(s) that was probably derived from the soil organic matter. With the addition of acetate, nitrate reduction was followed in turn by removal of aqueous Cr(VI) from solution, and then iron reduction. Removal of ~300uM aqueous Cr(VI) from solution was microbially mediated, probably by reductive precipitation, and occuredoccurs over a few months. Thus, in soil that has had time to acclimatize to the prevailing pH value and Cr(VI) concentration, microbially mediated Cr(VI) reduction can be stimulated at a pH value of 10.5 on a time scale compatible with engineering intervention at COPR contaminated sites

Biogeochemical reduction processes in a hyper-alkaline leachate affected soil profile

Hyperalkaline surface environments can occur naturally or because of contamination by hydroxide-rich wastes. The high pH produced in these areas has the potential to lead to highly specialised microbial communities and unusual biogeochemical processes. This paper reports an investigation into the geochemical processes that are occurring in a buried, saturated, organic–rich soil layer at pH 12.3. The soil has been trapped beneath calcite precipitate (tufa) that is accumulating where highly alkaline leachate from a lime kiln waste tip is emerging to atmosphere. A population of anaerobic alkaliphilic bacteria dominated by a single, unidentified species within the Comamonadaceae family of β-proteobacteria has established itself near the top of the soil layer. This bacterial population appears to be capable of nitrate reduction using electron donors derived from the soil organic matter. Below the zone of nitrate reduction a significant proportion of the 0.5N HCl extractable iron (a proxy for microbial available iron) is in the Fe(II) oxidation state indicating there is increasing anoxia with depth and suggesting that microbial iron reduction is occurring

Chromate reduction in highly alkaline groundwater by zerovalent iron: Implications for its use in a permeable reactive barrier

It is not currently known if the widely used reaction of zerovalent iron (ZVI) and Cr(VI) can be used in a permeable reactive barrier (PRB) to immobilize Cr leaching from hyperalkaline chromite ore processing residue (COPR). This study compares Cr(VI) removal from COPR leachate and chromate solution by ZVI at high pH. Cr(VI) removal occurs more rapidly from the chromate solution than from COPR leachate. The reaction is first order with respect to both [Cr(VI)] and the iron surface area, but iron surface reactivity is lost to the reaction. Buffering pH downward produces little change in the removal rate or the specific capacity of iron until acidic conditions are reached. SEM and XPS analyses confirm that reaction products accumulate on the iron surface in both liquors, but that other surface precipitates also form in COPR leachate. Leachate from highly alkaline COPR contains Ca, Si, and Al that precipitate on the iron surface and significantly reduce the specific capacity of iron to reduce Cr(VI). This study suggests that, although Cr(VI) reduction by ZVI will occur at hyperalkaline pH, other solutes present in COPR leachate will limit the design life of a PRB