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 specialized microbial communities and unusual biogeochemical processes. This article 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. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental files.
This paper reports an investigation of microbially mediated Cr(VI) reduction in a hyperalkaline, chromium-contaminated soil-water system representative of the conditions at a chromite ore processing residue (COPR) 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 a 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 ∼300 μM aqueous Cr(VI) from solution was microbially mediated, probably by reductive precipitation, and occured 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 of 10.5 on a time scale compatible with engineering intervention at COPR-contaminated sites.
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