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Bioimmobilization of Cr in Contaminated Aquifers

Source: Harry Beller, Dan Hawkes

Harry Beller (top) and Peter Nico (bottom)

In studying reductive chromium immobilization, ESD Bioenergy Program Head Harry Beller, ESD Geochemistry Department Head Peter Nico, and their colleagues found that flow-through columns constructed with homogenized aquifer sediment—and continuously infused with lactate, chromate, and various native electron acceptors—diverged to have very different Cr(VI)-reducing biogeochemical regimes, characterized by either denitrifying or fermentative conditions (as indicated by effluent chemical data, 16S rRNA pyrotag data, and metatranscriptome data). Despite the two different biogeochemical environments that evolved in the columns, these regimes created similar Cr(III)–Fe(III) hydroxide precipitates as the predominant Cr(VI) reduction product, as characterized by micro-X-ray fluorescence and micro-X-ray absorption near-edge structure analysis.

In a recently published article, Beller et al. (2014) discuss two conflicting scenarios of microbially mediated formation of Cr(III)–Fe(III) precipitates, each of which is both supported and contradicted by different lines of evidence: (1) enzymatic reduction of Cr(VI) to Cr(III) followed by coprecipitation of Cr(III) and Fe(III) and (2) both regimes generated at least small amounts of Fe(II), which abiotically reduced Cr(VI) to form a Cr–Fe precipitate. Evidence of zones with different levels of Cr(VI) reduction suggest that local heterogeneity may have confounded interpretation of processes based on bulk measurements. This study indicates that the bulk redox status and biogeochemical regime, as categorized by the dominant electron-accepting process, do not necessarily control the final product of Cr(VI) reduction.

Figure 6. XRF maps of typical Cr “hotspots” from a fermentative (A) and denitrifying (B) column. Traces below images show summed detector counts for the indicated shaded areas for Cr (green) and Fe (red). Pixel size ∼1 μm (A) and ∼1.25 μm (B).

Along with Beller and Nico, the team included ESD scientists Li Yang, Charu Varadharajan, Ruyang Han, Hsiao Chien Lim, Ulas Karaoz, Sergi Molins, Eoin Brodie, and Carl Steefel.

To read further, go to:

Citation: Beller, H., L. Yang, C. Varadharajan, R. Han, H.C. Lim, U. Karaoz, S. Molins, M. Marcus, E. Brodie, C. Steefel, and P. Nico (2014), Divergent aquifer biogeochemical systems converge on similar and unexpected Cr(VI) reduction products. Environmental Science & Technology, 48 (18), 10669–10706; DOI: 10.1021/es5016982.

Funding Source: BER, SBR SFA