We are studying the biogeochemistry of anaerobic flow-through columns containing Hanford 100H aquifer sediment and exposed to synthetic groundwater containing chromate, lactate, and site-relevant electron acceptors (nitrate, sulfate).  All columns contained native, bioreducible Fe(III) phases.  Substantial Cr(VI) reduction occurred in nitrate-amended (denitrifying) columns and in sulfate-amended columns in which fermentative conditions prevailed.  Despite the very different biogeochemical regimes in the denitrifying and fermentative columns, spectroscopic studies of both kinds of columns revealed very similar Cr(III)-containing minerals, approximating Fe0.75Cr0.25(OH)3.  Formation of these minerals is consistent with bacterial Fe(III) reduction and subsequent abiotic reduction of Cr(VI) by biogenic Fe(II).  In contrast, other evidence supports enzymatic Cr(VI) reduction: (1) both denitrifying and fermentative Hanford bacterial isolates catalyze rapid Cr(VI) reduction in iron-free systems, (2) strong correlations between nitrate and chromate reduction suggest that Cr(VI) reduction is co-metabolic with denitrification, (3) metatranscriptomic data clearly indicate active denitrification, which is thermodynamically inconsistent with Fe(III) reduction.

Competing evidence for enzymatic versus abiotic reduction of Cr(VI)   in flow-through columns containing Hanford 100H aquifer sediment

243rd National Meeting (2012)

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