Chromite ore processing residue (COPR), a waste product from chromium extraction, is highly alkaline and is toxic due to the soluble Cr(VI) it contains. Unregulated dumping during the previous century led to many sites being
contaminated with this waste. As Cr(VI) migrates in groundwater flows, reduction to less toxic and insoluble Cr(III) seems a prudent remediation strategy. Zero valent iron permeable reactive barriers (ZVIPRBs) and
biobarriers are both capable of in-situ reduction reactions and were thus investigated for their suitability to treat COPR impacted sites.
Reduction of Cr(VI) by ZVI was possible at pH 12 however the rate was slow and unsustained. Amending pH resulted in increased reaction rates only in the acidic range (<pH 4). Comparative experiments showed that reduction from simple Cr(VI) solutions occurred quicker and for longer than from COPR leachate. Investigative surface analysis of ZVI exposed to both liquors revealed that whilst each sample harboured Cr(III), the iron exposed to COPR leachate had many other solutes precipitated on the surface (chiefly
Ca and Si). These precipitates were blocking potential reaction sites, leading to premature inhibition of the iron. Thus whilst ZVI will reduce Cr(VI) at high
pH, inhibition reactions may make a ZVIPRB prohibitively expensive.
A community of alkaliphilic Fe(III)-reducing bacteria (principally Tissierella, Clostridium XI and Alkaliphilus sp.) have been isolated from highly alkaline soil found beneath a 19th century COPR site. This community was able to
reduce soluble Fe(III) into Fe(II) in anaerobic alkaline media – an important process as Fe(II) can reduce Cr(VI) abiotically. Bacterial growth occurred contemporaneously with the accumulation of riboflavin, a soluble electron
shuttle. Bacteria grown in media spiked with riboflavin reduced Fe(II) at a faster rate and with less lag. Due to the link between iron reduction and flavin concentration, it is likely that riboflavin has an import role in
extracellular metal reduction by this alkaliphilic community.
In Fe(III) growth media spiked with chromate, Cr(VI) reduction was recorded in media where the initial chromate concentration was up to 8500μmol.L-1. Population data shows that as Cr(VI) is introduced into the growth media the
diversity of the community is reduced as the Tissierella, sp. appear to have a low tolerance for Cr(VI). Cr(VI) reduction was also observed in media with Cr(VI) as the sole electron donor, although growth in this media was weak.The only species to be able to use Cr(VI) as the sole electron donor were Clostridium XI. Both the original Fe(III)-reducing consortium, and the Cr(VI)-reducing Clostridium XI sp. were also able to reduce solid phase Fe(III) contained within aquifer material from the site from which they were isolated. Growth of the Fe(III)-reducing consortium on solid phase iron favoured a single species of Tissierella which released riboflavin; probably because this
electron shuttling compound can mediate reduction of the extracellular solid phase iron.
Results from this study indicate that biobarriers hold significant promise for the successful treatment of COPR impacted sites. It shows that bioreduction of Cr(VI) can occur as a direct enzymatically mediated process, but this
process may not support long term growth. Cr(VI) reduction also occurs as part of Fe(III)/Fe(II) cycling that does support long term growth, even in the presence of high Cr(VI) concentrations. Therefore, it is likely that the indirect mechanism involving microbial Fe(III) reduction will be the more important mechanism for reducing Cr(VI) mobility a at COPR impacted sites.
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