Hexavalent chromium (Cr(VI)) is a highly toxic and soluble substance present in a wide range of industrial effluents. An effective treatment method is the biochemical Cr(VI) reduction to the less toxic trivalent chromium (Cr(III)), and such a transformation has recently been demonstrated in bioelectrochemical systems. However, depending on the pH of the catholyte, a biocatalyst might be required in the cathode and also the process can be very much inhibited by Cr(III) products which tend to form on the electrode surface and deactivate it. Herein is demonstrated how an electrophilic bacterium, that is Shewanella oneidensis MR-1, can be used as a bacterial catalyst in Cr(VI) reducing cathodes of bioelectrochemical systems. Starting with potentiostatically controlled experiments (-500 mV vs. Ag/AgCl for 4 h), the abiotic cathode’s (AC) performance was shown to be affected by the Cr(VI)-reduction products that are deactivating the cathode and are severely inhibiting further Cr(VI) reduction. The presence of metal chelators like lactate delayed this deactivating effect and enhanced the system’s performance to a large extent; in the presence of 30 mM lactate, the AC delivered approximately 3 times more electrons to Cr(VI). In addition, approximately 15 times more electrons were delivered when 5 μM of the electron shuttle riboflavin was also added in the AC. However, the presence of riboflavin did not have any effect in the absence of lactate. The MR-1 biocathode also exhibited an enhanced current production and Cr(VI) reduction, though the pre-treatment conditions were found to be important. When pre-treated in an aerated chamber with a poised electrode at +300 mV vs. Ag/AgCl, the MR-1 biocathode mediated 70% more electrons than the AC with 30 mM lactate, and only 39% more electrons when the electrode was initially poised at -500 mV vs. Ag/AgCl. Cr(VI) reduction was also enhanced, with a decrease in concentration over the 4 h operating period of 9 mg L-1 Cr(VI) in the aerobically pretreated MR-1 chamber, compared to only 1 and 3 mg L-1 in the AC without lactate and in the AC with 30 mM lactate respectively. On the other hand, when pre-treated anaerobically in the presence or absence of Cr(VI), the performance of the MR-1 biocathode was not much different than that of the AC with 30 mM lactate. The positive effect of lactate was further demonstrated in microbial fuel cell (MFC) cathodes, where maximum power densities produced were up to 44 times the power densities reported elsewhere for abiotic cathodes (8.8 mW m-2 vs. 0.2 mW m-2 at pH 7) and at similar levels to the power densities of biotic cathodes at pH 6 and 7. Considerable Cr(VI) reduction was also observed at alkaline pH abiotic cathodes and maximum power densities were 31 times the ones reported elsewhere for biotic cathodes at pH 8 (21.4 mW m-2 vs. 0.7 mW m-2). In MFCs, the presence of MR-1 enhanced the performance of pH 7 cathodes; in the presence of 30 mM lactate, the MR-1 biocathode bioelectrochemically reduced 3 times the amount of Cr(VI) reduced by the AC with the same amount of lactate. Compared to the results in the absence of an electrode, the MFC results suggested that different Cr(VI) reduction pathways could be utilised by MR-1 when the electron donor is in the poised electrode form rather than in the lactate form. In conclusion, effective and continuous Cr(VI) reduction with associated current production were achieved when MR-1 and lactate were both present in the biocathodes.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:595583 |
| Date | January 2014 |
| Creators | Xafenias, Nikolaos |
| Contributors | Banks, Charles |
| Publisher | University of Southampton |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://eprints.soton.ac.uk/363130/ |
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