Effect of set potential on hexavalent chromium reduction and electricity generation from biocathode microbial fuel cells

  • Huang L
  • Chai X
  • Chen G
 et al. 
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A microbial fuel cell (MFC) is a device that extracts energy from wastes and wastewaters through biocatalytic reactions. 1 Biocathode MFCs, which utilize electrochemically active micro-organisms on both the cathode and anode as catalysts, have attracted much attention and are holding great promise for bioremediation and waste treatment as they are self-regenerating and sustainable. 2À4 Aerobic biocathode MFCs have been inves-tigated to avoid the need for a metal catalyst on the cathode, 5À9 but other reactions at the cathode can accomplish reductive removal of contaminants or the production of value-added products. The range of biocathode applications has therefore expanded to include reduction of pollutants such as NO 3 À , chloroethenes, 2-chlorophenol, ClO 4 À , U(VI), Cr(VI) 10À16 and CO 2 , 17 and reduction for the value-added product of methane. 18,19 Improvements in the performance of anaerobic biocathode systems are needed to increase the current densities and rates of chemical reduction in these systems. Setting electrode potentials is a useful approach for controlling the performance of a bioelectrochemical system. 20 Either elec-trode potential can be set in an MFC or a microbial electrolysis cell (MEC). The main difference is that the overall reaction is not spontaneous in an MEC without this energy input. Only one of the MFC electrode potentials can be controlled, and thus the potential of the other electrode can vary to maintain current required for the reaction at the set electrode potential. The system being examined here is an MFC because the current flow is spontaneous in the absence of the added energy. Setting the anode potential has been shown to promote enrichment of the biofilm and reduce the startup time of the system, and to increase the subsequent power density due to better acclimation of the exoelectrogenic bacteria. 20À25 Relatively fewer studies have been conducted with set cathode potentials, 5,8 and bacterial growth on the cathode is not yet well studied. 4 Optimized biocathode potentials of 242 mV and 345 mV have been shown to reduce the time for startup and enhance the performance of aerobic biocathode, 5,8 but there are no studies on set cathode potentials in the absence of oxygen. Setting the biocathode potential should improve system performance for anaerobic cathodes in a way similar to that achieved for bioanodes. Cr(VI) is a priority toxic chemical present in wastewaters from electroplating, pigment, and lumber and wood product processes. Anaerobic biocathodes MFCs have been used to achieve reduction of Cr(VI) to much less toxic Cr(III) with simultaneous electricity generation, providing a promising application for the reductive treatment of oxidized Cr(VI) to precipitated Cr(OH) 3 . 15,26 Bio-cathode performance in terms of startup time, Cr(VI) reduction rate as well as electricity production need to be improved. In this study, we investigated the effect of different set biocathode potentials on anaerobic Cr(VI) reduction. Several different poten-tials (À450, À300, À150 and þ200 mV vs a standard hydrogen

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