Evaluation of carbon-based materials in tubular biocathode microbial fuel cells in terms of hexavalent chromium reduction and electricity generation

Biocathode microbial fuel cells (MFCs) are of great potentials in bioremediation of Cr(VI)-contaminant sites due to their low operating cost, self-regenerating ability and sustainable power supply. The improvement of Cr(VI) reduction rates and power generation, however, remains to be a challenging project. In the present study, graphite fibers, graphite felt and graphite granules were evaluated as biocatalytic cathode materials in tubular MFCs in terms of Cr(VI) reduction and electricity generation. At cathode to anode surface area ratio (C/A) of 3, graphite fiber was found superior to graphite felt or graphite granule. Specific Cr(VI) reduction rates ranging from 12.4 to 20.6mgg-1VSSh-1and power generation from 6.8 to 15Wm-3(20-48Am-3) were achieved in the biocatalytic graphite fiber cathode MFCs. Under a temperature of 22°C and pH 7.0, Cr(VI) reduction followed pseudo-first-order kinetic model with the rate constant being 0.451±0.003h-1. In comparison with pH 7.0, an acidic pH of 5.0 improved Cr(VI) reduction of 27.3% and power generation of 61.8% whereas an alkaline pH of 8.0 decreased Cr(VI) reduction of 21.2% and power generation of 6.0%. It was found that the products formed whether as dissolved and/or precipitated Cr(III) was heavily pH dependent. Elevating temperature from 22 to 50°C increased Cr(VI) reduction with the apparent activation energy (Ea) obtained as 10.6kJmol-1. At high temperature of 50°C, however, a decreased power generation was observed mainly because of the increase in anode potential. These results indicate that an optimal condition exists for efficient biocathode MFCs with quick Cr(VI) reduction and simultaneous high power generation.