External resistance as a potential tool for influencing azo dye reductive decolourisation kinetics in microbial fuel cells

Azo moieties of polar and highly charged azo dyes are thought to undergo reductive degradation into constituent amines in the extracellular milieu of bacterial cells due to their inability to penetrate into cellular interior. In this study, experiments were conducted in microbial fuel cells (MFCs) to investigate the influence of applied external resistance (R-ext) on the reductive decolourisation kinetics of three structurally different commercial azo dyes. The results indicate that at very high current densities (low R-ext) and very low current densities (high R-ext), the reductive decolourisation kinetic constants were lower for all three tested azo dyes in comparison to a moderate optimum R-ext (2.2 k Omega), close to the internal resistance of the MFC systems. PCR-DGGE of the 16s rRNA gene microbial community fingerprints were distinctly different between experiments that utilised different R-exts. Molecular phylogenetic microbial profiling indicated that the microbial communities selected at different R-exts were distinctly different. The enhanced decolourisation kinetic constants and good effluent quality in terms of chemical oxygen demand (COD) reduction obtained using a moderate R-ext indicates that R-ext can be used as a potential tool for influencing azo dye reductive degradation in MFCs. (C) 2013 Elsevier Ltd. All rights reserved.