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

• Rext influences reductive decolourisation kinetics of azo dyes in MFCs.
• Rext influenced power production in MFCs.
• Using an optimum Rext has the potential to enhance azo dye degradation in MFCs.
• Distinctly different microbial communities developed under different Rexts.

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 (Rext) on the reductive decolourisation kinetics of three structurally different commercial azo dyes. The results indicate that at very high current densities (low Rext) and very low current densities (high Rext), the reductive decolourisation kinetic constants were lower for all three tested azo dyes in comparison to a moderate optimum Rext (2.2 kΩ), 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 Rexts. Molecular phylogenetic microbial profiling indicated that the microbial communities selected at different Rexts were distinctly different. The enhanced decolourisation kinetic constants and good effluent quality in terms of chemical oxygen demand (COD) reduction obtained using a moderate Rext indicates that Rext can be used as a potential tool for influencing azo dye reductive degradation in MFCs.