Decolourisation of Acid orange 7 in a microbial fuel cell with a laccase-based biocathode: Influence of mitigating pH changes in the cathode chamber

Biocathodes may be a suitable replacement of platinum in microbial fuel cells (MFCs) if the cost of MFCs is to be reduced. However, the use of enzymes as bio-cathodes is fraught with loss of activity as time progresses. A possible cause of this loss in activity might be pH increase in the cathode as pH gradients in MFCs are well known. This pH increase is however, accompanied by simultaneous increase in salinity; therefore salinity may be a confounding variable. This study investigated various ways of mitigating pH changes in the cathode of MFCs and their effect on laccase activity and decolourisation of a model azo dye Acid orange 7 in the anode chamber. Experiments were run with catholyte pH automatically controlled via feedback control or by using acetate buffers (pH 4.5) of various strength (100 mM and 200 mM), with CMI7000 as the cation exchange membrane. A comparison was also made between use of CMI7000 and Nafion 117 as the transport properties of cations for both membranes (hence their potential effects on pH changes in the cathode) are different. Results show that using Nafion 117 membrane limits salinity and pH changes in the cathode (100 mM acetate buffer as catholyte) leading to prolonged laccase activity and faster AO7 decolourisation compared to using CMI7000 as a membrane; similarly automatic pH control in the cathode chamber was found to be better than using 200 mM acetate buffer. It is suggested that while pH control in the cathode chamber is important, it does not guarantee sustained laccase activity; as salinity increases affect the activity and it could be mitigated using a cation selective membrane.