Microbial fuel cells (MFCs) hold great promise for simultaneous wastewater treatment and electricity production. However, their performance is currently hampered by several challenges including high cathodic potential losses and cost of platinum (and other materials) used as a catalyst for the oxygen reduction reaction. These challenges could be overcome by using biological catalysts (oxidoreductase enzymes e.g. laccase & microbes). This work investigated the treatment of azo dye (Acid orange 7, AO7) in MFC utilising biological catalysts as replacements for platinum.
Various ways of mitigating pH changes in the cathode of MFCs and its effect on laccase activity were studied initially. Use of Nafion 117 membrane limited salinity and pH changes in the cathode (0.1 M acetate buffer) leading to prolonged laccase activity and faster anodic dye decolourization compared to using CMI7000 membrane; similarly, automatic pH control was better than using a higher acetate buffer strength (0.2 M). To improve robustness and shelf life, laccase was immobilized by different approaches using polyaniline (PANI), copper alginate beads and Nafion polymer. PANI-laccase showed the highest activity, producing a power density of 38.2±1.7 mW m-2 compared to 28±1 mW m-2 freely suspended enzyme. There was 81% activity retained after 1 cycle (5 days) for PANI laccase compared to 23.8% for freely suspended laccase. The cathodic dye decolourization was over 85% for freely suspended laccase, 81% for Cu-alginate systems,76% for PANI, and 73% for Nafion-immobilized laccase.
The efficiency and mechanism of dye degradation were compared between feeding the dye in the anode (S. oneidensis) and the cathode chamber (laccase). Power density and decolourization rate were better when dye was fed in the cathode (> 80% decolourization in 24 h, Pmax 50±4 mW m-2) compared to anode (20% decolourization in 24 h, Pmax 42.5±2.6 mW m-2). GC-MS analysis revealed benzoic acid and hexanoic acid for laccase degradation products, whereas S. oneidensis produced colourless unstable aromatic amines that underwent auto-oxidation to produce colour. Further, to improve the efficiency of laccase, two natural redox mediators (syringaldehyde (Syr), acetosyringone (As)) and artificial mediator ABTS were used in the cathode. The presence of ABTS and As increased power density from 54.7±3.5 mW m-2 to 77.2±4.2 mW m-2 and 62.5±3.7 mW m-2 respectively. The power decreased to 23.2±2.1 mW m-2 for laccase with syr. There was increase in decolourization by 20% with addition of mediators. Thus, the natural mediator As improved dye decolourization and power.
To develop an efficient microbial biocathode, activated sludge from textile treatment plant was enriched by potentiostatic method. The biofilm produced a power density of 64.6±3.5 mW m-2 compared to platinum (Pt) 72.7±1.2 mW m-2 in a MFC. The rate of dye decolourization at the anode was similar in both Pt and biocathode MFCs. The microbial community analysis revealed a selection of chemolithoautotrophic organisms that fix CO2 for their metabolism. Altogether, these results suggest that laccase and microbial biocathode have the potential to be an excellent catalyst for ORR in MFC with efficiency equivalent to Pt.