Probing mechanisms for microbial extracellular electron transfer (EET) using electrochemical and microscopic characterisations

Microbial fuel cell (MFC) is a bioreactor and a power generator that directly converts chemical energy (i.e. organic-inorganic matters in wastewater) to electrical energy through the metabolic activity of the microorganisms. This not only helps cutting the energy costs but also accomplishes wastewater treatment simultaneously. A MFC system has a high efficiency in wastewater treatment; still, an amount of power generated is extremely low and insufficient to power even a pumping unit in the system. One of the key factors to the power generation is the electrochemically active microorganisms that are effective in electron transfer to a solid anode. However, the exact mechanisms by which electrons are transferred between the electrode surface and the microbial metabolism still remain unclear. This work, therefore, aimed to experimentally define the possible mechanism(s) of microbial extracellular electron transfer (EET) that governs the anodic reaction efficiency through the simultaneous investigations of cell voltage, power generation and %COD removal at different inoculation/immobilisation time (t). Ex situ microbial inoculation was initially performed to investigate the microbial growth rate and growth behaviour under simulated MFC conditions using mixed microbial cultures, obtained from an up-flow anaerobic sludge blanket at Cho-heng rice vermicelli industry, Thailand. This preliminary study directed towards the optimal conditions for in situ microbial inoculation/immobilisation in a mediator- and membrane-less MFC reactor built-in house. The effect of each EET mechanism on the production of electric current was assessed at different growth states by electrochemical characterisations (including cyclic voltammetry and electrochemical impedance spectroscopy (EIS)). The cyclic voltammograms of the microbe suspension with or without the presence of soluble electron shuttle confirmed that the microbes themselves were relatively inactive to respire solid electron acceptors through the direct electron transfer (DET) via the OM cytochromes but required exogenous/endogenous mediators to carry electrons from the microbes by diffusive transport to the electrode surface. That was an explanation why the microbe-free medium exhibited superior electrochemical activity in comparison with other microbial components (including medium-free inoculum and inoculated SS mesh electrode) for the first 80 h of inoculation. The peak current was increased with the presence of extracellular biofilm matrix on the electrode surface at longer t (i.e. t = 7 and 14 days) when the extent of biofilm formation on electrode surface was found to increase in the microscopic level as visualised by environmental scanning electron microscopy (ESEM). The EIS results also suggested that the charge transfer resistance between the electrode surface and microbial metabolisms decreased greatly with the extent of biofilm coverage. Hence, the corresponding MFC performance including maximum power density and %COD removal was maximised to 465 mW cm−3 and 100% respectively at t = 7 days once the EET mechanism was mainly driven by DET via the extracellular biofilm matrix corresponding with the highest current response of inoculated SS electrode in the LSCV measurement. However, the depletion of fresh nutrient under a closed-loop and continuous MFC operation resulted in the reduction of power output dramatically to 186 mW cm−3 at the end of experimental period (t = 12 days). The MFC performance could be maintained at its maximum values under realistic MFC operation with an opened-loop and continuous feeding mode which would be contributed to the development of an economical and commercially viable MFC in the future.