Electricity Generation And Application Of Electroactive Biofilms In The Bioelectrochemical Systems

Electroactive biofilms (EAB) can be defined as a type of microbial biofilms that exchange electrons with their conductive solid substratum through oxidation and/or reduction reactions. In particular, EAB can be exploited as bioelectrocatalysts in microbial bioelectrochemical systems (BES) such as microbial fuel cells (MFC) and microbial electrolyte cells (MEC), which has currently attracted growing interest and brought promising opportunities for energy recovery during wastewater treatment, pollution degradation and environmental monitoring.To effectively apply BES in practice, challenges including low power output and high cost have to be solved first. The electron transfer activity of EAB to the solid electrode became the limiting factor of a higher power output BES. Most of the electrode materials used in MFC are commercially available carbon cloth, carbon felt and graphite plates for anodes and Pt/C for cathodes, which are cost-prohibitive at a large scale. However, biochar has been found to be a promising inexpensive alternative material for the electrodes of BES. Therefore, all these mentioned above urged the investigation of different BES with EAB to solve the problem. The main conclusions are as follows:1. Air-cathode single-chamber BESs were fabricated with Pt/C catalyst cathodes and carbon cloth anodes. Several experimental approaches, including cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron microscopy, and confocal laser scanning microscopy, have been used to characterize the effects of Ca2+on the structural and electron transfer properties of mixed culture EAB grown in BES. As revealed by electrochemical measurements, a lower current is obtained from the EAB formed in the presence of higher Ca2+concentrations. The decline of the current is due to the dominance of non-active cells and non-exoelectrogens and to the relatively high extracellular polymer substances in the biofilms formed at high Ca+ concentrations. While the increase of the current is due to the dominance of active cells and exoelectrogens and to the relatively low charge transfer resistance in the biofilms formed at low Ca2+ concentrations.2. In this study, we proposed a new application of the sewage sludge (SS)-derived biochar as advanced bifunctional electrode material (anode and cathode) in a microbial fuel cell (MFC). To function as an anode, the SS amended with various amounts of coconut shell was pressed into a mold and then converted into SS-derived carbon monoliths (SMs) by heat treatment. Meanwhile, powdered SMs (PSMs) were used as the catalysts for oxygen reduction in the cathodes of the MFC. The maximum power density of 969 ± 28 mW/m2 was achieved from the MFC with a SM anode and a PSM cathode, which was ca.2.4 times that of the MFC with a graphite anode and a Pt cathode. The enhanced electrical conductivity of the SMs caused by amending the coconut shell resulted in the enrichment of exoelectrogens and the decrease in electron transfer resistance. This study demonstrated that the SS-derived bifunctional materials were suitable for use as high-performance electrodes for the electrical power generation of air-cathode MFC.3. The MFC with a photosynthetic algae cathode is constructed, which is maintained by self-capturing CO2 released from the anode and utilizing solar energy as energy input. With this system, a maximum power density of 187 mW/m2 is generated when the anode off gas is piped into the catholyte under light illumination, which is higher than that of 21 mW/m2 in the dark, demonstrating the vital contribution of the algal photosynthesis. However, an unexpected maximum power density of 146 mW/m2 is achieved when the anode off gas is not piped into the catholyte. Measurements of cathodic microenvironments reveal that algal photosynthesis still takes place for oxygen production under this condition, suggesting the occurrence of CO2 crossover from anode to cathode through the Nafion membrane.4. Electrochemistry (electro-assisted) is introduced to the EAB reduction of pentachlorophenol (PCP). The PCP degradation efficiency of electro-assisted system is compared, which is driven by potentiostat and solar cell under different potential. When initial potential is set as-0.45 V vs. SCE, PCP degradation efficiency of electro-assisted system driven by potentiostat and solar cell is 97.1% and 78%, respectively. After many cycles of PCP degradation, the morphologies of EAB compared with the control were not changed with SEM scans. It is shown that the morphologies of EAB could not be affected by PCP toxicity when the EAB formation is stable. Further investigation by GC-MS analysis indicated that the intermediate products were degraded totally in 24 h, which lead to the detection of PCP and phenol derivatives.