Role played by the physical structure of carbon anode materials in MFC biosensor for BOD measurement

Microbial fuel cell (MFC) has been extensively studied as a biosensor for determining biochemical oxygen demand (BOD). The method for quantifying BOD by employing coulombic yield (Q) of a bio-electrochemical degradation process obtained from MFC biosensors is referred to as BOD_Q. The physical structures of anode materials greatly affect the sensitivity and accuracy of the biosensor. In this work, the effects of carbon cloth (CC) and carbon felt (CF) as anode substrate materials on the BOD_Q determination efficiencies were studied. The CF-MFC biosensor showed higher BOD_Q response than that of the CC-MFC within 25-400 mg L^-1 BOD concentration range, and the test value was very close to the theoretical BOD. The difference is resulting from higher coulombic efficiency (CE) of CF-MFC (64.89-65.38 %) than CC-MFC (55.58-63.51 %). It should be noted that for water samples with low BOD concentrations the physical structures of anode materials play a leading role in CE. For synthetic wastewaters with 25 mg L^-1 BOD, the CE of CF-MFC (65.38 %) was 17.63 % higher than that of CC-MFC (55.58 %). In contrast to the densely woven CC coated with thick biofilm, CF with loose carbon fiber and thin biofilm makes it good for organic diffusion and electron transportation, thus contributing to higher and more stable CE. These results indicate that the CF-MFC is more suitable for determining BOD_Q values over a wide concentration range. This work provides a useful strategy for selecting desirable MFC's anode material as the BOD biosensor. MFC biosensors with high-porosity biological anodes can obtain more accurate BOD test values.