Higher performances of open vs. closed circuit microbial fuel cell sensor for nitrate monitoring in water

Microbial fuel cell (MFC) sensor exhibits attractive prospects for real-time monitoring of nitrate (NO3-) in water, with unique advantages of self-powering and simple structure; but its stability and sensitivity are unsatisfactory. In an attempt to address the limitations, here we employed an open-circuit MFC (O-MFC) sensor for real-time monitoring of NO3- in water, compared to a conventional closed-circuit MFC (C-MFC) sensor. The stability of the O-MFC sensor was confirmed when subject to the continuous fluctuation of interfering substances including organic matter (acetate, 2-20 mM), SO42- (250-400 mg/L) and Fe3+ (0.3-0.6 mg/L). For one-time and continuous monitoring of NO3- at a concentration of 1-40 mg/L, the O-MFC sensor both achieved higher sensitivity than that of the C-MFC sensor; especially at a low NO3- concentration of 1 mg/L, the changes of electrical signal (ΔE) reached 10.0 ± 1.00 mV and 7.0 ± 0.85 mV respectively. In contrast, the C-MFC sensor failed to monitor at low NO3- concentrations of 1 and 5 mg/L. Moreover, good linear correlation between ΔE and NO3- concentration (R2 = 0.907 and 0.981) was obtained from the O-MFC sensor. However, owing to the highly activity of EAB that mainly grows in the outer layer of biofilm, the response time of the O-MFC sensor is too long. Microbial community analysis further revealed that the open-circuit condition is more suitable for NO3- monitoring in water, providing a new way to improve the performance of MFC sensors.