Efficacy of electrode position in microbial fuel cell for simultaneous Cr(VI) reduction and bioelectricity production

Microbial fuel cells (MFCs) that are bio-energy transducers capture bioelectricity produced from the oxidation of organic matter by using the electro-active bacteria grown on the biofilm attached on anode. Previous studies explored the effect of several limiting factors, such as electrode material, catalyst type, membrane structure, and electrolyte, on the electrochemical performance of MFCs. However, the effects of electrode position on Cr(VI) reduction and bioelectricity production remain unknown. In this study, MFCs with different electrode positions (i.e., 4 cm (MFC-4), 3 cm (MFC-3), 2 cm (MFC-2), and 1 cm (MFC-1)) were designed and fabricated to evaluate the overall performance of MFCs. The results of electrochemical analysis confirmed that MFC-2 exhibited low exchange transfer resistance (4.9 Ω) and strong conductivity, resulting in optimal electrochemical performance. In addition, Cr(VI) was completely removed within 11 h in MFC-2 with a large reduction rate of 0.91 g/m³·h. and COD removal efficiency of 78.25%. The overall performance of MFC-2 was comparatively higher than those of MFC-1 (0.80 g/m³·h and 68.82%), MFC-3 (0.64 g/m³·h and 61.67%), and MFC-4 (0.52 g/m³·h and 39.85%). Meanwhile, MFC-2 generated high open voltage (1.02 V) and power density (535.4 mW/m²), which are 1.4- and 3.1-fold larger than those of MFC-4 (0.72 V and 171.3 mW/m²). High COD removal and power density indicated the strong electrochemical activity of electroactive bacteria in the anode chamber of the MFCs, which was due to the low resistance in the MFCs could accelerate electron transfer and boost electrochemical reaction. Consequently, the optimal electrode spacing in MFCs was 2 cm. Further studies confirmed that Cr(VI) was removed and deposited in the form of Cr(III) on the electrode surface. High-throughput analysis suggested Pseudomonas species are the key electroactive bacteria for electricity generation.