Modulation of catholyte ionic strength for enhanced bioelectricity generation and bioremediation of tannery wastewater in photosynthetic microbial fuel cells

Abstract

Photosynthetic microbial fuel cells (PMFCs) represent an emerging class of bio-electrochemical systems capable of converting organic pollutants into electricity through microbial catalysis. Catholyte ionic strength is a principal determinant of PMFC performance, governing electrolyte conductivity, internal resistance, and microbial metabolic activity. To investigate this relationship systematically, four catholyte solutions were evaluated: ferric sulfate (Fe2(SO4)3), potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7), and distilled water (DW). The constant anolyte comprising tannery wastewater, Spirulina A. platensis, and a bacterial inoculum was maintained across all configurations. Biological baseline controls using Spirulina, bacterial inoculum, and raw wastewater individually confirmed the synergistic contribution of the algal-bacterial consortium to electricity generation. The highest electrochemical performance was attained by MFC-5 under illuminated conditions with KMnO4 as the catholyte, producing an open-circuit voltage of 981.00 ± 1.00 mV, a power density of 233.81 ± 1.87 mW m−2, a coulombic efficiency of 81.92%, and a 51.60% reduction in biochemical oxygen demand. MFC-5 also achieved the highest removal of heavy metals, including chromium, cobalt, cadmium, copper, manganese, and iron, together with near-complete removal of bromide, nitrate, sulfate, and phosphate, reaching up to 99.97% for phosphate. X-ray diffraction and Fourier-transform infrared analyses of the recovered electrode deposits further identified quartz and silica, potassium–manganese oxide phases, and their reduction products, supporting the proposed pollutant removal mechanisms. These findings demonstrate how catholyte ionic strength governs PMFC performance and highlight the technology's promise for combined bioenergy recovery and sustainable industrial wastewater treatment.