DEVELOPMENT OF PHOTOSYNTHETIC MICROBIAL FUEL CELLS FOR AZO DYE DEGRADATION

Azo dyes are xenobiotic, cytotoxic, and often mutagenic compounds commonly found in textile industry effluents. Their recalcitrant nature presents a major challenge to conventional wastewater treatment plants (WWTPs), which predominantly rely on the energy-intensive activated sludge process. This challenge is further compounded by the increasing global pressure to tighten effluent discharge regulations and reduce the carbon and energy footprint of wastewater treatment. Existing dye removal strategies such as adsorption, coagulation/flocculation, electrochemical oxidation, and anaerobic digestion often suffer from limitations including incomplete degradation, high operational costs, formation of toxic byproducts, or secondary pollution. In this study, photosynthetic microbial fuel cells (PMFCs) were investigated as a sustainable biological treatment strategy capable of degrading azo dyes while simultaneously generating bioelectricity. The model dye Acid Orange-7 (AO-7) was treated using Rhodopseudomonas palustris in a photosynthetic microbial fuel cell (PMFC). R. palustris was selected due to its metabolic versatility and potential for complete dye degradation. Results demonstrated complete decolourisation and irreversible degradation of AO-7 within 120 hours, achieving 100% dye removal efficiency. A maximum power density of 2.67 mW/m² was recorded, and partial degradation of aromatic amine byproducts was observed. Additionally, cytotoxicity assays indicated a substantial reduction in effluent toxicity post-treatment compared to the influent, highlighting the potential of PMFCs for simultaneous detoxification and energy recovery. To address the energy demands associated with conventional abiotic cathodes, alternative low-energy systems were explored. An H-type dual-chamber MFC employing Shewanella oneidensis as the anodic biocatalyst and Chlorella vulgaris as an in situ oxygen supplier in the cathode was evaluated. Algae-assisted MFCs exhibited enhanced AO-7 decolourisation efficiency of up to 87%, compared to 53% in abiotic cathode- MFC. Chemical oxygen demand (COD) removal reached significantly to 41% compared to 15% in the abiotic-cathode MFC. Although power output from algae-based cathodes was lower (2.75 mW/m² versus 6.92 mW/m²), the systems offered a more sustainable and cost-effective alternative by eliminating the need for external aeration. Further, a novel two-stage treatment strategy combining PMFCs with post-treatment aerobic systems was developed to achieve complete AO-7 degradation and detoxification. Two aerobic systems were tested: an oxygen-rich algal photobioreactor and an immobilised laccase bioreactor. Both were integrated with the PMFC acting as the initial reductive unit. In both setups, complete AO-7 decolourisation and degradation into non-toxic metabolites were achieved. Overall COD removal reached 74% in the PMFC-photobioreactor system and 68% in the PMFC-laccase bioreactor. Cytotoxicity assays revealed a 7-fold and 5-fold reduction, respectively, in final effluent toxicity compared to the primary influent. These findings demonstrate that MFC-based systems, particularly when coupled with biologically driven post-treatment steps, offer a viable and energy-efficient alternative to conventional WWTPs. The study highlights their potential for integrated dye degradation, detoxification, and renewable energy generation in the context of sustainable wastewater treatment.