Integrated electrochemical–biological treatment for efficient removal of metformin and its by-products: Optimization, mineralization, and toxicity assessment

Metformin is one of the most used pharmaceuticals, and its transformation products frequently appear in elevated concentrations within aquatic environments, where they may pose toxicity risks to non-target organisms. This study combines electrochemical and biological treatments to improve the removal of metformin and its associated by-products. The electrochemical process achieved 97.98% removal within 60 minutes, and the treated effluent was subsequently introduced into a biological system to target remaining intermediates, including dimethyl formamide, which was reduced by up to 95% within five days. Scavenger tests and photoluminescence analysis confirmed the central role of hydroxyl radicals in pollutant degradation. Enzyme activity measurements, including naphthalene dioxygenase and laccase, suggested their involvement in the biodegradation mechanism. Operational variables were optimized using Response Surface Methodology, and the energy consumption required for complete electrochemical decomposition of metformin was 1.33 kWh/m 3 . Following electrochemical treatment, TOC and COD removals were 48% and 55%, respectively, and these values increased to 90% and 95% after the combined process. Additional experiments assessed metformin removal in different water matrices and evaluated phytotoxicity to verify the effectiveness of the integrated system. Finally, the degradation pathway of metformin within the combined treatment system is presented in this integrated treatment study. • RSM-optimized electrochemical oxidation achieved 97.98% removal of metformin. • GC/MS confirmed dimethylformamide as a major by-product of the process. • Biological treatment with Enterobacter cloacae eliminated dimethylformamide. • Combined process increased COD and TOC removal from 55% and 48% to 95% and 90%. • Phytotoxicity tests verified improved environmental safety of the treated effluent.