Microbial reductive dehalogenation accelerated by multi-pathway electron transfer: The mediating role of novel multivalent iron-modified biochar redox mediators

The long-term accumulation of halogenated organic compounds (HOCs) can cause severe environmental harm, and while anaerobic digestion is widely used to treat such wastewater, its efficiency is limited by electron transfer rate. This study synthesized a novel multivalent iron-modified biochar redox mediator (Fe⁰/Feⁿ⁺-CRMs), which enhanced microorganism-pollutant electron transfer through multi-pathway mechanisms to promote halogenated organic pollutant degradation. Fe⁰/Feⁿ⁺-CRMs were successfully loaded with Fe₃O₄ and ZVI on their surface, demonstrating excellent electrochemical performance. The addition of Fe⁰/Feⁿ⁺-CRMs resulted in a 15–20% increase in the removal rate of HOCs and approximately a 10% rise in methane production compared to Feⁿ⁺-CRMs and Fe⁰-CRMs alone. Moreover, stable and highly efficient dehalogenation performance was maintained throughout the entire experimental period. Furthermore, Fe⁰/Feⁿ⁺-CRMs facilitated the enrichment of functional microorganisms (e.g. Georgenia). Research revealed that the electron transfer pathways of Fe⁰/Feⁿ⁺-CRMs primarily include: (ⅰ) the conductive role of biochar's graphitic structure; (ii) electron donation by ZVI as a potent reducing agent; (iii) Fe (II/III) redox cycling. This study fabricated Fe⁰/Feⁿ⁺-CRMs and proposed a multi-pathway electron transfer mechanism to accelerate microorganism-pollutant electron exchange, thereby alleviating the limiting effects of extracellular electron transfer on microbial anaerobic reductive dehalogenation efficiency, offering a viable strategy for enhancing halogenated organic pollutant removal in AD systems.