Magnetic biochar-facilitated azo dye degradation in bioelectrochemical system: Mechanistic insights from carbon metabolism, electron transfer, and molecular docking

In this study, Fe-based magnetic biochar (FMB) derived from straw was synthesized and comprehensively investigated for its role in enhancing azo dye degradation in a bioelectrochemical system integrated with a membrane-aerated biofilm reactor. Experimental results revealed that FMB with a 30 % iron loading achieved the highest decolorization and mineralization efficiency of alizarin yellow R (AYR) by lowering the biofilm redox potential and charge transfer resistance. Characterization of FMB indicated that surface functional groups (e.g., C[dbnd]C, C[dbnd]O) and Fe₃O₄ particles enhanced electron transfer within the system. Moreover, extracellular polymeric substances (EPS) exhibited an increased content of redox-active components, and the α-helix content in proteins increased by 4.61 %, enhancing the electroactivity of the biofilm. Metatranscriptomic analysis indicated that FMB modulated carbon metabolism, the electron transport chain and heme biosynthesis to facilitate electron flow toward azoR, the gene encoding azo reductase, and upregulate its expression. Additionally, Aquaspirillum sp. LM1 and Paralcaligenes sp. KSB-10 were identified as the dominant azoR-expressing species in the cathodic and anodic biofilms, respectively. Molecular docking further demonstrated that FMB could spontaneously bind to azoR, reducing the binding free energy of the azoR-AYR complex and accelerating azo bond cleavage. This study provides insights for understanding the mechanisms of magnetic biochar in enhancing the treatment of azo dye wastewater.