Biosynthetic composite iron minerals enhance electroactive biofilm activity and stability by reducing charge transfer resistance and buffering the internal acid inhibition

Electroactive biofilms (EAB) enable organic degradation and energy recovery in microbial electrochemical systems (MES). Nevertheless, internal proton accumulation during microbial metabolism impairs EAB activity, while slow electron transfer between EAB and electrodes further limits MES performance. Efficient in-situ strategies to mitigate acidification and accelerate electron transfer are critically required to stabilize EAB performance. Biosynthetic composite iron minerals (BCIM) consisting of Fe₃O₄ and FeCO₃ was synthesized via microbial reduction of amorphous Fe(III) oxyhydroxide, integrating electrical conductivity and proton buffering function. BCIM-modified carbon cloth anodes (1 and 4 mg/cm², group C1 and group C4) were evaluated against bare controls. Results showed that the carbon cloth anode loaded with 1 mg/cm² BCIM achieved a maximum power density of 1166 mW/m², 28% higher than the control group, and cut the startup period from 8 days to 1.6 days. BCIM reduced charge transfer resistance, boosted the bioelectrochemical activity and increased electroactive sites of EAB. Meanwhile, BCIM dramatically raised the relative abundance of Geobacter sp. from 31.1% in the control group to 48.8% in Group C1, and also upregulated the abundance of functional genes associated with pili and flagellum. The FeCO₃ component provided localized pH regulation to relieve biofilm proton accumulation and enhance cell activity. With dual functions of accelerating electron transfer and alleviating acidification, BCIM presents a promising material to optimize EAB performance.