Homoacetogenesis inhibition and electrode regulation strengthen an in situ hydrogen-retaining reducing environment for Fe³⁺ reduction-driven vivianite formation

Vivianite, a valuable secondary Fe-P mineral, recovery from waste activated sludge provides a promising strategy for sustainable phosphorus (P) recovery and value-added utilization. However, competitive H₂ consumption by homoacetogenesis and the insufficient reducing environment for Fe³⁺ reduction limit the vivianite formation. This study introduces chloroform (0.02% (v/v)) to suppress homoacetogenesis and further optimizes iron anode voltage to improve H₂ retention, electron transfer, and Fe-P mineralization. Chloroform addition improved Fe²⁺ utilization efficiency, while further synergistic regulation of iron anode voltage increased Fe²⁺ utilization efficiency to 70.5%, and both contributed to improving vivianite quality. In particular, the chloroform-operated at 0.6 V (C-0.6 V) series achieved the highest vivianite size, crystallinity, and purity of 89.3 μm, 95.8%, and 94.7%, respectively. Chloroform reduced acetate accumulation, and the relative abundance of homo-acetogens decreased from 51.1% in the 0.8 V without chloroform (Control-0.8 V) series to 15.8% in the chloroform-operated at 0.8 V (C-0.8 V) series, whereas C-0.6 V further enriched electroactive bacteria (EAB) and dissimilatory iron-reducing bacteria (DIRB), indicating a shift of electron flow from competitive H₂ consumption toward Fe³⁺ reduction processes. Meanwhile, 0.6 V provided a favorable electrochemical window, strengthening Fe³⁺ reduction-related electron transfer pathways, and then promoted efficient electron delivery from intracellular metabolism to extracellular Fe³⁺ acceptors. Chloroform reshaped electron allocation by suppressing homoacetogenesis, while the optimized iron anode voltage strengthened the coupling between Fe³⁺ reduction and electron transfer. Coordinated regulation of H₂ fate and electrochemical electron transfer can effectively drive the preferential formation of vivianite, providing a new strategy for simultaneous phosphorus recovery and H₂ accumulation from sludge.