Interrupting Fe(II)-Catalyzed Ferrihydrite Phase Transformation Enables Sustained Dissimilatory Iron Reduction for Enhanced Anaerobic Digestion

Dissimilatory iron reduction (DIR) offers a promising pathway to enhance anaerobic digestion (AD) performance, yet its efficiency is limited by rapid Fe(III) passivation. Beyond initial Fe(III) bioavailability, Fe(II)-catalyzed phase transformation of Fe(III) into inert minerals emerges as another limiting factor. To address this issue, an anti-passivation strategy was developed by integrating spatial isolation with catecholate-Fe(III) coordination, using polydopamine (PDA) as a model matrix. The results showed that the PDA matrix embedded ferrihydrite nanoparticles within microspheres via catecholate coordination, achieving spatial isolation that physically prevented nanoparticle aggregation and lattice alignment. This strategy achieved 82% Fe(III) reduction efficiency, outperforming conventional ferrihydrite reduction efficiency (27%), while suppressing inert Fe(III) formation by 36%. Density functional theory calculations revealed that PDA weakened the adsorption energy of Fe(II) at Fe(III) sites and lengthened the distance between adjacent Fe(II) and Fe(III) from 2.97 to 3.20 Å. In the AD of waste activated sludge, Fe@PDA increased methane production by 50%. Furthermore, the enhanced DIR facilitated microbial-material electron transfer, improved energy metabolism, enriched iron-reducing bacteria, and upregulated TCA-cycle genes. This work aims to address the long-standing limitations of Fe(III) oxide utilization in DIR-based anaerobic biotechnologies to enhance renewable energy recovery from organic wastes.