Physical Contact between Bacteria and Carbonaceous Materials: The Key Switch Triggering Activated Carbon and Biochar to Promote Microbial Iron Reduction

Carbonaceous materials, including activated carbon and pyrolytic carbon, have been recognized for about over a decade as effective electron shuttles or conductive materials in promoting microbial Fe(III) mineral reduction. However, recent studies reveal inhibitory effects, sparking debates about their overall impact. We hypothesized that the physical contact between bacteria and carbon is an overlooked yet critical factor in determining whether carbon promotes or inhibits microbial Fe(III) reduction. Using systems containing Shewanella oneidensis MR-1, activated carbon, and ferrihydrite, we investigated how carbon–iron oxide aggregate structure affects Fe(III) reduction kinetics. At low activated carbon-to-iron oxide ratios (C/Fe = 5:7 by mass), ferrihydrite aggregated with carbon, forming carbon-encapsulated particles that suppressed Fe(III) reduction rates. Conversely, at higher ratios (C/Fe = 100:7), the ferrihydrite dispersed on the carbon surface, enhancing both the rate and extent of Fe(III) reduction. Tests with 11 different carbonaceous materials (activated carbon and biochar) all confirmed that the microstructure of iron oxides─whether encapsulating or dispersed─on carbon surfaces is critical for determining Fe(III) reduction rates. This insight resolves the debate on whether carbonaceous materials promote or inhibit Fe(III) mineral reduction and enhances our understanding of their roles in biogeochemical processes and environmental remediation.