Dual role of citric acid: facilitate the bioconversion of CO2 to CH4 and enhance bioavailability of zero-valent iron.

Studies on the bioconversion of CO2 to CH4 using Fe0 have reported that a fraction of CO2 is converted to FeCO3, forming a passivation layer on the Fe0 surface, which reduces its reactivity and limits process efficiency. To mitigate FeCO3 precipitation and enhance CO2-to-CH4 conversion using Fe0, this study evaluates the role of citric acid (CA) as a ligand to Fe2+, which helps maintain iron in a soluble form and prevents its rapid conversion to insoluble iron carbonate (FeCO3). Batch experiments with 30 mM Fe0, anaerobic granular sludge (AGS), and NaHCO3 with CA as a ligand showed that citrate biodegraded into acetic acid and CO2 gas. To prevent citrate degradation and elucidate its effect on Fe0-mediated methanogenesis, 5 % v/v of antibiotic antimycotic solution (100x) (10,000 units penicillin, 10 mg streptomycin and 25 μg amphoctericin per ml) were introduced as a research tool, selectively inhibiting bacterial activity without affecting hydrogenotrophic methanogens. Methane production was notably enhanced only in the sample with NaHCO3, CA, antibiotics, and Fe0, reaching 38 ml by day 50, whereas the corresponding sample without Fe0 produced only 22 ml, resulting in a net methane production of 16 ml. The results indicate that CA enhances methane production by maintaining iron solubility and inhibiting FeCO3 precipitation, thereby facilitating continuous iron oxidation and sustained electron release, which supports the microbial reduction of CO2 to CH4. While antibiotics provided a controlled environment to uncover these mechanisms, their use is not a viable long-term solution. Based on the findings of this study, future work may explore short-term CA exposure (<12 h) or alternative ligands to minimize biodegradation without relying on antibiotics.