Influence of the continuous addition of zero-valent iron (ZVI) and nano-scaled zero-valent iron (nZVI) on the anaerobic biomethanation of carbon dioxide

Abstract Both zero valent iron (ZVI) and nano-scaled ZVI (nZVI) have the potential for biomethanation of CO2 under anaerobic condition, but when they were continuously added to a semi-continuous anaerobic bioreactor, exhibited different effects on the microbial mechanisms and system performances. With continuous addition of ZVI and nZVI, dissolved input CO2 was transformed into CH4 at up to 1.71 and 0.59 mmol/day, respectively. The abiotic corrosion rate and H2 evolution rate by ZVI were 27 times lower than those by nZVI. Active biomethanation occurred earlier in the period of operation with nZVI, and later with ZVI. As nZVI accumulated in the bioreactor, biomethanation stopped, without utilization of both the H2 and VFAs generated. Based on microbial community analysis, biomethanation by using both irons depended on hydrogenotrophic methanogenesis, but in the case of ZVI, direct use of electron from ZVI by methanogens was also significant. Estimation of CO2 conversion demonstrated that compared to nZVI, ZVI could be more effectively and stably used for biomethanation, with less loss in the form of chemical precipitates.