Exploration of Fe speciation preference for aerobic methane oxidation by using isotopic Fe-modified zeolites

Methane-oxidizing bacteria (MOB) dominate the natural consumption of fugitive methane (CH4), which is largely produced by methanogens in aerobic-anaerobic habitats. Although the influence of iron (Fe) on MOB activities has been examined, the underlying mechanism in terms of the preference remains to be clarified, the key to which may be the extracellular uptake of Fe during CH4 oxidation. In this study, isotopic ferric chloride was used to treat stable raw zeolites (Fe-Z) through a pyrolytic procedure (Fe-Z600). All samples were allowed to develop for 52 d, and the CH4 oxidation rate and contents of Fe and extracellular polymers (EPS) in different MOB periods were determined. The cumulative oxidation mass of CH4 in Fe-Z600 was two times that in Fe-Z, and the abundance of Methylocystis in Fe-Z600 was 21% higher. More uniformly dispersed alpha-Fe2O3 nanoparticles were formed during the pyrolysis of Fe-Z600, which accelerated the extracellular migration and stimulated MOB growth. In addition, Fe-Z600 presented the strongest extracellular polysaccharides (ECPs) inhibition; the total amount of ECPs was about 53.9% lower than that of the Raw group. Simultaneously, Fe was more likely to be stored in loosely bound EPS (LB-EPS), with a maximum cumulative amount of 38% during the stable period of CH4 oxidation in Fe-Z600, decreasing LB-EPS and tightly bound EPS by 47.4% and 75.4%, respectively. The presented findings can provide guidance for alleviating EPS blockage in long-term activities of MOB to ensure their efficient and durable CH4 oxidation.

Automated summary

Methane-oxidizing bacteria (MOB) play a dominant role in the natural consumption of fugitive methane (CH4) produced by methanogens. The mechanism of iron (Fe) influence on MOB activities and the extracellular uptake of Fe during CH4 oxidation needs further exploration. This study used isotopic ferric chloride to treat stable raw zeolites (Fe-Z) through a pyrolytic procedure (Fe-Z600) and found that Fe-Z600 exhibited higher CH4 oxidation rate and Methylocystis abundance compared to Fe-Z. The formation of uniformly dispersed alpha-Fe2O3 nanoparticles during pyrolysis accelerated the extracellular migration and stimulated MOB growth, while Fe-Z600 also showed stronger inhibition of extracellular polysaccharides (ECPs). Fe was more likely to be stored in loosely bound EPS (LB-EPS) in Fe-Z600, contributing to the efficient and durable CH4 oxidation.