Hematite-facilitated microbial ammoxidation for enhanced nitrogen removal in constructed wetlands

Highlights

● H-CWs synergistically eliminate NH₄⁺ and NO₃⁻ while reducing N₂O emissions.

● Inhibitors and isotope incubations are used to prove the Feammox process.

● Feammox contributes approximately 40% to ammonia removal in H-CWs.

● Nanowires on the hematite suggest ammoxidation likely linked to EET.

● H-CWs enhance the abundance of nitrogen-metabolizing microorganisms and genes.

Abstract

Constructed wetlands (CWs) are widely applied for decentralized wastewater treatment. However, achieving efficient removal of ammonia (NH₄⁺–N) has proven challenging due to insufficient oxygen. In this study, natural hematite (Fe₂O₃) was employed as a CW substrate (H-CWs) for the first time to drive anaerobic ammonia oxidation coupled with iron(III) reduction (Feammox). Compared to gravel constructed wetlands (G-CWs), ammonia removal was enhanced by 38.14% to 54.03% and nitrous oxide (N₂O) emissions were reduced by 34.60% in H-CWs. The synergistic removal of ammonia and nitrate by H-CWs also resulted in the absence of ammoxidation by-products. Inhibitor and ¹⁵N isotope tracer incubations showed that Feammox accounting for approximately 40% of all ammonia removal in the H-CWs. The enrichment of iron phosphate (Fe₃Fe₄(PO₄)₆) promoted the accumulation of the Feammox intermediate compound FeOOH. Microbial nanowires were observed on the surface of H-CW substrates as well, suggesting that the observed biological ammoxidation was most likely related to extracellular electron transfer (EET). Microbial and metagenomics analysis revealed that H-CWs elevated the integrity and enhanced the abundance of functional microorganisms and genes associated with nitrogen metabolism. Overall, the efficient ammonia removal in the absence of O₂ together with a reduction in N₂O emissions as described in this study may provide useful guidance for hematite-mediated anaerobic ammonia removal in CWs.

Graphical abstract

Keywords

Constructed wetlands / Nitrogen removal / Feammox / Hematite / Iron cycle

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CRediT Authorship Contribution Statement

Hao Qin: Conceptualization, Methodology, Investigation, Software, and Writing – original draft. Wenbo Nie, Dongxu Yang, and Mengli Chen: Methodology. Duo Yi and Tao Liu: Investigation. Yi Chen: Conceptualization, Supervision, Writing – review and editing, and Project administration.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (No. 52170150), Chongqing Science Fund for Distinguished Young Scholars (China) (No. CSTB2022NSCQ-JQX0023) and Fundamental Research Funds for the Central Universities (China) (No. 2020CDJDPT002). The authors would like to thank the Analytical and Testing Center of Chongqing University (China), AiMi Academic Services for English language editing and review services and Shiyanjia Laboratory for various characterizations.

Conflict of Interests

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11783-024-1842-1 and is accessible for authorized users.

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