Self-Sustaining Lattice Chlorine Cycle for Efficient Ammonium Oxidation with High Chloride Utilization

Guoshuai Liu; Jiaju Yang; Jiahui Xu; Mengxuan Wang; Ze Zhu; Yifeng Zhang; Xuedong Zhang; Gang Liu

doi:10.1016/j.eng.2026.07.012

Engineering ›› :202607012 DOI: 10.1016/j.eng.2026.07.012

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Self-Sustaining Lattice Chlorine Cycle for Efficient Ammonium Oxidation with High Chloride Utilization

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Abstract

Electrochemical chlorination is a promising strategy for ammonium (NH₄⁺) removal from wastewater, yet its practical implementation is hindered by inadequate Faradaic efficiency (FE) and low chloride (Cl⁻) utilization. To address these concerns, this study presents a novel Co₂(OH)₃Cl/Co₃O₄ composite anode to achieve highly efficient NH₄⁺ oxidation via lattice Cl⁻–mediated HClO generation. At a Cl⁻ concentration of 50 mmol·L⁻¹, the FE of free chlorine generation for the Co₂(OH)₃Cl/Co₃O₄ anode is 95.45%, surpassing those of pristine Co₃O₄ (79.37%) and Co₂(OH)₃Cl (15.56%) anodes. Consequently, the Co₂(OH)₃Cl/Co₃O₄ anode achieves approximately 100% NH₄⁺ oxidation within 3.5 h, outperforming Co₃O₄ (72.25%) and Co₂(OH)₃Cl (58.66%) benchmarks. Electron paramagnetic resonance and quenching studies confirm that free chlorine serves as the primary active species driving NH₄⁺ oxidation. Operando characterization and theoretical calculations reveal that interfacial Co³⁺ sites on Co₃O₄ oxidize lattice Cl⁻ from Co₂(OH)₃Cl to generate free chlorine. The resulting vacancies are spontaneously replenished by Cl⁻ in the electrolyte. This self-sustaining lattice chlorine cycle enables the oxidation of NH₄⁺ to environmentally benign N₂ at low Cl⁻ concentrations. Concurrently, the Co³⁺ sites in Co₃O₄ are thermodynamically favorable for the chlorine evolution reaction via the Volmer–Krishtalik pathway, minimizing chlorine radical formation and eliminating the risk of generating harmful chlorination byproducts. Furthermore, the Co₂(OH)₃Cl/Co₃O₄ anode demonstrates exceptional operational stability in piggery wastewater, exhibiting remarkably low cobalt leaching (< 0.3 wt%) while reducing the NH₄⁺-N concentration from 355.9 to 19.1 mg·L⁻¹. This study provides fundamental insights into lattice Cl⁻–mediated electrocatalysis and offers a viable strategy for sustainable wastewater treatment.

Keywords

Electrochemical chlorination / Co₂(OH)₃Cl/Co₃O₄ / Lattice chlorine cycle / High FE

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Guoshuai Liu, Jiaju Yang, Jiahui Xu, Mengxuan Wang, Ze Zhu, Yifeng Zhang, Xuedong Zhang, Gang Liu. Self-Sustaining Lattice Chlorine Cycle for Efficient Ammonium Oxidation with High Chloride Utilization. Engineering 202607012 DOI:10.1016/j.eng.2026.07.012

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