Boosting arsenic removal with metastable Fe²⁺/Mn³⁺ redox process in MnFe₂O₄/rGO composites for high capacity and stability

Fe-Mn oxides exhibit significant potential in the application of chemical and electrochemical remediation of groundwater arsenic contamination. However, the mechanism controlling the equilibrium between chemisorption inhibition and capacitive adsorption enhancement at ferromanganese oxide electrodes is unclear, posing significant challenges to achieving both electrochemical arsenic removal efficiency and cycle stability. Here, we introduce for the first time a defect engineering strategy to synthesize defect-rich, reduced graphene oxide-anchored MnFe₂O₄ composites (MnFe₂O₄/rGO). The electrochemically efficient arsenic removal capacity (102.6 mg·g⁻¹) and sustained cycling stability (30 cycles with >95 % efficiency) are achieved through the synergistic pseudocapacitive effect of metastable Fe-Mn bimetallic. 80 % of the arsenic removal is due to pseudocapacitive effects driven by reversible redox reactions of metastable Fe²⁺/Mn³⁺ in MnFe₂O₄ tetrahedral coordination revealed by X-ray photoelectron spectrum (XPS). The electronic microenvironment of iron site is modulated by Mn atom reducing the arsenic adsorption energy on MnFe₂O₄/rGO electrode based on electronic impedance spectrum (EIS) and density function theory (DFT). Continuous flow experiments reveal that this electrochemical system deeply purifies 5 L arsenic-laden groundwater (1 mg·L⁻¹) below World Health Organization′s (WHO) drinking water guidelines with lower energy consumption and high selectivity. This study provides valuable insights for tailoring effective, stable electrodes in electrochemical arsenic removal.