Enhancing arsenic (III) removal by integrated electrocatalytic oxidation and electrosorption reactions on nano-textured bimetal composite of iron oxyhydroxide and manganese dioxide polymorphs (α-, γ-, β-, and ε-MnxFe1−xO)

A composite electrode of manganese oxide (MnO 2 ) incorporated with iron oxide (α-FeOOH) was synthesized for arsenite (As(III)) oxidation and subsequent arsenate (As(V)) electrosorption. The crystal structure and chemical state of MnO 2 polymorphs were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and BET surface area. The redox couple of Mn(III)/Mn(IV) mediated the catalytic electron transfer with respect to As(III)/As(V) redox equilibrium. The Mn site contributed a high diffusive current to the redox capacitance, meanwhile the Fe site better provided the double-layer capacitive deionization for arsenic species. Electrolysis of arsenite under constant anodic potential mode (+1.0 V vs. Ag/AgCl) enabled assess the performance of the electrodes. Among the polymorphs, γ-Mn 0.2 Fe 0.8 O exhibited the best arsenic adsorption capacity of 48 mg-As g −1 , compared to that of α-FeOOH NPs (15 mg-As g −1 ) and γ-MnO 2 (7 mg-As g −1 ), based on multilayer Langmuir adsorption model. • Enhancing As(III) removal via electrooxidation over Mn x Fe 1−x O bimetal oxide was studied. • MnO 2 polymorphs contributed redox capacitance to effective As(III) oxidation. • Nano-goethite enhanced arsenic adsorption capacity of bimetal composite. • Multi-layer adsorption isotherm described electrosorption of arsenic well. • Mn x Fe 1−x O exhibited synergistic effect on adsorption capacity for total arsenic.