Arsenic has emerged as an environmental contaminant globally in the last decade because of its highly toxic and carcinogenicity nature. Electrosorption is an effective separation process for the purification of saline waters. This work investigated the arsenite (As(III)) oxidation and subsequent arsenate (As(V)) adsorption using mixed oxide electrodes. The composite of manganese oxide (MnO₂) incorporated with iron oxide (α-FeOOH) was synthesized via chemical precipitation and calcination. The crystal structure and chemical state of MnO₂ polymorphs, precipitated on goethite nanoparticles, were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and BET surface area. The voltammetry results showed that the redox couple of Mn(III)/Mn(IV) mediate the catalytic electron transfer with respect to As(III)/As(V) redox equilibrium. On the composite electrode, 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. The consecutive reaction kinetics was derived to determine the effect of Mn to Fe molar ratio on the rate of electrochemical oxidation and adsorption of arsenic. Among the polymorphs, γ-Mn_0.2Fe_0.8O 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-MnO₂ (7 mg-As g^-1), based on multilayer Langmuir adsorption model.