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Here, we prepared a novel nanostructured Fe–Cu–Mn composite oxide (FCMO x) adsorbent using an ultrasonic coprecipitation method. The maximum adsorption capacity of As(III) and As(V) reached 158.5 and 115.2 mg/g under neutral conditions, respectively. The effects of several environmental factors (coexisting ions, solution pH, etc.) on the removal of inorganic arsenic using FCMO x were studied through batch experiments. The results showed that except for PO 4 3– and high initial pH, it was not significantly affected by ionic strength and other existing anions, implying a higher selectivity and adaptability. Combined with EPR, FTIR, and XPS analysis, we concluded that the Cu component and the reactive oxygen species (ROS) it generates played a decisive role in maintaining the stability of the redox cycle between Mn(IV)/Mn(III)/Mn(II) and enhancing the oxidation efficiency of As(III). Meanwhile, the adsorption mechanism of As(V) was mainly through the replacement of the FCMO x surface –OH to form stable inner-sphere arsenic complexes, while the removal mechanism of As(III) may involve the process of synergistic oxidation and chemisorption coupling. Additionally, the effective removal of As from the simulated As-contaminated water and its satisfactory reuse performance make FCMO x adsorbents favorable candidates for the removal of As-contaminated water in the future. [Display omitted] ● A novel FCMO x adsorbent was employed to remove As(III) and As(V) from aqueous solutions. ● The FCMO x exhibited excellent photocatalytic activity under visible-light irradiation. ● The redox cycle between Mn(II)/Mn(III)/Mn(IV) was mainly driven by the Cu component in FCMO x. ● The coupling effect of photocatalytic oxidation, chemical oxidation and adsorption enhanced the removal of As(III). [ABSTRACT FROM AUTHOR]