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The preparation of easily synthesized and cheap composite materials for the efficient removal of toxic oxoanions still remains challenging in sewage treatment. Herein, a new carbonized manganese-crosslinked sodium alginate (Mn/SA-C) was fabricated for the removal of arsenite (As(III)), arsenate (As(V)) and hexavalent chromium (Cr(VI)) in water. The results indicated that the Mn/SA-C pretreated with MnSO 4 solution (Mn/SA-C-S) exhibited a rapid adsorption toward As(III) and As(V) with the removal efficiency of >98% within 10 min, and had a high adsorption capacity toward As(III), As(V), and Cr(VI) with the maximum value of 189.29, 193.29, and 104.50 mg/g based on the Langmuir model, respectively. The removal efficiency of As(III), As(V), and Cr(VI) could be further significantly enhanced by coupling a photocatalytic process. For example, the time in which >98% of Cr(VI) (10 mg/L) was removed dramatically shortened from 360 min (adsorption) to 45 min (adsorption-photocatalysis), and the removal efficiency of As(III) increased by ∼10% within initial 5 min. This was primarily attributed to the Mn-catalyzed production of the photocatalytic excitons for Cr(VI) reduction, and the superoxide (•O 2 −) and hydroxyl (•OH) radicals for As(III) oxidation. The adsorption removal of arsenic (As) was primarily ascribed to surface complexation with MnO and precipitation by MnS 2 , and oxidative adsorption because of Mn valence cycle. The removal mechanisms of Cr(VI) mainly contained reduction by MnO and MnS 2 , complexation with MnO and carboxyl/hydroxyl groups as well as Cr(OH) 3 precipitation. Our research provides a promising Mn/SA-C-S material for rapid and efficient removal of As(III), As(V), and Cr(VI) in contaminated water through an adsorption-photocatalysis synergistic strategy. [Display omitted] • A novel carbonized manganese-crosslinked sodium alginate (Mn/SA-C) was prepared. • >98% of As(III) and As(V) was rapidly adsorbed within 10 min under dark condition. • As(III), As(V) and Cr(VI) adsorption fitted well to pseudo-second-order models. • The removal of As(III), As(V) and Cr(VI) was enhanced by coupling a photocatalysis. • Adsorption-photocatalysis mechanisms of As(III), As(V) and Cr(VI) were proposed. [ABSTRACT FROM AUTHOR]