Efficient removal of chromium from water by Mn₃O₄@ZnO/Mn₃O₄ composite under simulated sunlight irradiation: Synergy of photocatalytic reduction and adsorption

A novel Mn₃O₄@ZnO/Mn₃O₄ composite was designed to remove Cr (VI) and Cr (III) from water by concurrent photocatalysis and adsorption. The Mn₃O₄@ZnO photocatalyst with core-shell nano-sphere structure, was first fabricated by precisely controlled process using a mild hydrothermal method combined with atomic layer deposition (ALD). The wurtzite ZnO layer was uniformly deposited on hausmannite Mn₃O₄ surface, which formed a typical II-type heterojunction, separating the photo-generated electron-hole pairs effectively. Hence, rapid Cr (VI) reduction by Mn₃O₄@ZnO under simulated sunlight irradiation was achieved at 95.3% in 110 min. The pure Mn₃O₄ as adsorbent could subsequently adsorb the reduced Cr (III) from aqueous solution. Noticeably, the total Cr removal efficiency was enhanced to 92.0% within 70 min by concurrent photocatalysis and adsorption compared to 88.8% even within 120 min in the separate two processes. Mechanism exploration proposed that more active sites on Mn₃O₄@ZnO surface would be released for continuous photocatalytic reduction of Cr (VI) after the transfer of reduced Cr (III) from Mn₃O₄@ZnO onto Mn₃O₄ surface. In addition, the synergy of photocatalysis and adsorption on Cr removal was the strongest at pH = 6.0 and the highest removal efficiency of 96.0% was realized within 70 min. The outstanding synergistic effect of photocatalytic reduction and adsorption makes the innovative Mn₃O₄@ZnO/Mn₃O₄ composite a promising candidate for Cr remediation in aquatic environment.