Degradation of thiosulfate ions in sodium aluminate solution photocatalyzed by Co-TiO2@ZIF-8

Abstract

The strongly alkaline sodium aluminate solution generated from high-sulfur bauxites during their refinement through the Bayer process contains sulfur, mainly in the form of thiosulfate (S₂O₃²⁻) ions, which affects the quality of the target product (alumina). Consequently, the large-scale use of high-sulfur bauxites is severely limited, and methods of desulfurizing sodium aluminate solutions are urgently needed. To address this need, we herein prepared metal–organic framework (ZIF-8)-supported Co-doped TiO₂ (Co-TiO₂@ZIF-8) using a sol–gel method and low-temperature calcination under N₂ and examined the ability of this composite to photocatalyze the oxidative degradation of thiosulfate ions in sodium aluminate solutions. Co-TiO₂@ZIF-8 retained the large specific surface area and rich porosity of ZIF-8, which reduced the agglomeration of TiO₂ particles on the surface. The loading of Co-TiO₂ endowed Co-TiO₂@ZIF-8 with a mesoporous structure and thus increased its adsorption capacity. Compared with ZIF-8, Co-TiO₂@ZIF-8 featured new functional groups, and the formation of Zn–C and N–Ti–O bonds confirmed the successful integration of Co-TiO₂ with the ZIF-8 carrier. Consequently, the thiosulfate removal rates by Co-TiO₂@ZIF-8 were 42 % and 26 % higher than those observed for the blank sample and pure TiO₂ (light source = xenon arc lamp, oxidant = O₃, reaction time = 60 min), respectively. This high performance is primarily due to Co doping, which enhanced the efficiency of visible light utilization, and the photogenerated electron–hole pairs on the photocatalyst surface and hydroxyl radicals formed by the interaction of holes with the solution-phase OH⁻ ions.