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

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 N-2 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-TiO(2 )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 = O-3, 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.