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 bonds confirmed the successful integration of Co-TiO2 with the ZIF-8 carrier. Co doping enhanced the efficiency of visible light utilization, resulting in thiosulfate removal rates 42% and 26% higher than those observed for the blank sample and unsupported Co-doped TiO2 (light source = xenon arc lamp, oxidant = O3, reaction time = 60 min), respectively. This high performance primarily originated from the photogenerated electron–hole pairs on the photocatalyst surface and hydroxyl radicals formed by the interaction of holes with the solution-phase OH− ions.

Keywords: Bayer process, TiO2, ZIF-8, photocatalysis, thiosulfate