ZIF-8-derived porous carbon composite photocatalysts for the removal of thiosulfate ions from sodium aluminate solutions

The Bayer process is the primary method for alumina production. However, thiosulfate ions (S 2 O 3 2− ) present in the involved sodium aluminate solution are difficult to remove and adversely affect this process, thereby limiting the large-scale application of high‑sulfur bauxite. To address this issue, this study designed and synthesized a composite photocatalyst Co-TiO 2 @C based on ZIF-8-derived porous carbon as a carrier. Moreover, the study combined wet oxidation and photocatalytic oxidation to investigate the photocatalytic S 2 O 3 2− degradation activity and stability of this photocatalyst in a sodium aluminate solution. Results indicated that the ZIF-8-derived porous carbon carrier had a high specific surface area and highly porous structure, which were conducive to the loading and dispersion of Co-TiO 2 catalytic components. Moreover, it formed a composite photocatalytic material with good adsorption performance and abundant active sites. High-temperature pyrolysis led to the formation of graphite structures with excellent electron transfer properties on porous carbon supports, while Co doping reduced the bandgap energy of the loaded nano-TiO 2 , resulting in excellent photocatalytic performance of the composite catalyst under visible-light irradiation (The bandgap is 2.78 eV, and the photocurrent density under xenon lamp can reach 2.41 μA/cm 2 ). The adsorption degradation of S 2 O 3 2− ions by Co-TiO 2 @C/900 reached 29.6% within 30 min. After 60 min of light irradiation, the photocatalytic degradation rate of S 2 O 3 2− ions by Co-TiO 2 @C/900 can reach 92.5% under xenon lamp irradiation, which is 38.4% higher than that of P25. Moreover, the degradation efficiency of S 2 O 3 2− ions by Co-TiO 2 @C/900 only decreased by 9.5% after 6 cycles of experiences. The results of quenching experiments and paramagnetic electron resonance indicated that the degradation mechanism of S 2 O 3 2− depended on the generation of reactive oxygen species, such as hydroxyl radicals and superoxide radicals, under light irradiation.