Auto-photocatalytic valorization of ZnS into ZnS/ZnO nanocomposite using O/UV oxidation for photocatalytic dye degradation.

Cost-effective valorization of zinc sulfide (ZnS), by-product from desulfurization of zinc oxide (ZnO), poses a significant challenge due to energy requirement under high temperature and reaction time. This study introduces an innovative synthesis of ZnS/ZnO nanocomposites (NCs) from ZnS nanoparticles (NPs) for environmental remediation. Aqueous colloidal solution of ZnS NPs (10 g/L) was oxidized in an airlift photoreactor (5 L) at room-temperature using ozone (O) and ozone/ultraviolet (O/UV), respectively. The effects of pH (3-11), time (10-90 min), and oxidation modes were explored on oxygen content (OC) of NCs. The optimization results using RSM showed that higher pHs and longer reaction times led to increased OC of NCs up to 7.14 and 11.12 wt% in O and O/UV oxidation, respectively. Interaction of O/UV and ZnS exerts a dual-effect involving O-induced oxidation of ZnS, and promoted-photocatalytic role of ZnS precursor (auto-photocatalytic effect). This effect facilitates ZnS to ZnO conversion and enhances reaction kinetics across a broader pH range. Optical bandgap analysis revealed a decrease in the bandgap energy from 3.70 eV for ZnS NPs to 3.41 eV for ZnS/ZnO NCs synthesized under O/UV (pH: 11 and time: 90 min). The proposed method was subsequently applied to spent ZnS NPs, yielding in ZnS/ZnO NCs with an OC of 9.32 wt%. The NCs synthesized from virgin and spent ZnS NPs were used for photocatalytic degradation of Congo red dye and the efficiencies of 91 and 98 % were achieved, respectively. Both NCs demonstrated reusable and environmentally friendly waste valorization, retaining 62-72 % efficiency after four recycling cycles. Cost-effectiveness of the suggested approach was also demonstrated in photocatalytic dye degradation.