Simultaneous oxidation of As(III) and reduction of Cr(VI) by NiS-CdS@biochar through efficient oxalate activation: The key role of enhanced generation of reactive oxygen species

The reutilization of exhausted biochar is attracting extensive interest among researchers. In this study, the biochar generated from Chinese fir with natural regular porous structure that adsorbed Cd2+/Ni2+ at different concentration levels was used as the precursor, and then combined with simple hydrothermal vulcanization and ion deposition to generate the p-n heterojunction between NiS and CdS compounds (NiS-CdS@C) in situ. The hybrids with 3 cycles of NiS deposition reduced the interfacial transmission resistance from 80 Ω to 40 Ω, and increased photocurrent density by 5 times, thus effectively promoting the separation of photogenerated electrons and holes. The simultaneous removal of As(III) and Cr(VI) was selected to evaluate the oxidation and reduction capacity of the visible light/NiS-CdS@C/oxalate system. The results indicated that 10 mg/L As(III) and Cr(VI) were completely and simultaneously removed with 0.75 mM oxalate addition within 40 min in the system, and the NiS-CdS@C presented good durability and stability for oxalate activation. Electron paramagnetic resonance (EPR) and quenching experiments demonstrated that oxalate was activated by holes under light to produce •CO2− and enhanced the generation of additional •OH and •O2−, further contributing to the oxidation of As(III) and reduction of Cr(VI).