Visible-light-driven photocatalytic inactivation of Escherichia coli K-12 over thermal treated natural magnetic sphalerite: Band structure analysis and toxicity evaluation

The cost-effective natural magnetic sphalerite (NMS) was thermally treated to further enhance its photocatalytic activity for water disinfection. Within 6 h visible light irradiation, the calcined NMS700 exhibited enhanced Escherichia coli inactivation with 6 log10 cfu/mL of cell reduction, while the pristine NMS (NMS0) only showed 2.5 log10 cfu/mL of cell reduction. After calcination, the pristine binary composite of ZnS/FeS for NMS0 was transformed into a ternary ZnS/ZnFe2O4/ZnO composite for NMS700. The optimum photocatalytic activity of NMS700 was attributed to the efficient charge separation originated from the three-level electron transfer system, well evidenced by the highest photocurrent and fastest electron-hole separation among all the calcined NMS samples. Scavenger study and probe determination demonstrated that the dominant bactericidal agent was changed from superoxide radical (O2−) for NMS0 to hole (h+) and hydroxyl radical (OH) for NMS700. Moreover, vibrating sampling magnetizer (VSM) analysis revealed that the saturated magnetism was enhanced from 0.03 emu/g for NMS0 to 0.15 emu/g for NMS700, due to the appearance of magnetic component like ZnFe2O4. In addition, NMS0 were not genotoxic but cytotoxic, while NMS700 was toxic of both, indicating the superior stability of NMS700 was obtained after calcination treatment. The study demonstrated that the calcination is an effective strategy to modify natural mineral-photocatalyst into highly effective photocatalyst activated by solar energy.