Sulfidized state and formation: Enhancement of nanoscale zero-valent iron on biochar（S-nZVI/BC） in activating peroxymonosulfate for Acid Red 73 degradation

To improve the performance of the nanoscale zero-valent iron (nZVI)-based advanced oxidation process (AOPs), a sulfidation pre-modification endowed nZVI with stable reduction nature mechanism was proposed, and a coupled system of activating PMS with sulfidized nanoscale zero-valent iron supported by biochar(S-nZVI/BC@PMS) was constructed in this study. It could be found that sulfidation state and formation, largely dependent on the sulfidized modification method, had a great impact on the enhancement for nZVI/BC in activating PMS for Acid Red 73 degradation. Compared with original nZVI/BC, the formed FeS shell blocked the external air to protect the internal Fe0, and S-nZVI/BC became relatively hydrophobic, which could slow down the material corrosion by water and dissolved oxygen to enhance the reaction durability. Appropriate sulfidation reduced the electrochemical impedance and enhanced the electrical conductivity of the catalyst, which facilitated the electron transfer in the system, and the participation of reducing sulfur species enabled the reaction to form a positive cycle and ensure a prolonged duration. Excessive sulfidation caused the material to become unstable and thus susceptible to air erosion, reducing the efficiency of contaminant removal. Due to the FeS protective shell, hydrophobicity of S-nZVI/BC and the promoted electron transfer in the system, S-nZVI/BC@PMS system secured more free radicals and had longer reaction time under the same conditions. In addition, S-nZVI/BC had good antioxidant properties, universality, and reusability for practical applications. These findings not only elucidated the superiority of S-nZVI/BC@PMS system but also provided new insights into cost-saving and efficient advanced oxidation processes.