Extending semiconductor-based photo-fenton reaction to circumneutral pH using chelating agents: The overlooked role of pH on the reduction mechanism of Fe3+

• A novel vis/H 2 O 2 /Fe 3+ /NTA/g-C 3 N 4 system could broaden the pH application range. • The pH-dependent Fe 3+ reduction mechanism achieved in this system. • Superoxide radical (O 2 •− ) mediated Fe 3+ reduction process at pH 7.0. • H 2 O 2 played a triple role in this system. • The system has a superior recyclability and strong resistance to practical water. In this work, we conceived a novel vis/H 2 O 2 /Fe 3+ /nitrilotriacetic acid (NTA)/g-C 3 N 4 system to address the bottlenecks of traditional homogeneous photo-Fenton reactions, i.e., the narrow pH application range and unsustainable electron supply by the ligand to metal charge transfer (LMCT) process. A strong synergistic effect was observed with efficient organic contaminant degradation in a broad pH range (3-8), and hydroxyl radical (HO•) was proved to be the primary active species. Intriguingly, we found that the mechanism of rate-limiting Fe 3+ reduction process was highly pH-dependent. At acidic condition (pH 3.0), the reduction of Fe 3+ was dominated by direct photogenerated electron (e − ) transfer and LMCT reaction. However, as the pH increased to 7.0, it gradually converted into a superoxide radical (O 2 •− ) mediated reduction process, which was mainly attributed to the enhanced electrostatic repulsion between g-C 3 N 4 and iron species, and the species transformation from protonated HO 2 • to more active deprotonated O 2 •− . Notably, H 2 O 2 played a triple role in this system including (1) serving as the precursor of O 2 •− (2) scavenging photogenerated holes (h + ) to complete the closed-loop of photocatalytic reaction and (3) boosting the generation of HO•. Finally, the vis/H 2 O 2 /Fe 3+ /NTA/g-C 3 N 4 system also featured with a strong water matrix resistance capacity (e.g., chloride, bicarbonate, Ca 2+ /Mg 2+ and humic acid), which enables it a promising platform for practical wastewater purification.