Overlooked Complexation\nand Competition Effects of\nPhenolic Contaminants in a Mn(II)/Nitrilotriacetic Acid/Peroxymonosulfate\nSystem: Inhibited Generation of Primary and Secondary High-Valent\nManganese Species

Organic contaminants with lower Hammett constants are\ntypically\nmore prone to being attacked by reactive oxygen species (ROS) in advanced\noxidation processes (AOPs). However, the interactions of an organic\ncontaminant with catalytic centers and participating ROS are complex\nand lack an in-depth understanding. In this work, we observed an abnormal\nphenomenon in AOPs that the degradation of electron-rich phenolics,\nsuch as 4-methoxyphenol, acetaminophen, and 4-presol, was unexpectedly\nslower than electron-deficient phenolics in a Mn(II)/nitrilotriacetic\nacid/peroxymonosulfate (Mn(II)/NTA/PMS) system. The established quantitative\nstructure–activity relationship revealed a volcano-type dependence\nof the degradation rates on the Hammett constants of pollutants. Leveraging\nsubstantial analytical techniques and modeling analysis, we concluded\nthat the electron-rich phenolics would inhibit the generation of both\nprimary (Mn(III)NTA) and secondary (Mn(V)NTA) high-valent manganese\nspecies through complexation and competition effects. Specifically,\nthe electron-rich phenolics would form a hydrogen bond with Mn(II)/NTA/PMS\nthrough outer-sphere interactions, thereby reducing the electrophilic\nreactivity of PMS to accept the electron transfer from Mn(II)NTA,\nand slowing down the generation of reactive Mn(III)NTA. Furthermore,\nthe generated Mn(III)NTA is more inclined to react with electron-rich\nphenolics than PMS due to their higher reaction rate constants (8314\n± 440, 6372 ± 146, and 6919 ± 31 M<sup>–1</sup> s<sup>–1</sup> for 4-methoxyphenol, acetaminophen, and 4-presol,\nrespectively, as compared with 671 M<sup>–1</sup> s<sup>–1</sup> for PMS). Consequently, the two-stage inhibition impeded the generation\nof Mn(V)NTA. In contrast, the complexation and competition effects\nare insignificant for electron-deficient phenolics, leading to declined\nreaction rates when the Hammett constants of pollutants increase.\nFor practical applications, such complexation and competition effects\nwould cause the degradation of electron-rich phenolics to be more\nsusceptible to water matrixes, whereas the degradation of electron-deficient\nphenolics remains largely unaffected. Overall, this study elucidated\nthe intricate interaction mechanisms between contaminants and reactive\nmetal species at both the electronic and kinetic levels, further illuminating\ntheir implications for practical treatment.