Peroxymonosulfate activation by iron(III)-tetraamidomacrocyclic ligand for degradation of organic pollutants via high-valent iron-oxo complex

Abstract

Herein, we proposed a new catalytic oxidation system, i.e., iron(III)-tetraamidomacrocyclic ligand (FeIII-TAML) mediated activation of peroxymonosulfate (PMS), for highly efficient organic degradation using p-chlorophenol (4-CP) as a model one. PMS/FeIII-TAML is capable of degrading 4-CP completely in 9 min at the initial 4-CP of 50 μM and pH = 7, whereas the recently explored system, H2O2/FeIII-TAML, could only result in ∼22% 4-CP removal in 20 min under otherwise identical conditions. More attractively, inorganic anions (i.e., Cl−, SO42−, NO3−, and HCO3−) exhibited insignificant effect on 4-CP degradation, and the negative effect of natural organic matters (NOM) on the degradation of 4-CP in PMS/FeIII-TAML is much weaker than the sulfate radical-based oxidation process (PMS/Co2+). Combined with in-situ XANES spectra, UV-visible spectra, electron paramagnetic resonance (EPR) spectra, and radical quenching experiments, high-valent iron-oxo complex (FeIV(O)TAML) instead of singlet oxygen (1O2), superoxide radical (O2•−), sulfate radicals (SO4•−) and hydroxyl radicals (HO•) was the key active species responsible for 4-CP degradation. The formation rate (kI) and consumption rate (kII) of the FeIV(O)TAML in PMS/FeIII-TAML were pH-dependent in the range of 6.0–11.5. As expected, increasing the FeIII-TAML and PMS dosage resulted in a higher steady-state concentration of FeIV(O)TAML and enhanced the 4-CP degradation accordingly. In addition, the oxidation capacity of PMS was almost totally utilized in PMS/FeIII-TAML for 4-CP oxidation due to the two-electron abstraction from 4-CP by one PMS. We believe this study will shed new light on effective PMS activation by Fe-ligand complexes to efficiently degrade organic contaminants via nonradical pathway.