Phosphate-mediated degradation of organic pollutants in water with peroxymonosulfate revisited: Radical or non-radical oxidation?

Whilst it is generally recognized that phosphate enables to promote the removal of some organic pollutants with peroxymonosulfate (PMS) oxidation, however, there is an ongoing debate as to whether free radicals are involved. By integrating different methodologies, here we provide new insights into the reaction mechanism of the binary mixture of phosphates (i.e., NaH₂PO₄, Na₂HPO₃, and NaH₂PO₂) with peroxymonosulfate (PMS) or hydrogen peroxide (H₂O₂). Enhanced degradation of organic pollutants and observation of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) adducts (i.e. DMPO[sbnd]OH and 5,5-dimethyl-2-oxopyrroline-1-oxyl (DMPOX)) with electron paramagnetic resonance (EPR) in most phosphates/PMS system seemly support a radical-dominant mechanism. However, fluorescence probe experiments confirm that no significant amount of hydroxyl radicals (^•OH) are produced in such reaction systems. PMS in the phosphate solutions (without any organics) remains relatively stable, but is only consumed while organic substrates are present, which is distinct from a typical radical-dominant Co²⁺/PMS system where PMS is continuously decomposed. Through density functional theory (DFT) calculation, the energy barriers of the phosphates/PMS reaction processes are greatly decreased when non-radical mechanism dominates. Complementary evidence suggests that the reactive intermediates of PMS-phosphate complex, rather than the free radicals, are capable of oxidizing electron-rich substrates such as DMPO and organic pollutants. Taking the case of phosphate/PMS system as an example, this study demonstrates the necessity of acquisition of lines of evidence for resolving paradoxes in identifying EPR adducts.