Are Free Radicals the Primary Reactive Species in Co(II)-Mediated Activation of Peroxymonosulfate? New Evidence for the Role of the Co(II)–Peroxymonosulfate Complex

Abstract

The catalytic activation of peroxymonosulfate (PMS) is under intensive investigation with potentials as an alternative advanced oxidation process (AOP) in wastewater treatment. Among all catalysts examined, Co(II) exhibits the highest reactivity for the activation of PMS, following the conventional Fenton-like mechanism, in which free radicals (i.e., sulfate radicals and hydroxyl radicals) are reckoned as the reactive species. Herein, we report that the primary reactive species (PRS) is proposed to be a Co(II)−PMS complex (Co(II)−OOSO 3 − ), while free radicals and Co(III) species act as the secondary reactive species (SRS) that play a minor role in the Co(II)/PMS process. This Co(II)−OOSO 3 − exhibits several intriguing properties including ability to conduct both one-electrontransfer and oxygen-atom-transfer reactions with selected molecules, both nucleophilic and electrophilic in nature, and strongly pHdependent reactivity. This study provides novel insights into the chemical nature of the Co(II)-catalyzed PMS activation process.

New Insights into the Activation of Peracetic Acid by Co(II): Role of Co(II)-Peracetic Acid Complex as the Dominant Intermediate Oxidant

“…In our recent work, we reported in a Co­(II)-mediated PMS activation process that the Co­(II)-PMS complex is the primary reactive species for the degradation of contaminants, and the Co­(II)-PMS complex could conduct both one-electron transfer and oxygen-atom transfer pathways. Results in this study show that the Co­(II)/PAA process could also conduct both H-abstraction and an oxygen-atom transfer reaction, highly analogous to the metal-peroxy species . Therefore, we speculate that the Co­(II)-PAA complex (i.e., Co­(II)-OO­(O)­CCH 3 ) is the primary reactive species in the Co­(II)/PAA process.…”

“…As shown in Figure S7 in SI, the signal belonging to DMPOX is clearly observed in the Co­(III) solution at pH = 7.0. It should be noted that Co­(III) mainly exists in the form of dimeric Co species containing a O–O bond at pH > 2.0. , Therefore, the Co­(II)-peroxide species is postulated to be one of the reactive species involved in the Co­(II)/PAA process.…”

“…It should be noted that Co(III) mainly exists in the form of dimeric Co species containing a O−O bond at pH > 2.0. 42,43 Therefore, the Co(II)-peroxide species is postulated to be one of the reactive species involved in the Co(II)/PAA process.…”

Mechanisms of Reaction Between Co(II) Complexes and Peroxymonosulfate

“…It is of interest to note that recently, after this manuscript was written, a report claiming, based on the nature of the final product of oxidation of an organic substrate, that reaction [1] does not occur and that Co II (SO 5 )(H 2 O) 2 is the reactive oxidizing agent, appeared. [22] Surprisingly, even ligands that stabilize Co III aq , i. e. PP; TPP & ATP, do not enable the direct oxidation of the central cobalt cation by HSO 5…”

Rapid pollutant degradation by peroxymonosulfate via an unusual mediated-electron transfer pathway under spatial-confinement

“… 14 Concurrently, Co( ii )-PMS complex (Co 2+ –OOSO 3 − ) was proposed to be the predominant reactive species according to the analysis on its capability of conducting both one-electron-transfer and oxygen-atom-transfer reactions. 15 …”