Reaction of Fe_aq^II with Peroxymonosulfate and Peroxydisulfate in the Presence of Bicarbonate: Formation of Fe_aq^IV and Carbonate Radical Anions

Abstract

Many advanced oxidation processes (AOPs) use Fenton-like reactions to degrade organic pollutants by activating peroxymonosulfate (HSO5 –, PMS) or peroxydisulfate (S2O8 2–, PDS) with Fe­(H2O)6 2+ (Feaq II). This paper presents results on the kinetics and mechanisms of reactions between Feaq II and PMS or PDS in the absence and presence of bicarbonate (HCO3 –) at different pH. In the absence of HCO3 –, Feaq IV, rather than the commonly assumed SO4 •–, is the dominant oxidizing species. Multianalytical methods verified the selective conversion of dimethyl sulfoxide (DMSO) and phenyl methyl sulfoxide (PMSO) to dimethyl sulfone (DMSO2) and phenyl methyl sulfone (PMSO2), respectively, confirming the generation of Feaq IV by the Feaq II-PMS/PDS systems without HCO3 –. Significantly, in the presence of environmentally relevant concentrations of HCO3 –, a carbonate radical anion (CO3 •–) becomes the dominant reactive species as confirmed by the electron paramagnetic resonance (EPR) analysis. The new findings suggest that the mechanisms of the persulfate-based Fenton-like reactions in natural environments might differ remarkably from those obtained in ideal conditions. Using sulfonamide antibiotics (sulfamethoxazole (SMX) and sulfadimethoxine (SDM)) as model contaminants, our study further demonstrated the different reactivities of Feaq IV and CO3 •– in the Feaq II-PMS/PDS systems. The results shed significant light on advancing the persulfate-based AOPs to oxidize pollutants in natural water.

How Do Metal Oxides Mislead Spin-Trapping Electron Paramagnetic Resonance Analysis?

“…Buffers are frequently utilized to stabilize pH levels in the exploration of catalytic oxidation mechanisms. However, these buffers could interact with the dominate oxidative species or activate the oxidants through different pathways. − In addition, these buffers could alter the solubility equilibrium of metal ions in MO suspensions, which might further change the transformation pathway of DMPO. Thus, we examined the effects of different buffers, including acetate buffer solution (ABS, pH 4.00), potassium hydrogen phthalate (KHP, pH 4.01), phosphate buffer solution (PBS, pH 6.86), and borate buffer solution (BBS, pH 9.18), on the DMPO-trapping signals (Figure c).…”

On The Nature of FeIV=Oaq in Aqueous Media: A DFT analysis

“…Clearly one cannot state that Fe IV (H 2 O) 5 O 2+ or Fe IV =O 2+ aq are the only form of Fe IV =O aq formed via reactions (1)–(4). The observation that Fe IV =O aq in neutral solutions reacts with DMSO & PMSO to form the corresponding sulfones [1,2,4,8,10] indicates that the alkaline forms of Fe IV =O aq react via analogous mechanisms to the reactions of Fe IV (H 2 O) 5 O 2+ with the same substrates [7] …”

“…Recently it was also shown that the reactions of Fe(H 2 O) 6 2+ with HSO 5 − or S 2 O 8 2− in neutral solutions yield Fe IV =O aq. [9,10] Due to the important role of Fe IV =O aq in AOPs, [11,12] and probably in biological processes [5,13,14] it seemed of interest to determine the reduction potential of Fe IV =O aq , and as this potential depends on the pH, pKa of the water ligands of Fe IV =O aq should be determined as well. As Fe IV =O aq is short lived, its lifetime in neutral solutions is unknown, it is difficult to determine experimentally these values.…”

Overlooked Formation of Carbonate Radical Anions in the Oxidation of Iron(II) by Oxygen in the Presence of Bicarbonate

“…It should be noted that Fe IV =O aq is expected to oxidize HCO 3 − forming CO 3 ⋅ − [39] . Furthermore, Fe IV =O aq might also be formed via the Fenton reaction in neutral media [22] .…”