Is Sulfate Radical Really Generated from Peroxydisulfate Activated by Iron(II) for Environmental Decontamination?

Abstract

It is well documented that the traditional Fenton reagent (i.e., the combination of Fe(II) and HO) produces hydroxyl radical (OH) under acidic conditions, while at near-neutral pH the reactive intermediate converts to ferryl ion (Fe(IV)) that can oxidize sulfoxides to produce corresponding sulfones, markedly differing from their OH-induced products. However, it remains unclear whether Fe(IV) is generated in the Fe(II) activated peroxydisulfate (PDS) process, where sulfate radical (SO) is long recognized as the dominant intermediate in literature. Here we demonstrated that SO oxidized methyl phenyl sulfoxide (PMSO, a model sulfoxide) to produce biphenyl compounds rather than methyl phenyl sulfone (PMSO). Interestingly, the formation of PMSO was observed when PMSO was treated by the Fe(II)/PDS system over a wide pH range, and the yields of PMSO were quantified to be ∼100% at acidic pH 3-5. The identification of Fe(IV) in the Fe(II)/PDS system could also reasonably explain the literature results on alcohol scavenging effect and ESR spectra analysis. Further, a Fe(IV)-based kinetic model was shown to accurately simulate the experimental data. This work urges re-evaluation of the Fe(II)/PDS system for environmental decontamination, given that Fe(IV) would have different reactivity toward environmental contaminants compared with SO and/or OH.

Fe(III)-Activated Sulfite Revisited: The Overlooked Role of High-Valent Iron(IV)-Oxo Complex (Fe(IV)) in Water Treatment

“…This inference was supported byWang et al (2018) who found that DMPO • -OH adduct signal also occurred in DMPO trapped EPR spectra of Fe(II)/O 3 , Fe(II)/PDS, and Fe(II)/PMS systems, which were identified as Fe(IV)-dominated AOPs. Moreover, Zong et al (2021) also reported that • OH, formed by Fe(IV) self-decay, was responsible for DMPO • -OH adduct signal occurrence in the EPR spectra of Fe(II)/periodate system.…”

“…1995 (Wang et al 2018, Wang et al 2019b, Wang et al 2019c, Wang et al 2020. In these studies, Wang et al firstly identified that Fe(IV) oxidized PMSO to methyl phenyl sulfone (PMSO 2 ) through oxygen-atom transfer, markedly different from the SO 4 •− and • OH involved oxidation products of PMSO (Wang et al 2018). Further, they observed that PMSO was almost completely transformed to PMSO 2 , the indicative of Fe(IV) formation, in Fe(II)/HSO 5 − (Eq.…”

Ultrasonic role to activate persulfate/chlorite with foamed zero-valent-iron: Sonochemical applications and induced mechanisms

“…Catalytic iron center formed a highly oxidizing oxo-iron species Fe IV/V (namely, O = Fe IV and O = Fe V ) [65] which had the reaction with the sulfoxide in Scheme 5 to generate the corresponding sulfone by oxygen atom transfer step [61] , [62] . Formally, O = Fe V was best described as an oxo-Fe(IV)-porphyrin radical cation, namely, O = Fe IV (Por ) • + (O = Fe IV • + ) [65] , [66] .…”

Insight into the comparison of thermally and Fe(II) activated persulfate on sludge dewaterability and disintegration

“…ESR spectra of Fe(II) and thermally activated PPS was showed in Figure 7. The result showed that the intensity of the DMPO-SO 4 adduct signal was very weak, while the relatively strong DMPO-OH adduct signal appeared in the activated PPS system, but the free radicals were mainly sulfate radicals (Wang et al 2018b). The mechanism based on the persulfate oxidation system to improve sludge dewatering and disintegration performance was discussed by detecting the sludge extracellular polymeric substances (EPS) and Zeta potential, preliminarily determining the oxidation of target organic substances by activated persulfate (Guo et al 2019).…”