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.

A novel electro-Fenton (E-Fenton) process was developed, in which the desired pH for an effective E-Fenton reaction and for a neutral treated media could be obtained by utilizing the reaction-released H+ and OH- in stead of chemical addition. In the laboratory-scale process using three chambers, the substrate solution pH > 4.0 was designed to be adjusted in situ through three sequencing steps: (I) pH reduction, (II) pH keeping for the effective E-Fenton reaction, and (III) pH recovery to neutral while the E-Fenton reaction continued. Experimental results demonstrated that such three-step pH adjustment was successfully achieved in this novel E-Fenton process, and that the pH adjustment was controlled by the E-Fenton reaction process. The performance of the novel process was assessed in terms of dimethyl phthalate (DMP) degradation in aqueous solution. The results revealed that the novel process was effective to reduce the DMP concentration and the total organic carbon (TOC) at steps II and III. Also, through experiments, the initial DMP solution pH > 4.0 was selected to be reduced to 3.5 in Step I of the process. This pH adjustment not only allowed the E-Fenton reaction to occur in its favorable pH range, but also benefited any potential subsequent biological treatment process or a final discharge. Moreover, the iron species could be recycled in the process.

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