Revisiting the Oxidizing Capacity of the Periodate–H₂O₂ Mixture: Identification of the Primary Oxidants and Their Formation Mechanisms

Abstract

This study reexamined the mechanisms for oxidative organic degradation by the binary mixture of periodate and H2O2 (PI/H2O2) that was recently identified as a new advanced oxidation process. Our findings conflicted with the previous claims that (i) hydroxyl radical (•OH) and singlet oxygen (1O2) contributed as the primary oxidants, and (ii) •OH production resulted from H2O2 reduction by superoxide radical anion (O2 •–). PI/H2O2 exhibited substantial oxidizing capacity at pH < 5, decomposing organics predominantly by •OH. The likelihood of a switch in the major oxidant under varying pH conditions was revealed. IO4 – as the major PI form under acidic conditions underwent one-electron reduction by H2O2 to yield radical intermediates, whereas H2I2O10 4– preferentially occurring at pH > 7 caused 1O2 generation through two-electron oxidation of H2O2. PI reduction by O2 •– was suggested to be a key reaction in •OH production, on the basis of the electron paramagnetic resonance detection of methyl radicals in the dimethyl sulfoxide solutions containing PI and KO2, and the absence of deuterated and 18O-labeled hydroxylated intermediates during PI activation using D2O and H2 18O2. Finally, simple oxyanion mixing subsequent to electrochemical PI and H2O2 production achieved organic oxidation, enabling a potential strategy to minimize the use of chemicals.

Photosensitized Transformation of Hydrogen Peroxide in Dissolved Organic Matter Solutions under Simulated Solar Irradiation

“…Hydrogen peroxide (H 2 O 2 ) is an important reactive oxidant in aquatic redox processes and holds a unique position in aquatic environments, affecting the speciation and biogeochemistry of transition metals , as well as contributing to the degradation of natural and anthropogenic organic compounds. − H 2 O 2 is ubiquitously detected in various water environments, including freshwater, rainwater, estuarine, and marine environments, at concentrations varying from 0.01 to 30 μM. − Generally, the formation of H 2 O 2 in sunlit surface water is primarily from the dismutation of superoxide radicals (HO 2 · /O 2 · – ), which are produced by one-electron reduction of dioxygen. , Meanwhile, previous studies reported that the addition of external phenols would enhance the photoproduction of H 2 O 2 because the presence of external phenols as electron donors essentially promoted the photoproduction rate of O 2 · – . It is commonly accepted that the fate of H 2 O 2 is to generate a hydroxyl radical ( · OH), which is also critical in aquatic environments as a nonselective oxidant. − The formation of · OH from H 2 O 2 induced by transition metal catalysis (Fenton and Fenton-like reactions) or UV-irradiation is well known and has been extensively investigated. − However, the investigation of metal-independent and UV-light free production of · OH from H 2 O 2 to date is insufficient. Consequently, the detailed transformation mechanism of H 2 O 2 in aquatic environments is not fully understood.…”

Understanding the Importance of Periodate Species in the pH-Dependent Degradation of Organic Contaminants in the H2O2/Periodate Process

“…− was the major species of periodate at pH < 8.0, [2][3][4]7,10,12 the predominance of IO 4 − was not clearly evidenced. 18 Instead, several lines of evidence have revealed that the periodate species are more likely present as H 4 IO 6 − and H 3 IO 6 − under acidic and neutral conditions, wherein IO 4…”

“…21 On the contrary, in periodate-based AOPs, the change of periodate speciation was often overlooked, and only a few studies have tried to elucidate its potential effects on organic contaminant degradation. 4,10,12 − was also considered to be the dominant periodate species at pH < 8, which could transform into less-reactive H 2 I 2 O 10 4− species at high pH levels. Although the abovementioned studies and many other studies supposed that IO 4…”

“…1−4 Periodate can be activated by energy (e.g., ultraviolet and ultrasound), 1,5 reduced transition-metal ions (e.g., Mn 2+ and Fe 2+ ), 6,7 metal oxides, 3,8 and metal-free reductants (e.g., hydroxylamine and H 2 O 2 ). 2,4,9,10 Depending on the activation methods, various reactive oxidizing species (ROS) such as iodate radicals (IO 3…”

“…However, being limited by the speed of manual sampling, the rapid kinetics of organic contaminant degradation by the H 2 O 2 /periodate process at different pH values are not yet actually determined. 4,9,10 Very recently, Kim et al reexamined the mechanisms for organic contaminant degradation in the H 2 O 2 /periodate process, and they proposed that HO • was the major ROS at pH < 5.0, while 1 O 2 became dominant at high pH values. 10 The authors claimed that the switch of major ROS from HO • to 1 O 2 (pH 6.0) accounted for the pH-dependent decontamination performance of the H 2 O 2 /periodate process.…”