First Direct and Unequivocal Electron Spin Resonance Spin-Trapping Evidence for pH-Dependent Production of Hydroxyl Radicals from Sulfate Radicals

Abstract

Recently, the sulfate radical (SO4 • –) has been found to exhibit broad application prospects in various research fields such as chemical, biomedical, and environmental sciences. It has been suggested that SO4 • – could be transformed into a more reactive hydroxyl radical (•OH); however, no direct and unequivocal experimental evidence has been reported yet. In this study, using an electron spin resonance (ESR) secondary radical spin-trapping method coupled with the classic spin-trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and the typical •OH-scavenging agent dimethyl sulfoxide (DMSO), we found that •OH can be produced from three SO4 • –-generating systems from weakly acidic (pH = 5.5) to alkaline conditions (optimal at pH = 13.0), while SO4 • – is the predominant radical species at pH < 5.5. A comparative study with three typical •OH-generating systems strongly supports the above conclusion. This is the first direct and unequivocal ESR spin-trapping evidence for •OH formation from SO4 • – over a wide pH range, which is of great significance to understand and study the mechanism of many SO4 • –-related reactions and processes. This study also provides an effective and direct method for unequivocally distinguishing •OH from SO4 • –.

Enhanced Oxidation of Organic Contaminants by Iron(II)-Activated Periodate: The Significance of High-Valent Iron–Oxo Species

“…Periodate (PI, IO 4 – )-based advanced oxidation processes (AOPs) have recently received increasing attention for the abatement of aqueous contaminants. − Compared with liquid-form oxidants (e.g., hydrogen peroxide (H 2 O 2 ), hypochlorous acid (HClO), and peracetic acid (PAA)) as the precursor of reactive species, solid-form oxidants (e.g., persulfate (PS) and PI) are relatively stable and impose much less safety risks during the transportation and storage process, which were even more preferred under certain circumstances. , For PI-based AOPs, despite the conclusive approach for the detection of iodate radical ( · IO 3 ) is still unavailable, · IO 3 and reactive oxygen species (e.g., hydroxyl radical ( · OH), atomic oxygen radical anion ( · O – ), superoxide radical ( · O 2 – ), and singlet oxygen ( 1 O 2 )) were generally recognized as the dominant active intermediates generated via the inter- and/or intramolecular electron transfer mechanism (eqs –). ,,− …”

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

“…6 a), however, the peak ( m / z 209) was clearly detected but the peak ( m / z 130) was hardly detected, suggesting that the main free radicals of O SC were SO 4 • − . Actually, because of the Scheme 3 [35] , the difficulty of free radical identification was increased [43] . However, by PN-HRMS with ROC detecting, the detection (consumption) was along with the reaction (from “without US originally” to “after introducing US”) to gain the variations of peaks at m / z 130 and 209 in real time, illustrating that i) •OH mainly originated from reaction of US-O SC , not Scheme 3 ; ii) after introducing US, the type of dominant free radicals tended to be •OH in the system and the transformation may be mainly initiated from SO 4 • − (being consistent with Section 3.5.1 -iiii); and iii) the PN-HRMS with ROC detecting (comparing with traditional detecting means, liquid chromatograph/mass spectrometer (LC/MS) or EPR with intermittent sample testing one by one) at a certain degree could avoid the potential misjudgement due to Scheme 3 .…”

Filter‐membrane treatment of continuous‐flow tetracycline through photocatalysis‐assisted peroxydisulfate oxidation

“…It is commonly believed that SO 4 ˙could be easily converted to ˙OH under alkaline conditions (Equation 4), and the produced ˙OH possesses a shorter life and lower oxidation capacity than SO 4 ˙-. 33 In addition, the leaching of Fe and Co became more serious in acidic solution (Figure S5A), but the leached amount was still very low even after experiencing five cycles at initial pH of 3.6…”