In Situ EPR Spin Trapping and Competition Kinetics Demonstrate Temperature-Dependent Mechanisms of Synergistic Radical Production by Ultrasonically Activated Persulfate

Abstract

Ultrasound coupled with activated persulfate can synergistically degrade aqueous organic contaminants. Here, in situ electron paramagnetic resonance spin trapping was used to compare radicals produced by ultrasonically activated persulfate (US-PS) and its individual technologies, ultrasound alone (US) and heat-activated persulfate (PS), with respect to temperature. Radicals were trapped using 5,5-dimethyl-1-pyrroline-N-oxide, DMPO, to form detectable nitroxide adducts. Using initial rates of radical adduct formation, and compared to US and PS, US-PS at 40 and 50 °C resulted in the largest synergistic production of radicals. Radicals generated from US were reasonably consistent from 40 to 70 °C, indicating that temperature had little effect on cavitational bubble collapse over this range. However, synergy indexes calculated from initial rates showed that ultrasonic activation of persulfate at the bubble interface changes with temperature. From these results, we speculate that higher temperatures enhance persulfate uptake into cavitation bubbles via nanodroplet injection. DMPO-OH was the predominant adduct detected for all conditions. However, competition modeling and spin trapping in the presence of nitrobenzene and atrazine probes showed that SO4 •– predominated. Therefore, the DMPO-OH signal is derived from SO4 •– trapping with subsequent DMPO-SO4 – hydrolysis to DMPO-OH. Spin trapping is effective in quantifying total radical adduct formation but limited in measuring primary radical speciation in this case.

Trace Cu(II)-Mediated Selective Oxidation of Benzothiazole: The Predominance of Sequential Cu(II)–Cu(I)–Cu(III) Valence Transition and Dissolved Oxygen

“…This hypothesis is consistent with the observed oxygen-dependent BTH removal and the maximal stoichiometric ratio of BTH/PDS. Unfortunately, O 2 •– was not identified by ESR, possibly because the DMPO-trapping rate (10 M –1 s –1 ) is 8 orders of magnitude lower than the reaction of eq . As the hyperfine splitting constants of DMPO–OOH (α N = 14.36 G, α β H = 11.24 G, α γ H = 1.07 G) approximate those of DMPO–OCH 3 (α N = 14.4 G, α β H = 10.2 G, α γ H = 1.3 G), O 2 •– cannot be discerned from the ESR spectrum even if it was trapped by DMPO (simulated with EasySpin in Figure S18, Supporting Information).…”

A simple and convenient strategy for the oxidation of C(sp3)–H bonds based on γ-valerolactone

“…The EPR spectrum of the reaction solution indicates the presence of ˙OOH species (hydroperoxyl free radical, A N = 14.4 G, A Hβ = 11 G, A Hγ = 1.6 G) 36 and ˙OH species ( A Hβ = A N = 15.6 G). 37 In contrast, only ˙OOH species was observed in the pure GVL system, suggesting that ˙OOH species formed during the reaction. Combined with the ability of GVL to accelerate the oxidation reaction, we speculated that the reaction was initiated via a process in which triplet-state molecular O 2 directly abstracts the H atom from GVL (Fig.…”

Enhancing polyol/sugar cascade oxidation to formic acid with defect rich MnO2 catalysts

“…MnO 2 -D exhibits a high O sur-sub content (77.6%) and a minimum O lat content (9.5%), indicating that the defective structure induces more active oxygen species (Supplementary Table 5 ). Low-temperature electron paramagnetic resonance (EPR) also shows that MnO 2 -D exhibits a stronger oxygen vacancy feature at a g-factor of 2.003 64 , 65 than MnO 2 -P (Supplementary Fig. 4 ).…”