Kinetics and Mechanism of Ultrasonic Activation of Persulfate: An in Situ EPR Spin Trapping Study

Abstract

Ultrasound (US) was shown to activate persulfate (PS) providing an alternative activation method to base or heat as an in situ chemical oxidation (ISCO) method. The kinetics and mechanism of ultrasonic activation of PS were examined in aqueous solution using an in situ electron paramagnetic resonance (EPR) spin trapping technique and radical trapping with probe compounds. Using the spin trap, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), hydroxyl radical (OH) and sulfate radical anion (SO) were measured from ultrasonic activation of persulfate (US-PS). The yield of OH was up to 1 order of magnitude greater than that of SO. The comparatively high OH yield was attributed to the hydrolysis of SO in the warm interfacial region of cavitation bubbles formed from US. Using steady-state approximations, the dissociation rate of PS in cavitating bubble systems was determined to be 3 orders of magnitude greater than control experiments without sonication at ambient temperature. From calculations of the interfacial volume surrounding cavitation bubbles and using the Arrhenius equation, an effective mean temperature of 340 K at the bubble-water interface was estimated. Comparative studies using the probe compounds tert-butyl alcohol and nitrobenzene verified the bubble-water interface as the location for PS activation by high temperature with OH contributing a minor role in activating PS to SO. The mechanisms unveiled in this study provide a basis for optimizing US-PS as an ISCO technology.

Coupling Ultrasound to the Electro‐Oxidation of Methyl Paraben Synthetic Wastewater: Effect of Frequency and Supporting Electrolyte

“…These is in agreement with the observations of Ferkous et al., which studied the persulfate oxidation of naphthol blue black enhanced by ultrasound and stated that persulfate‐enhanced effect was more remarkable at 20 kHz than at 585 kHz. In addition, most studies of ultrasound activation of persulfate only studied low frequency, mainly 20 kHz, obtaining very promising results …”

Toward understanding the mechanism of pure CO2‐quenching sonochemical processes

“…However, their estimated amounts were very low as compared with that of • OH, which is mainly due to the fact that such radicals reacts quickly to form nitrous and nitric acids (HNO 2 and HNO 3 ) as final products of air sonolysis inside the bubble . Therefore, our research on the effect of CO 2 will be centered only on the yield of hydroxyl radical because: (i) it was the major species formed inside the bubble and (ii) • OH is well identified as the primary radicals responsible for the chemical effect of ultrasound, particularly in the field of water decontamination, as confirmed by EPR‐spin trapping, chemical dosimetries, chemiluminescence of luminol and identification of hydroxylation intermediates …”

Biochar‐mediated oxidation of phenol by persulfate activated with zero‐valent iron

“…and Supporting Information Fig. S3, spectra of DMPO‐·OH and DMPO‐ SO 4 − · were determined from their hyperfine splitting constants (DMPO‐·OH: aH = aN = 14.98 G; DMPO‐ SO 4 − ·: aN = 13.5 G, aH = 9.7 G, aH = 1.48 G, and aH = 0.75 G; G is unit of magnetic field strength) . The EPR spectra supported the formation of ·OH and SO 4 − · in the Fe(0)–carbonaceous material–persulfate systems.…”