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

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.