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.
Ultrasonic treatment serves as an alternative and/or complementary remediation technology for contaminated soils and sediments in comparison to traditional methods, such as dredging and
in situ
capping. Ultrasonic waves of alternating rarefaction and compression cycles create cavitation bubbles over the course of microseconds. Cavitational collapse produces enormous temperatures and pressures that initiate thermolysis of water and subsequent radical oxidation. Physical effects of ultrasound increase mass transfer rates, whether it be via desorption/dissolution of contaminants or solute transport in porous media. Both of these processes are integral in remediation. Desorption and dissolution ensure that contaminants are available for reaction in the aqueous phase. Increased solute transport by ultrasound enhances amendment delivery to contaminated sites. The purpose of this article is to describe both the potential for and challenges of utilizing ultrasound as a remediation technology. Much of this knowledge has been laid by the literature exploring the sonochemical effects on pollutants in bench‐scale sediment slurries. Both bench‐scale ultrasonic studies on remediation of contaminated soils and sediments and enhanced oil recovery using ultrasound have provided both mechanistic understandings and insights for pilot‐scale systems. Although challenges exist in scaled‐up systems, ultrasound is a multifunctional treatment technology with unique features compared to other remediation technologies.
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