Quantification of hydroxyl radicals in photocatalysis and acoustic cavitation: Utility of coumarin as a chemical probe

In-situ hydroxyl radical production is a characteristic of advanced oxidation processes (AOPs) and its quantification is crucial for assessing the performance of these processes. Direct OH quantification is however unlikely due to their high reactivity and short life-time (~10⁻⁶ s). Therefore, chemical probes (like coumarin) are often used to indirectly quantify OH radicals. The resulting hydroxylated products of these probe molecules are relatively stable and detectable (like 7-hydroxycoumarin; 7OHC); their concentration is usually related to the OH radical concentration. Reported literature suggests the use of a constant fraction (6.1%) as the yield of 7OHC from total OH radicals generated (δ). This was originally determined from γ-irradiation studies of coumarin and translated across to other AOPs. It is however highly unlikely that this fraction is same across other AOPs. To address this gap and as a means of comparing AOPs, in the present work, we used coumarin as a probe to quantify 7OHC during acoustic cavitation (AC) and photocatalysis (PC) and determined the OH radical yields. Since δ cannot be determined directly, an alternative and simple means of comparison based on the amount of OH radicals utilised for desired reactions i.e., hydroxylation was exploited. Based on the proposed means of comparison, it was determined that OH radicals utilised by the probe was higher for PC than AC by ~ 25 times under the considered process operating conditions. The presented results will be useful for researchers and engineers interested in quantifying hydroxyl radicals and optimising AOPs.