Direct Determination of Absolute Radical Quantum Yields in Hydroxyl and Sulfate Radical-Based Treatment Processes

The absolute radical quantum yield ($$) is a critical parameter to evaluate the efficiency of radical-based processes in engineered water treatment. However, measuring $$ is fraught with challenges, as current quantification methods lack selectivity, specificity, and anti-interference capabilities, resulting in significant error propagation. Herein, we report a direct and reliable time-resolved technique to determine $$ at pH 7.0 for commonly used radical precursors in advanced oxidation processes. For H₂O₂ and peroxydisulfate (PDS), the values of $$_•OH and $$ at 266 nm were measured to be 1.10 ± 0.01 and 1.46 ± 0.05, respectively. For peroxymonosulfate (PMS), we developed a new approach to determine $$ with terephthalic acid as a trap-and-trigger probe in the nonsteady state system. For the first time, the $$ value was measured to be 0.56 by the direct method, which is stoichiometrically equal to $$ (0.57 ± 0.02). Additionally, radical formation mechanisms were elucidated by density functional theory (DFT) calculations. The theoretical results showed that the highest occupied molecular orbitals of the radical precursors are O-O antibonding orbitals, facilitating the destabilization of the peroxy bond for radical formation. Electronic structures of these precursors were compared, aiming to rationalize the tendency of the $$ values we observed. Overall, this time-resolved technique with specific probes can be used as a reliable tool to determine $$, serving as a scientific basis for the accurate performance evaluation of diverse radical-based treatment processes.