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

Abstract

The absolute radical quantum yield ( italicΦ ) is a critical parameter to evaluate the efficiency of radical-based processes in engineered water treatment. However, measuring italicΦ 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 italicΦ at pH 7.0 for commonly used radical precursors in advanced oxidation processes. For H2O2 and peroxydisulfate (PDS), the values of italicΦ •OH and normalΦ normalSO 4 • − 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 normalΦ • normalOH normalPMS with terephthalic acid as a trap-and-trigger probe in the nonsteady state system. For the first time, the normalΦ • normalOH normalPMS value was measured to be 0.56 by the direct method, which is stoichiometrically equal to Φ SO 4 • − PMS (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 italicΦ values we observed. Overall, this time-resolved technique with specific probes can be used as a reliable tool to determine italicΦ , serving as a scientific basis for the accurate performance evaluation of diverse radical-based treatment processes.