Kinetics and Mechanism of Degradation of Reactive Radical-Mediated Probe Compounds by the UV/Chlorine Process:...

The UV/chlorine process, by combining chlorination with UV irradiation, has been recently considered as a highly efficient advanced oxidation process (AOP) technology in water treatment. Nitrobenzene (NB), benzoic acid (BA), and p-chlorobenzoic acid (pCBA) are widely used as model probe compounds in the UV/chlorine system to calculate the second-order rate constants of the specific radical reaction with target contaminates by a competitive kinetics method. A comprehensive understanding of probe compounds' reaction mechanism with reactive radicals is critical for investigation of the UV/chlorine reaction system. Here, we evaluated the radical-mediated reaction kinetics and mechanism of NB, BA, and pCBA in the UV/chlorine process using theoretical calculations and experimental studies. The main reactive radicals ^•OH, ^•ClO, and ^•Cl in the UV/chlorine process for the initial reaction with NB, BA, and pCBA can be explained by H-abstraction and addition pathways. The ΔE ^0,≠ values for the ^•OH reaction with NB, BA, and pCBA were in the range of 5.0-8.0, 3.7-8.2, and 3.4-8.2 kcal mol^-1, respectively. The ΔE ^0,≠ values for ^•ClO and ^•Cl reactions with these three probe compounds were higher than those of ^•OH, indicating slower ^•ClO- and ^•Cl-initiated reactions than that of the ^•OH-initiated reaction. The theoretically calculated radical-mediated reaction kinetic rate constants (k _CP ^C) for NB, BA, and pCBA were 4.58 × 10^-3, 1.28 × 10^-2, and 1.6 × 10^-2 s^-1, respectively, which was consistent with the experimentally determined pseudo-first-order rate constant (k _CP ^RR) in the UV/chlorine process. Interestingly, theoretical calculations showed that ^•ClO and ^•Cl played an important role in subsequent reactions of NB-OH radicals, converting to hydroxylated and chlorinated products, which were further confirmed by experimental products' identification. The findings from this study indicated that quantum chemistry calculations provide an effective means to investigate the reaction kinetics and mechanism of chemicals in the UV/chlorine process.