Effective Species in UV/H2O2 Advanced Oxidation Process: A Case of Degradation of 3,3′-Diethylthiadicarbocyanine Iodide (DTDCI)

Steady-state and time-resolved absorption spectroscopic methods were employed to investigate the degradation of 3,3′-diethylthiadicarbocyanine iodide (DTDCI) to unveil the mechanism of the UV/H₂O₂ advanced oxidation process (AOP) and the corresponding kinetics at different pHs. Upon successive 266 nm pulsed irradiation of the H₂O₂/DTDCI mixture, the degradation of DTDCI was intensified under acidic conditions. Upon 266 nm pulsed laser excitation of the H₂O₂/DTDCI mixture probed by transient absorption spectroscopy, the decay of the absorbance of DTDCI at 638 nm was strengthened at acidic pH. As the prepared concentration of H₂O₂ was much higher than that of DTDCI, OH^• generated from the photolysis of H₂O₂ could quickly react with H₂O₂ to form HO₂^• under acidic conditions while the population of HO₂^•/O₂^•– (superoxide) was increased by lowering pH. Thus, the acidity-assisted DTDCI decay was majorly caused by HO₂^•, which is more reactive than O₂^•–. The bimolecular rate coefficient of DTDCI and HO₂^• was estimated to be (2.0 ± 0.2) × 10⁷ M^–1 s^–1 upon numerical analyses. On the basis of these kinetic analyses, the roles and reactivities of OH^•, HO₂^•, and O₂^•– in UV/H₂O₂ AOP and the relevant mechanisms should be carefully evaluated and interpreted for miscellaneous chemical systems.