Bimolecular versus Trimolecular Reaction Pathways for H2O2 with Hypochlorous Species and Implications for Wastewater Reclamation.

The benchmark advanced oxidation technology (AOT) that uses UV/H2O2 integrated with hypochlorous species exhibits great potential in removing micropollutants and enhancing wastewater treatability for reclamation purposes. Although efforts have been made to study the reactions of H2O2 with hypochlorous species, there exist great discrepancies in the order of reaction kinetics, the rate constants, and the molecule-level mechanisms. This results in an excessive use of hypochlorous reagents and system underperformance during treatment processes. Herein, the titled reaction was investigated systematically through complementary experimental and theoretical approaches. Stopped-flow spectroscopic measurements revealed a combination of bi- and trimolecular reaction kinetics. The bimolecular pathway dominates at low H2O2 concentrations, while the trimolecular pathway dominates at high H2O2 concentrations. Both reactions were simulated using direct dynamics trajectories, and the pathways identified in the trajectories were further validated by high-level quantum chemistry calculations. The theoretical results not only supported the spectroscopic data but also elucidated the molecule-level mechanisms and helped to address the origin of the discrepancies. In addition, the impact of the environmental matrix was evaluated by using two waters with discrete characteristics, namely municipal wastewater and ammonium-rich wastewater. Municipal wastewater had a negligible matrix effect on the reaction kinetics of H2O2 and the hypochlorous species, making it a highly suitable candidate for this integration technique. The obtained in-depth reaction mechanistic insights will enable the development of a viable and economical technology for safe water reuse.

An experimental and theoretical study on the degradation of clonidine by hydroxyl and sulfate radicals.

Emerging contaminants such as pharmaceuticals that cannot be completely removed by traditional biological treatments are ubiquitously present in water bodies with detected concentrations ranging from ng L-1 to mg L-1. Advanced oxidation technologies (AOTs) are promising, efficient, and environmentally friendly for the removal of these pharmaceuticals. In this study, we investigated the degradation kinetics of a model pharmaceutical, clonidine (CLD), via hydroxyl radical (OH) in UV/H2O2 and sulfate radical (SO4•-) in UV/peroxydisulfate (PS) systems for the first time. The second-order rate constants (k) of protonated cationic CLD with OH and SO4•- were measured to be (2.15 ±â€¯0.07) × 109 M-1 s-1 and (1.12 ±â€¯0.03) × 109 M-1 s-1, respectively. We also calculated the pKa value of CLD and thermodynamic behaviors for reactions of CLD/HCLD+ with OH and SO4•- at M05-2X/6-311++G**//M05-2X/6-31+G** level with SMD solvation model. The pKa value was calculated to be 8.14, confirming the literature value. H atom abstraction pathway was the most favorable pathway for both OH and SO4•-, while single electron transfer pathway was thermodynamically feasible only for SO4•- for CLD but not for HCLD+. In addition, the reactivities of both tautomeric forms of CLD (i.e., amino and imino CLD) with both radicals were also investigated. This study contributed to a better understanding on the degradation mechanisms of CLD and proposed the possibilities of the elimination of pharmaceuticals by applying AOTs during wastewater treatment processes.

Experimental and theoretical insight into hydroxyl and sulfate radicals-mediated degradation of carbamazepine.

Carbamazepine (CBZ), a widely detected pharmaceutical in wastewaters, cannot currently be treated by conventional activated sludge technologies, as it is highly resistant to biodegradation. In this study, the degradation kinetics and reaction mechanisms of CBZ by hydroxyl radical (OH) and sulfate radical ()-based advanced oxidation processes (AOPs) were investigated with a combined experimental/theoretical approach. We first measured the UV absorption spectrum of CBZ and compared it to the theoretical spectrum. The agreement of two spectra reveals an extended π-conjugation system on CBZ molecular structure. The second-order rate constants of OH and with CBZ, measured by competition kinetics method, were (4.63 ±â€¯0.01) × 109 M-1 s-1 and (8.27 ±â€¯0.01) × 108 M-1 s-1, respectively at pH 3. The energetics of the initial steps of CBZ reaction with OH and were also calculated by density functional theory (DFT) at SMD/M05-2X/6-311++G**//M05-2X/6-31 + G**level. Our results reveal that radical addition is the dominant pathway for both OH and . Further, compared to the positive ΔGR0 value for the single electron transfer (SET) reaction pathway between CBZ and OH, the ΔGR0 value for SET reaction between CBZ and is negative, showing that this reaction route is thermodynamically favorable. Our results demonstrated the remarkable advantages of AOPs for the removal of refractory organic contaminants during wastewater treatment processes. The elucidation of the pathways for the reaction of OH and with CBZ are beneficial to predict byproducts formation and assess associated ecotoxicity, providing an evaluation mean for the feasibility of AOPs application.