Mechanism and Kinetics of the Reaction of Nitrosamines with OH Radical: A Theoretical Study

Treating disinfection byproducts with UV or solar irradiation and in UV advanced oxidation processes: A review

This review focuses on the degradation kinetics and mechanisms of disinfection byproducts (DBPs) under UV and solar irradiation and in UV-based advanced oxidation processes (AOPs). A total of 59 such compounds are discussed. The processes evaluated are low pressure, medium pressure and vacuum UV irradiation, solar irradiation together with UV/hydrogen peroxide, UV/persulfate and UV/chlorine AOPs. Under UV and solar irradiation, the photodegradation rates of N-nitrosamines are much higher than those of halogenated DBPs. Among halogenated DBPs, those containing iodine are photodegraded more rapidly than those containing bromine or chlorine. This is due to differences in their bond energies (E_N-N < E_C-I < E_C-Br < E_C-Cl). Molar absorption coefficients at 254 nm and energy gaps can be used to predict the photodegradation rates of DBPs under low pressure UV irradiation. But many DBPs of interest cannot be degraded to half their original concentration with less than a 500 mJ cm^-2 dose of low pressure UV light. HO^• generally contributes to less than 30% of the degradation of DBPs except iodo-DBPs in UV/H₂O₂ AOPs. Reaction mechanisms under UV irradiation and in HO^•-mediated oxidation are also summarized. N-N bond cleavage initiates their direct UV photolysis of N-nitrosamines as C-X cleavage does among halogenated compounds. HO^• generally initiates degradation via single electron transfer, addition and hydrogen abstraction pathways. Information on the reaction rate constants of SO₄^•- and halogen radicals with DBPs is rather limited, and little information is available about their reaction pathways. Overall, this review provides improved understanding of UV, solar and AOPs.

Oxidation pathways, kinetics and branching ratios of chloromethyl ethyl ether (CMEE) initiated by OH radicals and the fate of its product radical: an insight from a computational study

The OH-initiated oxidation reactions of chloromethyl ethyl ether (CH2ClOCH2CH3) have been presented by using quantum calculation methods. The Minnesota functional (M06-2X) of the density functional theory method along with a polarization and diffuse 6-311++G(d,p) basis set is chosen for optimization and frequency calculations for H-abstractions from CH2ClOCH2CH3 molecules by OH radicals. Furthermore, the CCSD(T) method along with the same basis set is used for energy refinement of all optimized structures to obtain more accurate energies of the species. Our thermo-chemical calculation results show that the C˙HClOCH2CH3 product radical is more stable, corresponding to hydrogen atom abstraction from the -CH2Cl site, than others while the energy profile results indicate that the H-atom abstracted from the -OCH2 site follows the minimum energy path compared to other channels. The rate constants are computed using canonical transition state theory (CTST) within the temperature range of 250-450 K at 1 atm. The overall rate constant (at 298 K) for the abstraction reactions is found to be consistent with the earlier reported rate constant. The percentage branching ratios of different abstraction channels and the lifetime of chloromethyl ethyl ether are also given herein. We further investigated the unimolecular decomposition pathways of the CH2ClOCH(O˙)CH3 radical and found that unimolecular C-C bond scission is the kinetically and thermodynamically more feasible pathway compared to other unimolecular decomposition reactions.

Detection and structural characterization of nitrosamide H2NNO: A central intermediate in deNOx processes

The structure and bonding of H₂NNO, the simplest N-nitrosamine, and a key intermediate in deNO_x processes, have been precisely characterized using a combination of rotational spectroscopy of its more abundant isotopic species and high-level quantum chemical calculations. Isotopic spectroscopy provides compelling evidence that this species is formed promptly in our discharge expansion via the NH₂ + NO reaction and is collisionally cooled prior to subsequent unimolecular rearrangement. H₂NNO is found to possess an essentially planar geometry, an NNO angle of 113.67(5)°, and a N-N bond length of 1.342(3) Å; in combination with the derived nitrogen quadrupole coupling constants, its bonding is best described as an admixture of uncharged dipolar (H₂N-N=O, single bond) and zwitterion (H₂N⁺=N-O^-, double bond) structures. At the CCSD(T) level, and extrapolating to the complete basis set limit, the planar geometry appears to represent the minimum of the potential surface, although the torsional potential of this molecule is extremely flat.

The atmospheric oxidation mechanism and kinetics of 1,3,5-trimethylbenzene initiated by OH radicals – a theoretical study

The atmospheric fate of 1,3,5-trimethylbenzene is determined by OH-radical addition, and subsequent bicyclic peroxy radical ring closure and ring breaking pathways.