Theoretical studies on degradation mechanism for OH-initiated reactions with diuron in water system

The theoretical investigation of OH-induced degradation mechanisms of isoproturon

Herbicides are widely applied in agriculture, which causes emerging concern for contaminating surface water and groundwater. The degradation of isoproturon, being a typical phenylurea herbicide, has been studied much with advanced oxidation processes. However, little information on the detailed degradation mechanism from the theoretical level is available. In the present work, the degradation mechanisms of OH-initiated reactions of isoproturon were studied at the MPWB1K/6-311+G(3df, 2p)//MPWB1K/6-31+G(d, p) level. The calculation results display that H-atom abstraction, ·OH addition, and ·OH substitution pathways are found for ·OH and isoproturon reactions. For H-atom abstractions, the results show that aliphatic H atoms are more easily abstracted by ·OH than the aromatic H atoms, and the pathway R6 has the lowest energy barrier of 6.65 kcal mol^-1, indicating that H in the -CH₃ group attached to the N2 atom could be most favorably abstracted. Six pathways for ·OH addition to the six sites of the phenyl ring are identified with low energy barriers, which are in good accordance with the fact that the additions of ·OH to the phenyl ring were the important steps from experimental results. For ·OH substitution reactions, two pathways with high activation energies are found, and the corresponding products have been also detected from previous experimental studies, suggesting that the substitution reactions could also occur.

Mechanism and kinetics of diuron oxidation by hydroxyl radical addition reaction

Diuron is a phenyl urea herbicide used to control weeds in agricultural lands. The degradation of diuron in the atmosphere takes place dominantly via reaction with OH radicals. In this work, the OH addition reaction of diuron has been studied by using density functional theory methods M06-2X, ωB97X-D and MPWB1K with 6-31G(d,p) basis set. The calculated thermochemical parameters show that OH addition reaction occurs favourably at C₂ position of diuron. The rate constant is calculated for the favourable initial reaction pathway by using canonical variational transition state theory with small curvature tunnelling (SCT) correction over the temperature range of 200-1000 K. The reaction of initially formed diuron-OH adduct intermediate with O₂ leads to the formation of peroxy radical intermediate. The reaction of peroxy radical intermediate with HO₂ and NO_x (x = 1, 2) radicals is studied in detail. The results obtained from time-dependent density functional theory (TDDFT) calculations show that the intermediates and products formed from oxidation of diuron can be easily photolyzed in the sunlight. This study provides thermodynamical and kinetic data for the atmospheric oxidation of diuron by OH radical addition reaction and demonstrates the atmospheric chemistry of diuron and its derivatives.