Ferrate(vi) is an efficient and environmentally friendly oxidant for the degradation of organic micropollutants. However, the related mechanism for the degradation is ambiguous and can hardly be elucidated empirically due to the rapid oxidation process and unstable intermediates for experimental trapping. Herein we performed density function theory (DFT) calculations to unveil the mechanism of ferrate(vi)-mediated degradation, taking sulfamethoxazole as a model compound. The results show that nucleophilic attack (rather than electrophilic attack) of HFeO on the isoxazole moiety of sulfamethoxazole initiates the subsequent degradations, and ferrate(vi) rather than the water molecule provides O atoms for the oxidation of the nitroso group and isoxazole moiety. Electron delocalization from the Fe atom to the isoxazole moiety is crucial for the ring-opening of isoxazole, and organometallic intermediates suggested previously are not the necessary ones in the oxidation of sulfamethoxazole by HFeO. Thus, this study has theoretically clarified the ferrate(vi) oxidation mechanisms for a representative sulfonamide, which were also partially corroborated by the intermediates and products observed in the previous experimental studies for phosphite and tryptophan. This study provides an exemplification on the application of quantum chemical calculations to clarify the degradation pathways of organic micropollutants, which is important for the prediction of degradation products needed in their engineering design.

The degradation of azo dyes has attracted many research efforts not only due to the resulting environmental problems but also because the azo compounds with various substituents may show different degradation mechanism. It has been computationally found here, for the first time, that the HO• initiated cleavages of C-N and N-N bonds of alizarin yellow R with carboxyl group are kinetically competitive. In view of the formation of HO• adducts, the C-N and N-N bond cleavages of the hydrazone tautomer of alizarin yellow R are also kinetically competitive, but the former is more thermodynamically favorable. This result is different from that previously reported for the hydrozone tautomers of Acid Orange 7 and Acid Orange 8 containing hydroxyl and azo groups in neighboring positions, which are favorable to follow C-N bond cleavage mechanism both kinetically and thermodynamically. The decarboxylation occurs via an attack of HO• to the benzene ring carbon connecting to the carboxyl group rather than a direct attack of HO• to the carboxyl carbon atom. The anion form has higher reactivity than the neutral form in all of the reactions investigated. In addition, a water molecule as a proton relay reagent could significantly reduce the energy barrier for the N-N bond cleavage of alizarin yellow R. Figure 2. Optimized structures (distances in Å) and relative energies in the solution for HO• attacking the C3 and C6 sites of AH to result in the cleavage of the C-N bond (energies in kcal/mol relative to the energy sum of AH and HO•) X. JIAO ET AL. wileyonlinelibrary.com/journal/poc a The energies are relative to the energy sum of AH (or A) and HO•. The AH_Ci_TS and AH_Ci_P (i = 1, 2, 17, 18 and 21, atom labeling shown in Chart 3) denote the HO• addition transition states and resulting adducts, respectively. The same is true for the A case. DFT STUDY ON DEGRADATION MECHANISM OF AZO COMPOUNDS

Diarylamines ( Ar 2 NH ) are generally used as antioxidants to inhibit or retard the auto-oxidation degradation of lubricating oil by trapping ROO• radicals. In the present study, 20 kinds of 4,4′-disubstituted diphenylamine compounds were investigated through density functional theory (DFT) calculations. The results indicate that the N – H bond dissociation enthalpy (BDE) linearly correlates its one-electron oxidation potential, the difference in Mulliken atomic charge on the two atoms of N – H bond, the reaction rate constant of hydrogen transfer from Ar 2 NH to peroxy radical, and the chemical hardness of the resulted Ar 2 N • radical, respectively. The substitution of alkyl groups (electron-donating groups) decreases the N – H BDE, one-electron oxidation potential and the reaction rate constant, while that of significant electron-withdrawing groups such as - NO 2 and - COOCH 3 increases these three parameters. The electron-donating groups such as alkyls could improve the antioxidation performance of 4,4′-disubstitued diphenylamines whereas electron-withdrawing groups have the contrary effect. In addition, the frontier molecular orbital of Ar 2 NH has been also analyzed.