The bacteriostatic antibiotics, sulfamethoxazole (SMX) and trimethoprim (TMP), have frequently been found in wastewater and surface water, which raises the concerns about their ecotoxicological effects. The indirect photochemical transformation has been proven to be an efficient way to degrade SMX and TMP. In this study, the reaction mechanisms of the degradation by SMX and TMF by OH radicals were investigated by theoretical calculations. Corresponding rate constants were determined and the eco-toxicity of SMX and TMP and its degradations products were predicted using theoretical models. The results indicate that the most favorable pathways for the transformation of SMX and TMP are both •OH-addition reaction of benzene ring site with lowest Gibbs free energy barriers (6.86 and 6.21 kcal mol⁻¹). It was found that the overall reaction rate constants of •OH-initial reaction of SMX and TMP are 1.28 × 10⁸ M⁻¹ s⁻¹ and 6.21 × 10⁸ M⁻¹ s⁻¹ at 298 K, respectively. When comparing the eco-toxicity of transformation products with parent SMX and TMP, it can be concluded that the acute and chronic toxicities of the degraded products are reduced, but some products remain harmful for organisms, especially for daphnid (toxic or very toxic level). This study can give greater insight into the degradation of SMX and TMP by •OH through theoretical calculations in aquatic environment.

Hexafluoropropylene oxide dimer acid (HFPO-DA) is widely used as a substitute for perfluorooctanoic acid (PFOA). HFPO-DA exhibits high water solubility and low adsorption potential, conferring significant fluidity in aquatic environments. Given that the toxicity of HFPO-DA is similar to PFOA, it is necessary to control its content in aquatic environments. Electrochemical and thermally-activated persulfates have been successfully used to degrade HFPO-DA, but UV-activated persulfates cannot degrade the compound. Given that research on degradation mechanisms is still incomplete and lacks kinetic research, the mechanism and kinetic calculations of oxidative degradation were studied in detail using DFT calculations. And the toxicity of HFPO-DA degradation intermediates and products was evaluated to reveal the feasibility of using advanced oxidation process (AOP) technology based on persulfate to degrade HFPO-DA in wastewater. The results showed that the committed step of HFPO-DA degradation was initiated by the electron transfer reaction of SO₄^•- radicals. This reaction is not spontaneous at room temperature and requires sufficient electrical or thermal energy to be absorbed from the external environment. The perfluoroalcohol produced during this reaction can subsequently undergo four possible reactions: H atom abstraction from alcohol groups by an OH radical; H atom abstraction by SO₄^•-; direct HF removal; and HF removal with water as the catalyst. The final degradation products of HFPO-DA mainly include CO₂, CF₃CF₂COOH, CF₃COOH, FCOOH and HF, which has been identified through previous experimental analysis. Ecotoxicity assessment indicates that degradation does not produce highly toxic intermediates, and that the final products are non-toxic, supporting the feasibility of persulfate-based AOP technologies.

Effect of H₂O and NH₃ on the synergistic oxidation reaction of SO₂ and NO₂ is investigated by theoretical calculation using the molecule system SO₂-2NO₂-nH₂O (n = 0, 1, 2, 3) and SO₂-2NO₂-nH₂O-mNH₃ (n = 0, 1, 2; m = 1, 2). Calculated results show that SO₂ is oxidized to SO₃ by N₂O₄ intermediate. The additional H₂O in the systems can reduce the energy barrier of oxidation step. The increasing number of H₂O molecules in the systems enhances the effect and promotes the production of HONO. When the proportion of H₂O to NH₃ is 1:1, with NH₃ included in the system, the energy barrier is lower than two pure H₂O molecules in the oxidation step. The present study indicates that the H₂O and NH₃ have thermodynamic effects on promoting the oxidation reaction of SO₂ and NO₂, and NH₃ has a more significant role in stabilizing product complexes. In these hydrolysis reactions, nethermost barrier energy (0.29 kcal/mol) can be found in the system SO₂-2NO₂-H₂O. It is obvious that the production of HONO is energetically favorable. A new reaction mechanism about SO₂ oxidation in the atmosphere is proposed, which can provide guidance for the further study of aerosol surface reactions.

We present a study of benzoic acid with OH, NO³ and SO₄⁻ radicals in the atmospheric environment.