Quantum chemical investigation on photodegradation mechanisms of sulfamethoxypyridazine with dissolved inorganic matter and hydroxyl radical

Photodegradation fate of different dissociation species of antidepressant paroxetine and the effects of metal ion Mg2+: Theoretical basis for direct and indirect photolysis

Paroxetine (abbreviated as PXT) has been widely used as one of the standard antidepressants for the treatment of depression. PXT has been detected in the aqueous environment. However, the photodegradation mechanism of PXT remains unclear. The present study aimed to use density functional theory and time-dependent density functional theory to study the photodegradation process of two dissociated forms of PXT in water. The main mechanisms include direct and indirect photodegradation via reaction with ·OH and ¹O₂ and photodegradation mediated by the metal ion Mg²⁺. Based on the calculations, PXT and PXT-Mg²⁺ complexes in water are photodegraded mainly indirectly and directly. It was found that PXT and PXT-Mg²⁺ complexes were photodegraded by H-abstraction, OH-addition and F-substitution. The main reaction of PXT indirect photolysis is OH-addition reaction, while the main reaction of PXT⁰-Mg²⁺ complex is H-abstraction. All the reaction pathways of H-abstraction, OH-addition and F-substitution are exothermic. PXT⁰ reacts more readily with ·OH or ¹O₂ in water than PXT⁺. However, the higher activation energy of PXT with ¹O₂ indicates that the ¹O₂ reaction plays a minor role in the photodegradation pathway. The direct photolysis process of PXT includes ether bond cleavage, defluorination, and dioxolane ring-opening reaction. In the PXT-Mg²⁺ complex, the direct photolysis process occurs via a dioxolane ring opening. Additionally, Mg²⁺ in water has a dual effect on the direct and indirect photolysis of PXT. In other words, Mg²⁺ can inhibit or promote their photolytic reactions. Overall, PXT in natural water mainly undergo direct and indirect photolysis reactions with ·OH. The main products include direct photodegradation products, hydroxyl addition products and F-substitution products. These findings provide critical information for predicting the environmental behavior and transformation of antidepressants.

Influence of pH on the Kinetics and Products of Photocatalytic Degradation of Sulfonamides in Aqueous Solutions

The aims of the study were to determine the kinetics of the photocatalytic degradation of six sulfonamides in the presence of TiO₂-P25 in acidic, neutral, and alkaline solutions and to identify the structures of the stable products. It was stated that the pH of the solution significantly affected the photocatalytic degradation rate of sulfonamides in acidic and alkaline environments, and the effect likely depended on the susceptibility of sulfonamides to attack by hydroxyl radicals. In the post-reaction mixture, we identified the compounds resulting from the substitution of the aromatic rings with a hydroxyl group; the amide hydrolysis products; the hydroxylamine-, azo, and nitro derivatives; and the compounds formed via the elimination of the sulfone group. Moreover, previously unknown azo compounds were detected. Some degradation products of sulfonamides may exhibit marked bacteriostatic activity and high phytotoxicity. The azo and nitro compounds formed in an acidic environment may be potentially more toxic to aquatic ecosystems than the initial compounds.

Comparison of chemical and biological degradation of sulfonamides: Solving the mystery of sulfonamide transformation

Sulfonamides (SAs) are widespread in aquatic environments and pose serious environmental risks. The removal efficiencies and degradation mechanisms of SAs in both chemical and biological degradation systems were comprehensively reviewed. Density functional theory (DFT) was utilized to decipher the reaction types and reactive sites of both degradation mechanisms at the electron level. In chemical degradation, the rate of the reactive oxidants to degrade SAs follows the order SO₄•^- ≈ •OH > O₃ > ¹O₂ > ClO₂ ≈ Fe(VI) ≈ HOCl > peroxymonosulfate. pH affects the oxidation-reduction potentials of oxidants, the reactivity of SAs, and the intermolecular force between oxidants and SAs, thereby affecting the chemical degradation efficiencies and mechanisms. In biological degradation, oxidoreductase produced by bacteria, fungi, algae, and plants can degrade SAs. The catalytic activity of the enzyme is affected by the enzyme system, reaction conditions, and type of SAs. Despite the different reaction modes and removal efficiencies of SAs in chemical degradation and biological degradation, the transformation pathways and products show commonalities. Modification of the amino (N¹H₂-) moiety and destruction of sulfonamide bridge (-SO₂-N¹¹H-) moiety are the main pathways for both chemical and biological degradation of SAs. Most oxidants or enzymes can react with the N¹H₂- moiety. Reactions of the -SO₂-N¹¹H- moiety are mainly initiated by the cleavage of S-N bonds for five-membered heterocyclic ring-substituted SAs, and by SO₂ extrusion for six-membered heterocyclic ring-substituted SAs. Chlorine substitution and coupling on the N¹H₂- moiety, hydroxylation of the benzene moiety, oxidation of methyl, and isomerization of the R substituents are the transformation pathways unique to chemical degradation. Formylation/acetylation, glycosylation, pterin conjugation, and deamination of the N¹H₂- moiety are the transformation pathways unique to biological degradation. DFT studies revealed the same reaction types and the same reactive sites of SAs in chemical and biological degradation. Electrophiles are mostly prone to attack the N¹ atom on the amino moiety of neutral SAs and the N¹¹ atom on the sulfonamide bridge moiety of anionic SAs, leading to nitration or electrophilic substitution of the amino moiety and the cleavage of S-N bonds or SO₂ extrusion of the sulfonamide bridge moiety. Reactions on the -SO₂-N¹¹H- moiety eliminate antibacterial activity in the SA degradation process. This review elucidated SA transformation by comparing the chemical and biological degradation of SAs. This could provide theoretical guidance for the development of more efficient and economical treatment technologies for SAs.

Wavelength Dependence of the Transformation Mechanism of Sulfonamides Using Different LED Light Sources and TiO2 and ZnO Photocatalysts

The comparison of the efficiency of the commercially available photocatalysts, TiO2 and ZnO, irradiated with 365 nm and 398 nm light, is presented for the removal of two antibiotics, sulfamethazine (SMT) and sulfamethoxypyridazine (SMP). The •OH formation rate was compared using coumarin, and higher efficiency was proved for TiO2 than ZnO, while for 1,4-benzoquinone in O2-free suspensions, the higher contribution of the photogenerated electrons to the conversion was observed for ZnO than TiO2, especially at 398 nm irradiation. An extremely fast transformation and high quantum yield of SMP in the TiO2/LED398nm process were observed. The transformation was fast in both O2 containing and O2-free suspensions and takes place via desulfonation, while in other cases, mainly hydroxylated products form. The effect of reaction parameters (methanol, dissolved O2 content, HCO3- and Cl-) confirmed that a quite rarely observed energy transfer between the excited state P25 and SMP might be responsible for this unique behavior. In our opinion, these results highlight that "non-conventional" mechanisms could occur even in the case of the well-known TiO2 photocatalyst, and the effect of wavelength is also worth investigating.

Using general computational chemistry strategy to unravel the reactivity of emerging pollutants: An example of sulfonamide chlorination

Increasing number of emerging pollutants in environments requires an effective approach which can facilitate the prediction of reactivity and provide insights into the reaction mechanisms. Computational chemistry is exactly the tool to fulfill this demand with its good performance in theoretical investigation of chemical reactions at molecular level. In this study, chlorination of sulfonamide antibiotics is used as an illustration to present a systematic strategy demonstrating how computational chemistry can be applied to investigate the reaction behavior of emerging pollutants. Sulfonamides is a class of micropollutants that contain the common structure of 4-aminobenzenesulfonmaide while differ in their heterocycles. Based on the calculated conceptual DFT indices, the reactive sites of sulfonamide are successfully predicted, which locate on their common structure of 4-aminobenzenesulfonmaide. Therefore, all sulfonamides follow the similar reaction pathway. Product identification by LTQ-Orbitrap MS further verifies the in silico prediction. Three critical pathways are discovered, i.e., S-N bond cleavage, Cl-substitution onto aniline-N, and the following rearrangement to lose -SO₂- group, among which Cl-substitution is the key step due to its lowest free energy barrier. Heterocycles impact the reaction rate by affecting the electronic density of aniline group. In general, the more electron-donating the heterocycle is, the more readily sulfonamides to be chlorinated.

Effect of substituents in hydroxyl radical-mediated degradation of azo pyridone dyes: Theoretical approaches on the reaction mechanism

Understanding the degradation behavior of azo dyes in photocatalytic wastewater treatment is of fundamental and practical importance for their application in textile-processing and other coloration industries. In this study, quantum chemistry, as density functional theory, was used to elucidate different degradation pathways of azo pyridone dyes in a hydroxyl radical (HO^•)-initiated photocatalytic system. A series of substituted azo pyridone dyes were synthesized by changing the substituent group in the para position of the benzene moiety, ranging from strong electron-donating to strong electron-withdrawing groups. The effect of dye molecular structure on the photocatalytic degradation reaction mechanism was analyzed and quantification of substituent effects on the thermodynamic and kinetics parameters was performed. Potential energy surface analysis revealed the most susceptible reaction site for the HO^• attack. The calculated reaction barriers are found to be strongly affected by the nature of substituent group with a good correlation using Hammett σ_p constants and experimentally determined reaction rates. The stability of pre-reaction complexes and transition state complexes were analyzed applying the distortion-interaction model. The increased stability of the transition state complexes with the distancing from the substituent group has been established.

Mechanistic Insights into Dissolved Organic Sulfur Photomineralization through the Study of Cysteine Sulfinic Acid

Photochemical reactions convert dissolved organic matter (DOM) into inorganic and low-molecular-weight organic products, contributing to its cycling across environmental compartments. However, knowledge on the formation mechanisms of these products is still scarce. In this work, we investigate the triplet-sensitized photodegradation of cysteine sulfinic acid, a (photo)degradation product of cysteine, to sulfate (SO₄^2-). We use kinetic analysis, targeted experiments, and previous literature from several fields of chemistry to explain the elementary steps that lead to the release of sulfate. Our analysis indicates that triplet sensitizers act as one-electron oxidants on the sulfinate S lone pair. The resulting radical undergoes C-S fragmentation to form SO₂, which becomes hydrated to sulfite/bisulfite (S(IV)). S(IV) is further oxidized to SO₄^2- in the presence of triplet sensitizers and oxygen. We point out that the reaction sequence SO₂ ⇌ S(IV) → SO₄^2- is valid independently of the chemical structure of the model compound and might represent a sulfate photoproduction mechanism with general validity for DOS. Our mechanistic investigation revealed that amino acids in general might also be photochemical precursors of CO₂, ammonia, acetaldehyde, and H₂O₂ and that reaction byproducts can influence the rate and mechanism of S(IV) (photo)oxidation.

The importance of reactive oxygen species on the aqueous phototransformation of sulfonamide antibiotics: kinetics, pathways, and comparisons with direct photolysis

Sulfonamide antibiotics (SAs) are increasingly detected as aquatic contaminants and exist as different dissociated species depending on the pH of the water. Their removal in sunlit surface waters is governed by photochemical transformation. Here we report a detailed examination of the hydroxyl radical (•OH) and singlet oxygen (¹O₂) mediated photooxidation of nine SAs: sulfamethoxazole, sulfisoxazole, sulfamethizole, sulfathiazole, sulfamethazine, sulfamerazine, sulfadiazine, sulfachloropyridazine and sulfadimethoxine. Both •OH and ¹O₂ oxidation kinetics varied depending on the dominant protonated states of the SA in question (H₂SAs⁺, HSAs⁰ and SAs^-) as a function of pH. Based on competition kinetic experiments and matrix deconvolution calculations, HSAs⁰ or SAs^- (pH ∼5-8) were observed to be more highly reactive towards •OH, while SAs^- (pH ∼8) react the fastest with ¹O₂ for most of the SAs tested. Using the empirically derived rates of reaction for the speciated forms at different pHs, the environmental half-lives were determined using typical ¹O₂ and •OH concentrations observed in the environment. This approach suggests that photochemical ¹O₂ oxidation contributes more than •OH oxidation and direct photolysis to the overall phototransformation of SAs in sunlit waters. Based on the identification of key photointermediates using tandem mass spectrometry, ¹O₂ oxidation generally occurred at the amino moiety on the molecule, whereas •OH reaction experienced multi-site hydroxylation. Both these reactions preserve the basic parent structure of the compounds and raise concerns that the routes of phototransformation give rise to intermediates with similar antimicrobial potency as the parent SAs. We therefore recommend that these phototransformation pathways are included in risk assessments concerning the presence and fate of SAs in waste and surface waters.

DFT/TDDFT insights into effects of dissociation and metal complexation on photochemical behavior of enrofloxacin in water

Elucidation of the mechanisms underlying the effects of different dissociated forms and metal ion complexation on the photochemical behavior of antibiotics in aqueous media is a key problem and requires further research. We examined the mechanism of the direct photolysis of enrofloxacin (ENRO) in different dissociated forms in water and the impact of metal ions (Mg²⁺) on the photolysis of ENRO using density functional theory and time-dependent density functional theory. The results showed that different dissociated forms of ENRO exhibited diverse maximum electronic absorbance wavelengths (ENRO³⁺ (264 nm) < ENRO^- (278 nm) < ENRO⁰ (280 nm) < ENRO²⁺ (282 nm) < ENRO⁺ (306 nm)). The calculations of the reaction pathways and activation energies (E_a) in the photolysis of ENRO⁰/ENRO⁺/ENRO^- showed that defluorination was the main reaction pathway. The removal of cyclopropane was the main reaction pathway for the direct photolysis of ENRO²⁺/ENRO³⁺. Furthermore, the presence of Mg²⁺ was observed to change the order of the maximum electronic absorbance wavelengths and increases the intensities of the ENRO absorbance peaks. Calculations of the photolysis reaction pathways showed that the presence of Mg²⁺ increased the E_a for the most direct photolysis pathways of ENRO, while its presence decreased the E_a for several partial direct photolysis pathways such as the pathway in which the piperazine ring moiety of ENRO⁰/ENRO³⁺ is damaged and the pathway in which cyclopropane is released from ENRO³⁺. The findings on the photolysis behavior of ENRO in water system have provided useful information on the risk assessment of antibiotics.