Kinetic and mechanistic insight into the OH-initiated atmospheric oxidation of 2,3,7,8-tetrachlorodibenzo-p-dioxin via OH-addition and hydrogen abstraction pathways: A theoretical investigation

A comprehensive Review into Emission Sources, Formation Mechanisms, Ecological Effects, and Biotransformation Routes of Halogenated Polycyclic Aromatic Hydrocarbons (HPAHs)

Halogenated polycyclic aromatic hydrocarbons (HPAHs, H = F, Cl, Br) are a new class of PAHs derivatives that mainly originate from the incomplete combustion of halogen-laden materials and via metallurgical operations. These compounds circulate extensively in various environmental matrices. This survey provides a comprehensive review on governing synthesis routes of HPAHs, their environmental occurrence, and their health and ecological effects. The review comprehensively enlists and presents emission sources of these emerging organic pollutants into the air that serves as their main reservoir. The formation of HPAHs ensues through successive addition reactions of related precursors accompanied by ring cyclization steps; in addition to direct unimolecular fragmentation of parents halogenated. Halogenation of parent PAHs rapidly occurs in saline ecosystems, thus multiplying the availability of these notorious compounds in the environment. Certain HPAHs appear to be more carcinogenic than dioxins. Transmission routes of HPAHs from their emission sources to water bodies, soil, aquatic life, plants, terrestrial animals, and humans are well-documented. Later, the direct and indirect diffusion of HPAHs from air to the biotic (plants, animals, humans) and abiotic components (soil, water, sediments) are described in detail. The study concludes that HPAHs are permeable to the carbon matrices resulting in the alleviation of the source-to-sink interface. As a potential future perspective, understanding the transmission interfaces lays a foundation to intervene in the introduction of these toxicants into the food chain.

Influence of acid-base equilibrium on interactions of some monofunctional coumarin Pd(II) complexes with biologically relevant nucleophiles-comprehensive kinetic study

This aimed to develop a comprehensive theoretical protocol for examining substitution reaction processes. The researchers used a theoretical quantum-mechanical protocol based on the QM-ORSA approach, which estimates the kinetic parameters of thermodynamically favourable reaction pathways. This theoretical protocol was validated by experimentally investigating substitution mechanisms in two previously synthesised Pd(II) complexes: chlorido-[(3-(1-(2-hydroxypropylamino)ethylidene)chroman-2,4-dione)]palladium(II) (C1) and chlorido-[(3-(1-(2-mercaptoethylamino)-ethylidene)-chroman-2,4dione)]palladium(II) (C2), along with biologically relevant nucleophiles, namely L-cysteine (l-Cys), L-methionine (l-Met), and guanosine-5'-monophosphate (5'-GMP). Reactions were investigated under pseudo-first-order conditions, monitoring nucleophile concentration and temperature changes using stopped-flow UV-vis spectrophotometry. All reactions were conducted under physiological conditions (pH = 7.2) at 37 °C. The reactivity of the studied nucleophiles follows the order: l-Cys > l-Met > 5'-GMP, and the reaction mechanism is associative based on the activation parameters. The experimental and theoretical data showed that C2 is more reactive than C1, confirming that the complexes' structural and electronic properties greatly affect their reactivity with selected nucleophiles. The study's findings have confirmed that the primary interaction occurs with the acid-base species L-Cys, mostly through the involvement of the partially negative sulfur atom (87.2%). On the other hand, C2 has a higher propensity for reacting with L-Cys-, primarily through the partially negative oxygen atom (92.6%). The implementation of this theoretical framework will significantly restrict the utilization of chemical substances, hence facilitating cost reduction and environmental protection.

Theoretical investigation on the mechanism and kinetics of the OH•‒initiated atmospheric degradation of p-chloroaniline: Addition of ∑g-3O2 and isomerization of peroxy radicals

Atmospheric oxidation of the p-chloroaniline-OH• adduct [C6H4ClNH2-OH]• (AD-C2) by ∑g-3O2 and internal isomerization processes of peroxy radical [C6H4ClNH2-OH]•-O2 are theoretically investigated at the M06-2X/aug-cc-pVTZ and CBS-QB3//M06-2X/aug-cc-pVTZ level of theories. Potential energy surfaces (PESs) for the most efficient pathways indicated that the oxidation process begins via the complexation of individual reactants in syn mode forming PRCy-iOO-syn (y = 2,5) in an exothermic and endogenic step. The syn mode addition is favored over the anti one due to the formation of internal hydrogen bond between the hydroxyl and peroxy groups. Formation of new C5-OO bond in PRCy-iOO-syn complex is an unimolecular process which is exothermic and exoergic. This pathway is predominated over other internal conversions due to the presence of stronger intramolecular hydrogen bond. Cyclization of the produced [C6H4ClNH2-OH]•-O2 peroxy radical AD-C2-5OO-syn into the bicyclic peroxy radical AD-C2-5,6OO-syn is the last step which is strongly endothermic and endogenic. The rate coefficients are calculated by means of the RRKM theory over the temperature range 250-350 K and at a pressure range of 0.1 bar to the high-pressure limit. The RRKM rate coefficients at the M06-2X/aug-cc-pVTZ level for the first bimolecular and last unimolecular steps are in order of 10-16 cm3 molecule-1 s-1 and 10-7 s-1, respectively, while the obtained rate coefficients at the CBS-QB3//M06-2X/aug-cc-pVTZ are overestimated about two order of magnitude.

New insights into the degradation mechanism of ibuprofen in the UV/H2O2 process: role of natural dissolved matter in hydrogen transfer reactions

Ibuprofen (IBU), a widely used antipyretic and analgesic, has been frequently detected in various natural water systems. Advanced oxidation processes (AOPs) are effective ways to remove pollutants from water. The degradation of IBU under UV/H2O2 conditions in the presence of various kinds of natural dissolved matter was investigated using density functional theory (DFT). The eco-toxicological properties were predicted based on a quantitative structure-activity relationship (QSAR) model. The calculated results showed that two H-abstraction reactions occurring at the side chain are predominant pathways in the initial reaction. H2O, NH3, CH3OH, C2H5OH, HCOOH and CH3COOH can catalyze the H transfer in the degradation process through decreasing the energy barriers and the catalysis effects follow the order of NH3 > alcohols > acids > H2O. The catalysis effects differ under acid or alkaline conditions. The overall rate coefficient of the reaction of IBU with ˙OH is calculated to be 5.04 × 109 M-1 s-1 at 298 K. IBU has harmful effects on aquatic organisms and human beings and the degradation process cannot significantly reduce its toxicity. Among all products, 2-(4-formylphenyl)propanoic acid, which is more toxic than IBU, is the most toxic with acute and chronic toxicity, developmental toxicity, mutagenicity, genotoxic carcinogenicity and irritation/corrosivity to skin. The findings in this work provide new insights into the degradation of IBU and can help to assess its environmental risks.

A new insight into the CaO-induced inhibition pathways on PCDD/F formation: Metal passivation, dechlorination and hydroxide substitution

Ca-based inhibitors (especially CaO) for PCDD/F (polychlorinated dibenzo-p-dioxin and dibenzofuran) formation are considered as economic inhibitors with low toxicity and strong adsorption of acidic gases (e.g., HCl, Cl₂, and SO_x), whereas the insight understanding of its inhibition mechanisms is scarcely explored. Herein, CaO was used to inhibit the de novo reaction for PCDD/F formation (250-450 °C). The evolution of key elements (C, Cl, Cu, and Ca) combined with theoretical calculations was systematically investigated. The concentrations and distribution of PCDD/Fs demonstrated the significant inhibition effect of CaO on I-TEQ (international toxic equivalency) concentrations of PCDD/Fs (inhibition efficiencies: > 90 %) and hepta~octa chlorinated congeners (inhibition efficiencies: 51.5-99.8 %). And the conditions (5-10 % CaO, 350 °C) were supposed to be the preferred conditions applied in real MSWIs (municipal solid waste incinerators). CaO significantly suppressed the chlorination of carbon matrix (superficial organic Cl (CCl) reduced from 16.5 % to 6.5-11.3 %) and the formation of unsaturated hydrocarbons or aromatic carbon (superficial CC decreased from 6.7 % to 1.3-2.1 %). Also, CaO promoted the dechlorination of Cu-based catalysts and Cl solidification (e.g., conversion of CuCl₂ to CuO, and formation of CaCl₂). The dechlorination phenomenon was validated by the dechlorination of highly chlorinated PCDD/F-congeners (via DD/DF chlorination pathways). Density functional theory calculations revealed that CaO facilitated the substitution of Cl by -OH on the benzene ring to inhibit the polycondensation of the chlorobenzene and chlorophenol (Gibbs free energy reduced from +74.83 to -36.62 and - 148.88 kJ/mol), which also indicates the dechlorination effect of CaO on de novo synthesis.

Atmospheric degradation mechanism of anthracene initiated by OH•: A DFT prediction

Density functional theory (DFT) calculations at the M06-2X/def2-TZVP level have been employed to investigate the atmospheric oxidation mechanism of anthracene (ANT) initiated by HO^•. Direct hydrogen atom abstraction from the ANT using HO^• takes place hardly at ambient conditions while addition of HO^• to the C1, C2, and C4 sites are thermodynamically and kinetically more advantageous. The addition reactions are controlled by the aromaticity and the kinetic trends were justified by resonance stabilization energies. The rate constants were calculated by using the Rice-Ramsperger-Kassel-Marcus (RRKM) and canonical transition state theory (CTST) methods in conjugation with zero curvature tunneling (ZCT). The overall RRKM-bimolecular rate constant at ambient conditions is 6.72 × 10^-12 cm³ molecule^-1 s^-1, is negatively dependent on the temperature and can be expressed as k_250-350^1bar=3.92×10^-14exp(1534.9T). Contribution of the AD-C4 path in the overall reaction is about 70-80%, implying that the dependence of overall rate constant on pressure can be ignored. The kinetic data exhibit that the ANT is degraded during its long-range transport in the atmosphere and cannot be classified as persistent organic pollutants.

Formation pathways, gas-solid partitioning, and reaction kinetics of PCDD/Fs associated with baghouse filters operated at high temperatures: A case study

The application of the 3T method during combustion (i.e., a Temperature > 850 °C, a residence Time > 2 s, and sufficient Turbulence) can lead to elevated operating temperature in the baghouse filter for the municipal solid waste incineration (MSWI) systems without sufficient heat exchange capacity, which is potentially detrimental to the emission control of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Herein, a field study focusing on the distribution and variation of PCDD/Fs in gaseous and solid phases in a baghouse filter with high operating temperature (225-230 °C) was carried out. The concentration of PCDD/Fs in gases at the outlet of the baghouse filter was around 1 order of magnitude higher than that in inlet gases (i.e., noticeable memory effect of PCDD/Fs), because of the significant PCDD/Fs formation in filter fly ash (primarily contributed by the precursor pathway) followed by PCDD/Fs desorption. In addition, the mechanisms and factors resulting in the memory effect of PCDD/Fs were identified based on a laboratory study that carefully investigated the formation and desorption of PCDD/Fs at potential operating temperatures of baghouse filters (i.e., 180, 200, and 225 °C). The temperature was identified as the key factor inducing the memory effect of PCDD/Fs, because: i) PCDD/Fs memory effect was not observed for baghouse filters with low operating temperatures of ~150 °C in previous studies; ii) both the formation and desorption of PCDD/Fs were noticeably favored by rising temperature from 180 to 225 °C; iii) increasing temperature appeared to facilitate the transformation from inorganic Cl to organic Cl and the conversion from aliphatic carbon to aromatic carbon or unsaturated hydrocarbons, both of which were favorable to PCDD/Fs formation; and iv) the release rate of PCDD/Fs from fly ash was exponentially dependent on temperature based on the modeling results of reaction kinetics.

Theoretical evidence for the formation of perfluorocarboxylic acids form atmospheric oxidation degradation of fluorotelomer acrylates

The atmospheric oxidation degradation of fluorotelomer acrylates (FTAcs) has been proposed as a potential source of perfluorocarboxylic acids (PFCAs) in remote locations. In this paper, detailed reactions of the main oxidant OH radicals with 4:2 FTAc in the atmosphere have been investigated by using density functional theory (DFT) calculation. All possible pathways involved in the oxidation process were presented and discussed. Based on the mechanism, transition state theory (TST) was used to predict the rate constants of the key elementary steps including the initial reactions of OH radical with n:2 FTAcs and the subsequent reactions of the main intermediates. Studies show that the reaction processes of OH radical addition to C = C bond are dominant and the fluorotelomer glyoxylate and formaldehyde are the major products. At 296 K, the calculated overall rate constant of 4:2 FTAc with OH radical is 1.19 × 10^-11 cm³ molecule^-1 s^-1 with an atmospheric lifetime of 23.3 h. In the atmosphere, fluorotelomer glyoxylate will continue to be oxidized, which will lead to the formation of PFCAs ultimately. In addition, atmospheric reactions of more carbons FTAc (C_nF_2n+1CH₂CH₂OC(O)CH = CH₂, n = 6, 8, 10) are also discussed in the presence of O₂/NO_x.

Mechanistic insights of the degradation of an O-anisidine carcinogenic pollutant initiated by OH radical attack: theoretical investigations

O-Anisidine (O-AND) is one of the amino organic compounds that harm human health, and is considered as a carcinogenic chemical.

Enhanced photocatalytic reduction for the dechlorination of 2-chlorodibenzo-p-dioxin by high-performance g-C3N4/NiO heterojunction composites under ultraviolet-visible light illumination

Polychlorinated dibenzo-p-dioxins (PCDDs), characterized by their high persistency and bioaccumulation, are widely detected in the environment. In this study, high-performance g-C₃N₄/NiO heterojunctions were fabricated to degrade 2-chlorodibenzo-p-dioxin (2-CDD) under ultraviolet-visible (UV-vis) light illumination. Experiments revealed that the pure g-C₃N₄ and range of g-C₃N₄/NiO heterojunctions were synthesized by the mixing and heating method, and then were characterized by XRD, TEM, XPS and PL etc. The composites exhibited enhanced dechlorination activities under anoxic conditions. After comparison, the g-C₃N₄/NiO (4:6) showed optimal dechlorination performance such that 70.4% of 2-CDD was removed within 8 h and 52.3% of 2-CDD was transformed to dibenzo-p-dioxin (DD), about fourfold higher than the pristine g-C₃N₄. The transformation of 2-CDD was accompanied by the resale of Cl ion, and the additional oxygen was proven to be able to consume electrons and hydrogen ions, thus greatly inhibiting the degradation of PCDD in systems. The g-C₃N₄/NiO (4:6) can be reused at least seven times, and the mechanism was proposed in detail to promote photoinduced electrohole separation and provide active sites. This study extends the use range of g-C₃N₄/NiO heterojunctions and develops a new technology to degrade PCDDs with striking activity and stability.

Theoretical study on the mechanisms, kinetics and ecotoxicity assessment of OH-initiated reactions of guaiacol in atmosphere and wastewater

The OH-initiated transformation mechanisms, kinetics and ecotoxicity assessment of guaiacol (2-methoxyphenol) in the presence of O₂/NO_x were investigated both in atmosphere and wastewater. The solvent effect lowers the energy barriers of initial OH-addition reactions more than H-abstraction reactions, leading to much higher addition branching ratio (Γ_add) of 0.92 in aqueous solution than that of 0.42 in gas-phase. At 298 K, the overall rate constants of the title reactions in atmosphere and wastewater are 5.56 × 10^-12 and 1.41 × 10^-11 cm³ molecule^-1 s^-1 with corresponding half-lives of 34.6 h and 0.82 s, respectively. In atmosphere, all the proposed favorable products including nitroguaiacols, methoxybenzoquinone, 2-hydroxyphenyl formate, 2-methoxybenzene-1, 3-diol and dialdehyde could contribute to secondary organic aerosols (SOAs). In wastewater, NO₂ addition reactions lead to higher toxicity of products (nitroguaiacols and 2-methoxybenzene-1, 4-diol) than that of parental guaiacol. However, O₂/NO addition pathways may generate less harmful products except for methoxybenzoquinone (P3) which is with higher toxicity than guaiacol. Therefore, more attention should be focused on the products formed from OH-initiated reactions of guaiacol both in atmosphere and wastewater.

A theoretical study on the formation and oxidation mechanism of hydroxyalkylsulfonate in the atmospheric aqueous phase

Hydroxymethanesulfonate (HMS) is an important organosulfur compound in the atmosphere. In this work, we studied the formation mechanism of HMS via the reaction of formaldehyde with dissolved SO₂ using the quantum chemistry calculations. The results show that the barrier (9.7 kcal mol⁻¹) of the HCHO + HSO₃⁻ reaction is higher than that (1.6 kcal mol⁻¹) of the HCHO + SO₃²⁻ reaction, indicating that the HCHO + SO₃²⁻ reaction is easier to occur. For comparison, the reaction of acetaldehyde with dissolved SO₂ also was discussed. The barriers for the CH₃CHO + HSO₃⁻ reaction and CH₃CHO + SO₃²⁻ reaction are 16.6 kcal mol⁻¹, 2.5 kcal mol⁻¹, respectively. This result suggests that the reactivity of HCHO with dissolved SO₂ is higher than that of CH₃CHO. The further oxidation of CH₂(OH)SO₃⁻ and CH₃CH(OH)SO₃⁻ by an OH radical and O₂ shows that the SO₅˙⁻ radical can be produced.

We report the formation of an important organosulfur compound HMS and its oxidation using theoretical calculation.