Overlooked Transformation of Nitrated Polycyclic Aromatic Hydrocarbons in Natural Waters: Role of Self-Photosensitization

Insights into the catalytic effect of atmospheric organic trace species on the hydration of Criegee intermediates

The bimolecular reactions between Criegee intermediates (CIs) and atmospheric trace species have been extensively investigated, with a particular focus on the reaction with water, while the catalytic role of atmospheric organic compounds in hydration reactions was often neglected. In this study, we employed quantum chemical calculations and Born-Oppenheimer molecular dynamics (BOMD) simulations to investigate the catalytic effects of atmospheric organic amines, organic acids, and alcohols on the hydration reactions of CIs in the gas phase and at the gas-liquid interface. The catalytic reactions were found to follow a cyclic catalytic structure and a stepwise reaction mechanism. Gas-phase studies revealed that organic acids exhibited stronger catalytic effects compared to amines and alcohols, and the catalytic efficiency of amines and alcohols was similar to those of single water molecule. In addition, the catalytic reaction barriers of organic acids and alcohols were positively correlated with their gas-phase acidity (R2 = 0.94 to 0.97). A negative correlation was observed between the catalytic reaction barrier of amines and their gas-phase basicity (R2 = 0.84 to 0.90) and proton affinity (R2 = 0.84 to 0.92). At the gas-liquid interface, organic acids promoted the formation of hydroxyethyl hydroperoxide (HEHP, CH3CH(OH)(OOH)), organic acid ions, and H3O+, whereas the catalytic hydration of CIs by organic amines resulted in the formation of CH3CH(OH)OO and amine ions. Both HEHP and CH3CH(OH)OO can be further decomposed to form OH and HO2, or participate in new particles formation as precursors. This study complements the research gap on the reaction of CIs with water, providing valuable insights into the atmospheric sources of HEHP and HOx as well as the formation of secondary organic aerosols (SOAs).

Insight into the photodegradation of methylisothiazolinone and benzoisothiazolinone in aquatic environments

Methylisothiazolinone (MIT) and Benzisothiazolinone (BIT) are two widely used non-oxidizing biocides of isothiazolinones. Their production and usage volume have sharply increased since the pandemic of COVID-19, inevitably leading to more release into water environment. However, their photochemical behaviors in water environment are still unclear. Therefore, this study investigated photodegradation properties of MIT and BIT in natural water under simulated sunlight. The results demonstrated that direct photolysis was mainly responsible for their photodegradation which occurred through their excited singlet states rather than triplet states. The quantum yields of MIT and BIT photodegradation were 11 - 13.6 × 10-4 and 2.43 - 5.79 × 10-4, respectively. pH had almost no effect on the photodegradation of MIT, while the photodegradation of BIT was significantly promoted under alkaline condition due to abundance of BIT in its deprotonated form (BIT-N-). Cl-, NO3- and dissolved organic matter (DOM) in natural water inhibited the photodegradation of both MIT and BIT, with the light screening effect of DOM being the most significantly inhibitory factor. The addition of other isothiazolinones, which possibly coexisted with MIT and BIT in actual condition, slightly inhibited the photodegradation of MIT and BIT. The estimated half-life under natural sunlight at a 30°N latitude was estimated to be approximately 1.1 days. The photodegradation pathways of MIT and BIT are similar, primarily initiated from the ring-opening at the N-S bond, with Frontier electron densities (FED) calculations suggesting the likelihood of oxidation and ·OH addition reactions at the O, N, and S sites. While the photodegradation products exhibited significantly reduced acute toxicity compared to their parent compounds, they nonetheless posed substantial chronic toxicity. These insights are vital for assessing the ecological impacts of MIT and BIT in aquatic environments.

Differences in the Reaction Mechanisms of Chlorine Atom and Hydroxyl Radical with Organic Compounds: From Thermodynamics to Kinetics

Hydroxyl radical (HO•) and chlorine atom (Cl•) are common reactive species in aqueous environments. However, the intrinsic difference in their reactions with organic compounds has not been revealed. This study compared the reaction mechanisms of HO• and Cl• with 13 aromatic and 11 aliphatic compounds by quantum chemical calculation and laser flash photolysis. Both HO• and Cl• can spontaneously react with aromatic compounds via radical adduct formation (RAF), hydrogen atom transfer (HAT), and single electron transfer (SET) pathways. The SET reactions of Cl• were more thermodynamically favorable than HO•, but contrary results were obtained for HAT reactions. According to the free energy of activation (ΔGaq‡), the dominant oxidation mechanisms of aromatic compounds were RAF and SET by HO• and SET by Cl•. The important role of SET in the HO• reactions with aromatic compounds was further verified by accurately calculating the solvation free energy of HO•/HO- and experimentally tracking the radical cations, which were generally neglected in previous studies. Meanwhile, the ΔGaq‡ value of each reaction pathway of Cl• was lower than that of HO•, resulting in higher rate constants of Cl• with aromatic compounds than HO•. For saturated aliphatic compounds, HAT was found to be the only mechanism accounting for their transformation by HO• and Cl•. This study proposed general rules for the reaction mechanisms of HO• and Cl• and unraveled their differences in the aspects of thermodynamics and kinetics, providing fundamental information for understanding contaminant transformation in processes involving HO• and Cl•.

Oxidation of Polycyclic Aromatic Hydrocarbons (PAHs) Triggered by a Photochemical Synergistic Effect between High- and Low-Molecular-Weight PAHs

Photooxidation of polycyclic aromatic hydrocarbons (PAHs), which are widely observed in atmospheric particulate matter (PM), largely determines their atmospheric fate. In the environment, PAHs are highly complex in chemical composition, and a great variety of PAHs tend to co-occur. Despite extensive investigation on the photochemical behavior of individual PAH molecules, the photochemical interaction among these coexisting PAHs is still not well understood. Here, we show that during photooxidation, there is a strong photochemical synergistic effect among PAHs extracted from soot particles. We find that neither small PAHs with low molecular weights of 200-350 Da and 4-8 aromatic rings (named PAHsmall) nor large PAHs with high molecular weights of 350-600 Da and 8-14 aromatic rings (named PAHlarge) undergo photooxidation under red-light irradiation (λ = 648 nm), even though PAHlarge can absorb light with this wavelength. Interestingly, when PAHlarge is mixed with PAHsmall, substantial photooxidation is observed for both PAHlarge and PAHsmall. Comparisons of in situ infrared (IR), high-resolution mass spectrometry, and electron paramagnetic resonance analysis indicate that the presence of PAHsmall inhibits the light quenching effect arising from the π-π stacking of PAHlarge. This leads to the formation of singlet oxygen (1O2), which initiates the photooxidation. Our findings reveal a new mechanism for the photooxidation of PAHs and suggest that complex atmospheric PAHs exhibit distinct photoreactivity from simple systems.

The reactivity of organic radicals in the performic, peracetic, perpropionic acids-based advanced oxidation process: A case study of sulfamethoxazole

Advanced oxidation processes (AOPs) based on peracetic acid (PAA) displayed great potential in removing emerging contaminants by generating HO• and organic radicals. Performic and perpropionic acids (PFA and PPA) also act as disinfectants, but their application potential has not been investigated yet. Here, we investigated the degradation mechanism and kinetics of sulfamethoxazole (SMX) by HO•, RC(O)O• species (including HC(O)O•, CH3C(O)O• and CH3CH2C(O)O•) and RC(O)OO• species (including HC(O)OO•, CH3C(O)OO• and CH3CH2C(O)OO•). The results show that the calculated reaction rate constants of SMX follow the order of HC(O)O• > CH3C(O)O• > CH3CH2C(O)O• > HO• > HC(O)OO• > CH3C(O)OO• > CH3CH2C(O)OO•. The reactivity towards SMX is strongly correlated with the redox potentials of reactive radicals. Hence, the RCOO• species play dominant roles in the purification of SMX in PFA/PAA/PPA-based AOPs. The degradation of SMX mainly proceeds via addition at the benzene ring, the hydrogen abstraction from the -NH2 group as well as the single electron transfer reaction. This study highlights the fundamental aspects of PFA, PAA, and PPA in the purification of sulfamethoxazole and enhances the role of organic radicals in the AOPs based on organic peracetic acids.

Effect of Dissolved Organic Matter on the Photodegradation of Decachlorobiphenyl (PCB-209) in Heterogeneous Systems: Experimental Analysis and Excited-State Theory Calculations

Dissolved organic matter (DOM) can affect the transformation of pollutants through photosensitization, but most current research focuses on hydrophilic pollutants, making it such that less attention is paid to hydrophobic pollutants. In this paper, the effect and action mechanism of coexisting DOM on the photodegradation of decachlorobiphenyl (PCB-209) on suspended particles collected from the Yellow River were systematically investigated in a heterogeneous system using DOM standards and model compounds. Through molecular probe experiments, mass spectrometry analysis and theoretical calculations, we found that the excited triplet state of DOM (3DOM*) could excite PCB-209 to undergo dechlorination reaction. Due to the different modes of electron transition, the presence of carbonyl groups decreased the energy of 3DOM*, whereas the electron-donating groups made the energy of 3DOM* higher. DOM containing phenolic hydroxyl groups led to a higher steady-state concentration of •OH, and DOM containing phenyl ketone structures had a stronger ability to produce •O2-. Compared with aqueous •OH, •O2- produced from hydrophobic microregions could react more readily with PCB-209. This study deepens the understanding of the role of different functional groups of DOM in the photosensitized transformation of hydrophobic compounds.

Investigation of the photolysis process of benzo(a)anthracene (BaA) on polyvinyl chloride (PVC) and polystyrene (PS) microplastics: Plastics aging effect, transformation products and toxicity assessment

Microplastics (MPs) and persistent pollutants (POPs) are new pollutants that are extensively studied worldwide. To fill the gaps that the degradation processes and mechanisms of polycyclic aromatic hydrocarbons (PAHs) on the surface of most MPs are still unclear, the photochemical transformation of benzo(a)anthracene (BaA) on polyvinyl chloride (PVC) MPs and polystyrene (PS) MPs in water were investigated and compared. The photolysis of BaA on the surface of PS in water proceeded easier than that on PVC within the 48 h irradiation period, with the pseudo-first-order rate constant of 0.0489 min-1 and 0.0181 min-1, respectively, which can be ascribed to the smaller particle size and more OH production of PS MPs. Due to the light competition between the chromophore and BaA as well as the light-shielding effect, aged MPs showed an inhibitory effect on the degradation of BaA compared with pristine MPs. For BaA/PVC MPs system, the degradation of BaA in real water was not significantly affected by coexisting ions and humic acid (HA) (p < 0.05), while slight inhibitory effect on the degradation of BaA appeared for PS MPs in different water matrices (UP: 86.97 %, YR: 84.47 %, PR: 81.42 % and HR: 83.21 %). According to the electron paramagnetic resonance (EPR) test, quenching experiment and probe experiment, the relative contribution of direct photolysis (PVC: 82.02 %; PS: 69.54 %) and indirect photolysis (PVC: 17.98 %; PS: 30.46 %) was confirmed. A total of 14 products were identified, and the product types were not affected by plastics aging. The results of the toxicity assessment indicated that although some intermediate products remained toxic to aquatic organisms, the toxicity of most products was lower than that of BaA. This study provides new insights into the environmental fate of PAHs and the role of MPs in the photolysis process of contaminants in surface water.

Direct Determination of Absolute Radical Quantum Yields in Hydroxyl and Sulfate Radical-Based Treatment Processes

The absolute radical quantum yield (Φ ) is a critical parameter to evaluate the efficiency of radical-based processes in engineered water treatment. However, measuring Φ is fraught with challenges, as current quantification methods lack selectivity, specificity, and anti-interference capabilities, resulting in significant error propagation. Herein, we report a direct and reliable time-resolved technique to determine Φ at pH 7.0 for commonly used radical precursors in advanced oxidation processes. For H2O2 and peroxydisulfate (PDS), the values of Φ •OH and Φ SO 4 • - at 266 nm were measured to be 1.10 ± 0.01 and 1.46 ± 0.05, respectively. For peroxymonosulfate (PMS), we developed a new approach to determine Φ • OH PMS with terephthalic acid as a trap-and-trigger probe in the nonsteady state system. For the first time, the Φ • OH PMS value was measured to be 0.56 by the direct method, which is stoichiometrically equal to Φ SO 4 • - PMS (0.57 ± 0.02). Additionally, radical formation mechanisms were elucidated by density functional theory (DFT) calculations. The theoretical results showed that the highest occupied molecular orbitals of the radical precursors are O-O antibonding orbitals, facilitating the destabilization of the peroxy bond for radical formation. Electronic structures of these precursors were compared, aiming to rationalize the tendency of the Φ values we observed. Overall, this time-resolved technique with specific probes can be used as a reliable tool to determine Φ , serving as a scientific basis for the accurate performance evaluation of diverse radical-based treatment processes.

Estrogenic disruption effects and formation mechanisms of transformation products during photolysis of preservative parabens

The ubiquitous presence of emerging contaminants (ECs) in the environment and their associated adverse effects has raised concerns about their potential risks. The increased toxicity observed during the environmental transformation of ECs is often linked to the formation of their transformation products (TPs). However, comprehension of their formation mechanisms and contribution to the increased toxicity remains an unresolved challenge. To address this gap, by combining quantum chemical and molecular simulations with photochemical experiments in water, this study investigated the formation of TPs and their molecular interactions related to estrogenic effect using the photochemical degradation of benzylparaben (BZP) preservative as a representative example. A non-targeted analysis was carried out and three previously unknown TPs were identified during the transformation of BZP. Noteworthy, two of these novel TPs, namely oligomers BZP-o-phenol and BZP-m-phenol, exhibited higher estrogenic activities compared to the parent BZP. Their IC50 values of 0.26 and 0.50 μM, respectively, were found to be lower than that of the parent BZP (6.42 μM). The binding free energies (ΔGbind) of BZP-o-phenol and BZP-m-phenol (-29.71 to -23.28 kcal·mol-1) were lower than that of the parent BZP (-20.86 kcal·mol-1), confirming their stronger binding affinities toward the estrogen receptor (ER) α-ligand binding domain. Subsequent analysis unveiled that these hydrophobic residues contributed most favorably to ER binding, with van der Waals interactions playing a significant role. In-depth examination of the formation mechanisms indicated that these toxic TPs primarily originated from the successive cleavage of ester bonds (OCH2C6H5 and COO group), followed by their combination with BZP*. This study provides valuable insight into the mechanisms underlying the formation of toxic TPs and their binding interactions causing the endocrine-disrupting effects. It offers a crucial framework for elucidating the toxicological patterns of ECs with similar structures.

Humic substances mediated superior photochemical pollutant conversion on defective TiO2 in environmentally relevant matrices: The key roles of oxygen vacancy in surface interactions, oxidant activation and radical generation

Ubiquitous humic substances usually exhibit strong interfering effects on target pollutant removal in advanced water purification. This work aims to develop a photochemical conversion system on the nonstoichiometric TiO2 for pollutant removal in environmentally relevant matrices. In this synergistic reaction system, the redox-reactive humic substances and defective oxygen vacancies can serve as the organic electron transfer mediator and the key surface reactive sites, respectively. This system achieves a superior pollutant degradation in real surface water at low oxidant concentrations. Reactive oxygen vacancies on the TiO2 surface and sub-surface are of considerable interest for this photochemical reaction system. By engineering defective oxygen vacancies on high-energy {001} polar facet, the surface and electronic interactions between tailored TiO2 and humic substances are greatly strengthened for the promoted electron transfer and oxidant activation. Rendered by the strong surface affinity and molecular activation, defective oxygen vacancies thermodynamically and dynamically promote reactive chain reactions for free radical formation, including the selective O2 reduction to ·O2- and the H2O2 activation to ·OH. Our findings take new insights into environmental geochemistry, and provide an effective strategy to in-situ boost the humic substances-mediated water purification without secondary pollution. ENVIRONMENTAL IMPLICATION: Humic substances are widely distributed in aquatic environment, thus playing important roles in environmental geochemistry. For example, humic substances can achieve good surface adsorption through electrostatic adsorption, ligand exchange and electronic interactions with typical TiO2 to form reactive ligand-metal charge transfer complexes for pollutant degradation. Inspired by the unique properties of surface and sub-surface oxygen vacancies, the defective TiO2 was designed to refine the humic substances-mediated photochemical reactions. A superior reactivity was measured for pollutant degradation. Our findings provide an effective strategy to boost naturally photochemical decontamination in environmentally relevant matrices.

Bimolecular versus Trimolecular Reaction Pathways for H2O2 with Hypochlorous Species and Implications for Wastewater Reclamation

The benchmark advanced oxidation technology (AOT) that uses UV/H2O2 integrated with hypochlorous species exhibits great potential in removing micropollutants and enhancing wastewater treatability for reclamation purposes. Although efforts have been made to study the reactions of H2O2 with hypochlorous species, there exist great discrepancies in the order of reaction kinetics, the rate constants, and the molecule-level mechanisms. This results in an excessive use of hypochlorous reagents and system underperformance during treatment processes. Herein, the titled reaction was investigated systematically through complementary experimental and theoretical approaches. Stopped-flow spectroscopic measurements revealed a combination of bi- and trimolecular reaction kinetics. The bimolecular pathway dominates at low H2O2 concentrations, while the trimolecular pathway dominates at high H2O2 concentrations. Both reactions were simulated using direct dynamics trajectories, and the pathways identified in the trajectories were further validated by high-level quantum chemistry calculations. The theoretical results not only supported the spectroscopic data but also elucidated the molecule-level mechanisms and helped to address the origin of the discrepancies. In addition, the impact of the environmental matrix was evaluated by using two waters with discrete characteristics, namely municipal wastewater and ammonium-rich wastewater. Municipal wastewater had a negligible matrix effect on the reaction kinetics of H2O2 and the hypochlorous species, making it a highly suitable candidate for this integration technique. The obtained in-depth reaction mechanistic insights will enable the development of a viable and economical technology for safe water reuse.

New insight into the removal process of benzotriazole UV stabilizers by UV/H2O2: Integrating quantum chemical calculation with CFD simulation

Benzotriazole UV stabilizers (BT-UVs) are important UV absorbers. As high-production chemicals and potential hazards, their ubiquitous presence in aquatic environments is of greatly pressing concern. Herein, the removal of six typical BT-UVs by UV/H2O2 was comprehensively investigated by quantum chemistry calculation integrated with CFD simulation. Utilizing such a micro and macro incorporated model in treating contaminants is the first report. From the micro-view, degradation mechanisms of BT-UVs by •OH oxidation were determined, and corresponding rate constants were obtained with values of 109∼1010 M-1s-1. In a macroscopic aspect, combining the established kinetic model and CFD simulation, the effects of UV lamp power (P), volumetric flow rate (Qv), and H2O2 dosage ([H2O2]0) on removal yields of BT-UVs were expounded, increasing P or [H2O2]0 or decreasing Qv are effective in improving removal yields of BT-UVs, but the enhancement was abated when P or [H2O2]0 increased to a certain level. When [H2O2]0 is 5 mg/L and Qv is decreased from 0.1 to 0.05 m3/h, the removal yields of BT-UVs could achieve more than 95% (P = 150 W) and 99% (P = 250 W), respectively. This work provides a new interdisciplinary insight for investigating organic contaminant removal in potential industrial applications of UV/H2O2 systems.