The role of indirect photochemical degradation in the environmental fate of pesticides: a review

5-Amino-1,2,4-triazol-3-one Degradation by Indirect Photolysis: A Density Functional Theory Study

Sunlight irradiation induces formation of reactive oxygen species (superoxide, hydroperoxyl radical, singlet oxygen, etc.), which readily take part in degradation of environmental pollutants. Being a primary ingredient in a suite of insensitive munition formulations, NTO (5-nitro-1,2,4-triazol-3-one) can be released onto training range soils and reduced to ATO (5-amino-1,2,4-triazol-3-one) by soil bacteria or iron-contained minerals. ATO can be dissolved in surface water and groundwater due to its good water solubility and then undergo further decomposition. A detailed investigation of possible mechanisms for ATO decomposition in water induced by superoxide, hydroperoxyl radical, and singlet oxygen as pathways for ATO environmental degradation was performed by computational study at the PCM(Pauling)/M06-2X/6-311++G(d,p) level. Hydrolysis and degradation of ATO induced by superoxide are unlikely to occur due to the high activation energy or endergonicity of the processes. The hydroperoxyl radical causes rapid and reversible hydrogen transfer from ATO, while an attachment of the hydroperoxyl radical to ATO can induce decomposition of ATO, leading to its mineralization. Singlet oxygen shows a higher reactivity toward ATO than the hydroperoxyl radical. Decomposition of ATO was found to be a multistep process that begins with singlet oxygen attachment to the carbon atom of the C═N double bond. The intermediate that is formed undergoes recyclization, cycle opening, and sequential elimination of nitrogen gas, ammonia, and carbon(IV) oxide. Isocyanic acid, which arises intermediately, hydrolyzes into ammonia and carbon(IV) oxide. Calculated activation energies and high exergonicity of the studied processes support the contribution of singlet oxygen and the hydroperoxyl radical to ATO degradation into low-weight inorganic compounds in the environment.

Enhancing photodegradation of thiamethoxam insecticide using SnS2/NCL as a photocatalyst: Mechanistic insights and environmental implications

The current research highlights the fabrication of a novel SnS2/CO32-@Ni-Co LDH (SnS2/NCL) by precipitating Ni-Co LDH over hydrothermally synthesized SnS2 nanoparticles for the enhanced degradation of thiamethoxam (THM) insecticide through the advanced oxidation process. The effect of several reaction parameters was optimized, and a maximum degradation of 98.1 ± 1.2 % with a rate constant of 0.0541 min-1 of 10 ppm THM was reached at a catalyst loading of 0.16 gL-1 using 0.3 mM of H2O2 within 70 min of visible light irradiation. The effect of metal cations, inorganic anions, dissolved organic matter, organic compounds and water samples on the photodegradation performance of SnS2/NCL nanocomposite was also examined to evaluate the prepared photocatalyst's suitability for use in actual wastewater conditions. The metal cations blocked the active sites of the photocatalyst and reduced the degradation efficiency except for Fe2+ ions, since it is a Fenton reagent and increased the production of hydroxyl radicals. Inorganic anions are the scavengers of hydroxyl radicals and hinder photocatalytic activity. Meanwhile, lake water containing varying degrees of co-existing ions shows the lowest degradation efficiency among other water samples. The SnS2/NCL nanocomposite could be reused for five cycles while maintaining a photocatalytic efficiency of 83.6 ± 0.3 % in the fifth run. The prepared SnS2/NCL nanocomposite also showed excellent photodegradation of several other emerging organic pollutants with an efficiency of over 80 % under optimum conditions. Incorporating Ni-Co LDH with SnS2 helped to delocalize photoinduced charges, leading to increased photocatalytic activity and a slower electron-hole recombination rate. The present research highlights the photocatalytic activity of SnS2/NCL photocatalysts for the photocatalytic degradation of emerging contaminants from wastewater.

Direct and indirect photodegradation in aquatic systems mitigates photosensitized toxicity in screening-level substance risk assessments of selected petrochemical structures

Photochemical processes are typically not incorporated in screening-level substance risk assessments due to the complexity of modeling sunlight co-exposures and resulting interactions on environmental fate and effects. However, for many substances, sunlight exerts a profound influence on environmental degradation rates and ecotoxicities. Recent modeling advances provide an improved technical basis for estimating the effect of sunlight in modulating both substance exposure and toxicity in the aquatic environment. Screening model simulations were performed for 25 petrochemical structures with varied uses and environmental fate properties. Model predictions were evaluated by comparing the ratios of predicted exposure concentrations with and without light to the corresponding ratios of toxicity thresholds under the same conditions. The relative ratios of exposure and hazard in light vs. dark were then used to evaluate how inclusion of light modulates substance risk analysis. Results indicated that inclusion of light reduced PECs by factors ranging from 1.1- to 63-fold as a result of photodegradation, while reducing PNECs by factors ranging from 1- to 49-fold due to photoenhanced toxicity caused by photosensitization. Consequently, the presence of light altered risk quotients by factors that ranged from 0.1- to 17-fold, since the predicted increase in substance hazard was mitigated by the reduction in exposure. For many structures, indirect photodegradation decreases environmental exposures independently of the direct photolysis pathway which is associated with enhanced phototoxicity. For most of the scenarios and chemicals in the present work, photosensitization appears to be mitigated by direct and indirect degradation from sunlight exposure.

Attenuation photochemical potential of Pontal of Paranapanema river waters (Brazil) from agrochemicals: geographical and temporal assessment

Knowledge of the environmental photochemical fate of pesticides is essential to assess their potential impacts. However, there are few studies in the literature focused on the photochemical attenuation of micropollutants in Brazilian rivers. In this context, this study characterized the surface waters of the Pontal of Paranapanema region (region which concentrates more than 80% of Brazilian sugarcane cultivations), in order to determine its photochemical attenuation potential against micropollutants in different seasons. Thus, the steady-state concentrations of the photochemically produced reactive intermediates (PPRIs) (hydroxyl radical, HO•; singlet oxygen, 1O2, and triplet-excited state chromophoric dissolved organic matter, 3CDOM*), formed in the rivers, were simulated by using the APEX model (Aquatic Photochemistry of Environmentally-occurring Xenobiotics), considering the sunlight irradiance, water chemistry, and depth. Based on our simulations, these concentrations vary between 0.35 × 10-15 and 4.52 × 10-14 mol L-1 for HO•, 1.3 × 10-15 and 1.2 × 10--14 mol L-1 for 3CDOM*, and 2.5 × 10-15 and 2.5 × 10-14 mol L-1 for 1O2. Finally, mathematical simulations were used for predicting persistence of pesticides atrazine (ATZ) and diuron (DIR) in Pontal of Paranapanema surface waters and the half-life times (t1/2) of the pollutants ranged from a few hours to one week.

Wavelength-dependent direct and indirect photochemical transformations of organic pollutants

Sunlight-induced photochemical transformations greatly affect the persistence of organic pollutants in natural environment. Whereas sunlight intensity is well-known to affect pollutant phototransformation rates, the reliance of pollutant phototransformation kinetics on sunlight spectrum remains poorly understood, which may greatly vary under different spatial-temporal, water matrix, and climatic conditions. Here, we systematically assessed the wavelength-dependent direct and indirect phototransformations of 12 organic pollutants. Their phototransformation rates dramatically decreased with light wavelength increasing from 375 to 632 nm, with direct photolysis displaying higher wavelength-dependence than indirect photolysis. Remarkably, UV light dominated both direct (90.4-99.5 %) and indirect (64.6-98.7 %) photochemical transformations of all investigated organic pollutants, despite its minor portion in sunlight spectrum (e.g., 6.5 % on March 20 at the equator). Based on wavelength-dependent rate constant spectrum, the predicted phototransformation rate of chloramphenicol (4.5 ± 0.7 × 10-4 s-1) agreed well with the observed rate under outdoor sunlight irradiation (4.3 ± 0.0 × 10-4 s-1), and there is no significant difference between the predicted rate and the observed rate (p-value = 0.132). Moreover, rate constant and quantum yield coefficient (QYC) spectrum could be applied for facilely investigate the influence of spectral changes on the phototransformation of pollutants under varying spatial-temporal (e.g., season, latitude) and climatic conditions (e.g., cloud cover). Our study highlights the wavelength-dependence of both direct and indirect phototransformation of pollutants, and the UV part of natural sunlight plays a decisive role in the phototransformation of pollutants.

Theoretical explanation of direct photolysis and indirect photolysis of bendazone with •OH, •SO4-, and •CO3- in water: mechanism insights and ecotoxicity evaluation

Bendazone (BNTE) is an herbicide and a highly concerned pollutant in aquatic environments. Understanding the photochemical behavior of BNTE in water is crucial for evaluating its photochemical conversion process in aquatic environments. This study analyzed the direct photolysis and indirect photolysis pathways of two dissociated forms of BNTE in water through density functional theory and time-dependent density functional theory method. The results show that the reaction types of indirect photolysis of BNTE with free radicals (•OH, •SO4-, and •CO3-) are OH- addition, SO4- addition, and CO3- addition. In the process of indirect photolysis of BNTE and free radicals, the photolysis of •OH and BNTE was the easiest, followed by •SO4-. In addition, the active site of BNTE reacting with •OH is C8, and the active site of BNTE reacting with •SO4- is C10. However, the photolysis effect of •CO3- on BNTE is very small, indicating that •CO3- in water plays a secondary role in the indirect photolysis of BNTE. In the direct photolysis of BNTE, N1-C6 bond breaking is difficult to occur spontaneously in the environment due to its high endothermic property and energy barrier. The direct photolysis pathway of BNTE involves the break of the N1-S2/S2-N3/N3-C12 bond. In addition, the ecological toxicity evaluation showed that toxicity of most of the degradation products were reduced, but the toxicity level was still maintained at a harmful level. Our findings provide the photochemical fate of BNTE in aquatic environments and will help to more accurately understand their photochemical conversion mechanisms in the environment.

Next-generation hybrid technologies for the treatment of pharmaceutical industry effluents

Water and industries are intangible units of the globe that are always set to meet the population's demand. The global population depends on one-third of freshwater increasing the demand. The increase in population along with urbanization has polluted the fresh water resources. The pharmaceutical industry is marked as an emerging contaminant of water pollution. The most common type of pharmaceutical drugs that are detected in the environment includes antibiotics, analgesics, NSAIDs, and pain-relieving drugs. These drugs alter the food chain of the organisms causing chaos mainly in the marine ecosystem. Pharmaceutical drugs are found only in shallow amounts (ng/mg) they have a huge impact on the living system. The consumption of water contaminated with pharmaceutical ingredients can disrupt reproduction, hormonal imbalance, cancer, and respiratory problems. Various methods are used to remove these chemicals from the environment. In this review, we mainly focused on the emerging hybrid technologies and their significance in the effective treatment of pharmaceutical wastewater. This review paper primarily elaborates on the merits and demerits of existing conventional technologies helpful in developing integrated technologies for the modern era of pharmaceutical effluent treatment. This review paper further in detail discusses the various strategies of eco-friendly bioremediation techniques namely biostimulation, bioaugmentation, bacterial degradation, mycoremediation, phytoremediation, and others for the ultimate removal of pharmaceutical contaminants in wastewater. The review makes clear that targeted and hybrid solutions are what the world will require in the future to get rid of these pharmacological prints.

Degradation of the strobilurin fungicide mandestrobin in illuminated water-sediment systems

The degradation behavior of mandestrobin (1) was investigated in aerobic aquatic water-sediment systems exposed to continuous artificial sunlight (λ>290 nm). [14C]mandestrobin uniformly labeled at the phenoxy or benzyl ring was individually applied to the overlying water of the system at a rate equivalent to 262.5 g a.i./ha. The transformation of 1 was mainly proceeded via photoinduced bond cleavage at the benzyl phenyl ether and the subsequent rearrangement reaction. Interestingly, some of the photodegradates and microbial metabolites of 1 observed in the aquatic photodegradation and water-sediment (dark) studies, respectively, were never detected. Furthermore, the observed photoproducts were less formed and were steadily degraded or metabolized to carbon dioxide or strongly adsorbed to bottom sediment. The fate of 1 and its degradates in illuminated water-sediment systems was considered to reflect realistic conditions more precisely, as it accounts for various effects attributed to sunlight.

Fluorinated Pharmaceutical and Pesticide Photolysis: Investigating Reactivity and Identifying Fluorinated Products by Combining Computational Chemistry,19F NMR, and Mass Spectrometry

Fluorinated breakdown products from photolysis of pharmaceuticals and pesticides are of environmental concern due to their potential persistence and toxicity. While mass spectrometry workflows have been shown to be useful in identifying products, they fall short for fluorinated products and may miss up to 90% of products. Studies have shown that 19F NMR measurements assist in identifying and quantifying reaction products, but this protocol can be further developed by incorporating computations. Density functional theory was used to compute 19F NMR shifts for parent and product structures in photolysis reactions. Computations predicted NMR spectra of compounds with an R2 of 0.98. Computed shifts for several isolated product structures from LC-HRMS matched the experimental shifts with <0.7 ppm error. Multiple products including products that share the same shift that were not previously reported were identified and quantified using computational shifts, including aliphatic products in the range of -80 to -88 ppm. Thus, photolysis of fluorinated pharmaceuticals and pesticides can result in compounds that are polyfluorinated alkyl substances (PFAS), including aliphatic-CF3 or vinyl-CF2 products derived from heteroaromatic-CF3 groups. C-F bond-breaking enthalpies and electron densities around the fluorine motifs agreed well with the experimentally observed defluorination of CF3 groups. Combining experimental-computational 19F NMR allows quantification of products identified via LC-HRMS without the need for authentic standards. These results have applications for studies of environmental fate and analysis of fluorinated pharmaceuticals and pesticides in development.

Photobleaching affects the carbon sequestration of dissolved black carbon on ferrihydrite: Perspective from molecular fractionation

Photobleaching generally changes the structure and properties of dissolved black carbon (DBC), which further affects distribution of DBC at mineral-water interface. Here, we investigated the effect mechanism by which DBC photobleaching on its sequestration on ferrihydrite (Fh) from perspective of molecular fractionation. Results indicated that continuous sunlight irradiation led to the photolysis of aromatic humic- and fulvic-like components and the carboxylation of the functional structure. DBC could be considerably sequestered on the Fh surface, and photobleached DBC (pDBC) with longer sunlight irradiation durations had lower adsorption capacity on Fh. The photo-absorption and photo-activity ability of residual DBC/pDBCs after adsorption significantly weakened, indicating that the photo-liable components with great photochemical properties were preferentially sequestered on Fh during adsorption fractionation at Fh-water interface. Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) results showed high molecular weight, high O contents and high unsaturation compounds (such as polycyclic aromatics and polyphenols) were preferentially sequestered on Fh through ligand exchange between iron-coordinated hydroxyl and substituted carboxyl/hydroxyl in DBC. Among high unsaturation compounds, aromatic ring structures (C=C) were with greater affinity with Fh surface than CO in carboxyl/ester/quinone. Photobleaching caused the decrease in aromatic ring structures and the increase in CO in carboxyl, which was the key for weakening of sequestration of pDBC on Fh. Our findings prove that the photo-liable components of DBC are more tend to be sequestered on mineral, and promote the understanding of geochemical behavior of DBC in the solid earth interfaces.

Bandgap engineering approach for synthesising photoactive novel Ag/HAp/SnS2 for removing toxic anti-fungal pharmaceutical from aqueous environment

The present work shed light on synthesising a novel ternary Z-scheme Ag/HAp/SnS2 (AHS) nano photocatalyst to degrade metronidazole (MTZ) in wastewater through H2O2-assisted AOP under natural sunlight. HAp extracted from the fish scales of rohu fish through alkaline treatment was decorated with Ag nanoparticles using ascorbic acid as a bio-reductant. Tin disulphide (SnS2) was anchored over Ag/HAp to prevent agglomeration and enhance photocatalytic activity by delaying the electron-hole recombination rate. After 45 min of irradiation, a degradation efficiency of 98.85 ± 1.86% for 15 ppm MTZ could be achieved. The performance of the prepared photocatalyst in real wastewater was investigated by introducing several metal cations and anions in the photodegradation process. The degradation products were identified by HRLCMS analysis, and the breakdown mechanism of MTZ was proposed. The present study enlightens the importance of SnS2-based photocatalysts for organic pollutant degradation under natural sunlight through an advanced oxidation process. The characterization results showed that the enhanced photodegradation efficiency of AHS is attributed to the formation of an all-solid-state Z-scheme heterojunction with Ag nanoparticles acting as charge transfer medium and as electron accumulators helping in delaying charge recombination.

Photodegradation and photocatalysis of per- and polyfluoroalkyl substances (PFAS): A review of recent progress

Per- and polyfluoroalkyl substances (PFAS) are oxidatively recalcitrant organic synthetic compounds. PFAS are an exceptional group of chemicals that have significant physical characteristics due to the presence of the most electronegative element (i.e., fluorine). PFAS persist in the environment, bioaccumulate, and have been linked to toxicological impacts. Epidemiological and toxicity studies have shown that PFAS pose environmental and health risks, requiring their complete elimination from the environment. Various separation technologies, including adsorption with activated carbon or ion exchange resin; nanofiltration; reverse osmosis; and destruction methods (e.g., sonolysis, thermally induced reduction, and photocatalytic dissociation) have been evaluated to remove PFAS from drinking water supplies. In this review, we will comprehensively summarize previous reports on the photodegradation of PFAS with a special focus on photocatalysis. Additionally, challenges associated with these approaches along with perspectives on the state-of-the-art approaches will be discussed. Finally, the photocatalytic defluorination mechanism of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) following complete mineralization will also be examined in detail.

Prediction of Photochemical Properties of Dissolved Organic Matter Using Machine Learning

Apparent quantum yields (Φ) of photochemically produced reactive intermediates (PPRIs) formed by dissolved organic matter (DOM) are vital to element cycles and contaminant fates in surface water. Simultaneous determination of ΦPPRI values from numerous water samples through existing experimental methods is time consuming and ineffective. Herein, machine learning models were developed with a systematic data set including 1329 data points to predict the values of three ΦPPRIs (Φ3DOM*, Φ1O2, and Φ·OH) based on DOM spectral parameters, experimental conditions, and calculation parameters. The best predictive performances for Φ3DOM*, Φ1O2, and Φ·OH were achieved using the CatBoost model, which outperformed the traditional linear regression models. The significances of the wavelength range and spectral parameters on the three ΦPPRI predictions were revealed, suggesting that DOM with lower molecular weight, lower aromatic content, and a more autochthonous portion possessed higher ΦPPRIs. Chain models were constructed by adding the predicted Φ3DOM* as a new feature into the Φ1O2 and Φ·OH models, which consequently improved the predictive performance of Φ1O2 but worsened the Φ·OH prediction likely due to the complex formation pathways of ·OH. Overall, this study offered robust ΦPPRI prediction across interlaboratory differences and provided new insights into the relationship between PPRIs formation and DOM properties.

Exploring the Complexities of Dissolved Organic Matter Photochemistry from the Molecular Level by Using Machine Learning Approaches

Dissolved organic matter (DOM) sustains a substantial part of the organic matter transported seaward, where photochemical reactions significantly affect its transformation and fate. The irradiation experiments can provide valuable information on the photochemical reactivity (photolabile, photoresistant, and photoproduct) of molecules. However, the inconsistency of the fate of irradiated molecules among different experiments curtailed our understanding of the roles the photochemical reactions have played, which cannot be properly addressed by traditional approaches. Here, we conducted irradiation experiments for samples from two large estuaries in China. Molecules that occurred in irradiation experiments were characterized by the Fourier transform ion cyclotron resonance mass spectrometry and assigned probabilistic labels to define their photochemical reactivity. These molecules with probabilistic labels were used to construct a learning database for establishing a suitable machine learning (ML) model. We further applied our well-trained ML model to "un-matched" (i.e., not detected in our irradiation experiments) molecules from five estuaries worldwide, to predict their photochemical reactivity. Results showed that numerous molecules with strong photolability can be captured solely by the ML model. Moreover, comparing DOM photochemical reactivity in five estuaries revealed that the riverine DOM chemistry largely determines their subsequent photochemical transformation. We offer an expandable and renewable approach based on ML to compatibly integrate existing irradiation experiments and shed insight into DOM transformation and degradation processes.

Quantifying the Role of Simultaneous Transformation Pathways in the Fate of the Novel Aquatic Herbicide Florpyrauxifen-Benzyl

Predicting the fate of organic compounds in the environment is challenging due to the inability of laboratory studies to replicate field conditions. We used the intentionally applied aquatic herbicide florpyrauxifen-benzyl (FPB) as a model compound to investigate the contribution of multiple transformation pathways to organic compound fate in lakes. FPB persisted in five Wisconsin lakes for 5-7 days with an in-lake half-life of <2 days. FPB formed four transformation products, with the bioactive product florpyrauxifen persisting up to 30 days post-treatment. Parallel laboratory experiments showed that FPB degrades to florpyrauxifen via base-promoted hydrolysis. Hydroxy-FPB and hydroxy-florpyrauxifen were identified as biodegradation products, while dechloro-FPB was identified as a photoproduct. Material balance calculations using both laboratory rates and field product concentrations demonstrated that hydrolysis (∼47% of loss), biodegradation (∼20%), sorption (∼13%), and photodegradation (∼4%) occurred on similar timescales. Furthermore, the combined results demonstrated that abiotic and plant-catalyzed hydrolysis of FPB to florpyrauxifen, followed by biodegradation of florpyrauxifen to hydroxy-florpyrauxifen, was the dominant transformation pathway in lakes. This study demonstrates how combined field and laboratory studies can be used to elucidate the role of simultaneous and interacting pathways in the fate of organic compounds in aquatic environments.

Dissolved Organic Matter Composition Determines Its Susceptibility to Complete and Partial Photooxidation within Lakes

Dissolved organic matter (DOM) plays an important role in carbon cycling within inland surface waters. Under sunlight irradiation, DOM undergoes complete photooxidation to produce carbon dioxide (CO2) and partial photooxidation that alters the molecular composition of DOM. However, a mechanistic understanding of the relationship between DOM composition and its susceptibility to partial and complete photooxidation in surface waters is currently lacking. This work combines light exposure experiments with high-resolution mass spectrometry to investigate DOM photooxidation using two DOM isolates and DOM from 16 lakes that vary in trophic status and size. High ratios of oxygen consumption to dissolved inorganic carbon (DIC) production demonstrate that all samples undergo extensive partial photooxidation. At the molecular level, more oxidized, aromatic DOM formulas are associated with oxygen consumption and DIC production. Bulk level measurements indicate that DOM becomes less aromatic and lower in apparent molecular weight following partial photooxidation, and there is molecular level evidence of oxygen addition and loss of CO2 in all samples. However, formulas most susceptible to photooxidation vary depending on the initial DOM composition. Collectively, this work provides insights into the relationship between DOM composition and photooxidation, which has important implications for carbon cycling in diverse surface waters.

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

Photochemical transformation is an important process that involves trace organic contaminants (TrOCs) in sunlit surface waters. However, the environmental implications of their self-photosensitization pathway have been largely overlooked. Here, we selected 1-nitronaphthalene (1NN), a representative nitrated polycyclic aromatic hydrocarbon, to study the self-photosensitization process. We investigated the excited-state properties and relaxation kinetics of 1NN after sunlight absorption. The intrinsic decay rate constants of triplet (³1NN*) and singlet (¹1NN*) excited states were estimated to be 1.5 × 10⁶ and 2.5 × 10⁸ s^-1, respectively. Our results provided quantitative evidence for the environmental relevance of ³1NN* in waters. Possible reactions of ³1NN* with various water components were evaluated. With the reduction and oxidation potentials of -0.37 and 1.95 V, ³1NN* can be either oxidized or reduced by dissolved organic matter isolates and surrogates. We also showed that hydroxyl (^•OH) and sulfate (SO₄^•-) radicals can be generated via the ³1NN*-induced oxidation of inorganic ions (OH^- and SO₄^2-, respectively). We further investigated the reaction kinetics of ³1NN* and OH^- forming ^•OH, an important photoinduced reactive intermediate, through complementary experimental and theoretical approaches. The rate constants for the reactions of ³1NN* with OH^- and 1NN with ^•OH were determined to be 4.22 × 10⁷ and 3.95 ± 0.01 × 10⁹ M^-1 s^-1, respectively. These findings yield new insights into self-photosensitization as a pathway for TrOC attenuation and provide more mechanistic details into their environmental fate.

Inhibition of photosensitized degradation of organic contaminants by copper under conditions typical of estuarine and coastal waters

Dissolved organic matter (DOM) driven-photochemical processes play an important role in the redox cycling of trace metals and attenuation of organic contaminants in estuarine and coastal ecosystems. In this study, we evaluate the effect of Cu on 4-carboxybenzophenone (CBBP) and Suwannee River natural organic matter (SRNOM)-photosensitized degradation of seven target contaminants (TCs) including phenols and amines under pH conditions and salt concentrations typical of those encountered in estuarine and coastal waters. Our results show that trace amounts of Cu(II) (25 -500 nM) induce strong inhibition of the photosensitized degradation of all TCs in solutions containing CBBP. The influence of TCs on the photo-formation of Cu(I) and the decrease in the lifetime of transformation intermediates of contaminants (TC^•+/ TC^•_(-H)) in the presence of Cu(I) indicated that the inhibition effect of Cu was mainly due to the reduction of TC^•+/ TC^•_(-H) by the photo-produced Cu(I). The inhibitory effect of Cu on the photodegradation of TCs decreased with the increase in Cl^- concentration since less reactive Cu(I)-Cl complexes dominate at high Cl^- concentrations. The impact of Cu on the SRNOM-sensitized degradation of TCs is less pronounced compared to that observed in CBBP solution since the redox active moieties present in SRNOM competes with Cu(I) to reduce TC^•+/ TC^•_(-H). A detailed mathematical model is developed to describe the photodegradation of contaminants and Cu redox transformations in irradiated SRNOM and CBBP solutions.

Global modeling of lake-water indirect photochemistry based on the equivalent monochromatic wavelength approximation: The case of the triplet states of chromophoric dissolved organic matter

Chromophoric dissolved organic matter (CDOM) plays key role as photosensitizer in sunlit surface-water environments, and it is deeply involved in the photodegradation of contaminants. It has recently been shown that sunlight absorption by CDOM can be conveniently approximated based on its monochromatic absorption at 560 nm. Here we show that such an approximation allows for the assessment of CDOM photoreactions on a wide global scale and, particularly, in the latitude belt between 60°S and 60°N. Global lake databases are currently incomplete as far as water chemistry is concerned, but estimates of the content of organic matter are available. With such data it is possible to assess global steady-state concentrations of CDOM triplet states (³CDOM*), which are predicted to reach particularly high values at Nordic latitudes during summer, due to a combination of high sunlight irradiance and elevated content of organic matter. For the first time to our knowledge, we are able to model an indirect photochemistry process in inland waters around the globe. Implications are discussed for the phototransformation of a contaminant that is mainly degraded by reaction with ³CDOM* (clofibric acid, lipid regulator metabolite), and for the formation of known products on a wide geographic scale.

Dissolved Organic Matter Photoreactivity Is Determined by Its Optical Properties, Redox Activity, and Molecular Composition

Predicting the formation of photochemically produced reactive intermediates (PPRI) during the irradiation of dissolved organic matter (DOM) has remained challenging given the complex nature of this material and differences in PPRI formation mechanisms. We investigate the role of DOM composition in photoreactivity using 48 samples that span the range of DOM in freshwater systems and wastewater. We relate quantum yields for excited triplet-state organic matter (fTMP), singlet oxygen (Φ1O2), and hydroxylating species (Φ•OH) to DOM composition determined using spectroscopy, Fourier-transform ion cyclotron resonance mass spectrometry, and electron-donating capacity (EDC). fTMP and Φ1O2 follow similar trends and are correlated with bulk properties derived from UV-vis spectra and EDC. In contrast, no individual bulk property can be used to predict Φ•OH. At the molecular level, the subset of DOM that is positively correlated to both Φ•OH and EDC is distinct from DOM formulas related to Φ1O2, demonstrating that •OH and 1O2 are formed from different DOM fractions. Multiple linear regressions are used to relate quantum yields of each PPRI to DOM composition parameters derived from multiple techniques, demonstrating that complementary methods are ideal for characterizing DOM because each technique only samples a subset of DOM.

Evaluation of Probes to Measure Oxidizing Organic Triplet Excited States in Aerosol Liquid Water

Oxidizing triplet excited states of organic matter (³C*) drive numerous reactions in fog/cloud drops and aerosol liquid water (ALW). Quantifying oxidizing triplet concentrations in ALW is difficult because ³C* probe loss can be inhibited by the high levels of dissolved organic matter (DOM) and copper in particle water, leading to an underestimate of triplet concentrations. In addition, illuminated ALW contains high concentrations of singlet molecular oxygen (¹O₂*), which can interfere with ³C* probes. Our overarching goal is to find a triplet probe that has low inhibition by DOM and Cu(II) and low sensitivity to ¹O₂*. To this end, we tested 12 potential probes from a variety of compound classes. Some probes are strongly inhibited by DOM, while others react rapidly with ¹O₂*. One of the probe candidates, (phenylthiol)acetic acid (PTA), seems well suited for ALW conditions, with mild inhibition and fast rate constants with triplets, but it also has weaknesses, including a pH-dependent reactivity. We evaluated the performance of both PTA and syringol (SYR) as triplet probes in aqueous extracts of particulate matter. While PTA is less sensitive to inhibition than SYR, it results in lower triplet concentrations, possibly because it is less reactive with weakly oxidizing triplets.

Elucidation of the environmental reductive metabolism of the herbicide tritosulfuron assisted by electrochemistry and mass spectrometry

The environmental impact of pesticides and other pollutants is, to a great extent, determined by degradation and accumulation processes. Consequently, degradation pathways of pesticides have to be elucidated before approval by the authorities. In this study, the environmental metabolism of the sulfonylurea-herbicide tritosulfuron was investigated using aerobic soil degradation studies, during which a previously unidentified metabolite was observed using high performance liquid chromatography and mass spectrometry. The new metabolite was formed by reductive hydrogenation of tritosulfuron but the isolated amount and purity of the substance were insufficient to fully elucidate its structure. Therefore, electrochemistry coupled to mass spectrometry was successfully applied to mimic the reductive hydrogenation of tritosulfuron. After demonstrating the general feasibility of electrochemical reduction, the electrochemical conversion was scaled up to the semi-preparative scale and 1.0 mg of the hydrogenated product was synthesized. Similar retention times and mass spectrometric fragmentation patterns proved that the same hydrogenated product was formed electrochemically and in soil studies. Using the electrochemically generated standard, the structure of the metabolite was elucidated by means of NMR spectroscopy, which shows the potential of electrochemistry and mass spectrometry in environmental fate studies.

Photoinduced production of substances with humic-like fluorescence, upon irradiation of water samples from alpine lakes

Evidence is here provided that irradiation of some lake water samples can trigger the formation of fluorophores with humic-like properties, at the same time increasing water absorbance. This phenomenon is the opposite of photobleaching, which is often observed when natural waters are irradiated. The photoproduced humic-like fluorophores observed here would be of autochthonous rather than allochthonous origin, which marks a difference with the fraction of humic substances that derives from terrestrial sources. Photogeneration of humic-like compounds can be highlighted in water samples where the fluorescence signal of initially occurring humic substances is low, so that their photobleaching is minimised. Samples that are most likely to show photoinduced formation of humic-like fluorophores are in fact characterised by high values of protein-like vs. humic-like contribution ratios to fluorescence, as evidenced by parallel factor (PARAFAC) analysis. Mountain lakes in late summer appear to be suitable candidates to highlight the described phenomenon. In some cases, lake-water irradiation caused a decrease in the spectral slope of the absorbance that, together with increasing absorbance values, is consistent with an increase in molecular mass and aromaticity of organic matter. The absorbance increase triggered by irradiation might play a role in screening biologically harmful UV radiation, in mountain environments that would otherwise be characterised by very clear water that allows for easy transmission of UV light along the water column.

Role of Molecular Singlet Oxygen in Photochemical Degradation of NTO: DFT Study

NTO (5-nitro-1,2,4-triazol-3-one), an energetic material used in military applications, may be released to the environment and dissolved in surface water and groundwater due to its good water solubility. Singlet oxygen is an important reactive oxygen species produced in the aquatic environment under sunlight irradiation. A detailed investigation of the possible mechanism for NTO decomposition in water induced by singlet oxygen as one of the pathways for NTO environmental degradation was performed by a computational study at PCM(Pauling)/M06-2X/6-311++G(d,p) level. Decomposition of NTO was found to be a multistep process that may begin with singlet oxygen attachment to the carbon atom of the C═N double bond. The formed intermediate undergoes cycle opening, and nitrogen gas, nitrous acid, and carbon (IV) oxide elimination. Isocyanic acid, arisen transiently, hydrolyzes into ammonia and carbon (IV) oxide. The obtained results show a significant increase in reactivity of the anionic form of NTO as compared to its neutral form. The calculated activation energies and high exothermicity of the studied processes support the contribution of singlet oxygen to NTO degradation into low-weight inorganic compounds in the environment.

Photochemical Implications of Changes in the Spectral Properties of Chromophoric Dissolved Organic Matter: A Model Assessment for Surface Waters

Chromophoric dissolved organic matter (CDOM) is the main sunlight absorber in surface waters and a very important photosensitiser towards the generation of photochemically produced reactive intermediates (PPRIs), which take part in pollutant degradation. The absorption spectrum of CDOM (ACDOM(λ), unitless) can be described by an exponential function that decays with increasing wavelength: ACDOM(λ) = 100 d DOC Ao e− Sλ, where d [m] is water depth, DOC [mgC L−1] is dissolved organic carbon, Ao [L mgC−1 cm−1] is a pre-exponential factor, and S [nm−1] is the spectral slope. Sunlight absorption by CDOM is higher when Ao and DOC are higher and S is lower, and vice versa. By the use of models, here we investigate the impact of changes in CDOM spectral parameters (Ao and S) on the steady-state concentrations of three PPRIs: the hydroxyl radical (•OH), the carbonate radical (CO3•−), and CDOM excited triplet states (3CDOM*). A first finding is that variations in both Ao and S have impacts comparable to DOC variations on the photochemistry of CDOM, when reasonable parameter values are considered. Therefore, natural variability of the spectral parameters or their modifications cannot be neglected. In the natural environment, spectral parameters could, for instance, change because of photobleaching (prolonged exposure of CDOM to sunlight, which decreases Ao and increases S) or of the complex and still poorly predictable effects of climate change. A second finding is that, while the steady-state [3CDOM*] would increase with increasing ACDOM (increasing Ao, decreasing S), the effect of spectral parameters on [•OH] and [CO3•−] depends on the relative roles of CDOM vs. NO3− and NO2− as photochemical •OH sources.

The Degradation Process of Typical Neonicotinoid Insecticides in Tidal Streams in Subtropical Cities: A Case Study of the Wuchong Stream, South China

Neonicotinoid insecticides (NEOs) are commonly used to prevent unwanted insects in urban fields. Degradation processes have been one of the important environmental behaviors of NEOs in an aquatic environment. In this research, hydrolysis, biodegradation, and photolysis processes of four typical NEOs (i.e., thiacloprid (THA), clothianidin (CLO), acetamiprid (ACE), and imidacloprid (IMI)) were examined through the adoption of response surface methodology-central composite design (RSM-CCD) for an urban tidal stream in South China. The influences of multiple environmental parameters and concentration levels on the three degradation processes of these NEOs were then evaluated. The results indicated that the three degradation processes of the typical NEOs followed a pseudo-first-order reaction kinetics model. The primary degradation process of the NEOs were hydrolysis and photolysis processes in the urban stream. The hydrolysis degradation rate of THA was the highest (1.97 × 10^-5 s^-1), and that of CLO was the lowest (1.28 × 10^-5 s^-1). The temperature of water samples was the main environmental factor influencing the degradation processes of these NEOs in the urban tidal stream. Salinity and humic acids could inhibit the degradation processes of the NEOs. Under the influence of extreme climate events, the biodegradation processes of these typical NEOs could be suppressed, and other degradation processes could be further accelerated. In addition, extreme climate events could pose severe challenges to the migration and degradation process simulation of NEOs.

Biodegradation of sulfoxaflor and photolysis of sulfoxaflor by ultraviolet radiation

Sulfoxaflor (SUL, [N-[methyloxido[1-[6-(trifluoromethyl)-3-pyridinyl] ethyl]-λ⁴-sulfanylidene] cyanamide]) is a widely used systemic insecticide, and its residue has frequently been detected in the environment, posing a potential threat to the environment. In this study, Pseudaminobacter salicylatoxidans CGMCC 1.17248 rapidly converted SUL into X11719474 via a hydration pathway mediated by two nitrile hydratases (AnhA and AnhB). Extensive (96.4%) degradation of 0.83 mmol/L SUL was achieved by P. salicylatoxidans CGMCC 1.17248 resting cells within 30 min (half-life of SUL 6.4 min). Cell immobilization by entrapment into calcium alginate remediated 82.8% of the SUL in 90 min, and almost no SUL was observed in surface water after incubation for 3 h. P. salicylatoxidans NHases AnhA and AnhB both hydrolyzed SUL to X11719474, although AnhA exhibited much better catalytic performance. The genome sequence of P. salicylatoxidans CGMCC 1.17248 revealed that this strain could efficiently eliminate nitrile-containing insecticides and adapt to harsh environments. We firstly found that UV irradiation transforms SUL to the derivatives X11719474 and X11721061, and the potential reaction pathways were proposed. These results further deepen our understanding of the mechanisms of SUL degradation as well as the environmental fate of SUL.

Global Corrections to Reference Irradiance Spectra for Non-Clear-Sky Conditions

Photochemical reactions in surface waters play important roles in element cycling and in the removal of organic contaminants, among other processes. A central environmental variable affecting photochemical processes in surface waters is the incoming solar irradiance, as this initiates these processes. However, clear-sky incident irradiance spectra are often used when evaluating the fate of aquatic contaminants, leading to an overestimation of contaminant decay rates due to photochemical transformation. In this work, incident irradiance satellite data were used to develop global-scale non-clear-sky correction factors for commonly used reference irradiance spectra. Non-clear-sky conditions can decrease incident irradiance by over 90% depending on the geographic location and time of the year, with latitudes above 40°N being most heavily affected by seasons. The impact of non-clear-sky conditions on contaminant half-lives was illustrated in a case study of triclosan in lake Greifensee, which showed a 39% increase in the triclosan half-life over the course of a year under non-clear-sky conditions. A global annual average correction factor of 0.76 was determined as an approximate way to account for non-clear-sky conditions. The correction factors are developed at monthly and seasonal resolutions for every location on the globe between 70°N and 60°S at a 4 km spatial resolution and can be used by researchers, practitioners, and regulators who need improved estimates of incident irradiance.

Humic Substance Photosensitized Degradation of Phthalate Esters Characterized by 2H and 13C Isotope Fractionation

The photosensitized transformation of organic chemicals is an important degradation mechanism in natural surface waters, aerosols, and water films on surfaces. Dissolved organic matter including humic-like substances (HS), acting as photosensitizers that participate in electron transfer reactions, can generate a variety of reactive species, such as OH radicals and excited triplet-state HS (³HS*), which promote the degradation of organic compounds. We use phthalate esters, which are important contaminants found in wastewaters, landfills, soils, rivers, lakes, groundwaters, and mine tailings. We use phthalate esters as probes to study the reactivity of HS irradiated with artificial sunlight. Phthalate esters with different side-chain lengths were used as probes for elucidation of reaction mechanisms using ²H and ¹³C isotope fractionation. Reference experiments with the artificial photosensitizers 4,5,6,7-tetrachloro-2',4',5',7'-tetraiodofluorescein (Rose Bengal), 3-methoxy-acetophenone (3-MAP), and 4-methoxybenzaldehyde (4-MBA) yielded characteristic fractionation factors (-4 ± 1, -4 ± 2, and -4 ± 1‰ for ²H; 0.7 ± 0.2, 1.0 ± 0.4, and 0.8 ± 0.2‰ for ¹³C), allowing interpretation of reaction mechanisms of humic substances with phthalate esters. The correlation of ²H and ¹³C fractions can be used diagnostically to determine photosensitized reactions in the environment and to differentiate among biodegradation, hydrolysis, and photosensitized HS reaction.

Green photocatalytic remediation of Fenthion using composites with natural red clay and non-toxic metal oxides with visible light irradiation

In the present work, composites with non-toxic metal oxides, such as TiO₂ and ZnO, and a natural red clay (taua) reach in hematite were used in the photocatalytic degradation of Fenthion. The composite TiO₂/Taua (0.5:1 wt. ratio) and pure TiO₂ were prepared by sol-gel method while ZnO/Taua (0.5:1 wt. ratio) and pure ZnO were prepared by Pechini method. The materials were characterized by XRD, SEM, EDX, and DRS. The anatase phase was formed in both pure TiO₂ and TiO₂/Taua, while the hexagonal phase was formed in pure ZnO and ZnO/Taua. The bandgap energies for the two composites were narrowed compared to the respective pure oxides as consequence of the hematite (α-Fe₂O₃, E_g = 2.1 eV) in the red clay, reaching 2.1 eV for TiO₂/Taua and 2.0 eV for ZnO/Taua, while the bandgap energies for pure TiO₂ and ZnO were 3.2 and 3.0 eV, respectively. Fenthion was not degraded in the dark, but the concentration droped 20% after 180 min under visible light irradiation without photocatalyst and 60% after 210 min in the presence of the pure red clay. Both TiO₂/Taua and ZnO/Taua composites were also photocatalytic active to degrade Fenthion (λ > 420 nm), with degradation of 78% (in 180 min) and 85% (in 210 min) respectively. In the optimized conditions (pH 2, 100 mg L^-1 of H₂O₂ and 30 mg L^-1 of Fenthion), the ZnO/Taua composite was the most efficient, reaching 89% degradation in up to 30 min, with Fenthion sulfoxide as the degradation product.

Evaluation of the Environmental Fate of a Semivolatile Transformation Product of Ibuprofen Based on a Simple Two-Media Fate Model

Partitioning between surface waters and the atmosphere is an important process, influencing the fate and transport of semi-volatile contaminants. In this work, a simple methodology that combines experimental data and modeling was used to investigate the degradation of a semi-volatile pollutant in a two-phase system (surface water + atmosphere). 4-Isobutylacetophenone (IBAP) was chosen as a model contaminant; IBAP is a toxic transformation product of the non-steroidal, anti-inflammatory drug ibuprofen. Here, we show that the atmospheric behavior of IBAP would mainly be characterized by reaction with ^•OH radicals, while degradation initiated by ^•NO₃ or direct photolysis would be negligible. The present study underlines that the gas-phase reactivity of IBAP with ^•OH is faster, compared to the likely kinetics of volatilization from aqueous systems. Therefore, it might prove very difficult to detect gas-phase IBAP. Nevertheless, up to 60% of IBAP occurring in a deep and dissolved organic carbon-rich water body might be eliminated via volatilization and subsequent reaction with gas-phase ^•OH. The present study suggests that the gas-phase chemistry of semi-volatile organic compounds which, like IBAP, initially occur in natural water bodies in contact with the atmosphere is potentially very important in some environmental conditions.

Stimulated Raman generation of aqueous singlet oxygen without photosensitizers

Singlet oxygen is traditionally produced via photosensitizer molecules such as methylene blue, which function as catalysts. Here we investigate stimulated Raman generation of singlet oxygen from dissolved oxygen in both water (H₂O) and heavy water (D₂O) using nanosecond-pulsed visible blue laser light in the 400-440 nm spectral region without singlet oxygen photosensitizers. We report an oxygen-dependent Stokes peak in the red spectrum (600-670 nm) that is identical when produced in H₂O and D₂O. These red Stokes photons are not detected when an oxygen quencher is present. Temporal photodepletion of the uric acid absorbance peak at 294 nm is consistent with singlet oxygen generation. We postulate that a two-photon stimulated Raman process produces singlet oxygen from O₂ dissolved within the solvents. We note that the energy difference between input and output photons of 0.97 eV is precisely the energy needed to excite O₂ to its singlet state.

Finding Fluorine: Photoproduct Formation during the Photolysis of Fluorinated Pesticides

The photolysis of pesticides with different fluorine motifs was evaluated to quantify the formation of fluorinated products in buffered aqueous systems, advanced oxidation (AOP) and reduction processes (ARP), and river water. Simulated sunlight quantum yields at pH 7 were 0.0033, 0.0025, 0.0015, and 0.00012 for penoxsulam, florasulam, sulfoxaflor, and fluroxypyr, respectively. The bimolecular rate constants with hydroxyl radicals were 2 to 5.7 × 1010 M-1 s-1 and, with sulfate radicals, 1.6 to 2.6 × 108 M-1 s-1 for penoxsulam, florasulam, and fluroxypyr, respectively. The rate constants of sulfoxaflor were 100-fold lower. Using quantitative 19F-NMR, complete fluorine mass balances were obtained. The maximum fluoride formation was 53.4 and 87.4% for penoxsulam and florasulam under ARP conditions, and 6.1 and 100% for sulfoxaflor and fluroxypyr under AOP conditions. Heteroaromatic CF3 and aliphatic CF2 groups were retained in multiple fluorinated photoproducts. Aryl F and heteroaromatic F groups were readily defluorinated to fluoride. CF3 and CF2 groups formed trifluoroacetate and difluoroacetate, and yields increased under oxidizing conditions. 19F-NMR chemical shifts and coupling analysis provided information on hydrogen loss on adjacent bonds or changes in chirality. Mass spectrometry results were consistent with the observed 19F-NMR products. These results will assist in selecting treatment processes for specific fluorine motifs and in the design of agrochemicals to reduce byproduct formation.

Degradation of anthranilic diamide insecticide tetrachlorantraniliprole in water: Kinetics, degradation pathways, product identification and toxicity assessment

In the present study, aqueous behavior and fate of diamide insecticide tetrachlorantraniliprole (TCTP) were investigated under laboratory-controlled conditions. Half-lives of TCTP photolysis in natural water and pH buffers were 1.4-2.8 h, comparing with those of 1.2-231 d for hydrolysis. Both processes were highly influenced by pH with respect to degradation kinetics and routes. The hydrolysis rate of TCTP was accelerated by elevated temperatures. The presence of nitrate enhanced TCTP photolysis while fulvic acid exhibited suppression, with the extent of both effects as a function of concentration. Four degradation products were identified using a variety of spectroscopic approaches. Key reactions involved in the degradation pathways include intramolecular substitution and cyclization. There was a reduction in the acute toxicity of all four products to Daphnia magna by comparison with TCTP, whereas they were still classified as category 1 or 2 hazardous substances to the aquatic environment according to the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) standards.

Synthesizing Laboratory and Field Experiments to Quantify Dominant Transformation Mechanisms of 2,4-Dichlorophenoxyacetic Acid (2,4-D) in Aquatic Environments

Laboratory studies used to assess the environmental fate of organic chemicals such as pesticides fail to replicate environmental conditions, resulting in large errors in predicted transformation rates. We combine laboratory and field data to identify the dominant loss processes of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) in lakes for the first time. Microbial and photochemical degradation are individually assessed using laboratory-based microcosms and irradiation studies, respectively. Field campaigns are conducted in six lakes to quantify 2,4-D loss following large-scale herbicide treatments. Irradiation studies show that 2,4-D undergoes direct photodegradation, but modeling efforts demonstrated that this process is negligible under environmental conditions. Microcosms constructed using field inocula show that sediment microbial communities are responsible for degradation of 2,4-D in lakes. Attempts to quantify transformation products are unsuccessful in both laboratory and field studies, suggesting that their persistence is not a major concern. The synthesis of laboratory and field experiments is used to demonstrate best practices in designing laboratory persistence studies and in using those results to mechanistically predict contaminant fate in complex aquatic environments.

A Model Assessment of the Occurrence and Reactivity of the Nitrating/Nitrosating Agent Nitrogen Dioxide (•NO2) in Sunlit Natural Waters

Nitrogen dioxide (•NO2) is produced in sunlit natural surface waters by the direct photolysis of nitrate, together with •OH, and upon the oxidation of nitrite by •OH itself. •NO2 is mainly scavenged by dissolved organic matter, and here, it is shown that •NO2 levels in sunlit surface waters are enhanced by high concentrations of nitrate and nitrite, and depressed by high values of the dissolved organic carbon. The dimer of nitrogen dioxide (N2O4) is also formed in the pathway of •NO2 hydrolysis, but with a very low concentration, i.e., several orders of magnitude below •NO2, and even below •OH. Therefore, at most, N2O4 would only be involved in the transformation (nitration/nitrosation) of electron-poor compounds, which would not react with •NO2. Although it is known that nitrite oxidation by CO3•− in high-alkalinity surface waters gives a minor-to-negligible contribution to •NO2 formation, it is shown here that NO2− oxidation by Br2•− can be a significant source of •NO2 in saline waters (saltwater, brackish waters, seawater, and brines), which offsets the scavenging of •OH by bromide. As an example, the anti-oxidant tripeptide glutathione undergoes nitrosation by •NO2 preferentially in saltwater, thanks to the inhibition of the degradation of glutathione itself by •OH, which is scavenged by bromide in saltwater. The enhancement of •NO2 reactions in saltwater could explain the literature findings, that several phenolic nitroderivatives are formed in shallow (i.e., thoroughly sunlit) and brackish lagoons in the Rhône river delta (S. France), and that the laboratory irradiation of phenol-spiked seawater yields nitrophenols in a significant amount.

Kinetics of the photolysis of pyridaben and its main photoproduct in aqueous environments under simulated solar irradiation

The photolytic fate of pyridaben and its main photolysis product was investigated in different aqueous solutions. Results showed that the photolysis of pyridaben followed pseudo first-order kinetics or the hockey-stick model. In buffer solutions, the half-life of pyridaben was the shortest at pH 4, while the degradation rate within 24 h was the highest at pH 9. Humic acids (HA) at concentrations of 1-20 mg L^-1 favored the photolysis of pyridaben while fulvic acids (FA) did not have a significant effect. Nitrate at low concentrations (0.01 mM) accelerated the photolysis and Fe(iii) at high concentrations (0.01 and 0.1 mM) significantly inhibited the photolysis. The photolysis rate of pyridaben in rainwater, tap water, and river water was significantly higher than that in distilled water. The half-lives in distilled water, rainwater, tap water, river water, and pond water were 2.36, 1.36, 1.61, 1.77, and 2.68 h, respectively. Ultra-high-performance liquid chromatography/high-resolution mass spectrometry identified M328 as a photolysis product. The degradation of M328 followed pseudo first-order kinetics in distilled water, buffer solutions and aqueous solutions fortified with HA. The half-lives of M328 were in the range of 7.07-13.95 h. These results are essential for further environmental risk assessment of pyridaben.

Environmental photochemical fate of pesticides ametryn and imidacloprid in surface water (Paranapanema River, São Paulo, Brazil)

In addition to direct photolysis studies, in this work the second-order reaction rate constants of pesticides imidacloprid (IMD) and ametryn (AMT) with hydroxyl radicals (HO^●), singlet oxygen (¹O₂), and triplet excited states of chromophoric dissolved organic matter (³CDOM^*) were determined by kinetic competition under sunlight. IMD and AMT exhibited low photolysis quantum yields: (1.23 ± 0.07) × 10^-2 and (7.99 ± 1.61) × 10^-3 mol Einstein^-1, respectively. In contrast, reactions with HO^● radicals and ³CDOM* dominate their degradation, with ¹O₂ exhibiting rates three to five orders of magnitude lower. The values of k_IMD,HO● and k_AMT,HO● were (3.51 ± 0.06) × 10⁹ and (4.97 ± 0.37) × 10⁹ L mol^-1 s^-1, respectively, while different rate constants were obtained using anthraquinone-2-sulfonate (AQ2S) or 4-carboxybenzophenone (CBBP) as CDOM proxies. For IMD this difference was significant, with k_IMD,3AQ2S* = (1.02 ± 0.08) × 10⁹ L mol^-1 s^-1 and k_IMD,3CBBP* = (3.17 ± 0.14) × 10⁸ L mol^-1 s^-1; on the contrary, the values found for AMT are close, k_AMT,3AQ2S* = (8.13 ± 0.35) × 10⁸ L mol^-1 s^-1 and k_AMT,3CBBP* = (7.75 ± 0.80) × 10⁸ L mol^-1 s^-1. Based on these results, mathematical simulations performed with the APEX model for typical levels of water constituents (NO₃^-, NO₂^-, CO₃^2-, TOC, pH) indicate that the half-lives of these pesticides should vary between 24.1 and 18.8 days in the waters of the Paranapanema River (São Paulo, Brazil), which can therefore be impacted by intensive agricultural activity in the region.

Fate and transport of pharmaceuticals in water systems: A processes review

Pharmaceuticals are globally consumed by humans and animals to support daily health and to treat disease. Following consumption, they may reach the aquatic environment either directly through the discharge of untreated wastewater to water bodies, or indirectly via treated wastewater as a result of their incomplete removal from wastewater treatment plants. This paper reviews the processes that control the occurrence and fate of pharmaceuticals in water systems, including sorption, photodegradation, hydrolysis and biodegradation. The degree to which these four processes occur is influenced by pharmaceutical types and their chemical structure as well as environmental factors such as sunlight, water depth, organic matter content, water chemistry, sediment properties, and type and abundance of microorganisms. Depending on the complex interactions of these factors, pharmaceutical compounds may be mineralized, partially degraded, or remain intact because they are resistant to degradation. Kinetic rate parameters and the half-life of a variety of pharmaceutical products are provided herein for the above processes under different environmental conditions. Usually, photodegradation and biodegradation represent dominant reaction processes, while hydrolysis only affects some pharmaceuticals, particularly antibiotics. The identified sorption and reaction rate parameters can be incorporated into a concise modeling framework to assess and predict longitudinal concentration profiles of pharmaceutical products in the manmade and natural systems, particularly when large amounts of pharmaceuticals are discharged during abnormal events such as a virus outbreak. Finally, future research is suggested, including the fate of transformed products (intermediates) in water systems.

Effects of dissolved organic matter characteristics on the photosensitized degradation of pharmaceuticals in wastewater treatment wetlands

Wastewater treatment wetlands are aquatic systems where diverse dissolved organic matter (DOM) compositions physically interact. Complex photochemical behaviors ensue, leading to uncertainties in the prediction of indirect photodegradation rates for organic contaminants. Here, we evaluate the photosensitization ability of whole water DOM samples from a treatment wetland and wastewater treatment plant (WWTP) in North Carolina to photodegrade target pharmaceuticals. Optical characterization using ultraviolet-visible and excitation-emission matrix spectroscopy shows that wetland DOM has higher aromaticity than WWTP DOM and that WWTP secondary treatment processes increase aromaticity, overall molecular weight, and humic character of wastewater DOM. Our application of a reversed-phase HPLC method to assess DOM polarity distinctly reveals that a subset of the wetland samples possesses an abundance of hydrophobic DOM moieties. Hydroxyl radicals (˙OH) mediate the majority (>50%) of the indirect photodegradation for amoxicillin (AMX), atenolol (ATL), and 17α-ethinylestradiol (EE2), while singlet oxygen (¹O₂) is presumed to be solely responsible for the photodegradation of cimetidine (CME). Our findings suggest that hydrophobic interactions and improved accessibility to photogenerated reactive intermediates lead to significant increases in photosensitization efficiencies and overall indirect photodegradation rates of AMX, ATL, and EE2 for the hydrophobic wetland samples. In contrast, CME photosensitization yields are unaffected by polarity and trend positively with optical indicators of sunlight-induced DOM photobleaching and humification, suggesting that wetland processing favors faster ¹O₂ photogeneration. These relationships highlight the uncertainties in photosensitization yields and effects of DOM optical properties and polarity on the photochemical fate of organic contaminants.

Riparian strips as attenuation zones for the toxicity of pesticides in agricultural surface runoff: Relative influence of herbaceous vegetation and terrain slope on toxicity attenuation of 2,4-D

Pesticides reach aquatic ecosystems via surface runoff becoming one of the main contributors to their deterioration. Among the strategies to mitigate these impacts, the use of riparian strips is recommended, but the knowledge of how much each ecosystemic variable contributes to the process is still incipient. We analyzed the influence of terrain slope and vegetation in the attenuation of 2,4-Dichlorophenoxyacetic acid (2,4-D) toxicity in surface runoff using Lactuca sativa as a diagnostic organism. In addition, the differential effects of this herbicide were examined under laboratory conditions, with standardized water and ambient water as a dilution medium. The study was conducted in plots with different terrain slopes and presence/absence of vegetation. The herbicide was applied to each plot and rain was subsequently simulated. The runoff was collected at regular distances and the toxicity was measured. The runoff toxicity decreased with the distance from the application area in all plots, this reduction being greater in low-slope plots. No differences in attenuation of runoff toxicity were found between plots with and without vegetation. The data were incorporated into models to estimate the minimum widths of safety to reduce the toxicity of 2,4-D by 90% under these conditions, suggesting distances of 5 and 20 m for low-slope and high-slope zones, respectively. In laboratory experiments, lower relative toxicity of 2,4-D was detected when natural water was used as solvent. These results contribute to the design of sustainable agricultural practices.

Photochemical interactions between pesticides and plant volatiles

Among the numerous studies devoted to the photodegradation of pesticides, very scarce are those investigating the effect of plant volatiles. Yet, pesticides can be in contact with plant volatiles after having been spread on crops or when they are transported in surface water, making interactions between the two kinds of chemicals possible. The objectives of the present study were to investigate the reactions occurring on plants. We selected thyme as a plant because it is used in green roofs and two pesticides: the fungicide chlorothalonil for its very oxidant excited state and the insecticide imidacloprid for its ability to release the radical NO₂ under irradiation. Pesticides were irradiated with simulated solar light first in a solvent ensuring a high solubility of pesticides and plant volatiles, and then directly on thyme's leaves. Analyses were conducted by headspace gas chromatography-mass spectrometry (HS-GC-MS), GC-MS and liquid chromatography-high resolution mass spectrometry (LC-HRMS). In acetonitrile, chlorothalonil photosensitized the degradation of thymol, α-pinene, 3-carene and linalool with high quantum yields ranging from 0.35 to 0.04, and was photoreduced, while thymol underwent oxidation, chlorination and dimerization. On thyme's leave, chlorothalonil was photoreduced again and products arising from oxidation and dimerization of thymol were detected. Imidacloprid photooxidized and photonitrated thymol in acetonitrile, converting it into chemicals of particular concern. Some of these chemicals were also found when imidacloprid was irradiated dispersed on thyme's leaves. These results show that photochemical reactions between pesticides and the plants secondary metabolites can take place in solution as on plants. These findings demonstrate the importance to increase our knowledge on these complex scenarios that concern all the environmental compartments.

Contrasting Impacts of Photochemical and Microbial Processing on the Photoreactivity of Dissolved Organic Matter in an Adirondack Lake Watershed

Photochemical and microbial processing are the prevailing mechanisms that shape the composition and reactivity of dissolved organic matter (DOM); however, prior research has not comparatively evaluated the impacts of these processes on the photoproduction of reactive intermediates (RIs) from freshly sourced terrestrial DOM. We performed controlled irradiation and incubation experiments with leaf and soil samples collected from an acid-impacted lake watershed in the Adirondack Mountain region of New York to examine the effects of DOM processing on the apparent quantum yields of RIs (Φ_app,RI), including excited triplet states of DOM (³DOM*), singlet oxygen (¹O₂), and hydroxyl radicals (^•OH). Photodegradation led to net reductions in Φ_app,¹O2, Φ_app,³DOM*, and Φ_app,^•OH, whereas (photo-)biodegradation resulted in increases in Φ_app,¹O2 and Φ_app,³DOM*. Photodegradation and (photo-)biodegradation also shifted the energy distribution of ³DOM* in different directions. Multivariate statistical analyses revealed the potential relevance of photo-biodegradation in driving changes in Φ_app,¹O2 and Φ_app,³DOM* and prioritized five bulk DOM optical and redox properties that best explained the variations in Φ_app,¹O2 and Φ_app,³DOM* along the watershed terrestrial-aquatic continuum. Our findings highlight the contrasting impacts of photochemical and microbial processes on the photoreactivity of freshly sourced terrestrial DOM and invite further studies to develop a more holistic understanding of their implications for aquatic photochemistry.

Inhibition by phenolic antioxidants of the degradation of aromatic amines and sulfadiazine by the carbonate radical (CO3•-)

The carbonate radical CO3•- and the excited triplet states of chromophoric dissolved organic matter play an important role in the photodegradation of some easily oxidized pollutants in surface waters, such as the aromatic amines. Anilines and sulfadiazine are known to undergo back-reduction processes when their degradation is mediated by the excited triplet states of photosensitizers (triplet sensitization). Back-reduction, which inhibits photodegradation, means that phenols or the antioxidant (mostly phenolic) moieties occurring in the natural dissolved organic matter of surface waters reduce, back to the parent compounds, the radical species derived from the mono-electronic oxidation of anilines and sulfadiazine. Here we show that a similar process takes place as well in the case of substrate oxidation by CO3•-. The carbonate radical was here produced upon oxidation of HCO3-/CO32- by either HO•, generated by nitrate photolysis, or SO4•-, obtained by photolysis of persulfate. Back-reduction was observed in both cases in the presence of phenols, but at different extents as far as the details of reaction kinetics are concerned, and the occurrence of additional reductants might affect the efficacy by which phenols carry out the reduction process. In particular, when the carbonate radicals were produced by NO3- photolysis in the presence of HCO3-/CO32-, the numerical values of [PhOH]1/2 (the phenol concentration that halves the photodegradation rate of the substrate) were 2.19 ± 0.23 µM for aniline, 1.15 ± 0.25 µM for 3-chloroaniline, 1.18 ± 0.26 µM for 4-chloroaniline, and 1.18 ± 0.22 µM for 3,4-dichloroaniline. In contrast, when CO3•- was produced by photolysis of persulfate in the presence of HCO3-/CO32-, the corresponding values were 0.28 ± 0.02 µM for aniline and 0.79 ± 0.10 µM for sulfadiazine. Back-reduction has the potential to significantly inhibit photodegradation by CO3•- and excited triplet states in natural waters, and to comparatively increase the importance of HO•-mediated degradation that is not affected by the same phenomenon.

Emerging investigator series: quantifying the impact of cloud cover on solar irradiance and environmental photodegradation

Environmental photodegradation is dependent on the solar irradiance that reaches the Earth's surface, and photodegradation half-lives of contaminants are typically estimated assuming clear sky (i.e., cloudless) conditions. In this work, the effect of cloud cover on solar irradiance was investigated. Data from the National Renewable Energy Laboratory (NREL), which spanned 3 years of observations (10/2017 to 12/2020), were used to train two machine learning models to predict irradiance based on three inputs - day of year, time of day, and percentage of the sky that was cloudy. Results showed a non-linear relationship between cloud cover and irradiance. Solar irradiance was minimally impacted up to ≈50% cloud cover but decreased by ≈67% at 100% cloud cover. Both random forest and artificial neural network models performed well with relative root mean squared errors of 26-31%, which varied depending on the source of cloud cover data and the spectral region being modeled. Daily irradiance values for a whole year were predicted for varying cloud conditions using the machine learning models; this result was approximated using a quadratic fit of y = 1 - 0.00243x - (4.24 × 10^-5)x² where y is the fraction of clear sky irradiance expected and x is the percentage of cloud cover in the sky. In addition, the model results supported that there was no wavelength dependence for the effect of cloud cover. Therefore, decreases in both direct and indirect photodegradation rates should be proportional to the decrease in irradiance, which has a non-linear dependence on cloud cover.

Direct and indirect photodegradation of atrazine and S-metolachlor in agriculturally impacted surface water and associated C and N isotope fractionation

Knowledge of direct and indirect photodegradation of pesticides and associated isotope fractionation can help to assess pesticide degradation in surface waters. Here, we investigated carbon (C) and nitrogen (N) isotope fractionation during direct and indirect photodegradation of the herbicides atrazine and S-metolachlor in synthetic agriculturally impacted surface waters containing nitrates (20 mg L^-1) and dissolved organic matter (DOM, 5.4 mg_C L^-1). Atrazine and S-metolachlor were quickly photodegraded by both direct and indirect processes (half-lives <5 and <7 days, respectively). DOM slowed down photodegradation while nitrates increased degradation rates. The analysis of transformation products showed that oxidation mediated by hydroxyl radicals (HO˙) predominated during indirect photodegradation. UV light (254 nm) led to significant C and N isotope fractionation, yielding isotopic fractionation values ε_C = 2.7 ± 0.3 and 0.8 ± 0.1‰, and ε_N = 2.4 ± 0.3 and -2.6 ± 0.7‰ for atrazine and S-metolachlor, respectively. In contrast, photodegradation under simulated sunlight led to negligible C and slight N isotope fractionation, emphasizing the effect of the radiation wavelengths on the isotope fractionation induced by direct photodegradation. Altogether, these results highlight the importance of using simulated sunlight to obtain environmentally-relevant isotopic fractionation values and to distinguish photodegradation and other dissipation pathways in surface waters.

Effect of Solution pH on the Dual Role of Dissolved Organic Matter in Sensitized Pollutant Photooxidation

Dissolved organic matter (DOM) has a dual role in indirect phototransformations of aquatic contaminants by acting both as a photosensitizer and an inhibitor. Herein, the pH dependence of the inhibitory effect of DOM and the underlying mechanisms were studied in more than 400 kinetic irradiation experiments over the pH range of 6-11. Experiments employed various combinations of one of three DOM isolates, one of two model photosensitizers, the model antioxidant phenol, and one of nine target compounds (TCs), comprising several aromatic amines, in particular anilines and sulfonamides, and 4-cyanophenol. Using model photosensitizers without antioxidants, the phototransformation of most TCs increased with increasing pH, even for TCs for which pH did not affect speciation. This trend was attributed to pH-dependent formation yields of TC-derived radicals and their re-formation to the parent TC. Analogous trends were observed with DOM as a photosensitizer. Comparison of model and DOM photosensitizer data sets showed increasing inhibitory effects of DOM on TC phototransformation kinetics with increasing pH. In systems with anilines as a TC and phenol as a model antioxidant, pH trends of the inhibitory effect could be rationalized based on the reduction potential difference (ΔE_red) of phenoxyl/phenol and anilinyl/aniline couples. Our results indicate that the light-induced transformation of aromatic amines in the aquatic environment is governed by the pH-dependent inhibitory effects of antioxidant phenolic moieties of DOM and pH-dependent processes related to the formation of amine oxidation intermediates.

Photochemical characterization of paddy water during rice cultivation: Formation of reactive intermediates for As(III) oxidation

Although the photochemical behavior of surface water and its effects on pollutant transformation have been studied extensively in recent years, the photochemistry of paddy water remains largely unknown. In this study, we examined the photochemical processes involving paddy water samples collected at four different cultivation stages of rice. Triplet dissolved organic matter (³DOM*), singlet oxygen (¹O₂), and hydroxyl radicals (^•OH) were found to be the dominant reactive intermediates (RIs), and their apparent quantum yields and steady-state concentrations were quantified. Compared with the typical surface water, quantum yields of ³DOM* and ^•OH were comparable, while quantum yields of ¹O₂ were about 2.4-6.7 times higher than those of surface water. Fluorescence emission-excitation matrix (EEM) spectra, Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), and statistical analysis revealed that DOM properties and nitrite concentration were the main factor influencing RIs generation. The results suggest that DOM with lower molecular weight and humification extent generated more RIs, and nitrite contributed to 23.9%-100% of ^•OH generation. EEM and FTICR-MS data showed that DOM with more saturated and less aromatic formulas could produce more ³DOM* under the irradiation, while the polyphenolic components of DOM inhibited the formation of RIs. Moreover, RIs significantly enhanced arsenite (As(III)) oxidation with oxidation rate increased by 1.8-4.1 times in paddy water, and ^•OH and ³DOM* were the main RIs responsible for As(III) oxidation. This study provides new insight into the pathways of arsenite abiotic transformation in paddy soil and water.

Characterization and biogeochemical implications of dissolved organic matter in aquatic environments

Dissolved organic matter (DOM) is viewed as one of the most chemically active organic substances on earth. It plays vital roles in the fate, bioavailability and toxicity of aquatic exogenous chemical species (e.g., heavy metals, organic pollutants, and nanomaterials). The characteristics of DOM such low concentrations, salt interference and complexity in aquatic environments and limitations of pretreatment for sample preparation and application of characterization techniques severely limit understanding of its nature and environmental roles. This review provides a characterization continuum of aquatic DOM, and demonstrate its biogeochemical implications, enabling in-depth insight into its nature and environmental roles. A synthesis of the effective DOM pretreatment strategies, comprising extraction and fractionation methods, and characterization techniques is presented. Additionally, the biogeochemical dynamics of aquatic DOM and its environmental implications are discussed. The findings indicate the collection of representative DOM samples from water as the first and critical step for characterizing its properties, dynamics, and environmental implications. However, various pretreatment procedures may alter DOM composition and structure, producing highly variable recoveries and even influencing its subsequent characterization. Therefore, complimentary use of various characterization techniques is highly recommended to obtain as much information on DOM as possible, as each characterization technique exhibits various advantages and limitations. Moreover, DOM could markedly change the physical and chemical properties of exogenous chemical species, influencing their transformation and mobility, and finally altering their potential bioavailability and toxicity. Several research gaps to be addressed include the impact of pretreatment on the composition and structure of aquatic DOM, molecular-level structural elucidation for DOM, and assessment of the effects of DOM dynamics on the fate, bioavailability and toxicity of exogenous chemical species.