Assessing the photodegradation potential of compounds derived from the photoinduced weathering of polystyrene in water

Benzoate (Bz^-) and acetophenone (AcPh) are aromatic compounds known to be produced by sunlight irradiation of polystyrene aqueous suspensions. Here we show that these molecules could react with ^•OH (Bz^-) and ^•OH + CO₃^•- (AcPh) in sunlit natural waters, while other photochemical processes (direct photolysis and reaction with singlet oxygen, or with the excited triplet states of chromophoric dissolved organic matter) are unlikely to be important. Steady-state irradiation experiments were carried out using lamps, and the time evolution of the two substrates was monitored by liquid chromatography. Photodegradation kinetics in environmental waters were assessed by a photochemical model (APEX: Aqueous Photochemistry of Environmentally-occurring Xenobiotics). In the case of AcPh, a competitive process to aqueous-phase photodegradation would be volatilisation followed by reaction with gas-phase ^•OH. As far as Bz^- is concerned, elevated dissolved organic carbon (DOC) levels could be important in protecting this compound from aqueous-phase photodegradation. Limited reactivity of the studied compounds with the dibromide radical (Br₂^•-, studied by laser flash photolysis) suggests that ^•OH scavenging by bromide, which yields Br₂^•-, would be poorly offset by Br₂^•--induced degradation. Therefore, photodegradation kinetics of Bz^- and AcPh should be slower in seawater (containing [Br^-] ~ 1 mM) compared to freshwaters. The present findings suggest that photochemistry would play an important role in both formation and degradation of water-soluble organic compounds produced by weathering of plastic particles.

Insights into generation mechanisms of halogen radicals from excited triplet state of dissolved organic matter

Triplet states of dissolved organic matter (³DOM*) can sensitize the generation of halogen radicals in marine water. The generation pathways of halogen radicals from ³DOM* is still not fully understood. In this study, the generation of halogen radicals from DOM was investigated with Suwanee River fulvic acid (SRFA) as a representative and detailed generation pathways were further revealed with anthraquinone-2-sodium sulfonate (AQ2S) as a triplet sensitizer. The results showed that in SRFA solutions with halogen ions, various halogen radicals can be generated. Among which, Br is formed by the reaction of Br^- with ³AQ2S*, and Cl is produced by the reaction of Cl^- with AQ2S⁺ that generated in the presence of dissolved oxygen (DO). Cl₂^- and Br₂^- were generated via the subsequent combination of Cl/Br with another Cl^-/Br^-. In solutions without DO, BrCl^- is mainly generated through the combination of Br with Cl^-, and BrCl^- could also be generated through the combination of Cl with Br^- in solutions with DO. This study provides deep insights into the generation mechanisms of different halogen radicals from ³DOM* and is helpful for understanding the photochemical processes of halogen radicals in marine waters.

A model assessment of the role played by the carbonate (CO3-) and dibromide (Br2-) radicals in the photodegradation of glutathione in sunlit fresh- and salt-waters

Glutathione (GLU) is a peptidic thiol that plays important anti-oxidant roles in organisms and that occurs in both freshwater and seawater, where it can undergo both bio- and photodegradation. Recent results have elucidated the role played by OH, ¹O₂, H₂O₂ and other yet unidentified transients in GLU photochemistry, but very little is known of the role of CO₃^-. This is an important gap because CO₃^- is usually very reactive towards electron-rich compounds including thiols and mercaptans. Very little is also known on the environmental importance of the reaction between GLU and Br₂^-, which could account for the literature finding that GLU phototransformation is enhanced in simulated seawater compared to freshwater. By means of a photochemical model approach based on the APEX software (Aqueous Photochemistry of Environmentally-occurring Xenobiotics), here we provide an assessment of the role that several photoreactants, including most notably CO₃^- and Br₂^-, have in the photodegradation of GLU (both the whole substance and the separate neutral and mono-anionic species) under representative fresh- and saltwater conditions. Our model suggests that CO₃^- would dominate the photodegradation of GLU in low-DOC and high-pH freshwater, which are the only freshwater conditions that really ensure GLU photodegradation to be competitive with biotransformation. This result supports the potential key importance of CO₃^- in the environmental photochemistry of GLU. In surface seawater and in brackish water, GLU phototransformation might be dominated by the Br₂^- reaction (the role of additional halogen species such as Cl₂^- and ClBr^- is still unknown).

Effects of the antioxidant moieties of dissolved organic matter on triplet-sensitized phototransformation processes: Implications for the photochemical modeling of sulfadiazine

Previous studies have shown that the photodegradation of some pollutants, induced by the excited triplet states of chromophoric dissolved organic matter (³CDOM*), can be inhibited by back-reduction processes carried out by phenolic antioxidants occurring in dissolved organic matter (DOM). Here, for the first time to our knowledge, we included such an inhibition effect into a photochemical model and applied the model predictions to sulfadiazine (SDZ), a sulfonamide antibiotic that occurs in surface waters in two forms, neutral HSDZ and anionic SDZ^- (pK_a = 6.5). The input parameters of the photochemical model were obtained by means of dedicated experiments, which showed that the inhibition effect was more marked for SDZ^- than for HSDZ. Compared to the behavior of 2,4,6-trimethylphenol, which does not undergo antioxidant inhibition when irradiated in natural water samples, the back-reduction effect on the degradation of SDZ was proportional to the electron-donating capacity of the DOM. According to the model results, direct photolysis and OH reaction would account for the majority of both HSDZ and SDZ^- photodegradation in waters having low dissolved organic carbon (DOC < 1 mgC L^-1). With higher DOC values (>3-4 mgC L^-1) and despite the back-reduction processes, the ³CDOM* reactions are expected to account for the majority of HSDZ phototransformation. In the case of SDZ^- at high DOC, most of the photodegradation would be accounted for by direct photolysis. The relative importance of the triplet-sensitized phototransformation of both SDZ^- and (most importantly) HSDZ is expected to increase with increasing DOC, even in the presence of back reduction. An increase in water pH, favoring the occurrence of SDZ^- with respect to HSDZ, would enhance direct photolysis at the expense of triplet sensitization. SDZ should be fairly photolabile under summertime sunlight, with predicted half-lives ranging from a few days to a couple of months depending on water conditions.

Modelling the photochemical attenuation pathways of the fibrate drug gemfibrozil in surface waters

Gemfibrozil (GFZ) is a relatively persistent pollutant in surface-water environments and it is rather recalcitrant to biological degradation. The GFZ photochemical lifetimes are relatively short in shallow waters with low levels of dissolved organic carbon (DOC), but they can reach the month-year range in deep and high-DOC waters. The main reason is that GFZ undergoes negligible reaction with singlet oxygen or degradation sensitised by the triplet states of chromophoric dissolved organic matter, which are the usually prevalent photochemical pathways in deep and high-DOC sunlit waters. Nitrate and nitrite scarcely affect the overall GFZ lifetimes, but they can shift photodegradation from direct photolysis to the OH process. These two pathways are the main GFZ phototransformation routes, with the direct photolysis prevailing in shallow environments during summer. Under these conditions the GFZ photochemical lifetimes are also shorter and the environmental significance of photodegradation correspondingly higher. The direct photolysis of GFZ under UVB irradiation yielded several transformation intermediates deriving from oxidation or cleavage of the aliphatic lateral chain. A quinone derivative (2,5-dimethyl-1,4-benzoquinone), a likely oxidation product of the transformation intermediate 2,5-dimethylphenol, is expected to be the most acutely and chronically toxic compound arising from GFZ direct photolysis. Interestingly, literature evidence suggests that the same toxic intermediate would be formed upon OH reaction.

Assessing the phototransformation of diclofenac, clofibric acid and naproxen in surface waters: Model predictions and comparison with field data

Phototransformation is important for the fate in surface waters of the pharmaceuticals diclofenac (DIC) and naproxen (NAP) and for clofibric acid (CLO), a metabolite of the drug clofibrate. The goal of this paper is to provide an overview of the prevailing photochemical processes, which these compounds undergo in the different conditions found in freshwater environments. The modelled photochemical half-life times of NAP and DIC range from a few days to some months, depending on water conditions (chemistry and depth) and on the season. The model indicates that direct photolysis is the dominant degradation pathway of DIC and NAP in sunlit surface waters, and potentially toxic cyclic amides were detected as intermediates of DIC direct phototransformation. With modelled half-life times in the month-year range, CLO is predicted to be more photostable than DIC or NAP and to be degraded mainly by reaction with the ^•OH radical and with the triplet states of chromophoric dissolved organic matter (³CDOM*). The CLO intermediates arising from these processes and detected in this study (hydroquinone and 4-chlorophenol) are, respectively, a chronic toxicant to aquatic organisms and a possible carcinogen for humans. Hydroquinone is formed with only ∼5% yield upon CLO triplet-sensitised transformation, but it is highly toxic for algae and crustaceans. In contrast, the formation yield of 4-chlorophenol reaches ∼50% upon triplet sensitisation and ∼10% by ^·OH reaction. The comparison of model predictions with field data from a previous study yielded a very good agreement in the case of DIC and, when using 4-carboxybenzophenone as proxy for triplet sensitisation by CDOM, a good agreement was found for CLO as well. In the case of NAP, the comparison with field data suggests that its direct photolysis quantum yield approaches or even falls below the lower range of literature values.

A model assessment of the ability of lake water in Terra Nova Bay, Antarctica, to induce the photochemical degradation of emerging contaminants

The shallow lakes located in Terra Nova Bay, Antarctica, are free from ice for only up to a couple of months (mid December to early/mid February) during the austral summer. In the rest of the year, the ice cover shields the light and inhibits the photochemical processes in the water columns. Previous work has shown that chromophoric dissolved organic matter (CDOM) in these lakes is very reactive photochemically. A model assessment is here provided of lake-water photoreactivity in field conditions, based on experimental data of lake water absorption spectra, chemistry and photochemistry obtained previously, taking into account the water depth and the irradiation conditions of the Antarctic summer. The chosen sample contaminants were the solar filter benzophenone-3 and the antimicrobial agent triclosan, which have very well known photoreactivity and have been found in a variety of environmental matrices in the Antarctic continent. The two compounds would have a half-life time of just a few days or less in the lake water during the Antarctic summertime, largely due to reaction with CDOM triplet states ((3)CDOM*). In general, pollutants that occur in the ice and could be released to lake water upon ice melting (around or soon after the December solstice) would be quickly photodegraded if they undergo fast reaction with (3)CDOM*. With some compounds, the important (3)CDOM* reactions might favour the production of harmful secondary pollutants, such as 2,8-dichlorodibenzodioxin from the basic (anionic) form of triclosan.

Long-term trends of chemical and modelled photochemical parameters in four Alpine lakes

Based on long-term trends of water chemistry parameters of photochemical significance from four lakes located in the Alps (Iseo, Garda, Piburgersee, Geneva), we calculated the corresponding steady-state concentrations of photoinduced transient species with an ad-hoc photochemical model. Such transients were the hydroxyl ((•)OH) and carbonate (CO3(-•)) radicals, singlet oxygen ((1)O2), and the triplet states of chromophoric dissolved organic matter ((3)CDOM*). Among the investigated lakes, Lake Iseo, for example, showed a long-term near-stability in chemical parameters that resulted in a photochemical stability. By contrast, Piburgersee underwent important chemical modifications, but the interplay of compensation (parallel increase of both inorganic and organic carbon) and near-saturation effects (organic matter as main (•)OH source and sink) prevented the modelled photochemistry to undergo significant shifts over time. This result suggests the occurrence of a sort of "photochemical buffering" in some lake ecosystems, which would dampen modifications of the steady-state concentration of the photochemically-formed reactive transients, even in the case of significant changes in water chemistry. Finally, in lakes Garda and Geneva, long-term changes in water chemistry had an effect on photochemistry. While in Lake Garda the small increase in DOM was associated to a small increase in (1)O2 and (3)CDOM*, in Lake Geneva, the increases in pH and bicarbonate and the decrease in nitrite resulted in an (•)OH decrease. Overall, our results predict very different lake photochemistry patterns in relation to alterations in water chemistry parameters caused by climate change, such as changes in water alkalinity and dissolved organic carbon concentration.

A model assessment of the importance of direct photolysis in the photo-fate of cephalosporins in surface waters: Possible formation of toxic intermediates

The direct and indirect photodegradation of six cephalosporins was predicted using a photochemical model, on the basis of literature values of photochemical reactivity. Environmental photodegradation would be important in surface water bodies with depth ⩽ 2-3m, and/or in deeper waters with low values of the dissolved organic carbon (DOC ⩽ 1 mg C L(-1)). The half-life times would range from a few days to a couple of weeks in summertime. In deeper and higher-DOC waters and/or in different seasons, hydrolysis could prevail over photodegradation. The direct photolysis of cephalosporins is environmentally concerning because it is known to produce toxic intermediates. It would be a major pathway for cefazolin, an important one for amoxicillin and cefotaxime and, at pH<6.5, for cefapirin as well. In contrast, direct photolysis would be negligible for cefradine and cefalexin. The DOC values would influence the fraction of photodegradation accounted for by direct photolysis in shallow water, to a different extent depending on the role of sensitisation by the triplet states of chromophoric dissolved organic matter.

Photogeneration of reactive transient species upon irradiation of natural water samples: Formation quantum yields in different spectral intervals, and implications for the photochemistry of surface waters

Chromophoric dissolved organic matter (CDOM) in surface waters is a photochemical source of several transient species such as CDOM triplet states ((3)CDOM*), singlet oxygen ((1)O2) and the hydroxyl radical (OH). By irradiation of lake water samples, it is shown here that the quantum yields for the formation of these transients by CDOM vary depending on the irradiation wavelength range, in the order UVB > UVA > blue. A possible explanation is that radiation at longer wavelengths is preferentially absorbed by the larger CDOM fractions, which show lesser photoactivity compared to smaller CDOM moieties. The quantum yield variations in different spectral ranges were definitely more marked for (3)CDOM* and OH compared to (1)O2. The decrease of the quantum yields with increasing wavelength has important implications for the photochemistry of surface waters, because long-wavelength radiation penetrates deeper in water columns compared to short-wavelength radiation. The average steady-state concentrations of the transients ((3)CDOM*, (1)O2 and OH) were modelled in water columns of different depths, based on the experimentally determined wavelength trends of the formation quantum yields. Important differences were found between such modelling results and those obtained in a wavelength-independent quantum yield scenario.

New insights into the environmental photochemistry of 5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan): reconsidering the importance of indirect photoreactions

Triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) is a widely used antimicrobial agent that undergoes fairly slow biodegradation. It is often found in surface waters in both the acidic (HTric) and basic (Tric(-)) forms (pKa ∼8), and it can undergo direct photodegradation to produce several intermediates including a dioxin congener (2,8-dichlorodibenzodioxin, hereafter 28DCDD). The latter is formed from Tric(-) and causes non-negligible environmental concern. Differently from current literature reports, in this paper we show that the direct photolysis would not be the only important transformation pathway of triclosan in surface waters. This is particularly true for HTric, which could undergo very significant reactions with (•)OH and, if the laser-derived quenching rate constants of this work are comparable to the actual reaction rate constants, with the triplet states of chromophoric dissolved organic matter ((3)CDOM*). Model calculations suggest that reaction with (3)CDOM* could be the main HTric phototransformation pathway in deep waters with high dissolved organic carbon (DOC), while reaction with (•)OH could prevail in low-DOC waters. In the case of Tric(-) the direct photolysis is much more important than for HTric, but triplet-sensitised transformation could produce 28DCDD + 27DCDD with higher yield compared to the direct photolysis, and it could play some role as dioxin source in deep waters with elevated DOC.

Photochemical transformation of phenylurea herbicides in surface waters: a model assessment of persistence, and implications for the possible generation of hazardous intermediates

This work models the phototransformation kinetics in surface waters of five phenylurea herbicides (diuron, fenuron, isoproturon, metoxuron and chlortoluron), for which important photochemical parameters are available in the literature (direct photolysis quantum yields and reaction rate constants with ·OH, CO3(-·) and the triplet states of chromophoric dissolved organic matter, (3)CDOM*). Model calculations suggest that isoproturon and metoxuron would be the least photochemically persistent and diuron the most persistent compound. Reactions with ·OH and (3)CDOM* would be the main phototransformation pathways for all compounds in the majority of environmental conditions. Reaction with CO3(-) could be important in waters with low dissolved organic carbon (DOC), while direct photolysis would be negligible for fenuron, quite important for chlortoluron, and somewhat significant for the other compounds. The direct photolysis of metoxuron and diuron is known to increase toxicity, and such a photoreaction pathway would be enhanced at intermediate DOC values (1-4 mg C L(1)). The reaction between phenylureas and ·OH is known to produce toxic intermediates, differently from (3)CDOM*. Therefore, the shift of reactivity from ·OH to (3)CDOM* with increasing DOC could reduce the environmental impact of photochemical transformation.

Photochemical generation of photoactive compounds with fulvic-like and humic-like fluorescence in aqueous solution

The irradiation of L-tryptophan, L-tyrosine and 4-phenoxyphenol in aqueous solution produced compounds with similar fluorescence properties as humic substances, and with absorption spectra that were significantly extended into the UVA and visible regions compared to the starting compounds. The irradiated systems had photosensitizing properties, as proven by the photodegradation of 2,4,6-trimethylphenol and furfuryl alcohol (probes of excited triplet states and (1)O2, respectively). The described photochemical processes could constitute an additional pathway for the formation of humic substances in clear and shallow water bodies, which would be added to the complex network of reactions involving dissolved organic matter.

Photochemical processes involving the UV absorber benzophenone-4 (2-hydroxy-4-methoxybenzophenone-5-sulphonic acid) in aqueous solution: reaction pathways and implications for surface waters

The sunlight filter benzophenone-4 (BP-4) is present in surface waters as two prevailing forms, the singly deprotonated (HA-) and the doubly deprotonated one (A(2-)), with pKa2 = 7.30 ± 0.14 (μ ± σ, by dissociation of the phenolic group). In freshwater environments, BP-4 would mainly undergo degradation by reaction with ·OH and direct photolysis. The form HA(-) has a second-order reaction rate constant with ·OH (k(·OH)) of (1.87 ± 0.31)·10(10) M(-1) s(-1) and direct photolysis quantum yield Φ equal to (3.2 ± 0.6)·10(-5). The form A(2-) has (8.46 ± 0.24)·10(9) M(-1) s(-1) as the reaction rate constant with ·OH and (7.0 ± 1.3)·10(-5) as the photolysis quantum yield. The direct photolysis of HA(-) likely proceeds via homolytic breaking of the O-H bond of the phenolic group to give the corresponding phenoxy radical, as suggested by laser flash photolysis experiments. Photochemical modelling shows that because of more efficient direct photolysis (due to both higher sunlight absorption and higher photolysis quantum yield), the A(2-) form can be degraded up to 3 times faster than HA(-) in surface waters. An exception is represented by low-DOC (dissolved organic carbon) conditions, where the ·OH reaction dominates degradation and the transformation kinetics of HA(-) is faster compared to A(2-). The half-life time of BP-4 in mid-latitude summertime would be in the range of days to weeks, depending on the environmental conditions. BP-4 also reacts with Br2(·-), and a rate constant k(Br2(·-),BP-4) = (8.05 ± 1.33)·10(8) M(-1) s(-1) was measured at pH 7.5. Model results show that reaction with Br2(·-) could be a potentially important transformation pathway of BP-4 in bromide-rich (e.g. seawater) and DOM-rich environments.