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.

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.

Geographical and temporal assessment of the photochemical decontamination potential of river waters from agrochemicals: A first application to the Piedmont region (NW Italy).

This work shows the potential of using photochemical modelling to assess the river-water ability to photodegrade agrochemicals on a geographic and temporal scale. The case of flowing water requires different data treatment compared to more stationary water bodies (e.g., lakes), but it could allow for the identification of particularly vulnerable environments. Five pesticides were considered here, and the photodegradation rate followed the order bentazon > isoproturon > dimethomorph âˆ¼ chlortoluron > atrazine. The modelled photodegradation kinetics was particularly fast in the river Po, which receives significant input of agricultural nitrate from groundwater and features higher steady-state [•OH] than most other rivers in the region. The fact that the Po eventually collects all river waters in Piedmont is positive, from the point of view of comprehensive photodegradation of pesticides. However, this paradoxical situation of agricultural pollution (nitrate) helping fight pollution from the same source (pesticides) has two important limitations: (i) when compared to the parent compounds, some intermediates deriving from •OH reactions are either more harmful (N-formyl derivatives of phenylureas), or about as harmful (desethyl atrazine); (ii) banned atrazine is no longer sprayed over fields during the plant growth season, but it reaches surface waters from legacy groundwater inputs. The latter are operational also during winter, when photochemistry is least active. Therefore, photochemistry might not ensure considerable attenuation of atrazine during wintertime. Overall, bentazon would be the safest among the studied pesticides because of fast degradation by direct photolysis, and of low ecotoxicological impact of its phototransformation intermediates.

Possible Effect of Climate Change on Surface-Water Photochemistry: A Model Assessment of the Impact of Browning on the Photodegradation of Pollutants in Lakes during Summer Stratification. Epilimnion vs. Whole-Lake Phototransformation.

Water browning in lakes (progressive increase of the content of chromophoric dissolved organic matter, CDOM) has the potential to deeply alter the photodegradation kinetics of pollutants during summer stratification. Browning, which takes place as a consequence of climate change in several Nordic environments, causes the thermocline to be shallower, because higher CDOM decreases the penetration of sunlight inside the water column. Using a model approach, it is shown in this paper that pollutants occurring in the epilimnion would be affected differently depending on their main photodegradation pathway(s): almost no change for the direct photolysis, slight decrease in the degradation kinetics by the hydroxyl radicals (•OH, but the resulting degradation would be too slow for the process to be effective during summer stratification), considerable decrease for the carbonate radicals (CO3•-), increase for the excited triplet states of CDOM (3CDOM*) and singlet oxygen (1O2). Because it is difficult to find compounds that are highly reactive with CO3•- and poorly reactive with 3CDOM*, the degradation rate constant of many phenols and anilines would show a minimum with increasing dissolved organic carbon (DOC), because of the combination of decreasing CO3•- and increasing 3CDOM* photodegradation. In contrast, overall photodegradation would always be inhibited by browning when the whole water column (epilimnion + hypolimnion) is considered, either because of slower degradation kinetics in the whole water volume, or even at unchanged overall kinetics, because of unbalanced distribution of photoreactivity within the water column.

Mapping the Photochemistry of European Mid-Latitudes Rivers: An Assessment of Their Ability to Photodegrade Contaminants.

The abiotic photochemical reactions that take place naturally in sunlit surface waters can degrade many contaminants that pose concern to water bodies for their potentially toxic and long-term effects. This works aims at assessing the ability of European rivers to photoproduce reactive transient intermediates, such as HO• radicals and the excited triplet states of chromophoric dissolved organic matter (3CDOM*), involved in pollutant degradation. A photochemical mapping of the steady-state concentrations of these transients was carried out by means of a suitable modeling tool, in the latitude belt between 40 and 50°N. Such a map allowed for the prediction of the photochemical lifetimes of the phenylurea herbicide isoproturon (mostly undergoing photodegradation upon reaction with HO• and especially 3CDOM*) across different European countries. For some rivers, a more extensive dataset was available spanning the years 1990-2002, which allowed for the computation of the steady-state concentration of the carbonate radicals (CO3•-). With these data, it was possible to assess the time trends of the photochemical half-lives of further contaminants (atrazine, ibuprofen, carbamazepine, and clofibric acid). The calculated lifetimes were in the range of days to weeks, which might or might not allow for efficient depollution depending on the river-water flow velocity.

Photochemical consequences of prolonged hydrological drought: A model assessment of the Lower Lakes of the Murray-Darling Basin (Southern Australia).

The prolonged "Millennium" drought affecting Australia in the 2000s had important consequences on surface-water bodies, including the Lower Lakes (Lake Alexandrina and Lake Albert) located at the terminal end of the River Murray system. Shallower water depths, limited solute dilution and altered geochemical processes ensured that the concentration values of several water constituents increased considerably during drought, including the water parameters of photochemical significance (nitrate, bicarbonate, carbonate and the dissolved organic carbon, DOC). The aim of this study was to model the photochemical processes in the Lower Lakes during the drought and post-drought periods, to provide insight into the changes that photoinduced reactions can undergo in periods of water scarcity. Among the photochemical processes involved in the light-assisted transformation of dissolved compounds, an important role is played by indirect photochemistry where degradation is triggered by photogenerated transient species such as hydroxyl (OH) and carbonate (CO3-) radicals, and the triplet states of chromophoric dissolved organic matter (3CDOM*). Results of photochemical modelling suggest that the reactions induced by 3CDOM* would be enhanced during drought, while the processes triggered by OH and CO3- would be less modified. For compounds undergoing efficient degradation with 3CDOM*, enhanced photochemistry during drought could offset the higher concentration values resulting from lower dilution. In contrast, for compounds mainly degraded by OH or CO3- the drought period could produce a concentration increase not balanced by an increment in the photochemical reactivity of the water body.

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).

A model assessment of the potential of river water to induce the photochemical attenuation of pharmaceuticals downstream of a wastewater treatment plant (Guadiana River, Badajoz, Spain).

We predicted the possible direct and indirect phototransformation kinetics of carbamazepine (CBZ), ibuprofen (IBU) and diclofenac (DIC) in river water, based on data of water chemistry obtained for the Guadiana River near Badajoz (Southwestern Spain) during a year-round sampling campaign. The three compounds were chosen, (i) because they occurred at the outlet of the wastewater treatment plant (WWTP) in Badajoz, as well as in river water sampled 1 km downstream of the WWTP, and (ii) because their photochemical fate in surface waters is known well enough to be modelled. The predicted phototransformation kinetics would be negligible in winter and fastest in April-August, with comparable rate constants in April through August despite differences in sunlight irradiance. Favourable water chemistry would in fact offset the lower irradiance, and vice versa. Half-life times of at least three weeks - one month are predicted for CBZ and IBU. Photodegradation may be an important attenuation pathway for biorecalcitrant CBZ, while IBU photochemistry is unlikely to be competitive with other processes including biodegradation. The predicted DIC photochemical half-life times of 7-10 days in April-August would be comparable with the biodegradation kinetics data reported in the literature. Photochemistry might not induce extensive phototransformation of xenobiotics in the Guadiana River under normal flow conditions, but it could become important in the case of low flow produced by water scarcity.

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.

Imidazolium-Based Ionic Liquids in Water: Assessment of Photocatalytic and Photochemical Transformation.

The photoinduced transformation of two ionic liquids, 1-methylimidazolium hydrogensulfate (HMIM) and 1-ethyl-3-methylimidazolium hydrogensulfate (EMIM), was investigated under photocatalytic conditions in the presence of irradiated TiO2. We monitored substrate disappearance, transformation products (TPs), degree of mineralization, and toxicity of the irradiated systems. Acute toxicity measures suggested in both cases the occurrence of more toxic TPs than the parent molecules. A total of five TPs were detected by HPLC-HRMS from HMIM and nine from EMIM. Complete mineralization and stoichiometric release of nitrogen was achieved for both compounds within 4 h of irradiation. The photochemical transformation kinetics and pathways in surface waters (direct photolysis and indirect photoreactions) were studied for EMIM, to assess its persistence in sunlit water bodies such as rivers or lakes. Environmental phototransformation would be dominated by direct photolysis, with half-life times of up to one month under fine-weather conditions.

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.

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.

Modelling lake-water photochemistry: three-decade assessment of the steady-state concentration of photoreactive transients (·OH, CO3(-·) and (3)CDOM(∗)) in the surface water of polymictic Lake Peipsi (Estonia/Russia).

Over the last 3-4 decades, Lake Peipsi water (sampling site A, middle part of the lake, and site B, northern part) has experienced a statistically significant increase of bicarbonate, pH, chemical oxygen demand, nitrate (and nitrite in site B), due to combination of climate change and eutrophication. By photochemical modelling, we predicted a statistically significant decrease of radicals ·OH and CO3(-·) (site A, by 45% and 35%, respectively) and an increase of triplet states of chromophoric dissolved organic matter ((3)CDOM(∗); site B, by ∼25%). These species are involved in pollutant degradation, but formation of harmful by-products is more likely with (3)CDOM(∗) than with ·OH. Therefore, the photochemical self-cleansing ability of Lake Peipsi probably decreased with time, due to combined effects of climate change and eutrophication. In different environments (e.g. Lake Maggiore, NW Italy), ecosystem restoration policies had the additional advantage of enhancing sunlight-driven detoxification, suggesting that photochemical self-cleansing would be positively correlated with lake water quality.

Photochemical transformation of anionic 2-nitro-4-chlorophenol in surface waters: laboratory and model assessment of the degradation kinetics, and comparison with field data.

Anionic 2-nitro-4-chlorophenol (NCP) may occur in surface waters as a nitroderivative of 4-chlorophenol, which is a transformation intermediate of the herbicide dichlorprop. Here we show that NCP would undergo efficient photochemical transformation in environmental waters, mainly by direct photolysis and reaction with OH. NCP has a polychromatic photolysis quantum yield Φ(NCP)=(1.27±0.22)·10(-5), a rate constant with OH k(NCP,)(OH)=(1.09±0.09)·10(10) M(-1) s(-1), a rate constant with (1)O(2)k(NCP,1O2)=(2.15±0.38)·10(7) M(-1) s(-1), a rate constant with the triplet state of anthraquinone-2-sulphonate k(NCP,3AQ2S*)=(5.90±0.43)·10(8) M(-1) s(-1), and is poorly reactive toward CO(3)(-). The k(NCP,3AQ2S*) value is representative of reaction with the triplet states of chromophoric dissolved organic matter. The inclusion of photochemical reactivity data into a model of surface-water photochemistry allowed the NCP transformation kinetics to be predicted as a function of water chemical composition and column depth. Very good agreement between model predictions and field data was obtained for the shallow lagoons of the Rhône delta (Southern France).

Photodegradation of phenol and salicylic acid by coated rutile-based pigments: a new approach for the assessment of sunscreen treatment efficiency.

The treatments used in many commercial sunscreen pigments (organic additives and inorganic surface coating) to lower the pigments' ability to degrade molecules under photocatalytic conditions are effective in inhibiting the degradation of phenol, but much less effective towards the degradation of salicylic acid. The reason is in the different degradation pathways that phenol and salicylic acid follow under photocatalytic conditions. The treatments are thus effective in inhibiting the degradation pathways initiated by Ti(IV)-*OH(surf) (also named *OH(ads)), as in the case of phenol, but much less effective towards electron-transfer processes involving surface complexes, as in the case of salicylic acid. These results indicate that the techniques currently adopted by the cosmetic industry to develop organic additives for use as pigment treatments are likely to be inadequate. The problem most likely is that treatments are optimized following the inhibition of the photocatalytic degradation of a single model molecule, which results in blocking just one photocatalytic degradation pathway (usually the one initiated by Ti(IV)-*OH(surf)) and not the other (electron transfer processes). The possible implications for sunscreen pigments are briefly discussed, and a new approach for the evaluation of the photocatalytic activity of inorganic sunscreens is proposed.