A Critical View of the Application of the APEX Software (Aqueous Photochemistry of Environmentally-Occurring Xenobiotics) to Predict Photoreaction Kinetics in Surface Freshwaters

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.

Biodegradation, photolysis, and sorption of antibiotics in aquatic environments: A scoping review

The presence of antibiotics in surface waters is a potential driver of antibiotic resistance and thus of concern to human and environmental health. Key factors driving the potential impact of antibiotics are their persistence and transport in rivers and lakes. The goal of this study was to describe the peer-reviewed published literature on the photolysis (direct and indirect), sorption, and biodegradation of a selected group of antibiotic compounds following a scoping review methodology. Primary research from 2000 to 2021 was surveyed to compile information on these processes for 25 antibiotics from 6 classes. After compilation and assessment of the available parameters, the results indicate that information is present to predict the rates of direct photolysis and reaction with hydroxyl radical (an indirect photolysis process) for most of the selected antibiotics. There is insufficient or inconsistent information for including other indirect photolysis processes, biodegradation, or removal via sorption to settling particles for most of the targeted antibiotic compounds. Future research should focus on collecting fundamental parameters such as quantum yields, second-order rate constants, normalized biodegradation rates, and organic carbon or surface area normalized sorption coefficients rather than pseudo-first order rate constants or sorption equilibrium constants that apply only to specific conditions/sites.

Possible Effects of Changes in Carbonate Concentration and River Flow Rate on Photochemical Reactions in Temperate Aquatic Environments

In temperate environments, climate change could affect water pH by inducing enhanced dissolution of CaSO4 followed by biological sulphate reduction, with the potential to basify water due to H+ consumption. At the same time, increased atmospheric CO2 could enhance weathering of carbonate rocks (e.g., dolomite) and increase the total concentration of dissolved carbonate species. Both processes enhance phototransformation by the carbonate radical (CO3•-), as shown for the non-steroidal anti-inflammatory drug paracetamol, provided that the dissolved organic carbon of water does not undergo important fluctuations. Climate change could also affect hydrology, and prolonged drought periods might considerably decrease flow rates in rivers. This is a substantial problem because wastewater pollutants become less diluted and, as a result, can exert more harmful effects due to increased concentrations. At the same time, in low-flow conditions, water is also shallower and its flow velocity is decreased. Photochemical reactions become faster because shallow water is efficiently illuminated by sunlight, and they also have more time to occur because water takes longer to cover the same river stretch. As a result, photodegradation of contaminants is enhanced, which offsets lower dilution but only at a sufficient distance from the wastewater outlet; this is because photoreactions need time (which translates into space for a flowing river) to attenuate pollution.

Insights into the mechanisms of natural organic matter on the photodegradation of indomethacin under natural sunlight and simulated light irradiation

Indomethacin (INDO) is an antipyretic and analgesic pharmaceutical that has been widely detected in the aquatic environment. Photodegradation is an essential pathway for removal of INDO in sunlit surface water, however the effect of dissolved organic matter (DOM) on its photodegradation and the ecotoxicity of photodegradation products are largely unknown. In this study, the effect of DOM on the photodegradation of INDO under both natural and simulated light irradiation was studied. The results showed that indirect photolysis is the main photodegradation pathway of INDO in presence of DOM where 3DOM* plays the most important promoting role. Compared to commercial DOM (SRNOM and SRFA), DOM extracted from local-lake water (SLDOM) promoted the photodegradation to the highest extent. Although the steady-state concentrations of 3DOM* of SRNOM and SRFA were higher than SLDOM, their inhibition effect surpassed SLDOM namely higher light screening effect and phenolic antioxidant concentrations. The photodegradation pathway in pure water is different from that in DOM system where the decarboxylation of acetic acid chain and the oxidative fracture of indole ring are the main degradation pathways. Density Functional Theory (DFT) calculation further supports the proposed degradation pathways of INDO. ECOSAR calculation showed that the toxicity of INDO photodegradation products to aquatic organisms may maintain or even exceed its parent compound. Therefore, comprehensive understanding of the impact of DOM on the photodegradation of INDO is of crucial significance for evaluating its ecological risk in the natural environment.

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.

Evaluating the Microheterogeneous Distribution of Photochemically Generated Singlet Oxygen Using Furfuryl Amine

Singlet oxygen (1O2) is an important reactive species in natural waters produced during photolysis of dissolved organic matter (DOM). Prior studies have demonstrated that 1O2 exhibits a microheterogeneous distribution, with [1O2] in the interior of DOM macromolecules ∼30 to 1000-fold greater than in bulk solution. The [1O2] profile for DOM-containing solutions has been determined mainly by the use of hydrophobic probes, which are not commercially available. In this study, we employed a dual-probe method combining the widely used hydrophilic 1O2 probe furfuryl alcohol (FFA) and its structural analogue furfuryl amine (FFAm). FFAm exists mainly as a cation at pH <9 and was therefore hypothesized to have an enhanced local concentration in the near-DOM phase, whereas FFA will be distributed homogeneously. The probe pair was used to quantify apparent [1O2] in DOM samples from different isolation procedures (humic acid, fulvic acid, reverse osmosis) and diverse origins (aquatic and terrestrial) as a function of pH and ionic strength, and all samples studied exhibited enhanced reactivity of FFAm relative to FFA, especially at pH 7 and 8. To quantify the spatial distribution of [1O2], we combined electrostatic models with Latch and McNeill's three-phase distribution model. Modeling results for Suwannee River humic acid (SRHA) yield a surface [1O2] of ∼60 pM, which is ∼96-fold higher than the aqueous-phase [1O2] measured with FFA. This value is in agreement with prior reports that determined 1-3 orders of magnitude higher [1O2] in the DOM phase compared to bulk solution. Overall, this work expands the knowledge base of DOM microheterogeneous photochemistry by showing that diverse DOM isolates exhibit this phenomenon. In addition, the dual-probe approach and electrostatic modeling offer a new way to gain mechanistic insight into the spatial distribution of 1O2 and potentially other photochemically produced reactive intermediates.

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.

Semiconducting mineral induced photochemical conversion of PAHs in aquatic environment: Mechanism study and fate prediction

Semiconducting minerals (such as iron sulfides) are highly abundant in surface water, but their influences on the natural photochemical process of contaminants are still unknown. By simulating the natural water environment under solar irradiation, this work comprehensively investigated the photochemical processes of anthracene (a typical Polycyclic Aromatic Hydrocarbons) in both freshwater and seawater. The results show that the natural pyrite (NP) significantly promotes the degradation of anthracene under solar illumination via 1) NP induced photocatalytic degradation of anthracene, and 2) Fenton reaction due to the NP induced photocatalytic generation of H₂O₂. The material characterization and theoretical calculation reveal that the natural impurity in NP enlarges its band gap, which limits the utilization of solar spectra to shorter wavelength. The contribution of generated reactive intermediates on anthracene degradation follows the order of ¹O₂ >OH > O₂^- in freshwater and O₂^- >¹O₂ >OH in seawater. The photochemically generated H₂O₂ is a vital source for OH generation (from Fenton reaction). The steady-state concentration of OH, ¹O₂ and O₂^- in freshwater were monitored as 3.0 × 10^-15 M, 1.1 × 10^-13 M, and 4.5 × 10^-14 M, respectively. However, the OH concentration in seawater can be negligible due to the quenching effects by halides, and the ¹O₂ and O₂^- concentrations are higher than that in freshwater. An anthracene degradation kinetic model was built based on the experimentally determined reactive intermediates concentration and its second order rate constant with anthracene. Moreover, the anthracene degradation pathway was proposed based on intermediates analysis and DFT calculation, and its toxicity evolution during the photochemical process was assessed by quantitative structure-activity relationship (QSAR) based prediction. This finding suggests that the natural semiconducting minerals can affect the fate and environmental risks of contaminants in natural water.

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.

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.

Sunlight-driven environmental photodegradation of 2-chlorobiphenyl (PCB-1) in surface waters: kinetic study and mathematical simulations

Polychlorinated biphenyls (PCBs) are a family of highly toxic, resistant, and persistent organic pollutants, among which 2-chlorobiphenyl (PCB-1) is one of the simplest. Most studies on PCBs' photochemistry are limited to their direct photolysis, while the important role of reactive photo-induced species (RPS) (hydroxyl radicals, HO^●; singlet oxygen, ¹O₂; and triplet excited states of chromophoric dissolved organic matter, ³CDOM*) in removing PCBs in natural waters through indirect photolysis has not yet been evaluated. In this work, the rate constants of the reactions between aqueous PCB-1 and RPS were obtained under simulated solar radiation (450-W Xenon lamp and an AM 1.5 global filter) by competition kinetics, and the effects of the initial pollutant concentration and the physicochemical characteristics of the water were investigated. The direct photolysis quantum yield of PCB-1 in the range 290-800 nm was found as 1.60 × 10^-2 mol Einstein^-1. The value of k_PCB-1,HO● = (6.80 ± 0.09) × 10⁹ L mol^-1 s^-1 is in good agreement with the literature. For ¹O₂, k_PCB-1,1O2 = (1.13 ± 0.20) × 10⁶ L mol^-1 s^-1, while for ³CDOM*, k_PCB-1,3CBBP* = (2.44 ± 0.04) × 10⁹ L mol^-1 s^-1 and k_PCB-1,3AQ2S* = (3.36 ± 0.04) × 10⁹ L mol^-1 s^-1 were obtained using 4-benzoylbenzoic acid (CBBP) and anthraquinone-2-sulfonate (AQ2S) as CDOM proxies, respectively. These results show that the main pathways involved in PCB-1 photodegradation are the reactions with HO^● and ³CDOM* together with direct photolysis. In addition, the photodegradation of PCB-1 in sunlit waters was simulated using the kinetic model APEX (Aqueous Photochemistry of Environmentally Occurring Xenobiotics). According to simulations, a greater influence of the water depth and dissolved organic carbon concentration (DOC) on the persistence of PCB-1 is expected, being only slightly influenced by the concentrations of nitrite, nitrate, and bicarbonate. Finally, based on data reported for Brazilian surface waters, the average half-life (t_1/2) of PCB-1 is expected to vary from 2 to 14 days. In particular, the t_1/2 in the Paranapanema River is estimated at 7 to 8 days.

Insights into the Time Evolution of Slowly Photodegrading Contaminants

Photochemical degradation plays an important role in the attenuation of many recalcitrant pollutants in surface freshwaters. Photoinduced transformation kinetics are strongly affected by environmental conditions, where sunlight irradiance plays the main role, followed by water depth and dissolved organic carbon (DOC). Apart from poorly predictable weather-related issues, fair-weather irradiance has a seasonal trend that results in the fastest photodegradation in June and the slowest in December (at least in temperate areas of the northern hemisphere). Pollutants that have first-order photochemical lifetimes longer than a week take more than one month to achieve 95% photodegradation. Consequently, they may experience quite different irradiance conditions as their photodegradation goes on. The relevant time trend can be approximated as a series of first-order kinetic tracts, each lasting for one month. The trend considerably departs from an overall exponential decay, if degradation takes long enough to encompass seasonally varying irradiance conditions. For instance, sunlight irradiance is higher in July than in April, but increasing irradiance after April and decreasing irradiance after July ensure that pollutants emitted in either month undergo degradation with very similar time trends in the first 3-4 months after emission. If photodegradation takes longer, pollutants emitted in July experience a considerable slowdown in photoreaction kinetics as winter is approached. Therefore, if pollutants are photostable enough that their photochemical time trend evolves over different seasons, degradation acquires some peculiar features than cannot be easily predicted from a mere analysis of lifetimes in the framework of simple first-order kinetics. Such features are here highlighted with a modelling approach, taking the case of carbamazepine as the main example. This contaminant is almost totally biorecalcitrant, and it is also quite resistant to photodegradation.

Foreseen Effects of Climate-Impacted Scenarios on the Photochemical Fate of Selected Cyanotoxins in Surface Freshwaters

Cyanobacteria populate most water environments, and their ability to effectively exploit light and nutrients provide them with a competitive advantage over other life forms. In particular conditions, cyanobacteria may experience considerable growth and give rise to the so-called harmful algal blooms (HABs). HABs are often characterized by the production of cyanotoxins, which cause adverse effects to both aquatic organisms and humans and even threaten drinking water supplies. The concentration of cyanotoxins in surface waters results from the budget between production by cyanobacteria and transformation, including photodegradation under sunlight exposure. Climate change will likely provide favorable conditions for HABs, which are expected to increase in frequency over both space and time. Moreover, climate change could modify the ability of some surface waters to induce phototransformation reactions. Photochemical modeling is here carried out for two cyanotoxins of known photoreaction kinetics (microcystin-LR and cylindrospermopsin), which follow different phototransformation pathways and for particular freshwater scenarios (summertime stratification in lakes, water browning, and evaporative water concentration). On this basis, it is possible to quantitatively predict that the expected changes in water-column conditions under a changing climate would enhance photodegradation of those cyanotoxins that are significantly transformed by reaction with the triplet states of chromophoric dissolved organic matter (³CDOM*). This is known to be the case for microcystin-LR, for which faster photodegradation in some environments would at least partially offset enhanced occurrence. Unfortunately, very few data are currently available for the role of ³CDOM* in the degradation of other cyanotoxins, which is a major knowledge gap in understanding the link between cyanotoxin photodegradation and changing climate.

Factors affecting the mixed-layer concentrations of singlet oxygen in sunlit lakes

The steady-state concentration of singlet oxygen within a lake ([¹O₂]_SS) is an important parameter that can affect the environmental half-life of pollutants and environmental fate modelling. However, values of [¹O₂]_SS are often determined for the near-surface of a lake, and these values typically do not represent the average over the epilimnia of lakes. In this work, the environmental and physical factors that have the largest impact on [¹O₂]_SS within lake epilimnia were identified. It was found that the depth of the epilimnion has the largest impact on depth-averaged [¹O₂]_SS, with a factor of 8.8 decrease in [¹O₂]_SS when epilimnion depth increases from 2 m to 20 m. The next most important factors are the wavelength-dependent singlet oxygen quantum yield relationship and the latitude of the lake, causing variations in [¹O₂]_SS by factors of 3.2 and 2.5 respectively, over ranges of representative values. For a set of representative parameters, the depth-averaged value of [¹O₂]_SS within an average epilimnion depth of 9.0 m was found to be 5.8 × 10^-16 M and the near-surface value of [¹O₂]_SS was found to be 1.9 × 10^-14 M. We recommend a range of 6 × 10^-17 to 5 × 10^-15 M as being more representative of [¹O₂]_SS values within the epilimnia of lakes globally and potentially more useful for estimating pollutant lifetimes than those calculated using [¹O₂]_SS values that correspond to near-surface, summer midday values. This work advances our understanding of [¹O₂]_SS inter-lake variability in the environment, and provides estimates of [¹O₂]_SS for practitioners and researchers to assess environmental half-lives of pollutants due to reaction with singlet oxygen.

Solar Water Disinfection to Produce Safe Drinking Water: A Review of Parameters, Enhancements, and Modelling Approaches to Make SODIS Faster and Safer

Solar water disinfection (SODIS) is one the cheapest and most suitable treatments to produce safe drinking water at the household level in resource-poor settings. This review introduces the main parameters that influence the SODIS process and how new enhancements and modelling approaches can overcome some of the current drawbacks that limit its widespread adoption. Increasing the container volume can decrease the recontamination risk caused by handling several 2 L bottles. Using container materials other than polyethylene terephthalate (PET) significantly increases the efficiency of inactivation of viruses and protozoa. In addition, an overestimation of the solar exposure time is usually recommended since the process success is often influenced by many factors beyond the control of the SODIS-user. The development of accurate kinetic models is crucial for ensuring the production of safe drinking water. This work attempts to review the relevant knowledge about the impact of the SODIS variables and the techniques used to develop kinetic models described in the literature. In addition to the type and concentration of pathogens in the untreated water, an ideal kinetic model should consider all critical factors affecting the efficiency of the process, such as intensity, spectral distribution of the solar radiation, container-wall transmission spectra, ageing of the SODIS reactor material, and chemical composition of the water, since the substances in the water can play a critical role as radiation attenuators and/or sensitisers triggering the inactivation process.

Photochemistry of dissolved organic matter extracted from coastal seawater: Excited triplet-states and contents of phenolic moieties

Coastal seawater constitutes an important ecosystem receiving inputs of organic micropollutants (OMPs) such as sulfa antibiotics from land-based sources or mariculture activities. It is necessary to investigate photodegradation of OMPs in coastal seawaters for assessing their environmental fate and risks. However, effects of coastal seawater dissolved organic matter (S-DOM) on OMPs photodegradation are largely unknown, given that chemical compositions of S-DOM are different from those of freshwater DOM. Herein, photochemical characteristics of S-DOM extracted from Dalian coastal seawaters were investigated by simulating photochemical experiment adopting sulfachloropyridazine as a case. Results show that S-DOM accelerates the photodegradation mainly through excited triplet-state DOM (³DOM*) with an apparent rate constant (4.43 × 10⁸ M^-1 s^-1) ten folds of that of freshwater DOM, which is mainly due to much lower phenol contents detected in the S-DOM (0.022 mg-Gallic acid mg-C^-1). The S-DOM impacted by mariculture can photogenerate more high-energy ³DOM* than those less impacted by mariculture, further contributing to the high ³DOM* reactivity. The study shows that to accurately predict photolytic persistence of OMPs in field water bodies, it is of significance to determine the second-order reaction rate constants between ³DOM* and target OMPs using DOM extracted from relevant water bodies.

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.