Anthraquinones-based photocatalysis: A comprehensive review

In recent years, there has been significant interest in photocatalytic technologies utilizing semiconductors and photosensitizers responsive to solar light, owing to their potential for energy and environmental applications. Current efforts are focused on enhancing existing photocatalysts and developing new ones tailored for environmental uses. Anthraquinones (AQs) serve as redox-active electron transfer mediators and photochemically active organic photosensitizers, effectively addressing common issues such as low light utilization and carrier separation efficiency found in conventional semiconductors. AQs offer advantages such as abundant raw materials, controlled preparation, excellent electron transfer capabilities, and photosensitivity, with applications spanning the energy, medical, and environmental sectors. Despite their utility, comprehensive reviews on AQs-based photocatalytic systems in environmental contexts are lacking. In this review, we thoroughly describe the photochemical properties of AQs and their potential applications in photocatalysis, particularly in addressing key environmental challenges like clean energy production, antibacterial action, and pollutant degradation. However, AQs face limitations in practical photocatalytic applications due to their low electrical conductivity and solubility-related secondary contamination. To mitigate these issues, the design and synthesis of graphene-immobilized AQs are highlighted as a solution to enhance practical photocatalytic applications. Additionally, future research directions are proposed to deepen the understanding of AQs' theoretical mechanisms and to provide practical applications for wastewater treatment. This review aims to facilitate mechanistic studies and practical applications of AQs-based photocatalytic technologies and to improve understanding of these technologies.

An anthraquinone-based bismuth-iron metal-organic framework as an efficient photoanode in photoelectrochemical cells

Metal-organic frameworks (MOFs) are appealing candidate materials to design new photoelectrodes for use in solar energy conversion because of their modular nature and chemical versatility. However, to date there are few examples of MOFs that can be directly used as photoelectrodes, for which they must be able to afford charge separation upon light absorption, and promote the catalytic dissociation of water molecules, while maintaining structural integrity. Here, we have explored the use of the organic linker anthraquinone-2, 6-disulfonate (2, 6-AQDS) for the preparation of MOFs to be used as photoanodes. Thus, the reaction of 2, 6-AQDS with Bi(iii) or a combination of Bi(iii) and Fe(iii) resulted in two new MOFs, BiPF-10 and BiFePF-15, respectively. They display similar structural features, where the metal elements are disposed in inorganic-layer building units, which are pillared by the organic linkers by coordination bonds through the sulfonic acid groups. We show that the introduction of iron in the structure plays a crucial role for the practical use of the MOFs as a robust photoelectrode in a photoelectrochemical cell, producing as much as 1.23 mmol H2 cm-2 with the use of BiFePF-15 as photoanode. By means of time-resolved and electrochemical impedance spectroscopic studies we have been able to unravel the charge transfer mechanism, which involves the formation of a radical intermediate species, exhibiting a longer-lived lifetime by the presence of the iron-oxo clusters in BiFePF-15 to reduce the charge transfer resistance.

Unveiling the enhancement mechanisms of algogenic extracellular organic matters on chlortetracycline photodegradation: Constitutive relationships of compound components and reactive oxygen species generation

Algogenic extracellular organic matters (EOMs) have been found to play a crucial role in the photodegradation of antibiotics. However, the specific molecular structure compositions of EOMs have not been fully characterized, and the intrinsic association between the structure and the production of ROS remains unclear. In this study, EOMs from Chlorella Vulgaris were characterized using FT-ICR-MS. Based on the FT-ICR-MS results, nine representative model compounds (MCs, i.e., benzene, naphthalene, anthracene, phenanthrene, glucose, l-glutamic, triglyceride, tannic and lignin) were applied to investigate the physicochemical properties of EOMs and the ROS changes induced by the photoreaction of chlortetracycline (CTC). With the help of quenching assays, nine MCs were classified into prone-ROS and non-prone-ROS fractions. Prone-ROS compounds generate O2- upon electron transfer to 3O2, which then produces ·OH after disproportionation to generate hydrogen peroxide. The formation of 1O2 is attributed to energy transfer from prone-ROS to 3O2. Density functional theory revealed that prone-ROS exhibited higher reactivity compared to non-prone-ROS, this finding is as well supported by the result of steady-state photolysis measurement. Our study gives a new insight into photochemical fate of CTC in aquatic environments, providing theoretical basis for assessing antibiotics' ecological risk accurately.

Wavelength trends of photoproduction of reactive transient species by chromophoric dissolved organic matter (CDOM), under steady-state polychromatic irradiation

The formation quantum yields of photochemically produced reactive intermediates (PPRIs) by irradiated CDOM (in this study, Suwannee River Natural Organic Matter and Upper Mississippi River Natural Organic Matter) decrease with increasing irradiation wavelength. In particular, the formation quantum yields of the excited triplet states of CDOM (³CDOM*) and of singlet oxygen (¹O₂) have an exponentially decreasing trend with wavelength. The ^•OH wavelength trend is different, because more effective ^•OH production occurs under UVB irradiation than foreseen by a purely exponential function. We show that the parameter-adjustable Weibull function (which adapts to both exponential and some non-exponential trends) is suitable to fit the mentioned quantum yield data, and it is very useful when CDOM irradiation is carried out under polychromatic lamps as done here. Model calculations suggest that, thanks to the ability of CDOM to also absorb visible radiation, and despite its decreasing quantum yield of ^•OH generation with increasing wavelength, CDOM would be able to trigger ^•OH photogeneration in deep waters, to a higher extent than UVB-absorbing nitrate or UVB + UVA-absorbing nitrite.

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.

Ultrafast transient absorption spectroelectrochemistry: femtosecond to nanosecond excited-state relaxation dynamics of the individual components of an anthraquinone redox couple

Many photoactivated processes involve a change in oxidation state during the reaction pathway and formation of highly reactive photoactivated species. Isolating these reactive species and studying their early-stage femtosecond to nanosecond (fs-ns) photodynamics can be challenging. Here we introduce a combined ultrafast transient absorption-spectroelectrochemistry (TA-SEC) approach using freestanding boron doped diamond (BDD) mesh electrodes, which also extends the time domain of conventional spectrochemical measurements. The BDD electrodes offer a wide solvent window, low background currents, and a tuneable mesh size which minimises light scattering from the electrode itself. Importantly, reactive intermediates are generated electrochemically, via oxidation/reduction of the starting stable species, enabling their dynamic interrogation using ultrafast TA-SEC, through which the early stages of the photoinduced relaxation mechanisms are elucidated. As a model system, we investigate the ultrafast spectroscopy of both anthraquinone-2-sulfonate (AQS) and its less stable counterpart, anthrahydroquinone-2-sulfonate (AH2QS). This is achieved by generating AH2QS in situ from AQS via electrochemical means, whilst simultaneously probing the associated early-stage photoinduced dynamical processes. Using this approach we unravel the relaxation mechanisms occurring in the first 2.5 ns, following absorption of ultraviolet radiation; for AQS as an extension to previous studies, and for the first time for AH2QS. AQS relaxation occurs via formation of triplet states, with some of these states interacting with the buffered solution to form a transient species within approximately 600 ps. In contrast, all AH2QS undergoes excited-state single proton transfer with the buffered solution, resulting in formation of ground state AHQS- within approximately 150 ps.

Photo-ammonification of low molecular weight dissolved organic nitrogen by direct and indirect photolysis

The photo-ammonification process plays a crucial role in the transformation of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN). However, previous studies have primarily focused on DON biotransformation than on abiotic processes. This study investigated the photo-ammonification process of nine model low molecular weight (LMW) DON molecules (e.g., amino acids, nucleotides, and urea) under the influence of different light sources. The results showed that photo-ammonification of model DON was mainly induced by UV light, while negligible contribution by visible light was found. Depending on their molecular structures, amino acids yielded different ammonia amounts, whereas negligible photo-ammonification was observed for nucleotides and urea. As for the reactive species, OH promoted ammonia yields of all the model amino acids; ³CDOM^⁎ contributed to the photo-ammonification of six amino acids; ¹O₂ only had a positive impact on ammonification of tryptophan, histidine, and tyrosine; and CO₃^- accelerated ammonia generation from histidine and methionine. In natural water samples, tryptophan, tyrosine, histidine, and methionine generated significant ammonia. OH and ¹O₂ were speculated as the contributing reactive species based on kinetic studies as well as significant fluorescent humic-like and tyrosine-like substances degradation in irradiated samples compared to the raw samples characterized by the EEM-PARAFAC analysis. The negative linear correlations between photo-ammonification rates and the E_LUMO-E_HOMO of the amino acids emphasized the importance of the role of the molecular structure. Overall, these results revealed the LMW DON photo-ammonification mechanism in sunlit surface waters and highlighted its significance in the nitrogen biogeochemical cycle as well as water quality management.

Singlet Oxygen Quantum Yields in Environmental Waters

Singlet oxygen (¹O₂) is a reactive oxygen species produced in sunlit waters via energy transfer from the triplet states of natural sensitizers. There has been an increasing interest in measuring apparent ¹O₂ quantum yields (Φ_Δ) of aquatic and atmospheric organic matter samples, driven in part by the fact that this parameter can be used for environmental fate modeling of organic contaminants and to advance our understanding of dissolved organic matter photophysics. However, the lack of reproducibility across research groups and publications remains a challenge that significantly limits the usability of literature data. In the first part of this review, we critically evaluate the experimental techniques that have been used to determine Φ_Δ values of natural organic matter, we identify and quantify sources of errors that potentially explain the large variability in the literature, and we provide general experimental recommendations for future studies. In the second part, we provide a qualitative overview of known Φ_Δ trends as a function of organic matter type, isolation and extraction procedures, bulk water chemistry parameters, molecular and spectroscopic organic matter features, chemical treatments, wavelength, season, and location. This review is supplemented with a comprehensive database of Φ_Δ values of environmental samples.

Photolysis of atrazine: Role of triplet dissolved organic matter and limitations of sensitizers and quenchers

Atrazine, a widely used herbicide, is susceptible to photolysis. The role of triplet excited states of chromophoric dissolved organic matter (³CDOM*) in the photolysis of atrazine, however, is not well understood. The direct photolysis of atrazine under irradiation sources (natural sunlight/environmentally relevant simulated solar light) and its indirect photochemical reactivity with model triplet photosensitizers (benzophenone, 2-acetonaphthone, 3'-methoxy-acetophenone, 4-carboxybenzophenone, rose bengal, methylene blue, and anthraquinone-2-sulphonate) was investigated. The reactivity of the model sensitizers and DOM (Suwannee River natural organic matter, river/lake water, and wastewater effluent), were compared. The direct photolysis quantum yield was determined as 0.0196 mol Einstein^-1 in a solar simulator and 0.00437 mol Einstein^-1 under natural sunlight. Considerable photosensitization was induced by triplet state (n-π*) model sensitizers, while insignificant effects on atrazine loss were discerned in natural organic matter even when oxygen, a triplet quencher, was removed. The triplet sensitizers benzophenone and 2-acetylnaphthone reacted with L-histidine and 2-propanol that were intended to quench/ scavenge ¹O₂ and hydroxyl radical ^•OH, respectively, and benzophenone reacted with NaN₃ as a ¹O₂ scavenger and furfuryl alcohol as a ¹O₂ trapping agent, indicating quenchers may have unanticipated effects when using model sensitizers. Atrazine loss via reaction with ³DOM* will be relevant only in selected conditions, and this work provides a more comprehensive view on the use of model photosensitizers to mimic triplet ³DOM*.

Development of a quantitative structure-activity relationship model for mechanistic interpretation and quantum yield prediction of singlet oxygen generation from dissolved organic matter

Singlet oxygen (¹O₂) is capable of degrading organic contaminants and inducing cell damage and inactivation of viruses. It is mainly generated through the interaction of dissolved oxygen with excited triplet states of dissolved organic matter (DOM) in natural waters. The present study aims at revealing the underlying mechanism of ¹O₂ generation and providing a potential tool for predicting the quantum yield of ¹O₂ (Φ_1O2) generation from DOM by constructing a quantitative structure-activity relationship (QSAR) model. The determined Φ_1O2 values for the selected DOM-analogs range from (0.54 ± 0.23) × 10^-2 to (62.03 ± 2.97) × 10^-2. A QSAR model was constructed and was proved to have satisfactory goodness-of-fit and robustness. The QSAR model was successfully used to predict the Φ_1O2 of Suwannee River fulvic acid. Mechanistic interpretation of the descriptors in the model showed that hydrophobicity, molecular complexity and the presence of carbonyl groups in DOM play crucial roles in the generation of ¹O₂ from DOM. The presence of other heteroatoms besides O, such as N and S, also affects the generation of ¹O₂. The results of this study provide valuable insights into the generation of ¹O₂ from DOM in sunlit natural waters.

Kinetic modeling of lag times during photo-induced inactivation of E. coli in sunlit surface waters: Unraveling the pathways of exogenous action

This work presents a kinetic analysis of the exogenous photo-induced disinfection of E. coli in natural waters. Herein, the inactivation of bacteria by light and photo-generated transient species, i.e., hydroxyl radical (HO^•), excited triplet states of organic matter (³CDOM*) and singlet oxygen (¹O₂), was studied. It was found that the exogenous disinfection of E. coli proceeds through a lag time, followed by an exponential phase triggered by photo-generated HO^•, ¹O₂ and ³CDOM*. Also, we report that the concentration increased of transient species (and especially HO^•) precursors decreased the lag times of bacteria inactivation. Due to the limitations of the competition kinetics methodology to include the lag phase, an alternative strategy to study the interaction between E. coli and photo-generated transient species was proposed, considering the log-linear pseudo-first order rate constants and lag-times. On this basis and by using APEX software, a full kinetic analysis of exogenous bacterial inactivation, taking into account both lag-time and exponential decay, was developed. This approach provided insights into the conditions that could make exogenous inactivation competitive with the endogenous process for the E. coli inactivation in natural sunlit waters. Hence, this research contributes to the understanding of fundamental kinetic aspects of photoinduced bacterial inactivation, which is the basis for light-assisted processes such as the solar disinfection (SODIS).

Photoinduced disinfection in sunlit natural waters: Measurement of the second order inactivation rate constants between E. coli and photogenerated transient species

This work uncovers the implications of the estimation of exogenous inactivation rates for E. coli after the initial lag phase, and presents a strategy for the determination of the second-order inactivation rate constants (k^2nd) of these bacteria with relevant transient species promoted by solar light in natural waters. For this purpose, specific precursors were considered (nitrate ion, rose bengal, anthraquinone-2-sulfonate) as well as the respective photo-generated transient species (i.e., hydroxyl radical (•OH), singlet oxygen (¹O₂) and triplet excited states). Under these conditions and by using suitable reference compounds (acesulfame K and 2,4,6-trimethylphenol in different series of experiments), the k^2nd values were obtained after developing a proper competition kinetics methodology. The k^2nd values were (2.5 ± 0.9) × 10¹¹, (3.8 ± 1.6) × 10⁷ and (1.8 ± 0.7) × 10¹⁰ M^-1 s^-1 for the inactivation of E. coli by •OH, ¹O₂ and the triplet state of anthraquinone-2-sulfonate (³AQ2S*), respectively. The measurement of a reaction rate constant that is higher than the diffusion-control limit for small molecules in aqueous solution implies that bacteria behave differently from molecules, e.g., because of the large size difference between bacteria and the transients. The obtained k^2nd values were used for the modeling of the bacteria inactivation kinetics in outdoor systems (both water bodies and SODIS bottles), limited to the exponential decay phase that follows the initial lag time. Afterwards, the role of dissolved organic matter (DOM) as precursor of transient species for bacterial elimination was systematically studied. The interaction of different sunlight wavelength regions (UVB, UV-A, blue, green and yellow light) with Suwannee river (SW) and Nordic Lake organic matter (ND) was tested, and the photoinduced disinfection exerted by DOM isolates (SW DOM, Suwannee River Humic Acid, Suwannee River Fulvic Acid or Pony Lake Fulvic Acid) was compared. It was not possible to achieve a complete differentiation of the individual contributions of DOM triplet states (³DOM*) and ¹O₂ to bacterial inactivation. However, the application of competition kinetics to E. coli under solar irradiation in the presence of SW led to a k^2nd value of (2.17 ± 0.40) × 10¹⁰ M^-1 s^-1, which is very near the value for inactivation by ³AQ2S* and suggests that the latter behaved very similar to SW-³DOM* and was a good ³DOM* proxy in the present case. The determination of the second-order inactivation rate constants of E. coli with •OH, ³DOM* and ¹O₂ represents a significant progress in the understanding of the external inactivation pathways of bacteria. It also allows predicting that, after the lag phase, ¹O₂ would contribute to photoinactivation to a far lesser extent than •OH and ³DOM*.

Ultraviolet absorption redshift induced direct photodegradation of halogenated parabens under simulated sunlight

As disinfection by-products of parabens, halogenated parabens are frequently detected in aquatic environments and exhibit higher persistence and toxicity than parabens themselves. An interesting phenomenon was found that UV absorption redshift (∼45 nm) occurs after halogenation of parabens at circumneutral pH, leading to overlap with the spectrum of terrestrial sunlight. This work presents the first evidence on the direct photodegradation of seven chlorinated and brominated parabens under simulated sunlight. These halogenated parabens underwent rapid direct photodegradation, distinguished from the negligible degradation of the parent compounds. The photodegradation rate depended on their forms and substituents. The deprotonation of halogenated parabens facilitated the direct photodegradation. Brominated parabens exhibited higher degradation efficiency than chlorinated parabens, and mono-halogenated parabens had higher degradation than di-halogenated parabens. The pseudo-first-order rate constants (k_obs) for brominated parabens (0.075-0.120 min^-1) were approximately 7-fold higher than those of chlorinated parabens (0.011-0.017 min^-1). A quantitative structure-activity relationship (QSAR) model suggested that the photodegradation was linearly correlated with the C-X bond energies, electronic and steric effects of halogen substituents. The photodegradation products were identified using QTOF-MS analyses and a degradation pathway was proposed. The yeast two-hybrid estrogenicity assay revealed that the estrogenic activities of the photoproducts were negligible. These findings are important for the removal of halogenated parabens and predictions of their fate and potential impacts in surface waters.

An experimental methodology to measure the reaction rate constants of processes sensitised by the triplet state of 4-carboxybenzophenone as a proxy of the triplet states of chromophoric dissolved organic matter, under steady-state irradiation conditions

By a combination of transient absorption spectroscopy and steady-state irradiation experiments, we investigated the transformation of phenol and furfuryl alcohol (FFA) sensitised by irradiated 4-carboxybenzophenone (CBBP). The latter is a reasonable proxy molecule to assess the reactivity of the excited triplet states of the chromophoric dissolved organic matter that occurs in natural waters. The main reactive species for the transformation of both phenol and FFA was the CBBP triplet state, despite the fact that FFA is a commonly used probe for 1O2. In the case of FFA it was possible to develop a simple kinetic model that fitted well the experimental data obtained by steady-state irradiation, in a wide range of FFA concentration values. In the case of phenol the model was made much more complex by the likely occurrence of back reactions between radical species (e.g., phenoxyl and superoxide). This problem can be tackled by considering only the experimental data at low phenol concentration, where the degradation rate increases linearly with concentration. We do not recommend the use of 1O2 scavengers/quenchers such as sodium azide to elucidate CBBP photoreaction pathways, because the azide provides misleading results by also acting as a triplet-state quencher. Based on the experimental data, we propose a methodology for the measurement of the CBBP triplet-sensitisation rate constants from steady-state irradiation experiments, allowing for a better assessment of the triplet-sensitised degradation of emerging contaminants.

Photochemical transformation of dimethyl phthalate (DMP) with N(iii)(H2ONO+/HONO/NO2−) in the atmospheric aqueous environment

The primary step of photochemical transformation of dimethyl phthalate (DMP) with N(iii) is ˙OH-addition on the aromatic ring of DMP to form a DMP–OH adduct, followed by several decay channels and corresponding rate constants are determined.

Phototransformation of Acesulfame K in surface waters: Comparison of two techniques for the measurement of the second-order rate constants of indirect photodegradation, and modelling of photoreaction kinetics

By use of photochemical modelling we show that acesulfame K (ACE) can undergo photodegradation in surface waters, mainly by reaction with OH and possibly ³CDOM* (the triplet states of chromophoric dissolved organic matter). With the possible exception of shallow water bodies containing low dissolved organic carbon, we predict ACE to be a refractory compound in environmental waters which agrees well with many literature reports. We used two methods to measure the photoreactivity parameters of ACE, of which one is based on the monitoring of the time evolution of ACE alone and the other is based on the monitoring of both ACE and a reference compound (hereafter, they are referred to as substrate-only and substrate + reference method, respectively). The substrate + reference method can be time-saving, but it is potentially prone to interferences. In this work, ibuprofen and atrazine were used as reference compounds of known behaviour to study the photoreactivity of ACE by competition kinetics in the substrate + reference method. The two methods gave overall comparable results, partially because two different reference compounds instead of only one were used in the substrate + reference method. By so doing, however, one loses part of the time-saving advantage of the substrate + reference method.

Photoinduced transformation of pyridinium-based ionic liquids, and implications for their photochemical behavior in surface waters

The photochemical reactivity of three ionic liquids (1-ethylpyridinium tetrafluoroborate, 1-butyl-4-methylpyridinium tetrafluoroborate, and 1-(3-cyanopropyl)pyridinium chloride) was studied by combining laboratory experiments and photochemical modeling, to get insight into the possible behavior in surface-water environments. Among the studied compounds, phototransformation in sunlit surface waters could be an important attenuation pathway for 1-butyl-4-methylpyridinium tetrafluoroborate (BMPOTFB). In this case the reaction with the carbonate radicals (CO₃^-) would prevail at low values of the dissolved organic carbon (DOC), while the direct photolysis would be important at intermediate to high DOC values. The sensitization by the triplet states of chromophoric dissolved organic matter could play a significant role at high DOC, especially in the presence of a considerable fraction of highly photoreactive pedogenic organic matter derived from soil runoff. The main processes that account for the phototransformation of BMPOTFB and produce the main detected transformation products (TPs) are hydroxylation, detachment/shortening of the butyl chain and double bond formation. Interestingly, there is a considerable overlap between the TPs formed by direct photolysis and those produced by indirect photochemistry. Some of the TPs formed from BMPOTFB are more toxic than the parent compound towards Vibrio fischeri bacteria, and account for the increase in toxicity of the irradiated mixtures. Differently from BMPOTFB, in the case of the other two studied ionic liquids the photodegradation would play a negligible role in environmental attenuation, with the possible exception of very shallow waters with low DOC.

Phototransformation of the Herbicide Propanil in Paddy Field Water

When irradiated in paddy-field water, propanil (PRP) undergoes photodegradation by direct photolysis, by reactions with ^•OH and CO₃^•-, and possibly also with the triplet states of chromophoric dissolved organic matter. Irradiation also inhibits the nonphotochemical (probably biological) degradation of PRP. The dark- and light-induced pathways can be easily distinguished because 3,4-dichloroaniline (34DCA, a transformation intermediate of considerable environmental concern) is produced with almost 100% yield in the dark but not at all through photochemical pathways. This issue allows an easy assessment of the dark process(es) under irradiation. In the natural environment, we expect PRP photodegradation to be important only in the presence of elevated nitrate and/or nitrite levels, e.g., [NO₃^-] approaching 1 mmol L^-1 (corresponding to approximately 60 mg L^-1). Under these circumstances, ^•OH and CO₃^•- would play a major role in PRP phototransformation. Because flooded paddy fields are efficient denitrification bioreactors that can achieve decontamination of nitrate-rich water used for irrigation, irrigation with such water would both enhance PRP photodegradation and divert PRP dissipation processes away from the production of 34DCA, at least in the daylight hours.

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.

Triplet state dissolved organic matter in aquatic photochemistry: reaction mechanisms, substrate scope, and photophysical properties

Excited triplet states of chromophoric dissolved organic matter (³CDOM*) play a major role among the reactive intermediates produced upon absorption of sunlight by surface waters. After more than two decades of research on the aquatic photochemistry of ³CDOM*, the need for improving the knowledge about the photophysical and photochemical properties of these elusive reactive species remains considerable. This critical review examines the efforts to date to characterize ³CDOM*. Information on ³CDOM* relies mainly on the use of probe compounds because of the difficulties associated with directly observing ³CDOM* using transient spectroscopic methods. Singlet molecular oxygen (¹O₂), which is a product of the reaction between ³CDOM* and dissolved oxygen, is probably the simplest indicator that can be used to estimate steady-state concentrations of ³CDOM*. There are two major modes of reaction of ³CDOM* with substrates, namely triplet energy transfer or oxidation (via electron transfer, proton-coupled electron transfer or related mechanisms). Organic molecules, including several environmental contaminants, that are susceptible to degradation by these two different reaction modes are reviewed. It is proposed that through the use of appropriate sets of probe compounds and model photosensitizers an improved estimation of the distribution of triplet energies and one-electron reduction potentials of ³CDOM* can be achieved.

Photochemical transformation of sunscreen agent benzophenone-3 and its metabolite in surface freshwater and seawater

The occurrence of sunscreen agents and their metabolites in surface waters gives rise to public concerns. However, little is known about the environmental fate of these pollutants at present, especially for their metabolites. In this study, we investigated the photochemical of sunscreen agents and their metabolites in natural waters, adopting benzophenone-3 (BP-3) and its human metabolite 4-hydroxybenzophenone (4-OH-BP3) as examples. Results show that only anionic forms of both BP-3 and 4-OH-BP3 can undergo direct photodegradation. The photolytic rates of both compounds in natural waters are faster as compared to those in pure water. Radical scavenging experiments revealed that triplet-excited dissolved organic matter ((3)DOM(∗)) was responsible for the indirect photodegradation of BP-3 and 4-OH-BP3 in seawater, whereas in freshwater, the indirect photodegradation of these two compounds was attributed to (3)DOM(∗) and ·OH. (1)O2 plays a negligible role in their photodegradation because of the weak (1)O2 reactivity. Furthermore, we probed the contribution of ·OH and (3)DOM(∗) to the photodegradation of both compounds in freshwater, and the results revealed that ·OH accounted for 56% and 59% of the observed photodegradation for BP-3 and 4-OH-BP3, respectively, whereas (3)DOM(∗) accounted for 43% and 12% of the observed photodegradation for BP-3 and 4-OH-BP3, respectively. These results are helpful in assessing the ecological risk of BP-3 and its metabolite in the aquatic environment.

Modeling the photochemical transformation of nitrobenzene under conditions relevant to sunlit surface waters: Reaction pathways and formation of intermediates

Nitrobenzene (NB) would undergo photodegradation in sunlit surface waters, mainly by direct photolysis and triplet-sensitized oxidation, with a secondary role of the *OH reaction. Its photochemical half-life time would range from a few days to a couple of months under fair-weather summertime irradiation, depending on water chemistry and depth. NB phototransformation gives phenol and the three nitrophenol isomers, in different yields depending on the considered pathway. The minor *OH role in degradation would make NB unsuitable as *OH probe in irradiated natural water samples, but the selectivity towards *OH could be increased by monitoring the formation of phenol from NB+*OH. The relevant reaction would proceed through ipso-addition of *OH on the carbon atom bearing the nitro-group, forming a pre-reactive complex that would evolve into a transition state (and then into a radical addition intermediate) with very low activation energy barrier.

Co-occurrence of Photochemical and Microbiological Transformation Processes in Open-Water Unit Process Wetlands

The fate of anthropogenic trace organic contaminants in surface waters can be complex due to the occurrence of multiple parallel and consecutive transformation processes. In this study, the removal of five antiviral drugs (abacavir, acyclovir, emtricitabine, lamivudine and zidovudine) via both bio- and phototransformation processes, was investigated in laboratory microcosm experiments simulating an open-water unit process wetland receiving municipal wastewater effluent. Phototransformation was the main removal mechanism for abacavir, zidovudine, and emtricitabine, with half-lives (t1/2,photo) in wetland water of 1.6, 7.6, and 25 h, respectively. In contrast, removal of acyclovir and lamivudine was mainly attributable to slower microbial processes (t1/2,bio = 74 and 120 h, respectively). Identification of transformation products revealed that bio- and phototransformation reactions took place at different moieties. For abacavir and zidovudine, rapid transformation was attributable to high reactivity of the cyclopropylamine and azido moieties, respectively. Despite substantial differences in kinetics of different antiviral drugs, biotransformation reactions mainly involved oxidation of hydroxyl groups to the corresponding carboxylic acids. Phototransformation rates of parent antiviral drugs and their biotransformation products were similar, indicating that prior exposure to microorganisms (e.g., in a wastewater treatment plant or a vegetated wetland) would not affect the rate of transformation of the part of the molecule susceptible to phototransformation. However, phototransformation strongly affected the rates of biotransformation of the hydroxyl groups, which in some cases resulted in greater persistence of phototransformation products.

Isotope Fractionation Associated with the Indirect Photolysis of Substituted Anilines in Aqueous Solution

Organic micropollutants containing aniline substructures are susceptible to different light-induced transformation processes in aquatic environments and water treatment operations. Here, we investigated the magnitude and variability of C and N isotope fractionation during the indirect phototransformation of four para-substituted anilines in aerated aqueous solutions. The model photosensitizers, namely 9,10-anthraquinone-1,5-disulfonate and methylene blue, were used as surrogates for dissolved organic matter chromophores generating excited triplet states in sunlit surface waters. The transformation of aniline, 4-CH3-, 4-OCH3-, and 4-Cl-aniline by excited triplet states of the photosensitizers was associated with inverse and normal N isotope fractionation, whereas C isotope fractionation was negligible. The apparent 15N kinetic isotope effects (AKIE) were almost identical for both photosensitizers, increased from 0.9958±0.0013 for 4-OCH3-aniline to 1.0035±0.0006 for 4-Cl-aniline, and correlated well with the electron donating properties of the substituent. N isotope fractionation is pH-dependent in that H+ exchange reactions dominate below and N atom oxidation processes above the pKa value of the substituted aniline's conjugate acid. Correlations of C and N isotope fractionation for indirect phototransformation were different from those determined previously for the direct photolysis of chloroanilines and offer new opportunities to distinguish between abiotic degradation pathways.

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.

Investigation of humic substance photosensitized reactions via carbon and hydrogen isotope fractionation

Humic substances (HS) acting as photosensitizers can generate a variety of reactive species, such as OH radicals and excited triplet states ((3)HS*), promoting the degradation of organic compounds. Here, we apply compound-specific stable isotope analysis (CSIA) to characterize photosensitized mechanisms employing fuel oxygenates, such as methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE), as probes. In oxygenated aqueous media, Λ (Δδ(2)H/Δδ(13)C) values of 23 ± 3 and 21 ± 3 for ETBE obtained by photosensitization by Pahokee Peat Humic Acid (PPHA) and Suwannee River Fulvic Acid (SRFA), respectively, were in the range typical for H-abstraction by OH radicals generated by photolysis of H2O2 (Λ = 24 ± 2). However, (3)HS* may become a predominant reactive species upon the quenching of OH radicals (Λ = 14 ± 1), and this process can also play a key role in the degradation of ETBE by PPHA photosensitization in deoxygenated media (Λ = 11 ± 1). This is in agreement with a model photosensitization by rose bengal (RB(2-)) in deoxygenated aqueous solutions resulting in one-electron oxidation of ETBE (Λ = 14 ± 1). Our results demonstrate that the use of CSIA could open new avenues for the assessment of photosensitization pathways.

Conceptual model and experimental framework to determine the contributions of direct and indirect photoreactions to the solar disinfection of MS2, phiX174, and adenovirus

Sunlight inactivates waterborne viruses via direct (absorption of sunlight by the virus) and indirect processes (adsorption of sunlight by external chromophores, which subsequently generate reactive species). While the mechanisms underlying these processes are understood, their relative importance remains unclear. This study establishes an experimental framework to determine the kinetic parameters associated with a virus' susceptibility to solar disinfection and proposes a model to estimate disinfection rates and to apportion the contributions of different inactivation processes. Quantum yields of direct inactivation were determined for three viruses (MS2, phiX174, and adenovirus), and second-order rate constants associated with indirect inactivation by four reactive species ((1)O2, OH(•), CO3(•-), and triplet states) were established. PhiX174 exhibited the greatest quantum yield (1.4 × 10(-2)), indicating that it is more susceptible to direct inactivation than MS2 (2.9 × 10(-3)) or adenovirus (2.5 × 10(-4)). Second-order rate constants ranged from 1.7 × 10(7) to 7.0 × 10(9) M(-1) s(-1) and followed the sequence MS2 > adenovirus > phiX174. A predictive model based on these parameters accurately estimated solar disinfection of MS2 and phiX174 in a natural water sample and approximated that of adenovirus within a factor of 6. Inactivation mostly occurred by direct processes, though indirect inactivation by (1)O2 also contributed to the disinfection of MS2 and adenovirus.

Phototransformation pathways of the fungicide dimethomorph ((E,Z) 4-[3-(4-chlorophenyl)-3-(3,4-dimethoxyphenyl)-1-oxo-2-propenyl]morpholine), relevant to sunlit surface waters

Dimethomorph (DMM) is a widely used fungicide that shows low toxicity for birds and mammals but can be quite toxic to aquatic organisms. The persistence of DMM in surface waters is thus of high importance, and this work modelled its water half-life time due to photochemical processes. Depending on environmental conditions (e.g. water chemistry, depth, season), DMM lifetime could vary from a few days to a few months. For lifetimes of a few weeks or shorter, photochemistry would be an important pathway for DMM attenuation in surface waters. Such conditions could be reached in summer, in shallow water bodies with low dissolved organic carbon (DOC) and high nitrate and/or nitrite. The main pathways accounting for DMM photodegradation in environmental waters would be the reactions with OH and with the triplet states of chromophoric dissolved organic matter, (3)CDOM* (under the hypothesis that (3)CDOM* reactivity is well described by the triplet state of anthraquinone-2-sulphonate), while direct photolysis would be less important. The OH pathway would be favoured in low-DOC waters, while the opposite conditions would favour (3)CDOM*. It was possible to detect and identify some intermediates formed upon reaction between DMM and (3)CDOM*, namely N-formylmorpholine, 4-chloroacetophenone and 4-chlorobenzoic acid. The transformation of DMM into the detected compounds would not increase the acute toxicity of the fungicide towards mammals, and the acute effects for freshwater organisms could be decreased significantly.

Photosensitised humic-like substances (HULIS) formation processes of atmospheric significance: a review

Photosensitised reactions can produce compounds that closely resemble the humic-like substances (HULIS) occurring in atmospheric aerosols. The relevant processes have been observed in the laboratory, in both gas-solid systems and the aqueous phase. They involve triplet sensitisers (such as benzophenones, anthraquinones and nitroaromatic compounds, which yield reactive triplet states after sunlight absorption) or photogenerated oxidants like (•)OH, in the presence of substrates that undergo oligomerisation reactions upon oxidation. Formation of higher molecular weight compounds, modification of the wettability properties of organic films and photoproduction of substances with humic-like fluorescence properties have been observed as a consequence of the photosensitised reactions. Ozone plays an important but still not completely clear role in gas-solid systems.

Indirect photochemistry in sunlit surface waters: photoinduced production of reactive transient species

This paper gives an overview of the main reactive transient species that are produced in surface waters by sunlight illumination of photoactive molecules (photosensitizers), such as nitrate, nitrite, and chromophoric dissolved organic matter (CDOM). The main transients (˙OH, CO3(-˙) , (1)O2, and CDOM triplet states) are involved in the indirect phototransformation of a very wide range of persistent organic pollutants in surface waters.

Assessing the photochemical transformation pathways of acetaminophen relevant to surface waters: transformation kinetics, intermediates, and modelling

This work shows that the main photochemical pathways of acetaminophen (APAP) transformation in surface waters would be direct photolysis (with quantum yield of (4.57 ± 0.17)⋅10(-2)), reaction with CO3(-·) (most significant at pH > 7, with second-order rate constant of (3.8 ± 1.1)⋅10(8) M(-1) s(-1)) and possibly, for dissolved organic carbon higher than 5 mg C L(-1), reaction with the triplet states of chromophoric dissolved organic matter ((3)CDOM*). The modelled photochemical half-life time of APAP in environmental waters would range from days to few weeks in summertime, which suggests that the importance of phototransformation might be comparable to biodegradation. APAP transformation by the main photochemical pathways yields hydroxylated derivatives, ring-opening compounds as well as dimers and trimers (at elevated concentration levels). In the case of (3)CDOM* (for which the triplet state of anthraquinone-2-sulphonate was used as proxy), ring rearrangement is also hypothesised. Photochemistry would produce different transformation products (TPs) of APAP than microbial biodegradation or human metabolism, thus the relevant TPs might be used as markers of APAP photochemical reaction pathways in environmental waters.

Formation and reactivity of the dichloride radical (Cl2(-·)) in surface waters: a modelling approach

The occurrence of Cl2(-·) in natural waters would depend on the budget between triplet-sensitised photogeneration (which might have second-order rate constant of 10(7)-10(9) M(-1) s(-1)) and scavenging by dissolved organic matter (DOM, with possible rate constant of 10-10(3) L (mg C)(-1) s(-1)). The steady-state [Cl2(-·)] in brackish to saline waters might be in the range of 10(-14)-10(-12) M in mid-latitude summertime, coherently with data of phenol photochlorination in seawater. Steady-state [Cl2(-·)] would be enhanced by chloride (up to a plateau above 0.1 M Cl(-)) and inhibited by DOM. The radical Cl2(-·) would also be a major oxidant of nitrite to the nitrating agent (·)NO2 in brackish- and salt-water. This issue may explain the sustained formation of nitrophenols in phenol-spiked seawater and in natural brackish waters impacted by phenolic pollutants (Rhône delta, Southern France).

Photochemical transformation of atrazine and formation of photointermediates under conditions relevant to sunlit surface waters: laboratory measures and modelling

By combination of laboratory experiments and modelling, we show here that the main photochemical pathways leading to the transformation of atrazine (ATZ, 2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) in surface waters would be direct photolysis, reaction with ·OH and with the triplet states of chromophoric dissolved organic matter ((3)CDOM*). Reaction with (3)CDOM* would be favoured by elevated water depth and dissolved organic carbon content, while opposite conditions would favour direct photolysis and OH reaction. Desethylatrazine (DEA, 4-amino-2-chloro-6-isopropylamino-1,3,5-triazine) was the main detected intermediate of ATZ phototransformation. Its formation yield from ATZ (ratio of DEA formation to ATZ transformation rate) would be 0.93 ± 0.14 for ·OH, 0.55 ± 0.05 for (3)CDOM*, and 0.20 ± 0.02 for direct photolysis. Direct photolysis and ·OH reaction also yielded 4-amino-2-hydroxy-6-isopropylamino-1,3,5-triazine (DEAOH) and 6-amino-2-chloro-4-ethylamino-1,3,5-triazine (DIA). Reaction with excited triplet states also produced 2-hydroxy-4,6-diamino-1,3,5-triazine (AN) and 2-chloro-4,6-diamino-1,3,5-triazine (CAAT). Therefore, if biological processes can be neglected and if the low formation yields do not prevent detection, DEAOH and DIA could be used as markers of ATZ direct photolysis and ·OH reaction, while AN and CAAT could be markers of ATZ reaction with (3)CDOM*. Model predictions concerning ATZ phototransformation were compared with available field data from the literature. When sufficiently detailed field information was provided, good agreement was found with the model.

Photochemical transformation of ibuprofen into harmful 4-isobutylacetophenone: pathways, kinetics, and significance for surface waters

The harmful compound 4-isobutylacetophenone (IBAP) can be formed photochemically from the anti-inflammatory drug ibuprofen (IBP), upon direct photolysis (yield 25 ± 7%, μ ± σ), reaction with ·OH (yield 2.3 ± 0.1%) and reaction with triplet states of chromophoric dissolved organic matter, (3)CDOM* (yield 31 ± 4%). In the latter case, anthraquinone-2-sulphonate was used as CDOM proxy. The three processes would account for most of the photochemical transformation of IBP and IBAP in surface waters. IBAP formation from IBP involves the propanoic acid chain, which is more reactive than the aromatic ring as shown by quantum mechanical calculations. IBAP is expected to undergo slightly faster photochemical transformation than IBP in surface waters, with a modelled pseudo-first order rate constant that is higher by 1.5-1.9 times compared to IBP. Due to fairly high formation yields and depending on IBP emission scenarios, photochemical modelling suggests that IBAP could reach concentration values up to ~15% of IBP in surface waters, thus being a potentially important transformation intermediate. This issue prompts for the need of field studies that provide information on IBAP environmental occurrence, which is virtually unknown at the present moment.

Production of photo-oxidants by dissolved organic matter during UV water treatment

Dissolved organic matter (DOM) irradiated by sunlight generates photo-oxidants that can accelerate organic contaminant degradation in surface waters. However, the significance of this process to contaminant removal during engineered UV water treatment has not been demonstrated, partly due to a lack of suitable methods in the deep UV range. This work expands methods previously established to detect (1)O2, HO•, H2O2, and DOM triplet states ((3)DOM*) at solar wavelengths to irradiation at 254 nm, typical of UV water treatment. For transient intermediates, the methods include a photostable probe combined with selective scavengers. Quantum yields for (1)O2, (3)DOM* and H2O2 were in the same range as for solar-driven reactions but were an order of magnitude higher for HO•, which other experiments indicate is due to H2O2 reduction. With the quantum yields, the degradation of metoxuron was successfully predicted in a DOM solution irradiated at 254 nm. Further modeling showed that the contribution of DOM sensitization to organic contaminant removal during UV treatment should be significant only at high UV fluence, characteristic of advanced oxidation processes. Of the reactive species studied, (3)DOM* is predicted to have the greatest general influence on UV degradation of contaminants.

Photochemical formation of hydroxyl radical from effluent organic matter: role of composition

The photochemical formation of hydroxyl radical (HO(•)) from effluent organic matter (EfOM) depends upon the chemical properties of this heterogeneous mixture. In this study, two EfOM samples collected from wastewater treatment plants (WWTP A and B) were fractionated by both hydrophobicity (bulk and non-humic) and apparent molecular weight (AMW). The apparent quantum yield for HO(•) formation (ΦHO(•)) and the maximum fluorescence quantum yield (ΦF) were subsequently measured for each subfraction. The formation rates of HO(•) (considering only the hydrogen-peroxide-independent pathways) for the bulk waters were 4.8 × 10(-10) and 9.6 × 10(-11) M s(-1) for WWTP A and B, respectively. For the AMW fractions, the values of ΦHO(•) increased as the AMW of the material decreased. For the WWTP A sample, the ΦHO(•) increased from 2.54 × 10(-4) (bulk water) to 6.29 × 10(-4) for the <1 kDa fraction, and for the WWTP B sample, the value of ΦHO(•) increased from 6.50 × 10(-5) for bulk water to 3.45 × 10(-4) for the <1 kDa fraction. In the case of fluorescence, the values of ΦF ranged from 2.37 × 10(-4) (bulk water) to 3.48 × 10(-4) (<1 kDa fraction) for WWTP A and 3.19 × 10(-4) (bulk water) to 5.75 × 10(-4) (<1 kDa fraction) for WWTP B. There was a linear correlation between ΦHO(•) and ΦF, suggesting that different photophysical processes occur in the chemical components of the fractions. Understanding the formation of HO(•) from EfOM is essential for understanding wastewater-impacted aquatic systems because these results influence the photochemical degradation and mineralization of trace organic contaminants.

Phototransformation of the sunlight filter benzophenone-3 (2-hydroxy-4-methoxybenzophenone) under conditions relevant to surface waters

The UV filter benzophenone-3 (BP3) has UV photolysis quantum yield ΦBP3=(3.1±0.3)·10(-5) and the following second-order reaction rate constants: with (•)OH, k(BP3,(•)OH)=(2.0±0.4)·10(10) M(-1) s(-1); with the triplet states of chromophoric dissolved organic matter ((3)CDOM*), K(BP3,(3)CDOM*)=(1.1±0.1)·10(9) M(-1) s(-1); with (1)O2, k(BP3,(1)O2)=(2.0±0.1)·10(5) M(-1) s(-1), and with CO3(-•), k(BP3,CO3(-•))<5·10(7) M(-1) s(-1). These data allow the modelling of BP3 photochemical transformation, which helps filling the knowledge gap about the environmental persistence of this compound. Under typical surface-water conditions, direct photolysis and reactions with (•)OH and (3)CDOM* would be the main processes of BP3 phototransformation. Reaction with (•)OH would prevail at low DOC, direct photolysis at intermediate DOC (around 5 mg C L(-1)), and reaction with (3)CDOM* at high DOC. If the reaction rate constant with CO3(-•) is near the upper limit of experimental measures (5·10(7) M(-1) s(-1)), the CO3(-•) degradation process could be somewhat important for DOC<1 mg C L(-1). The predicted half-life time of BP3 in surface waters under summertime conditions would be of some weeks, and it would increase with increasing depth and DOC. BP3 transformation intermediates were detected upon reaction with (•)OH. Two methylated derivatives were tentatively identified, and they were probably produced by reaction between BP3 and fragments arising from photodegradation. The other intermediates were benzoic acid (maximum concentration ~10% of initial BP3) and benzaldehyde (1%).

Anthraquinone-2-sulfonic acid (AQ2S) is a novel neurotherapeutic agent

Anthraquinone derivatives such as emodin have recently been shown to protect in models of beta amyloid β (Aβ) and tau aggregation-induced cell death. The mechanisms of action possibly involve preconditioning effects, anti-aggregation properties, and/or enhancing the phosphatidylinositol-3-kinase (PI3K)/AKT survival mechanism. We studied several natural (emodin, rhein, and aloin) and synthetic (AQ2S) anthraquinones, to screen for post-treatment therapeutic benefit in two models of neuronal death, namely hydrogen peroxide (H(2)O(2)) and staurosporine (STS)-induced injury. Treatment with emodin, rhein, or aloin failed to reduce H(2)O(2) injury. Moreover, consistent with emodin behaving like a mild toxin, it exacerbated oxidative injury at the highest concentration used (50 μM) in our post-treatment paradigm, and potently inhibited AKT. In contrast, AQ2S was neuroprotective. It reduced H(2)O(2) injury at 50 and 75 μM. In addition, AQ2S potently inhibited staurosporine (STS)-induced injury. The mechanisms of action involve caspase inhibition and AKT activation. However, blockade of AKT signaling with LY294002 failed to abolish AQ2S-mediated protection on the STS assay. This is the first study to report that AQ2S is a new neuroprotective compound and a novel caspase inhibitor.

Could triplet-sensitised transformation of phenolic compounds represent a source of fulvic-like substances in natural waters?

Here we show that fluorescent compounds that could be classified as "M-like" (marine-like) fulvic acids are formed upon phototransformation of phenol by a triplet sensitiser (anthraquinone-2-sulphonate, AQ2S). The relevant process most likely involves phenol oxidation to phenoxyl radical by triplet AQ2S, followed by dimerisation of phenoxyl radicals into phenoxyphenols and dihydroxybiphenyls. It might be the first step of an oligomerization process that bears resemblance with the expected formation pathways of humic-like substances (HULIS) in the atmosphere. Such a process could account for the formation in surface waters of compounds having similar fluorescence properties as "M-like" fulvic acids. Presently it is thought that such species are formed upon photo-fragmentation of larger humic and fulvic acids ("top-down" pathway), and we propose that an opposite, "bottom-up" pathway could also be operational.

Assessing the occurrence of the dibromide radical (Br₂⁻•) in natural waters: measures of triplet-sensitised formation, reactivity, and modelling

The triplet state of anthraquinone-2-sulphonate (AQ2S) is able to oxidise bromide to Br(•)/Br(2)(-•), with rate constant (2-4)⋅10(9)M(-1)s(-1) that depends on the pH. Similar processes are expected to take place between bromide and the triplet states of naturally occurring chromophoric dissolved organic matter ((3)CDOM*). The brominating agent Br(2)(-•) could thus be formed in natural waters upon oxidation of bromide by both (•)OH and (3)CDOM*. Br(2)(-•) would be consumed by disproportionation into bromide and bromine, as well as upon reaction with nitrite and most notably with dissolved organic matter (DOM). By using the laser flash photolysis technique, and phenol as model organic molecule, a second-order reaction rate constant of ~3⋅10(2)L(mg C)(-1)s(-1) was measured between Br(2)(-•) and DOM. It was thus possible to model the formation and reactivity of Br(2)(-•) in natural waters, assessing the steady-state [Br(2)(-•)]≈10(-13)-10(-12)M. It is concluded that bromide oxidation by (3)CDOM* would be significant compared to oxidation by (•)OH. The (3)CDOM*-mediated process would prevail in DOM-rich and bromide-rich environments, the latter because elevated bromide would completely scavenge (•)OH. Under such conditions, (•)OH-assisted formation of Br(2)(-•) would be limited by the formation rate of the hydroxyl radical. In contrast, the formation rate of (3)CDOM* is much higher compared to that of (•)OH in most surface waters and would provide a large (3)CDOM* reservoir for bromide to react with. A further issue is that nitrite oxidation by Br(2)(-•) could be an important source of the nitrating agent (•)NO(2) in bromide-rich, nitrite-rich and DOM-poor environments. Such a process could possibly account for significant aromatic photonitration observed in irradiated seawater and in sunlit brackish lagoons.

Photochemical fate of carbamazepine in surface freshwaters: laboratory measures and modeling

It is shown here that carbamazepine (CBZ) would undergo direct photolysis and reaction with (•)OH as the main phototransformation pathways in surface waters. Environmental lifetimes are expected to vary from a few weeks to several months, and predictions are in good agreement with available field data. Acridine (I) and 10,11-dihydro-10,11-trans-dihydroxy-CBZ (V) are the main quantified phototransformation intermediates upon direct photolysis and (•)OH reaction, respectively. The photochemical yield of mutagenic I from CBZ is in the 3-3.5% range, and it is similar for both direct photolysis and (•)OH reaction: it would undergo limited variation with environmental conditions. In contrast, the yield of V would vary in the 4-8.5% range depending on the conditions, because V is formed from CBZ by (•)OH (9.0% yield) more effectively than upon direct photolysis (1.4% yield). Other important photointermediates, mostly formed from CBZ upon (•)OH reaction, are an aromatic-ring-dihydroxylated CBZ (VI) and N,N-bis(2-carboxyphenyl)urea (VII). Compounds VI and VII are formed by photochemistry and are not reported as human metabolites; thus, they could be used as tracers of CBZ phototransformation in surface waters. Interestingly, VI has recently been detected in river water.

Chemical and optical phototransformation of dissolved organic matter

Dissolved organic matter represents the main reservoir of organic carbon in most aquatic ecosystems. In the present study, we determined the optical changes and the quantum yields of transient species formation for chromophoric dissolved organic matter (CDOM) samples undergoing photodegradation. The results show that the triplet states (3)CDOM* are potentially key players in CDOM photodegradation and that such transformations are strongly influenced by small differences in CDOM sources and sinks. In contrast, ·OH radicals are very unlikely to play a key role in phototransformation. These results represent an important first step in combining optical and transient species analyses to understand photodegradation processes of dissolved organic matter.

Modelling the photochemical fate of ibuprofen in surface waters

We show that the main photochemical processes involved in the phototransformation of anionic ibuprofen (IBP) in surface waters are the reaction with (•)OH, the direct photolysis and possibly the reaction with the triplet states of chromophoric dissolved organic matter ((3)CDOM). These conclusions were derived by use of a model of surface water photochemistry, which adopted measured parameters of photochemical reactivity as input data. The relevant parameters are the polychromatic UVB photolysis quantum yield (Φ(IBP) = 0.33 ± 0.05, μ±σ), the reaction rate constant with (•)OH (k(IBP,•OH)=(1.0 ± 0.3)⋅10(10) M(-1) s(-1)), the (1)O(2) rate constant (k(IBP,)( ¹O₂)= (6.0 ± 0.6)⋅10(4) M(-1) s(-1)), while the reaction with CO(3)(-•) can be neglected. We adopted anthraquinone-2-sulphonate (AQ2S) and riboflavin (Ri) as CDOM proxies and the reaction rate constants with the respective triplet states were k(IBP,3AQ2S)=(9.7 ± 0.2)⋅10(9) M(-1) s(-1) and k(IBP,3Ri) = 4.5⋅10(7) M(-1) s(-1). The reaction with (3)CDOM can be an important IBP sink if its rate constant is comparable to that of (3)AQ2S, while it is unimportant if the rate constant is similar to the (3)Ri* one. The photochemical pathways mainly lead to the transformation (oxidation and/or shortening) of the propanoic lateral chain of IBP, which appears to be significantly more reactive than the isobutyl one. Interestingly, none of the detected intermediates was produced by substitution on the aromatic ring.

Formation of hydroxyl radicals by irradiated 1-nitronaphthalene (1NN): oxidation of hydroxyl ions and water by the 1NN triplet state

The excited triplet state of 1-nitronaphthalene ((3)1NN*) reacts with OH(-) with a second-order reaction rate constant of (1.66 ± 0.08)×10(7) M(-1) s(-1) (μ±σ). The reaction yields the ˙OH radical and the radical anion 1NN(-)˙. In aerated solution, the radical 1NN(-)˙ would react with O(2) to finally produce H(2)O(2) upon hydroperoxide/superoxide disproportionation. The photolysis of H(2)O(2) is another potential source of ˙OH, but such a pathway would be a minor one in circumneutral (pH 6.5) or in basic solution ([OH(-)] = 0.3-0.5 M). The oxidation of H(2)O by (3)1NN*, with rate constant 3.8 ± 0.3 M(-1) s(-1), could be the main ˙OH source at pH 6.5.