Direct Photolysis Quantum Yield Research Articles

Laser flash photolysis and quantum chemical studies of UV degradation of pharmaceutical drug chloramphenicol: Short-lived intermediates, quantum yields and mechanism of photolysis

Using methods of time-resolved and stationary photolysis, HPLC-MS and quantum-chemical calculations by the DFT method, the mechanism of direct UV photolysis of the antibiotic chloramphenicol (CAP) was established. For the first time, short-lived intermediates formed during photolysis were detected. The primary photoprocess is the cleavage of the β-C-C bond relative to the aromatic system with the formation of 4-nitrobenzylalcohol radical and residual aliphatic radical. The first radical in deoxygenated solutions predominantly transforms into para-nitrobenzaldehyde and its secondary photolysis products. In the presence of oxygen, the aromatic radical and para-nitrobenzaldehyde are transformed into para-nitrosobenzoic and para-nitrobenzoic acids as a result of reaction with reactive oxygen species (ROS). Formation of ROS is provoked by reactions of aliphatic radical with dissolved oxygen, so this radical is very important for CAP degradation. The quantum yield of direct photolysis of CAP is ∼3% and does not depend on the presence of dissolved oxygen and on the change of the excitation wavelength in the range of 254–308 nm. Obtained data are important for further understanding of the transformation pathways of CAP and similar PPCP in natural and wastewaters under the action of sunlight and artificial UV radiation.

Photochemical environmental persistence of venlafaxine in an urban water reservoir: A combined experimental and computational investigation

The photochemical behavior of venlafaxine (VNX) in surface water was investigated using a hybrid theoretical-experimental-kinetic modeling approach. In addition to the direct photolysis quantum yield, the rate constants of the reactions between VNX and reactive photo-induced species (hydroxyl radicals, HO·; singlet oxygen, 1O2; and triplet excited states of chromophoric dissolved organic matter, 3CDOM*) in sunlit water were measured under simulated solar radiation using competition kinetics. To elucidate possible degradation products, first-principles calculations were applied, followed by toxicity estimation by quantitative structure-activity relationship (QSAR) calculations. Furthermore, kinetic simulations of VNX persistence in an urban water reservoir of the São Paulo Metropolitan Region, Brazil, were performed. The results indicate low direct photolysis of VNX, with ΦVNX = (1.06 ± 0.18) × 10−2 mol Einstein−1. The measured values of the second-order reaction rate constants are kVNX,3CDOM* = (3.98 ± 0.28) × 109 L mol−1 s−1 and kVNX,1O2 = (2.09 ± 0.17) × 107 L mol−1 s−1, which are lower than kVNX, HO·obs = (6.92 ± 0.37) × 109 L mol−1 s−1 and kVNX, HO·theo = 4.98 × 109 L mol−1 s−1. O-desmethylvenlafaxine, the most thermodynamically stable metabolite predicted by theoretical calculations, revealed potentially greater toxicity and, therefore, higher environmental risk to aquatic ecosystems. The estimated half-lives for VNX range from 9 to 62 days, depending on the season, local weather conditions and water quality parameters, the effects of which can be satisfactorily simulated with the aid of the modified photochemical fate model proposed in this work.

UVC-induced degradation of cilastatin in natural water and treated wastewater

This work reports for the first time the UVC photodegradation of cilastatin, a renal dehydropeptidase inhibitor co-adminstered with the imipenem antibiotic. Initially, solutions of cilastatin at varying concentrations were prepared in ultra-pure water and the direct photolysis of cilastatin was monitored under 254-nm irradiation. Degradation was slower at higher initial cilastatin concentrations, due to absorption saturation. Of the different eluting photoproducts, only one was tentatively identified as oxidized cilastatin bearing a sulfoxide group. UV-254 photolysis occurred faster at lower pH values, because the protonated forms of the molecule (H3A+, H2A) have both higher absorption coefficients and higher photolysis quantum yields than the non-protonated ones (HA−, A2−). The direct photolysis of cilastatin does not involve •OH, as excluded by experiments in which t-butanol was added as •OH scavenger, whereas the presence of humic acids inhibited photolysis due to competition for radiation absorption. The same explanation partially accounts for the observation that the photolysis kinetics of cilastatin was slower in tap water, river water and treated wastewater samples compared to ultra-pure water. Moreover, the direct photolysis quantum yield was also lower in water matrices compared to ultra-pure water. Similar findings reported for triclosan and the herbicide 2-methyl-4-chlorophenoxyacetic acid in previous studies might suggest that the water matrix components could carry out either physical quenching of cilastatin's excited states or back-reduction to cilastatin of the partially oxidized degradation intermediates. Overall, the present results demonstrate that UVC irradiation is a fast and efficient process for the degradation of cilastatin in natural water and treated wastewater.

Phototransformation study of the antidepressant paroxetine in surface waters

In this work, the photochemistry of the antidepressant paroxetine (PXT) and its photochemical fate in surface waters were investigated. The direct photolysis quantum yield, as well as the second-order rate constants of the reactions between PXT and the photochemically produced reactive intermediates (i.e., HO•, 3CDOM* and 1O2) were assessed with steady-state irradiation experiments, while the reaction rate constant with CO3•− was known from the literature. Using these results, the PXT photochemical fate (i.e., pseudo-first order photodegradation rate constant and half-life time) was modelled in a surface-water scenario by varying the chemical composition of water and its depth. Nine transformation products were identified, formed upon PXT direct and indirect photolysis, and a photodegradation pathway was proposed that is initiated by photohydrolysis of PXT to 4-(4-fluorophenyl)-3-(hydroxymethyl)piperidine.The results showed that PXT reacts with HO• radicals approaching a diffusion-controlled kinetics, while the direct photolysis quantum yield is ΦPXT = (4.03 ± 0.25) × 10−2. Direct photolysis, together with reactions with HO• and CO3•− radicals would be the main photodegradation pathways for PXT in surface waters. Reaction with CO3•− is particularly important for low amounts of dissolved organic matter (DOM), high pH and high inorganic carbon, which are for instance typical of surface waters in Slovenia. Finally, PXT reaction with 3CDOM* is little important because it is significantly inhibited in the presence of antioxidant compounds, which reduce the partially oxidized intermediates back to the parent PXT.

A comprehensive dynamic kinetic model for the UVC/H2O2 process: application to zinc bacitracin degradation in wastewater as a case study.

A dynamic kinetic model is presented for the UVC/H2O2-driven process. The model comprises 103 reactions, including background species, such as HCO3-/CO32-, NO2-, NO3-, SO42-, Cl-, and H2PO4-/HPO42/PO43- anions, and effluent organic matter (EfOM) was validated based on experimental data obtained for the photooxidation of the nonribosomal peptide antibiotic zinc bacitracin (Zn-Bc, 34 μmol L-1). The set of ordinary differential equations for 38 species was combined with the molar balances describing the recirculating tubular photoreactor used. Predictions for the photolytic and UVC/H2O2 processes confirmed the good agreement with experimental data, enabling the estimation of fundamental kinetic parameters, such as the direct photolysis quantum yield (Ф254 nm, Zn-Bc = 0.0143 mol Einstein-1) and the second-order rate constants for the reactions of Zn-Bc with HO•, HO2•, and O2•- radicals (2.64 × 109, 1.63 × 103, and 1.49 × 104 L mol-1 s-1, respectively). The predicted optimum process conditions correspond to [H2O2]0 = 6.8 mmol L-1 and a specific photon emission rate of 11.1 × 10-6 Einstein L-1 s-1. Zn-Bc photooxidation was significantly impacted by wastewater constituents, particularly EfOM and HCO3-/CO32- (i.e., alkalinity), resulting in a degradation rate about 32% lower compared to that obtained in deionized water. In particular, EfOM acts as a strong radical scavenger and inner filter. In addition, simulations pointed out the continuous tubular photochemical reactor as the best configuration for treating Zn-Bc-containing wastewater. This study hence provides a comprehensive modeling approach, especially useful for predicting the effect of complex water matrices on the performance of the UVC/H2O2 treatment process.

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*.

Modeling and validation of a modular multi-lamp photo-reactor for cetylpyridinium chloride degradation by UV and UV/H2O2 processes

UV-C assisted disinfection processes are among the well-known tertiary processes for water reuse, wastewater reclamation and domestic water disinfection. However, treated wastewaters normally contain several organic compounds and pollutants that can be partially degraded under UV-C irradiation to different by-products, often more toxic than their parent compounds, and/or, for longer treatment times, completely mineralized. For this reason, kinetic modeling of pollutants degradation in UV-C assisted processes is of crucial importance in plants design. This work aims to the characterization of a reactor configuration consisting of 6 lamps placed following a symmetric hexagonal geometry. Specifically, a mathematical model is proposed to estimate the mean optical length and degradation kinetics for UV-C and UV-C/H2O2 processes. The model has been tested with different reference organic compounds and represents a significant characterization of a modular geometric unit used in a great variety of applications. Finally, the characterized reactor was used to estimate the quantum yield of direct photolysis at 254 nm (Φ254CPC=(8.04±0.36)·10-3mol·ein-1) and kinetic constant of reaction with hydroxyl radical of cetylpyridinium chloride (CPC) ((1.11±0.0778)·109L·mol-1·s-1), an emerging pollutant of wide interest.

Photolysis of atorvastatin in aquatic environment: Influencing factors, products, and pathways

Atorvastatin (ATV), a second generation cholesterol-lowering drug, is detected frequently in natural water because of its extensive use and incomplete removal from wastewater. In this study, the photochemical behavior of ATV under simulated solar irradiation was systematically investigated in order to assess the potential of photolysis as its transformation pathway in aquatic environment. The quantum yield of ATV direct photolysis was determined to be 0.0041. Among various water components investigated, including pH, Suwannee River Fulvic Acid (SRFA), Fe3+, HCO3−, SO42− and NO3−, the major factors contributing to the indirect photolysis of ATV were SRFA and NO3−, and the co-existence of SRFA and NO3− showed no interaction in synthetic water containing the above water components. The results were further verified in natural water samples. Singlet oxygen (1O2) played dominant role in the indirect photolysis of ATV, and the contributions of 1O2 and ·OH to the photolysis of ATV in the solution with optimum combination of water components were calculated to be 67.14% and 0.66%, respectively. Nine phototransformation intermediates were identified by liquid chromatography - time-of-flight - mass spectrometry (LC-TOF-MS), and the degradation pathways were speculated as hydroxyl addition, pyrrole-ring open and debenzamide reactions. In addition, the evolution of products in the degradation process showed that the ring-opened product P416 and hydroxylation product P575 still remained at a certain level after two days of photodegradation, which may accumulate and cause additional ecological risks. This study provides significant information for understanding the risk and fate of ATV in aquatic environment.

Reversible Triplet Excitation Transfer in a Trimethylene-Linked Thioxanthone and Benzothiophene-2-Carboxanilide that Photochemically Expels Leaving Group Anions.

The triplet excited state of thioxanthone produced by photolysis undergoes reversible triplet energy transfer with a trimethylene-linked benzothiophene-2-carboxanilide ring system. The ensuing electrocyclic ring closure of the anilide moiety produces a putative zwitterionic intermediate that is capable of expelling leaving groups (LG-) from the C-3 position of the benzothiophene ring. Stern-Volmer quenching studies with cyclohexadiene as quencher furnish the rate constants for the triplet excitation transfer in the forward and reverse directions, which can be expressed as an equilibrium constant K = 0.058. Overall, the rate of the triplet excited state reaction becomes K × kr = 5.7 × 104 s-1 for LG- = Cl-, where kr is the triplet decay rate of the C-3 chloro-substituted benzothiophene-2-carboxanilide, found through Stern-Volmer quenching. The high quantum efficiencies found for the trimethylene-linked systems are due to K × kr being competitive with the triplet excited state decay of the thioxanthone of kd = 7.7 × 104 s-1. On the basis of Φisc = 0.68, the overall expected quantum yield for direct photolysis should be 0.50 for LG- = Cl- as compared to 0.41 at 25 °C experimentally. Φ decreases with increasing basicity of the leaving group (LG- = Cl-, (EtO)2PO2-, PhCH2CO2-, PhS-, and PhCH2S-).

Direct photolysis of oxytetracycline: Influence of initial concentration, pH and temperature

Abstract Oxytetracycline (OTC) commonly undergoes direct photolysis as the main elimination pathway in surface waters. The photolysis of OTC was studied as a function of initial concentration, pH and temperature. The experiments were performed in the absence of oxygen to avoid oxygen induced indirect photolysis. The photolysis rate decreased when increasing the initial OTC concentration, and was accelerated as raising the temperature. pH was the key factor controlling the photolysis rate. By measuring the UV–vis absorption spectra of OTC under various pH conditions and fitting them with global analysis, four OTC species H3L＋, H2L±, HL−, and L2− (L represents OTC) were decomposed, meanwhile, the acid dissociation constant of OTC and the molar absorption coefficient of each species were resolved. Because apparent photolysis rate constants under varied light source conditions are often incomparable, therefore the direct photolysis quantum yield of OTC species were calculated by a mathematical approach which based on the molar absorption coefficients, the species composition and apparent kinetic rates of OTC under experimental pH conditions. The result shows the photolysis quantum yield of OTC followed the order L2－ > H2L± > HL－ > H3L＋. The direct photolysis rates of OTC were tightly correlated to the energy dissipation processes at the excited states, e.g., the excited states inter-/intramolecular proton transfer (ESIPT), and the molecular conformation transfer induced internal energy transmission between ground state and excited state. Fluorescence emission was not an important energy dissipation process. The results of this study provide the useful parameters for predicting the environmental fate of OTC in surface waters. And the approach for calculating the photolysis quantum yield is also available for other antibiotics with multiple pKa values.

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.

Photodegradation of Mefenamic Acid in Aqueous Media: Kinetics, Toxicity and Photolysis Products.

The present study investigated the photolytic behavior and photodegradation products of mefenamic acid (MEF) under ultraviolet-C irradiation. The results demonstrated that the photodegradation of MEF followed pseudo-first-order kinetics and the direct photolysis quantum yield of mefenamic acid was observed to be 2.63 ± 0.28 × 10⁻³. Photodegradation of MEF included degradation by direct photolysis and by self-sensitization that the contribution rates of self-sensitized photodegradation were 5.70, 11.25 and 18.96 % for ·OH, ¹O₂ and O·₂⁻ , respectively. Primary transformation products of MEF were identified using ultra performance liquid chromatography and quadrupole time-of-flight mass spectrometer (UPLC-Q-TOF-MS). The identified transformation products suggested three possible pathways of MEF photodegradation: dehydrogenation, hydroxylation, and ketonized reactions. Toxicity of phototransformation products were evaluated using the Microtox test, which revealed that photodegradation likely provides a critical pathway for MEF toxicity reduction in drinking water and wastewater treatment facilities.

Photochemical degradation of imazosulfuron under simulated California rice field conditions.

The photodegradation of imazosulfuron (IMZ), a potent broad-spectrum herbicide, was investigated under simulated rice field conditions. Previous reports have indicated that it is photolabile, but have failed to report radiation intensity or determine a quantum yield, precluding extrapolation to environmental rates. Therefore, the objective of this investigation was to determine the photolytic rate of IMZ under simulated rice field conditions and how it is influenced by environmental factors such as turbidity, salinity and temperature. IMZ was efficiently photolyzed in all solutions and fitted pseudo-first-order kinetics. Degradation was faster in HPLC-grade water than in field water. Field-relevant variances in temperature, turbidity and salinity did not significantly influence degradation. The experimentally derived quantum yield for direct photolysis (2.94 × 10(3) ) was used to predict the half-life of IMZ in a California rice field (3.6 days). Aqueous photolysis is predicted to be an important process in the overall degradation of IMZ in the environment, regardless of variances in salinity, organic matter and temperature. Based on the predicted half-life of IMZ in a California rice field (3.6 days), state-mandated holding periods for field water post-IMZ application (30 days) are expected to allow for sufficient clearance of the herbicide (>98%), preventing significant contamination of the environment upon release of tailwater. © 2015 Society of Chemical Industry.

The role of direct photolysis and indirect photochemistry in the environmental fate of ethylhexyl methoxy cinnamate (EHMC) in surface waters

The aquatic environmental fate of ethylhexyl methoxy cinnamate (EHMC), one of the most used UVB filters worldwide, was studied by assessing its environmental persistence and photoinduced transformations. The role of direct and indirect photolysis was evaluated. Direct photolysis was shown to play a key role, and this process is expected to be the main attenuation route of EHMC in sunlit surface waters. In contrast, the reaction with OH radicals would be negligible and that with 3CDOM* would at most be a secondary process.The measurement of the quantum yield of direct photolysis and of the rate constants of reaction with photogenerated transient species (or, sometimes, the use of reasonable values for the latter) allowed the prediction of the EHMC half-life time in surface waters, by means of a validated photochemical model. The predicted EHMC lifetime is of the order of hours to a few days in fair-weather summertime, and the main factors controlling the EHMC phototransformation in sunlit surface waters would be the water depth and the dissolved organic carbon (DOC) content.The formation of transformation products (TPs) was followed as well via HPLC/HRMS. Three TPs were detected in the samples exposed to UVA radiation, while one additional TP was detected in the samples exposed to UVB radiation. The detected TPs comprised 4-methoxybenzaldehyde, a hydroxylated derivative and dimeric species. Through the use of heterogeneous photocatalysis with TiO2, seven additional TPs were identified, most of them resulting from the further degradation of primary TPs formed through direct photolysis and that might be detected in aquatic systems as well. The photodegradation of EHMC in the presence of TiO2 yielded more toxic TPs than the parent compound (as determined with the Vibrio fischeri Microtox assay). The increased toxicity is partially accounted for by the formation of 4-methoxybenzaldehyde.

Direct photolysis of benzoylecgonine under UV irradiation at 254 nm in a continuous flow microcapillary array photoreactor

Benzoylecgonine (BE) is the major metabolite of cocaine and a contaminant of emerging concern often detected in sewage treatment plant (STP) effluents and surface waters. In this study, an innovative microcapillary film (MCF) array photoreactor made of fluoropolymer material was used to determine the direct photolysis quantum yield at 254nm of benzoylecgonine. The photolysis quantum yield of BE was found to be (6.22±0.19)⋅10−3molein−1. The proposed methodology was validated by estimating the quantum yield of caffeine (7.48±0.64)⋅10−4molein−1, which was found in agreement with results published in literature. The MCF uses a very small sample volume (in the order of 330μL per meter length of material) and allows extremely rapid photolysis with a short contact time ranging from a fraction of seconds to few minutes. The novel microfluidic-based approach presented in this study is particularly useful for determining the photochemical behavior of highly priced pharmaceuticals, illicit drugs, metabolites and uncommon or regulated substances.

Degradation of florfenicol in water by UV/Na2S2O8 process

UV irradiation-activated sodium persulfate (UV/PS) was studied to degrade florfenicol (FLO), a phenicol antibiotic commonly used in aquaculture, in water. Compared with UV/H2O2 process, UV/PS process achieves a higher FLO degradation efficiency, greater mineralization, and less cost. The quantum yield for direct photolysis of FLO and the second-order rate constant of FLO with sulfate radicals were determined. The effects of various factors, namely PS concentration, anions (NO3 (-), Cl(-), and HCO3 (-)), ferrous ion, and humic acid (HA), on FLO degradation were investigated. The results showed that the pseudo-first-order rate constant increased linearly with increased PS concentration. The tested anions all adversely affected FLO degradation performance with the order of HCO3 (-) > Cl(-) > NO3 (-). Coexisting ferrous ions enhanced FLO degradation at a Fe(2+)/PS molar ratio under 1:1. HA significantly inhibited FLO degradation due to radical scavenging and light-screening effect. Toxicity assessment showed that it is capable of controlling the toxicity for FLO degradation. These findings indicated that UV/PS is a promising technology for water polluted by antibiotics, and the treatment is optimized only after the impacts of water characteristics are carefully considered.

APEX (Aqueous Photochemistry of Environmentally occurring Xenobiotics): a free software tool to predict the kinetics of photochemical processes in surface waters

The APEX software predicts the photochemical transformation kinetics of xenobiotics in surface waters as a function of: photoreactivity parameters (direct photolysis quantum yield and second-order reaction rate constants with transient species, namely ˙OH, CO₃(-)˙, (1)O₂ and the triplet states of chromophoric dissolved organic matter, (3)CDOM*), water chemistry (nitrate, nitrite, bicarbonate, carbonate, bromide and dissolved organic carbon, DOC), and water depth (more specifically, the optical path length of sunlight in water). It applies to well-mixed surface water layers, including the epilimnion of stratified lakes, and the output data are average values over the considered water column. Based on intermediate formation yields from the parent compound via the different photochemical pathways, the software can also predict intermediate formation kinetics and overall yield. APEX is based on a photochemical model that has been validated against available field data of pollutant phototransformation, with good agreement between model predictions and field results. The APEX software makes allowance for different levels of knowledge of a photochemical system. For instance, the absorption spectrum of surface water can be used if known, or otherwise it can be modelled from the values of DOC. Also the direct photolysis quantum yield can be entered as a detailed wavelength trend, as a single value (constant or average), or it can be defined as a variable if unknown. APEX is based on the free software Octave. Additional applications are provided within APEX to assess the σ-level uncertainty of the results and the seasonal trend of photochemical processes.

A critical assessment of the photodegradation of pharmaceuticals in aquatic environments: defining our current understanding and identifying knowledge gaps

This work presents a critical assessment of the state and quality of knowledge around the aquatic photochemistry of human- and veterinary-use pharmaceuticals from laboratory experiments and field observations. A standardized scoring rubric was used to assess relevant studies within four categories: experimental design, laboratory-based direct and indirect photolysis, and field/solar photolysis. Specific metrics for each category are defined to evaluate various aspects of experimental design (e.g., higher scores are given for more appropriate characterization of light source wavelength distribution). This weight of evidence-style approach allowed for identification of knowledge strengths and gaps covering three areas: first, the general extent of photochemical data for specific pharmaceuticals and classes; second, the overall quality of existing data (i.e., strong versus weak); and finally, trends in the photochemistry research around these specific compounds, e.g. the observation of specific and consistent oversights in experimental design. In general, those drugs that were most studied also had relatively good quality data. The four pharmaceuticals studied experimentally at least ten times in the literature had average total scores (lab and field combined) of ≥29, considered decent quality; carbamazepine (13 studies; average score of 31), diclofenac (12 studies; average score of 31), sulfamethoxazole (11 studies; average score of 34), and propranolol (11 studies; average score of 29). Major oversights and errors in data reporting and/or experimental design included: lack of measurement and reporting of incident light source intensity, lack of appropriate controls, use of organic co-solvents in irradiation solutions, and failure to consider solution pH. Consequently, a number of these experimental parameters were likely a cause of inconsistent measurements of direct photolysis rate constants and quantum yields, two photochemical properties that were highly variable in the literature. Overall, the assessment rubric provides an objective and scientifically-defensible set of metrics for assessing the quality of a study. A major recommendation is the development of a method guideline, based on this rubric, for conducting and reporting on photochemical studies that would produce consistent and reliable data for quantitative comparison across studies. Furthermore, an emphasis should be placed on conducting more dual-fate studies involving controlled photolysis experiments in natural sunlight, and whole system fate studies in either natural or artificial systems. This would provide accurate data describing the actual contribution of photolysis to the overall fate of pharmaceuticals in the environment.

Photodegradation routes of the herbicide bromoxynil in solution and sorbed on silica nanoparticles

Some organic contaminants dissolved in natural waters tend to adsorb on suspended particles and sediments. In order to mimic the photodegradation routes in natural waters of bromoxynil (BXN) adsorbed on silica, we here prepare and characterize silica nanoparticles modified with BXN (NP-BXN). We measure the direct photolysis quantum yield of aqueous BXN at 307 nm (0.064 ± 0.001) and detect the formation of bromide ions as a reaction product. Under similar conditions the photolysis quantum yield of BXN bonded to NP-BXN is much lower (0.0021 ± 0.0004) and does not lead to formation of bromide ions. The rate constant of the reaction of NP-BXN with the excited triplet states of riboflavin, a molecule employed as a proxy of chromophore dissolved organic matter (DOM) was measured in laser flash-photolysis experiments. The rate constants for the overall (kt) and chemical interaction (kr) of singlet oxygen with NP-BXN were also measured. Kinetic computer simulations show that the relevance of the direct and indirect (through reactions with reactive species generated in photoinduced processes) photodegradation routes of BXN is very much affected by sorption on silica. Immobilization of the herbicide on the particles, on one hand, affects the photolysis mechanism and lowers its photolysis quantum yield. On the other hand, the results obtained in aqueous suspensions indicate that immobilization also lowers the rate of collisional encounter, which affects the quenching rate constants of excited triplet states and singlet oxygen with the herbicide.

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

The sunlight filter benzophenone-4 (BP-4) is present in surface waters as two prevailing forms, the singly deprotonated (HA−) and the doubly deprotonated one (A2−), with pKa2 = 7.30 ± 0.14 (μ ± σ, by dissociation of the phenolic group). In freshwater environments, BP-4 would mainly undergo degradation by reaction with OH and direct photolysis. The form HA− has a second-order reaction rate constant with OH (kOH) of (1.87 ± 0.31)·1010 M−1 s−1 and direct photolysis quantum yield Φ equal to (3.2 ± 0.6)·10−5. The form A2− has (8.46 ± 0.24)·109 M−1 s−1 as the reaction rate constant with OH and (7.0 ± 1.3)·10−5 as the photolysis quantum yield. The direct photolysis of HA− likely proceeds via homolytic breaking of the O–H bond of the phenolic group to give the corresponding phenoxy radical, as suggested by laser flash photolysis experiments. Photochemical modelling shows that because of more efficient direct photolysis (due to both higher sunlight absorption and higher photolysis quantum yield), the A2− form can be degraded up to 3 times faster than HA− in surface waters. An exception is represented by low-DOC (dissolved organic carbon) conditions, where the OH reaction dominates degradation and the transformation kinetics of HA− is faster compared to A2−. The half-life time of BP-4 in mid-latitude summertime would be in the range of days to weeks, depending on the environmental conditions. BP-4 also reacts with Br2−, and a rate constant kBr2−,BP−4=(8.05±1.33)·108M−1s−1 was measured at pH 7.5. Model results show that reaction with Br2− could be a potentially important transformation pathway of BP-4 in bromide-rich (e.g. seawater) and DOM-rich environments.