Long-term phototransformation of microplastics under simulated sunlight irradiation in aquatic environments: Roles of reactive oxygen species

Microplastics may experience photoaging and breakdown into nanoplastics in aquatic environment as a result of long-term light irradiation. However, the underlying mechanisms responsible for the photodegradation of microplastics are largely overlooked. In this study, the photodegradation of microplastics, utilizing polystyrene microplastic (PS-MP) as a model, was investigated under irradiation with simulated solar light for as long as 150 d. A large amount of reactive oxygen species (ROS), including O₂^•-, ¹O₂, H₂O₂ and •OH, were detected in the PS-MP suspension due to light irradiation, which displayed significant relationships with the generated environmentally persistent free radicals (EPFRs). Distinct photoaging of PS-MP was observed with increased surface roughness and decreased particle size. However, these photoaging effects were significantly inhibited by ROS quenchers, suggesting that the generation ROS played a vital role in the PS-MP phototransformation. In addition, ROS induced formation of more oxidative functional groups on the PS-MP, thus enhancing the negative surface potential and the stability of PS-MP in water. This study elucidated the mechanism of formation of ROS by simulated solar light irradiated MPs and their subsequent roles in the phototransformation of MP, thus expanding current knowledge on the fate of MPs in aquatic environments.

Sunlight-induced transformation of sulfadiazine and sulfamethoxazole in surface waters and wastewater effluents

Sulfadiazine (SD) and sulfamethoxazole (SMX) are widely used sulfonamide antibiotics, which are present as contaminants in surface waters and are known to undergo phototransformation. This kinetic study was conducted to identify the processes responsible for their phototransformation in sunlit surface waters. Water samples from the Thur River (Switzerland) and from a pilot wastewater treatment plant, as well as aqueous solutions of two well-characterized natural dissolved organic matter (DOM) extracts, namely Suwannee River and Pony Lake fulvic acids (SRFA, PLFA), were examined. Both sulfonamides were found to undergo direct and indirect phototransformation, with contributions of excited triplet states of DOM and of effluent organic matter (EfOM) and possibly of hydroxyl radical and other unidentified reactive species. Under simulated sunlight, SMX mainly reacted through direct phototransformation, with a certain contribution of indirect phototransformation occurring for a wastewater effluent. The behavior of SD was found to be more diverse. For river waters, wastewater effluents and PLFA solutions, indirect phototransformation was predominant, while for SRFA solutions direct phototransformation prevailed. The rates of phototransformation of SD were interpreted as the result of a complex interplay between the photosensitizing and the inhibitory effect of DOM/EfOM, with an additional component related to the nitrite ion as a source of photoproduced hydroxyl radical. For typical conditions found in surface waters comparable to the Thur River, phototransformation half-lives on the order of 3-13 d were estimated for the two studied sulfonamides.

Photochemical behavior of benzophenone sunscreens induced by nitrate in aquatic environments

Benzophenones (BPs), which are widely used UV filters, have aroused considerable public concern owing to their potential endocrine-disrupting activities. Herein, we systematically investigated their photochemical behavior and fate, which is mediated by nitrate in aquatic environments. The results showed that 10 μM of 3 BPs can be completely degraded within 4 h of simulated sunlight irradiation in a 10 mM nitrate solution at pH 8.0, and 2,4-dihydroxybenzophenone (BP-1) has a 31.6% mineralization rate after 12 h irradiation. Their photolytic rates (k_obs) presented a significant linear correlation with the logarithmic values of the nitrate concentration for 0.1-10 mM (R² > 0.98), and in three actual waters, the rates of BP-1 were also positively related to the intrinsic nitrate concentration. Furthermore, higher transformation rates under alkaline condition were observed, especially for BP-1, with its k_obs at pH 10 being 8.3-fold higher than that at pH 6.0. Moreover, dissolved oxygen (DO) also has an impact on the reaction kinetics to some degree. According to the quenching experiments, we found that three reactive oxygen species (ROS), namely, •OH, •NO, and •NO₂, participated in this photolysis of BPs, and the contribution of •OH accounted for 32.1%. Furthermore, we selected BP-1 as the model molecule to study the transformation pathways and toxicity changes in this system. Four main transformation pathways including hydroxylation, nitrosylation, nitration, and dimerization were proposed, based on liquid chromatography quadrupole time-of-flight mass spectrometry (LC-Q-TOF-MS) analysis and density functional theory (DFT). According to the toxicity test, the formed intermediates were more toxic to Photobacterium phosphoreum than the parent BP-1. Therefore, these results can help reveal primary phototransformation mechanisms and evaluate the potential ecological risks of BPs in aquatic environments.

Phototransformation of halophenolic disinfection byproducts in receiving seawater: Kinetics, products, and toxicity

Flushing toilet with seawater is an effective method for preserving freshwater resources, but it introduces iodide and bromide ions into domestic wastewater. During chlorine disinfection, iodide and bromide ions in the saline wastewater effluent lead to the formation of iodinated and brominated aromatic disinfection byproducts (DBPs). Examples of aromatic DBPs include iodophenolic, bromophenolic and chlorophenolic compounds, which generally display substantially higher toxicity than haloaliphatic DBPs. This paper presented for the first time the rates of phototransformation of 21 newly identified halophenolic DBPs in seawater, the receiving waterbody of the wastewater effluent. The phototransformation rate constants (k) were in the range from 7.75 × 10^-4 to 4.62 × 10^-1 h^-1, which gave half-lives of 1.5-895 h. A quantitative structure-activity relationship was established for the phototransformation of halophenolic DBPs as logk=-0.0100×ΔG_f⁰+5.7528×logMW+0.3686×pKa-19.1607, where ΔG_f⁰ is standard Gibbs formation energy, MW is molecular weight, and pK_a is dissociation constant. This model well predicted the k values of halophenolic DBPs. Among the tested DBPs, 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol were found to exhibit relatively high risks on marine organisms, based on toxicity indices and half-lives. In seawater, the two DBPs underwent photonucleophilic substitutions by bromide, chloride and hydroxide ions, resulting in the conversion to their bromophenolic and chlorophenolic counterparts (which are less toxic than the parent iodophenolic DBPs) and to their hydroxyphenolic counterparts (iodo(hydro)quinones, which are more toxic than the parent iodophenolic DBPs). The formed iodo(hydro)quinones further transformed to hydroxyl-iodo(hydro)quinones, which have lower toxicity than the parent compounds.

Is the phototransformation of pharmaceuticals a natural purification process that decreases ecological and human health risks?

Sunlight photodegradation has long been considered a significant process in lowering the concentrations of pharmaceuticals in surface waters and thus decreasing the ecological risk. For the first time, this study identified the significance of investigating the environmental photodegradation of a pharmaceutical residue mixture (rather than a single compound) and the associated toxicity of transformation byproducts in environmental waters, including rivers, hospital wastewaters, and effluents from wastewater treatment plants and pharmaceutical production facilities. Pharmaceuticals undergo phototransformation rather than mineralization (11-23% in 34 h). Pharmaceutical mixtures could possibly act as dissolved organic matter for each individual compound and subsequently affect the photolysis rates. The increased toxicity of irradiated pharmaceutical mixtures challenges the validity of the current understanding of sunlight photolysis. The implications of this work suggest that current knowledge concerning the occurrence, natural attenuation, ecotoxicity, and human health risks of pharmaceuticals is far from complete; photolysis is not necessarily a purification process.

Sunlight and UVC-254 irradiation induced photodegradation of organophosphorus pesticide dichlorvos in aqueous matrices

Dichlorvos (DDVP) is an organophosphorus pesticide that has been classified as highly hazardous chemical by the World Health organization. In this study, the fate of the pesticide DDVP in natural water compartments was examined under simulated sunlight. Moreover, the effect of UV-254 irradiation on DDVP depletion was also studied. In deionized water, DDVP was photodegraded only in the presence of dissolved molecular oxygen. The photodegradation during the first 6 h of sunlight irradiation occurred with pseudo first-order kinetics, and the rate constants were 0.040 h^-1 at pH 7 and 0.064 h^-1 at pH 3. A reaction mechanism for the generation of reactive oxygen species (ROS) via DDVP photoabsorption was proposed. Humic acids (HA) played a double role as photosensitizer and inhibitor, observing an enhancement on DDVP photodegradation at low HA concentration (TOC = 2 mg L^-1). The depletion of DDVP under 254 nm UV irradiation was ascribed to direct photodegradation and oxygen mediated photoinduced reactions. Direct photodegradation of DDVP decreased with 254 nm irradiation reduction, highlighting the importance of radical mediated mechanisms at low irradiation doses. Based on LC/MS data, the main photoproducts under simulated solar light and UV-C irradiation were identified and potential reaction pathways were postulated. The three main identified products were o-methyl 2,2-dichlorovinyl phosphate, dichloroacetaldehyde and dimethylphosphate. Moreover, the toxicity of samples was evaluated along the irradiation exposure time using Microtox® assays. This study brings new insights into the role of oxygen in the photodegradation of DDVP and the induced and inhibition mechanisms involved in the presence of the humic acids in natural waters.

Phototransformation of the red-light-absorbing form of undegraded pea phytochrome by laser flash excitation

The phototransformation of undegraded pea phytochrome, which was estimated to be about 30 to 40% pure, from its red-light-absorbing form to its far-red-light-absorbing form was examined at 2-4°C after laser flash excitation at 655 nm using a custom-designed multichannel transient spactra analyzer. A difference spectrum measured 10 μsec after the flash showed an absorbance increase at 692 nm, a decrease at 660 nm and a slight decrease at 604 nm. With time in darkness, the peak at 692 nm gradually decreased in magnitude and shifted to 695 nm. A decay curve of absorbance at 692 nm between 10 and 500 μsec after the flash could be resolved into two reaction components, one proceeding with a rate constant of 46,000 sec(-1) and the other with one of 2,500 sec(-1). The faster component has not been reported previously. Difference spectra also indicated that a small but significant increase in absorbance between 400 and 410 nm and decrease around 360 nm took place 10-260 μsec after the flash.

Mechanism and kinetics of photochemical transformation of ketoprofen and its degradation intermediates

Ketoprofen, 2-(3-benzoylphenyl)-propionic acid, a widely used non-steroidal anti-inflammatory drug, is considered as an important water pollutant. Kinetics and mechanism of its photolytic transformation in aqueous solutions was studied experimentally and partial reaction steps were modelled by means of quantum chemistry methods. While the rate of ketoprofen photolysis was not significantly affected by its acid-base equilibrium, a marked influence of pH on the subsequent degradation reactions was observed. At pH 1.3, two oxygenated primary products were identified, that underwent fast photolysis. Deprotonated form of ketoprofen was transformed preferentially to ethylbenzophenone and further degradation proceeded substantially slower. Oxygen participated on photolytic processes both as a reactant and the triplet state quencher. The active involvement of water molecules in the reaction mechanism was investigated by comparative experiments in acetonitrile. The phototransformation mechanism proposed based on the experimental data corresponded well with the theoretical results.

Environmental fate processes of antimicrobial peptides daptomycin, bacitracins, and polymyxins

Antimicrobial peptides (AMPs) are increasingly important as a last resort against multi-drug resistant bacteria due to resistance formation towards conventional antibiotics. However, many AMPs were introduced to the market before environmental risk assessment was required, e.g., by the European Medicines Agency (EMA) since 1998. While AMPs have been administered as antibiotics and growth promotors in feedstock since the 1960s and were reconsidered for human medicine by the EMA in 2013, details about their mobility and persistence in the environment remain unknown. This study investigated the environmental fate of three commonly used AMPs: bacitracins, daptomycin, and polymyxins B and E (Colistin). We observed moderate sorption affinity of daptomycin to standard European soils (K_d = 20.6-48.6), while polymyxins adsorbed irreversibly. Bacitracin variants sorbed slightly to sandy soil (K_d = 5.8-8) and significantly to clayey soil (K_d = 169-250). We further investigated photochemical and microbial transformation processes relevant in surface waters. We demonstrated that phototransformation of all AMPs was enhanced in the presence of dissolved organic matter and fast bimolecular reaction rate constant with singlet oxygen contributed largely to indirect phototransformation (15-41%). Phototransformation product analysis for daptomycin was consistent with expected modifications of the tryptophan and kynurenine moieties. Moreover, riverine biofilm communities demonstrated biotransformation potential for all AMPs. Our findings of sorption behaviour, photo- and biotransformation suggest that these processes play a critical role in the fate of bacitracins, daptomycin, and polymyxins in environmental systems.

What happens with organic micropollutants during UV disinfection in WWTPs? A global perspective from laboratory to full-scale

The phototransformation of 18 organic micropollutants (OMPs) commonly detected in wastewater treatment plant (WWTP) effluents was examined attempting to explain their fate during UV disinfection in WWTPs. For this purpose, a lab-scale UV reactor (lamp emitting at 254nm) was used to study the influence of the operational conditions (UV dose, temperature and water matrix) on OMPs abatement and disinfection efficiency. Chemical properties of OMPs and the quality of treated effluent were identified as key factors affecting the phototransformation rate of these compounds. Sampling campaigns were carried out at the inlet and outlet of UV systems of three WWTPs, and the results evidenced that only the most photosensitive compounds, such as sulfamethoxazole and diclofenac, are eliminated. Therefore, despite UV treatment is an effective technology to phototransform OMPs, the UV doses typically applied for disinfection (10-50mJ/cm²) are not sufficient to remove them. Consequently, small modifications (increase of UV dose, use of catalysts) should be applied in WWTPs to enhance the abatement of OMPs in UV systems.

Photolytic degradation of ciprofloxacin in solid and aqueous environments: kinetics, phototransformation pathways, and byproducts

Many lipophilic pharmaceuticals may be sorbed in solid phases, leading to different photochemical behaviors. This study investigated the photochemistry of ciprofloxacin in a solid-phase system and compared it to that in a water-phase system. Kaolinite was used as the model solid matrix. The photolysis of ciprofloxacin in kaolinite fits pseudo-first-order kinetics for thicknesses less than 199 μm, and the rate constants k _p decreased from 0.0154 to 0.0016 min^-1 as the thickness of the layer increased. Unlike the aqueous phase, two-step degradation processes were observed for all kaolinite layer thicknesses (14-199 μm), and the pseudo-first-order constant at the surface of the kaolinite layer was smaller than that in the water phase. Comparatively , a similar photolysis rate constant of ciprofloxacin in a kaolinite suspension was also observed, and it was an order of magnitude smaller than that of the direct photodegradation (0.035 min^-1) in water. The results indicate that ciprofloxacin is likely more stable when it is adsorbed on kaolinite and that the half-lives of ciprofloxacin in kaolinite and a kaolinite suspension are 2-25 times longer than that in deionized water (20 min) under simulated sunlight. Direct photolysis is proposed to be the main photodegradation mechanism for ciprofloxacin in kaolinite, and the cleavage of a piperazine ring is the main degradation pathway. However, the interaction between ciprofloxacin and kaolinite reduces the direct photolysis and leads to a higher light stability. In association with the reduction in photolysis, the yields of norfloxacin and defluorinated byproduct decreased significantly. Consequently, the interaction increases the persistence of ciprofloxacin and thus the ecological risk to the environment.

Photodegradation of sulfadiazine in different aquatic environments - Evaluation of influencing factors

The presence of antibiotics, such as sulfadiazine (SDZ), in the aquatic environment contributes to the generation of antimicrobial resistance, which is a matter of great concern. Photolysis is known to be a major degradation pathway for SDZ in surface waters. Therefore, influencing factors affecting SDZ photodegradation in different aquatic environments were here evaluated in order to have a better knowledge about its persistence in the environment. Photodegradation of SDZ was found to be more efficient at higher pH (t_1/2 = 6.76 h, at pH = 7.3; t_1/2 = 12.2 h, at pH = 6.3), in the presence of humic substances (HS) (t_1/2 between 1.76 and 2.42 h), as well as in the presence of NaCl (t_1/2 = 1.00 h) or synthetic sea salts (t_1/2 = 0.78 h). Using ˙OH and ¹O₂ scavengers, it was possible to infer that direct photolysis was the main pathway for SDZ photodegradation in ultrapure water. Furthermore, results under N₂ purging confirmed that ¹O₂ was not relevant in the phototransformation of SDZ. Then, the referred observations were used for the interpretation of results obtained in environmental matrices, namely the final effluent of a sewage treatment plant (STPF), fresh and brackish water (t_1/2 between 2.3 and 3.48 h), in which SDZ photodegradation was found to be much faster than in ultrapure water (t_1/2 = 6.76 h).

Photochemical oxidation of phenols and anilines mediated by phenoxyl radicals in aqueous solution

Reactive intermediates formed upon irradiation of chromophoric dissolved organic matter (CDOM) contribute to the degradation of various organic contaminants in surface waters. Besides well-studied "short-lived" photooxidants, such as triplet state CDOM (3CDOM*) or singlet oxygen, CDOM-derived "long-lived" photooxidants (LLPO) have been suggested as key players in the transformation of electron-rich contaminants. LLPO were hypothesized to mainly consist of phenoxyl radicals derived from phenolic moieties in the CDOM. To test this hypothesis and to better characterize LLPO, the transformation kinetics of selected target compounds (phenols and anilines) induced by a suite of electron-poor model phenoxyl radicals was studied in aerated aqueous solution at pH 8. The phenoxyl radicals were generated by photosensitized oxidation of the parent phenols using aromatic ketones as photosensitizers. Under steady-state irradiation, the presence of any of the electron-poor phenols lead to an enhanced abatement of the phenolic target compounds (at an initial concentration of 1.0 × 10-7 M) compared to solutions containing the photosensitizer but no electron-poor phenol. A trend of increasing reactivity with increasing one-electron reduction potential of the electron-poor phenoxyl radical (range: 0.85‒1.12 V vs. standard hydrogen electrode) was observed. Using the excited triplet state of 2-acetonaphthone as a selective oxidant for phenols, it was observed that the reactivity correlated with the concentration of electron-poor phenoxide present in solution. The rates of transformation of anilines induced by the 4-cyanophenoxyl radical were an order of magnitude smaller than for the phenolic target compounds. This was interpreted as a reduction of the radical intermediates back to the parent compound by the superoxide radical anion. Laser flash photolysis measurements confirmed the formation of the 4-cyanophenoxyl radical in solutions containing 2-acetonaphthone and 4-cyanophenol, and yielded values of (2.6 - 5.3) × 108 M-1 s-1 for the second-order rate constant for the reaction of this radical with 2,4,6-trimethylphenol. These and further results indicate that electron-poor model phenoxyl radicals generated through photosensitized oxidation are useful models to understand the photoreactivity of LLPO as part of the CDOM.

Insights into the transport of pristine and photoaged graphene oxide-hematite nanohybrids in saturated porous media: Impacts of XDLVO interactions and surface roughness

The transport behaviors of nanomaterials, in especial multifunctional nanohybrids have not been well disclosed until now. In this study, environmentally relevant conditions, including cation types, ionic strength and pH, were selected to investigate the transport and retention of graphene oxide-hematite (GO-Fe₂O₃) nanohybrids and a photoaged product in saturated sandy columns. Results show that more hybridization of hematite led to decreased negative surface charge, while increased particle size and hydrophobicity of the nanohybrids, which depressed their transport according to extented Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. However, the inhibitory transport of photoaged nanohybrids was attributed to their distinct surface roughness caused by relatively high hybridization and photoirradiation. Notably the restrained transport was alleviated in the CaCl₂ saturated media, since the less surface O-functional groups of the corresponding nanohybrids reduced the cation bridging effect caused by Ca²⁺. Similarly, increasing pH promoted the transport of the nanohybrids in NaCl saturated media, particularly for the nanohybrids that contained rich O-functional groups, but exerted inconspicuous effect on mobility of the nanohybrids in CaCl₂ saturated media. These observations highlight that both XDLVO interactions and surface roughness may work together to impact the transport and fate of the burgeoning, versatile nanohybrids in the environment.

Phototransformation of roxithromycin in the presence of dissolved organic matter: Characteriazation of the degradation products and toxicity evaluation

Roxithromycin (ROX) is a widely used macrolide antibiotic and its environmental fate and ecotoxicity have attracted considerable attention. In this study, the phototransformation kinetics and products of ROX under the irradiation of simulated sunlight were investigated. The ecotoxicity of ROX before and after phototransformation were also examined using the bioluminescence bioassay and algae growth inhibition test. The results showed that ROX underwent direct photolysis and indirect photolysis in the presence of Suwannee River humic acid (SRHA) and Suwannee River natural organic matter (SRNOM). The kinetic rate constant of the photodegradation of ROX in the presence of 20 mg·L^-1 SRHA and SRNOM were 4.0 and 3.6 times higher than direct photolysis in the absence of dissolved organic matter (DOM). A total of 20 phototransformation products (PTPs) formed as a result of the photodegradation of ROX by simulated solar irradiation were identified, and 10 of them were reported for the first time. The PTPs were generally formed through the N-demethylation, O-demethylation or direct cleavage of the side chain, desosamine or cladinose moiety from ROX. Solutions containing ROX and its PTPs showed an increased toxicity to Vibrio fischeri, demonstrating some PTPs were more toxic to V. fischeri. On the other hand, the toxicity of ROX after irradiation to Chlorella pyrenoidosa decreased, suggesting the phototransformation of ROX in the environment may be a positive outcome in the context of the growth of green algae.

Insight into phototransformation mechanism and toxicity evolution of novel and legacy brominated flame retardants in water: A comparative analysis

The novel brominated flame retardants (NBFRs) have become widespread as a consequence of the prohibition on the use of polybrominated diphenyl ethers (PBDEs). However, the transformation mechanism and potential environmental risk are largely unclear. In this study, we have explored the phototransformation behavior of the most abundant NBFRs, 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE) in water under ultraviolet (UV) irradiation. Meanwhile, the legacy 2,2',4,4',6,6'-hexabromodiphenyl ether (BDE155) with similar structure was investigated contrastively. Results show that novel BTBPE is more persistent than legacy BDE155, with nearly four times slower photodegradation rate constants (0.0120 min^-1and 0.0447 min^-1, respectively). 18 products are identified in the phototransformation of BTBPE. Different from the only debrominated products formed in legacy BDE155 transformation, the ether bond cleavage photoproducts (e.g. bromophenols) are also identified in novel BTBPE transformation. Compound-specific stable isotope analysis (CSIA) confirms the phototransformation mechanism is mainly via debromination accompanying with the breaking of ether bond. Computational toxicity assessment implies that transformation products of BTBPE still have the high kidney risks. Especially the bromophenols formed via the ether bond cleavage could significantly increase the health effects on skin irritation. This study emphasizes the importance of understanding the photolytic behavior and potential risks of novel NBFRs and other structurally similar analogues.

Photochemical transformation of zearalenone in aqueous solutions under simulated solar irradiation: Kinetics and influence of water constituents

The presence of estrogenic mycotoxins, such as zearalenone (ZEN), in surface waters is an emerging environmental issue. Little is known about its phototransformation behavior, which may influence its environmental fate. In this context, the phototransformation of ZEN was investigated in pure water, river water and estuarine water using simulated sunlight irradiation. Kinetic studies revealed that two concomitant processes contribute to the fate of ZEN under solar irradiation: photoisomerization and photodegradation. This phototransformation followed a pseudo-first order kinetics. ZEN degrades quickly in natural waters and slowly in deionized water, with half-lives (t_1/2) of 28 ± 4 min (estuarine water), 136 ± 21 min (river water) and 1777 ± 412 min (deionized water). The effects of different water constituents on the phototransformation of ZEN in aqueous solution have been assessed (NaCl, Ca²⁺, Mg²⁺, Fe³⁺, NO₃^- and oxalate ions, synthetic seawater, Fe(III)-oxalate and Mg(II)-oxalate complexes, humic acids, fulvic acids and XAD-4 fraction). In the presence of synthetic seawater salt (t_1/2 = 18 ± 5 min) and Fe(III)-oxalate complexes (t_1/2 = 61 ± 9 min), the transformation rate increased considerably in relation to other water constituents tested. The solution pH also had a considerable effect in the kinetics with maximum transformation rates occurring around pH 8.5. These results allow us to conclude that phototransformation by solar radiation can be an important degradation pathway of ZEN in natural waters.

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

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

Photochemical reactivity of aqueous fullerene clusters: C60 versus C70

Over the past few years, there has been a strong interest in exploring the potential impact of fullerenes in the environment. Despite that both C₆₀ and C₇₀ have been detected in environmental matrices, the research on the impact of higher fullerenes, such as C_70, has been largely missing. This study evaluated and compared the phototransformation of aqueous C₆₀ and C₇₀ clusters (nC₆₀ and nC₇₀) and their ¹O₂ production under sunlight and lamp light irradiation (315nm, 360nm and 420nm). The nC₆₀ and nC₇₀ samples formed by direct mixing with water adopted a face-centered cubic (FCC) crystal structure. The apparent quantum yields (AQYs) of fullerene phototransformed were relatively constant over the examined wavelengths, while ¹O₂ production AQYs decreased with increased wavelengths. The long-term fate studies with outdoor sunlight indicated that both nC₆₀ and nC₇₀ lost considerable organic carbon contents (>80%) in water after ∼8 months of irradiation and that the intermediate photoproducts of nC₆₀ and nC₇₀ exhibited a progressively increased level of oxygen-containing functionalities. Overall, the study indicates that nC₇₀ can be photochemically removed under sunlight conditions and that the photoreactivity of nC₆₀ based on AQYs is greater than that of nC₇₀.

Naproxen and Its Phototransformation Products: Persistence and Ecotoxicity to Toad Tadpoles (Anaxyrus terrestris), Individually and in Mixtures

Although pharmaceutical pollution is a global environmental concern, much remains unknown about the transformation of pharmaceuticals in the wild and their effects on wildlife. In the environment, pharmaceuticals typically transform to some extent into different, structurally related compounds. Pharmaceutical transformation products resulting from exposure to sunlight (i.e., ultraviolet radiation) in surface waters are of particular concern; these products can be more hydrophobic, persistent, and toxic than their parent compounds. In the present study, naproxen, a widely used nonsteroidal anti-inflammatory drug, and its phototransformation products were studied to assess the overall persistence and photochemical fate of naproxen. Southern toad (Anaxyrus terrestris) larvae were used as model aquatic vertebrates to evaluate the acute toxicity of naproxen and its phototransformation products singly and in mixtures. The phototransformation products were observed to be more persistent and more toxic than naproxen itself. The slower phototransformation of the phototransformation products relative to naproxen suggests a greater potential to accumulate in the environment, particularly when naproxen is continually released. Mixtures of naproxen and its phototransformation products, in ratios observed during phototransformation, were more toxic than naproxen alone, as predicted by the model of concentration addition and the greater toxicity of the phototransformation products. Together, these results indicate that the ecological risk of naproxen may be underestimated by considering environmental levels of naproxen alone. Environ Toxicol Chem 2019;38:2008-2019. © 2019 SETAC.

Inhibitory effects of antioxidant moieties in humic substances on phototransformation of chlortetracycline mediated by the algae extracellular organic matter

In algae rich waters, sunlight-driven transformation of antibiotics could be accelerated via sensitization by algae extracellular organic matter (EOM), and this photosensitization process will be affected by coexisting humic substances. In this study, we explored the effect and mechanism of humic substances on photodegradation of chlortetracycline (CTC) mediated by EOM. We found that humic substances exhibited a marked inhibitory effect on the EOM-mediated photodegradation of CTC. Given that humic substances exhibited little effects on the EOM-mediated formation of triplet state species, the quenching effect of humic substances on reactive species was excluded. The inhibitory effect of humic substances was mainly attributed to the back reduction of CTC oxidation intermediates by the antioxidant moieties in humic substances. The ozone oxidation treatment for humic substances was applied to destroy antioxidant moieties. After ozonation, the inhibitory effects of humic substances were greatly decreased, confirming the dominant role of antioxidant moieties in humic substances, which inhibited CTC photodegradation mediated by EOM via reducing oxidation intermediates of CTC. This back reduction was further verified to be exergonic via reactive Gibbs free energy, indicating the back reduction by humic substances of CTC oxidation intermediates could occur spontaneously. The present study will be helpful for predicting the fate and risk of CTC in algae rich water environments, and is of great significance for the study of phototransformation of other antibiotics.

Aquatic transformation of phosphite under natural sunlight and simulated irradiation

The phototransformation of phosphite (HPO₃^2-, H₂PO₃^-, +3) from Lake Taihu water (THW) under natural sunlight was evaluated. No direct phosphite photoreaction was observed under sunlight. Suspended solids were shown to play important roles in the indirect photoreaction of phosphite in lake water. The phototransformation of phosphite followed pseudo-first-order reaction kinetics and the kinetics constants (k, d^-1) decreased as: 0.0324 (original THW), 0.0236 (sterilized THW), 0.0109 (filtered THW) and 0.0102 (sterilized filtered THW). Original THW with 1 mmol L^-1 NO₃^- added was used to simulate the phosphite removal in lakes with serious N pollution. The results showed that the phototransformation was accelerated (with k increased to 0.0386-0.0463 d^-1), and sterilization or filtration shown little effect to the transformation, as the half-lives of phosphite drew closer. Under simulated irradiation in NO₃^- system, increasing NO₃^- concentration or decreasing pH value promoted phototransformation. The addition of Fe³⁺ or Fe²⁺ accelerated photooxidation, while the addition of Mn²⁺ or Cd²⁺ inhibited phototransformation. Br^-, NO₂^- and HCO₃^- in environmental concentrations decreased phototransformation, and HCO₃^- showed the strongest inhibition. Suwannee River humic acid or Suwannee River fulvic acid strongly inhibited the photooxidation process, and the inhibiting effects varied with their structure. Phosphite photooxidation was strongly inhibited by adding isopropanol or sodium azide as reactive oxygen species (ROS) quenchers. Electron spin resonance analysis indicated that OH was a main oxidant produced in this system. The increased amount of phosphate coincided with the decreased amount of phosphite, which indicated that the transformation product of phosphite was phosphate. Phosphite is a considerable component of the P redox cycle in Lake Taihu.

Polystyrene microplastics reduce Cr(VI) and decrease its aquatic toxicity under simulated sunlight

Microplastics (MPs) serve as vectors for chromium (Cr), influencing its fate and toxicity in aquatic environments, and have attracted much attention recently. However, it is still unknown whether MPs mediate Cr species transformation under sunlight irradiation. This study confirmed that polystyrene (PS) MPs could reduce Cr(VI) to Cr(III) under sunlight irradiation, with a photoreduction rate constant of 0.0023 h^-1. PS MPs-mediated Cr(VI) reduction was predominantly dependent on O₂^•- and simultaneously suppressed by ¹O₂, •OH and ³PS* . Aged PS MPs were exposed to simulated sunlight irradiation for 0, 200, 500, and 800 h, and Cr(VI) reduction was hindered by increased ¹O₂ and •OH formation and light-screening effects (decreased photon absorption). The size, functional groups and concentration of PS MPs and environmental factors (e.g., humic acid, pH, Mg²⁺, Fe³⁺ and O₂) strongly affected Cr(VI) reduction. Furthermore, Cr(VI) reduction induced by PS MPs could occur in reservoir water, and the reduction rate was faster than that in double distilled (DD) water. Correspondingly, PS MPs (1 mg/L) decreased the oxidative stress induced by Cr(VI) to Lemna minor in reservoir water after 96 h of sunlight irradiation. This study provided deep insight into how PS MPs affect Cr species transformations and hazardous effects in realistic aquatic environments under sunlight conditions.

Persistent phototransformation products of vardenafil (Levitra®) and sildenafil (Viagra®)

As pharmaceutically active compounds (PhACs) are increasingly detected in the aquatic environment, the importance of investigating their transformation products-products of naturally occurring hydrolysis, biodegradation, and solar-mediated photochemical reactions - for persistence and ecotoxicity remains an important part of assessing the pharmaceutical's environmental impact and fate. In this study, the solar phototransformation of vardenafil (active ingredient in Levitra®) and sildenafil (Viagra®), two structurally similar compounds used to treat erectile dysfunction, was studied, with special attention to potentially persistent phototransformation (PT) products. PT products for vardenafil are identified here for the first time. Dilute solutions were prepared in buffered (pH=7.4) aqueous solutions, both with and without Suwanee River humic acid (SRHA) and fulvic acid (SRFA), and exposed to simulated sunlight. The PT products were identified via Liquid Chromatography-Electrospray Ionization-Mass Spectrometry (LC-ESI-MS) and confirmed by MS/MS. Both SRHA and SRFA were observed to lead to more extensive degradation of VRD while having minimal effect on SLD. At least two PT products, SLD-392 and VRD-392, were observed to be notably persistent, indicating their potential impact in the aquatic environment and highlighting the need for investigations of transformation products in natural water samples.

Multiple transformation pathways of p-arsanilic acid to inorganic arsenic species in water during UV disinfection

p-Arsanilic acid (p-ASA) is widely used in China as livestock and poultry feed additive for promoting animal growth. The use of organoarsenics poses a potential threat to the environment because it is mostly excreted by animals in its original form and can be transformed by UV-Vis light excitation. This work examined the initial rate and efficiency of p-ASA phototransformation under UV-C disinfection lamp. Several factors influencing p-ASA phototransformation, namely, pH, initial concentration, temperature, as well as the presence of NaCl, NH4(+), and humic acid, were investigated. Quenching experiments and LC-MS were performed to investigate the mechanism of p-ASA phototransformation. Results show that p-ASA was decomposed to inorganic arsenic (including As(III) and As(V)) and aromatic products by UV-C light through direct photolysis and indirect oxidation. The oxidation efficency of p-ASA by direct photosis was about 32%, and those by HO and (1)O2 were 19% and 49%, respectively. Cleavage of the arsenic-benzene bond through direct photolysis, HO oxidation or (1)O2 oxidation results in simultaneous formation of inorganic As(III), As(IV), and As(V). Inorganic As(III) is oxidized to As(IV) and then to As(V) by (1)O2 or HO. As(IV) can undergo dismutation or simply react with oxygen to produce As(V) as well. Reactions of the organic moieties of p-ASA produce aniline, aminophenol and azobenzene derivatives as main products. The photoconvertible property of p-ASA implies that UV disinfection of wastewaters from poultry and swine farms containing p-ASA poses a potential threat to the ecosystem, especially agricultural environments.