Photodegradation of bisphenol A and related compounds under natural-like conditions in the presence of riboflavin: kinetics, mechanism and photoproducts

Estimating the temporal and spatial distribution and threats of bisphenol A in temperate lakes using machine learning models

Bisphenol A (BPA) is easily enriched in many human-disturbed watersheds, particularly lakes with poor water mobility, which is posing a threat to aquatic biota. While previous studies have focused on the concentration of BPA in water and its toxicity to aquatic organisms, a small amount of measured data is not enough to reveal the temporal and spatial distribution and threats of BPA, and estimate the ecological risk in watersheds. Therefore, we collected 164 measured BPA data points from Taihu Lake to develop machine learning models using random forest (RF), support vector machine (SVM) and least square regression (LSR) and created month-by-month watershed prediction maps in temperate lakes to estimate the spatiotemporal distribution and threats of BPA. Due to RF's superior robustness to noisy data, the RF model exhibits the best performance among the three algorithms. The RF model showed acceptable predictive performance on the modeling dataset (coefficients of determination and root-mean-square error for the training set were 0.927 and 17.499, respectively, and 0.607, 39.645 for the validation set, respectively). The maps indicated that areas susceptible to anthropogenic activities were more severely polluted by BPA, and rainy climate may favor the migration of BPA to aquatic ecosystems. The model was also applied to predict 42 data points of BPA collected from Dianchi Lake, and the results showed that most predicted data were within a factor of 10 of the measured data, but the prediction accuracy of the model has declined. The ecological risks in the two lakes were evaluated and attention should be paid to the regions with higher risks. Our study provided a novel idea for comprehensive monitoring of an unconventional trace pollutant with endocrine disrupting effects in aquatic ecosystems and analyzing their spatiotemporal distribution, which will contribute to the scientific assessment of the ecological risk of BPA.

Direct and indirect photodegradation of bisphenol A in the presence of natural water components

The impacts and mechanisms of natural water constituents, such as humic acid (HA), nitrates, iron and chloride ions, to the photodegradation of bisphenol A (BPA) were investigated in aqueous media under UV light irradiation. Due to the contributions of ·OH, 1O2, O2- and BPA* to BPA photodegradation in pure water in 13.4, 7.7, 22.9 and 47.9%, respectively, BPA was attenuated through the reaction pathway of direct photodegradation more than self-sensitized photodegradation. About indirect photodegradation, BPA photolysis through inhibitory effect from HA was mainly by light screening effect and quenching effect was insignificant. NO- 3 and NO- 2 both showed inhibitory effect but due to different reactive oxidization species (ROS). The photodegradation of BPA was significantly enhanced by the addition of iron from the formation of ·OH and H2O2, due to iron-assisted indirect photolysis for the degradation process. A dual effect of chloride depending on the different concentration levels involved quenching and promotion effect on reactive photo-induced species (RPS). A simple linear model revealed that BPA photodegradation was significantly impacted by the interaction of the above factors. In natural water, the decreased photolytic rate of BPA was mainly attributed to triple-excited dissolved organic matter (3DOM*), indicating that indirect photolysis was the primary transformation pathway of BPA. The detected photolysis products, such as nitrate and chlorinated products, suggest that there might be potential ecological risk of BPA photodegradation.

A novel eco-friendly polymeric photosensitizer based on chitosan and flavin mononucleotide

Flavin mononucleotide (FMN) is a dye belonging to the flavin family. These dyes produce photosensitized degradation of organic compounds via reaction with the excited states of the dye or with reactive oxygen species photogenerated from the triplet of the dye. This article presents a new polymeric dye (FMN-CS) composed of the photosensitizer FMN covalently bonded to chitosan polysaccharide (CS). FMN-CS obtained has a molecular weight of 230 × 103 g mol-1 and a deacetylation degree of 74.8%. The polymeric dye is an environmentally friendly polymer with spectroscopic and physicochemical properties similar to those of FMN and CS, respectively. Moreover, under sunlight, it is capable of generating 1O2 with a quantum yield of 0.31. FMN-CS, like CS, is insoluble in basic media. This allows easy recovery of the polymeric dye once the photosensitized process has been carried out and makes FMN-CS a suitable photosensitizer for the degradation of pollutants in contaminated waters. To evaluate whether FMN-CS may be used for pollutant degradation, the photosensitized degradation of two trihydroxybenzenes by FMN-CS was studied.

Photochemical behavior of constructed wetlands-derived dissolved organic matter and its effects on Bisphenol A photodegradation in secondary treated wastewater

Free water surface flow (FWS) constructed wetlands (CWs) have been broadly applied for polishing secondary treated effluents. Dissolved organic matter derived from FWS CWs (WDOM) plays key roles in contaminants transformations. Conversely, photodegradation could shape the quantity and quality of WDOM, thereby affecting its roles in the photolysis of organic micropollutants (OMPs). Nevertheless, whether and how solar irradiation-induced photodegradation modify the properties of WDOM, and the effects of WDOM on the photodegradation of OMPs remain unclear. This study elucidates the photochemical behavior of two WDOM isolated from field-scale FWS CWs for effluent polishing under simulated sunlight irradiation using spectroscopic tools and high-resolution mass spectra. Furthermore, the roles of WDOM in the photodegradation of Bisphenol A (BPA), as a representative endocrine-disrupting compound (EDC), were comprehensively investigated. Solar irradiation was demonstrated to lower the molecular weight and aromaticity of WDOM, as well as weaken its light absorption. Ultrahigh-resolution mass spectra further confirmed that aromatic and unsaturated structures were susceptible to solar irradiation-induced photodegradation reactions. Subsequently, less aromatic and more saturated structures eventually formed under sunlight irradiation, consistent with the result from spectroscopic characterization. The reactive species produced from WDOM significantly enhanced the photodegradation of BPA with the k_obs noticeably increasing 4-fold compared with the k_obs for direct photolysis. Additionally, ³WDOM* was identified as the dominant reactive species leading to the photolysis of BPA in the presence of WDOM. These findings improve understanding of the phototransformation behavior of WDOM under sunlight irradiation and the roles that WDOM plays in the photochemical fate of coexisting OMPs in CWs treatment systems.

Studying the combined influence of microplastics' intrinsic and extrinsic characteristics on their weathering behavior and heavy metal transport in storm runoff

The weathering and contaminant transport behavior of both primary (PMPs) and secondary microplastics (SMPs) are interrelated to their original physiochemical features and variations within the environment. This study examines the influence of PMPs' intrinsic characteristics (polymer structure and crystallinity) and SMPs' extrinsic features (surface oxidation and external sediments attachment) on the photodegradation kinetics, and subsequently Pb(II) and Zn(II) uptake from stormwater. For this purpose, high density polyethylene (HDPE) and low density polyethylene (LDPE) with different degrees of crystallinities were produced as PMPs, and their photodegradation behaviors were compared with original polymers. Furthermore, the SMPs generated by abrasion and surface oxidation of PMPs and the virgin PMPs underwent accelerated photodegradation, and the changes of their crystallinity, surface chemistry, and morphology were examined. Scanning electron microscopy (SEM) imaging and X-ray photoelectron (XPS) studies revealed the formation of cracks and different oxidized functionalities on MPs surface due to UV photodegradation. The vinyl and carbonyl indices calculated using Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy demonstrated an elevated photodegradation rate for SMPs compared to the PMPs. Moreover, the Differential Scanning Colorimetry (DSC) demonstrated an increasing percentage of crystallinity in all MPs due to the photodegradation. The percent crystallinity of HDPE pellets increased after photodegradation from 49.8 to 62.6 and it increased from 17.2 to 38.9 for LDPE pellets respectively. The greater level of increase in crystallinity for LDPE in comparison to HDPE upon photodegradation was referred to as LDPE's greater amorphous content and branched structure. A greater level of metal uptake was obtained for photodegraded LDPE pellets as 2526 μg/m² for Pb(II) and 2028 μg/m² for Zn(II) respectively.

Photocatalytic Degradation of Recalcitrant Pollutants of Greywater

These days, many countries have a water shortage and have limited access to clean water. To overcome this, a new treatment is emerging, namely, the photocatalytic processing of greywater. Photocatalytic processes to remove the organic matter from different greywater sources are critically reviewed. Their efficiency in degrading the organic matter in greywater is scrutinized along with factors that can affect the activity of photocatalysts. Modified TiO2, ZnO and TiO2 catalysts show great potential in degrading organic materials that are present in greywater. There are several methods that can be used to modify TiO2 by using sol-gel, microwave and ultrasonication. Overall, the photocatalytic approach alone is not efficient in mineralizing the organic compounds, but it works well when the photocatalysis is combined with oxidants and Fe3+. However, factors such as pH, concentration and catalyst-loading of organic compounds can significantly affect photocatalytic efficiency.

Synthesis of new composite based on TiO2 immobilized in glass fibers for photo-catalytic degradation of chlorobenzene in aqueous solutions

In this study photo-catalytic degradation of chlorobenzene from aqueous solutions using CQD decorated Fe-doped TiO₂ immobilized in Glass Fibers (GF) was investigated. Characteristics of the synthesized photo-catalyst were determined by EF-SEM, EDX, BET, XRD, FTIR, and DRS analysis. Additionally, DRS analysis demonstrated adding CQD to the TiO₂-Fe reduced its band gap energy from 2.96 eV to 2.91eV, while that was 3.10 eV for undoped TiO₂. Among that three photo-catalysts, GF/CQD_(4.5 wt%) decorated Fe-TiO₂ composite had performance nearly 100.0%, when pH was 5 and low concentration of chlorobenzene. In addition, GF/CQD_(4.5 wt%) decorated Fe-TiO₂ composite show it could be well applied for five times and with a little reduction on the performance. Also, no detectable Fe found to be released from the composite. Minimum inhibitory concentration (MIC) for E. coli bacteria was 12.2 mg L^-1 of chlorobenzene residual. Our findings show the catalyst was successful for chlorobenzene removal in the wastewater effluent. In conclusion, present hybrid composite could successfully and safely remove chlorobenzene from synthetic aqueous solution.

Toxicity and biotransformation of bisphenol S in freshwater green alga Chlorella vulgaris

Safety and environmental behavior of bisphenol A (BPA) alternatives have attracted considerable attention because of their wide use. In the present study, toxicity and biotransformation of bisphenol S (BPS), a primary alternative to BPA, in Chlorella vulgaris were investigated. BPS had a significant inhibition on the growth rate of C. vulgaris with an inhibition rate of 41.6%, 103.7% and 238.4% under exposure of 1, 10 and 100 mg L^-1 BPS, respectively. BPS (2 d EC₅₀: 3.16 mg L^-1) had a higher acute toxicity to C. vulgaris than BPA (2 d EC₅₀: 41.43 mg L^-1), but its toxicity was gradually lower than BPA as the exposure time increased. BPS underwent rapid degradation and was more recalcitrant to degradation by C. vulgaris than BPA at 5 and 10 mg L^-1. BPS was less accumulated in algal cells than BPA (p < 0.05), suggesting that it may pose less risk than BPA on the aquatic algophagous organisms and other high-trophic-level predators through the food chain. In addition, six new metabolites of BPS were identified in algal cells using high-resolution mass spectrometry. This is the first time that degradation pathway for BPS in algae is described, and these results represent a significant advance in understanding the fate of BPS and other BPA substitutes in the aquatic environment.

Riboflavin-Mediated Photooxidation of Gold Nanoparticles and Its Effect on the Inactivation of Bacteria

Photodynamic inactivation (PDI) of microorganisms, based on the ability of photosensitizers to produce reactive oxygen species (ROS) under adequate irradiation, emerges as a promising technique to face the increasing bacterial resistance to conventional antimicrobials. In this work, we analyze the combined action of Riboflavin (Rf) and pectin-coated gold nanoparticles (PecAuNP) on Staphylococcus aureus (S. aureus) and Pseudomonas aeruginosa (P. aeruginosa) as suitable PDI strategy. We demonstrate that gold ions can be generated upon Rf-photosensitized oxidation of PecAuNP. Transient absorption spectroscopy shows that the Rf cationic radical can accept an electron from the nanoparticles to yield Au(I) ions, which in aqueous medium is disproportionate to yield Au⁰ and Au(III). Microbiological assays showed that the presence of PecAuNP enhanced the antibacterial activity of photoirradiated Rf toward S. aureus and P. aeruginosa, in line with the well-known antibacterial activity of gold ions. Moreover, the irradiation of Rf solutions containing about 100 μM PecAuNP enabled the solutions to be bactericidal against both bacteria.

Bisphenol A attenuation in natural microcosm: Contribution of ecological components and identification of transformation pathways through stable isotope tracing

Residues of bisphenol A (BPA) are ubiquitously detected in the surface water due to its widespread usage. This study systematically investigated the dissipation and kinetics of BPA under simulated hydrolysis, direct and indirect photolysis, bacterial degradation, microbial degradation and natural attenuation in microcosm. Structural equation modeling (SEM) by using partial least square method in path coefficient analysis suggested that the microbial degradation was the major factor involved in the natural attenuation of BPA. The potential transformation products were identified by using liquid chromatography high-resolution mass spectrometry (LC-HRMS) and stable isotope tracing technique by simultaneous performing gas chromatography combustion isotope ratio mass spectrometry (GC-C-IRMS) and gas chromatography mass spectrometry (GC-MS). A total of fourteen including three novel transformation products of BPA were identified to indicate five possible pathways. An increased yield of labeled (δ¹³C) CO₂ and detection of ¹³C-labeled phospholipid fatty acids (PLFAs) indicated the mineralization of BPA and possible utilization of BPA or its transformation products by microbes for cellular membrane synthesis, respectively.

Direct and indirect photolysis of the antibiotic enoxacin: kinetics of oxidation by reactive photo-induced species and simulations

The purpose of this study was to investigate the aqueous phase photochemical behavior of enoxacin (ENO), an antibiotic selected as a model pollutant of emerging concern. The second-order reaction rate constants of ENO with hydroxyl radicals (HO^●) and singlet oxygen (¹O₂) were determined at pH 3, 7, and 9. Also, the rate constants of the electron transfer reaction between ENO and triplet states of chromophoric dissolved organic matter (³CDOM*) are reported for the first time, based on anthraquinone-2-sulfonate (AQ2S) as CDOM proxy. The sunlight-driven direct and indirect ENO degradation in the presence of dissolved organic matter (DOM) is also discussed. The results show that direct photolysis, which occurs more rapidly at higher pH, along with the reactions with HO^● and ³AQ2S*, is the key pathway involved in ENO degradation. The ENO zwitterions, prevailing at pH 7, show k_ENO, HO^●, k_ENO,1O2, and k_ENO,3AQ2S* of (14.0 ± 0.8) × 10¹⁰, (3.9 ± 0.2) × 10⁶, and (61.5 ± 0.7) × 10⁸ L mol^-1 s^-1, respectively, whose differences at pH 3, 7, and 9 are due to ENO pH-dependent speciation and reactivity. These k values, along with the experimental ENO photolysis quantum yield, were used in mathematical simulations for predicting ENO persistence in sunlit natural waters. According to the simulations, dissolved organic matter and water depth are expected to have the highest impacts on ENO half-life, varying from a few hours to days in summertime, depending on the concentrations of relevant waterborne species (organic matter, NO₃^-, NO₂^-, HCO₃^-).

Photo-Fenton and Riboflavin-photosensitized Processes of the Isoxaflutole Herbicide

The proherbicide Isoxaflutole (IXF) hydrolyzes spontaneously to diketonitrile (DKN) a phytotoxic compound with herbicidal activity. In this work, the sensitized degradation of IXF using Riboflavin (Rf), a typical environmentally friendly sensitizer, Fenton and photo-Fenton processes has been studied. The results indicate that only the photo-Fenton process produces a significant degradation of the IXF. Photolysis experiments of IXF sensitized by Riboflavin is not a meaningful process, IXF quenches the Rf excited triplet (³ Rf*) state with a quenching rate constant of 1.5 · 10⁷  m^-1  s^-1 and no reaction is observed with the species O₂ (¹ Δ_g ) or O 2 · - generated from ³ Rf*. The Fenton reaction produces no changes in the IXF concentration. While the photo-Fenton process of the IXF, under typical conditions, it produces a degradation of 99% and a mineralization to CO₂ and H₂ O of 88%. A rate constant value of 1.0 × 10⁹  m^-1  s^-1 was determined for the reaction between IXF and HO˙. The photo-Fenton process degradation products were identified by UHPLC-MS/MS analysis.

Occurrence, endocrine-related bioeffects and fate of bisphenol A chemical degradation intermediates and impurities: A review

In recent decades, increasing attention has been directed toward the effects of bisphenol A (BPA) as an environmental pollutant, primarily due to its demonstrated endocrine-disruptive effects. A growing body of evidence indicates that many BPA derivatives also exhibit endocrine activity and other adverse biological properties. A review of the published literature was performed to identify BPA degradation intermediates resulting from chemical degradation processes of BPA, as well as BPA's associated co-pollutants. Products of biological metabolism were not included in this study. Seventy-nine chemicals were identified. Of these chemicals, a subset - those containing two 6-membered aromatic rings connected by a central ring-linking carbon - was identified, and a further literature review was conducted to identify demonstrated biological effects associated with the chemicals in this subset. The objectives of this review were to assess the potential risks to human and environmental health associated with BPA derivatives, characterize our current understanding of BPA's degradation intermediates and co-pollutants, and aid in the identification of compounds of interest that have received insufficient scrutiny.

Reactive Oxygen Species-mediated Degradation of Antidiabetic Compounds: Cytotoxic Implications of Their Photodegradation Products

Reactive oxygen species (ROS) have been described in their double physiological function, helping in the maintenance of health as well as contributing to oxidative stress. Diabetes mellitus is a chronical disease nearly related to oxidative stress, whose treatment (in type II variant) consists in the administration of antidiabetic compounds (Andb) such as Gliclazide (Gli) and Glipizide (Glip). In this context, as Andb may be exposed to high ROS concentrations in diabetic patients, we have studied the potential ROS-mediated degradation of Gli and Glip through photosensitized processes, in the presence of Riboflavin (Rf) vitamin. We found that singlet oxygen (O₂ (¹ ∆_g )) participated in the Rf-sensitized photodegradation of both Andb, and also superoxide radical anion in the case of Gli. Two principal products derived from O₂ (¹ ∆_g )-mediated degradation of Gli were identified and their chemical structures characterized, through HPLC mass spectrometry. O₂ (¹ ∆_g )-mediated degradation products and their toxicity was assayed on Vero cell line. These studies demonstrated that neither Gli nor its photoproducts caused cytotoxic effect under the experimental conditions assayed. Our results show strong evidences of ROS-mediated Andb degradation, which may involve the reduction or loss of their therapeutic action, as well as potential cytotoxicity derived from their oxidation products.

Inhibitory effects of natural organic matter on methyltriclosan photolysis kinetics

This study evaluated the effects and related mechanisms of natural organic matter (NOM) on the photolysis of methyltriclosan (MTCS), a metabolite of triclosan. Addition of two representative NOM isolates, Pony Lake fulvic acid (PLFA-microbial origin) and Suwannee River fulvic acid (SRFA-terrestrial origin), significantly inhibited the direct photolytic rate of MTCS by ∼70%. The MTCS photolytic rate in the presence of PLFA was greater than for SRFA. NOM not only suppressed photolysis by light-shielding, but also produced ROS to oxidatively degrade MTCS and/or triplet NOM (³NOM*) to sensitize degradation. The dual effects of light-screening and photo-sensitization led to an overall decrease in photolysis of MTCS with a positive concentration-dependence. Upon addition of NOM, EPR documented the occurrence of ¹O₂ and ˙OH in the photolytic process, and the bimolecular k value for the reaction of ¹O₂ with MTCS was 1.86 × 10⁶ M⁻¹ s⁻¹. ROS-quenching experiments indicated that the contribution of ˙OH (19.1–29.5%) to indirect photolysis of MTCS was lower than for ¹O₂ (38.3–58.7%). Experiments with D₂O further demonstrated that ¹O₂ participated in MTCS photodegradation. Moreover, the addition of sorbic acid and O₂ gas to the reaction confirmed the participation of ³NOM* as a key reactant in the photochemical transformation of MTCS. This is the first comprehensive analysis of NOM effects on the indirect photolysis of MTCS, which provides new insights for understanding the environmental fate of MTCS in natural environments.

We demonstrate that PLFA and SRFA inhibit the MTCS photolysis by synergistic effects of light-shielding and photo-sensitization.

QSARs for phenols and phenolates: oxidation potential as a predictor of reaction rate constants with photochemically produced oxidants

Quantitative structure-activity relationships (QSARs) for prediction of the reaction rate constants of phenols and phenolates with three photochemically produced oxidants, singlet oxygen, carbonate radical, and triplet excited state sensitizers/organic matter, are developed. The predictive variable is the one-electron oxidation potential (E₁), which is calculated for each species using density functional theory. The reaction rate constants are obtained from the literature, and for singlet oxygen, are augmented with new experimental data. Calculated E₁ values have a mean unsigned error compared to literature values of 0.04-0.06 V. For singlet oxygen, a single linear QSAR that includes both phenols and phenolates is developed that predicts experimental rate constants, on average, to within a factor of three. Predictions for only 6 out of 87 compounds are off by more than a factor of 10. A more limited data set for carbonate radical reactions with phenols and phenolates also gives a single linear QSAR with prediction of rate constant being accurate to within a factor of three. The data for the reactions of phenols with triplet state sensitizers demonstrate that two sensitizers, 2-acetonaphthone and methylene blue, most closely predict the reactivity trend of triplet excited state organic matter with phenols. Using sensitizers with stronger reduction potentials could lead to overestimation of rate constants and thus underestimation of phenolic pollutant persistence.

Fate of Bisphenol A in Terrestrial and Aquatic Environments

Bisphenol A (2,2-bis[4-hydroxyphenyl]propane, BPA), the monomer used to produce polycarbonate plastic and epoxy resins, is weakly estrogenic and therefore of environmental and human health interest. Due to the high production volumes and disposal of products made from BPA, polycarbonate plastic and epoxy resins, BPA has entered terrestrial and aquatic environments. In the presence of oxygen, diverse taxa of bacteria, fungi, algae and even higher plants metabolize BPA, but anaerobic microbial degradation has not been documented. Recent reports demonstrated that abiotic processes mediate BPA transformation and mineralization in the absence of oxygen, indicating that BPA is susceptible to degradation under anoxic conditions. This review summarizes biological and nonbiological processes that lead to BPA transformation and degradation, and identifies research needs to advance predictive understanding of the longevity of BPA and its transformation products in environmental systems.

Aqueous photochemical degradation of BDE-153 in solutions with natural dissolved organic matter

The compound 2,2',4,4',5,5'-hexabrominated diphenyl ether (BDE-153) is an intermediate photolytic product in the degradation of highly brominated diphenyl ethers to lower brominated forms. Herein, we report the effects of two natural organic matter (NOM) sources, Suwannee River fulvic acid (SRFA) and Pony Lake fulvic acid (PLFA), on BDE-153 photolysis in water. The rate constant (k) and half-life of BDE-153 was 2.26 × 10(-2) min(-1) and 30.72 min under UV-Vis irradiation (direct photolysis at λ > 290 nm). The k value for BDE-153 decreased markedly in the presence of NOM with a larger decrease in the presence of PLFA than SRFA. Electron spin resonance (ESR) demonstrated generation of free radicals in the photolytic process that mainly involved (1)O2 and OH. The biomolecular k values for reaction of (1)O2 and OH with BDE-153 were 3.65 × 10(6) and 7.70 × 10(8) M(-1) s(-1), respectively. The contribution of OH (28.7-31.0%) to the indirect photolysis of BDE-153 was higher than for (1)O2 (12.9-14.9%). The photolytic rate of BDE-153 in oxygen-rich (aerated) solution was much slower than in oxygen-poor (nitrogen-sparged) conditions, demonstrating that (3)NOM* is a more effective reagent for degradation of BDE-153 than (1)O2. Addition of sorbic acid (a (3)NOM* quencher) significantly reduced the photolytic rate of BDE-153 confirming the important role of (3)NOM* in indirect photolysis. In the presence of NOM, BDE-153 indirect photolysis was facilitated mainly by reaction with (3)NOM* and OH. To the best of our knowledge, this is the first comprehensive investigation of indirect photolysis of BDE-153 in water containing NOM.

A new trick (hydroxyl radical generation) of an old vitamin (B2) for near-infrared-triggered photodynamic therapy

A new NIR-light-triggered PDT method has been developed using an old vitamin (vitamin B₂) integrated with the upconversion nanotechnology.

Vitamin B2-sensitized degradation of the multifunctional drug Evernyl, in the presence of visible light – microbiological implications

Taking into consideration the importance of the photooxidative effects in complex bio- environments, this paper reports on the visible-light-promoted interactions between Evernyl (methyl 2, 4-dihydroxy-3,6-dimethylbenzoate, Ev) and vitamin B2 (riboflavin, Rf). Ev is a phenolic derivative, transparent to visible light, that possesses important antimicrobial activity. This compound is the first known natural, complete and specific human androgen receptor antagonist. Ev is profusely employed in personal-care products and synthesized as a secondary metabolite by several lichen and plant species. In both sceneries, acting Ev as a cosmetic component for topic applications or as a lichen constituent, may Rf behave as a native visible-light absorber pigment. In this context, kinetic and mechanistic aspects of the Rf-sensitized photooxidation of Ev, has been studied in aqueous solution, irradiating with blue LED light (463–471 nm) and employing stationary and time resolved methods. Results indicate that Ev reacts with the photogenerated reactive oxygen species (ROS) singlet molecular oxygen with a rate constant of kr=1.1±0.2×106 M−1s−1. In parallel Ev also quenches the electronically excited singlet and triplet excited states of Rf with rate constants close to the difussion limit. As a result the ROS superoxide radical anion and hydrogen peroxide are generated and the latter subsequently reacts with Ev. Possible implications of these photoreactions on the antimicrobial activity of Ev have been investigated employing a Candida albicans (CA) strain, isolated from human skin infection. The simultaneous presence of Rf, Ev in a sub-MIC, and blue-light irradiation produced a significant antimycotic effect, attributed to ROS photogeneration.

Riboflavin-sensitized photooxidation of Ceftriaxone and Cefotaxime. Kinetic study and effect on Staphylococcus aureus

Trace amounts of the widely used β-lactam antibiotics (Atbs) in waste water may cause adverse effects on the ecosystems and contribute to the proliferation of antibiotic-resistant bacteria. On these grounds, kinetic and mechanistic aspects of photosensitized degradation of Ceftriaxone (Cft) and Cefotaxime (Ctx), have been studied in pure water by stationary and time-resolved techniques. Additionally, possible implications of these photoprocesses on the antimicrobial activity of the Atbs have also been investigated. Photoirradiation of aqueous solutions of Cft and Ctx produces the degradation of both Atbs in the presence of Riboflavin (vitamin B2), a well known pigment dissolved in natural aquatic systems. The process occurs through Type I and Type II mechanisms, with effective prevalence of the former. The participation of O2(-), OH and O2((1)Δg) is supported by experiments of oxygen consumption carried out in the presence of specific scavengers for such reactive oxygen species. Microbiological assays exhibit a parallelism between the rate of Cft and Ctx photodegradation and the loss of their bactericidal capacity on Staphylococcus aureus strains. Results contribute to both understanding kinetic and mechanism aspects of the degradation and predicting on natural decay of Atbs waste water-contaminants.

Photobleaching-induced changes in photosensitizing properties of dissolved organic matter

Photosensitizing properties of different dissolved organic matter (DOM) were investigated according to their performance in singlet oxygen ((1)O2), triplet state of DOM ((3)DOM*), and hydroxyl radical (·OH) productions. The photobleaching of DOM solutions after irradiation was characterized by fluorescence excitation-emission matrix and UV-Vis spectroscopy. The photosensitizing properties of pre-irradiated DOM solutions were changed in a sunlight simulator. The performance of DOMs in photosensitized degradation of several contaminants was investigated. For a 20 h exposure, the observed degradation rate constant (kobs) of some contaminants decreased as a function of exposure time, and highly depended on the properties of both DOM and contaminant. Degradation of contaminants with lower kobs was more susceptible to DOM photobleaching-induced decrease in kobs. Under the current experimental conditions, the photobleaching-induced decrease of DOM photo-reactivity in contaminant degradation was mainly attributed to indirect phototransformation of DOM caused by the interactions between photo-inductive DOM moieties and photochemically-produced reactive species. Reactive contaminants can inhibit DOM indirect photobleaching by scavenging reactive species, photosensitized degradation of these contaminants exhibited a stable kobs as a result. This is the first study to report DOM photobleaching-induced changes in the simultaneous DOM photosensitized degradation of contaminants and the inhibitory effect of reactive contaminants on DOM photobleaching.

Degradation of organic pollutants in/on snow and ice by singlet molecular oxygen (¹O₂*) and an organic triplet excited state

Singlet molecular oxygen (¹O₂*) can be a significant sink for a variety of electron-rich pollutants in surface waters and atmospheric drops. We recently found that ¹O₂* concentrations are enhanced by up to a factor of 10(4) on illuminated ice compared to in the equivalent liquid solution, suggesting that ¹O₂* could be an important oxidant for pollutants in snow. To examine this, here we study the degradation of three model organic pollutants: furfuryl alcohol (to represent furans), tryptophan (for aromatic amino acids), and bisphenol A (for phenols). Each compound was studied in illuminated aqueous solution and ice containing Rose Bengal (RB, a sensitizer for ¹O₂*) and sodium chloride (to adjust the concentration of total solutes). The RB-mediated loss of each organic compound is enhanced on illuminated ice compared to in solution, by factors of 6400 for furfuryl alcohol, 8300 for tryptophan, and 50 for bisphenol A for ice containing 0.065 mM total solutes. Rates of loss of furfuryl alcohol and tryptophan decrease at a higher total solute concentration, in qualitative agreement with predictions from freezing-point depression. In contrast, the loss of bisphenol A on ice is independent of total solute concentration. Relative to liquid tests, the enhanced loss of tryptophan on ice during control experiments made with deoxygenated solutions and solutions in D₂O show that the triplet excited state of Rose Bengal may also contribute to loss of pollutants on ice.

Singlet oxygen generation by the genetically encoded tag miniSOG

The genetically encodable fluorescent tag miniSOG is expected to revolutionize correlative light- and electron microscopy due to its ability to produce singlet oxygen upon light irradiation. The quantum yield of this process was reported as ΦΔ = 0.47 ± 0.05, as derived from miniSOG's ability to photooxidize the fluorescent probe anthracene dipropionic acid (ADPA). In this report, a significantly smaller value of ΦΔ = 0.03 ± 0.01 is obtained by two methods: direct measurement of its phosphorescence at 1275 nm and chemical trapping using uric acid as an alternative probe. We present insight into the photochemistry of miniSOG and ascertain the reasons for the discrepancy in ΦΔ values. We find that miniSOG oxidizes ADPA by both singlet oxygen-dependent and -independent processes. We also find that cumulative irradiation of miniSOG increases its ΦΔ value ~10-fold due to a photoinduced transformation of the protein. This may be the reason why miniSOG outperforms other fluorescent proteins reported to date as singlet oxygen generators.

Stability of bisphenol A (BPA) in oil-in water emulsions under riboflavin photosensitization

Effects of riboflavin photosensitization on the degradation of bisphenol A (BPA) were determined in oil-in-water (O/W) emulsions containing ethylenediaminetetraacetic acid (EDTA) or sodium azide, which are a metal chelator or a singlet oxygen quencher, respectively. Also, the distribution of BPA between the continuous and dispersed phases in O/W emulsions was analyzed by high-performance liquid chromatography (HPLC). The concentration of BPA in O/W emulsions significantly decreased by 38.6% after 2 h under visible light irradiation and in the presence of riboflavin (P < 0.05). Addition of EDTA and sodium azide protected the decomposition of BPA significantly in a concentration dependent manner (P < 0.05), which implies both transition metals and singlet oxygen accelerate the photodegradation of BPA in O/W emulsions. Approximately 21.7% of the BPA was distributed in the 2.5% (w/v) dispersed lipid particles and 78.3% was in the continuous aqueous phase of the emulsions. The amount of BPA in aqueous phase decreased faster than the amount of BPA in the lipid phase during riboflavin photosensitization (P < 0.05). Thus, the BPA in the aqueous phase was the major target of riboflavin photodegradation in O/W emulsions.

PRACTICAL APPLICATION

Concentration of BPA, an endocrine disrupting chemical, was decreased significantly in oil-in-water emulsions under riboflavin and visible light irradiation. BPA in continuous aqueous phase was major target of riboflavin photosensitization. However, BPA was distributed more densely in lipid phase and more protected from riboflavin photosensitized O/W emulsions. This study can help to decrease the level of BPA in foods made of O/W emulsions containing riboflavin, which could be displayed under visible light irradiation.

Direct and indirect photolysis of the phytoestrogens genistein and daidzein

Genistein and daidzein are two estrogenic compounds derived from plants, especially legumes. This research begins to explore their environmental fate, focusing on direct and indirect photolysis. UV-visible spectra for both compounds at varying pH values were taken, the pK(a) values for both compounds were measured, and UV-visible spectra for each protonation state were determined. The loss of both compounds in deionized water was observed upon exposure to natural sunlight, and the quantum yields were determined for each protonation state. In Mississippi River water, direct photolysis does not account for all of the loss of genistein and daidzein. The mechanism of indirect photolysis was probed using quenchers and sensitizers, and results suggest that daidzein is transformed mainly via direct photolysis and singlet oxygenation, while genistein is transformed mainly via reaction with triplet-state natural organic matter. The parameters determined in this study will allow for estimation of the concentration of genistein and daidzein in sunlit surface waters, which will allow for assessment of any risks posed to aquatic wildlife.

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.

Photosensitized degradation in water of the phenolic pesticides bromoxynil and dichlorophen in the presence of riboflavin, as a model of their natural photodecomposition in the environment

Within the context of environmentally friendly methods for the elimination of surface-water pollutants, the photodegradation of the phenolic pesticides bromoxynil (BXN) and dichlorophen (DCP) under simulated natural conditions has been studied. The work was done in the presence of the visible-light absorber photosensitizer riboflavin (Rf), usually present in trace quantities in natural waters. Under aerobic conditions, an efficient photooxidation of both pesticides was observed. The relatively intricate photochemical mechanism involves pesticide and oxygen consumption and, to a lesser extent, Rf degradation. The kinetic and mechanistic study supports that both H(2)O(2) and singlet molecular oxygen, O(2)((1)Δ(g)), are involved in the process. Kinetic data for the O(2)((1)Δ(g))-mediated oxidation indicate that BXN and DCP are photodegraded with this species faster than the parent compound phenol, very frequently employed as a model for aquatic contaminants, likely due to their lower pK(a) values. This observation allows the design of phenolic pesticides with different photodegradation rates under environmental conditions.

Effect of atmosphere and catalyst on reducing bisphenol A (BPA) emission during thermal degradation of polycarbonate

Bisphenol A (BPA), a well-known endocrine disruptor, is one of the major products in the thermal degradation of polycarbonate (PC) and is also leached out from various PC products. Because of the high acute toxicity of BPA, reducing BPA production during degradation of PC is an important topic. A combined Infrared Image Furnace with Ion attachment mass spectrometry technique was used to investigate the evolution of BPA from a PC sample during heating in either nitrogen or air atmosphere and with or without a CuCl(2) catalyst. Thermal treatment in the presence of CuCl(2), in nitrogen atmospheres and at lower degradation temperatures, substantially reduced the BPA emission.