Photosensitized degradation of acetaminophen in natural organic matter solutions: The role of triplet states and oxygen

Bioinspired metal oxide particles as efficient wet air oxidation and photocatalytic oxidation catalysts for the degradation of acetaminophen in aqueous phase

The catalytic performances of the biomimetic metal oxides were tested in photo Fenton-like oxidation and catalytic wet air oxidation processes. Biomimetic copper oxide, iron oxide, and cobalt oxide catalysts were prepared by using pollen grains as biotemplate. The surface characteristics of the biomimetic metal oxides were characterized. SEM micrographs of the biomimetic catalysts demonstrated that pollen grains were successfully mimicked by metal oxide structures. The influences of UV light intensity, catalyst loading, and the initial hydrogen peroxide concentration on acetaminophen degradation were investigated in the photo Fenton-like oxidation process whereas the effects of reaction temperature and catalyst loading were investigated in catalytic wet air oxidation process. The biomimetic copper oxide was the most effective catalyst for the removal of acetaminophen in both of the advanced oxidation processes. The highest acetaminophen degradation efficiency was 86.9% in photo Fenton-like oxidation process when the initial acetaminophen concentration, catalyst loading, and the initial H₂O₂ concentrations were 10 mg/L, 0.1 g/L and 1 mM, respectively, at room temperature. In the catalytic wet air oxidation process, 98.3% degradation was achieved for the treatment of 100 mg/L acetaminophen solutions at 180 °C and 10 bar by using 1 g/L of catalyst loading at the same reaction time as photo Fenton-like oxidation. Mineralization analysis and the toxicity tests indicated that the biomimetic copper oxide catalysts were promising for the acetaminophen removal in catalytic wet air oxidation processes.

Enhanced Photodegradation of Extracellular Antibiotic Resistance Genes by Dissolved Organic Matter Photosensitization

Extracellular antibiotic resistance genes (eARGs) contribute to antibiotic resistance, and as such, they pose a serious threat to human health. eARGs, regarded as an emerging contaminant, have been widely detected in various bodies of water. Degradation greatly weakens their distribution potential and environmental risks. Dissolved organic matter (DOM), mainly consisted of humic substances, carbohydrates, and organic acids, is ubiquitous in diverse waters and significantly affects the degradation of coexisting contaminants. However, the photodegradation of eARGs in natural water, especially regarding the roles of DOM in this process, remains unknown. Herein, we investigated the eARGs photodegradation in waters with and without DOM. Illumination has been found to effectively photodegrade eARGs, and this process was significantly enhanced by DOM. Further experiments revealed that photosensitization of DOM produced hydroxyl radicals (^•OH) to enhance plasmid strand breaks and produced singlet oxygen (¹O₂) to accelerate the guanine oxidation, which in turn promoted the photodegradation of plasmid-carried eARGs. Transformation assays indicated that eARGs transformation efficiencies were reduced after their photodegradation. The presence of DOM accelerated the decreases of eARGs transformation efficiencies under illumination. DOM concentration and some ions (e.g., NO₃^-, NO₂^-, HCO₃^-, Br^-, and Fe³⁺) affected ^•OH or ¹O₂ levels, further influencing the photodegradation of eARGs. Overall, eARGs photodegradation in aquatic environments is a crucial process both in the reduction of eARGs concentrations and in transformation efficiencies. This work facilitated us to better understand the fate of eARGs in waters.

Furan Carboxamides as Model Compounds To Study the Competition between Two Modes of Indirect Photochemistry

Singlet oxygen (¹O₂) and triplet chromophoric dissolved organic matter (³CDOM*) are photochemically produced reactive intermediates responsible for the photodegradation of several micropollutants in the sunlit surface waters. However, elucidating the mechanism of reactions involving both ¹O₂ and ³CDOM* can be complicated by the deeply interconnected nature of these two reactive species. In this work, we synthesized a series of model compounds inspired by the chemical structure of fenfuram, a fungicide used in the 1980s, and used them to investigate structure-reactivity relationships in photodegradation reactions involving ¹O₂ and ³CDOM*. A combination of steady-state and time-resolved approaches was employed to successfully predict the extent of ¹O₂-induced degradation. Conversely, the prediction of triplet-induced reactivity was complicated by the presence of repair mechanisms whose extent and relative importance were difficult to predict. The results of our work indicate that bimolecular rate constants measured via time-resolved techniques alone are not sufficient to accurately predict environmental half-lives, as intrinsic differences in the reaction mechanism can amplify the importance of secondary degradation pathways.

Assessment of oxidative stress of paracetamol to Daphnia magna via determination of Nrf1 and genes related to antioxidant system

Paracetamol (APAP) is one of the most widely used anti-inflammatory and analgesic drugs in human being health care and has been universally detected in various aquatic environments. However, its potential adverse effects and toxic mechanisms on freshwater invertebrates still remain unclear. In the present study, the effects of APAP on the expressions of Nrf1 and the antioxidant related genes including GCLC, GST, GPX, CAT, TRX, TrxR and Prx1 in Daphnia magna (D. magna) were evaluated after 24, 48 and 96 h, and the changes of GPX, GST and CAT enzyme activities, as well as the GSH and MDA content under APAP exposure for 48 h were also determined. Results showed that paracetamol affected the expressions of Nrf1 and antioxidant related genes in D. magna, which were related to the exposure time and concentration of APAP. Nrf1 was inhibited at 48 h, but induced at 96 h under the APAP exposure, being about two fold of the control in 5.0 μg/L. CAT were significantly induced in all treatments. But Prx decreased in an concentration-dependent manner in all treatments. In comparison with the mRNA expression, antioxidant enzymes activity displayed less changes in D. magna. Overall, APAP exposure altered the expression of Nrf1 and genes related to antioxidant system and disturbed the redox homeostasis of D. magna.

Synergistic removal of ammonium by monochloramine photolysis

The presence of ammonium (NH₄⁺) in drinking water treatment results in inhibition of disinfection efficiency and formation of nitrogenous disinfection by-products. Our previous study found monochloramine (NH₂Cl) photolysis under 254 nm UV irradiation can be effective for removal of NH₄⁺; however, the mechanisms of NH₄⁺ degradation in this process were unknown. The kinetics and fundamental radical chemistry responsible for NH₄⁺ removal in the UV/NH₂Cl process were investigated in this study. The results showed that the pseudo first-order rate constant for NH₄⁺ degradation in the UV/NH₂Cl process ranged between 3.6 × 10^-4 to 1.8 × 10^-3 s^-1. Solution pH affected radical conversion and a higher NH₄⁺ degradation efficiency was achieved under acidic conditions. The effects of chloride were limited; however, the presence of either bicarbonate or natural organic matter scavenged radicals and inhibited NH₄⁺ removal. NH₂Cl photolysis generated an aminyl radical (NH₂^•) and a chlorine radical (Cl^•) that further transformed to a chlorine dimer (Cl₂^•-) and a hydroxyl radical (HO^•). The second-order rate constants for Cl^• and Cl₂^•- reacting with NH₄⁺ were estimated as 2.59 × 10⁸ M^-1s^-1 and 3.45 × 10⁵ M^-1s^-1 at pH 3.9, respectively. Cl^•, Cl₂^•-, and HO^• contributed 95.2%, 3.5%, and 1.3% to NH₄⁺ removal, respectively, at the condition of 3 mM NH₂Cl and pH 7.5. Major products included nitrite and nitrate, possibly accompanied by nitrogen-containing gases. This investigation provides insight into the photochemistry of NH₄⁺ degradation in the UV/NH₂Cl process and offers an alternative method for drinking water production.

Disparate effects of DOM extracted from coastal seawaters and freshwaters on photodegradation of 2,4-Dihydroxybenzophenone

Dissolved organic matter (DOM) plays an important role in degradation of organic pollutants by photochemically-produced reactive intermediates (RIs), such as excited triplet-states of DOM (³DOM*), singlet oxygen (¹O₂) and hydroxyl radical (·OH). However, it is not clear whether DOM extracted from coastal seawaters (CS-DOM) and DOM derived from freshwaters (FW-DOM) exhibit similar effects on photodegradation of organic micropollutants. Herein, 2,4-dihydroxybenzophenone (BP-1) was adopted as a model compound to probe the effects of different DOM on photodegradation kinetics of organic micropollutants. Results show that the CS-DOM promotes the photodegradation of BP-1 mainly via the pathway involving ³DOM*; while ³DOM*, ¹O₂ and ·OH are responsible for BP-1 photodegradation in the presence of the FW-DOM. Compared with the FW-DOM, the CS-DOM undergoes more photobleaching, and contains less aromatic C=C and C=O functional groups. Although ³DOM* formation quantum yields for the CS-DOM are relatively higher than those for the FW-DOM, the CS-DOM has lower rates of light absorption, leading to lower steady-state RI concentrations for the CS-DOM. BP-1 photodegradation in the presence of the CS-DOM is faster than in the presence of the FW-DOM, due to higher second-order reaction rate constants between BP-1 and CS-³DOM* and fewer antioxidants contained in the CS-DOM.

Enhanced Phototransformation of Tetracycline at Smectite Clay Surfaces under Simulated Sunlight via a Lewis-Base Catalyzed Alkalization Mechanism

As an important class of soil minerals and a key constituent of colloidal particles in surface aquifers, smectite clays can strongly retain tetracyclines due to their large surface areas and high cation exchange capacities. However, the research on phototransformation of tetracyclines at smectite clay surfaces is rarely studied. Here, the phototransformation kinetics of tetracycline preadsorbed on two model smectite clays (hectorite and montmorillonite) exchanged with Na⁺, K⁺, or Ca²⁺ suspended in aqueous solution under simulated sunlight was compared with that of tetracycline dissolved in water using batch experiments. Adsorption on clays accelerated tetracycline phototransformation (half-lives shortened by 1.1-5.3 times), with the most significant effects observed for Na⁺-exchanged clays. Regardless of the presence or absence of clay, the phototransformation of tetracycline was facilitated by increasing pH from 4 to 7. Inhibition or enhancement of photolysis-induced reactive species combined with their measurement using scavenger/probe chemicals indicate that the facilitated production of self-photosensitized singlet oxygen (¹O₂) was the key factor contributing to the clay-enhanced phototransformation of tetracycline. As evidenced by the red shifts and the increased molar absorptivity in the UV-vis absorption spectra, the complexation of tetracycline with the negatively charged (Lewis base) sites on clay siloxane surfaces led to formation of the alkalized form, which has larger light absorption rate and is more readily to be oxidized compared to tetracycline in aqueous solution at equivalent pH. Our findings indicate a previously unrecognized, important phototransformation mechanism of tetracyclines catalyzed by smectite clays.

Enhanced removal of organic contaminants in water by the combination of peroxymonosulfate and carbonate

In this study, a favorable CO₃^2-/PMS system for efficient degradation of organic contaminants (acid orange 7 (AO7), acetaminophen, para-aminobenzoic acid, phenol, methylene orange, methylene blue) in water was firstly reported. Under optimal conditions, the decolorization ration of AO7 was 100% within 40 min. Data fitting showed that the AO7 decolorization could be described by the pseudo-first-order kinetics, and the rates constant values ranging from 0.0006 to 0.2297 min^-1 depending on the operating parameters (initial PMS, CO₃^2-, AO7 concentrations). Radical scavenging studies revealed that superoxide anion radical (O₂^-) and singlet oxygen (¹O₂) rather than sulfate (SO₄^-) nor hydroxyl (HO) were the dominant oxidants might be responsible for AO7 degradation. The presence of NO₃^-, HPO₄^2- and low concentration of Cl^-, NO₂^-, HCO₃^-, H₂PO₄^-, HA had no significantly effect on the decolorization of AO7. Adding a higher Cl^- concentration displayed favorable effects on the removal efficiencies of AO7, but adding a higher NO₂^-, HCO₃^-, H₂PO₄^- and HA concentration apparently inhibited this process. The decolorization of AO7 was lower in wastewater in comparison to other natural waters and ultrapure water, which was probably due to the presence of higher concentration of colloids in wastewater. Nevertheless, up to 94.8%, 97.0% and 85.1% of AO7 were degraded from the filtrate, permeate, and retentate phases of wastewater within 60 min, respectively. Consequently, CO₃^2-/PMS would be promising for removal methodology for AO7 in wastewater containing considerable colloids. Finally, three intermediates were identified and degradation pathways of AO7 were proposed.

Mechanistic Study on the Role of Soluble Microbial Products in Sulfate Radical-Mediated Degradation of Pharmaceuticals

The role of soluble microbial products (SMP), the most important component of effluent organic matter from municipal wastewater treatment plants, in sulfate radical (SO₄^•-)-based advanced oxidation technologies (AOTs) remains substantially unclear. In this study, we first utilized a suite of macro- and microanalytical techniques to characterize the SMP from a membrane bioreactor for its fundamental molecular, spectroscopic, and reactivity properties. The degradation kinetics of three representative pharmaceuticals (i.e., naproxen, gemfibrozil, and sulfadiazine) in the presence of SMP was significantly reduced as compared to in its absence. Possible mechanisms for the interference by SMP in degrading these target compounds (TCs) were investigated. The low percentage of bound TCs to SMP ruled out the cage effect. The measurement of steady-state ¹O₂ concentration indicated that formation of ¹O₂ upon UV irradiation on SMP was not primarily responsible for the degradation of TCs. However, the comparative and quenching results reveal that SMP absorbs UV light acting as an inner filter toward the TCs, and meanwhile scavenges SO₄^•- with a high second-order rate constant of 2.48 × 10⁸ M_C^-1 s^-1.

Transformation of acetaminophen in natural surface water and the change of aquatic microbes

The kinetics and transformation pathway of acetaminophen (APAP) in natural surface water (one sample from the Yangtze River and three others from different lakes), and the changes of aquatic microbes in surface water were revealed in this study. Both photochemical and microbial reactions contributed to the transformation of APAP under irradiance of 1.0-250 mW/cm². Microbial compositions were significantly different among surface water, and same microbial transformation product (1,4-bezoquinone) was detected as the predominant biotransformation intermediate in four studied surface water, but the lag phase (12-50-h) for the transformation was highly dependent on the aquatic microbial abundance and composition. The lag phase no longer existed with irradiance increased to 5.9 mW/cm². Aquatic microbial abundance and composition were influenced by the presence of APAP and radiation, and the influence extent was dependent on microbial species. The findings demonstrated that the individual contribution of biotic and abiotic process to the overall transformation of APAP and maybe other phenol in surface water varied as the background composition of surface water and the external environment changed, and biotransformation dominated (>73%) the overall transformation of APAP in surface water.

The role of reactive oxygen species and carbonate radical in oxcarbazepine degradation via UV, UV/H2O2: Kinetics, mechanisms and toxicity evaluation

Oxcarbazepine (OXC) is ubiquitous in the aqueous environment. And due to its ecotoxicological effects and potential risks to human, an effective way to eliminate OXC from aqueous environment has aroused public concerns in recent years. Radical-based reactions have been shown to be an efficient way for OXC destruction, but the reactions of OXC with reactive oxygen species (ROS) and carbonate radical (CO₃^•-) are still unclear. In this study, we focused the degradation of OXC and ROS, CO₃^•- generation mechanism, and their roles in OXC degradation via UV and UV/H₂O₂. The triplet state of oxcarbazepine (³OXC^∗) was found to play an important role in OXC degradation via UV. And hydroxyl radicals (^•OH) and singlet oxygen (¹O₂) were found to be the dominant ROS in OXC degradation. Superoxide radical (O₂^•-) did not react with OXC directly, but it may react with intermediate byproducts. Generation of CO₃^•- played a positive role on OXC degradation for both UV and UV/H₂O₂. In addition to ^•OH, ³OXC* also contribute to CO₃^•- production. The second-order rate constants of OXC with ^•OH and CO₃^•- were 1.7 × 10¹⁰ M^-1 s^-1 and 8.6 × 10⁷ M^-1 s^-1, respectively. Potential OXC degradation mechanisms by ^•OH were proposed and included hydroxylation, α-ketol rearrangement, and benzylic acid rearrangement. Compared with non-selective ^•OH, the reactions involving CO₃^•- are mainly electron transfer and hydrogen abstraction. And the acute toxicity of OXC was lower after UV/H₂O₂ and UV/H₂O₂/HCO₃^- treatments, which was confirmed by luminescent bacterial assay (Vibrio fischeri bacterium).

Degradation of acetaminophen in aqueous solution by UV and UV-activated sludge processes

Acetaminophen (N-acetyl-p-aminophenol, APAP) is one of the most common antipyretic analgesics used to treat common ailments throughout the world. Recently, APAP has been frequently detected in wastewater effluent and groundwater, resulting in potential risks to the environment. Current methods for eliminating APAP are complicated and cost-prohibitive. This study examined APAP degradation by ultraviolet-C (UV-C) and UV-C irradiation combined with activated sludge (UV/AS) to evaluate potential applications in wastewater treatment. The results of this study indicate that UV-C irradiation reached an APAP degradation efficiency of more than 52% and a degradation rate of 0.0012-0.0013 min^-1 during 720 min of exposure, while the initial APAP concentration exhibited only a nominal effect on the degradation rate. However, the UV/AS treatment demonstrated an APAP degradation rate that was 9.6 times the rate of the UV-C-only treatment, with a degradation efficiency of 99% over the same UV irradiation period. The results further indicated that APAP photolysis efficiency was more effective when applied to sterilized AS than when applied to unsterilized AS. Finally, excessive dosage of both AS and humic acid inhibited APAP photolysis efficiency.

A facile electrochemical aptasensor for lysozyme detection based on target-induced turn-off of photosensitization

The quantification of proteins is essential in fundamental research or clinical applications. Here, we developed a facile electrochemical aptasensor based on target-induced turn-off of photosensitization for label-free and ultrasensitive detection of protein (exemplified by lysozyme). EB (ethidium bromide) molecules that were embedded in dsDNA between lysozyme binding aptamer and complementary DNA immobilized on the electrode, could photo-cleave the dsDNA via singlet oxygen (O₂¹) during photosensitization, resulting in a high voltammetry current of the [Fe(CN)₆]^3-/4-. Upon recognition of the lysozyme by aptamer, the EB molecules were released from dsDNA, and its photosensitization activity was turned off. As a result, more amount of complementary DNA was retained on the Au nanoparticles modified carbon nanotube paste electrode (AuNPs-CNPE), leading to a declined voltammetry current. Such a sensing strategy allowed detection of 10 pM-1 µM lysozyme with a low detection limit (about 2 pM). Besides, the sensor was free of labeling procedure as well as extra signal amplification step, and the CNPE modification was quite simple, only with AuNPs. The sensor also showed excellent selectivity toward lysozyme in the presence of interfering proteins, such as thrombin, bovine serum albumin, myoglobin, etc. The proposed sensor was applied to the determination of lysozyme in urine samples with the recoveries ranging from 96.6% to 101%. The proposed biosensor holds a great promise in developing other electrochemical sensors based on photosensitization.

Dissolved Black Carbon as an Efficient Sensitizer in the Photochemical Transformation of 17β-Estradiol in Aqueous Solution

Dissolved black carbon (DBC) is an important component of the dissolved organic matter (DOM) pool. Nonetheless, little is known about its role in the photochemical processes of organic contaminants. This study investigated the effect of DBC on the phototransformation of 17β-estradiol in aqueous solutions under simulated sunlight. Four well-studied dissolved humic substances (DHS) were included as comparisons. DBC acted as a very effective sensitizer to facilitate the phototransformation of 17β-estradiol. The apparent quantum yield for 17β-estradiol phototransformation mediated by DBC was approximately six times higher than that by DHS at the same carbon concentration. Quenching experiments suggested that direct reaction with triplet-excited state DBC (³DBC*) was the predominant pathway of 17β-estradiol phototransformation. The higher mediation efficiency of DBC than DHS is likely due to the higher contents of aromatic groups and smaller molecular sizes, which facilitated the generation of ³DBC*. The apparent quantum yield of triplet-excited states production for DBC was 4-8 times higher than that for DHS. The results suggest that ³DBC* may have a considerable contribution to the overall photoreactivity of triplet-excited state DOM in aquatic systems. Our findings also imply that DBC can play an important role in the phototransformation of organic contaminants in the environments.

Contribution of the Excited Triplet State of Humic Acid and Superoxide Radical Anion to Generation and Elimination of Phenoxyl Radical

Contributions of excited triplet state of humic acid (³HA*) and superoxide radical anion (O₂^•-), which is mainly generated via the reaction of O₂ with HA-derived reducing intermediates (HA^•-), to phenol transformation were revealed using acetaminophen, 2,4,6-trimethylphenol and tyrosine as probe molecules. Phenol transformation was initiated by ³HA*, leading to the formation of the phenoxyl radical (PhO•), but the distribution of transformation intermediates was codetermined by ³HA* and HA^•-. The influence of HA^•- essentially resulted from the production of O₂^•-, which affected the fate of PhO•. PhO• could undergo dimerization, or react with O₂^•-, leading to either phenol peroxide formation (radical addition) or phenol regeneration (electron transfer). In addition, PhO• could bind to HA or react with HA radicals, particularly in the absence of O₂ and O₂^•-. These PhO• reactions were dependent on the reduction potential and structure of PhO•. This study also proved that the reaction of phenol with ¹O₂ and the reaction of PhO• with O₂^•- lead to the same oxidation product. The contributions of ³HA* and its generated ¹O₂, HA^•- and its generated O₂^•- to phenol transformation were pH-dependent.

Influence of DOM components, salinity, pH, nitrate, and bicarbonate on the indirect photodegradation of acetaminophen in simulated coastal waters

The indirect photodegradation behaviors of acetaminophen (APAP) were investigated in the presence of four kinds of dissolved organic matter (DOM) and were also assessed in the presence of seawater components and conditions such as salinity, pH, nitrate and bicarbonate. The results showed three important findings: firstly, in the indirect photolysis of APAP, the contributions of ³DOM*, ·OH and ¹O₂ were >85.0%, 2.3-9.9% and 0.8-2.6% at pH 8.0. Secondly, DOM was divided into four terrestrial humic-like components by Excitation-emission matrix spectroscopy (EEMs) combined with parallel factor analysis (PARAFAC). This study showed a good linearity between DOM fluorescence components and the indirect photodegradation of APAP (R² = 0.92) and the differences in photodegradation rates of APAP among various DOM solutions were due to the diverse compositions of DOM. Finally, salinity was an important factor influencing the removal of APAP, and the APAP photodegradation rate constants increased from (3.33 ± 0.07) × 10^-5 s^-1 to (1.25 ± 0.05) × 10^-4 s^-1 with increasing salinity. The increased pseudo-first-order rate constants for photolysis of APAP with increasing salinity, pH and nitrate were attributed to the enhanced generation of reactive intermediates (RI) and easier reactions between RI and APAP. The increased APAP removal rate constant with increasing bicarbonate was likely ascribed to the yield of ∙CO₃^-. This is the first report of the roles of DOM components and salinity on the indirect photolysis of APAP. These findings would be essential to predict the photochemical fate of APAP and would also allow for a better understanding of the environmental fate of other phenolic contaminants.

Copper Inhibition of Triplet-Induced Reactions Involving Natural Organic Matter

The triplet excited state of natural organic matter (³NOM*) is an important reactive intermediate in sensitizing transformation of a wide range of environmentally relevant organic compounds, but the impact of trace metals on the fate and reactivity of ³NOM* is poorly understood. In this study, we investigate the effect of low concentrations of copper on ³NOM*-mediated oxidation (electron transfer) and energy transfer reactions. The oxidative efficiency of ³NOM* from Suwannee River NOM (SRNOM) and the widely used model triplet sensitizer 4-carboxybenzophenone were determined by measuring the photooxidation of 2,4,6-trimethylphenol (TMP). The pseudo-first-order photooxidation rate constants of TMP decreased markedly in the presence of trace amounts of Cu(II) (25-500 nM) with the decrease associated with the continuous reduction of the oxidation intermediates of TMP (i.e., TMP^•_(-H)) by the photochemically produced Cu(I). A kinetic model is developed that adequately describes the Cu inhibition effect in TMP photooxidation in irradiated SRNOM solutions. The ³NOM* energy transfer ability was assessed by measuring the isomerization of sorbic acid with the rate of this process markedly retarded in the presence of significantly higher (micromolar) concentrations of Cu(II) than previously used. This result is attributed to (i) decreased formation of high energy ³NOM* due to formation of Cu-NOM complexes and (ii) increased loss of ³NOM* as a result of quenching by Cu. Since ³NOM* is the precursor to singlet oxygen (¹O₂) formation, the steady-state concentrations of ¹O₂ also decreased in the presence of micromolar concentrations of Cu(II) with the quenching rate constant of ³NOM* by Cu calculated to be 1.08 × 10¹⁰ M^-1 s^-1.

Photoreactivity of graphene oxide in aqueous system: Reactive oxygen species formation and bisphenol A degradation

The phototransformation and environmental implications of graphene oxide (GO) have been widely studied in order to understand its implications upon release into the environment. However, very little is known about the formation of reactive oxygen species (ROS) by GO under solar irradiation. Currently there are no studies on the mechanism of ROS formation by GO or the amount of ROS catalyzed by the nanomaterials in the environment. In this study, we carefully investigated the different types and formation mechanisms of ROS generated by GO in the presence of simulated solar irradiation. The effect of GO's photoactivity on bisphenol A (BPA), a representative organic co-pollutant, was also studied. The conduction band electron (e_aq^-) of GO led to the formation of different ROS including OH, O₂^-, and ¹O₂. Among the three types of ROS investigated, O₂^- was the most abundant species generated during simulated solar irradiation of GO. BPA was degraded, mainly due to the oxidative potential of the valence band holes produced during solar irradiation of GO. This study advances understanding of the photoactivity of GO and its potential impact on other possible environmental co-pollutants.

Predicting apparent singlet oxygen quantum yields of dissolved black carbon and humic substances using spectroscopic indices

Dissolved black carbon (DBC) is ubiquitous in aquatic systems, being an important subgroup of the dissolved organic matter (DOM) pool. Nevertheless, its aquatic photoactivity remains largely unknown. In this study, a range of spectroscopic indices of DBC and humic substance (HS) samples were determined using UV-Vis spectroscopy, fluorescence spectroscopy, and proton nuclear magnetic resonance. DBC can be readily differentiated from HS using spectroscopic indices. It has lower average molecular weight, but higher aromaticity and lignin content. The apparent singlet oxygen quantum yield (Φ_{singlet oxygen}) of DBC under simulated sunlight varies from 3.46% to 6.13%, significantly higher than HS, 1.26%-3.57%, suggesting that DBC is the more photoactive component in the DOM pool. Despite drastically different formation processes and structural properties, the Φ_{singlet oxygen} of DBC and HS can be well predicted by the same simple linear regression models using optical indices including spectral slope coefficient (S_275-295) and absorbance ratio (E₂/E₃) which are proxies for the abundance of singlet oxygen sensitizers and for the significance of intramolecular charge transfer interactions. The regression models can be potentially used to assess the photoactivity of DOM at large scales with in situ water spectrophotometry or satellite remote sensing.

Aquatic photodegradation of clofibric acid under simulated sunlight irradiation: kinetics and mechanism analysis

Clofibric acid is one of the most frequently detected pharmaceuticals in various aquatic environments.

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.

Environmental photochemistry of fenamate NSAIDs and their radical intermediates

Fenamates are a class of nonsteroidal anti-inflammatory drugs (NSAIDs) that are not fully removed during wastewater treatment and can be released to surface waters. Here, near-surface photochemical half-lives were evaluated to range from minutes to hours of four fenamates and the closely related diclofenac. While quantum yields for direct photochemical reactions at the water surface vary widely from 0.071 for diclofenac to <0.001 for mefenamic acid, all fenamates showed significant reactivity towards singlet oxygen and hydroxyl radical with bimolecular reaction rate constants of 1.3-2.8 × 10⁷ M^-1 s^-1 and 1.1-2.7 × 10¹⁰ M^-1 s^-1, respectively. Photodecay rates increased in the presence of dissolved organic matter (DOM) for diclofenac (+19%), tolfenamic acid (+9%), and mefenamic acid (+95%), but decreased for flufenamic acid (-2%) and meclofenamic acid (-14%) after accounting for light screening effects. Fast reaction rate constants of all NSAIDs with model triplet sensitizers were quantified by laser flash photolysis. Here, the direct observation of diphenylamine radical intermediates by transient absorption spectroscopy demonstrates one-electron oxidation of all fenamates. Quenching rate constants of these radical intermediates by ascorbic acid, a model antioxidant, were also quantified. These observations suggest that the balance of oxidation by photoexcited triplet DOM and quenching of the formed radical intermediates by antioxidant moieties determines whether net sensitization or net quenching by DOM occurs in the photochemical degradation of fenamates.