10 research outputs found

    Phenolic Antioxidants Inhibit the Triplet-Induced Transformation of Anilines and Sulfonamide Antibiotics in Aqueous Solution

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    Recent studies have shown that dissolved organic matter (DOM) may inhibit the excited triplet-induced oxidation of several aromatic water contaminants, in particular those containing an aniline functionality. Such an inhibition was ascribed to antioxidant moieties of DOM. The present study was conducted with the aim of verifying whether well-defined antioxidants could act as inhibitors in analogy to DOM. Various substituted phenols exhibiting antioxidant character were able, at micromolar concentration, to slow down the photoinduced depletion of several anilines and sulfonamides in aerated aqueous solution containing 2-acetonaphthone as the photosensitizer. A concomitant accelerated degradation of the phenols in the presence of such contaminants was observed. This reinforces the hypothesis of reduction of oxidation intermediates of the contaminants by the phenols. Phenol (unsubstituted) was found to be a useful inhibitor even in the case of DOM-photosensitized transformations. Phenolic antioxidants are proposed as diagnostic tools to investigate the aquatic photochemistry of aromatic amines

    Probing the Photosensitizing and Inhibitory Effects of Dissolved Organic Matter by Using <i>N</i>,<i>N</i>‑dimethyl-4-cyanoaniline (DMABN)

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    Dissolved organic matter (DOM) can act as a photosensitizer and an inhibitor in the phototransformation of several nitrogen-containing organic contaminants in surface waters. The present study was performed to select a probe molecule that is suitable to measure these antagonistic properties of DOM. Out of nine studied nitrogen-containing aromatic compounds, 4-cyanoaniline, <i>N</i>,<i>N</i>-dimethyl-4-cyanoaniline (DMABN), sotalol (a β-blocker) and sulfadiazine (a sulfonamide antibiotic) exhibited a marked photosensitized transformation that could be substantially inhibited by addition of phenol as a model antioxidant. The photosensitized transformation of DMABN, the selected probe compound, was characterized in detail under UV-A and visible irradiation (λ > 320 nm) to avoid direct phototransformation. Low reactivity of DMABN with singlet oxygen was found (second-order rate constant <2 × 10<sup>7</sup> M<sup>–1</sup> s<sup>–1</sup>). Typically at least 85% of the reactivity of DMABN could be inhibited by DOM or the model antioxidant phenol. The photosensitized transformation of DMABN mainly proceeded (>72%) through demethylation yielding <i>N</i>-methyl-4-cyanoaniline and formaldehyde as primary products. In solutions of standard DOM extracts and their mixtures the phototransformation rate constant of DMABN was shown to vary nonlinearly with the DOM concentration. Model equations describing the dependence of such rate constants on DOM and model antioxidant concentrations were successfully used to fit experimental data

    Quenching of Excited Triplet States by Dissolved Natural Organic Matter

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    Excited triplet states of aromatic ketones and quinones are used as proxies to assess the reactivity of excited triplet states of the dissolved organic matter (<sup>3</sup>DOM*) in natural waters. <sup>3</sup>DOM* are crucial transients in environmental photochemistry responsible for contaminant transformation, production of reactive oxygen species, and potentially photobleaching of DOM. In recent photochemical studies aimed at clarifying the role of DOM as an inhibitor of triplet-induced oxidations of organic contaminants, aromatic ketones have been used in the presence of DOM, and the question of a possible interaction between their excited triplet states and DOM has emerged. To clarify this issue, time-resolved laser spectroscopy was applied to measure the excited triplet state quenching of four different model triplet photosensitizers induced by a suite of DOM from various aquatic and terrestrial sources. While no quenching for the anionic triplet sensitizers 4-carboxybenzophenone (CBBP) and 9,10-anthraquinone-2,6-disulfonic acid (2,6-AQDS) was detected, second-order quenching rate constants with DOM for the triplets of 2-acetonaphthone (2AN) and 3-methoxyacetophenone (3MAP) in the range of 1.30–3.85 × 10<sup>7</sup> L mol<sub>C</sub><sup>–1</sup> s<sup>–1</sup> were determined. On the basis of the average molecular weight of DOM molecules, the quenching for these uncharged excited triplet molecules is nearly diffusion-controlled, but significant quenching (>10%) in aerated water is not expected to occur below DOM concentrations of 22–72 mg<sub>C</sub> L<sup>–1</sup>

    Isotope Fractionation Associated with the Photochemical Dechlorination of Chloroanilines

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    Isotope fractionation associated with the photochemical transformation of organic contaminants is not well understood and can arise not only from bond cleavage reactions but also from photophysical processes. In this work, we investigated the photolytic dechlorination of 2-Cl- and 3-Cl-aniline to aminophenols to obtain insights into the impact of the substituent position on the apparent <sup>13</sup>C and <sup>15</sup>N kinetic isotope effects (AKIEs). Laboratory experiments were performed in aerated aqueous solutions at an irradiation wavelength of 254 nm over the pH range 2.0 to 7.0 in the absence and presence of Cs<sup>+</sup> used as an excited singlet state quencher. Photolysis of 2-Cl-anilinium cations exhibits normal C and inverse N isotope fractionation, while neutral 2-Cl-aniline species shows inverse C and normal N isotope fractionation. In contrast, the photolysis of 3-Cl-aniline was almost insensitive to C isotope composition and the moderate N isotope fractionation points to rate-limiting photophysical processes. <sup>13</sup>C- and <sup>15</sup>N-AKIE-values of 2-Cl-aniline decreased in the presence of Cs<sup>+</sup>, whereas those for 3-Cl-aniline were not systematically affected by Cs<sup>+</sup>. Our current and previous work illustrates that photolytic dechlorinations of 2-Cl-, 3-Cl-, and 4-Cl-aniline isomers are each accompanied by distinctly different and highly variable C and N isotope fractionation due to spin selective isotope effects

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

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    Organic micropollutants containing aniline substructures are susceptible to different light-induced transformation processes in aquatic environments and water treatment operations. Here, we investigated the magnitude and variability of C and N isotope fractionation during the indirect phototransformation of four <i>para</i>-substituted anilines in aerated aqueous solutions. The model photosensitizers, namely 9,10-anthraquinone-1,5-disulfonate and methylene blue, were used as surrogates for dissolved organic matter chromophores generating excited triplet states in sunlit surface waters. The transformation of aniline, 4-CH<sub>3</sub>-, 4-OCH<sub>3</sub>-, and 4-Cl-aniline by excited triplet states of the photosensitizers was associated with inverse and normal N isotope fractionation, whereas C isotope fractionation was negligible. The apparent <sup>15</sup>N kinetic isotope effects (AKIE) were almost identical for both photosensitizers, increased from 0.9958 ± 0.0013 for 4-OCH<sub>3</sub>-aniline to 1.0035 ± 0.0006 for 4-Cl-aniline, and correlated well with the electron donating properties of the substituent. N isotope fractionation is pH-dependent in that H<sup>+</sup> exchange reactions dominate below and N atom oxidation processes above the p<i>K</i><sub>a</sub> value of the substituted aniline’s conjugate acid. Correlations of C and N isotope fractionation for indirect phototransformation were different from those determined previously for the direct photolysis of chloroanilines and offer new opportunities to distinguish between abiotic degradation pathways

    Abatement of Polychoro-1,3-butadienes in Aqueous Solution by Ozone, UV Photolysis, and Advanced Oxidation Processes (O<sub>3</sub>/H<sub>2</sub>O<sub>2</sub> and UV/H<sub>2</sub>O<sub>2</sub>)

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    The abatement of 9 polychloro-1,3-butadienes (CBDs) in aqueous solution by ozone, UV–C­(254 nm) photolysis, and the corresponding advanced oxidation processes (AOPs) (i.e., O<sub>3</sub>/H<sub>2</sub>O<sub>2</sub> and UV/H<sub>2</sub>O<sub>2</sub>) was investigated. The following parameters were determined for 9 CBDs: second-order rate constants for the reactions of CBDs with ozone (<i>k</i><sub>O<sub>3</sub></sub>) (<0.1–7.9 × 10<sup>3</sup> M<sup>–1</sup> s<sup>–1</sup>) or with hydroxyl radicals (<i>k</i><sub><sup>•</sup>OH</sub>) (0.9 × 10<sup>9</sup> – 6.5 × 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup>), photon fluence-based rate constants (<i>k</i>′) (210–2730 m<sup>2</sup> einstein<sup>–1</sup>), and quantum yields (Φ) (0.03–0.95 mol einstein<sup>–1</sup>). During ozonation of CBDs in a natural groundwater, appreciable abatements (>50% at specific ozone doses of 0.5 gO<sub>3</sub>/gDOC to ∼100% at ≥1.0 gO<sub>3</sub>/gDOC) were achieved for tetra-CBDs followed by (<i>Z</i>)-1,1,2,3,4-penta-CBD and hexa-CBD. This is consistent with the magnitude of the determined <i>k</i><sub>O<sub>3</sub></sub> and <i>k</i><sub><sup>•</sup>OH</sub>. The formation of bromate, a potentially carcinogenic ozonation byproduct, could be significantly reduced by addition of H<sub>2</sub>O<sub>2</sub>. For a typical UV disinfection dose (400 J/m<sup>2</sup>), various extents of phototransformations (10–90%) could be achieved. However, the efficient formation of photoisomers from CBDs with <i>E</i>/<i>Z</i> configuration must be taken into account because of their potential residual toxicity. Under UV–C­(254 nm) photolysis conditions, no significant effect of H<sub>2</sub>O<sub>2</sub> addition on CBDs abatement was observed due to an efficient direct phototransformation of CBDs

    Chemical Oxidation of Dissolved Organic Matter by Chlorine Dioxide, Chlorine, And Ozone: Effects on Its Optical and Antioxidant Properties

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    In water treatment dissolved organic matter (DOM) is typically the major sink for chemical oxidants. The resulting changes in DOM, such as its optical properties have been measured to follow the oxidation processes. However, such measurements contain only limited information on the changes in the oxidation states of and the reactive moieties in the DOM. In this study, we used mediated electrochemical oxidation to quantify changes in the electron donating capacities (EDCs), and hence the redox states, of three different types of DOM during oxidation with chlorine dioxide (ClO<sub>2</sub>), chlorine (as HOCl/OCl<sup>–</sup>), and ozone (O<sub>3</sub>). Treatment with ClO<sub>2</sub> and HOCl resulted in comparable and prominent decreases in EDCs, while the UV light absorbances of the DOM decreased only slightly. Conversely, ozonation resulted in only small decreases of the EDCs but pronounced absorbance losses of the DOM. These results suggest that ClO<sub>2</sub> and HOCl primarily reacted as oxidants by accepting electrons from electron-rich phenolic and hydroquinone moieties in the DOM, while O<sub>3</sub> reacted via electrophilic addition to aromatic moieties, followed by ring cleavage. This study highlights the potential of combined EDC-UV measurements to monitor chemical oxidation of DOM, to assess the nature of the reactive moieties and to study the underlying reaction pathways

    Isotope Fractionation Associated with the Direct Photolysis of 4‑Chloroaniline

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    Compound-specific isotope analysis is a useful approach to track transformations of many organic soil and water pollutants. Applications of CSIA to characterize photochemical processes, however, have hardly been explored. In this work, we systematically studied C and N isotope fractionation associated with the direct photolysis of 4-Cl-aniline used as a model compound for organic micropollutants that are known to degrade via photochemical processes. Laboratory experiments were carried out at an irradiation wavelength of 254 nm over the pH range 2.0 to 9.0 as well as in the presence of Cs<sup>+</sup> as a quencher of excited singlet 4-Cl-aniline at pH 7.0 and 9.0. We observed considerable variation of C and N isotope enrichment factors, ϵ<sub>C</sub> and ϵ<sub>N</sub>, between −1.2 ± 0.2‰ to −2.7 ± 0.2‰ for C and −0.6 ± 0.2‰ to −9.1 ± 1.6‰ for N, respectively, which could not be explained by the speciation of 4-Cl-aniline alone. In the presence of 1 M Cs<sup>+</sup>, we found a marked increase of apparent <sup>13</sup>C-kinetic isotope effects (<sup>13</sup>C-AKIE) and decrease of 4-Cl-aniline fluorescence lifetimes. Our data suggest that variations of C and N isotope fractionation originate from heterolytic dechlorination of excited triplet and singlet states of 4-Cl-aniline. Linear correlations of <sup>13</sup>C-AKIE vs <sup>15</sup>N-AKIE were distinctly different for these two reaction pathways and may be explored further for the identification of photolytic aromatic dechlorination reactions

    Isoproturon Reappearance after Photosensitized Degradation in the Presence of Triplet Ketones or Fulvic Acids

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    Isoproturon (IPU) is a phenylurea herbicide used to control broad-leaf grasses on grain fields. Photosensitized transformation induced by excited triplet states of dissolved organic matter (<sup>3</sup>DOM*) has been identified as an important degradation pathway for IPU in sunlit waters, but the reappearance of IPU in the absence of light is observed after the initial photolysis. In this study, we elucidate the kinetics of this photodegradation and dark-reappearance cycling of IPU in the presence of DOM proxies (aromatic ketones and reference fulvic acids). Using mass spectrometry and nuclear magnetic resonance spectroscopic techniques, a semi-stable intermediate (IPU<sub>int</sub>) was found to be responsible for IPU reversion and was identified as a hydroperoxyl derivative of IPU. IPU<sub>int</sub> is photogenerated from incorporation of diatomic oxygen to IPU and is subjected to thermolysis whose rate depends on temperature, pH, the presence of DOM, and inorganic ions. These results are important to understand the overall aquatic fate of IPU and structurally similar compounds under diurnal conditions

    Organic Contaminant Abatement in Reclaimed Water by UV/H<sub>2</sub>O<sub>2</sub> and a Combined Process Consisting of O<sub>3</sub>/H<sub>2</sub>O<sub>2</sub> Followed by UV/H<sub>2</sub>O<sub>2</sub>: Prediction of Abatement Efficiency, Energy Consumption, and Byproduct Formation

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    UV/H<sub>2</sub>O<sub>2</sub> processes can be applied to improve the quality of effluents from municipal wastewater treatment plants by attenuating trace organic contaminants (micropollutants). This study presents a kinetic model based on UV photolysis parameters, including UV absorption rate and quantum yield, and hydroxyl radical (·OH) oxidation parameters, including second-order rate constants for ·OH reactions and steady-state ·OH concentrations, that can be used to predict micropollutant abatement in wastewater. The UV/H<sub>2</sub>O<sub>2</sub> kinetic model successfully predicted the abatement efficiencies of 16 target micropollutants in bench-scale UV and UV/H<sub>2</sub>O<sub>2</sub> experiments in 10 secondary wastewater effluents. The model was then used to calculate the electric energies required to achieve specific levels of micropollutant abatement in several advanced wastewater treatment scenarios using various combinations of ozone, UV, and H<sub>2</sub>O<sub>2</sub>. UV/H<sub>2</sub>O<sub>2</sub> is more energy-intensive than ozonation for abatement of most micropollutants. Nevertheless, UV/H<sub>2</sub>O<sub>2</sub> is not limited by the formation of <i>N</i>-nitrosodimethylamine (NDMA) and bromate whereas ozonation may produce significant concentrations of these oxidation byproducts, as observed in some of the tested wastewater effluents. The combined process of O<sub>3</sub>/H<sub>2</sub>O<sub>2</sub> followed by UV/H<sub>2</sub>O<sub>2</sub>, which may be warranted in some potable reuse applications, can achieve superior micropollutant abatement with reduced energy consumption compared to UV/H<sub>2</sub>O<sub>2</sub> and reduced oxidation byproduct formation (i.e., NDMA and/or bromate) compared to conventional ozonation
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