Effects of dissolved organic matter on the phototransformation of aromatic amines in surface waters

Dissolved Organic Matter (DOM) is a complex mixture of thousands of organic molecules ubiquitously present in surface waters. Its influence on the photodegradation of organic contaminants is a complicated topic. For decades DOM had been mostly assumed to have a positive effect (i.e., promoting the degradation of contaminants), but more recently it was discovered that for some compounds, DOM can also have negative effects (i.e., inhibiting the photodegradation of contaminants) in surface waters. These dual properties of DOM are the subject of the investigations of this thesis, in which DOM is considered for both its photosensitizing and inhibitory properties. The goal of this thesis is to achieve a better understanding of the photoreactivity of DOM towards contaminants in surface waters. To investigate the dual effect of DOM, an initial screening study was done to look for an optimal probe compound, subject to both photosensitizing and inhibitory effects. Nine candidate pharmaceuticals or model compounds, almost exclusively aromatic amines, were selected and their photodegradation evaluated under simulated sunlight in various water matrix. This study lead to the selec-tion of N,N-dimethyl-4-cyanoaniline (DMABN) as a probe compound. The indirect photodegradation of DMABN was characterized and shown to proceed mostly through excited triplet states of DOM. The indirect photodegradation kinetics of DMABN was also studied in various waters and mixtures of waters, and model equations describing the effect of the photosensitizing and inhibitory properties of DOM on photodegradation rates constants were developed. An indirect phototransformation pathway for DMABN was proposed, in which following initial oxidation of DMABN by triplet DOM, deprotonation of the formed radical cation of DMABN (DMABN+.) occurred. Subsequently, demethylation of DMABN took place, forming N-methyl-4-cyanoaniline and formaldehyde. The absence of deuterium kinetic isotope effect on the inhibitory effect of phenol, used as a surrogate for DOM, on the indirect photo-transformation of DMABN indicated that the inhibition proceeds by electron transfer. To study the inhibitory effect of DOM, laser flash photolysis (LFP) investigations to detect short-lived reaction intermediates and measure their reaction rate constants were performed. The rate constant for DMABN-induced quenching of a series of triplet-aromatic ketones, used as proxies for excited triplet DOM were found to follow well-known relationships in the frame of Marcus theory of electron transfer. DMABN+. was observed on a microsecond time scale as the primary species resulting from the reaction between the excited triplet photosensitizers and DMABN. The decay kinetics of this radical cation was studied in the presence and absence of phenols or DOM. DMABN+. was found, as expected, to be quenched by both phenols and DOM, confirming the postu-lated model for the inhibitory effect. Electron-rich phenols reacted faster than phenol with DMABN+., while the rate constants for quenching by DOM were proportional to the electron donating capacity of the DOM. Finally, a similar study as for DMABN was performed with sulfadiazine, a well-known antibiotic, as target compound. Results were qualitatively similar to those for DMABN but their interpretation was more complex probably due to deprotonation of the intermediate radical product

Computational Calculation of Dissolved Organic Matter Absorption Spectra

The absorption spectrum of dissolved organic matter (DOM) is a topic of interest to environmental scientists and engineers as it can be used to assess both the concentration and physicochemical properties of DOM. In this study, the UV&ndash;vis spectra for DOM model compounds were calculated using time-dependent density functional theory. Summing these individual spectra, it was possible to re-create the observed exponential shape of the DOM absorption spectra. Additionally, by predicting the effects of sodium borohydride reduction on the model compounds and then calculating the UV&ndash;vis absorbance spectra of the reduced compounds, it was also possible to correctly predict the effects of borohydride reduction on DOM absorbance spectra with a relatively larger decrease in absorbance at longer wavelengths. The contribution of charge-transfer (CT) interactions to DOM absorption was also evaluated, and the calculations showed that intra-molecular CT interactions could take place, while inter-molecular CT interactions were proposed to be less likely to contribute. &nbsp;</p

Iron Speciation in PM2.5 from Urban, Agriculture, and Mixed Environments in Colorado, USA

Atmospheric iron solubility varies depending on whether the particles are collected in rural or urban areas, with urban areas showing increased iron solubility. In this study, we investigate if the iron species present in different environments affects its ultimate solubility. Field data are presented from the Platte River Air Pollution and Photochemistry Experiment (PRAPPE), aimed at understanding the interactions between organic carbon and trace elements in atmospheric particulate matter (PM). 24-hr PM2.5 samples were collected during the summer and winter (2016–2017), at three different sites on the Eastern Colorado plains: an urban, agricultural, and a mixed site. Downtown Denver had an average total and water-soluble iron air concentration of 181.2 and 7.7 ng m−3, respectively. Platteville, the mixed site, had an average of total iron of 76.1 ng m−3, with average water-soluble iron concentration of 9.1 ng m−3. Jackson State Park (rural/agricultural) had the lowest total iron average of 31.5 ng m−3 and the lowest water-soluble iron average, 1.3 ng m−3. The iron oxidation state and chemical speciation of 97 samples across all sites and seasons was probed by X-ray absorption near edge structure (XANES) spectroscopy. The most common iron phases observed were almandine (Fe₃Al₂Si₃O₁₂) (Denver 21%, Platteville 16%, Jackson 24%), magnetite (Fe3O4) (Denver 9%, Platteville 4%, Jackson 5%) and Fe (III)dextran (Denver 5%, Platteville 13%, Jackson 5%), a surrogate for Fe-organic complexes. Additionally, native iron [Fe(0)] was found in significant amounts at all sites. No correlation was observed between iron solubility and iron oxidation state or chemical speciation

Laser flash photolysis study of the photoinduced oxidation of 4-(dimethylamino)benzonitrile (DMABN)

Aromatic amines are aquatic contaminants for which phototransformation in surface waters can be induced by excited triplet states of dissolved organic matter (3DOM*). The first reaction step is assumed to consist of a one-electron oxidation process of the amine to produce its radical cation. In this paper, we present laser flash photolysis investigations aimed at characterizing the photoinduced, aqueous phase one-electron oxidation of 4-(dimethylamino)benzonitrile (DMABN) as a representative of this contaminant class. The production of the radical cation of DMABN (DMABN˙+) after direct photoexcitation of DMABN at 266 nm was confirmed in accord with previous experimental results. Moreover, DMABN˙+ was shown to be produced from the reactions of several excited triplet photosensitizers (carbonyl compounds) with DMABN. Second-order rate constants for the quenching of the excited triplet states by DMABN were determined to fall in the range of 3 &times; 107&ndash;5 &times; 109 M&minus;1 s&minus;1, and their variation was interpreted in terms of electron transfer theory using a Rehm&ndash;Weller relationship. The decay kinetics of DMABN˙+ in the presence of oxygen was dominated by a second-order component attributed to its reaction with the superoxide radical anion (O2˙&minus;). The first-order rate constant for the transformation of DMABN˙+ leading to photodegradation of DMABN was estimated not to exceed &asymp;5 &times; 103 s&minus;1. </div

Photochemical Aging of Atmospheric Particulate Matter in the Aqueous Phase

This study focused on the photoaging of atmospheric particulate matter smaller than 2.5 &mu;m (PM2.5) in the aqueous phase. PM2.5 was collected during a winter, a spring, and a summer campaign in urban and rural settings in Colorado and extracted into water. The aqueous extracts were photoirradiated using simulated sunlight, and the production rate (r&bull;OH) and the effects of hydroxyl radicals (&bull;OH) were measured as well as the optical properties as a function of the photoaging of the extracts. r&bull;OH was seen to have a strong seasonality with low mean values for the winter and spring extracts (4.8 and 14 fM s&ndash;1 mgC&ndash;1 L, respectively) and a higher mean value for the summer extracts (65.4 fM s&ndash;1 mgC&ndash;1 L). For the winter extracts, &bull;OH was seen to mostly originate from nitrate photolysis while for the summer extracts, a correlation was seen between r&bull;OH and iron concentration. The extent of photobleaching of the extracts was correlated with r&bull;OH, and the correlation also indicated that non-&bull;OH processes took place. Using the &bull;OH measurements and singlet oxygen (1O2) measurements, the half-life of a selection of compounds was modeled in the atmospheric aqueous phase to be between 1.9 and 434 h. &nbsp;</p

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

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, N,N-dimethyl-4-cyanoaniline (DMABN), sotalol (a beta-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 (lambda > 320 nm) to avoid direct phototransformation. Low reactivity of DMABN with singlet oxygen was found (second-order rate constant 72%) through demethylation yielding N-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 an Aniline Radical Cation by Dissolved Organic Matter and Phenols: A Laser Flash Photolysis Study

Aromatic amines are relevant aquatic organic contaminants whose photochemical transformation is affected by dissolved organic matter (DOM). The goal of this study is to elucidate the underlying mechanism of the inhibitory effect of DOM on such reactions. The selected model aromatic amine, 4-(dimethylamino)benzonitrile (DMABN), was subjected to laser flash photolysis in the presence and absence of various model photosensitizers. The produced radical cation (DMABN(.+)) was observed to react with several phenols and different types of DOM on a time scale of similar to 100 mu s. The determined second-order rate constants for the quenching of DMABN(.+) by phenols were in the range of (1.4-26) X 10(8) M-1 s(-1) and increased with increasing electron donor character of the aromatic ring substituent. For DOM, quenching rate constants increased with the phenolic content of the DOM. These results indicate the reduction of DMABN(.+) to re-form its parent compound as the basic reaction governing the inhibitory effect. In addition, the photosensitized oxidation of the sulfonamide antibiotic sulfadiazine (SDZ) was studied. The observed radical intermediate of SDZ was quenched by 4-methoxyphenol less effectively than DMABN(.+), which was attributed to the lower reduction potential of the SDZ-derived radical compared to DMABN(.+)

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

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 × 10⁷ M^–1 s^–1). 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