Seasonal and spatial variabilities in the water chemistry of prairie pothole wetlands influence the photoproduction of reactive intermediates

Photoproduction of reactive intermediates from dissolved organic matter in coastal seawater around an urban metropolis in South China: Characterization and predictive modeling

Chromophoric dissolved organic matter (CDOM) is an important photochemical precursor to reactive intermediates (RIs) (e.g., excited triplet states of chromophoric dissolved organic matter (3CDOM⁎), hydroxyl radicals (·OH), and singlet oxygen (1O2)) in aquatic systems to drive the photodegradation of contaminants. There have been limited studies on the photoproduction of RIs in coastal seawater CDOM in Asia, which impedes our ability to model the lifetimes and fates of contaminants in these coastal seawater systems. Hong Kong is an urban metropolis in South China, whose coastal seawater is susceptible to anthropogenic activities from the surrounding areas and the nearby Pearl River. We investigated the photoproduction of RIs in seawater around Hong Kong during the wet vs. dry season. Higher intensities of fluorescent components, dissolved organic carbon concentration ([DOC]), apparent quantum yields of RIs (ΦRIs), and steady-state concentrations of photogenerated RIs ([RIs]ss) were observed for samples collected in the areas closest to the Pearl River during the wet season. Lower humification degrees and ΦRIs but higher intensities of fluorescent components and [RIs]ss were generally observed for the wet season samples compared to the dry season samples. Statistical analysis revealed strong significant correlations (Spearman |r| > 0.6, p < 0.05) between ΦRIs and the absorbance properties (including the absorbance ratio E2:E3, spectral slope coefficients S350-400, and spectral slope ratio SR) of CDOM, and between [RIs]ss and the quantity-reflected properties (including the fluorescence intensity of humic-like components) of CDOM. Our modeling analyses combining orthogonal partial least squares and stepwise multiple linear regression showed excellent prediction strengths for [1O2]ss and [3CDOM⁎]ss (R2adj > 0.7) when [DOC] and the chemical and optical properties of CDOM were used as predictor variables. These modeling results demonstrate the feasibility of predicting the concentrations and quantum yields of RIs in seawater around Hong Kong, and potentially other coastal cities in South China, from easily measurable chemical and optical properties.

Photo-production of excited triplet-state of dissolved organic matters in inland freshwater and coastal seawater

The excited triplet-state of dissolved organic matter (3DOM*) is a major reactive intermediate in sunlit waters. Its quantum yield is important in understanding the fate of organic micropollutants. The degradation efficiency of its chemical probe, 2,4,6-trimeythlphenol (fTMP), is generally used as a proxy of the quantum yield. However, fTMP has been described and modelled only for freshwater systems. Therefore, this study quantified fTMP in inland freshwater and coastal seawater sampled in Japan by conducting steady-state photochemical experiments. Optical properties of water were then used to model fTMP. Results indicated that the inland freshwater DOM originated mainly from terrestrial sources, while the coastal seawater DOM were microbial-dominated. On average, inland freshwater exhibited lower fTMP (61.2 M-1) than coastal seawater (79.7 M-1) and the coastal seawater exhibited significant variations in the proportion of high-energy 3DOM* (> 250 kJ/mol). In addition, E2:E3 (ratio of absorbance at 254 to 365 nm) was positively correlated with fTMP of inland freshwater, coastal seawater, and the overall dataset. Catchment conditions such as forest coverage also influenced the production of 3DOM* and high-energy 3DOM* in inland freshwater. Furthermore, the developed models estimated fTMP based on the optical properties of both freshwater and seawater, providing valuable insights about 3DOM* photochemistry in the aquatic environment.

Prediction of Photochemical Properties of Dissolved Organic Matter Using Machine Learning

Apparent quantum yields (Φ) of photochemically produced reactive intermediates (PPRIs) formed by dissolved organic matter (DOM) are vital to element cycles and contaminant fates in surface water. Simultaneous determination of ΦPPRI values from numerous water samples through existing experimental methods is time consuming and ineffective. Herein, machine learning models were developed with a systematic data set including 1329 data points to predict the values of three ΦPPRIs (Φ3DOM*, Φ1O2, and Φ·OH) based on DOM spectral parameters, experimental conditions, and calculation parameters. The best predictive performances for Φ3DOM*, Φ1O2, and Φ·OH were achieved using the CatBoost model, which outperformed the traditional linear regression models. The significances of the wavelength range and spectral parameters on the three ΦPPRI predictions were revealed, suggesting that DOM with lower molecular weight, lower aromatic content, and a more autochthonous portion possessed higher ΦPPRIs. Chain models were constructed by adding the predicted Φ3DOM* as a new feature into the Φ1O2 and Φ·OH models, which consequently improved the predictive performance of Φ1O2 but worsened the Φ·OH prediction likely due to the complex formation pathways of ·OH. Overall, this study offered robust ΦPPRI prediction across interlaboratory differences and provided new insights into the relationship between PPRIs formation and DOM properties.

Limitations of conventional approaches to identify photochemically produced reactive intermediates involved in contaminant indirect photodegradation

Dissolved organic matter (DOM) mediated indirect photodegradation can play an important role in the degradation of aquatic contaminants. Predicting the rate of this process requires knowledge of the photochemically produced reactive intermediates (PPRI) that react with the compound of interest, as well as the ability of individual DOM samples to produce PPRI. Key PPRI are typically identified using quencher studies, yet this approach often leads to results that are difficult to interpret. In this work, we analyze the indirect photodegradation of atorvastatin, carbamazepine, sulfadiazine, and benzotriazole using a diverse set of 48 waters from natural and engineered aquatic systems. We use this large data set to evaluate relationships between PPRI formation and indirect photodegradation rate constants, which are directly compared to results using standard quenching experiments. These data demonstrate that triplet state DOM (3DOM) and singlet oxygen (1O2) are critical PPRI for atorvastatin, carbamazepine, and sulfadiazine, while hydroxyl radical (˙OH) contributes to the indirect photodegradation of benzotriazole. We caution against relying on quenching studies because quenching of 3DOM limits the formation of 1O2 and all studied quenchers react with ˙OH. Furthermore, we show that DOM composition directly influences indirect photodegradation and that low molecular weight, microbial-like DOM is positively correlated with the indirect photodegradation rates of carbamazepine, sulfadiazine, and benzotriazole.

Apparent quantum yield for photo-production of singlet oxygen in reservoirs and its relation to the water matrix

Man-made reservoirs are important for human daily lives and offer different functions, however they are contaminated due to anthropogenic activities. Dissolved organic matter (DOM) from each reservoir is unique in composition, which further determines its photo-reactivity. Thus, this study aimed to investigate the photo-reactivity of reservoir DOM in terms of the quantum yield for photo-production of singlet oxygen (Ф1O2). We sampled surface water of 50 reservoirs in Japan and determined their Ф1O2 using simulated sunlight together with bulk water analysis. Their Ф1O2 ranged from 1.46 × 10-2 to 6.21 × 10-2 (mean, 2.55 × 10-2), which was identical to those of lakes and rivers reported in the literature, but lower than those of wetland water and wastewater. High-energy triplet-state of DOM accounted for 59.4% of the 1O2 production in the reservoir water on average. Among the bulk water properties, the spectral slope of wavelength from 350 to 400 nm (S350-400) was statistically detected as the most important predictor for Ф1O2. Furthermore, the multiple linear regression model employed S350-400 and the biological index as predictors with no intercorrelations and reasonable accuracy (r2 = 0.86), while the random forest model showed a better accuracy (r2 = 0.90). Overall, these major findings are beneficial for understanding the photo-reactivity of reservoir waters.

Dissolved Organic Matter Photoreactivity Is Determined by Its Optical Properties, Redox Activity, and Molecular Composition

Predicting the formation of photochemically produced reactive intermediates (PPRI) during the irradiation of dissolved organic matter (DOM) has remained challenging given the complex nature of this material and differences in PPRI formation mechanisms. We investigate the role of DOM composition in photoreactivity using 48 samples that span the range of DOM in freshwater systems and wastewater. We relate quantum yields for excited triplet-state organic matter (fTMP), singlet oxygen (Φ1O2), and hydroxylating species (Φ•OH) to DOM composition determined using spectroscopy, Fourier-transform ion cyclotron resonance mass spectrometry, and electron-donating capacity (EDC). fTMP and Φ1O2 follow similar trends and are correlated with bulk properties derived from UV-vis spectra and EDC. In contrast, no individual bulk property can be used to predict Φ•OH. At the molecular level, the subset of DOM that is positively correlated to both Φ•OH and EDC is distinct from DOM formulas related to Φ1O2, demonstrating that •OH and 1O2 are formed from different DOM fractions. Multiple linear regressions are used to relate quantum yields of each PPRI to DOM composition parameters derived from multiple techniques, demonstrating that complementary methods are ideal for characterizing DOM because each technique only samples a subset of DOM.

Integrated evaluation of the reactive oxygen species (ROS) production characteristics in one large lake under alternating flood and drought conditions

Reactive oxygen species (ROS) are omnipresent in natural aquatic environments, and play an important role in biogeochemical cycles. One of the dominant sources of ROS in surface waters was thought to be from dissolved organic matter (DOM) interacting with photochemical process. The properties of DOM were different between the flood and drought periods in lakes; yet, information on how these variations influence ROS photoproduction is unknown. Through a three-year study, the photochemical properties of DOM and the resultant ROS photoproduction between the flood and drought period were determined in the largest freshwater lake in China (Lake Poyang). Results found that quantum yield coefficients of excited triplets (³CDOM*), apparent quantum yields of singlet oxygen (¹O₂) and hydroxyl radicals (•OH) were holistically higher in the flood period than those in the drought period. The optical properties of DOM showed that DOM in the flood period featured an allochthonous input, accompanied by higher molecular size (E2/E3), aromatic content (SUVA₂₅₄), humification degree (HIX), while DOM in the drought period was mainly internal input. Fourier transform ion cyclotron resonance mass spectrometry (FI-ICR MS) further revealed that some refractory components, such as lignin-like and carboxyl-rich alicyclic molecules (CRAM) presented higher abundance in the flood period, and played the positive impacts on ROS production. Orthogonal partial least squares (OPLS) were used to build novel multivariate predictive models for indicating the spatio-temporal ROS production. Also, the relatively higher steady-state concentrations of ³CDOM* and ¹O₂ in the flood period could significantly diminish the half-lives of acetochlor. Considering the photochemical activity of DOM varied considerably at different periods, this study provided a new method to predict ROS production and contributed to a new insight into stage-specific emerging contaminants removing in natural aquatic environments.

The impact of natural sunlight irradiation on the biotoxicities of different molecular sizes EfOM/SRNOM and its relationship with spectral and molecular level parameters

Natural sunlight irradiation is regarded as an efficient and low-carbon method for controlling the biotoxicity of effluents from domestic wastewater treatment plants (WWTPs). Dissolved organic matter in WWTPs effluent (EfOM) is responsible for the non-specific biotoxicity of effluent. In the present study, the variation in spectral characteristics, molecular composition, luminescent bacteria toxicity, and genotoxicity of EfOM of different molecular sizes (MOSs) during natural sunlight irradiation were investigated from a systematic perspective, and the standard natural organic matter from the Suwannee River (SRNOM) was synchronously assessed for comparative purposes. To further explore the cause of the biotoxicity changes, the relationships between the spectral or molecular level parameters (obtained from FT-ICR MS analysis) and biotoxicity were assessed using correlation analysis. The molecules in <1 kDa EfOM with lower molecular weight, higher unsaturation degree, and higher humification and fluorescence had higher luminescent bacteria toxicity under sunlight irradiation. However, in the <1 kDa SRNOM, the molecules which were characterized by higher humification and fluorescence had higher luminescent bacteria toxicity. The notable genotoxicity reduction of EfOM under sunlight irradiation was attributed to the photochemical degradation of components with a high unsaturation degree. Such findings could enable ecological safety improvement of aquatic environments using natural sunlight.

Effects of dissolved organic matter characteristics on the photosensitized degradation of pharmaceuticals in wastewater treatment wetlands

Wastewater treatment wetlands are aquatic systems where diverse dissolved organic matter (DOM) compositions physically interact. Complex photochemical behaviors ensue, leading to uncertainties in the prediction of indirect photodegradation rates for organic contaminants. Here, we evaluate the photosensitization ability of whole water DOM samples from a treatment wetland and wastewater treatment plant (WWTP) in North Carolina to photodegrade target pharmaceuticals. Optical characterization using ultraviolet-visible and excitation-emission matrix spectroscopy shows that wetland DOM has higher aromaticity than WWTP DOM and that WWTP secondary treatment processes increase aromaticity, overall molecular weight, and humic character of wastewater DOM. Our application of a reversed-phase HPLC method to assess DOM polarity distinctly reveals that a subset of the wetland samples possesses an abundance of hydrophobic DOM moieties. Hydroxyl radicals (˙OH) mediate the majority (>50%) of the indirect photodegradation for amoxicillin (AMX), atenolol (ATL), and 17α-ethinylestradiol (EE2), while singlet oxygen (¹O₂) is presumed to be solely responsible for the photodegradation of cimetidine (CME). Our findings suggest that hydrophobic interactions and improved accessibility to photogenerated reactive intermediates lead to significant increases in photosensitization efficiencies and overall indirect photodegradation rates of AMX, ATL, and EE2 for the hydrophobic wetland samples. In contrast, CME photosensitization yields are unaffected by polarity and trend positively with optical indicators of sunlight-induced DOM photobleaching and humification, suggesting that wetland processing favors faster ¹O₂ photogeneration. These relationships highlight the uncertainties in photosensitization yields and effects of DOM optical properties and polarity on the photochemical fate of organic contaminants.

Radical chemistry of dissolved black carbon under sunlight irradiation: quantum yield prediction and effects on sulfadiazine photodegradation

Dissolved black carbon (DBC) is regarded as an important part of the natural organic matter pool. However, it is unclear about DBC's photochemical activity and the relationships between reactive intermediates (RIs) and the molecular structure of DBC remain unclear. In this study, we investigate the photochemical formation ability of RIs and spectral parameters (E₂/E₃, S_275-295) of DBC made from five types of plants at five pyrolysis temperatures. The results showed that there were good linear regressions between the RI quantum yields and the spectral parameters (E₂/E₃, S_275-295), and this was indicative of the RI generation prediction from DBC under solar irradiation. The DBC-mediated photochemical experiment of sulfadiazine revealed that ³DBC^* was the primary active species for the indirect photodegradation of sulfadiazine. Further studies indicated that a linear relationship was observed between the indirect photodegradation ability of sulfadiazine induced directly by ³DBC^* at different pyrolysis temperatures and the ³DBC^* quantum yields or E₂/E₃. These findings indicate that the simple models of the RI quantum yield as a function of spectral parameters can be used to evaluate the degradation of pollutants with known DBC spectral parameters.

Photochemistry of dissolved organic matter in water from the Pearl river (China): Seasonal patterns and predictive modelling

Photochemical properties of dissolved organic matter (DOM) vary widely in natural and engineered water systems due to the different dominant compositions. However, seasonal patterns of DOM photochemical properties in urban rivers remain unclear. In this study, two seasons (wet and dry) of water samples were collected from eleven sites throughout the Pearl River (China) to investigate the spatiotemporal variability of DOM optical and photochemical properties. The optical properties of DOM in the Pearl River were characterized by UV-vis and fluorescence spectroscopies, which showed the substantial decrease in absorption coefficient and fluorescence intensity and increase in absorbence ratio (E₂/E₃) and specific absorption coefficient (SUVA) from the wet to dry season. The photochemical properties in terms of the apparent quantum yields of ³DOM*, ¹O₂ and ·OH from DOM (Φ³_DOM*, Φ¹_O2 and Φ_{·OH, DOM}) under illumination also displayed a significant decrease from the wet to the dry season. Spearman's rank correlation analysis revealed the strongest relationships between Φ³_DOM*, Φ¹_O2 and Φ_{·OH, DOM} and the relative abundance of microbial humic-like component (C2%) derived from parallel factor analysis (PARAFAC). Partial least squares regression (PLSR) modelling exhibited an excellent prediction strength for steady-state concentrations of ¹O₂ ([¹O₂]_ss) and ·OH ([·OH]_ss) with adjusted R² values of 0.85 and 0.91, respectively, by using DOC concentration ([DOC]), optical properties, nitrate and nitrite concentrations as the response variables. In addition, the model identified that the F_max of humic-like component C4 (F_max-C4) was the most effective predictor amongst the used response variables. This study provides an approach to describe and predict the seasonal patterns of DOM photochemical properties in urbanized rivers, offering a good understanding of the formation mechanism of reactive species from river DOM.

Factors affecting the mixed-layer concentrations of singlet oxygen in sunlit lakes

The steady-state concentration of singlet oxygen within a lake ([¹O₂]_SS) is an important parameter that can affect the environmental half-life of pollutants and environmental fate modelling. However, values of [¹O₂]_SS are often determined for the near-surface of a lake, and these values typically do not represent the average over the epilimnia of lakes. In this work, the environmental and physical factors that have the largest impact on [¹O₂]_SS within lake epilimnia were identified. It was found that the depth of the epilimnion has the largest impact on depth-averaged [¹O₂]_SS, with a factor of 8.8 decrease in [¹O₂]_SS when epilimnion depth increases from 2 m to 20 m. The next most important factors are the wavelength-dependent singlet oxygen quantum yield relationship and the latitude of the lake, causing variations in [¹O₂]_SS by factors of 3.2 and 2.5 respectively, over ranges of representative values. For a set of representative parameters, the depth-averaged value of [¹O₂]_SS within an average epilimnion depth of 9.0 m was found to be 5.8 × 10^-16 M and the near-surface value of [¹O₂]_SS was found to be 1.9 × 10^-14 M. We recommend a range of 6 × 10^-17 to 5 × 10^-15 M as being more representative of [¹O₂]_SS values within the epilimnia of lakes globally and potentially more useful for estimating pollutant lifetimes than those calculated using [¹O₂]_SS values that correspond to near-surface, summer midday values. This work advances our understanding of [¹O₂]_SS inter-lake variability in the environment, and provides estimates of [¹O₂]_SS for practitioners and researchers to assess environmental half-lives of pollutants due to reaction with singlet oxygen.

Photo-ammonification in surface water samples: Mechanism and influencing factors

Dissolved organic nitrogen (DON) accounts for a large proportion of the total aquatic nitrogen. Compared with dissolved inorganic nitrogen (DIN), the reactivity of DON has received limited attention. Photo-ammonification contributes significantly to the transformation of DON to DIN. However, information on the mechanism of this process is limited. This study investigated the photo-ammonification process of different natural surface water samples. The effects of seasons and rainfall on this process were explored, and the contributing factors were identified. Results showed that the seasonal effect on photo-ammonification differed for different water samples, whereas rainfall increased the rates of photo-ammonification for most of the lakes. The concentrations of reactive species, including triplet states of chromophoric dissolved organic matter (³CDOM*) and singlet oxygen (¹O₂), were found to be significantly correlated with water optical-parameters. Multivariable linear regression analysis (R² = 0.617) revealed that the photo-ammonification of DON was mainly facilitated by ³CDOM* whereas ¹O₂ competed with ³CDOM* and showed an inhibiting effect. The components of dissolved organic matter (DOM) were identified by fluorescence excitation emission matrices coupled with parallel factor analysis and were found to be greatly influenced by the location. Allochthonous humic-like components were found to promote the production of reactive species while tryptophan-like component was found to be a reactive species consumer. This study revealed that the composition of DOM and the reactive species governed the rates of photo-ammonification.

Singlet Oxygen Quantum Yields in Environmental Waters

Singlet oxygen (¹O₂) is a reactive oxygen species produced in sunlit waters via energy transfer from the triplet states of natural sensitizers. There has been an increasing interest in measuring apparent ¹O₂ quantum yields (Φ_Δ) of aquatic and atmospheric organic matter samples, driven in part by the fact that this parameter can be used for environmental fate modeling of organic contaminants and to advance our understanding of dissolved organic matter photophysics. However, the lack of reproducibility across research groups and publications remains a challenge that significantly limits the usability of literature data. In the first part of this review, we critically evaluate the experimental techniques that have been used to determine Φ_Δ values of natural organic matter, we identify and quantify sources of errors that potentially explain the large variability in the literature, and we provide general experimental recommendations for future studies. In the second part, we provide a qualitative overview of known Φ_Δ trends as a function of organic matter type, isolation and extraction procedures, bulk water chemistry parameters, molecular and spectroscopic organic matter features, chemical treatments, wavelength, season, and location. This review is supplemented with a comprehensive database of Φ_Δ values of environmental samples.

Photolysis of atrazine: Role of triplet dissolved organic matter and limitations of sensitizers and quenchers

Atrazine, a widely used herbicide, is susceptible to photolysis. The role of triplet excited states of chromophoric dissolved organic matter (³CDOM*) in the photolysis of atrazine, however, is not well understood. The direct photolysis of atrazine under irradiation sources (natural sunlight/environmentally relevant simulated solar light) and its indirect photochemical reactivity with model triplet photosensitizers (benzophenone, 2-acetonaphthone, 3'-methoxy-acetophenone, 4-carboxybenzophenone, rose bengal, methylene blue, and anthraquinone-2-sulphonate) was investigated. The reactivity of the model sensitizers and DOM (Suwannee River natural organic matter, river/lake water, and wastewater effluent), were compared. The direct photolysis quantum yield was determined as 0.0196 mol Einstein^-1 in a solar simulator and 0.00437 mol Einstein^-1 under natural sunlight. Considerable photosensitization was induced by triplet state (n-π*) model sensitizers, while insignificant effects on atrazine loss were discerned in natural organic matter even when oxygen, a triplet quencher, was removed. The triplet sensitizers benzophenone and 2-acetylnaphthone reacted with L-histidine and 2-propanol that were intended to quench/ scavenge ¹O₂ and hydroxyl radical ^•OH, respectively, and benzophenone reacted with NaN₃ as a ¹O₂ scavenger and furfuryl alcohol as a ¹O₂ trapping agent, indicating quenchers may have unanticipated effects when using model sensitizers. Atrazine loss via reaction with ³DOM* will be relevant only in selected conditions, and this work provides a more comprehensive view on the use of model photosensitizers to mimic triplet ³DOM*.

Interfacial Molecular Fractionation on Ferrihydrite Reduces the Photochemical Reactivity of Dissolved Organic Matter

The selective sorption of dissolved organic matter (DOM) on minerals is a widespread geochemical process in the natural environment. Recent studies have explored the influence of this process on the molecular fractionation of DOM at water-mineral interfaces. However, it remains unclear how molecular fractionation affects the photochemistry of DOM. Here, we demonstrate that the adsorptive fractionation of DOM on ferrihydrite greatly reduces its photoproduction of reactive oxygen species (ROS) including ¹O₂, O₂^•-, and •OH normalized to organic carbon (ROS_OC). The ROS_OC for ¹O₂, O₂^•-, and •OH were positively correlated with the abundances of polyphenols and oxygenated polycyclic aromatics, which were also observed using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis to be preferentially sequestered by ferrihydrite. The molecules that preferentially remained in the solution after adsorption displayed low levels of ROS_OC. The molecular fractionation of DOM induced by adsorption on ferrihydrite therefore influenced the molecular components and also significantly reduced the photoreactive fractions of DOM in waters. These results are very important in promoting our understanding of the effects of molecular fractionation on the biogeochemical features, behaviors, and implications of DOM in the environment.

Geographical and temporal assessment of the photochemical decontamination potential of river waters from agrochemicals: A first application to the Piedmont region (NW Italy)

This work shows the potential of using photochemical modelling to assess the river-water ability to photodegrade agrochemicals on a geographic and temporal scale. The case of flowing water requires different data treatment compared to more stationary water bodies (e.g., lakes), but it could allow for the identification of particularly vulnerable environments. Five pesticides were considered here, and the photodegradation rate followed the order bentazon > isoproturon > dimethomorph ∼ chlortoluron > atrazine. The modelled photodegradation kinetics was particularly fast in the river Po, which receives significant input of agricultural nitrate from groundwater and features higher steady-state [^•OH] than most other rivers in the region. The fact that the Po eventually collects all river waters in Piedmont is positive, from the point of view of comprehensive photodegradation of pesticides. However, this paradoxical situation of agricultural pollution (nitrate) helping fight pollution from the same source (pesticides) has two important limitations: (i) when compared to the parent compounds, some intermediates deriving from ^•OH reactions are either more harmful (N-formyl derivatives of phenylureas), or about as harmful (desethyl atrazine); (ii) banned atrazine is no longer sprayed over fields during the plant growth season, but it reaches surface waters from legacy groundwater inputs. The latter are operational also during winter, when photochemistry is least active. Therefore, photochemistry might not ensure considerable attenuation of atrazine during wintertime. Overall, bentazon would be the safest among the studied pesticides because of fast degradation by direct photolysis, and of low ecotoxicological impact of its phototransformation intermediates.

Photochemical Characterization of Surface Waters from Lakes in the Adirondack Region of New York

The Adirondack Mountain region of New York, a historical hotspot for atmospheric sulfur and nitrogen deposition, features abundant lakes that are experiencing browning associated with recovery from acidification. Yet, much remains unknown about the photoreactivity of Adirondack lake waters. We quantified the apparent quantum yields (Φ_app,RI) of photochemically produced reactive intermediates (RIs), such as excited triplet states of dissolved organic matter (³DOM*), singlet oxygen (¹O₂), and hydroxyl radicals (^•OH), for surface waters collected from 16 representative Adirondack lakes. Φ_app,³DOM* and Φ_app,¹O2 for native Adirondack lake waters fell within ranges reported for whole waters and DOM isolates from various sources, while Φ_app,^•OH were substantially lower than those measured for other aquatic samples. Orthogonal partial least squares and multiple linear regression analyses identified the spectral slope coefficient from 290 to 400 nm (S_290-400) as the most effective predictor of Φ_app,RI among measured water chemistry parameters and bulk DOM properties. Φ_app,RI also exhibited divergent responses to controlled pH adjustment and aluminum or iron addition simulating hypothetical scenarios relevant to past and future water chemistry conditions of Adirondack lakes. This study highlights the need for continued research on changes in photoreactivity of acid-impacted aquatic ecosystems in response to browning and subsequent impacts on photochemical processes.

Study of the dissolved organic matter (DOM) of the Auzon cut-off meander (Allier River, France) by spectral and photoreactivity approaches

Wetlands are recognized for the importance of their hydrological function and biodiversity, and there is now a consensus to protect and restore them as well as to complete the knowledge on their functioning. Here, we studied the dissolved organic matter (DOM) of a wetland composed of the Auzon cut-off meander, the Allier River, the alluvial fluvial flow, and watershed aquifer. Water was sampled at different locations, in spring, summer, and autumn. For each sample, DOM was characterized for its chemical and optical properties and its photooxidant capacity through its ability to generate DOM triplet excited states (³DOM*) and singlet oxygen upon simulated solar light exposure. UV-visible and fluorescence indices revealed that DOM was mainly microbial-derived whatever the sampling sites with spatial and temporal variations in terms of aromaticity (5.5-22%), specific UV absorbance at 254 nm (0.28-2.82 L m^-1mgC^-1), ratio of the absorbance at 254 and 365 nm (4.6-10.8), fluorescence index (1.35-166), and biological index (0.812-2.25). All the samples generated ³DOM* and singlet oxygen, rates of formation of which showed parallel variations. Using principal component analysis (PCA), we found positive correlations between the sensitizing properties of DOM samples and parameters associated to the abundance of low molecular weight and low absorbing chromophores. Moreover, the parameter variation across the wetland reinforced the hydrological movements observed in a previous study, suggesting that these parameters could be used as water connection tracers.

Prediction of Photochemically Produced Reactive Intermediates in Surface Waters via Satellite Remote Sensing

Absorption of solar radiation by colored dissolved organic matter (CDOM) in surface waters results in the formation of photochemically produced reactive intermediates (PPRIs) that react with pollutants in water. Knowing the steady-state concentrations of PPRIs ([PPRI]_ss) is critical to predicting the persistence of pollutants in sunlit surface waters. CDOM levels (a₄₄₀) can be measured remotely for lakes over large areas using satellite imagery. Laboratory measurements of [PPRI]_ss and apparent quantum yields (Φ) of three PPRIs (³DOM*, ¹O₂, and ^•OH) were made for 24 lake samples under simulated sunlight. The total rate of light absorption by the water samples (R_a), the rates of formation (R_f), and [PPRI]_ss of ³DOM* and ¹O₂ linearly increased with increasing a₄₄₀. The production rate of ^•OH was linearly correlated with a₄₄₀, but the steady-state concentration was best fit by a logarithmic function. The relationship between measured a₄₄₀ and Landsat 8 reflectance was used to map a₄₄₀ for more than 10 000 lakes across Minnesota. Relationships of a₄₄₀ with R_f, [PPRIs]_ss, and R_a were coupled with satellite-based a₄₄₀ assessments to map reactive species production rates and concentrations as well as contaminant transformation rates. This study demonstrates the potential for using satellite imagery for estimating contaminant loss via indirect photolysis in lakes.

Emerging investigator series: critical review of photophysical models for the optical and photochemical properties of dissolved organic matter

Optical measurements (absorbance and fluorescence) are widely used to track dissolved organic matter (DOM) quantity and quality in natural and engineered systems. Despite many decades of research on the optical properties of DOM, there is a lack of understanding with regards to the underlying photophysical model that is the basis for these optical properties. This review both summarizes advances to date on the photophysical properties of DOM and seeks to critically evaluate the photophysical models for DOM optical properties. Recent studies have refined the quantitative understanding of DOM photophysical properties such as excited state lifetimes and energies, rates of different photophysical processes, and quantum yields. Considering fundamental models, more clarity is needed on whether DOM photophysical processes are due to a superposition of non-interacting components (superposition model), or whether a portion of optical signals can be ascribed to electronically interacting moieties, for example in the form of electron donor-acceptor complexes (charge transfer model). Multiple studies over more than two decades have provided evidence for the charge transfer model. Questions have been raised, however, about the broad applicability of the charge transfer model. The charge transfer and superposition model are critically reviewed in light of this current research. Recommendations are given for future studies to help clarify the accuracy of these competing photophysical models.

Dissolved Organic Matter Singlet Oxygen Quantum Yields: Evaluation Using Time-Resolved Singlet Oxygen Phosphorescence

Singlet oxygen (¹O₂) generation quantum yields from chromophoric dissolved organic matter (CDOM) have been reported for many samples over the past 4 decades. Yet even for standardized isolates such as those from the International Humic Substance Society (IHSS), wide-ranging values exist in the literature. In this manuscript, time-resolved ¹O₂ phosphorescence was used to determine the ¹O₂ quantum yields (Φ_Δ) of a variety of dissolved organic matter (DOM) isolates and natural waters. In general, the ¹O₂ quantum yield values in this study are in the middle, although below the median of the range of past reported values (e.g., for Suwannee River Natural Organic Matter IHSS isolate: 1.8% vs 0.23-2.89%). Notably, hydrophobic neutral fractions of DOM isolates were found to possess the highest ¹O₂ quantum yields, an interesting result given that these fractions are not retained in typical humic and fulvic acid isolation procedures that use XAD resins. The excitation wavelength dependence of ¹O₂ generation from CDOM was also examined, and an approximate linear decrease with longer excitation wavelength was observed. This work advances the understanding of CDOM photoprocesses, especially in relation to wavelength-dependent ¹O₂ production, which is valuable for assessing real-world environmental behavior.

The Role of Dissolved Organic Matter Composition in Determining Photochemical Reactivity at the Molecular Level

Dissolved organic matter (DOM) composition influences its ability to form photochemically produced reactive intermediates (PPRI). While relationships have been established between bulk DOM properties and triplet DOM (³DOM) and singlet oxygen (¹O₂) quantum yields, contradictory evidence exists for hydroxyl radical (^•OH) and hydroxylating species. Furthermore, little is known about these relationships at the molecular level. We evaluated DOM composition and photochemical reactivity of water samples from a wastewater treatment plant and the St. Louis River in Minnesota and Wisconsin, U.S.A. Bulk characterization using ultraviolet-visible spectroscopy demonstrates that color and apparent size of DOM decrease downstream, while molecular composition analysis using Fourier-transform ion cyclotron resonance mass spectrometry reveals that saturation and chemodiversity is highest near Lake Superior. ³DOM quantum yield coefficients and ¹O₂ quantum yields increase downstream and correlate strongly with saturated formulas. Similar results are observed for carbon-normalized photodegradation rate constants of atorvastatin, carbamazepine, and venlafaxine, which react primarily with ³DOM and ¹O₂. In contrast, ^•OH quantum yields are lowest downstream and correlate with less saturated, more oxygenated DOM, suggesting that ³DOM is not its major precursor. Mixed relationships are observed for DEET, which reacts with multiple PPRI. Molecular-level compositional data reveal insights into the differing formation pathways of individual PPRI, but information about specific contaminants is needed to predict their photochemical fate.

Quantifying photo-production of triplet excited states and singlet oxygen from effluent organic matter

The organic matter present in wastewater effluents (EfOM), is likely to have different properties than the organic matter present in the receiving water. The properties of EfOM will affect the fate of contaminants of emerging concern because EfOM is a source of photochemically produced reactive intermediates (PPRIs) capable of transforming contaminants. Effluent water samples were taken seasonally from sixteen wastewater treatment plants in Minnesota and two effluent dominated rivers in California and Arizona. Samples (n = 94) were tested for water chemistry, light absorption characteristics, and excited state triplet organic matter (³EfOM^∗) and singlet oxygen (¹O₂) production. Based on analysis of spectral parameters, EfOM had higher molecular weight and lower aromatic content than organic matter present in stormwaters from Minnesota, which are representative of human-impacted natural organic matter (NOM) containing waters. The second order rate constant for the reaction of the ³EfOM^∗ probe 2,4,6-trimethylephenol (k_T,TMP) for the effluents was 7.79 (±3.03) × 10⁸ M^-1s^-1, and this value was used to calculate apparent quantum yields for ³EfOM^∗ production, which ranged from 0.006 to 0.114. The quantum yield for the production of singlet oxygen ranged from 0.007 to 0.064. Processes in the wastewater treatment train, season, and water chemistry parameters did not serve as predictors of ³EfOM^∗ or ¹O₂ production. Among the parameters measured, Spearman rank correlations were strongest between quantum yields of ¹O₂ and ³EfOM^∗ and E2/E3 (absorption at 250 nm/absorption at 365 nm). This relationship, however, is weaker than that previously observed for NOM. The efficiency of ¹O₂ production from ³EfOM^∗ was 54%. Results indicate that 2,4,6-trimethylphenol samples nearly all of the triplets in EfOM that have sufficient energy to produce ¹O₂, which may not be the case for NOM.

Dissolved organic matter processing and photoreactivity in a wastewater treatment constructed wetland

Constructed wetlands have the capacity to degrade a host of contaminants of emerging concern through photodegradation via sunlight produced reactive oxygen species. Dissolved organic matter (DOM) is a critical intermediary in photodegradation as it influences the production of reactive oxygen species. In this study, the photochemical behavior of DOM of wastewater treated in constructed wetlands was characterized. Whole water samples and fractionated DOM were characterized using SUVA₂₅₄, spectral slope ratios, excitation emission matrix fluorescence spectroscopy (EEMs), and proton nuclear magnetic resonance (¹H NMR). Photoreactivity was assessed by measuring formation rates and steady state concentrations of hydroxyl radical (^•OH), singlet oxygen (¹O₂), and the triplet excited states of DOM (³DOM^⁎). The effluent was observed to transition from a microbially sourced protein-like DOM to a terrestrial DOM with higher aromaticity. Size exclusion chromatography revealed an 18% increase in larger molecular weight fractions of vegetated wetland effluent DOM. Additionally, wetland effluent DOM was observed to have a 32% increase in the aromatic region of ¹H NMR spectra as compared to untreated wastewater. ¹H NMR analysis also indicated an increase in the complexity of wetland effluent DOM. Fluorescence intensity fraction of the protein-like Peak T (Ex/Em:278/342 nm) of EEMs decreased by 16% from the untreated wastewater to wetland effluent. A negative correlation between the percent fluorescence of Peak T (Ex/Em:278/342 nm) and Peaks A (Ex/Em:245/460 nm), C (Ex/Em:336/435 nm), and M (Ex/Em:312/400 nm) of the excitation emission spectra confirmed the transition from a spectrum of pure wastewater to a spectrum characteristic of terrestrially derived DOM. Microbial uptake of bio-labile DOM and leaching of humic like substances from vegetated wetland cells were the predominant processes involved in this transition. This transition coincided with an increase in the formation rates of ¹O₂ and ³DOM^⁎ and in the steady state concentration of ¹O₂.

Spatio-temporal variations in physico-chemical parameters and potentially harmful elements (PHEs) of Uchalli Wetlands Complex (Ramsar site), Pakistan

Uchalli Wetlands Complex (UWC) is located in District Khushab, Pakistan, which comprised of three lakes named Khabeki, Uchalli, and Jahlar. The UWC Pakistan is one of the Ramsar sites of international importance. However, the information regarding water quality parameters and concentration of potentially harmful elements (PHEs) is relatively short. Present study focused on spatio-temporal variations in the physico-chemical parameters and PHE (Cd, Pb, Ni, Cu, Zn, Cr, As, Mn) concentrations in water and fish samples using inductively coupled plasma. Sampling was done in summer (August 2016) and winter (January 2017) seasons. The overall concentrations of PHEs in water were in the following order: Mn > Zn > Cu > Cr > Ni > Cd > Pb > As for Khabeki; As >Ni > Cr > Mn > Zn > Cu > Cd > Pb for Uchalli; and Mn > Zn > Ni > Cu > As > Cr > Cd > Pb for Jahlar Lake. PHE concentration in fish followed the order Ni > Cd > Mn > Pb > Cu > Zn > Cr > As. PHEs analysis showed that Mn; Ni and As; and Ni and Mn in summer were above the Pakistan Environmental Quality Standards (PEQS) and World Health Organization (WHO) standards in Khabeki, Uchalli, and Jahlar Lakes respectively while in winter, Mn; Cd, Ni, and As; and Ni and Mn were higher than standard values in Khabeki, Uchalli, and Jahlar Lakes respectively. In fish samples, only Cd (0.0942) was higher in summer as compared to winter (0.0512) while other seven PHEs observed were higher in winter. Conclusively, the metal pollution index showed that water quality of UWC is not very fit for human consumption directly. The bioconcentration factor results indicated potential to accumulate PHEs, i.e., Cd (29.4375 and 9.4814), Pb (16.66 and 4.375), and Ni (4.9875 and 6.206), in fish during both sampling campaigns. Target hazard quotient (THQ), target carcinogenic risk (TR), hazard index (HI), estimated daily intake (EDI), and international safe standard limits of PHEs for fish species indicated that fish from UWC is safe for human consumption. Variations in physic-chemical parameters and PHE concentration were observed spatially and temporally that could be caused by precipitation amount or natural geochemistry of the lakes' crust. The water quality was not suitable for direct human consumption. Fish was only found in Khabeki Lake that had potential to accumulate Cd, Pb, and Ni more as compared to other studied PHEs.

Singlet Oxygen Phosphorescence as a Probe for Triplet-State Dissolved Organic Matter Reactivity

Triplet-state chromophoric dissolved organic matter (³CDOM*) plays an important role in aquatic photochemistry, yet much remains unknown about the reactivity of these intermediates. To better understand the kinetic behavior and reactivity of ³CDOM*, we have developed an indirect observation method based on monitoring time-resolved singlet oxygen (¹O₂) phosphorescence kinetics. The underpinning principle of our approach relies on the fact that O₂ quenches almost all triplets with near diffusion limited rate constants, resulting in the formation of ¹O₂, which is kinetically linked to the precursors. A kinetic model relating ¹O₂ phosphorescence kinetics to triplet excited states produced from isolated humic substances and in whole natural-water samples (hereafter referred to as ³CDOM*) was developed and used to determine rate constants governing ³CDOM* natural lifetimes and quenching by oxygen and 2,4,6-trimethylphenol (TMP), a common triplet probe molecule. ³CDOM* was found to exhibit smaller O₂ and TMP quenching rate constants, ∼9 × 10⁸ and ∼8 × 10⁸ M^-1 s^-1, respectively, compared with model sensitizers, such as aromatic ketones. Findings from this report shed light on the fundamental photochemical properties of CDOM in organic matter isolates and whole waters and will help refine photochemical models to more accurately predict pollutant fate in the environment.

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.

High Pressure Size Exclusion Chromatography (HPSEC) Determination of Dissolved Organic Matter Molecular Weight Revisited: Accounting for Changes in Stationary Phases, Analytical Standards, and Isolation Methods

We reassessed the molecular weight of dissolved organic matter (DOM) determined by high pressure size exclusion chromatography (HPSEC) using measurements made with different columns and various generations of polystyrenesulfonate (PSS) molecular weight standards. Molecular weight measurements made with a newer generation HPSEC column and PSS standards from more recent lots are roughly 200 to 400 Da lower than initial measurements made in the early 1990s. These updated numbers match DOM molecular weights measured by colligative methods and fall within a range of values calculated from hydroxyl radical kinetics. These changes suggest improved accuracy of HPSEC molecular weight measurements that we attribute to improved accuracy of PSS standards and changes in the column packing. We also isolated DOM from wetlands in the Prairie Pothole Region (PPR) using XAD-8, a cation exchange resin, and PPL, a styrene-divinylbenzene media, and observed little difference in molecular weight and specific UV absorbance at 280 nm (SUVA₂₈₀) between the two solid phase extraction resins, suggesting they capture similar DOM moieties. PPR DOM also showed lower SUVA₂₈₀ at similar weights compared to DOM isolates from a global range of environments, which we attribute to oxidized sulfur in PPR DOM that would increase molecular weight without affecting SUVA₂₈₀.

Photochemical Transformation of Four Ionic Liquid Cation Structures in Aqueous Solution

Ionic liquids (ILs) are a new class of solvents expected to be used increasingly by the chemical industry in the coming years. Given their slow biodegradation and limited sorption affinities, IL cations have a high potential to reach aquatic environments. We investigated the fate of ILs in sunlit surface water by determining direct and indirect photochemical transformation rates of imidazolium, pyridinium, pyrrolidinium, and piperidinium cations. The photodegradation of all investigated IL cations was faster in solutions containing dissolved organic matter (DOM) than in ultrapure water, illustrating the importance of indirect photochemical processes. Experiments with model sensitizers and DOM isolates revealed that reactions with hydroxyl radicals dominated the transformation of tested IL cations. Bimolecular reaction rate constants with hydroxyl radicals ranged from (2.04 ± 0.37) × 10⁹ to (8.47 ± 0.97) × 10⁹ M^-1 s^-1 and showed an increase in rate constants with increasing carbon side-chain length. Consequently, average estimated half-lives of IL cations in sunlit surface water ranged from 32 ± 4 to 135 ± 25 days, highlighting the potential of IL cations to become persistent aquatic contaminants.

Reactivity of Triplet Excited States of Dissolved Natural Organic Matter in Stormflow from Mixed-Use Watersheds

Dissolved organic matter (DOM) quantity and composition control the rate of formation (R_f,T) of triplet excited states of dissolved natural organic matter (³DOM*) and the efficiency of ³DOM* formation (the apparent quantum yield, AQY_T). Here, the reactivity of ³DOM* in stormflow samples collected from watersheds with variable land covers is examined. Stormflow DOM reflects variability in DOM quantity and composition as a function of land cover and may be important in controlling the fate of cotransported pollutants. R_f,T and AQY_T were measured using 2,4,6-trimethylphenol in stormflow samples under simulated sunlight. The DOM source and composition was characterized using absorbance and fluorescence spectroscopies and high-resolution mass spectrometry. R_f,T and the total rate of light absorption by the water samples (R_a) increased with the dissolved organic carbon (DOC) concentration. AQY_T was independent of DOC concentration, but varied with DOM source: developed land cover (4-6%) ≈ open water > vegetated land cover (3%). AQY_T was positively related to an index for microbial/algal DOM content and negatively related to DOM molecular weight, DOM aromaticity, and the content of polyphenols. This work demonstrates that TMP is an effective probe for the determination of R_f,T and AQY_T in whole water samples after accounting for the inhibition of TMP photodegradation by DOM.

Predicting Reactive Intermediate Quantum Yields from Dissolved Organic Matter Photolysis Using Optical Properties and Antioxidant Capacity

The antioxidant capacity and formation of photochemically produced reactive intermediates (RI) was studied for water samples collected from the Florida Everglades with different spatial (marsh versus estuarine) and temporal (wet versus dry season) characteristics. Measured RI included triplet excited states of dissolved organic matter (³DOM*), singlet oxygen (¹O₂), and the hydroxyl radical (^•OH). Single and multiple linear regression modeling were performed using a broad range of extrinsic (to predict RI formation rates, R_RI) and intrinsic (to predict RI quantum yields, Φ_RI) parameters. Multiple linear regression models consistently led to better predictions of R_RI and Φ_RI for our data set but poor prediction of Φ_RI for a previously published data set,1 probably because the predictors are intercorrelated (Pearson's r > 0.5). Single linear regression models were built with data compiled from previously published studies (n ≈ 120) in which E2:E3, S, and Φ_RI values were measured, which revealed a high degree of similarity between RI-optical property relationships across DOM samples of diverse sources. This study reveals that ^•OH formation is, in general, decoupled from ³DOM* and ¹O₂ formation, providing supporting evidence that ³DOM* is not a ^•OH precursor. Finally, Φ_RI for ¹O₂ and ³DOM* correlated negatively with antioxidant activity (a surrogate for electron donating capacity) for the collected samples, which is consistent with intramolecular oxidation of DOM moieties by ³DOM*.

Phototransformation of the Herbicide Propanil in Paddy Field Water

When irradiated in paddy-field water, propanil (PRP) undergoes photodegradation by direct photolysis, by reactions with ^•OH and CO₃^•-, and possibly also with the triplet states of chromophoric dissolved organic matter. Irradiation also inhibits the nonphotochemical (probably biological) degradation of PRP. The dark- and light-induced pathways can be easily distinguished because 3,4-dichloroaniline (34DCA, a transformation intermediate of considerable environmental concern) is produced with almost 100% yield in the dark but not at all through photochemical pathways. This issue allows an easy assessment of the dark process(es) under irradiation. In the natural environment, we expect PRP photodegradation to be important only in the presence of elevated nitrate and/or nitrite levels, e.g., [NO₃^-] approaching 1 mmol L^-1 (corresponding to approximately 60 mg L^-1). Under these circumstances, ^•OH and CO₃^•- would play a major role in PRP phototransformation. Because flooded paddy fields are efficient denitrification bioreactors that can achieve decontamination of nitrate-rich water used for irrigation, irrigation with such water would both enhance PRP photodegradation and divert PRP dissipation processes away from the production of 34DCA, at least in the daylight hours.