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

A novel mathematical template for developing fDOM probe fluorescence signal correction models for freshwaters

The reliable application of field deployable fluorescent dissolved organic matter (fDOM) probes is hindered by several influencing factors which need to be compensated. This manuscript describes the corrections of temperature, pH, turbidity and inner filter effect on fluorescence signal of a commercial fDOM probe (fDOMs). For this, Australian waters with wide ranging qualities were selected, e.g. dissolved organic carbon (DOC) ranging from ∼1 to ∼30 mg/L, specific UV absorbance at 254 nm from ∼1 to ∼6 L/m/mg and turbidity from ∼1 to ∼ 350 FNU. Laboratory-based model calibration experiments (MCEs) were performed. A model template was developed and used for the development of the correction models. For each factor, data generated through MCEs were used to determine model coefficient (α) values by fitting the generated model to the experimental data. Four discrete factor models were generated by determination of a factor-specific α value. The α values derived for each water of the MCEs subset were consistent for each factor model. This indicated generic nature of the four α values across wide-ranging water qualities. High correlation between fDOMs and DOC were achieved after applying the four-factor compensation models to new data (r, 0.96, p < 0.05). Also, average biases (and %) between DOC predicted through fDOMs and actual DOC were decreased by applying the four-factor compensation model (from 3.54 (60.9%) to 1.28 (16.7%) mg/L DOC). These correction models were incorporated into a Microsoft EXCEL-based software termed EXOf-Correct for ready-to-use applications.

The substantial generation of photochemically produced reactive intermediates (PPRIs) in algae-type zones from one large shallow lake promoted the removal of organic pollutants

Photochemically produced reactive intermediates (PPRIs) are ubiquitously present in aquatic systems and hold significant importance in biogeochemical cycles. The photochemical reaction of dissolved organic matter (DOM), known as photosensitizers upon irradiation, is the main pathway for PPRIs generation. However, the PPRIs produced by algal-derived organic matter (ADOM) and their environmental effects remains elusive. This study confirmed that substantial PPRIs were generated by ADOM in the algal-derived areas. UV absorption spectra, fluorescence spectra and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were then indicated a significant positive correlation between the molecular weight of DOM and the quantum yield of PPRIs, with lower molecular weight of DOM exhibiting a higher potential for PPRIs generation. Orthogonal partial least squares (OPLS) were used to build novel multivariate predictive models for indicating the PPRIs production in algae-type zone. Also, the higher concentrations of PPRIs could significantly removal different kinds of organic pollutants, such as bisphenol A (BPA), sulfadiazine (SDZ) and 17α-ethinylestradiol (EE2). Quenching experiments further elucidated that 3DOM⁎ was the key specie for pollutants degradation, serving as the precursor to generate a series of PPRIs. This study highlighted the importance of PPRIs generated from ADOM in the natural attenuation of pollutants and provided a new insight for understanding the self-purification in aquatic system.

Formation of reactive intermediates in paddy water from different temperature zones for the promotion of abiotic ammonification

The paddy field is a hot area of biogeochemical process. The paddy water has a large capacity in photo-generation of reactive intermediates (RIs) due to abundant photosensitive dissolved organic matter (DOM), which is influenced by the spatial heterogeneity of paddy soils but rarely been explored. Our work presents the first investigation of the role of soil properties on photochemistry in paddy water. Soil organic matter (SOM), determined by the temperature, was the dominant factor for the photo-generation of RIs in paddy water of main rice producing areas. The RI concentrations generated with abundant SOM from cool regions are 0.05-8.71 times higher than those for the warm regions in China. The humic-like substance and aromatic-like compounds of DOM plays an essential role in RIs generation, which is abundant in paddy soils rich in SOM from Chinese cool regions. In addition, RIs can efficiently accelerate the photo-ammonification of urea and free amino acids by 15.2 %-164 %, leading to 0.13-0.17 mmol/L/d photo-produced ammonium after fertilization, which is preferentially absorbed by rice. The findings of this study will extend our knowledge of the geochemistry of global paddy field ecosystem. The potential role of RIs in nitrogen cycle should be highlighted in the agroecosystem.

Characterizing the Impact of Cyanobacterial Blooms on the Photoreactivity of Surface Waters from New York Lakes: A Combined Statewide Survey and Laboratory Investigation

Cyanobacterial blooms introduce autochthonous dissolved organic matter (DOM) into aquatic environments, but their impact on surface water photoreactivity has not been investigated through collaborative field sampling with comparative laboratory assessments. In this work, we quantified the apparent quantum yields (Φapp,RI) of reactive intermediates (RIs), including excited triplet states of dissolved organic matter (3DOM*), singlet oxygen (1O2), and hydroxyl radicals (•OH), for whole water samples collected by citizen volunteers from more than 100 New York lakes. Multiple comparisons tests and orthogonal partial least-squares analysis identified the level of cyanobacterial chlorophyll a as a key factor in explaining the enhanced photoreactivity of whole water samples sourced from bloom-impacted lakes. Laboratory recultivation of bloom samples in bloom-free lake water demonstrated that apparent increases in Φapp,RI during cyanobacterial growth were likely driven by the production of photoreactive moieties through the heterotrophic transformation of freshly produced labile bloom exudates. Cyanobacterial proliferation also altered the energy distribution of 3DOM* and contributed to the accelerated transformation of protriptyline, a model organic micropollutant susceptible to photosensitized reactions, under simulated sunlight conditions. Overall, our study provides insights into the relationship between the photoreactivity of surface waters and the limnological characteristics and trophic state of lakes and highlights the relevance of cyanobacterial abundance in predicting the photoreactivity of bloom-impacted surface waters.

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.

Reassessing the Quantum Yield and Reactivity of Triplet-State Dissolved Organic Matter via Global Kinetic Modeling

Measuring the quantum yield and reactivity of triplet-state dissolved organic matter (3DOM*) is essential for assessing the impact of DOM on aquatic photochemical processes. However, current 3DOM* quantification methods require multiple fitting steps and rely on steady-state approximations under stringent application criteria, which may introduce certain inaccuracies in the estimation of DOM photoreactivity parameters. Here, we developed a global kinetic model to simulate the reaction kinetics of the hv/DOM system using four DOM types and 2,4,6-trimethylphenol as the probe for 3DOM*. Analyses of residuals and the root-mean-square error validated the exceptional precision of the new model compared to conventional methods. 3DOM* in the global kinetic model consistently displayed a lower quantum yield and higher reactivity than those in local regression models, indicating that the generation and reactivity of 3DOM* have often been overestimated and underestimated, respectively. The global kinetic model derives parameters by simultaneously fitting probe degradation kinetics under different conditions and considers the temporally increasing concentrations of the involved reactive species. It minimizes error propagation and offers insights into the interactions of different species, thereby providing advantages in accuracy, robustness, and interpretability. This study significantly advances the understanding of 3DOM* behavior and provides a valuable kinetic model for aquatic photochemistry research.

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.

Photo-Reactivity of dissolved black carbon unveiled by combination of optical spectroscopy and FT-ICR MS analysis: Effects of pyrolysis temperature

Dissolved black carbon (DBC) has high photoactivity, which plays an important role in contaminants photodegradation. However, it is unclear how pyrolysis temperatures would affect the composition and photo-reactivity of DBC at the molecular level. Herein, we combined complementary techniques to study the characteristics of DBC pyrolyzed at 200 - 500 ℃, as well as the photoproduction of reactive species and the photodegradation of tetracycline (TC). Bulk composition characterization found that condensed aromatic carbonyl compounds (ConAC) with narrow molecular weights in DBC experienced an increase from 200 to 500 °C, which enhanced the photoproduction of 3DBC*,1O2, and ·OH. Molecular-level data suggested that 3DBC* and 1O2 were both related to the same DBC compounds. Comparatively, the patterns for ·OH were less pronounced, implying its precursor was not 3DBC* and had more complexity. Plentiful CHOx species of ConAC in DBC400 and DBC500 (DBCT, where T = pyrolysis temperature) accelerated the generation of 3DBC* and 1O2, enhancing the photodegradation of TC, and mainly triplet states of quinones reacted with TC. In contrast, DBC200 and DBC300 exhibited inhibition since massive CHOx species in lignin-like reduced 3TC* to TC. Our data revealed the diverse photochemical behavior mechanisms of DBC pyrolyzed at 200 - 500 ℃ at the molecular level and the implications for aquatic contaminants photochemistry.

Insights into the Enhanced Photogeneration of Hydroxyl Radicals from Chlorinated Dissolved Organic Matter

Free available chlorine has been and is being applied in global water treatment and readily reacts with dissolved organic matter (DOM) in aquatic environments, leading to the formation of chlorinated products. Chlorination enhances the photoreactivity of DOM, but the influence of chlorinated compounds on the photogeneration of hydroxyl radicals (•OH) has remained unexplored. In this study, a range of chlorinated carboxylate-substituted phenolic model compounds were employed to assess their •OH photogeneration capabilities. These compounds demonstrated a substantial capacity for •OH production, exhibiting quantum yields of 0.1-5.9 × 10-3 through direct photolysis under 305 nm and 0.2-9.5 × 10-3 through a triplet sensitizer (4-benzoylbenzoic acid)-inducing reaction under 365 nm LED irradiation. Moreover, the chlorinated compounds exhibited higher light absorption and •OH quantum yields compared to those of their unchlorinated counterparts. The •OH photogeneration capacity of these compounds exhibited a positive correlation with their triplet state one-electron oxidation potentials. Molecular-level compositional analysis revealed that aromatic structures rich in hydroxyl and carboxyl groups (e.g., O/C > 0.5 with H/C < 1.5) within DOM serve as crucial sources of •OH, and chlorination of these compounds significantly enhances their capacity to generate •OH upon irradiation. This study provides novel insights into the enhanced photogeneration of •OH from chlorinated DOM, which is helpful for understanding the fate of trace pollutants in chlorinated waters.

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.

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.

Determination of excited triplet states of dissolved organic matter using chemical probes: A comparative and mechanistic study

Dissolved organic matter (DOM) plays an important role in the biogeochemical cycle in natural waters. The determination and characterization of the excited triplet state of DOM (³DOM*) have attracted much attention recently. However, the underlying differences of determined ³DOM* through different pathways are not yet fully understood. In this study, the differences and underlying mechanisms of the determined ³DOM* using 2,4-hexadien-1-ol (HDO) through an energy transfer pathway and 2,4,6-trimethylphenol (TMP) through an electron transfer pathway, were investigated. The results showed that the determined quantum yields of ³DOM* (Φ_3DOM*) for four commercial and four isolated local DOMs are different using HDO ((0.04 ± 0.00) × 10^-2 to (2.9 ± 0.17) × 10^-2)) and TMP ((0.08 ± 0.01) × 10^-2 to (1.2 ± 0.17) × 10^-2), respectively. For 17 DOM-analogs, significant differences were also observed with the quantum yields of their ³DOM* determined using HDO (Φ_HDO) and the triplet-state quantum yield coefficients determined using TMP (f_TMP). It indicates the different reactivity of TMP and HDO with the excited triplet of the chromophores with different structures within the isolated DOM. Based on the experimental and predicted values of f_TMP and Φ_HDO for different DOM-analogs, the impact of substituents on differences in ³DOM* values were further revealed. These results demonstrated that the levels of ³DOM* depended on the chemical functionalities present in the DOM-analogs.

FT-ICR-MS combined with fluorescent spectroscopy reveals the driving mechanism of the spatial variation in molecular composition of DOM in 22 plateau lakes

Dissolved organic matter (DOM) is the largest carbon pool and directly affects the biogeochemistry in lakes. In the current study, fourier transform ion cyclotron mass spectrometry (FT-ICR-MS) combined with fluorescent spectroscopy was used to assess the molecular composition and driving mechanism of DOM in 22 plateau lakes in Mongolia Plateau Lakes Region (MLR), Qinghai Plateau Lakes Region (QLR) and Tibet Plateau Lakes Region (TLR) of China. The limnic dissolved organic carbon (DOC) content ranged from 3.93 to 280.8 mg L^-1 and the values in MLR and TLR were significantly higher than that in QLR. The content of lignin was the highest in each lake and showed a gradually decreasing trend from MLR to TLR. Random forest model and structural equation model implied that altitude played an important role in lignin degradation while the contents of total nitrogen (TN) and chlorophyll a (Chl-a) have a great influence on the increase of DOM Shannon index. Our results also suggested that the inspissation of DOC and the promoted endogenous DOM production caused by the inspissation of nutrient resulted in a positive relationship between limnic DOC content and limnic factors such as salinity, alkalinity and nutrient concentration. From MLR to QLR and TLR, the molecular weight and the number of double bonds gradually decreased but the humification index (HIX) also decreased. In addition, from the MLR to the TLR, the proportion of lignin gradually decreased, while the proportion of lipid gradually increased. Both above results suggested that photodegradation was dominated in lakes of TLR, while microbial degradation was dominated in lakes of MLR.

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.

A data-driven approach for understanding the structure dependence of redox activity in humic substances

Humic substances (HS) can facilitate electron transfer during biogeochemical processes due to their redox properties, but the structure-redox activity relationships are still difficult to describe and poorly understood. Herein, the linear (Partial Least Squares regressions; PLS) and nonlinear (artificial neural network; ANN) models were applied to monitor the structure dependence of HS redox activities in terms of electron accepting (EAC), electron donating (EDC) and overall electron transfer capacities (ETC) using its physicochemical features as input variables. The PLS model exhibited a moderate ability with R² values of 0.60, 0.53 and 0.65 to evaluate EAC, EDC and ETC, respectively. The variable influence in the projection (VIP) scores of the PLS identified that the phenols, quinones and aromatic systems were particularly important for describing the redox activities of HS. Compared with the PLS model, the back-propagation ANN model achieved higher performance with R² values of 0.81, 0.65 and 0.78 for monitoring the EAC, EDC and ETC, respectively. Sensitivity analysis of the ANN separately identified that the EAC highly depended on quinones, aromatics and protein-like fluorophores, while the EDC depended on phenols, aromatics and humic-like fluorophores (or stable free radicals). Additionally, carboxylic groups were the best indicator for evaluating both the EAC and EDC. Good model performances were obtained from the selected features via the PLS and sensitivity analysis, further confirming the accuracy of describing the structure-redox activity relationships with these analyses. This study provides a potential approach for identifying the structure-activity relationships of HS and an efficient machine-learning model for predicting HS redox activities.

A review of long-term change in surface water natural organic matter concentration in the northern hemisphere and the implications for drinking water treatment

Reduced atmospheric acid deposition has given rise to recovery from acidification - defined as increasing pH, acid neutralization capacity (ANC), or alkalinity in surface waters. Strong evidence of recovery has been reported across North America and Europe, driving chemical responses. The primary chemical responses identified in this review were increasing concentration and changing character of natural organic matter (NOM) towards predominantly hydrophobic nature. The concentration of NOM also influenced trace metal cycling as many browning surface waters also reported increases in Fe and Al. Further, climate change and other factors (e.g., changing land use) act in concert with reductions in atmospheric deposition to contribute to widespread browning and will have a more pronounced effect as deposition stabilizes. The observed water quality trends have presented challenges for drinking water treatment (e.g., increased chemical dosing, poor filter operations, formation of disinfection by-products) and many facilities may be under designed as a result. This comprehensive review has identified key research areas to be addressed, including 1) a need for comprehensive monitoring programs (e.g., larger timescales; consistency in measurements) to assess climate change impacts on recovery responses and NOM dynamics, and 2) a better understanding of drinking water treatment vulnerabilities and the transition towards robust treatment technologies and solutions that can adapt to climate change and other drivers of changing water quality.

Impact of water matrices on oxidation effects and mechanisms of pharmaceuticals by ultraviolet-based advanced oxidation technologies: A review

The binding between water components (dissolved organic matters, anions and cations) and pharmaceuticals influences the migration and transformation of pollutants. Herein, the impact of water matrices on drug degradation, as well as the electrical energy demands during UV, UV/catalysts, UV/O₃, UV/H₂O₂-based, UV/persulfate and UV/chlorine processes were systemically evaluated. The enhancement effects of water constituents are due to the powerful reactive species formation, the recombination reduction of electrons and holes of catalyst and the catalyst regeneration; the inhibition results from the light attenuation, quenching effects of the excited states of target pollutants and reactive species, the stable complexations generation and the catalyst deactivation. The transformation pathways of the same pollutant in various AOPs have high similarities. At the same time, each oxidant also can act as a special nucleophile or electrophile, depending on the functional groups of the target compound. The electrical energy per order (EEO) of drugs degradation may follow the order of EEO_UV > EEO_UV/catalyst > EEO_UV/H2O2 > EEO_UV/PS > EEO_UV/chlorine or EEO_UV/O3. Meanwhile, it is crucial to balance the cost-benefit assessment and toxic by-products formation, and the comparison of the contaminant degradation pathways and productions in the presence of different water matrices is still lacking.

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.

Forest and Freshwater Ecosystem Responses to Climate Change and Variability at US LTER Sites

Forest and freshwater ecosystems are tightly linked and together provide important ecosystem services, but climate change is affecting their species composition, structure, and function. Research at nine US Long Term Ecological Research sites reveals complex interactions and cascading effects of climate change, some of which feed back into the climate system. Air temperature has increased at all sites, and those in the Northeast have become wetter, whereas sites in the Northwest and Alaska have become slightly drier. These changes have altered streamflow and affected ecosystem processes, including primary production, carbon storage, water and nutrient cycling, and community dynamics. At some sites, the direct effects of climate change are the dominant driver altering ecosystems, whereas at other sites indirect effects or disturbances and stressors unrelated to climate change are more important. Long-term studies are critical for understanding the impacts of climate change on forest and freshwater ecosystems.

Straw return in paddy field alters photodegradation of organic contaminants by changing the quantity rather than the quality of water-soluble soil organic matter

Straw return, an important agricultural management practice, is worldwide adopted to enhance soil carbon sequestration and soil fertility. Although water-soluble soil organic matter (WSOM) in paddy field is known to affect the photodegradation of organic contaminants, how straw return regulates the photosensitization of WSOM by changing its properties remain unclear. Here, we determined the temporal variations in the content, chemical characteristics, and photosensitizing ability of WSOM after wheat straw return in a wheat-rice rotation system using optical spectroscopy and steady-state photodegradation tests. After straw return, the WSOM content first increased to a maximum and then gradually decreased to pre-return level at day 90. Nevertheless, the relative abundance of humic-like components in WSOM was not shifted by straw return, and protein-like component in WSOM just showed a slight decrease at day 45. All the WSOM samples inhibited sulfamethoxazole (SMX) photodegradation by light filtering, reactive species quenching and other mechanisms, while promoted diuron (DIU) degradation via reacting with •OH, ¹O₂ and excited triplet WSOM. The photodegradation of SMX and DIU was little affected by straw return changing WSOM composition and photochemical activity. However, straw return could decelerate SMX and DIU photodegradation by elevating WSOM content in a relatively short-term. This study emphasizes that straw return may reduce the photodegradation of organic contaminants by increasing WSOM concentration instead of altering WSOM chemical characteristics.

Mechanistic Study of the Effects of Agricultural Amendments on Photochemical Processes in Paddy Water during Rice Growth

The photochemical properties of paddy water might be affected by the commonly used amendments in rice fields owing to the associated changes in water chemistry; however, this important aspect has rarely been explored. We examined the effects of agricultural amendments on the photochemistry of paddy water during rice growth. The amendments significantly influenced the photogenerated reactive intermediates (RIs) in paddy water, such as triplet dissolved organic matter (³DOM*), singlet oxygen, and hydroxyl radicals. Compared with control experiments without amendments, the application of straw and lime increased the RI concentrations by up to 16.8 and 11.1 times, respectively, while biochar addition had limited effects on RI generation from paddy water in in situ experiments under sunlight. Fluorescence emission-excitation matrix spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry, and structural equation modeling revealed that upon the addition of straw and lime amendments, humified DOM substances contained lignins, proteins, and fulvic acids, which could produce more RIs under irradiation. Moreover, the amendments significantly accelerated the degradation rate of 2,4-dichlorophenol but led to the ³DOM*-mediated formation of more toxic and stable dimeric products. This study provides new insights into the effects of amendments on the photochemistry of paddy water and the pathways of abiotic degradation of organic contaminants in paddy fields.

Effect of permanganate oxidation on the photoreactivity of dissolved organic matter for photodegradation of typical pharmaceuticals

Permanganate has been widely used in the remediation of contaminated water due to its relatively strong oxidation properties and ease of use. The ubiquitous dissolved organic matter (DOM) in natural waters causes a significant sink of permanganate in treatments, which further impacts the photoformation of reactive species and the removal of trace pollutants by DOM. Significantly, the effect of permanganate oxidation on the photoreactivity of DOM remains unknown. The present paper investigated for the first time the photophysical and photochemical properties variation of DOM from different sources after permanganate oxidation. Results showed that the permanganate oxidation caused a decrease in UV absorbance, fluorescence intensity, aromaticity, and molecular weight for all tested DOM samples, as well as photoformation rate of DOM triplet states (³DOM^⁎), singlet oxygen (¹O₂), and hydroxyl radical (OH) under simulated sunlight. Quantum yield of ¹O₂ showed positively linear correlations with both triplet quantum yield coefficient (f_TMP) and E2/E3 (ratio of absorbance at 254 and 365 nm) for all the DOM samples before and after permanganate oxidation. The quantum yield of OH exhibited no significant correlation with f_TMP or E2/E3. Permanganate oxidation inhibited the DOM-photosensitized indirect photodegradation of pollutants that do not absorb sunlight (e.g., decreased by 15-29%). For the tested pollutants that undergo direct photolysis under sunlight, their photodegradation was promoted (e.g., increased by 1-19%) in the permanganate oxidized DOM solutions due to the decrease of light-screening effect by DOM. These findings suggest that permanganate oxidation affects the photoreactivity of DOM and the corresponding photochemical fate of organic pollutants in natural waters.

Photodegradation of chloramphenicol in micro-polluted water using a circulatory thin-layer inclined plate reactor

This study describes the photodegradation of chloramphenicol (CAP) in micro-polluted water with a thin-layer inclined plate reactor. Under simulated sunlight irradiation, the effect of reaction parameters including solution pH, initial CAP concentration, and co-existed humic acid (HA) or chloride was evaluated. The photodegradation of CAP was independent of initial pH in the range of 6.0-9.0, but sharply decreased by 25.5% with the increase of initial CAP concentration from 0.4 to 1.0 mg/L. The presence of HA exhibited a significant inhibitory effect, while Cl^- promoted the photoreaction. In this study, CAP was degraded through both direct and indirect photolysis, in which ¹O₂ was the main reactive species responsible for the indirect route. Its steady-state concentration in the micro-polluted water was determined to be 1.40 × 10^-13 mol/L. Transformation intermediates were identified to propose the degradation pathway of CAP, which substantially met the density functional theory (DFT) calculation results. Moreover, four other pharmaceuticals including tetracycline, doxycycline, oxytetracycline, and minocycline were also successfully photodegraded during 5 h irradiation. Therefore, the designed circulatory thin-layer inclined plate reactor is suggested to be effectively applied to the decontamination of organic micro-polluted water.

Contrasting Impacts of Photochemical and Microbial Processing on the Photoreactivity of Dissolved Organic Matter in an Adirondack Lake Watershed

Photochemical and microbial processing are the prevailing mechanisms that shape the composition and reactivity of dissolved organic matter (DOM); however, prior research has not comparatively evaluated the impacts of these processes on the photoproduction of reactive intermediates (RIs) from freshly sourced terrestrial DOM. We performed controlled irradiation and incubation experiments with leaf and soil samples collected from an acid-impacted lake watershed in the Adirondack Mountain region of New York to examine the effects of DOM processing on the apparent quantum yields of RIs (Φ_app,RI), including excited triplet states of DOM (³DOM*), singlet oxygen (¹O₂), and hydroxyl radicals (^•OH). Photodegradation led to net reductions in Φ_app,¹O2, Φ_app,³DOM*, and Φ_app,^•OH, whereas (photo-)biodegradation resulted in increases in Φ_app,¹O2 and Φ_app,³DOM*. Photodegradation and (photo-)biodegradation also shifted the energy distribution of ³DOM* in different directions. Multivariate statistical analyses revealed the potential relevance of photo-biodegradation in driving changes in Φ_app,¹O2 and Φ_app,³DOM* and prioritized five bulk DOM optical and redox properties that best explained the variations in Φ_app,¹O2 and Φ_app,³DOM* along the watershed terrestrial-aquatic continuum. Our findings highlight the contrasting impacts of photochemical and microbial processes on the photoreactivity of freshly sourced terrestrial DOM and invite further studies to develop a more holistic understanding of their implications for aquatic photochemistry.

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.

Photodegradation of three antidepressants in natural waters: Important roles of dissolved organic matter and nitrate

Antidepressants have become ubiquitous emerging organic pollutants. Therefore, it is essential to investigate photodegradation of the antidepressants in environment waters for their ecological risk assessment. However, photodegradation behavior of antidepressants varied from different structures and photodegradation mechanism was rarely known for most antidepressants. Herein, citalopram (CIT), paroxetine (PAR) and fluvoxamine (FLUVO) were employed to study the photodegradation behavior of antidepressants in lake water. Results show that direct photolysis of CIT decreased when pH increased from 6 to 9 while the pH effect was not obvious on direct photolysis of FLUVO and PAR. Photodegradation of CIT occurred from its triplet-state and can undergo self-photosensitization through reaction with the generated hydroxyl radical (·OH). In lake water, PAR and FLUVO are degraded mainly via direct photolysis, while CIT is transformed mainly via indirect photolysis. Dissolved organic matter (DOM) and NO₃^- was proved to be the main factors affecting antidepressants photodegradation in lake water. DOM and NO₃^- showed inhibition effect on photodegradation of FLUVO and PAR, while promoted CIT degradation. The promotion effect of CIT was mainly through reaction with ·OH and excited triplet-state of DOM while singlet oxygen played a minor role. Based on the photodegradation products identified by MS/MS, the photodegradation pathways were proposed for CIT and PAR, respectively. For FLUVO, one (Z)-isomer degradation product was also found. Understanding the photodegradation behavior of antidepressants is vital for providing data to do ecological risk assessment.

Photochemical characterization of paddy water during rice cultivation: Formation of reactive intermediates for As(III) oxidation

Although the photochemical behavior of surface water and its effects on pollutant transformation have been studied extensively in recent years, the photochemistry of paddy water remains largely unknown. In this study, we examined the photochemical processes involving paddy water samples collected at four different cultivation stages of rice. Triplet dissolved organic matter (³DOM*), singlet oxygen (¹O₂), and hydroxyl radicals (^•OH) were found to be the dominant reactive intermediates (RIs), and their apparent quantum yields and steady-state concentrations were quantified. Compared with the typical surface water, quantum yields of ³DOM* and ^•OH were comparable, while quantum yields of ¹O₂ were about 2.4-6.7 times higher than those of surface water. Fluorescence emission-excitation matrix (EEM) spectra, Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), and statistical analysis revealed that DOM properties and nitrite concentration were the main factor influencing RIs generation. The results suggest that DOM with lower molecular weight and humification extent generated more RIs, and nitrite contributed to 23.9%-100% of ^•OH generation. EEM and FTICR-MS data showed that DOM with more saturated and less aromatic formulas could produce more ³DOM* under the irradiation, while the polyphenolic components of DOM inhibited the formation of RIs. Moreover, RIs significantly enhanced arsenite (As(III)) oxidation with oxidation rate increased by 1.8-4.1 times in paddy water, and ^•OH and ³DOM* were the main RIs responsible for As(III) oxidation. This study provides new insight into the pathways of arsenite abiotic transformation in paddy soil and water.

Multiple roles of humic acid in the photogeneration of reactive bromine species using a chemical probe method

Photosensitization of natural organic matter (NOM) is an important natural source of reactive bromine species (RBrS) in the environment. Up to now, quantitative information about RBrS was mainly based on model sensitizers. Whether the behavior of model compounds could represent those of complex NOM remains unknown. In this study, we employed a chemical probe (3,5-dimethyl-1-H-pyrazole) to measure RBrS in humic acid (HA)-containing solutions and investigated their influential factors. The formation rate, decay rate constant, steady-state concentration, and lifetimes of RBrS were 3.87(±0.16) × 10^-13 mol L^-1·s^-1, 1.99(±0.20) × 10⁴ s^-1, 2.04(±0.13) × 10^-17 mol L^-1, and 5.06(±1.05) × 10^-5 s, respectively. Measured steady-state concentrations of RBrS were 3-5 orders of magnitude lower than those in model sensitizer system. Results showed that HA drove the RBrS generation, and about 0.12-0.70% of triplet-state HA (³HA*) would be transformed into RBrS. HA structures strongly affected this process. Phenolic-like groups suppressed the formation, while aromatic ketone-like moieties facilitated it. Last, HA also altered the transformation pathways. The contribution of ·OH dependent and direct oxidation pathways was almost equal, while the direct oxidation was predominant in the model system. Thus, careful consideration should be taken into photochemical formation of RBrS in NOM-involved solution, due to their complexity and multiple roles.

Photosensitized formation of sulfate and volatile sulfur gases from dissolved organic sulfur: Roles of pH, dissolved oxygen, and salinity

The photodegradation of dissolved organic sulfur (DOS) is a potential source of aqueous sulfate and its chemical precursors in surface water. However, the photochemical fate of DOS and factors that control its fate still remain unclear. Herein, we employed a DOS model featuring a photosensitizer (humic acids, HA) to investigate the photochemical degradation pathways of DOS in various natural water sources, from which we observed the substantial photosensitized formation of sulfate, methanesulfonic acid (MSA), carbonyl sulfide (COS), and carbon disulfide (CS₂). However, the photochemical production of sulfate and MSA tends to be more efficient than COS and CS₂. The formation of sulfur-containing photodegradation products was also strongly affected by the identity of the organic sulfur precursor, the oxygen concentration, and the pH, while the salinity did not significantly influence the production ratios. Our results revealed that the photosensitization of DOS contributed significantly to the overall production of sulfate and MSA production, especially in acidic and oxygen-enriched environments, which was attributed to the photochemical production of reactive intermediates, such as excited CDOM (³CDOM*) and reactive oxygen species (ROS). Considering the coexistence of DOS and photosensitizers in aquatic environments, photochemistry may play an essential role in the fate of aquatic DOS.

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.

Photogeneration of Reactive Species from Biochar-Derived Dissolved Black Carbon for the Degradation of Amine and Phenolic Pollutants

Due to agricultural waste combustion and large-scale biochar application, biochar-derived dissolved black carbon (DBC) is largely released into surface waters. The photogeneration of reactive species (RS) from DBC plays an important role in organic pollutant degradation. However, the mechanistic interactions between RS and pollutants are poorly understood. Here, we investigated the formation of DBC triplet states (³DBC*), singlet oxygen (¹O₂), and hydroxyl radical (^•OH) in straw biochar-derived DBC solutions and photodegradation of typical pharmaceuticals and personal care products (PPCPs). Laser flash photolysis and electron spin resonance spectrometry showed that DBC exhibited higher RS quantum yields than some well-studied dissolved organic matter. The RS caused rapid degradation of atenolol, diphenhydramine, and propylparaben, selected as target PPCPs in this study. The ³DBC* contributed primarily to the oxidation of selected PPCPs via one-electron-transfer interaction, with average reaction rate constants of 1.15 × 10⁹, 1.41 × 10⁹, and 0.51 × 10⁹ M^-1 s^-1, respectively. ^•OH also participated in the degradation and accounted for approximately 2.7, 2.5, and 18.0% of the total removal of atenolol, diphenhydramine, and propylparaben, respectively. Moreover, the photodegradation products were identified using high-resolution mass spectrometry, which further confirmed the electron transfer and ^•OH oxidation mechanisms. These findings suggest that DBC from the combustion process of agricultural biomass can efficiently induce the photodegradation of organic pollutants under sunlight in aquatic environments.

Mechanistic Investigation of Enhanced Photoreactivity of Dissolved Organic Matter after Chlorination

Chlorine is commonly used in disinfection processes in wastewater treatment plants prior to discharge of the effluents into receiving waters. Effluent organic matter and humic substances constitute up to 90% of dissolved organic matter (DOM) in receiving water, which induces photogeneration of reactive species (RS) such as excited triplet state of DOM (³DOM*), singlet oxygen (¹O₂), and hydroxyl radical (^•OH). The RS plays an important role in the attenuation of trace pollutants. However, the effect of chlorine disinfection on the photoreactivity of the DOM has remained unclear. Here, we investigated the physicochemical properties and subsequent RS variation after chlorination of DOM. Solid-state ¹³C cross-polarization/magic angle-spinning NMR and Fourier transform ion cyclotron resonance mass spectrometry verified that the aromaticity, electron-donating capacity (EDC), and average molecular weight of DOM decreased markedly after chlorination. It was found for the first time that the photoproduction of ³DOM*, ¹O₂, and ^•OH increased markedly after chlorination of DOM upon irradiation of simulated sunlight. The quantum yields of ³DOM*, ¹O₂, and ^•OH were positively correlated with E2/E3 (ratio of the absorbance at 254 to 365 nm) while negatively correlated with EDC before and after chlorination. These findings highlight the synergetic effect of chlorine disinfection on the photosensitization of DOM under irradiation of sunlight, which will promote the removal of trace pollutants in surface waters.

Photodegradation of cyanotoxins in surface waters

Cyanotoxin-producing harmful algal blooms (HABs) are a global occurrence and pose ecotoxicological threats to humans and animals alike. The presence of cyanotoxins can seriously harm or kill nearby wildlife and restrict a body of water's use as a drinking water supply and recreational site, making it imperative to fully understand their fate and transport in natural waters. Photodegradation contributes to the overall degradation of cyanotoxins in environmental systems, especially for those present in the photic zone of surface waters. This makes photochemical transformation mechanisms important factors to account for when assessing the persistence of cyanotoxins in environmental systems. This paper reviews current knowledge on the photodegradation rates and pathways of cyanotoxins that can occur over the course of HABs. Sensitized, or indirect, photolysis contributes to the degradation of all cyanotoxins addressed in this paper (anatoxins, cylindrospermopsins, domoic acids, microcystins, and nodularins), with hydroxyl radicals (•OH), excited triplet states formed from the absorption of light by dissolved organic matter (³DOM*), and photosynthetic pigment sensitized pathways being of primary interest. Direct photolysis pathways play a less significant role, but are still relevant for most of the cyanotoxins discussed in this paper.

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.