Photochemical behaviors of dissolved organic matter in aquatic environment: Generation, characterization, influencing factors and practical application

The oxidative effect of duckweed pond DOM and its mechanism in the photodegradation of neonicotinoid insecticides

Neonicotinoid pesticides (NNIs) are widely used worldwide and commonly detected in natural aquatic systems and in engineered systems, including urban or agriculture ponds. We investigated the photoreactivity of DOM and the photodegradation of NNIs in duckweed ponds (DWP) water under simulated sunlight. The molecular composition of DOM in DWP water typically contained more than 65% bulk compositional measurements of lignin-like, terrestrially derived molecules, revealed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). These compounds serve as critical candidates for the oxidizing 3DOM* in the DWP water. The steady-state concentrations of triplet-excited state DOM (3DOM*), singlet oxygen (1O2) and hydroxyl radical (·OH) were measured 6.00 × 10-14 M, 5.42 × 10-13 M, and 8.33 × 10-15 M, respectively, under 5 mg/L [DOC]. Enhanced removal rates of thiamethoxam (TMX) and dinotefuran (DIN) in the irradiated DWP water relative to the purity water demonstrated the importance of indirect photolysis pathways involving photochemically produced 3DOM*and 1O2. The findings demonstrated that in DWP water, the photolysis rate constants (k) of TMX and DIN increased to 0.3573 h-1 and 0.3237 h-1, respectively. Degradation of imidacloprid was not significantly promoted through the photochemical production of 3DOM* and 1O2. Results from this study underscore the role of DOM as photosensitizer in limiting the persistence of NNIs in duckweed ponds through photochemical reactions.

New insights into the environmental photochemistry of hydroxynaphthalene congeners in water and in ice: A distinct comparative study

Hydroxynaphthalene congeners (OH-Naps) are newly recognized contaminants, urging new insights into their photodegradation in water and in ice. In the study, the important differences between the aqueous and ice photochemistry of four OH-Naps were found. Under simulated sunlight irradiation (λ > 290 nm), they photolyzed faster in ice than in equivalent water in most cases, indicating that their photodegradation was related to whether they resided in water or ice. Meanwhile, the photolytic kinetics were influenced greatly by the substituent groups (-OH, -Cl, and -NO2) and positions, resulting in the fastest photolysis of 2-hydroxynaphthalene (2-OHN) or 4-chloro-1-hydroxynaphthalene (4-Cl-1-OHN), and the slowest photodegradation of 4-nitro-1-hydroxynaphthalene (4-NO2-1-OHN) in the two phases. Furthermore, their apparent photolysis was found to be faster at alkaline pH, attributing to the stronger photo-absorption, electron density and higher reactivities of the anionic forms. The •OH photooxidation kinetics also depended on the specific OH-Nap and the matrix type. Through the key photoproduct identification, the phototransformation of 4-Cl-1-OHN and 4-NO2-1-OHN involved different pathways in the two phases. Only in ice, the two OH-Naps underwent multi-hydroxylation, and 4-NO2-1-OHN suffered from photoisomerization as well. The bioassay to Vibrio fischeri indicated the higher photo-modified toxicity of most OH-Naps in ice than in water, attributing to the generation of more toxic multiple-hydroxyl adducts in ice. Based on extrapolating the lab-derived data to the real environment, the photochemical fate of OH-Naps highly depended on latitudes and solar intensities. These results are significant for evaluating the environmental persistence, fate and risk of the newly recognized contaminants.

Effects of microorganisms, temperature and irradiation on the stability of dissolved okadaic acid and dinophysistoxin-1 in seawater

Diarrhetic shellfish toxins (DSTs) are widespread in marine environments, posing potential threats to marine ecosystems, shellfish aquaculture, and human health. Despite their prevalence, knowledge of the stability of dissolved DSTs in seawater is still limited. This study aimed to investigate the effects of bacteria, temperature, and irradiation on the stability of dissolved okadaic acid (OA) and dinophysistoxin-1 (DTX1) in seawater. Results indicated that bacteria did not contribute to the biodegradation of OA and DTX1, while their growth was inhibited by the toxins over the 7-day experiment. During a 28-day period without irradiation at 4 °C, 20 °C, and 37 °C, no degradation of OA was observed, whereas significant degradation of DTX1 occurred, with concentrations dropping to 58%-78% of the initial concentration at the end of the experiment. Under xenon lamp irradiation at 1000 W, the concentrations of DTX1 decreased by 15%-19% in seawater after 160 min, while the concentrations of OA showed minimal change. Conversely, both OA and DTX1 underwent significant degradation under mercury lamp irradiation with an irradiation intensity-dependent pattern. Additionally, the degradation rates of OA and DTX1 increased with higher concentrations of dissolved organic matter in the range of 1.2-15 mg C L⁻1. This study enhances the understanding of DST stability in seawater under varying temperature and light conditions, highlighting the complexities involved in their degradation processes. The results of this study found that ultraviolet is an important driving environmental factor for OA and DTX1 degradation in the natural marine environment. In case of harmful algal blooms with associated phycotoxins, ultraviolet irradiation can be used as a removal method for OA and DTX1.

Novel Photoelectron-Assisted Microbial Reduction of Arsenate Driven by Photosensitive Dissolved Organic Matter in Mine Stream Sediments

The microbial reduction of arsenate (As(V)) significantly contributes to arsenic migration in mine stream sediment, primarily driven by heterotrophic microorganisms using dissolved organic matter (DOM) as a carbon source. This study reveals a novel reduction pathway in sediments that photosensitive DOM generates photoelectrons to stimulate diverse nonphototrophic microorganisms to reduce As(V). This microbial photoelectrophic As(V) reduction (PEAsR) was investigated using microcosm incubation, which showed the transfer of photoelectrons from DOM to indigenous sediment microorganisms, thereby leading to a 50% higher microbial reduction rate of As(V). The abundance of two marker genes for As(V) reduction, arrA and arsC, increased substantially, confirming the microbial nature of PEAsR rather than a photoelectrochemical process. Photoelectron ion is unlikely to stimulate photolithoautotrophic growth. Instead, diverse nonphototrophic genera, e.g., Cupriavidus, Sphingopyxis, Mycobacterium, and Bradyrhizobium, spanning 13 orders became enriched by 10-50 folds. Metagenomic binning revealed their genetic potential to mediate the photoelectron-assisted reduction of As(V). These microorganisms contain essential genes involved in respiratory As(V) reduction, detoxification As(V) reduction, dimethyl sulfoxide reductase family, c-type cytochromes, and multiple heavy-metal resistance but lack a complete photosynthesis system. The novel microbial PEAsR pathway offers new insights into the interaction between photoelectron utilization and nonphototrophic As(V)-reducing microorganisms, which may have profound implications for arsenic pollution transportation in mine stream sediment.

Molecular-scale investigation on the photochemical transformation of dissolved organic matter after immobilization by iron minerals with FT-ICR MS

The interaction between dissolved organic matter (DOM) and iron minerals has a significant effect on its stabilization and preservation in the environment. In this study, iron minerals with different crystal forms (crystalline goethite and amorphous ferrihydrite) were selected to investigate the photochemical transformation process for DOM immobilized on iron minerals under simulated sunlight irradiation at the molecular scale with the help of Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results showed that a total of 7148 molecules were detected in alkaline-extractable sedimentary DOM, of which 38.8% and 36.2% were adsorbed by ferrihydrite and goethite, respectively, while there was no selectivity difference between the two iron minerals in terms of DOM adsorption. After simulated sunlight irradiation, the DOM adsorbed by goethite was significantly degraded (58.3%), in which the H/C ratio of the mineral-immobilized DOM increased and the O/C ratio decreased, and the photodegradation primarily involved DOM molecules with high Kendrick mass defect (KMD) values. The results confirmed that the iron mineral types play an important role in the transportation and transformation of DOM, which adds to the understanding of the fate of DOM in natural environments.

Seasonal effects of fish, seaweed and abalone cultures on dissolved organic matter and carbon sequestration potential in Sansha Bay, China

Coastal bays serve as undeniable dissolved organic matter (DOM) reactors and the role of prevalent mariculture in DOM cycling deserves investigation. This study, based on four seasonal field samplings and a laboratory incubation experiment, examined the source and seasonal dynamics of DOM and fluorescent dissolved organic matter (FDOM) in the seawater of fish (Larimichthys crocea, LC), seaweed (Gracilaria lemaneiformis, GL) and abalone (Haliotis sp., HA) culturing zones in Sansha Bay, China. Using three-dimensional fluorescence spectroscopy coupled with parallel factor analysis (EEMs-PARAFAC), three fluorescent components were identified, i.e. protein-like C1, protein-like C2, and humic-like C3. Our results showed that mariculture activities dominated the DOM pool by seasonal generating abundant DOM with lower aromaticity and humification degrees. Accounting for 40-95 % of total fluorescent components, C1 (Ex/Em = 300/340 nm) was regarded the same as D1 (Ex/Em = 300/335 nm) identified in a 180-day degradation experiments of G. lemaneiformis detritus, indicating that the cultured seaweed modulated DOM through the seasonal production of C1. In addition, the incubation experiment revealed that 0.7 % of the total carbon content of seaweed detritus could be preserved as recalcitrant dissolved organic carbon (RDOC). However, fish culture appeared to contribute to liable DOC and protein-like C2, exerting a substantial impact on DOM during winter but making a negligible contribution to carbon sequestration, while abalone culture might promote the potential export and sequestration of seaweed-derived carbon to the ocean. Our results highlight the influences of mariculture activities, especially seaweed culture, in shaping DOM pool in coastal bays. These findings can provide reference for future studies on the carbon accounting of mariculture.

Photodegradation of the main synthetic musk (HHCB) in water: kinetic study and influencing factors

Galaxolide (HHCB) is the most common synthetic musk compound detected in numerous daily products. Despite its persistence in the aquatic environment, the photodegradation of HHCB remains poorly understood. In this study, we investigated the direct and indirect photolysis kinetics of HHCB under simulated sunlight and UVC light. Our aim was to determine the role of reactive oxygen species (ROS) responsible for HHCB degradation in the aquatic environment and to identify its transformation products. The influence of environmental factors on indirect photolysis was investigated by testing both synthetic waters (containing humic acid, carbonate (CO32-), and nitrate (NO3-)) and real waters (riverine and effluent). Hydrogen peroxide (H2O2/UVC) was tested to simulate the wastewater treatment process. Quencher experiments were conducted to identify the role of ROS in HHCB photodegradation, including hydroxyl radicals (˙OH), carbonate radicals (CO3˙-), triplet states of dissolved organic matter (3DOM*), and singlet oxygen (1O2). The results clearly indicated that HHCB was efficiently degraded by direct photolysis under both light conditions. The presence of H2O2 led to the most efficient HHCB degradation due to the high production of ˙OH induced under UVC. Indirect photolysis contribution was observed, induced by ˙OH, CO3˙-, 3DOM*, and 1O2 to different extents depending on the light and matrix composition. The experiments led to the detection of transformation products: HHCB lactone, a well-known transformation product, and two other substances with proposed structures. This study provides a comprehensive identification of the processes involved in the direct and indirect photodegradation of HHCB, which could serve as the basis for evaluating and modeling the fate of HHCB in aquatic environments.

Study of humic-like substances of dissolved organic matter using size exclusion chromatography and chemometrics

The study of dissolved organic matter (DOM) presents a significant challenge for environmental analyses and the monitoring of wastewater treatment plants (WWTPs). This is particularly true for the tracking of recalcitrant to biodegradation dissolved organic matter (rDOM) compounds, which is generated during the thermal pretreatment of sludge. This study aims to develop analytical and chemometric methods to differentiate melanoidins from humic acids (HAs), two components of rDOM that require monitoring at various stages of wastewater treatment processes due to their distinct biological effects. The developed method implements the separation of macromolecules through ultra-high-performance liquid chromatography size-exclusion chromatography (U-HPLC SEC) followed by online UV and fluorescence detection. UV detection was performed at 210, 254, and 280 nm, and fluorescence detection at six excitation/emission pairs: 230/355 nm, 270/355 nm, 240/440 nm, 270/500 nm, 330/425 nm, and 390/500 nm. Chromatograms obtained for each sample from these nine detection modes were integrated and separated into four molecular fractions: >40 kDa, 20-40 kDa, 10-20 kDa, and <10 kDa. To enhance analytical resolution and normalize the data, ratios were calculated from the areas of chromatographic peaks obtained for each detection mode. The results demonstrate the utility of these ratios in discriminating samples composed of HAs, melanoidins, and their mixtures, through principal component analysis (PCA). Low molecular weight fractions were found to be specific to melanoidins, while high molecular weight fractions were characteristic of HAs. For the detection modes specific to melanoidins, UV absorbance at 210, 254, and 280 nm were predominantly present in the numerators, with tryptophan-like fluorescence emissions in the denominators. Conversely, fluorescence emissions largely represented both numerators and denominators for HAs. This online method also enables the discrimination of pseudo-melanoidins, compounds revealing a nitrogen deficiency in their chemical structures.

Effect of Dissolved Organic Matter on the Photodegradation of Decachlorobiphenyl (PCB-209) in Heterogeneous Systems: Experimental Analysis and Excited-State Theory Calculations

Dissolved organic matter (DOM) can affect the transformation of pollutants through photosensitization, but most current research focuses on hydrophilic pollutants, making it such that less attention is paid to hydrophobic pollutants. In this paper, the effect and action mechanism of coexisting DOM on the photodegradation of decachlorobiphenyl (PCB-209) on suspended particles collected from the Yellow River were systematically investigated in a heterogeneous system using DOM standards and model compounds. Through molecular probe experiments, mass spectrometry analysis and theoretical calculations, we found that the excited triplet state of DOM (3DOM*) could excite PCB-209 to undergo dechlorination reaction. Due to the different modes of electron transition, the presence of carbonyl groups decreased the energy of 3DOM*, whereas the electron-donating groups made the energy of 3DOM* higher. DOM containing phenolic hydroxyl groups led to a higher steady-state concentration of •OH, and DOM containing phenyl ketone structures had a stronger ability to produce •O2-. Compared with aqueous •OH, •O2- produced from hydrophobic microregions could react more readily with PCB-209. This study deepens the understanding of the role of different functional groups of DOM in the photosensitized transformation of hydrophobic compounds.

Microplastic-derived dissolved organic matter: Generation, characterization, and environmental behaviors

Microplastics (MPs) represent a substantial and emerging class of pollutants distributed widely in various environments, sparking growing concerns about their environmental impact. In environmental systems, dissolved organic matter (DOM) is crucial in shaping the physical, chemical, and biological processes of pollutants while significantly contributing to the global carbon budget. Recent findings have revealed that microplastic-derived dissolved organic matter (MP-DOM) constitutes approximately 10 % of the DOM present on the ocean surface, drawing considerable attention. Hence, this study's primary objective is to explore, the generation, characterization, and environmental behaviors of MP-DOM. The formation and characteristics of MP-DOM are profoundly influenced by leaching conditions and types of MPs. This review delves into the mechanisms of the generation of MP-DOM and provides an overview of a wide array of analytical techniques, including ultraviolet-visible (UV-Vis) spectroscopy, fluorescence spectroscopy, Fourier transform infrared spectroscopy (FTIR), and mass spectroscopy, used to assess the MP-DOM characteristics. Furthermore, this review investigates the environmental behaviors of MP-DOM, including its impacts on organisms, photochemical processes, the formation of disinfection by-products (DBPs), adsorption behavior, and its interaction with natural DOM. Finally, the review outlines research challenges, perspectives for future MP-DOM research, and the associated environmental implications.

Photo-transformation of isoproturon under UV-A irradiation: The synergy of nitrite and natural organic matter

Isoproturon (IPU), a widely utilized phenylurea herbicide, is recognized as an emerging contaminant. Previous studies have predominantly attributed the degradation of IPU in natural waters to indirect photolysis by natural organic matter (NOM). Here, we demonstrate that nitrite (NO2-) also serves as an important photosensitizer that induces the photo-degradation of IPU. Through radical quenching tests, we identify hydroxyl radicals (•OH) and nitrogen dioxide radicals (NO2•) originating from NO2- photolysis as key players in IPU degradation, resulting in the generation of a series of hydroxylated and nitrated byproducts. Moreover, we demonstrate a synergistic effect on the photo-transformation of IPU when both NOM and NO2- are present in the reaction mixture. The observed rate constant (kobs) for IPU removal increases to 0.0179 ± 0.0002 min-1 in the co-presence of NO2- (50 μM) and NOM (2.5 mgC/L), surpassing the sum of those in the presence of each alone (0.0135 ± 0.0004 min-1). NOM exhibits multifaceted roles in the indirect photolysis of IPU. It can be excited by UV and transformed to excited triplet states (3NOM*) which oxidize IPU to IPU•+ that undergoes further degradation. Simultaneously, NOM can mitigate the reaction by reducing the IPU•+ intermediate back to the parent IPU. However, the presence of NO2- alters this dynamic, as IPU•+ rapidly couples with NO2•, accelerating IPU degradation and augmenting the formation of mono-nitrated IPU. These findings provide in-depth understandings on the photochemical transformation of environmental contaminants, especially phenylurea herbicides, in natural waters where NOM and NO2- coexist.

Recent advances in the role of dissolved organic matter during antibiotics photodegradation in the aquatic environment

The presence of residual antibiotics in the environment is a prominent issue. Photodegradation behavior is an important way of antibiotics reduction, which is closely related to dissolved organic matter (DOM) in water. The review provides an overview of the latest advancements in the field. Classification, characterization of DOM, and the dominant mechanisms for antibiotic photodegradation were discussed. Furthermore, it summarized and compared the effects of DOM on different antibiotics photodegradation. Moreover, the review comprehensively considered the factors influencing the photodegradation of antibiotics in the aquatic environment, including the characteristics of light, temperature, dosage of DOM, concentration of antibiotics, solution pH, and the presence of coexisting ions. Finally, potential directions were proposed for the development of predictive models for the photodegradation of antibiotics. Based on the review of existing literature, this paper also considered several pathways for the future study of antibiotic photodegradation. This study allows for a better understanding of the DOM's environmental role and provides important new insights into the photochemical fate of antibiotics in the aquatic environment.

Molecular and optical signatures of photochemical transformation of dissolved organic matter: Nonnegligible role of suspended particulate matter in urban river

Natural dissolved organic matter (DOM) is one of the Earth's dynamic carbon pools and a key intermediate in the global carbon cycle. Photochemical processes potentially affect DOM composition and activity in surface water. Suspended particulate matter (SPM) is the integral component of slow-moving rivers, and holds the potential for photochemical reactivity. To further investigate the influence of SPM on DOM photochemical transformation, this study conducted experiments comparing samples with and without SPM irradiated under simulated sunlight. Surface water samples from slow-moving urban rivers were collected. DOM optical characteristics and molecular features obtained by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were investigated. Photolabile DOM was enriched in unsaturated and highly aromatic terrestrial substances. Photoproduced DOM had low aromaticity and was dominated by saturated aliphatics, protein-like substances, and carbohydrates. Study results indicated that the presence of SPM had a nonnegligible impact on the molecular traits of DOM, such as composition, molecular diversity, photolability, and bioavailability during photochemical reactions. In the environment affected by SPM, molecules containing heteroatoms exhibit higher photosensitivity. SPM promotes the photochemical transformation of a wider range of chemical types of photolabile DOM, particularly nitrogen-containing compounds. This study provides an essential insight into the more precise simulation of photochemical reactions of DOM influenced by SPM occurring in natural rivers, contributing to our understanding of the global carbon cycle from new theoretical perspectives.