Photoproduction Rates of One-Electron Reductants by Chromophoric Dissolved Organic Matter via Fluorescence Spectroscopy: Comparison with Superoxide and Hydrogen Peroxide Rates

Effects of Tryptophan and Tyrosine on the Transformation of Monophenols in Chromophoric Dissolved Organic Matter Solutions: Enhance the Forward Transformation and Reduce the Reverse Transformation

Tryptophan (Trp) and tyrosine (Tyr) are the primary precursors of protein-like components in dissolved organic matter. Phenolic compounds are ubiquitous in aquatic environments and are considered the main electron donor in chromophoric dissolved organic matter (CDOM). Our results showed that Trp and Tyr (50 μM) enhanced the transformation of six monophenols (20 μM) with varying numbers of -CH3 and -OCH3 substituent groups by a factor of 1.0-1.8. The enhancement factor increased with the ratio of Trp (Tyr) to monophenols. In four different CDOM solutions (5 mg C/L, pH 8.0), a maximum enhancement factor of 3.2-6.7 was observed at a Trp/monophenol concentration ratio of 50. Conversely, monophenols greatly inhibited the transformation of Trp or Tyr. The enhancement factor decreased as the initial pH increased from 3.0 to 10.0. Additionally, the enhancement factor was not directly proportional to the oxidation potential of monophenol. We propose that the promotion effects are generated through the direct oxidation of monophenols by Trp (Tyr) radicals as well as through the reaction between Trp (Tyr) radicals and the one-electron reductant of CDOM.

Photochemical Transformation of Monochloramine Induced by Triplet State Dissolved Organic Matter

The photoexcited dissolved organic matter (DOM) could produce reactive intermediates, affecting chemical oxidant transformation in UV based advanced oxidation processes (AOPs). This study confirmed the critical role of triplet state DOM (3DOM*), generated from DOM photoexcitation, in the transformation of monochloramine (NH2Cl), a commonly used chemical oxidant and disinfectant in water treatment. NH2Cl (42.25 μM, as Cl2) was decayed by 17.4-73.4 % within 60 min, primarily due to 3DOM* , in DOM (2-30 mgC L-1) solutions irradiated by 365 nm, where NH2Cl has no absorption. The second-order quenching rate constants of triplet state model photosensitizers by NH2Cl were determined to be 0.95(± 0.04)-4.49(± 0.04)× 108 M-1 s-1 by using laser flash photolysis. As a reductant, 3DOM* reacted with NH2Cl through one-transfer mechanism, leading to amino radical (NH2•) generation, which then transferred to ammonia (NH4+, pKa 9.25) through H-abstraction by the phenolic moieties in DOM. Additionally, the intermediate product of 3DOM* oxidized by NH2Cl or those triplet state quinones can hydrolyze to form phenolic moieties, elevating NH4+ yield to higher than 99% upon 365 nm irradiation. These findings suggest that the widespread DOM can be applied to convert NH2Cl via 3DOM* with minimal toxic risks.

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.

Mechanistic and Kinetic Consideration of the Photochemically Generated Oxidative Organic Radicals in Dissolved Black Carbon Solutions under Simulated Solar Irradiation

The photochemically generated oxidative organic radicals (POORs) in dissolved black carbon (DBC) was investigated and compared with that in dissolved organic matter (DOM). POORs generated in DBC solutions exhibited higher one-electron reduction potential values (1.38-1.56 V) than those in DOM solutions (1.22-1.38 V). We found that the photogeneration of POORs from DBC is enhanced with dissolved oxygen (DO) increasing, while the inhibition of POORs is observed in reference to DOM solution. The behavior of the one-electron reducing species (DBC•-/DOM•-) was employed to explain this phenomenon. The experimental results revealed that the DO concentration had a greater effect on DBC•- than on DOM•-. Low DO levels led to a substantial increase in the steady-state concentration of DBC•-, which quenched the POORs via back-electron reactions. Moreover, the contribution of POORs to the degradation of 19 emerging organic contaminants (EOCs) in sunlight-exposed DBC and DOM solutions was estimated. The findings indicate that POORs play an important role in the photodegradation of EOCs previously known to react with triplets, especially in DBC solutions. Compared to DOM solutions, POOR exhibits a lower but considerable contribution to EOC attenuation. This study enhances the understanding of pollutant fate in aquatic environments by highlighting the role of DBC in photochemical pollutant degradation and providing insights into pollutant transformation mechanisms involving POORs.

Copper Safeguards Dissolved Organic Matter from Sunlight-Driven Photooxidation

Sunlight plays a crucial role in the transformation of dissolved organic matter (DOM) and the associated carbon cycle in aquatic environments. This study demonstrates that the presence of nanomolar concentrations of copper (Cu) significantly decreases the rate of photobleaching and the rate of loss of electron-donating moieties of three selected types of DOM (including both terrestrial and microbially derived DOM) under simulated sunlight irradiation. Employing Fourier transform ion cyclotron resonance mass spectrometry, we further confirm that Cu selectively inhibits the photooxidation of lignin- and tannin-like phenolic moieties present within the DOM, in agreement with the reported inhibitory impact of Cu on the photooxidation of phenolic compounds. On the basis of the inhibitory impact of Cu on the DOM photobleaching rate, we calculate the contribution of phenolic moieties to DOM photobleaching to be at least 29-55% in the wavelength range of 220-460 nm. The inhibition of loss of electrons from DOM during irradiation in the presence of Cu is also explained quantitatively by developing a mathematical model describing hydrogen peroxide (a proxy measure of loss of electrons from DOM) formation on DOM irradiation in the absence and presence of Cu. Overall, this study advances our understanding of DOM transformation in natural sunlit waters.

Direct Evidence of a Light-Dependent Sink of Superoxide within Chromophoric Dissolved Organic Matter

Superoxide (O2• -) is produced photochemically in natural waters by chromophoric dissolved organic matter (CDOM) via the reaction of molecular oxygen with photoproduced one-electron reductants (OERs) within CDOM. In the absence of other sinks (metals or organic radicals), O2• - is believed to undergo primarily dismutation to produce hydrogen peroxide (H2O2). However, past studies have implicated the presence of an additional light-dependent sink of O2• - that does not lead to H2O2 production. Here, we provide direct evidence of this sink through O2• - injection experiments. During irradiations, spikes of O2• - are consumed to a greater extent (∼85-30% loss) and are lost much faster (up to ∼0.09 s-1) than spikes introduced post-irradiation (∼50-0% loss and ∼0.03 s-1 rate constant). The magnitude of the loss during irradiation and the rate constant are wavelength-dependent. Analysis of the H2O2 concentration post-spike indicates that this light-dependent sink does not produce H2O2 at low spike concentrations. This work further demonstrates that simply assuming that the O2• - production is twice the H2O2 production is not accurate, as previously believed.

Phototransformation of Lignin-related Compounds in Chromophoric Dissolved Organic Matter Solutions

Lignin is a major terrestrial source of chromophoric dissolved organic matter (CDOM), and studying the phototransformation of lignin monomers and their related compounds can enhance our understanding of CDOM intramolecular interactions. Coniferyl aldehyde (Coni) and sinapaldehyde (Sina) form ground-state complexes with CDOM, with equilibrium constants of 7,800 (± 1,800) and 20,000 (± 2,000) M-1, respectively. In comparison, vanillin (Van) exhibits minimal affinity for CDOM complexation. The bimolecular reaction rate constants between singlet oxygen (1O2) and these phenolic carbonyl compounds ranged from 0.46 (± 0.02) to 1.8 (± 0.1) × 107 M-1s-1, which is approximately one order of magnitude lower than their reaction rate constants (0.51 (± 0.02)-1.25 (± 0.02) × 108 M-1s-1) with the triplet excited state of CDOM (3CDOM*). In acidic CDOM solutions (pH 5.0), 1O2, H2O2, and organic peroxyl radicals had negligible impact on the transformation. Comparing the initial transformation rate in the presence and in the absence of NaN3 or furfuryl alcohol led to an overestimation of the contribution of 1O2 to the transformation of Van, Coni, or Sina. 3CDOM* scavengers could not fully inhibit the transformation of Coni or Sina. The remaining transformation is considered to arise from either the unquenched intra-CDOM phase 3CDOM* or a fraction of Coni⊂CDOM or Sina⊂CDOM complex, which underwent intramolecular photoinduced chemical reactions.

Novel Near-Infrared Fluorescence Probe for Bioimaging and Evaluating Superoxide Anion Fluctuations in Ferroptosis-Mediated Epilepsy

Ferroptosis is an iron-regulated, caspase-mediated pathway of cell death that is associated with the excessive aggregation of lipid-reactive oxygen species and is extensively involved in the evolution of many diseases, including epilepsy. The superoxide anion (O2•-), as the primary precursor of ROS, is closely related to ferroptosis-mediated epilepsy. Therefore, it is crucial to establish a highly effective and convenient method for the real-time dynamic monitoring of O2•- during the ferroptosis process in epilepsy for the diagnosis and therapy of ferroptosis-mediated epilepsy. Nevertheless, no probes for detecting O2•- in ferroptosis-mediated epilepsy have been reported. Herein, we systematically conceptualized and developed a novel near-infrared (NIR) fluorescence probe, NIR-FP, for accurately tracking the fluctuation of O2•- in ferroptosis-mediated epilepsy. The probe showed exceptional sensitivity and outstanding selectivity toward O2•-. In addition, the probe has been utilized effectively to bioimage and evaluate endogenous O2•- variations in three types of ferroptosis-mediated epilepsy models (the kainic acid-induced chronic epilepsy model, the pentylenetetrazole-induced acute epilepsy model, and the pilocarpine-induced status epilepticus model). The above applications illustrated that NIR-FP could serve as a reliable and suitable tool for guiding the accurate diagnosis and therapy of ferroptosis-mediated epilepsy.

Production of reactive oxygen species from oxygenation of Fe(II)-carbonate complexes: The critical roles of carbonate

Hydroxyl radicals (•OH) production upon the oxygenation of reduced iron minerals at the oxic/anoxic interface has been well recognized. However, little is known in the influencing environmental factors and the involved mechanisms. In this study, much more •OH could be efficiently produced from oxygenation of Fe(II) with 20-200 mM carbonate. Both carbonate concentration and anoxic reaction time play a critical role in •OH production. High carbonate facilitates the formation of Fe(II)_{high reactivity}, i.e., surface-adsorbed and structural Fe(II) with low crystalline that is reactive toward O₂ reaction for •OH production, while long anoxic reaction time enables the transfer from Fe(II)_{high reactivity} to Fe(II)_{low reactivity}, i.e., Fe(II) at interior sites with high crystalline, that is hardly oxidized by O₂. Furthermore, the degradation pathway of p-nitrophenol (PNP) is highly dependent on the carbonate concentration that low carbonate facilitates •OH oxidation of PNP (80.2%) while high carbonate enhanced O₂^•- reduction of PNP (48.7%). Besides, carbonate also influences the structural evolution of Fe mineral during oxygenation by retarding its hydrolysis and following transformation. Our finding sheds new light on understanding the important role of oxyanions such as carbonate in iron redox cycles and directing contaminant attenuation in subsurface environment.

Photodegradation of organic micropollutants in aquatic environment: Importance, factors and processes

Photochemical reactions widely occur in the aquatic environment and play fundamental roles in aquatic ecosystems. In particular, solar-induced photodegradation is efficient for many organic micropollutants (OMPs), especially those that cannot undergo hydrolysis or biodegradation, and thus can mitigate chemical pollution. Recent reports indicate that photodegradation may play a more important role than biodegradation in many OMP transformations in the aquatic environment. Photodegradation can be influenced by the water matrix such as pH, inorganic ions, and dissolved organic matter (DOM). The effect of the water matrix such as DOM on photodegradation is complex, and new insights concerning the disparate effects of DOM have recently been reported. In addition, the photodegradation process is also influenced by physical factors such as latitude, water depth, and temporal variations in sunlight as these factors determine the light conditions. However, it remains challenging to gain an overview of the importance of photodegradation in the aquatic environment because the reactions involved are diverse and complex. Therefore, this review provides a concise summary of the importance of photodegradation and the major processes related to the photodegradation of OMPs, with particular attention given to recent progress on the major reactions of DOM. In addition, major knowledge gaps in this field of environmental photochemistry are highlighted.

Photosensitized Transformation of Hydrogen Peroxide in Dissolved Organic Matter Solutions under Simulated Solar Irradiation

Hydrogen peroxide plays an important role in photochemical processes in aquatic environments. However, whether it can be transformed by photoexcited chromophoric dissolved organic matter (CDOM) remains unclear. Therefore, this study examined the photosensitized degradation of H₂O₂ in CDOM-enriched solutions under simulated solar irradiation. Our results suggest that the presence of CDOM enhances the photodegradation rate of H₂O₂ via the photosensitization process and ^·OH is generated stoichiometrically with H₂O₂ attenuation. Experimental results with model photosensitizers indicate that one-electron reducing species of CDOM (CDOM^·-), not triplet CDOM, is the primary reactive species that reduces H₂O₂ to yield ^·OH. By monitoring the variation of CDOM^·-, the reaction rate constant of CDOM^·- with H₂O₂ was estimated to be 1.5-fold greater than that with O₂. Furthermore, a wastewater effluent was exposed to simulated solar irradiation with the addition of H₂O₂, and the results demonstrated that the photodegradation of trace organic contaminants (TrOCs) was significantly enhanced by the increased ^·OH level. Overall, the current study provided new insights into the photochemical formation of ^·OH via the one-electron reduction of H₂O₂ by CDOM^·-. The solar irradiation of wastewater with H₂O₂ enhancement could be a useful and economically beneficial advanced oxidation process for TrOC abatement.

Photochemical reactions of dissolved organic matter and bromide ions facilitate abiotic formation of manganese oxide solids

Manganese (Mn) oxide solids are ubiquitous in nature, acting as both electron donors and acceptors in diverse redox reactions in the environment. Reactions of Mn(III/IV) oxides with dissolved natural organic matter (DOM) are commonly described as reductive dissolutions that generate Mn²⁺(aq). In this study, we investigated the role of photochemical reactions of DOM in Mn²⁺(aq) oxidation and the resulting formation of Mn oxide solids. During the photolysis of DOM, reactive intermediates can be generated, including excited triplet state DOM (³DOM*), hydroxyl radicals (^•OH), superoxide radicals (O₂^•-), hydrogen peroxide, and singlet oxygen. Among these, we found that O₂^•- radicals were mainly responsible for Mn oxidation. The solution pH controlled the formation of Mn oxide solids by affecting both Mn²⁺ oxidation by O₂^•- during photolysis of DOM and reductive dissolutions of Mn oxide solids by DOM. Further, with the addition of bromide ions (Br^-), reactions between ³DOM* and Br^-, together with reactions between ^•OH and Br^-, can form reactive bromide radicals. The formed Br radicals also promoted Mn oxide formation. In DOM with more aromatic functional groups, more Mn²⁺ was oxidized to Mn oxide solids. This enhanced oxidation could be the result of promoted pathways from charge-transfer state DOM (DOM^•+/•-) to O₂^•-. These new observations advance our understanding of natural Mn²⁺ oxidation and Mn(III/IV) oxide formation and highlight the underappreciated oxidative roles of DOM in the oxidation of metal ions in surface water illuminated by sunlight.

Insights into generation mechanisms of halogen radicals from excited triplet state of dissolved organic matter

Triplet states of dissolved organic matter (³DOM*) can sensitize the generation of halogen radicals in marine water. The generation pathways of halogen radicals from ³DOM* is still not fully understood. In this study, the generation of halogen radicals from DOM was investigated with Suwanee River fulvic acid (SRFA) as a representative and detailed generation pathways were further revealed with anthraquinone-2-sodium sulfonate (AQ2S) as a triplet sensitizer. The results showed that in SRFA solutions with halogen ions, various halogen radicals can be generated. Among which, Br is formed by the reaction of Br^- with ³AQ2S*, and Cl is produced by the reaction of Cl^- with AQ2S⁺ that generated in the presence of dissolved oxygen (DO). Cl₂^- and Br₂^- were generated via the subsequent combination of Cl/Br with another Cl^-/Br^-. In solutions without DO, BrCl^- is mainly generated through the combination of Br with Cl^-, and BrCl^- could also be generated through the combination of Cl with Br^- in solutions with DO. This study provides deep insights into the generation mechanisms of different halogen radicals from ³DOM* and is helpful for understanding the photochemical processes of halogen radicals in marine waters.

Reactive oxygen species in the world ocean and their impacts on marine ecosystems

Reactive oxygen species (ROS) are omnipresent in the ocean, originating from both biological (e.g., unbalanced metabolism or stress) and non-biological processes (e.g. photooxidation of colored dissolved organic matter). ROS can directly affect the growth of marine organisms, and can also influence marine biogeochemistry, thus indirectly impacting the availability of nutrients and food sources. Microbial communities and evolution are shaped by marine ROS, and in turn microorganisms influence steady-state ROS concentrations by acting as the predominant sink for marine ROS. Through their interactions with trace metals and organic matter, ROS can enhance microbial growth, but ROS can also attack biological macromolecules, causing extensive modifications with deleterious results. Several biogeochemically important taxa are vulnerable to very low ROS concentrations within the ranges measured in situ, including the globally distributed marine cyanobacterium Prochlorococcus and ammonia-oxidizing archaea of the phylum Thaumarchaeota. Finally, climate change may increase the amount of ROS in the ocean, especially in the most productive surface layers. In this review, we explore the sources of ROS and their roles in the oceans, how the dynamics of ROS might change in the future, and how this change might impact the ecology and chemistry of the future ocean.

Photosensitized Transformation of Peroxymonosulfate in Dissolved Organic Matter Solutions under Simulated Solar Irradiation

Sulfate radical (SO₄^•-)-mediated advanced oxidation processes via peroxymonosulfate (PMS) activation have been extensively investigated. However, the phototransformation of PMS in sunlit dissolved organic matter (DOM) solution has not been previously examined. For the first time, the photosensitized transformation of PMS in DOM-enriched solutions under simulated solar irradiation was observed. The generation of reactive species, including ¹O₂, SO₄^•-, and ^•OH, was confirmed by electron paramagnetic resonance and quantified by chemical probes. SO₄^•- was the primary reactive species generated via the reaction of excited triplet DOM (³DOM*) with PMS. ³DOM* acted as a reactive reductant and was quickly oxidized by PMS, with an estimated reaction rate constant of (4.09 ± 0.21) × 10⁸ M^-1 s^-1. Compared to ³DOM*, one-electron-reducing DOM (DOM^•-) was a minor contributor to the photosensitized transformation of PMS, and the contribution of DOM^•- relied on the phenolic constituents. In addition, a series of different types of DOM, including terrestrial DOM, autochthonous DOM, and effluent organic matter and its fractions, were employed to examine the photosensitized transformation kinetics of PMS. Overall, the photosensitized transformation of PMS by irradiated DOM could be a useful and economical approach to generate SO₄^•- under environmentally relevant conditions.