Copper Inhibition of Triplet-Induced Reactions Involving Natural Organic Matter

From Quencher to Promoter: Revisiting the Role of 2,4,6-Trimethylphenol (TMP) in Triplet-State Photochemistry of Dissolved Organic Matter

Triplet-state dissolved organic matter (3DOM*) plays a crucial role in environmental aquatic photochemistry, with 2,4,6-trimethylphenol (TMP) frequently used as a chemical probe or quencher due to its high reactivity with 3DOM*. However, the influence of TMP-derived oxidation intermediates on the target photochemical reactions has not been comprehensively examined. This study investigated TMP's effect on the photolysis of sulfamethoxazole (SMX), a common antibiotic found in natural waters, in the presence of different DOM sources or model photosensitizer. Contrary to expectation, TMP significantly accelerated SMX photolysis, with the extent of enhancement depending on TMP and DOM concentrations. Laser flash photolysis and kinetic modeling suggested the long-lived TMP-derived reactive species (TMP-RS), including phenoxyl radicals, semiquinone radicals, and quinones, as the key factors in this process. Unlike 3DOM*, TMP-RS may react with SMX with the formation of non-SMX•+ intermediates. This process prevents the reduction of SMX•+ and the subsequent regeneration of SMX. The kinetic model successfully predicts the dynamic contributions of various factors to SMX oxidation during the reaction, highlighting the critical role of TMP-RS. This study advances our understanding of TMP's involvement in triplet-state photochemistry and suggests a reconsideration of the role long-lived organic RSs play in the transformation of environmental micropollutants.

Use of Copper in Evaluating the Role of Phenolic Moieties in the Photooxidation of Dissolved Organic Matter

In a recent study, copper was shown to act as a novel quencher for investigating the mechanism of the photooxidation and photobleaching of dissolved organic matter (DOM) by selectively quenching the one-electron oxidizing intermediates of DOM (DOMD•+). However, the capture of DOMD•+ by Cu is possibly partially due to strong competition from phenolic antioxidant moieties intrinsically present in DOM for DOMD•+ quenching. In this study, the extent of interaction between DOMD•+ and phenolic antioxidant moieties is quantified by measuring the inhibitory effect of Cu on DOM photooxidation and photobleaching under varying pH (5.2-10.0) conditions. The increase in pH facilitates formation of deprotonated phenolic moieties (pKa ∼ 9-10), increasing their quenching capacity of DOMD•+. Accordingly, our results indicate that the inhibitory effect of Cu on the DOM photobleaching and the loss of electron-donating moieties of DOM significantly decreased with an increase in pH, suggesting more pronounced competition for DOMD•+ from antioxidant phenolic moieties within DOM. Considering the precursors of DOMD•+ also originate from phenolic moieties of DOM, the findings of this study provide important insights into the long-distance charge transfer reactions occurring at different phenolic moiety sites during DOM photooxidation.

Mechanistic insight into multiple effects of copper ion on the photoreactivity of dissolved organic matter

Sunlight irradiation of dissolved organic matter (DOM) in surface water results in the production of photochemically produced reactive intermediates (PPRIs). This process is inevitably influenced by co-existing metal ions in aquatic environments; However, the underlying mechanism remains unclear. In this study, the effect of co-existing copper ion (Cu2 +) on PPRIs produced by irradiation of DOM was systematically investigated, because Cu2+ is a typical redox transient cation and has strong affinity to DOM. The findings demonstrated that Cu2+, acting as cation bridge, caused DOM to aggregate, and had impacts on the optical properties and conformation of DOM. The electron shuttle and catalyst effect of Cu2+ could accelerate the charge transfer processes for the increasing of quantum yield and steady concentrations of hydroxyl radical (·OH) with the increase of concentrations of e-aq, O2.-, hydrogen peroxide (H2O2) and charge separated states of DOM (DOM·+ or DOM·-); On the other hand, Cu2+, as excited state quencher, decrease of apparent quantum yield of triplet state of DOM (3DOM*) and singlet oxygen (1O2) through static quenching of singlet excited of DOM (1DOM*) and dynamic quenching of 3DOM*, respectively. The results provide a deeper understanding of the effect mechanism of Cu2+ on the DOM photochemistry in real environment and will be useful for assessment the photodegradation of organic contaminants in the presence of both DOM and Cu2+.

Photodegradation of ciprofloxacin and its interaction with Cu(II) in different water matrices: Insight into degradation pathways

Co-contamination of ciprofloxacin (CIP) and Cu(II) is common in marine aquaculture water. However, the transmission and transformation of these substances in natural water matrices are often overlooked. This study sought to assess the impact of Cu(II) on CIP degradation in distilled (DI) and simulated (SI) mariculture water, as well as to develop a relationship between Cu(II), CIP, and its degradation products. First, complexation assays and analog computations revealed that Cu (II) forms complexes by binding to the oxygen atoms of the carbonyl (C=O) and carboxyl (COOH) groups in the CIP molecule. Second, photodegradation experiments showed that Cu(II) significantly hindered the degradation effect of CIP in DI water, while Cu(II) did not significantly hinder the degradation of CIP in SI water. Furthermore, the effect of Cu(II) on the degradation mechanism of CIP was determined by combining quenching and EPR experiments, Materials Studio software calculations, and UPLC-MS results. It was demonstrated that Cu(II) enhanced the production of singlet oxygen (1O2), hydroxyl radicals (•OH), and superoxide radicals (•O2-) in DI water. In the presence of Cu(II), CIP undergoes hydroxylation and decarbonylation reactions, forming hydroxylated and nitroxylated products. Additionally, direct defluorination and cleavage of the piperazine ring occur, followed by complexation reactions with Cu(II). However, in SI water, the production of 1O2 depends on the indirect action of Cu(II) and the excited state transformation of organic matter. Experimental evidence has shown that CIP can create intermediate compounds that include O-O peroxide rings, with or without the presence of Cu(II). When Cu(II) is present, the cyclopropyl group of the CIP molecule is more prone to transformation and so degradation. Finally, the toxicity assessment results indicated that both Cu(II) and SI water increased the toxicity of the degradation products.

Multiple effects of relative humidity on heterogeneous ozonolysis of cooking organic aerosol proxies from heated peanut oil emissions

The exposure to cooking organic aerosols (COA) is closely related to people's daily lives. Despite extensive investigations into COA's model compounds like oleic acid, the intricacies of heterogeneous ozonolysis of real COA and the effects of ambient conditions like humidity remain elusive. In this work, the ozonolysis of COA proxies from heated peanut oil emissions was investigated using diffuse reflectance infrared Fourier transform (DRIFTS) spectroscopy, and proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). We found that humidity hinders the reaction between ozone and CC double bonds due to the competitive adsorption of water and ozone on COA. Although visible light has little influence on the ozonolysis of COA in the absence of humidity, the ozonolytic CO production is significantly promoted by visible light in the presence of humidity. It may be attributed to the formation of water-derived reactive oxygen species (ROS, mainly HO•) from the photosensitization of polycyclic aromatic hydrocarbons (PAHs) in COA. We also found that humidity can enhance the depolymerization of carboxylic acid dimers and hydrolysis of intrinsic acetals in the COA. Moreover, humidity promotes the release of VOCs during both the dark and light ozonolysis of COA. This work reveals the important roles of humidity-responsive and photo-responsive components in COA during its ozonolysis, and the change in VOC release may guide the control of human VOC exposure in indoor air.

New insight into wastewater treatment by activation of sulfite with humic acid under visible light irradiation

Sulfite (S(IV)), as an alternative to persulfate, has demonstrated its cost-effectiveness and environmentally friendly nature, garnering increasing attention in Advanced Oxidation Processes (AOPs). Dissolved organic matter (DOM) commonly occurred in diverse environments and was often regarded as an interfering factor in sulfite-based AOPs. However, less attention has been paid to the promotion of the activation of sulfite by excited DOM, which could produce various reactive intermediates. The study focused on the activation of sulfite using visible light (VL) - excited humic acid (HA) to efficiently degrade many common organic pollutants, which was better than peroxydisulfate (PDS) and peroxymonosulfate (PMS) systems. Quenching experiments and electron paramagnetic resonance (EPR) analysis revealed that the triplet states of HA (3HA*) activated sulfite through energy transfer, resulting in the production of SO4·-, O2·-, and 1O2. The most significant active species found in the degradation of roxarsone (ROX) was 1O2, which was a non-radical pathway and exhibits high selectivity for pollutant degradation. This non-radical pathway was not commonly observed in traditional sulfite-based AOPs. Additionally, the coexistence of various inorganic anions, such as NO3-, Cl-, SO42-, CO32-, and PO43-, had little effect on the degradation of ROX. Furthermore, DOM from different natural water demonstrated efficient activation of S(IV) under light conditions, opening up new possibilities for applying sulfite-based advanced oxidation to the remediation of organic pollution in diverse sites and water bodies. In summary, this research offered promising insights into the potential application of sulfite-based AOPs, facilitated by photo-excited HA, as a new strategy for efficiently degrading organic pollutants in various environmental settings.

Enhanced photolysis of tetracycline by Zn(II): Role of complexation

Zn(II) is a necessary additive during antibiotic production and aquaculture, leading to the coexistence of Zn(II) and antibiotics in aquatic environment, especially in receiving waters of pharmaceutical and aquaculture wastewater. However, the roles of Zn(II) in the photochemical behavior of antibiotics are still not clear, which limits the understanding of the fate of antibiotic in nature. In this study, tetracycline (TC) was selected as typical antibiotic to evaluate the effect of Zn(II) on antibiotic photolysis. The removal of TC was accelerated by 22.75 % with TC:Zn(II) molar ratio at 1:5. The mechanism of Zn(II)-induced TC photolysis was explored via reactive oxygen species (ROS) analysis and density functional theory (DFT) calculation for the first time. Zn(II) could enhance the formation of TC excited states and further produce more singlet oxygen (12.54 % higher than control group) to promote indirect photolysis. Besides, Zn(II) could react with TC via complexation, and the complex was more vulnerable to attack by reactive oxygen species due to more active sites. Furthermore, the structure and toxicity of intermediates were identified with mass spectrometer, T.E.S.T. and ECOSAR software. Zn(II) hardly changed the degradation path of TC, and TC was mainly degraded via ring opening, demethylation, deamidation, and hydrogen abstraction with more toxic intermediates than the parent molecule. This work is significant to better understand the environmental fate of antibiotics, and also provides new insight into wastewater treatment in the pharmaceutical and aquaculture industry.

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.

Modulating Cu valence state in Cu and graphene oxide composites for electrocatalytic tetracycline hydrochloride degradation

Cu and graphene oxide composites (Cu-GO) were designed by anchoring Cu+ via oxygen groups in GO based on the heavy co-relationships of copper (Cu) anode electrocatalytic activity with Cu valence state. With the consumption of oxygen groups under various pyrolysis temperatures, the Cu valence state changed from Cu ions (as CuCl2 and CuCl) to Cu oxide (CuO and Cu2O) and the final metallic Cu. In which the Cu+ in CuCl was more favorable for electrocatalytic oxidation than other Cu valence states. Due to the dramatic contribution of 1O2 and active chlorine, 100% degradation efficiency was achieved using tetracycline hydrochloride (TCH) as the target pollutant. Cu+ showed a selective preference for 1O2 and active chlorine triggering, rather than metallic Cu. Under the attack of 1O2 and active chlorine, the degradation intermediates of TCH were then provided by LC-MS results. The final results not only prove the feasibility of the Cu-GO/electrocatalysis system for pollution control but also shed light on the anode design via Cu valence state modulation.

The overlooked role of Cr(VI) in micropollutant degradation under solar light irradiation

Hexavalent chromium (Cr(VI)) is ubiquitous in natural environments, whereas its role in the transformation of coexisting contaminants may have been overlooked. In this work, it was reported for the first time that the irradiation of Cr(VI) by solar light (solar light/Cr(VI) system) could effectively degrade various micropollutants with different structures. The removal efficiency of selected micropollutants was increased by 13.3-64.8% by the solar light/Cr(VI) system compared to that by direct solar photolysis. Meanwhile, the oxidation rates were enhanced by 2.2-21.5 folds, while they were negligible by Cr(VI) oxidation alone. Experiments by specific scavengers, probe compounds, fluorescence absorbance, and electron spin resonance analysis demonstrated that hydroxyl radical (•OH) was the major reactive species in the solar light/Cr(VI) system. Further experiments showed that the generation of •OH was closely related to the intermediate Cr(V) generated from Cr(VI) reduction, and Cr(V) could be re-oxidized back to Cr(VI). Increasing solution pH negatively affected model micropollutant (carbamazepine (CBZ)) degradation by the solar light/Cr(VI) system, mainly due to the decreased quantum yield of •OH at higher pH. Coexisting sulfate ions showed negligible effect on CBZ degradation in the solar light/Cr(VI) system, while the presence of bicarbonate, chloride, and humic acid inhibited CBZ degradation to varying degrees, owing to their diverse scavenging effects on •OH. Furthermore, moderate CBZ degradation was also achieved by natural solar light photolysis of Cr(VI). This study demonstrated the pivotal role of Cr(VI) in the transformation of micropollutants under solar irradiation, which advances the understanding of the fate of micropollutants in natural environments.

Mechanistic insight into humic acid-enhanced sonophotocatalytic removal of 17β-estradiol: Formation and contribution of reactive intermediates

In this study, humic acid (HA) enhanced 17β-estradiol (17β-E2) degradation by Er3+-CdS/MoS2 (ECMS) was investigated under ultrasonic and light conditions. The degradation reaction rate of 17β-E2 was increased from (14.414 ± 0.315) × 10-3 min-1 to (122.677 ± 1.729) × 10-3 min-1 within 90 min sonophotocatalytic (SPC) reaction with the addition of HA. The results of quenching coupled with chemical probe experiments indicated that more reactive intermediates (RIs) including reactive oxygen species (ROSs) and triplet-excited states were generated in the HA-enhanced sonophotocatalytic system. The triplet-excited states of humic acid (3HA*), hydroxyl radical (•OH), and superoxide radical (•O2-) were the dominant RIs for 17β-E2 elimination. In addition, the energy- and electron-transfer process via coexisting HA also account for 12.86% and 29.24% contributions, respectively. The quantum yields of RIs in the SPC-ECMS-HA system followed the order of 3HA* > H2O2 > 1O2 > •O2-> •OH. Moreover, the spectral and fluorescence characteristics of HA were further analyzed during the sonophotocatalytic reaction process. The study expanded new insights into the comprehension of the effects of omnipresent coexisting HA and RIs formation for the removal of 17β-E2 during the sonophotocatalytic process.

Inhibiting mechanisms of metal ion complexation on photogenerated reactive intermediates derived from dissolved black carbon

Dissolved black carbon (DBC), an important photosensitizer in surface waters, can influence the photodegradation of various organic micropollutants. In natural water systems, DBC often co-occurs with metal ions as DBC-metal ion complexes; however, the influence of metal ion complexation on the photochemical activity of DBC is still unclear. Herein, the effects of metal ion complexation were investigated using common metal ions (Mn²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Zn²⁺, Al³⁺, Ca²⁺, and Mg²⁺). Complexation constants (logK_M) derived from three-dimensional fluorescence spectra revealed that Mn²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Zn²⁺, and Al³⁺ quenched the fluorescence components of DBC via static quenching. The steady-state radical experiment suggested that in the complex systems of DBC with various metal ions, Mn²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Zn²⁺ and Al³⁺ inhibited the photogeneration of ³DBC* via dynamic quenching, which reduced the yields of ³DBC*-derived ¹O₂ and O₂·^-. Moreover, ³DBC* quenching by metal ions was associated with the complexation constant. A strong positive linear relationship existed between logK_M and the dynamic quenching rate constant of metal ions. These results indicate that the strong complexation ability of metal ions enabled ³DBC quenching, which highlights the photochemical activity of DBC in natural aquatic environments enriched with metal ions.

Inhibition of photosensitized degradation of organic contaminants by copper under conditions typical of estuarine and coastal waters

Dissolved organic matter (DOM) driven-photochemical processes play an important role in the redox cycling of trace metals and attenuation of organic contaminants in estuarine and coastal ecosystems. In this study, we evaluate the effect of Cu on 4-carboxybenzophenone (CBBP) and Suwannee River natural organic matter (SRNOM)-photosensitized degradation of seven target contaminants (TCs) including phenols and amines under pH conditions and salt concentrations typical of those encountered in estuarine and coastal waters. Our results show that trace amounts of Cu(II) (25 -500 nM) induce strong inhibition of the photosensitized degradation of all TCs in solutions containing CBBP. The influence of TCs on the photo-formation of Cu(I) and the decrease in the lifetime of transformation intermediates of contaminants (TC^•+/ TC^•_(-H)) in the presence of Cu(I) indicated that the inhibition effect of Cu was mainly due to the reduction of TC^•+/ TC^•_(-H) by the photo-produced Cu(I). The inhibitory effect of Cu on the photodegradation of TCs decreased with the increase in Cl^- concentration since less reactive Cu(I)-Cl complexes dominate at high Cl^- concentrations. The impact of Cu on the SRNOM-sensitized degradation of TCs is less pronounced compared to that observed in CBBP solution since the redox active moieties present in SRNOM competes with Cu(I) to reduce TC^•+/ TC^•_(-H). A detailed mathematical model is developed to describe the photodegradation of contaminants and Cu redox transformations in irradiated SRNOM and CBBP solutions.

The photoactivity of complexation of DOM and copper in aquatic system: Implication on the photodegradation of TBBPA

The photoactivity of dissolved organic matter (DOM) has a great impact on the photodegradation of organic pollutants in natural waters. In this study, the photodegradation of TBBPA was investigated under simulated sunlight irradiation in the presence of copper ion (Cu²⁺), dissolved organic matter (DOM) and Cu-DOM complexation (Cu-DOM) to illustrate the effect of Cu²⁺ on photoactivity of DOM. The rate of photodegradation of TBBPA in the presence of Cu-DOM complex was 3.2 times higher than that in pure water. The effects of Cu²⁺, DOM and Cu-DOM on the photodegradation of TBBPA were highly pH dependent and hydroxyl radical(·OH) responded for the acceleration effect. Spectral and radical experiments indicated that Cu²⁺ had high affinity to fluorescence components of DOM, and acted as both the cation bridge and electron shuttle, resulting the aggregation of DOM and increasing of steady-state concentration of ·OH (·OH_ss). Simultaneously, Cu²⁺ also inhibited intramolecular energy transfer leading to the decrease of steady-state concentration singlet oxygen (¹O_2ss) and triplet of DOM (³DOM^⁎_ss). The interaction between Cu²⁺ and DOM followed the order of conjugated carbonyl CO, COO^- or CO stretching in phenolic groups and carbohydrate or alcoholic CO groups. With these results, a comprehensive investigation on the photodegradation of TBBPA in the presence of Cu-DOM was conducted, and the effect of Cu²⁺ on the photoactivity of DOM was illustrated. These findings helped to understanding the potential mechanism of interaction among metal cation, DOM and organic pollutants in sunlit surface water, especially for the DOM-induced photodegradation of organic pollutants.

Evaluation of Probes to Measure Oxidizing Organic Triplet Excited States in Aerosol Liquid Water

Oxidizing triplet excited states of organic matter (³C*) drive numerous reactions in fog/cloud drops and aerosol liquid water (ALW). Quantifying oxidizing triplet concentrations in ALW is difficult because ³C* probe loss can be inhibited by the high levels of dissolved organic matter (DOM) and copper in particle water, leading to an underestimate of triplet concentrations. In addition, illuminated ALW contains high concentrations of singlet molecular oxygen (¹O₂*), which can interfere with ³C* probes. Our overarching goal is to find a triplet probe that has low inhibition by DOM and Cu(II) and low sensitivity to ¹O₂*. To this end, we tested 12 potential probes from a variety of compound classes. Some probes are strongly inhibited by DOM, while others react rapidly with ¹O₂*. One of the probe candidates, (phenylthiol)acetic acid (PTA), seems well suited for ALW conditions, with mild inhibition and fast rate constants with triplets, but it also has weaknesses, including a pH-dependent reactivity. We evaluated the performance of both PTA and syringol (SYR) as triplet probes in aqueous extracts of particulate matter. While PTA is less sensitive to inhibition than SYR, it results in lower triplet concentrations, possibly because it is less reactive with weakly oxidizing triplets.

Mitigation of ultrafiltration membrane fouling by a simulated sunlight-peroxymonosulfate system with the assistance of irradiated NOM

Oxidation pretreatments prior to ultrafiltration are hindered by the need for energy input and sludge disposal. Herein, a simulated sunlight-induced natural organic matter (NOM) for peroxymonosulfate (PMS) activation was used as pretreatment to alleviate ultrafiltration membrane fouling caused by NOM itself in the Songhua River water. When light intensity was over 100 mW/cm², the pretreatment removed NOM effectively, characterized with UV₂₅₄, dissolved organic carbon (DOC) and maximum fluorescent intensity (F_max), and improved filtration flux. At 200 mW/cm² light intensity and 0.5 mM PMS, 57.5% of UV₂₅₄ and 18.5% of DOC were removed, and humic-like fluorescent component was degraded by 84%-94% while ∼60% for protein-like substance. Membrane flux was increased by 94%, and reversible and irreversible fouling resistances were reduced by 62.4% and 51.9%, respectively. Both total fouling index (TFI) and hydraulic irreversible fouling index (HIFI) were moderately correlated with the DOC, whereas they prominently correlated with the UV₂₅₄ and the F_maxs of all fluorescence components, which could be served as key indicators to predict and control membrane fouling. Mathematical modeling showed that the pretreatment alleviated the fouling in the membrane pores and cake layer. The simulated sunlight-induced NOM (³NOM* and e_aq^¯) could activate PMS to form active species, which enabled to oxidize high molecular weight (MW) substances and mineralize low MW compounds in NOM as well as hinder their linking with inorganic cations, thereby reducing organic and inorganic membrane fouling simultaneously. This study may provide a new strategy for decentralized potable water treatment, especially in a single household or community.

Significant Promotion of Light Absorption Ability and Formation of Triplet Organics and Reactive Oxygen Species in Atmospheric HULIS by Fe(III) Ions

Metal ions are key components in atmosphere that potentially affect the optical properties and photochemical reactivity of atmospheric humic-like substances (HULIS), while this mechanism is still unclear. In this study, we demonstrated that atmospheric HULIS coupled with Fe³⁺, Cu²⁺, Zn²⁺, and Al³⁺ exhibited distinct optical properties and reactive intermediates from that of HULIS utilizing three-dimensional fluorescence spectroscopy and electron paramagnetic resonance spectroscopy. The HULIS components showed light absorption that increased by 56% for the HULIS-Fe³⁺ system, fluorescence blue shift, and fluorescence quenching, showing a certain dose-effect relationship. These are mainly attributed to the fact that the highly oxidative HULIS chromophores have a stronger complexing ability with Fe³⁺ ions than the other metal ions. In addition, triplet organics (promoting ratio: 53%) and reactive oxygen species (promoting ratio: 82.6%) in the HULIS-Fe³⁺ system showed obvious generation promotion. Therefore, the main assumption of the photochemical mechanisms of atmospheric HULIS in the HULIS-Fe³⁺ system is that Fe³⁺ ions can form ³HULIS*-Fe³⁺ complexation with photoexcited ³HULIS* and then transition to the ground state through energy transfer, electron transfer, or nonradiative transition, accompanied by the formation of singlet oxygen and hydroxyl radicals. Our results provide references for evaluating the radiative forcing and aging effect of metal ions on atmospheric aerosols.

Enhancive and inhibitory effects of copper complexation on triplet dissolved black carbon-sensitized photodegradation of organic micropollutants

Excited-triplet dissolved black carbon (DBC) was deemed as a significant reactive intermediate in the phototransformation of environmental micropollutants, but the impacts of concomitant metal ions on photochemical behavior of excited-triplet DBC (³DBC*) are poorly understood. Here, the photolytic kinetics of sulfadiazine and carbamazepine induced by ³DBC* involving Cu²⁺ was explored. The presence of Cu²⁺ reduced the ³DBC*-induced photodegradation rate of sulfadiazine; whereas for carbamazepine, Cu²⁺ enhanced ³DBC*-induced photodegradation. Cu(II)-DBC complex was formed due to the decreasing fluorescence intensities of DBC in the presence of Cu²⁺. Cu²⁺ complexation caused the decrease of ³DBC* steady-state concentrations, which markedly reduced ³DBC*-induced photodegradation rate of sulfadiazine due to its high triplet reactivity. Kinetic model showed that ³DBC* quenching rate by Cu²⁺ was 7.98 × 10⁹ M^-1 s^-1. Cu²⁺ complexation can also enhance the electron transfer ability, thereby producing more ∙OH in Cu(II)-DBC complex, which explains the promoting effect of Cu²⁺ complexation on carbamazepine photodegradation in view of its low triplet reaction rate. These indicate that ³DBC* reactivity differences of organic micropollutants may explain their photodegradation kinetics differences in DBC system with/without Cu²⁺, which was supported by the linearized relationship between the photodegradation rate ratios of ten micropollutants with/without Cu²⁺ and their triplet reaction activity.

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.

Aqueous ·OH Oxidation of Highly Substituted Phenols as a Source of Secondary Organic Aerosol

Biomass burning (BB) releases large quantities of phenols (ArOH), which can partition into cloud/fog drops and aerosol liquid water (ALW), react, and form aqueous secondary organic aerosol (aqSOA). While simple phenols are too volatile to significantly partition into particle water, highly substituted ArOH partition more strongly and might be important sources of aqSOA in ALW. To investigate this, we measured the ^·OH oxidation kinetics and aqSOA yields for six highly substituted ArOH from BB. Second-order rate constants are high, in the range (1.9-14) × 10⁹ M^-1 s^-1 at pH 2 and (14-25) × 10⁹ M^-1 s^-1 at pH 5 and 6. Mass yields of aqSOA are also high, with an average (±1σ) value of 82 (±12)%. ALW solutes have a range of impacts on phenol oxidation by ^·OH: a BB sugar and some inorganic salts suppress oxidation, while a nitrate salt and transition metals enhance oxidation. Finally, we estimated rates of aqueous- and gas-phase formation of SOA from a single highly substituted phenol as a function of liquid water content (LWC), from conditions of cloud/fog (0.1 g-H₂O m^-3) to ALW (10 μg-H₂O m^-3). Formation of aqSOA is significant across the LWC range, although gas-phase ^·OH becomes dominant under ALW conditions. We also see a generally large discrepancy between measured and modeled aqueous ^·OH concentrations across the LWC range.

Dual roles of Cu2+ complexation with dissolved organic matter on the photodegradation of trace organic pollutants: Triplet- and OH-induced reactions

The triplet excited state of dissolved organic matter (³DOM^⁎) is highly effective in the photodegradation of a broad spectrum of trace organic pollutants (TOPs), and its photoactivity is affected by concomitant metal ions in surface waters. However, the impact of environmental metal ions on the ³DOM^⁎-induced photodegradation of TOPs has not been systemically explored. Herein, we investigated the effect of environmental Cu²⁺ on the ³DOM^⁎-induced photodegradation kinetics of 16 TOPs. A fluorescence quenching experiment showed that a Cu(II)-DOM complex was formed. For the TOPs with stronger electron-donating groups (triplet-labile moieties, e.g., phenols and anilines), Cu²⁺ complexation notably inhibited ³DOM^⁎-induced photodegradation. This may be ascribed to the decrease of ³DOM^⁎ steady-state concentration because Cu²⁺ complexation reduces its formation rates and enhances scavenging rates tested by sorbic acid isomerization experiment. Meanwhile, it was found that Cu²⁺ complexation facilitated the photolysis of refractory TOPs (lower triplet reactivity) because of enhanced electron transfer between DOM and Cu(II), causing photoinduced OH formation. These findings implied that ³DOM^⁎ reactivity differences in TOPs could affect the photodegradation rates in the complex system, which was confirmed via a linear correlation of photodegradation rate ratios for 16 TOPs induced by ³DOM^⁎ in the presence/absence of Cu²⁺ with their ³DOM^⁎ reactivity. These findings helped to improve our understanding of the photochemical reactivity of ³DOM^⁎ in natural waters, especially the effects of environmentally concomitant metal ions.

Pivotal role of water molecules in the photodegradation of pymetrozine: New insights for developing green pesticides

Photodegradation of the insecticide pymetrozine (PYM) was studied on surface of wax films, and in aqueous and nonaqueous phase. The half-life of PYM on the wax surface was approximately 250 times longer than in water. Scavenging experiments, laser flash photolysis, and spectra analysis indicated the first singlet excited state of PYM (S1 *_PYM) to be the most important photoinduced species initiating the photodegradation. Quantum chemistry calculations identified significant molecular torsion and changes in the structure C-CN-N of S1 *_PYM, and the absolute charges of the CN atoms increased and the bond strength weakened. Free energy surface analysis, and O¹⁸ labeling experiments further confirmed that the mechanism was two-step photoinduced hydrolysis. The first step is the hydrolysis of S1 *_PYM at CN upon reaction with 2-3 water molecules (one H₂O molecule as the catalyst). The second step is an intramolecular hydrogen transfer coupled with the cleavage of C-N bond and formation of two cyclic products. During the interactions, water molecules experience catalytic activation by transferring protons, while there is a negligible solvent effect. Clarifying the detailed photodegradation mechanisms of PYM is beneficial for the development of green pesticides that are photostable and effective on leaf surfaces, and photolabile and detoxified in the aquatic environment.

Nonpolar Side Chains Affect the Photochemical Redox Reactions of Copper(II)-Amino Acid Complexes in Aqueous Solutions

Photochemical redox reactions of Cu(II) complexes of eight amino acid ligands (L) with nonpolar side chains have been systematically investigated in deaerated aqueous solutions. Under irradiation at 313 nm, the intramolecular carboxylate-to-Cu(II) charge transfer within Cu(II)-amino acid complexes leads to Cu(I) formation and the concomitant decomposition of amino acids. All amino acid systems studied here can produce ammonia and aldehydes except proline. For the 1:1 Cu(II) complex species (CuL), the Cu(I) quantum yields at 313 nm (Φ_Cu(I),CuL) vary by fivefold and in the sequence (0.10 M ionic strength at 25 °C) alanine (0.094) > valine (0.059), leucine (0.059), isoleucine (0.056), phenylalanine (0.057) > glycine (0.052) > methionine (0.032) > proline (0.019). This trend can be rationalized by considering the stability of the carbon-centered radicals and the efficient depopulation of the photoexcited state, both of which are dependent on the side-chain structure. For the 1:2 Cu(II) complex species (CuL₂), the Cu(I) quantum yields exhibit a similar trend and are always less than those for CuL. The photoformation rates of ammonia, Cu(I), and aldehydes are in the ratio of 1:2.0 ± 0.2:0.7 ± 0.2, which supports the proposed mechanism. This study suggests that the direct phototransformation of Cu(II)-amino acid complexes may contribute to the bioavailable nitrogen for aquatic microorganisms and cause biological damage on cell surfaces in sunlit waters.

Relating natural organic matter conformation, metal complexation, and photophysics

We investigated the relationship between changes in fluorescence intensity and in fluorescence anisotropy for Suwannee River Natural Organic Matter (SRNOM) due to the formation of NOM-metal complexes with divalent and trivalent metals commonly present in both fresh water and sea water environments. We chose metal ions whose complexes give rise to both fluorescence quenching (Fe3+, Cu2+) and fluorescence enhancement (Al3+, Mg2+). Stern–Volmer type analyses quantified the changes in the SRNOM fluorescence as a function of metal concentration. All metals display strong complexation with SRNOM, associated with their effect on fluorescence. Experiments with Fe3+ further show strong effects due to NOM aggregation at all but the lowest metal concentrations studied here. There was little to no change in the conformation of SRNOM as inferred from fluorescence anisotropy caused by increasing metal concentration. These results suggest that there is no correlation between photophysical changes and conformational changes in NOM associated with complexation by the metal ions.

Modulating Triplet Excited-State Energy in Phosphorescent Carbon Dots for Information Encryption and Anti-Counterfeiting

Room-temperature phosphorescent carbon dots (RTP-CDs) may be used in anti-counterfeiting, information encryption, and optoelectronic devices, but modulating their triplet-state energy is still challenging. Here, a type of RTP-CDs was developed via hydrothermal polymerization-carbonization of azamacrocycle and poly(acrylic acid). The introduction of nitrogen heterocycle promotes the intersystem crossing from the singlet state to the triplet state, and the functional groups of CDs can form interdot hydrogen bonds to protect the triplet state. In addition, the uncarbonized heterocycle groups in the CDs provide coordination sites for metal ions. In this case, the excited triplet-state energy of CDs is quenched by paramagnetic ions (Co²⁺ and Cu²⁺) or transfers to luminescent ions (Tb³⁺ and Eu³⁺). Furthermore, the modulation of the triplet state by metal ion binding was demonstrated in information encryption and anti-counterfeiting applications.

Kinetics and Mass Yields of Aqueous Secondary Organic Aerosol from Highly Substituted Phenols Reacting with a Triplet Excited State

Biomass burning emits large amounts of phenols, which can partition into cloud/fog drops and aerosol liquid water (ALW) and react to form aqueous secondary organic aerosol (aqSOA). Triplet excited states of organic compounds (³C*) are likely oxidants, but there are no rate constants with highly substituted phenols that have high Henry's law constants (K_H) and are likely important in ALW. To address this gap, we investigated the kinetics of six highly substituted phenols with the triplet excited state of 3,4-dimethoxybenzaldehyde. Second-order rate constants at pH 2 are all fast, (2.6-4.6) × 10⁹ M^-1 s^-1, while values at pH 5 are 2-5 times smaller. Rate constants are reasonably described by a quantitative structure-activity relationship with phenol oxidation potentials, allowing rate constants of other phenols to be predicted. Triplet-phenol kinetics are unaffected by ammonium sulfate, sodium chloride, galactose (a biomass-burning sugar), or Fe(III). In contrast, ammonium nitrate increases the rate of phenol loss by making hydroxyl radicals, while Cu(II) inhibits phenol decay. Mass yields of aqueous SOA from triplet reactions are large and range from 59 to 99%. Calculations using our data along with previous oxidant measurements indicate that phenols with high K_H can be an important source of aqSOA in ALW, with ³C* typically the dominant oxidant.

Copper Inhibition of Triplet-Sensitized Phototransformation of Phenolic and Amine Contaminants

Excited triplet states of natural organic matter (³NOM*) are important reactive intermediates in phototransformation of organic contaminants in sunlit waters. The main goal of this study was to explore the influence of Cu on triplet-sensitized transformation rates of 20 selected phenolic and amine contaminants. Fourteen of the compounds examined exhibited a marked decrease in their 4-carboxybenzophenone (CBBP)-mediated phototransformation rate in the presence of trace amounts of Cu(II) (25-500 nM). Both mathematical modeling of these rate data and transient absorption spectroscopy measurements support the hypothesis that the decrease in the rate and extent of phototransformation of organic contaminants is due to the reduction of radical intermediates of the contaminants by photochemically formed Cu(I). The Cu-induced inhibition of oxidation of organic contaminants photosensitized by Suwannee River NOM (SRNOM) could also take place in the presence of nanomolar concentrations of Cu. The inhibitory effect of Cu on the oxidation rates of amine contaminants in SRNOM solutions was found to be significantly weaker compared to that in CBBP solutions, but little difference was observed on depletion of phenols. This behavior was attributed to the intrinsic inhibitory effect of the antioxidant moieties present in NOM on phototransformation of amine compounds, partially neutralizing the potential for further Cu inhibition.

Distinct effects of copper on the degradation of β-lactam antibiotics in fulvic acid solutions during light and dark cycle

This study revealed the dual roles of Cu(II) on the β-lactam antibiotics degradation in Suwannee River fulvic acid (SRFA) solution during day and night cycle. Amoxicillin (AMX) and ampicillin (AMP) were selected as the representative β-lactam antibiotics. Cu(II) played a key role in the dark degradation of AMX and AMP via catalytic hydrolysis and oxidation. However, Cu(II) mainly exhibited an inhibitory effect on SRFA-involved photochemical degradation of AMX and AMP. In the presence of 500 nM of Cu(II), the degradation rate of AMX and AMP in the light condition were around 5 times higher than that in the dark condition, suggesting the photodegradation of β-lactam antibiotics was much more pronounced than catalyzed hydrolysis and oxidation. The triplet excited state of SRFA (³SRFA∗) primarily contributed to AMX and AMP photodegradation. Hydroxyl radicals (^•OH) and singlet oxygen (¹O₂) exhibited limit impacts. The redox cycle of Cu(II)/Cu(I) restricted the electron transfer pathway of ³SRFA∗ with AMX and AMP. During the day and night cycles for 48 h, Cu(II) served as a stronger inhibitor rather than a promotor. These findings highlight the interactions between Cu(II) and SRFA are distinct under day and night conditions, which could further affect the fate of β-lactam antibiotics in natural environments.

Mechanistic Insight into the Effect of Metal Ions on Photogeneration of Reactive Species from Dissolved Organic Matter

The photogeneration of reactive species (RS) from dissolved organic matter (DOM) exhibits a great impact on the attenuation of pollutants in natural waters. However, the effect of metal ions on the photogeneration of excited triplet-state DOM (³DOM*), singlet oxygen (¹O₂), and hydroxyl radical (^•OH) by effluent organic matter (EfOM), fulvic acid (FA), and humic acid (HA) is poorly understood. Here, we provided the first evidence that the quenching of ³DOM* was positively correlated with the complexation capacity of metal ions with DOM. Generally, the paramagnetic metal ions (Cr³⁺, Mn²⁺, Fe³⁺, and Cu²⁺) exhibited higher conditional stability constants (log K_ML) with DOM and stronger inhibition for RS than the others (Mg²⁺, Ca²⁺, Al³⁺, and Zn²⁺). For DOM of different sources, the metal binding capacity increased in the order of EfOM < HA < FA and the humic substances were more susceptible to metal ions. The inhibition was attributed to both static and dynamic quenching of ³DOM* by metal ions. The dynamic quenching rate constants of metal ions for ³DOM* were estimated as ∼10⁹ M^-1 s^-1, which was positively related to the corresponding log K_ML. These findings highlight crucial links between metal-DOM complexation and ³DOM* quenching and, consequently, the inhibition of RS.

Intermittent light and microbial action of mixed endogenous source DOM affects degradation of 17β-estradiol day after day in a relatively deep natural anaerobic aqueous environment

All kinds of wastewaters containing steroid estrogens (SEs) and mixed endogenous source dissolved organic matter (DOM) enter natural water environments with intermittent illumination where microbial action occurs in a relatively deep natural aqueous environment. The role of mixed endogenous source DOM in SEs' biodegradation and photochemical degradation in such environments was studied using 17β-estradiol (E2) in laboratory experiments under anaerobic conditions. The experimental results show that microbial action can improve the optical properties and electron transfer capability of mixed endogenous source DOM, promoting photodegradation and biodegradation. Intermittent illumination attenuates DOM's electron transfer capacity and its chromophore groups, but it improves the bioavailability of low molecular weight dissolved organic matter which promotes microbial growth under anaerobic conditions. DOM-mediated co-degradation by light and microbial action over three days was better than either individually. The presence of Fe(III) promoted electron transfer, and Fe(III)-DOM complexes accelerated energy transfer under irradiation, enhancing photodegradation. Any remaining estrogens will continue to degrade, most effectively in well-aerated waters with sufficient illumination.

Enhanced Removal of Chlorophene and 17β-estradiol by Mn(III) in a Mixture Solution with Humic Acid: Investigation of Reaction Kinetics and Formation of Co-oligomerization Products

Reaction with soluble Mn(II) has been considered as a main decay pathway for superoxide in natural waters, accompanied by an important Mn redox cycling. In this study, the interaction of Mn(II) and humic acid (HA) was investigated in visible light irradiated water. Our results indicate that HA may play a dual role to act as a photosensitizer to produce superoxide anions (O₂^-) and as a strong ligand to stabilize the Mn(III), forming soluble Mn(III)_L species for substrate transformation. Furthermore, the reaction kinetics, products, and mechanisms of chlorophene (CP) and estradiol (E2) mixture in the Mn(II)/HA/visible light reaction systems were assessed. The removal of CP and E2 was enhanced by 24.3% and 13.2%, respectively, in mixture solution at initial concentration of 1.0 μM for each target contaminant, as compared to the case of single-compound degradation. Product identification and density functional theory calculations indicated that cross-coupling reaction of CP and E2 radicals was more likely to occur than the self-coupling reaction in mixture solution. In addition, estrogenic activities of initial reaction solution were also effectively decreased during the transformation process. These findings provide new insights into Mn(III)-mediated reactions to better understand the environmental fate of organic contaminant mixture in waters.