Probe Compounds to Assess the Photochemical Activity of Dissolved Organic Matter

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.

Evaluating the Microheterogeneous Distribution of Photochemically Generated Singlet Oxygen Using Furfuryl Amine

Singlet oxygen (1O2) is an important reactive species in natural waters produced during photolysis of dissolved organic matter (DOM). Prior studies have demonstrated that 1O2 exhibits a microheterogeneous distribution, with [1O2] in the interior of DOM macromolecules ∼30 to 1000-fold greater than in bulk solution. The [1O2] profile for DOM-containing solutions has been determined mainly by the use of hydrophobic probes, which are not commercially available. In this study, we employed a dual-probe method combining the widely used hydrophilic 1O2 probe furfuryl alcohol (FFA) and its structural analogue furfuryl amine (FFAm). FFAm exists mainly as a cation at pH <9 and was therefore hypothesized to have an enhanced local concentration in the near-DOM phase, whereas FFA will be distributed homogeneously. The probe pair was used to quantify apparent [1O2] in DOM samples from different isolation procedures (humic acid, fulvic acid, reverse osmosis) and diverse origins (aquatic and terrestrial) as a function of pH and ionic strength, and all samples studied exhibited enhanced reactivity of FFAm relative to FFA, especially at pH 7 and 8. To quantify the spatial distribution of [1O2], we combined electrostatic models with Latch and McNeill's three-phase distribution model. Modeling results for Suwannee River humic acid (SRHA) yield a surface [1O2] of ∼60 pM, which is ∼96-fold higher than the aqueous-phase [1O2] measured with FFA. This value is in agreement with prior reports that determined 1-3 orders of magnitude higher [1O2] in the DOM phase compared to bulk solution. Overall, this work expands the knowledge base of DOM microheterogeneous photochemistry by showing that diverse DOM isolates exhibit this phenomenon. In addition, the dual-probe approach and electrostatic modeling offer a new way to gain mechanistic insight into the spatial distribution of 1O2 and potentially other photochemically produced reactive intermediates.

The enhance mechanism of DOM on tetracyclines degradation by electrochemical technology: A improvement of treatment processes

Tetracyclines (TC) is a typical broad-spectrum antimicrobial agent, and excessive use of TC can lead to a large accumulation of residual tetracycline in water. DOM is organic substances that can pass through the 0.45 μm filter. While dissolved organic matter (DOM) is one of the most significant substances in water, which has an important effect on water treatment. In this study, ultraviolet and visible spectrophotometry (UV-Vis) is applied to explore DOM to the effect of the electrochemical degradation. Three-dimension excitation emission matrix fluorescence spectroscopy (3D-EEM) is used to identify the component variation of DOM after the electrochemical oxidation (EO). Liquid chromatograph mass spectrometer (LC-MS) is used to confirm the degradation pathway of TC whether spontaneous or electrochemical oxidation. High performance liquid chromatography (HPLC) suggests the ROS production by DOM in the electrochemical oxidation under different conditions. Results show that DOM can promote the degradation of TC in the electrochemical oxidation. Tailwater DOM containssubstances can produce persistent free radicals, which can promote the degradation under light and dark conditions, natural source DOM can produce more free radicals under light. Therefore, TC wastewater should be added tailwater to promote the degradation of TC before the further water treatment. Otherwise, TC can be degraded to differentpathways (light, electricity, and degrade spontaneously). This study provides a significant idea for practical water treatment of tetracyclines, and promotes the practical application of electrochemical technology.

Indirect photodegradation of sulfadimidine and sulfapyridine: Influence of CDOM components and main seawater factors

This study investigated the indirect photodegradation of sulfadimidine (SM₂) and sulfapyridine (SP) in the presence of chromophoric dissolved organic matter (CDOM), and studied the influences of main marine factors (salinity, pH, NO₃^- and HCO₃^-). Reactive intermediate (RI) trapping experiments demonstrated that triplet CDOM (³CDOM*) played a major role in the photodegradation of SM₂ with a 58% photolysis contribution, and the contributions to the photolysis of SP were 32%, 34% and 34% for ³CDOM*, hydroxyl radical (HO·) and singlet oxygen (¹O₂), respectively. Among the four CDOMs, JKHA, with the highest fluorescence efficiency, exhibited the fastest rate of SM₂ and SP photolysis. The CDOMs were composed of one autochthonous humus (C1) and two allochthonous humus (C2 and C3). C3, with the strongest fluorescence intensity, had the strongest capacity to generate RIs and accounted for approximately 22%, 11%, 9% and 38% of the total fluorescence intensity of SRHA, SRFA, SRNOM and JKHA, respectively, indicating the predominance of CDOM fluorescent components in the indirect photodegradation of SM₂ and SP. These results demonstrated the photolysis mechanism: The photosensitization of CDOM occurred after its fluorescence intensity decreased, and a large number of RIs (³CDOM*, HO· and ¹O₂, etc.) were generated by energy and electron transfer, then these RIs reacted with SM₂ and SP to cause photolysis. The increase in salinity stimulated the photolysis of SM₂ and SP consecutively. The photodegradation rate of SM₂ first increased and then decreased with increasing pH, whereas the photolysis of SP was remarkably promoted by high pH but remained stable at low pH. NO₃^- and HCO₃^- had little effect on the indirect photodegradation of SM₂ and SP. This research may contribute to a better understanding of the fate of SM₂ and SP in the ocean and provide new insights into the transformation of other sulfonamides (SAs) in marine ecological environments.

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.

Decoupling Optical Response and Photochemical Formation of Singlet Oxygen in Size Isolated Fractions of Ozonated Dissolved Organic Matter

The complex effects of ozonation on the photophysical and size-based properties of dissolved organic matter (DOM) were investigated using two DOM isolates, Suwannee River Fulvic Acid (SRFA) and Pony Lake Fulvic Acid (PLFA). A size exclusion chromatography system paired with absorbance, fluorescence, and total organic carbon detection was used to determine the fluorescence quantum yield (Φ_f) as a function of the apparent molecular weight (AMW). Size-based fractions of each isolate were collected and irradiated to measure singlet oxygen (¹O₂) quantum yield (Φ_1O2). Φ_f decreased with ozonation in low AMW fractions, while increasing in high AMW fractions. Φ_1O2 increased with ozone dose in low AMW fractions from ∼2 to ∼7% and ∼3 to ∼11% for PLFA and SRFA, respectively, indicating that these are the most photoreactive fractions of DOM. Decreases in Φ_f and concomitant increases in Φ_1O2 in low AMW fractions indicated that chemical transformations occurred, likely including the conversion of phenols to quinones, particularly in SRFA. Results further suggest that the photoactive and fluorescent fractions of DOM are likely independent pools of chromophores from different AMW fractions. In PLFA, a linear response in Φ_1O2, specific UV absorbance at wavelength 254 nm (SUVA₂₅₄), and Φ_f with ozonation indicated the equal distribution of ozone-reactive moieties.

Assessing the Use of Probes and Quenchers for Understanding the Reactive Species in Advanced Oxidation Processes

Advanced oxidation processes (AOPs) are increasingly applied in water and wastewater treatment. Understanding the role of reactive species using probes and quenchers is one of the main requirements for good process design. However, much fundamental kinetic data for the reactions of probes and quenchers with reactive species is lacking, probably leading to inappropriate probe and quencher selection and dosing. In this work, second-order rate constants for over 150 reactions of probes and quenchers with reactive species such as ^•OH, SO₄^•-, and Cl^• and chemical oxidants such as free chlorine and persulfate were determined. Some previously ill-quantified reactions (e.g., furfuryl alcohol and methyl phenyl sulfoxide reactions with certain chemical oxidants, nitrobenzene and 1,4-dioxane reactions with certain halogen radicals) were found to be kinetically favorable. The selection of specific probes can be guided by the improved kinetic database. The criteria for properly choosing dosages of probes and quenchers were proposed along with a procedure for quantifying reactive species free of interference from probe addition. The limitations of probe and quencher approaches were explicated, and possible solutions (e.g., the combination with other tools) were proposed. Overall, the kinetic database and protocols provided in this work benefit future research in understanding the radical chemistry in AOPs as well as other radical-involved processes.

Impacts of properties of dissolved organic matters on indirect photodegradation of genistein

Excited triplet states of dissolved organic matters (³DOM*) are one of the most important photochemically-produced reactive intermediates leading to transformation of organic contaminants. However, relationships of photodegradation kinetics of different dissociation states of phenolic organic contaminants with chemical components or properties of ³DOM* are largely unknown. In this study, roles of ³DOM* in photodegradation of polyhydroxy phenolic genistein (Gs) at pH 5, 8 and 12 were investigated taking five kinds of DOM from different sources as examples. Relationships between photodegradation kinetics constants and DOM properties were built. Results showed that the contributions of direct ³DOM*-induced reactions to the total indirect photodegradation of Gs and second-order reaction rate constants (k_DOM,Gs) of Gs with ³DOM* increased with pH increases. This was mainly attributed to decreases in vertical ionization energy of Gs at higher pH, endowing Gs with stronger electron donating capacities. k_DOM,Gs was found to positively correlate with the specific ultraviolet absorbance at 254 nm, reflecting aromaticity of DOM, and negatively correlate with the absorbance ratio at 254 and 365 nm and contents of dissociated acidic functional groups of DOM, representing molecular weights of DOM, antioxidants and the repulsive forces between ³DOM* and Gs. This study provided a new insight into relationship between DOM properties and indirect photodegradation kinetics of phenolic contaminants in aquatic environments.

Impact of chlorination and ozonation of dissolved organic matter on its photo-induced production of long-lived photooxidants and excited triplet states

Recent studies suggested that long-lived photooxidants (LLPO), which are reactive intermediates formed during irradiation of dissolved organic matter (DOM), may consist of phenoxyl radicals derived from phenolic moieties of the DOM. Besides the well-studied excited triplet states of chromophoric DOM (³CDOM*), LLPO presumably are important photooxidants for the transformation of electron-rich contaminants in surface waters. The main objective of this study was to further test the potential role of phenoxyl radical as LLPO. Suwannee River fulvic acid (SRFA) as a model DOM was pre-oxidised using the phenol-reactive oxidants chlorine and ozone, followed by its characterization by the specific UV absorption at 254 nm (SUVA₂₅₄), the ratio of absorbance at λ = 254 nm and λ = 365 nm (E2:E3), and the electron donating capacity (EDC). Subsequently, the photoreactivity of pre-oxidized SRFA was tested using 3,4-dimethoxyphenol (DMOP) as a LLPO probe compound at two initial concentrations ([DMOP]₀ = 0.1 and 5.0 μM). Linear inter-correlations were observed for the relative changes in SUVA₂₅₄, E2:E3, and EDC for increasing oxidant doses. Pseudo-first-order transformation rate constants normalized to the changing SRFA absorption rate (i.e., k_0.1^obs/r_CDOM^absand k_5.0^obs/r_CDOM^abs, for 0.1 and 5.0 µM, respectively) exhibited the following distinct trends: The LLPO-dominated k_0.1^obs/r_CDOM^absratio decreased with increasing oxidant dose and with decreasing SUVA₂₅₄ and EDC, while the ³CDOM*-dominated k_5.0^obs/r_CDOM^absratio positively correlated with E2:E3. Finally, it was concluded that precursors of ³CDOM* and LLPO are chemically modified differently by pre-oxidation of DOM, and LLPO precursors likely consist of phenolic moieties of DOM, suggesting phenoxyl radicals as LLPO.

Photochemical Implications of Changes in the Spectral Properties of Chromophoric Dissolved Organic Matter: A Model Assessment for Surface Waters

Chromophoric dissolved organic matter (CDOM) is the main sunlight absorber in surface waters and a very important photosensitiser towards the generation of photochemically produced reactive intermediates (PPRIs), which take part in pollutant degradation. The absorption spectrum of CDOM (ACDOM(λ), unitless) can be described by an exponential function that decays with increasing wavelength: ACDOM(λ) = 100 d DOC Ao e− Sλ, where d [m] is water depth, DOC [mgC L−1] is dissolved organic carbon, Ao [L mgC−1 cm−1] is a pre-exponential factor, and S [nm−1] is the spectral slope. Sunlight absorption by CDOM is higher when Ao and DOC are higher and S is lower, and vice versa. By the use of models, here we investigate the impact of changes in CDOM spectral parameters (Ao and S) on the steady-state concentrations of three PPRIs: the hydroxyl radical (•OH), the carbonate radical (CO3•−), and CDOM excited triplet states (3CDOM*). A first finding is that variations in both Ao and S have impacts comparable to DOC variations on the photochemistry of CDOM, when reasonable parameter values are considered. Therefore, natural variability of the spectral parameters or their modifications cannot be neglected. In the natural environment, spectral parameters could, for instance, change because of photobleaching (prolonged exposure of CDOM to sunlight, which decreases Ao and increases S) or of the complex and still poorly predictable effects of climate change. A second finding is that, while the steady-state [3CDOM*] would increase with increasing ACDOM (increasing Ao, decreasing S), the effect of spectral parameters on [•OH] and [CO3•−] depends on the relative roles of CDOM vs. NO3− and NO2− as photochemical •OH sources.

Photolysis of 3-Nitro-1,2,4-triazol-5-one: Mechanisms and Products

Insensitive munitions formulations that include 3-nitro-1,2,4-triazol-5-one (NTO) are replacing traditional explosive compounds. While these new formulations have superior safety characteristics, the compounds have greater environmental mobility, raising concern over potential contamination and cleanup of training and manufacturing facilities. Here, we examine the mechanisms and products of NTO photolysis in simulated sunlight to further inform NTO degradation in sunlit surface waters. We demonstrate that NTO produces singlet oxygen and that dissolved oxygen increases the NTO photolysis rate in deionized water. The rate of NTO photolysis is independent of concentration and decreases slightly in the presence of Suwannee River Natural Organic Matter. The apparent quantum yield of NTO generally decreases as pH increases, ranging from 2.0 × 10-5 at pH 12 to 1.3 × 10-3 at pH 2. Bimolecular reaction rate constants for NTO with singlet oxygen and hydroxyl radical were measured to be (1.95 ± 0.15) × 106 and (3.28 ± 0.23) × 1010 M-1 s-1, respectively. Major photolysis reaction products were ammonium, nitrite, and nitrate, with nitrite produced in nearly stoichiometric yield upon the reaction of NTO with singlet oxygen. Environmental half-lives are predicted to span from 1.1 to 5.7 days. Taken together, these data enhance our understanding of NTO photolysis under environmentally relevant conditions.

Effects of erythromycin and roxithromycin on river periphyton: Structure, functions and metabolic pathways

Macrolides have been frequently detected in the surface waters worldwide, posing a threat to the aquatic microbes. Several studies have evaluated the ecotoxicological effects of macrolides on single algal and bacterial strains. However, without considering the species interaction in the aquatic microbial community, these results cannot be extrapolated to the field. Thus, the present study aimed to evaluate the effects of two macrolides (erythromycin and roxithromycin) on the structure, photosynthetic process, carbon utilization capacity, and the antibiotic metabolic pathways in river periphyton. The colonized periphyton was exposed to the graded concentration (0 μg/L (control), 0.5 μg/L (low), 5 μg/L (medium), 50 μg/L (high)) of ERY and ROX, respectively, for 7 days. Herein, high levels of ERY and ROX altered the community composition by reducing the relative abundance of Chlorophyta in the eukaryotic community. Also, the Shannon and Simpson diversity indexes of prokaryotes were reduced, although similar effects were seldomly detected in the low and medium groups. In contrast to the unchanged carbon utilization capacity, the PSII reaction center involved in the periphytic photosynthesis was significantly inhibited by macrolides at high levels. In addition, both antibiotics had been degraded by periphyton, with the removal rate of 51.63-66.87% and 41.85-48.27% for ERY and ROX, respectively, wherein the side chain and ring cleavage were the main degradation pathways. Overall, this study provides an insight into the structural and functional toxicity and degradation processes of macrolides in river periphyton.

Mapping the diffusion pattern of 1O2 along DNA duplex by guanine photooxidation with an appended biphenyl photosensitizer

To realize nucleic acid-targeting photodynamic therapy, a photosensitizer should be attached at the optimal position on a complementary oligonucleotide, where a guanine photooxidation is maximized. Here we show the photooxidation of 22 DNA duplexes with varied lengths between a 1O2-generating biphenyl photosensitizer attached at a midchain thymine in a strand and the single guanine reactant in the other strand. The best photooxidation efficiencies are achieved at 9, 10, and 21 base intervals, which coincides with the pitch of 10.5 base pairs per turn in a DNA duplex. The low efficiencies for near and far guanines are due to quenching of the biphenyl by guanine and dilution of 1O2 by diffusion, respectively. The 1O2-diffusion mapping along DNA duplex provides clues to the development of efficient and selective photosensitizer agents for nucleic acid-targeting photodynamic therapy, as well as an experimental demonstration of diffusion of a particle along cylindrical surface in molecular level.

Photodegradation of polyolefin thin films in simulated freshwater conditions

Polypropylene (PP) and polyethylene (PE) are commonly used polyolefins in a variety of applications, which have resulted in their accumulation in the environment. Once in the environment, these polymers undergo various chemical and physical transformations as the result of environmental stressors such as sunlight. While photodegradation has been studied for decades, there are key gaps in knowledge on the phototransformations of polyolefins that occur under aqueous conditions. Therefore, the goal of this study is to characterize the phototransformations of PP and PE in simulated freshwater conditions. Polymer thin films were irradiated with 254 nm and 350 nm UV light in air, ultra-pure water, and solutions of dissolved organic matter (DOM) to simulate natural systems. Irradiated plastics were evaluated for oxidation and chain scission. It was observed using Fourier transform infrared spectroscopy (FTIR) that oxidation in aqueous environments happened at a slower rate compared to oxidations in air. However, photo-oxidation was accelerated in the presence of DOM compared to ultrapure water, with singlet oxygen and hydroxyl radical causing varied amounts of degradation depending on the polymer. The vinyl characteristic, a chain scission product, revealed an increased yield but the reaction rate showed that these photoproducts were more likely to occur when oxidation is less favorable. Compared to naturally weathered samples, lab observed transformations were on par with naturally degraded samples and support the importance of the in-lab measurements. This work quantifies the extent and rate of photodegradation pathways in PP and PE to demonstrate the importance of photodegradation in aquatic systems.

An inevitable but underestimated photoaging behavior of plastic waste in the aquatic environment: Critical role of nitrate

Photoaging is an important reaction for waste plastics in the aquatic environment and plays a key role in the lifetime of plastics. Nevertheless, when natural photosensitive substances such as nitrate participate in this process, the physiochemical changes in plastics and the corresponding reaction mechanisms are not well-understood. In this work, the photochemical behavior of polyethylene terephthalate (PET) bottles in deionized water and nitrate solution was systematically investigated under ultraviolet (UV) irradiation. The analyses of the surface physicochemical properties of the photoaged PET bottles indicated that, after 20 days of photo-irradiation, the presence of nitrate reduced the contact angle from 69.8 ± 0.9° to 60.0 ± 0.3°, and increased the O/C ratio from 0.23 to 0.32, respectively. The leaching rate of dissolved organic carbon (DOC), which was 0.0193 mg g^-1·day^-1 in nitrate solution, was twice that of 0.00941 mg g^-1·day^-1 in deionized water. Furthermore, fluorescence spectroscopy revealed that the increasing DOC had aromatic rings with hydroxyl on the side-chain formed after UV irradiation. The positive effect of nitrate on the degradation of PET bottles was mainly through the generation of hydroxyl radicals that were produced through the photolysis of nitrate. In addition, two-dimensional correlation spectroscopy analysis showed that the chain scission of PET plastics could be initiated by nitrate-induced ·OH attacking the carbon-oxygen bonds instead of forming peroxides with oxygen. This work elucidates the mechanism of photodegradation of plastics that was induced by nitrate and highlights the important role of natural photosensitive substances in the photoaging process of plastics.

Rapid Biotic and Abiotic Transformation of Toxins produced by Ostreopsis. cf. ovata

The dinoflagellate Ostreopsis cf. ovata produces several families of toxic polyketides. Despite only a few field measurements of these phycotoxins in seawater and aerosols, they are believed to be responsible for dermatitis and the toxic inhalations reported during blooms of this species. Therefore, the stability of these compounds in seawater is essential to understanding the causes of these symptoms, however, this has never been assessed. In the current study, the optimization of a solid phase extraction (SPE) procedure was first performed to ensure the most efficient extraction of all phycotoxins known to be produced by this strain, including the recently described liguriatoxins. The SPE cartridge SDBL® under non acidified conditions offered the best option. The stability of the ovatoxins and the liguriatoxins under biotic and abiotic stress was assessed by exposing the spent medium of a culture of Ostreopsis cf. ovata to its bacterial consortium and natural sunlight. A rapid biotic transformation was detected for both families of compounds. When exposed to bacteria, the half-lives of the ovatoxins were reached before 10 h and at 36 h, 97% of these toxins had been transformed. The half-lives of the liguriatoxins were 10 h under these conditions. Photolysis (abiotic degradation) of the ovatoxins (T1/2 < 36 h) was faster than for the liguriatoxins (T1/2 > 62 h). Although none of the catabolites of these phycotoxins were thoroughly identified, an untargeted metabolomics approach combined with molecular networking highlighted the presence of several compounds exhibiting structural similarities with the ovatoxins. Additional work should confirm the preliminary findings on these potential ovatoxins’ catabolites and their biological properties. The rapid transformation of O. cf. ovata’s phycotoxins introduces questions concerning their presence in seawater and their dispersion in the sea spray aerosols. The compounds involved in the toxic inhalations and dermatitis often experienced by beachgoers may stem from the catabolites of these toxins or even unrelated and as yet unidentified compounds.

Evaluation of the Environmental Fate of a Semivolatile Transformation Product of Ibuprofen Based on a Simple Two-Media Fate Model

Partitioning between surface waters and the atmosphere is an important process, influencing the fate and transport of semi-volatile contaminants. In this work, a simple methodology that combines experimental data and modeling was used to investigate the degradation of a semi-volatile pollutant in a two-phase system (surface water + atmosphere). 4-Isobutylacetophenone (IBAP) was chosen as a model contaminant; IBAP is a toxic transformation product of the non-steroidal, anti-inflammatory drug ibuprofen. Here, we show that the atmospheric behavior of IBAP would mainly be characterized by reaction with ^•OH radicals, while degradation initiated by ^•NO₃ or direct photolysis would be negligible. The present study underlines that the gas-phase reactivity of IBAP with ^•OH is faster, compared to the likely kinetics of volatilization from aqueous systems. Therefore, it might prove very difficult to detect gas-phase IBAP. Nevertheless, up to 60% of IBAP occurring in a deep and dissolved organic carbon-rich water body might be eliminated via volatilization and subsequent reaction with gas-phase ^•OH. The present study suggests that the gas-phase chemistry of semi-volatile organic compounds which, like IBAP, initially occur in natural water bodies in contact with the atmosphere is potentially very important in some environmental conditions.

Direct photolysis of contaminants in surface freshwaters, within the equivalent monochromatic wavelength (EMW) approximation

Abiotic photochemical reactions are usually very important degradation pathways for biorecalcitrant pollutants in surface freshwaters. Therefore, the assessment of photolytic lifetimes of contaminants helps estimate their impact on aquatic systems. This is commonly carried out by combining irradiation experiments and modelling, where the latter considers mathematical functions with polychromatic parameters, such as sunlight spectra, photolysis quantum yields (when Kasha's rule does not hold), and absorption coefficients. With the polychromatic approach, the photolytic lifetime is calculated by solving several integrals, which requires quite demanding modelling resources. In this work, we applied a recently developed approach, which is based on the equivalent monochromatic wavelength (EMW) approximation, to compute the direct-photolysis lifetimes of a range of >40 pollutants in inland waters. The EMW approximation allowed for easier modelling procedure, at the same time providing very good agreement with the polychromatic system. To further show EMW potentialities, lifetimes of three contaminants were mapped over the Piedmont region (NW Italy), as an example of how easy it becomes to geographically EMW-assess the potential of watercourses, to get photochemically decontaminated from pollutants.

Wavelength trends of photoproduction of reactive transient species by chromophoric dissolved organic matter (CDOM), under steady-state polychromatic irradiation

The formation quantum yields of photochemically produced reactive intermediates (PPRIs) by irradiated CDOM (in this study, Suwannee River Natural Organic Matter and Upper Mississippi River Natural Organic Matter) decrease with increasing irradiation wavelength. In particular, the formation quantum yields of the excited triplet states of CDOM (³CDOM*) and of singlet oxygen (¹O₂) have an exponentially decreasing trend with wavelength. The ^•OH wavelength trend is different, because more effective ^•OH production occurs under UVB irradiation than foreseen by a purely exponential function. We show that the parameter-adjustable Weibull function (which adapts to both exponential and some non-exponential trends) is suitable to fit the mentioned quantum yield data, and it is very useful when CDOM irradiation is carried out under polychromatic lamps as done here. Model calculations suggest that, thanks to the ability of CDOM to also absorb visible radiation, and despite its decreasing quantum yield of ^•OH generation with increasing wavelength, CDOM would be able to trigger ^•OH photogeneration in deep waters, to a higher extent than UVB-absorbing nitrate or UVB + UVA-absorbing nitrite.

Reducing properties of triplet state organic matter (3DOM*) probed via the transformation from chlorine dioxide to chlorite

The triplet states of dissolved organic matter (³DOM*) have been well known to oxidize various organic contaminants, but evidence of their reducing properties are largely scarce. In this work, chlorine dioxide (ClO₂) as a single-electron oxidant was used as a probe to evaluate the reduction property of ³DOM*. The reduction of ClO₂ to chlorite was observed in the solutions of model photosensitizers (i.e., 4-carboxybenzophenone, benzophenone, acetophenone, 3-methoxyacetophenone, naphthalene, and xanthone) during UV irradiation with the presence of ClO₂, though they are resistant to ClO₂ oxidation in the dark. The reducing property of the triplet states of photosensitizers was verified and their second-order reaction rate constants with ClO₂ were determined to be in the range of 1.45(± 0.03)× 10⁹ - 2.18(± 0.06) × 10⁹ M^-1 s^-1 at pH 7.0. The quenching tests excluded the role of other reactive species (e.g., HO^•, O(³P), Cl^•, ClO^• and HOCl/OCl^-, O₂^•- and e_aq^-) in ClO₂ reduction to chlorite when using model photosensitizers and DOM isolates. Chlorite formation was 48.1-90.4% and 4812.8-7721.8% higher during UV irradiation with the presence of ClO₂ and DOM than those without UV irradiation or without DOM present, respectively. The enhancement was attributed to the enhanced electron donating capacity (chlorite precursors) of DOM upon UV irradiation and also to ³DOM* acting as an electron donor reducing ClO₂ to chlorite. This study highlighted the important role of ³DOM* as a reductant.

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

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

Global review of macrolide antibiotics in the aquatic environment: Sources, occurrence, fate, ecotoxicity, and risk assessment

The extensive use of macrolide antibiotics (MCLs) has led to their frequent detection in aquatic environments, affecting water quality and ecological health. In this study, the sources, global distribution, environmental fate, ecotoxicity and global risk assessment of MCLs were analyzed based on recently published literature. The results revealed that there are eight main sources of MCLs in the water environment. These pollution sources resulted in MCL detection at average or median concentrations of up to 3847 ng/L, and the most polluted water bodies were the receiving waters of wastewater treatment plants (WWTPs) and densely inhabited areas. Considering the environmental fate, adsorption, indirect photodegradation, and bioremoval may be the main attenuation mechanisms in natural water environments. N-demethylation, O-demethylation, sugar and side chain loss from MCL molecules were the main pathways of MCLs photodegradation. Demethylation, phosphorylation, N-oxidation, lactone ring hydrolysis, and sugar loss were the main biodegradation pathways. The median effective concentration values of MCLs for microalgae, crustaceans, fish, and invertebrates were 0.21, 39.30, 106.42, and 28.00 mg/L, respectively. MCLs induced the generation of reactive oxygen species, that caused oxidative stress to biomolecules, and affected gene expression related to photosynthesis, energy metabolism, DNA replication, and repair. Moreover, over 50% of the reported water bodies represented a medium to high risk to microalgae. Further studies on the development of tertiary treatment technologies for antibiotic removal in WWTPs, the combined ecotoxicity of antibiotic mixtures at environmental concentration levels, and the development of accurate ecological risk assessment models should be encouraged.

Photo-Reactivity and Photo-Transformation of Algal Dissolved Organic Matter Unraveled by Optical Spectroscopy and High-Resolution Mass Spectrometry Analysis

The rapid proliferation of planktonic algae induced by eutrophication and climate warming make algae dissolved organic matter (AOM) an important source of dissolved organic matter (DOM) in surface waters, but the understanding of the link between AOM composition and photo-reactivity/photo-transformation of DOM in aquatic systems is limited. Here, intracellular organic matter (IOM) from Microcystis aeruginosa was extracted and subjected to molecular weight (MW) fractionation. Results indicated that IOM had lower aromaticity and higher photosensitive activity compared to Suwannee River fulvic acid (SRFA). The photosensitive activity of IOM relied on both its molecular weight distribution and fluorescence components. The IOM fraction with the highest MW proteins had the lowest quantum yields of reactive intermediates (Φ_RIs), which increased with the decrease of MW, while the fractions with more low-excitation tyrosine-like components had relatively higher Φ_RIs. Parallel factor analysis and high-resolution mass spectrometry revealed that light radiation of IOM resulted in the composition transformation from tryptophan-like and tyrosine-like components to humic-like components, forming less aromatic and more saturated recalcitrant dissolved organic carbon. Our findings provide new insights into the photo-reactivity and photo-transformation of algae-derived organic matters and help to predict DOM formation involved in carbon cycling in water environment.

Photochemical Production of Carbon Monoxide from Dissolved Organic Matter: Role of Lignin Methoxyarene Functional Groups

Carbon monoxide (CO) is the second most abundant identified product of dissolved organic matter (DOM) photodegradation after CO₂, but its formation mechanism remains unknown. Previous work showed that aqueous photodegradation of methoxy-substituted aromatics (ArOCH₃) produces CO considerably more efficiently than aromatic carbonyls. Following on this precedent, we propose that the methoxy aromatic groups of lignin act as the C source for the photochemical formation of CO from terrestrial DOM via a two-step pathway: formal hydrolytic demethylation to methanol and methanol oxidation to CO. To test the reasonableness of this mechanism, we investigated the photochemistry of eight lignin model compounds. We first observed that initial CO production rates are positively correlated with initial substrate degradation rates only for models containing at least one ArOCH₃ group, regardless of other structural features. We then confirmed that all ArOCH₃-containing substrates undergo formal hydrolytic demethylation by detecting methanol and the corresponding phenolic transformation products. Finally, we showed that hydroxyl radicals, likely oxidants to initiate methanol oxidation to CO, form during irradiation of all models. This work proposes an explicit mechanism linking ubiquitous, abundant, and easily quantifiable DOM functionalities to CO photoproduction. Our results further hint that methanol may be an abundant (yet overlooked) DOM photoproduct and a likely precursor of formaldehyde, formic acid, and CO₂ and that lignin photodegradation may represent a source of hydroxyl radicals.

Quantification of the diverse inhibitory effects of dissolved organic matter on transformation of micropollutants in UV/persulfate treatment

Dissolved organic matter (DOM), ubiquitous in natural waters, is known to inhibit the degradation of micropollutants in the advanced oxidation processes such as the UV/peroxydisulfate process. However, the quantitative understanding of the inhibitory pathways is missing. In this study, guanosine, aniline and catechol belonging to amines, purines and phenols were first investigated due to their resistance to UV irradiation at 254 nm and similar reactivity with SO₄^•- and HO^•, respectively. The presence of 0.5 mg_C L^-1 Suwannee River NOM (SRNOM) inhibited their degradation rates by 72.9%, 54.5%, and 32.4%, respectively, despite their similar degradation rates in the absence of SRNOM. The results highlight the importance of reverse reduction of oxidation intermediates to the parent compound by antioxidant moieties in SRNOM besides the inner filtering and radical scavenging effects. The three inhibitory pathways were quantified for 34 common micropollutants. In the presence of 0.5 mg_C L^-1 SRNOM, inner filtering effect was found to contribute less than 2.8% of the inhibitory percentages (IP). Radical scavenging effects contribute between 10.7% and 38.9% and compounds having lower reactivity with SO₄^•- (< 4.0 × 10⁹ M^-1 s^-1) tended to be inhibited more strongly. The IP of reverse reduction effects of SRNOM varied significantly from none up to 70.8%. It was linearly related with a micropollutant's reduction potential. Purines and amines generally exhibited more pronounced reverse reduction inhibition than phenols. The results of this study provide guidance on improving the elimination efficiency of micropollutants.

Phototransformation of the fungicide tebuconazole, and its predicted fate in sunlit surface freshwaters

The fungicide tebuconazole (TBCZ) is expected to undergo negligible direct photolysis in surface freshwaters, but it can be degraded by indirect photochemistry. TBCZ mainly reacts with hydroxyl radicals and, to a lesser extent, with the triplet states of chromophoric dissolved organic matter (³CDOM*). Indirect photochemistry is strongly affected by environmental conditions, and TBCZ lifetimes of about one week are expected in sunlit surface waters under favourable circumstances (shallow waters with low concentrations of dissolved organic carbon, DOC, during summer). In these cases, the time trend would follow pseudo-first order kinetics (mono-exponential decay). Under less favourable conditions, photoinduced degradation would span over a few or several months, and TBCZ phototransformation would depart from an exponential trend because of seasonally changing sunlight irradiance. The TBCZ phototransformation products should be less toxic than their parent compound,thus photodegradation has potential to decrease the environmental impact of TBCZ. Hydroxylation is a major TBCZ transformation route, due to either OH attack, or one-electron oxidation sensitised by ³CDOM*, followed by reaction of the oxidised transient with oxygen and water.

Enhanced photochemical production of reactive intermediates at the wetland soil-water interface

Photochemically produced reactive intermediates (PPRIs) formed by sunlight-irradiation of natural photosensitizers play critical roles in accelerating biogeochemical cycles on earth surface. Existing PPRI studies mostly focus on bulk phase reactions (e.g., bulk water), with PPRI processes at the environmental interfaces largely unexplored. Here, we report the wetland soil-water interface (SWI) as a widespread but previously unappreciated hotspot for PPRI productions. Massive productions of four important PPRI species (i.e., triplet-state excited organic matter (³OM*), singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂), and hydroxyl radical (^•OH)) were observed at the SWI. All four PPRI species exhibited higher productions at the SWI than those in bulk water, where ^•OH production was largely elevated by up to one order of magnitude. The enhanced PPRI productions at the SWI were caused by intensified photon absorption and vibrant Fe-mediated redox processes, where the light absorption by less- or non-photoactive soil substances partially offset the enhancement on PPRI productions. Nationwide wetland investigations demonstrate that the SWI was a ubiquitous hotspot for PPRI productions. Simulations on PPRIs-mediated reactions suggest that the enhanced PPRI productions could greatly affect the kinetics and transformation pathways of nutrients and pollutants. Given that the SWI also acts a hotspot for nutrient and pollutant accumulation, incorporating the SWI enhanced PPRI productions into biogeochemical process assessments is pivotal for advancing our understandings on the element cycles and pollutant dynamics in wetlands.

Multiple Roles of Dissolved Organic Matter in Advanced Oxidation Processes

Advanced oxidation processes (AOPs) can degrade a wide range of trace organic contaminants (TrOCs) to improve the quality of potable water or discharged wastewater effluents. Their effectiveness is impacted, however, by the dissolved organic matter (DOM) that is ubiquitous in all water sources. During the application of an AOP, DOM can scavenge radicals and/or block light penetration, therefore impacting their effectiveness toward contaminant transformation. The multiple ways in which different types or sources of DOM can impact oxidative water purification processes are critically reviewed. DOM can inhibit the degradation of TrOCs, but it can also enhance the formation and reactivity of useful radicals for contaminants elimination and alter the transformation pathways of contaminants. An in-depth analysis highlights the inhibitory effect of DOM on the degradation efficiency of TrOCs based on DOM's structure and optical properties and its reactivity toward oxidants as well as the synergistic contribution of DOM to the transformation of TrOCs from the analysis of DOM's redox properties and DOM's transient intermediates. AOPs can alter DOM structure properties as well as and influence types, mechanisms, and extent of oxidation byproducts formation. Research needs are proposed to advance practical understanding of how DOM can be exploited to improve oxidative water purification.

A Model Assessment of the Occurrence and Reactivity of the Nitrating/Nitrosating Agent Nitrogen Dioxide (•NO2) in Sunlit Natural Waters

Nitrogen dioxide (•NO2) is produced in sunlit natural surface waters by the direct photolysis of nitrate, together with •OH, and upon the oxidation of nitrite by •OH itself. •NO2 is mainly scavenged by dissolved organic matter, and here, it is shown that •NO2 levels in sunlit surface waters are enhanced by high concentrations of nitrate and nitrite, and depressed by high values of the dissolved organic carbon. The dimer of nitrogen dioxide (N2O4) is also formed in the pathway of •NO2 hydrolysis, but with a very low concentration, i.e., several orders of magnitude below •NO2, and even below •OH. Therefore, at most, N2O4 would only be involved in the transformation (nitration/nitrosation) of electron-poor compounds, which would not react with •NO2. Although it is known that nitrite oxidation by CO3•− in high-alkalinity surface waters gives a minor-to-negligible contribution to •NO2 formation, it is shown here that NO2− oxidation by Br2•− can be a significant source of •NO2 in saline waters (saltwater, brackish waters, seawater, and brines), which offsets the scavenging of •OH by bromide. As an example, the anti-oxidant tripeptide glutathione undergoes nitrosation by •NO2 preferentially in saltwater, thanks to the inhibition of the degradation of glutathione itself by •OH, which is scavenged by bromide in saltwater. The enhancement of •NO2 reactions in saltwater could explain the literature findings, that several phenolic nitroderivatives are formed in shallow (i.e., thoroughly sunlit) and brackish lagoons in the Rhône river delta (S. France), and that the laboratory irradiation of phenol-spiked seawater yields nitrophenols in a significant amount.

Dissolved Organic Matter Promotes the Aging Process of Polystyrene Microplastics under Dark and Ultraviolet Light Conditions: The Crucial Role of Reactive Oxygen Species

Microplastics (MPs) interact frequently with dissolved organic matter (DOM) commonly found in the environment, but information on the aging behavior of MPs under the participation of DOM is still lacking. Thus, the polystyrene microplastic (PSMP) aging process with DOM participation was systematically studied by electron paramagnetic resonance spectroscopy, high-performance liquid chromatography, Fourier transform infrared (FTIR) spectroscopy, and two-dimensional correlation spectroscopy analyses under dark and ultraviolet (UV) light conditions. DOM was found to promote electron transfer to generate reactive oxygen species (ROS) under dark conditions and the aging of PSMPs, while the process of DOM generating ROS under UV light was more susceptible to photoelectrons and accelerated the aging process of PSMPs. However, among the four DOM types, fulvic acid (FA) has a more significant promoting effect on the aging process of PSMPs than humic acid, which can be attributed to the stronger conversion ability of FA to semiquinone radicals. Density functional theory calculations are used to describe the difference in the aging process of different structures of plastics with the participation of DOM. This study provides a necessary theoretical basis for the study of the migration of MPs in groundwater and deep surface water.

Effect of disinfection on the photoreactivity of effluent organic matter and photodegradation of organic contaminants

Chlorine, UV₂₅₄, and ozone are three typical processes commonly used for wastewater disinfection, which could change the photoreactivity of dissolved organic matter (DOM) in effluents of wastewater treatment plants (WWTPs). The photoinduced reactive species (RS) from DOM, primarily including the excited triplet state of DOM (³DOM*), singlet oxygen (¹O₂), and hydroxyl radical (^•OH), play important roles in the attenuation of contaminants. However, the effect of disinfection processes on the photosensitized degradation of contaminants is poorly understood. This paper presents the first evidence that ³DOM*, ¹O₂, and ^•OH interaction with three typical contaminants (diphenhydramine, cimetidine, and N,N-diethyl-m-toluamide (DEET)) was largely impacted by DOM after disinfection. The results of electron spin resonance (ESR) spectrometry and laser flash photolysis (LFP) experiments demonstrated that the chlorination increased the formation rate of ³DOM* and ¹O₂, while UV₂₅₄ irradiation and ozonation decreased the formation rate of these RS. All these three disinfection processes promoted the photoproduction of ^•OH and increased the photodegradation rate constants (k_obs) of DEET by 26-361%. The k_obs of diphenhydramine, cimetidine, and DEET correlated positively with the formation rate of ³DOM*, ¹O₂, and ^•OH, respectively. The bimolecular reaction rate constant of ³DOM* with diphenhydramine increased by ∼41% after chlorination. These findings suggest that disinfection processes altered the photogeneration of RS from DOM, which significantly impacts the fate of trace pollutants in aquatic environments.

Hydroxyl radicals in anodic oxidation systems: generation, identification and quantification

Anodic oxidation has emerged as a promising treatment technology for the removal of a broad range of organic pollutants from wastewaters. Hydroxyl radicals are the primary species generated in anodic oxidation systems to oxidize organics. In this review, the methods of identifying hydroxyl radicals and the existing debates and misunderstandings regarding the validity of experimental results are discussed. Consideration is given to the methods of quantification of hydroxyl radicals in anodic oxidation systems with particular attention to approaches used to compare the electrochemical performance of different anodes. In addition, we describe recent progress in understanding the mechanisms of hydroxyl radical generation at the surface of most commonly used anodes and the utilization of hydroxyl radical in typical electrochemical reactors. This review shows that the key challenges facing anodic oxidation technology are related to i) the elimination of mistakes in identifying hydroxyl radicals, ii) the establishment of an effective hydroxyl radical quantification method, iii) the development of cost effective anode materials with high corrosion resistance and high electrochemical activity and iv) the optimization of electrochemical reactor design to maximise the utilization efficiency of hydroxyl radicals.

Mechanistic study on photochemical generation of I•/I2•- radicals in coastal atmospheric aqueous aerosol

The reactive iodine species may exhibit significant impacts on many global atmospheric issues and the I^•/I₂^•- radicals play key roles for inducing the formation of these reactive iodine species. However, the current understanding on the formation of I^•/I₂^•- radicals in atmospheric aqueous aerosol is still quite limited. The results reported herein suggest that I^•/I₂^•- can be produced simultaneously in aqueous aerosol by several sunlight-driven photochemical pathways including direct photo-dissociation of soluble organic iodine (SOI) at rates of 0.10-1.34 × 10^-9 M ns^-1 and 0.99-5.68 × 10^-7 M μs^-1, ^•OH-mediated oxidation of I^- at 0.03-1.41 × 10^-8 M ns^-1 and 0.05-4.10 × 10^-6 M μs^-1, and ³DOM^⁎-induced oxidation of I^- at 1.57-1.65 × 10^-9 M ns^-1 and 0.99-5.68 × 10^-7 M μs^-1 for generation of I^• and I₂^•-, respectively. Meanwhile, the pathway of e_aq^--initiated stepwise reduction of IO₃^- to I_2(aq) and further photolyzed into I^• plays negligible role in formation of I^•/I₂^•- due to the low reaction rates and severe quenching effect of e_aq^- by dissolved O₂. Our work presented the new data on mechanism and kinetics for comprehensive elucidation of I^•/I₂^•- formation in coastal atmospheric aqueous aerosol and would help to better understand the transformation mechanism of iodine species, pathways of iodine cycling and the associated environmental impacts involving atmospheric reactive iodine radicals.

Critical Review of UV-Advanced Reduction Processes for the Treatment of Chemical Contaminants in Water

UV-advanced reduction processes (UV-ARP) are an advanced water treatment technology characterized by the reductive transformation of chemical contaminants. Contaminant abatement in UV-ARP is most often accomplished through reaction with hydrated electrons (e_aq ^-) produced from UV photolysis of chemical sensitizers (e.g., sulfite). In this Review, we evaluate the photochemical kinetics, substrate scope, and optimization of UV-ARP. We find that quantities typically reported in photochemical studies of natural and engineered systems are under-reported in the UV-ARP literature, especially the formation rates, scavenging capacities, and concentrations of key reactive species like e_aq ^-. The absence of these quantities has made it difficult to fully evaluate the impact of operating conditions and the role of water matrix components on the efficiencies of UV-ARP. The UV-ARP substrate scope is weighted heavily toward contaminant classes that are resistant to degradation by advanced oxidation processes, like oxyanions and per- and polyfluoroalkyl substances. Some studies have sought to optimize the UV-ARP treatment of these contaminants; however, a thorough evaluation of the impact of water matrix components like dissolved organic matter on these optimization strategies is needed. Overall, the data compilation, analysis, and research recommendations provided in this Review will assist the UV-ARP research community in future efforts toward optimizing UV-ARP systems, modeling the e_aq ^--based chemical transformation kinetics, and developing new UV-ARP systems.

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

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

Enhanced photodegradation of antibiotics based on anoxygenic photosynthetic bacteria and bacterial metabolites: A sustainably green strategy for the removal of high-risk organics from secondary effluent

Antibiotic residues in effluents discharged from wastewater treatment plants (WWTPs) have been considered high-risk organics due to biorefractory property and potential toxicity. Secondary pollution and unsustainability existed in advanced treatment of secondary effluent are currently in urgent need of improvement. In this study, a sustainably green strategy based on Rhodopseudomonas palustris (R.palustris) by regulating the structure of extracellular polymeric substances (EPS) was proposed for the first time to achieve efficiently removal of sulfadiazine (SDZ). Results showed that 0.2 V was the optimal external potential for R.palustris to efficiently remove SDZ, where the biodegradation rate constant obtained at this potential was 4.87-folds higher than that in open-circuit mode and a complete removal was achieved within 58 h in the presence of EPS extracted at this potential. Three-dimensional excitation-emission matrix (3D-EEM) spectra analysis suggested that tryptophan protein-like, tyrosine protein-like, humic acid-like and fulvic acid-like substances present in EPS were the main effective components which was responsible for the indirect photodegradation of SDZ. The quenching experiments showed that ³EPS* was the dominant reactive species which accounted for 90% of SDZ removal. This study provides new implications for the advanced treatment of secondary effluent organic matters by developing eco-friendly bioaugmentation technology and biomaterials.

Atmospheric chemical processes of microcystin-LR at the interface of sea spray aerosol

Microcystins are the most toxic toxins released by cyanobacteria and they have adverse effects on aquatic ecosystems and even human health. Although the removal and detoxification of microcystins in various water bodies have been extensively studied, the interaction mechanism and reaction process of microcystins once they enter the atmosphere are largely unknown, especially at the organic-enriched sea spray aerosol (SSA) interface. Herein, using the surface technique of Langmuir trough coupled in-situ infrared reflection-absorption spectra, we studied the interfacial behavior of microcystin-LR (MC-LR) in artificial seawater containing humic acid and typical surfactants in the presence or absence of UV-irradiation. Zwitterionic 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and anionic stearic acid (SA) were chosen as typical film-forming species, results obtained from the surface pressure-area isotherms showed that MC-LR caused greater expansion of the DSPC monolayer. The comparable results of MC-LR in DSPC/SA-containing systems indicated that the interaction ability was closely related to the monolayer molecular structure and was regulated by electrostatic interaction. Furthermore, the presence of humic acid (HA) could enhance the interaction between microcystin and monolayer molecules. UV-irradiation experiments showed that the photosensitized reaction greatly promoted the removal of microcystin embedded in the SSA surface compared with the direct photolysis effect in the absence of HA. These findings highlight that the toxic effects of microcystins after entering the atmosphere may be weakened by photochemical reactions.

Promotive effects of vacuum-UV/UV (185/254 nm) light on elimination of recalcitrant trace organic contaminants by UV-AOPs during wastewater treatment and reclamation: A review

The use of vacuum-UV/UV (185/254 nm) for trace organic contaminants (TOrCs) elimination during wastewater treatments has attracted much attention. Advanced oxidation processes which combine VUV/UV and additional oxidants (vacuum-UV/UV-based advanced oxidation processes, VUV/UV-AOPs) provide a promising method for eliminating recalcitrant and toxic TOrCs for wastewater reclamation. Researches in this area are increasing but the promoting effects, mechanisms, and influencing factors have not been well summarized. A comprehensive discussion of the limitations of this technique and future research directions is needed. VUV/UV-AOPs have considerable synergistic effects by increasing usage of VUV/UV photons and the oxidant, which increases radical generation. In terms of elimination kinetics, VUV/UV-AOPs outperform conventional UV-AOPs and VUV/UV processes in most cases; a 1.2-87.7-fold increase of the fluence-based kinetic constant is achieved. In terms of energy efficiency per order (EE/O) of TOrCs elimination, the EE/O of VUV/UV-AOPs only accounts for 4% of UV-AOPs and 63% of VUV/UV. However, VUV/UV-AOPs still need to be further investigated. Firstly, although VUV and UV processes have similar radical formation pathways, limited information is available on the quantum yields of photolysis and radical formation of oxidants under VUV irradiation. Secondly, optimization of VUV/UV-AOPs operating conditions, especially oxidant dosage and water-flow patterns, is needed. Thirdly, VUV/UV-AOPs are significantly inhibited by organic and inorganic matters, but the mechanisms of inhibition on VUV/UV scattering, radical quenching, and radical conversion are not well understood. Such inhibition suggests that the use of VUV/UV-AOPs would be limited to relatively clear water treatment, e.g., reverse osmosis effluent for potable water reuse and ultrapure water production. Related research is needed to establish a clearer scheme for VUV/UV-AOPs in terms of the spatial distribution of radical species in the VUV/UV irradiation system and the relevant optimization method for promoting oxidation performance.

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.

Dissolved Organic Matter Enhanced the Aggregation and Oxidation of Nanoplastics under Simulated Sunlight Irradiation in Water

Nanoplastics (NPs) have become a new type of pollutant of high concern that is ubiquitous in aqueous environments. However, the transport and transformation of NPs in natural waters are not yet fully understood. In this study, the aggregation and photooxidation of NPs were assessed with nanosized polystyrene (PS) as an example, and the effects of dissolved organic matter (DOM) were investigated with Suwannee River fulvic acid (SRFA) as representative DOM. The results showed that simulated sunlight irradiation exhibited negligible effects on the aggregation of PS, while SRFA enhanced its heteroaggregation through hydrophobic interactions. In SRFA solutions, photooxidation of PS with a particle size of 200 nm was observed, which led to an increase in the O/C ratio on its surface at a rate of (2.20 ± 0.40) × 10^-2 h^-1. This indicates the promotional effect of SRFA on the oxidation of nanosized PS, which is attributed to the generation of the excited triplet state (³SRFA*), hydroxyl radicals (^•OH), and singlet oxygen (¹O₂). Among these reactive species, ¹O₂ played a crucial role in the oxidation of PS. The findings in this study are helpful for an in-depth understanding of the environmental behavior of NPs in natural waters.

Photoaged polystyrene microplastics serve as photosensitizers that enhance cimetidine photolysis in an aqueous environment

Microplastics (MPs) have received much attention in recent years because of their continuous photoaging process in aquatic environments. However, little research has been conducted on the photochemistry of aged microplastics and the associated effects on coexisting pharmaceuticals. This study investigated the photodegradation of cimetidine via aged polystyrene microplastics (PS-MPs) with different aging times (0-7 d) under simulated sunlight irradiation (700 W/m²). PS-MPs with 5 d of aging time resulted in much faster cimetidine degradation (>99%) after 2 h of irradiation than pristine PS-MPs (<8%). The enhanced photodegradation of cimetidine by aged PS-MPs was related to the increase in chromophoric oxygenated groups (CO, C-O) followed by redshifted absorbance through the photoaging process, which induced the formation of the environmentally persistent free radicals (EPFRs) OH, ¹O₂ and ³PS*. However, only ¹O₂ and ³PS* contributed to enhanced cimetidine photodegradation, with ¹O₂ playing a more important role in our case. This work also demonstrated that other compounds that are susceptible to indirect photolysis, such as codeine and morphine, are likewise significantly degraded under irradiation in the presence of aged PS-MPs. Although previous studies have reported how MPs can increase the persistence of contaminants, this study demonstrates that MPs can serve as photosensitizers and alter the fate of coexisting pharmaceuticals in aquatic environments.

Assessing the source of the photochemical formation of hydroxylating species from dissolved organic matter using model sensitizers

Dissolved organic matter (DOM) is ubiquitous in natural waters and can facilitate the chemical transformation of many contaminants through the photochemical production of reactive intermediates, such as singlet oxygen (¹O₂), excited triplet state DOM (³DOM*), and hydroxylating species (˙OH and other intermediates of similar reaction chemistry). The formation mechanism of most reactive intermediates is well understood, but this is not the case for the formation of hydroxylating species from DOM. To investigate this chemistry, DOM model sensitizers were irradiated with two different probe compounds (benzene and benzoic acid) at two irradiation wavelengths (254 and 320 nm). The ability of DOM model sensitizers to hydroxylate these arene probes was assessed by measuring rates of formation of the hydroxylated probe compounds (phenol and salicylic acid). Multiple classes of model sensitizers were tested, including quinones, hydroxybenzoic acids, aromatic ketones, and other triplet forming species. Of these classes of model sensitizers, only quinones and hydroxybenzoic acids had a hydroxylating capacity. Methanol quenching experiments were used to assess the reactivity of hydroxylating species. These results have several implications for the systems tested. First, they suggest that the hydroxylating intermediate produced from hydroxybenzoic acid photolysis may not be hydroxyl radical, but a different hydroxylating species. Also, these data prompted investigation of whether quinone photoproducts have a hydroxylating capacity. These results confirm that hydroxybenzoic acids and quinones are important to the photochemical production of hydroxylating species from DOM, but the mechanism by which this occurs for these classes of sensitizers is still elusive.

A model to predict the kinetics of direct (endogenous) virus inactivation by sunlight at different latitudes and seasons, based on the equivalent monochromatic wavelength approach

Sunlight plays an important role in the inactivation of pathogenic microorganisms such as bacteria and viruses in water. Here we present a model that is able to predict the kinetics of direct virus inactivation (i.e. inactivation triggered by sunlight absorption by the virion, without the role played by photochemically produced reactive intermediates generated by water-dissolved photosensitizers) on a global scale (from 60 °S to 60 °N latitude) and for the different months of the year. The model is based on the equivalent monochromatic wavelength (EMW) approach that was introduced recently, and which largely simplifies complex polychromatic calculations by approximating them with a monochromatic equation at the proper wavelength, the EMW. The EMW equation was initially established for mid-July conditions at a mid-latitude, and was then extended to different seasons and to the latitude belt where the day-night cycle is always observed throughout the year. By so doing, the first-order rate constant of direct virus photoinactivation can be predicted on a global scale, with the use of a relatively simple equation plus tables of pre-calculated input data, as a function of latitude, month, and key water parameters. The model was here applied to the virus organism phiX174, a somatic phage that is often used as proxy for pathogenic viruses undergoing fast direct inactivation, and for which a wide array of published inactivation data is available. Model predictions are validated by comparison with field data of inactivation of somatic phages by sunlight.

Humic Acid and Fulvic Acid Hinder Long-Term Weathering of Microplastics in Lake Water

We investigated the photoaging of polypropylene (PP) microplastics (MPs) in lake water. The results showed that photoaging of PP MPs was significantly inhibited in lake water compared with ultrapure water after 12 d of ultraviolet (UV) irradiation, and humic acid and fulvic acid, rather than carbonate (CO₃^2-), nitrate (NO₃^-), or chloride (Cl^-) ions, were identified as the primary contributors to the observed inhibition. Mechanisms for the roles of humic acid (Suwannee River humic acid) and fulvic acid (Pony Lake fulvic acid) in reducing the rates of photodegradation showed that humic acid and fulvic acid acted as both reactive oxygen species (ROS) scavengers (e.g., of ^•OH) (dominant contribution) and optical light filters. As ROS scavengers, humic acid and fulvic acid significantly decreased the capacity for the formation of ^•OH and O₂^•- by PP MPs under irradiation. In addition, the chromophores in humic acid and fulvic acid competed for photons with MPs through the light-shielding effect, thereby causing less fragmentation of PP particles and changes in other properties (melting temperature, contact angle, and surface zeta potential). The proposed mechanisms for inhibition by humic acid and fulvic acid will aid our efforts to assess the duration of aging and alterations of MP properties during long-term weathering in natural waters.

Inhibitory effects of antioxidant moieties in humic substances on phototransformation of chlortetracycline mediated by the algae extracellular organic matter

In algae rich waters, sunlight-driven transformation of antibiotics could be accelerated via sensitization by algae extracellular organic matter (EOM), and this photosensitization process will be affected by coexisting humic substances. In this study, we explored the effect and mechanism of humic substances on photodegradation of chlortetracycline (CTC) mediated by EOM. We found that humic substances exhibited a marked inhibitory effect on the EOM-mediated photodegradation of CTC. Given that humic substances exhibited little effects on the EOM-mediated formation of triplet state species, the quenching effect of humic substances on reactive species was excluded. The inhibitory effect of humic substances was mainly attributed to the back reduction of CTC oxidation intermediates by the antioxidant moieties in humic substances. The ozone oxidation treatment for humic substances was applied to destroy antioxidant moieties. After ozonation, the inhibitory effects of humic substances were greatly decreased, confirming the dominant role of antioxidant moieties in humic substances, which inhibited CTC photodegradation mediated by EOM via reducing oxidation intermediates of CTC. This back reduction was further verified to be exergonic via reactive Gibbs free energy, indicating the back reduction by humic substances of CTC oxidation intermediates could occur spontaneously. The present study will be helpful for predicting the fate and risk of CTC in algae rich water environments, and is of great significance for the study of phototransformation of other antibiotics.

Direct and indirect photodegradation of atrazine and S-metolachlor in agriculturally impacted surface water and associated C and N isotope fractionation

Knowledge of direct and indirect photodegradation of pesticides and associated isotope fractionation can help to assess pesticide degradation in surface waters. Here, we investigated carbon (C) and nitrogen (N) isotope fractionation during direct and indirect photodegradation of the herbicides atrazine and S-metolachlor in synthetic agriculturally impacted surface waters containing nitrates (20 mg L^-1) and dissolved organic matter (DOM, 5.4 mg_C L^-1). Atrazine and S-metolachlor were quickly photodegraded by both direct and indirect processes (half-lives <5 and <7 days, respectively). DOM slowed down photodegradation while nitrates increased degradation rates. The analysis of transformation products showed that oxidation mediated by hydroxyl radicals (HO˙) predominated during indirect photodegradation. UV light (254 nm) led to significant C and N isotope fractionation, yielding isotopic fractionation values ε_C = 2.7 ± 0.3 and 0.8 ± 0.1‰, and ε_N = 2.4 ± 0.3 and -2.6 ± 0.7‰ for atrazine and S-metolachlor, respectively. In contrast, photodegradation under simulated sunlight led to negligible C and slight N isotope fractionation, emphasizing the effect of the radiation wavelengths on the isotope fractionation induced by direct photodegradation. Altogether, these results highlight the importance of using simulated sunlight to obtain environmentally-relevant isotopic fractionation values and to distinguish photodegradation and other dissipation pathways in surface waters.

Photochemical Behavior of Microbial Extracellular Polymeric Substances in the Aquatic Environment

Microbially derived extracellular polymeric substances (EPSs) occupy a large portion of dissolved organic matter (DOM) in surface waters, but the understanding of the photochemical behaviors of EPS is still very limited. In this study, the photochemical characteristics of EPS from different microbial sources (Shewanella oneidensis, Escherichia coli, and sewage sludge flocs) were investigated in terms of the production of reactive species (RS), such as triplet intermediates (³EPS*), hydroxyl radicals (^•OH), and singlet oxygen (¹O₂). The steady-state concentrations of ^•OH, ³EPS*, and ¹O₂ varied in the ranges of 2.55-8.73 × 10^-17, 3.01-4.56 × 10^-15, and 2.08-2.66 × 10^-13 M, respectively, which were within the range reported for DOM from other sources. The steady-state concentrations of RS varied among different EPS isolates due to the diversity of their composition. A strong photochemical degradation of the protein-like components in EPS isolates was identified by excitation emission matrix fluorescence with parallel factor analysis, but relatively, humic-like components remained stable. Fourier-transform ion cyclotron resonance mass spectrometry further revealed that the aliphatic portion of EPS was resistant to irradiation, while other portions with lower H/C ratios and higher O/C ratios were more susceptible to photolysis, leading to the phototransformation of EPS to higher saturation and lower aromaticity. With the phototransformation of EPS, the RS derived from EPS could effectively promote the degradation of antibiotic tetracycline. The findings of this study provide new insights into the photoinduced self-evolution of EPS and the interrelated photochemical fate of contaminants in the aquatic environment.

Photochemical Aging of Atmospheric Particulate Matter in the Aqueous Phase

This study focused on the photoaging of atmospheric particulate matter smaller than 2.5 μm (PM_2.5) in the aqueous phase. PM_2.5 was collected during a winter, a spring, and a summer campaign in urban and rural settings in Colorado and extracted into water. The aqueous extracts were photoirradiated using simulated sunlight, and the production rate (r_•OH) and the effects of hydroxyl radicals (^•OH) were measured as well as the optical properties as a function of the photoaging of the extracts. r_•OH was seen to have a strong seasonality with low mean values for the winter and spring extracts (4.8 and 14 fM s^-1 mg_C^-1 L, respectively) and a higher mean value for the summer extracts (65.4 fM s^-1 mg_C^-1 L). For the winter extracts, ^•OH was seen to mostly originate from nitrate photolysis while for the summer extracts, a correlation was seen between r_•OH and iron concentration. The extent of photobleaching of the extracts was correlated with r_•OH, and the correlation also indicated that non-^•OH processes took place. Using the ^•OH measurements and singlet oxygen (¹O₂) measurements, the half-life of a selection of compounds was modeled in the atmospheric aqueous phase to be between 1.9 and 434 h.

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

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