Photochemical Oxidation of Water by 2-Methyl-1,4-benzoquinone: Evidence against the Formation of Free Hydroxyl Radical

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

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

Role of Direct and Sensitized Photolysis in the Photomineralization of Dissolved Organic Matter and Model Chromophores to Carbon Dioxide

This study addresses the fundamental processes that drive the photomineralization of dissolved organic matter (DOM) to carbon dioxide (CO2), deconvoluting the role of direct and sensitized photolysis. Here, a suite of DOM isolates and model compounds were exposed to simulated sunlight in the presence of various physical and chemical quenchers to assess the magnitude, rate, and extent of direct and sensitized photomineralization to CO2. Results suggest that CO2 formation occurs in a biphasic kinetic system, with fast production occurring within the first 3 h, followed by slower production thereafter. Notably, phenol model chromophores were the highest CO2 formers and, when conjugated with carboxylic functional groups, exhibited a high efficiency for CO2 formation relative to absorbed light. Simple polycarboxylated aromatic compounds included in this study were shown to be resistant to photomineralization. Quencher results suggest that direct photolysis and excited triplet state sensitization may be largely responsible for CO2 photoproduction in DOM, while singlet oxygen and hydroxyl radical sensitization may play a limited role. After 3 h of irradiation, the CO2 formation rate significantly decreased, and the role of sensitized reactions in CO2 formation increased. Together, the results from this study advance the understanding of the fundamental reactions driving DOM photomineralization to CO2, which is an important part of the global carbon cycle.

Dissolved organic matter isolates obtained by solid phase extraction exhibit higher absorption and lower photo-reactivity: Effect of components

Notable differences in photo-physical and chemical properties were found between bulk water and solid phase extraction (SPE) isolates for dissolved organic matter (DOM). The moieties extracted using modified styrene divinylbenzene cartridges, which predominantly consist of conjugated aromatic molecules like humic acids, contribute mainly to light absorption but exhibit lower quantum yields of fluorescence and photo-produced reactive intermediates (PPRIs). Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed lignin as the moieties displaying most significant variance in abundance. In Van Krevelen-Spearman plot, we observed molecules positively or negatively correlated with DOM's optical and photochemical properties (including SUVA254, steady-state concentrations of ·OH, 1O2 quantum yield, etc.) were confined to specific regions, which can be delineated using a threshold modified aromaticity index (AImod) of 0.3. Based on the relationships between optical properties and PPRI production, it is suggested that the energy gap between ground state and excited singlet state (△ES1→S0), governing the inner conversion rate, serves as a determinant for apparent quantum yield of PPRIs in DOM, with intra-molecular charge transfer (CT) interactions potentially playing a pivotal role. Regarding DOM's photoreactivity with pollutants, this study has revealed, for the first time, that protein/amino sugars/amino acids could act as antioxidant groups in addition to phenols on the photolysis of sulfadiazine. These findings provide valuable insights into DOM photochemistry and are expected to stimulate further research in this area.

UV-induced photodegradation of emerging para-phenylenediamine quinones in aqueous environment: Kinetics, products identification and toxicity assessments

Substituted para-phenylenediamine quinones (PPD-quinones) are a class of emerging contaminants frequently detected in the aqueous environment. One of them, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q), was found to cause acute toxicities to aquatic species at extremely low environmental levels. The ubiquitousness and ecotoxicity of such pollutants underscore the importance of their transformation and elimination. In this work, we demonstrated effective removals of five PPD-quinones in aqueous environments under UV irradiation, with up to 94% of 6PPD-Q eliminated after a 40-min treatment. By applying high-resolution mass spectrometry (HRMS) non-targeted screening in combination with isotope labeling strategies, a total of 22 transformation products (TPs) were identified. Coupling with the time-based dynamic patterns, potential transformation mechanisms were identified as an •OH-induced photocatalysis reaction involving bond cleavage, hydroxylation, and oxidation. Computational toxicity assessment predicted lower aquatic toxicity of the TPs than their parent PPD-quinones. Our results in parallel evidenced an obvious reduction of PPD-quinones accompanied by the presence of their TPs in the effluent after UV disinfection in real municipal wastewater. This work builds a comprehensive understanding of the fate, transformation products, and related toxicological characteristics of emerging PPD-quinone contaminants in the aqueous environment.

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.

Hydroxyl radicals induced mineralization of organic carbon during oxygenation of ferrous mineral-organic matter associations: Adsorption versus coprecipitation

The iron (Fe) phases have been widely proposed to preserve organic carbon (OC) via adsorption or coprecipitation pathways, however, such role of Fe phases might be largely reversed under redox-fluctuation conditions, especially for Fe(II) minerals-protected OC. In this study, we synthesized the Fe(II)-OC associations via adsorption and coprecipitation using FeCO₃ and three types of low-molecular-weight organic compounds (LMWOCs) at different C/Fe molar ratios, and investigated the OC mineralization induced by hydroxyl radicals (OH) during oxygenation processes. Abundant OH can be produced upon oxygenation of FeCO₃-LMWOCs associations within 96 h, giving values of 28.49-151.36 μM in adsorption and 12.63-76.41 μM in coprecipitation treatments depended on types of LMWOCs and C/Fe molar ratios. Fe(II) species in coprecipitates with hydroquinone (HQ) mainly transformed into Goethite-like phases after oxygenation, while adsorption samples induced more formation of lower-crystalline Fe phase (e.g., ferrihydrite). The surface-Fe(II) was the primary electron donors to O₂, which further induced hydrogen peroxide (H₂O₂) formation via one- and two-electron transfer pathways. Finally, the produced OH removed 0.55-9.65 and 0.16-85.54 mg L^-1 total OC in adsorption and coprecipitation treatments after oxygenation. Collectively, this study highlights that OC associated with Fe(II) minerals might be labile due to the oxidation of formed OH, and the role of Fe phases in OC sequestration may be further re-evaluated under redox fluctuation conditions.

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

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

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.

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

The Adirondack Mountain region of New York, a historical hotspot for atmospheric sulfur and nitrogen deposition, features abundant lakes that are experiencing browning associated with recovery from acidification. Yet, much remains unknown about the photoreactivity of Adirondack lake waters. We quantified the apparent quantum yields (Φ_app,RI) of photochemically produced reactive intermediates (RIs), such as excited triplet states of dissolved organic matter (³DOM*), singlet oxygen (¹O₂), and hydroxyl radicals (^•OH), for surface waters collected from 16 representative Adirondack lakes. Φ_app,³DOM* and Φ_app,¹O2 for native Adirondack lake waters fell within ranges reported for whole waters and DOM isolates from various sources, while Φ_app,^•OH were substantially lower than those measured for other aquatic samples. Orthogonal partial least squares and multiple linear regression analyses identified the spectral slope coefficient from 290 to 400 nm (S_290-400) as the most effective predictor of Φ_app,RI among measured water chemistry parameters and bulk DOM properties. Φ_app,RI also exhibited divergent responses to controlled pH adjustment and aluminum or iron addition simulating hypothetical scenarios relevant to past and future water chemistry conditions of Adirondack lakes. This study highlights the need for continued research on changes in photoreactivity of acid-impacted aquatic ecosystems in response to browning and subsequent impacts on photochemical processes.

Assessing the human risk and the environmental fate of pharmaceutical Tramadol

Tramadol (TRA) is a widely used human pharmaceutical and a well-established emerging pollutant and its potential genotoxic and cytotoxic effects on humans as well as its fate in aqueous systems demand full investigation. The present study is a multidisciplinary approach and provides important insights on the potential risks of Tramadol on humans accompanied by its photolytic transformation under simulated solar irradiation. The present study revealed that Tramadol can induce genotoxic and cytotoxic effects under the specific experimental conditions, significantly depended on the tested concentration. In addition, the photolytic transformation of Tramadol was investigated in detail under simulated solar irradiation in two different water matrices: ultrapure water (UW) and treated wastewater (WW). Differences in the degradation rates were observed between UW and WW, being slower in WW. The results showed that more than 70% of Tramadol was removed after 240 min in UW ([TRA] = 10 mg L^-1, I = 500 W m^-2). After this period, TOC removal was found to be about 40%. Transformation of N atoms into NO₃^- and NH₄⁺ followed a similar trend reaching up to 38% release. Τramadol degraded mainly by HO radicals and ¹O₂ through a self-sensitizing process while direct photolysis was also significant. Hydroxylation, demethylation and N-oxidation of the parent compound were found to be the main degradation pathways confirming the important role of HO and ¹O₂ in the photolytic process. Toxicity measurements showed a noticeable increase of the inhibition for Vibrio fischeri at the first stages which coincide with the formation of the major TPs.

Assessing the contribution of hydroxylation species in the photochemical transformation of primidone (pharmaceutical)

Pharmaceutical and personal care products (PPCPs) are a group of emerging contaminants that have frequently been detected in aqueous environments. Phototransformation driven by solar irradiation is one of the most important natural processes for the elimination of PPCPs. In this study, primidone (PMD) was chosen as a model "photorefractory" compound. A series of experiments were conducted to assess if reactive intermediates (RIs), such as hydroxyl radical (HO), singlet oxygen (¹O₂), and triplet states of dissolved organic matter (³DOM^⁎), inhibited or enhanced the photochemical transformation of PMD under simulated solar irradiation. The results indicate that HO plays a key role in the photodegradation of PMD and that dissolved oxygen can affect the degradation rate of PMD by promoting HO formation. Our results demonstrated that PMD can not only react with free HO (HO_-free) but also react with lower-energy hydroxylation agents (HO_-like). The contributions of HO_-free and HO_-like to PMD degradation in various dissolved organic matter (DOM) solutions were estimated by a methane-quenching experiment. The results indicated that the HO_-like species were important in the photodegradation of "photorefractory" compounds. The bimolecular reaction rate constant of the reaction of free HO with PMD was measured as (5.21 ± 0.02) × 10⁹ M^-1 s^-1 by using electron pulse radiolysis. Furthermore, PMD was used as a probe to estimate the steady-state concentration of HO_-free in various DOM solutions. Using the multivariate statistical strategies of orthogonal projection to latent structures discriminant analysis (OPLS-DA) and hierarchical clustering, 28 photochemical transformation products (TPs) of PMD were successfully identified from the DOM matrix.

Trimethyl Lock: A Multifunctional Molecular Tool for Drug Delivery, Cellular Imaging, and Stimuli-Responsive Materials

Trimethyl lock (TML) systems are based on ortho-hydroxydihydrocinnamic acid derivatives displaying increased lactonization reactivity owing to unfavorable steric interactions of three pendant methyl groups, and this leads to the formation of hydrocoumarins. Protection of the phenolic hydroxy function or masking of the reactivity as benzoquinone derivatives prevents lactonization and provides a trigger for controlled release of molecules attached to the carboxylic acid function through amides, esters, or thioesters. Their easy synthesis and possible chemical adaption to several different triggers make TML a highly versatile module for the development of drug-delivery systems, prodrug approaches, cell-imaging tools, molecular tools for supramolecular chemistry, as well as smart stimuliresponsive materials.

Photoreactivity of graphene oxide in aqueous system: Reactive oxygen species formation and bisphenol A degradation

The phototransformation and environmental implications of graphene oxide (GO) have been widely studied in order to understand its implications upon release into the environment. However, very little is known about the formation of reactive oxygen species (ROS) by GO under solar irradiation. Currently there are no studies on the mechanism of ROS formation by GO or the amount of ROS catalyzed by the nanomaterials in the environment. In this study, we carefully investigated the different types and formation mechanisms of ROS generated by GO in the presence of simulated solar irradiation. The effect of GO's photoactivity on bisphenol A (BPA), a representative organic co-pollutant, was also studied. The conduction band electron (e_aq^-) of GO led to the formation of different ROS including OH, O₂^-, and ¹O₂. Among the three types of ROS investigated, O₂^- was the most abundant species generated during simulated solar irradiation of GO. BPA was degraded, mainly due to the oxidative potential of the valence band holes produced during solar irradiation of GO. This study advances understanding of the photoactivity of GO and its potential impact on other possible environmental co-pollutants.

Predicting Reactive Intermediate Quantum Yields from Dissolved Organic Matter Photolysis Using Optical Properties and Antioxidant Capacity

The antioxidant capacity and formation of photochemically produced reactive intermediates (RI) was studied for water samples collected from the Florida Everglades with different spatial (marsh versus estuarine) and temporal (wet versus dry season) characteristics. Measured RI included triplet excited states of dissolved organic matter (³DOM*), singlet oxygen (¹O₂), and the hydroxyl radical (^•OH). Single and multiple linear regression modeling were performed using a broad range of extrinsic (to predict RI formation rates, R_RI) and intrinsic (to predict RI quantum yields, Φ_RI) parameters. Multiple linear regression models consistently led to better predictions of R_RI and Φ_RI for our data set but poor prediction of Φ_RI for a previously published data set,1 probably because the predictors are intercorrelated (Pearson's r > 0.5). Single linear regression models were built with data compiled from previously published studies (n ≈ 120) in which E2:E3, S, and Φ_RI values were measured, which revealed a high degree of similarity between RI-optical property relationships across DOM samples of diverse sources. This study reveals that ^•OH formation is, in general, decoupled from ³DOM* and ¹O₂ formation, providing supporting evidence that ³DOM* is not a ^•OH precursor. Finally, Φ_RI for ¹O₂ and ³DOM* correlated negatively with antioxidant activity (a surrogate for electron donating capacity) for the collected samples, which is consistent with intramolecular oxidation of DOM moieties by ³DOM*.

Mathematical description of pH-stat kinetic traces measured during photochemical quinone decomposition

Substituted 1,4-benzoquinone (QR) derivatives are photosensitive in aqueous solution and form hydroquinones (QR-H₂) and hydroxy-quinones (QR-OH), two weak acids in their photoreaction. For this reason, the kinetics of the photoreaction can be conveniently followed by the pH-stat titration technique. The mathematical description of the kinetic traces measured provides the two main parameters of the photoreaction: the differential quantum yield of the reaction (Φ) and the ratio of the two photo-products, i.e. the fraction of QR that is converted to QR-OH (α). These values are described in this paper for 2,5-dichloro-1,4-benzoquinone at different pH values, together with the detailed mathematical evaluation of the application limits of the pH-stat method for such reactions.

Photosensitized degradation of acetaminophen in natural organic matter solutions: The role of triplet states and oxygen

The photolysis of acetaminophen, a widely used pharmaceutical, in simulated natural organic matter solutions was investigated. The triplet states of natural organic matter (³NOM*) were found to play the dominant role in its photodegradation, while the contributions from hydroxyl radicals and singlet oxygen were negligible. Dissolved oxygen (DO) plays a dual role. From anaerobic to microaerobic (0.5 mg/L DO) conditions, the degradation rate of acetaminophen increased by 4-fold. That suggests the involvement of DO in reactions with the degradation intermediates. With increasing oxygen levels to saturated conditions (26 mg/L DO), the degradation rate became slower, mainly due to DO's quenching effect on ³NOM*. Superoxide radical (O₂^-) did not react with acetaminophen directly, but possibly quenched the intermediates to reverse the degradation process. The main photochemical pathways were shown to involve phenoxyl radical and N-radical cations, finally yielding hydroxylated derivatives, dimers and nitrosophenol. A reaction mechanism involving ³NOM*, oxygen and O₂^- is proposed.

Synthesis, characterization and photocatalytic activity of mesoporous Na-doped TiO2 nano-powder prepared via a solvent-controlled non-aqueous sol–gel route

The superior photo-catalytic activity of mesoporous Na doped TiO₂ attributed to the combined effect of electron–hole recombination rate, increased surface area and enhanced crystallinity.

First assessment on degradability of sodium p-perfluorous nonenoxybenzene sulfonate (OBS), a high volume alternative to perfluorooctane sulfonate in fire-fighting foams and oil production agents in China

First report on biotic and abiotic degradability assessment of the fluorinated surfactants OBS.

Probe Compounds to Assess the Photochemical Activity of Dissolved Organic Matter

The photochemical properties of dissolved organic matter (DOM) have been of interest to scientists and engineers since the 1970s. Upon light absorption, chromophoric DOM (CDOM) can sensitize the formation of different short-lived reactive intermediates (RIs), including hydroxyl radical (^•OH), singlet oxygen (¹O₂) and superoxide radical anion (O₂^•-). In addition, a fraction of the excited singlet states in CDOM decays into excited triplet states (³CDOM*), which are also important photochemical transients in environmental systems. These RIs have a significant impact on different processes in sunlit waters, including degradation of organic contaminants and the inactivation of pathogens. Due to their transient nature and low steady-state concentrations, the use of common analytical techniques for the direct measurement of these species is impractical. Therefore, specific probe compounds (PCs) are used. PCs include furfuryl alcohol for ¹O₂, and terephthalic acid for ^•OH. In this publication, we present a critical review of the use of PCs for the assessment of the formation of photochemically generated RIs. We first introduce the concept of a PC, including the kinetic treatment and necessary assumptions needed to conduct a specific measurement. Afterward, we present short overviews of the most studied RIs and review relevant issues regarding the use of specific PCs for their measurement. We finalize by offering recommendations regarding the use of PCs in environmental photochemistry.

Photomineralization of aqueous salicylic acids. Photoproducts characterization and formation of light induced secondary OH precursors (LIS-OH)

The photolysis of aqueous 5-chlorosalicylic acid (ClSA) and dihydroxybenzoic acid (DHBA), its main photoproduct, was studied to determine the extent of degradation caused by simulated solar light. Photoproducts identification was achieved using high resolution LC-MS and GC-MS. About 40 photoproducts from C₁₉ to C₁ were characterized, including a dihydroxycyclopentadienic acid, a ring contraction photoproduct, and numerous carbonyls and carboxylic acid derivatives that were detected thanks to derivatization. UV-visible spectral monitoring of the reactions revealed that ClSA and DHBA underwent photobleaching after developing a temporarily featureless absorbance between 300 and 500 nm. Measurement of OH radicals using terephtalic acid as a probe showed that OH radicals were generated with an average rate of 7 × 10^-9 M s^-1 and a total cumulated concentration of 10^-3 M, corresponding to ∼ 5-fold the initial concentration of DHBA. Furthermore, TOC analysis indicated that significant mineralization (51-90%) occurred. These findings are consistent with the formation of light induced secondary OH (LIS-OH) precursors such as featureless long wavelength absorbing compounds as well as non-absorbing hydroperoxides. The formation of LIS-OH illustrated here may also take place in the aqueous photodegradation of other substituted phenols likely present in dissolved organic matter and humic substructures, it deserves to be studied in more details.

Triplet state dissolved organic matter in aquatic photochemistry: reaction mechanisms, substrate scope, and photophysical properties

Excited triplet states of chromophoric dissolved organic matter (³CDOM*) play a major role among the reactive intermediates produced upon absorption of sunlight by surface waters. After more than two decades of research on the aquatic photochemistry of ³CDOM*, the need for improving the knowledge about the photophysical and photochemical properties of these elusive reactive species remains considerable. This critical review examines the efforts to date to characterize ³CDOM*. Information on ³CDOM* relies mainly on the use of probe compounds because of the difficulties associated with directly observing ³CDOM* using transient spectroscopic methods. Singlet molecular oxygen (¹O₂), which is a product of the reaction between ³CDOM* and dissolved oxygen, is probably the simplest indicator that can be used to estimate steady-state concentrations of ³CDOM*. There are two major modes of reaction of ³CDOM* with substrates, namely triplet energy transfer or oxidation (via electron transfer, proton-coupled electron transfer or related mechanisms). Organic molecules, including several environmental contaminants, that are susceptible to degradation by these two different reaction modes are reviewed. It is proposed that through the use of appropriate sets of probe compounds and model photosensitizers an improved estimation of the distribution of triplet energies and one-electron reduction potentials of ³CDOM* can be achieved.

Effects of C60 on the Photochemical Formation of Reactive Oxygen Species from Natural Organic Matter

Buckminsterfullerenes (C₆₀) are widely used nanomaterials that are present in surface water. The combination of C₆₀ and humic acid (HA) generates reactive oxygen species (ROS) under solar irradiation, but this process is not well understood. Thus, the present study focused on the photochemical formation of singlet oxygen (¹O₂), hydroxyl radical (HO^•)-like species, superoxide radicals (O₂^•-), hydrogen peroxide (H₂O₂), and triplet excited states (³C₆₀*/³HA*) in solutions containing both C₆₀ and HA. The quantum yield coefficients of excited triplet states (f_TMP) and apparent quantum yields of ROS were measured and compared to the calculated values, which were based on the conservative mixing model. Although C₆₀ proved to have only a slight impact on the ¹O₂ formation from HA, C₆₀ played a key role in the inhibition of O₂^•-. The photochemical formation of H₂O₂ followed the conservative mixing model due to the reaction of C₆₀^•- with HO₂^•/O₂^•-, and the biomolecular reaction rate constant has been measured as (7.4 ± 0.6) × 10⁶ M^-1 s^-1. The apparent f_TMP was significantly lower than the calculated value, indicating that the steric effect of HA was significant in the reaction of ³C₆₀* with the TMP probe. In contrast, C₆₀ did not have an effect on the photochemical formation of HO^• from HA, suggesting that HO^• is elevated from the hydrophilic surface of HA. The aforementioned results may be useful for predicting the photochemical influence of C₆₀ on aqueous environments.

Effects of pH and dissolved oxygen on the photodegradation of 17α-ethynylestradiol in dissolved humic acid solution

To probe the mechanisms responsible for pH and dissolved oxygen (DO) affecting the photodegradation of 17α-ethynylestradiol (EE2) in dissolved humic acid (HA) solution, EE2 aqueous solutions with pH values ranging from 3.0 to 11.0 and different DO conditions were irradiated by using a 300 W mercury lamp equipped with 290 nm light cutoff filters. In 5.0 mg L(-1) HA solutions (pH 8.0), EE2 was degraded at a rate of 0.0739 h(-1) which was about 4-fold faster than that in Milli-Q water. The degradation of EE2 was mainly caused by the oxidation of photogenerated reactive species (RS), and the contribution of direct photodegradation to EE2 degradation was always lower than 27%. Both the direct and indirect photodegradation of EE2 were closely dependent on the EE2 initial concentration, pH value and DO concentration. The photodegradation rate of EE2 decreased with increased initial concentration of EE2 due to the limitation of photon flux. With pH and DO increasing, the degradation rate of EE2 increased significantly due to the increase in the yields of excited EE2 and RS. Among the photogenerated RS, HO˙ and (3)HA* were determined to be the key contributors, and their global contribution to EE2 photodegradation was about 50%. Although HA could generate more (1)O2 than HO˙, the contribution of (1)O2 to EE2 degradation was lower than 13% due to its low reactivity towards EE2. This study could enlarge our knowledge on the photochemical behaviors of steroid estrogens in natural sunlit waters.

Physicochemical design and analysis of self-propelled objects that are characteristically sensitive to environments

The development of self-propelled motors that mimic biological motors is an important challenge for the transport of either themselves or some material in a small space, since biological systems exhibit high autonomy and various types of responses, such as taxis and swarming. In this perspective, we review non-living systems that behave like living matter. We especially focus on nonlinearity to enhance autonomy and the response of the system, since characteristic nonlinear phenomena, such as oscillation, synchronization, pattern formation, bifurcation, and hysteresis, are coupled to self-motion of which driving force is the difference in the interfacial tension. Mathematical modelling based on reaction-diffusion equations and equations of motion as well as physicochemical analysis from the point of view of the molecular structure are also important for the design of non-living motors that mimic living motors.

Temperature dependence of the photochemical formation of hydroxyl radical from dissolved organic matter

The temperature dependence of the photochemical production of the hydroxyl radical (•OH) from dissolved organic matter (DOM) was investigated by measuring the apparent temperature dependence of the quantum yield (Φa) for this process. Temperature dependent Φa values were analyzed using the Arrhenius equation. Apparent activation energies obtained for DOM isolates purchased from the International Humic Substances Society ranged from 16 to 34 kJ mol(-1). Addition of 40 units mL(-1) catalase, used to hinder the hydrogen peroxide (H2O2)-dependent pathway to •OH, did not impact the observed activation energy. However, an increase in activation energy was observed in lower molecular weight DOM obtained by size fractionation. We also measured the temperature dependence of p-benzoquionone photolysis as a model compound for DOM and observed no temperature dependence (slope p = 0.41) for the formation of phenol from oxidation of benzene (the •OH probe used), but a value of about 10 kJ mol(-1) for p-benzoquinone loss, which is consistent with formation of a quinone-water exciplex. These data provide insight into DOM photochemistry as well as provide parameters useful for modeling steady state •OH concentrations in natural systems.

Photoinduced water splitting via benzoquinone and semiquinone sensitisation

The splitting of water into H˙ and OH˙ radicals by sensitisation of a redox-active chromophore with sunlight may eventually become a viable way of producing unlimited, clean and sustainable energy.

Estimating hydroxyl radical photochemical formation rates in natural waters during long-term laboratory irradiation experiments

In this study it was observed that, during long-term irradiations (>1 day) of natural waters, the methods for measuring hydroxyl radical (˙OH) formation rates based upon sequentially determined cumulative concentrations of photoproducts from probes significantly underestimate actual ˙OH formation rates. Performing a correction using the photodegradation rates of the probe products improves the ˙OH estimation for short term irradiations (<1 day), but not long term irradiations. Only the 'instantaneous' formation rates, which were obtained by adding probes to aliquots at each time point and irradiating these sub-samples for a short time (≤2 h), were found appropriate for accurately estimating ˙OH photochemical formation rates during long-term laboratory irradiation experiments. Our results also showed that in iron- and dissolved organic matter (DOM)-rich water samples, ˙OH appears to be mainly produced from the Fenton reaction initially, but subsequently from other sources possibly from DOM photoreactions. Pathways of ˙OH formation in long-term irradiations in relation to H2O2 and iron concentrations are discussed.

Impact of halides on the photoproduction of reactive intermediates from organic matter

The excitation of dissolved organic matter (DOM) from sunlight produces a range of reactive intermediates, including triplet-excited state dissolved organic matter ((3)DOM*), hydroxyl radical (HO(•)), and singlet oxygen ((1)O2). These intermediates are important for the inactivation of pathogens and for the degradation of trace organic contaminants (OC) within natural and engineered systems. However, halides found in the background matrix can alter the photoproduction rates by promoting or quenching the formation of these intermediates. Apparent quantum yields (Φ(a)) for (1)O2, HO(•), and steady state (3)DOM* concentrations photoproduced from DOM isolates were determined with varying concentrations of chloride and bromide. Fluorescence quantum yields were measured as well to probe the photophysics of the system. The maximum fluorescence quantum yield (ΦF) decreased with the addition of halides, representing a quenching of the excited singlet state of DOM. In contrast, the steady state concentrations for (3)DOM* were enhanced, suggesting intersystem crossing from the singlet state to the triplet state was increased by the presence of halides. The Φ(a) for (1)O2 was increased with the addition of halides, which was expected following the (3)DOM* results because the mechanism for (1)O2 production occurs through the inactivation of (3)DOM* by dissolved oxygen. Although HO(•) production would be expected to follow (1)O2, the opposite trend was seen, which suggests the formation of HO(•) does not occur through the same precursor. Understanding the impact of specific quenchers on DOM could be a key to understanding the true formation potential for reactive intermediates and is especially important in estuaries and wastewater impacted aquatic systems.

Iron redox transformations in continuously photolyzed acidic solutions containing natural organic matter: kinetic and mechanistic insights

In this work, the various pathways contributing to the formation and decay of Fe(II) in photolyzed acidic solutions containing Suwannee River fulvic acid (SRFA) are investigated. Results of experimental and computational studies suggest that ligand to metal charge transfer (LMCT), superoxide-mediated iron reduction and interaction with reduced organic species that are present intrinsically in SRFA each contribute to Fe(III) reduction with LMCT the most likely dominant pathway under these conditions. Fe(II) oxidation occurs as a result of its interaction with a variety of light-generated species including (i) short-lived organic species, (ii) relatively stable semiquinone-like organic species, and (iii) hydroperoxy radicals. While not definitive, a hypothesis that the short-lived organic species are similar to peroxyl radicals appears most consistent with our experimental and modeling results. The semiquinone-like organic species formed during photolysis by superoxide-mediated oxidation of reduced organic moieties are long-lived in the dark but prone to rapid oxidation by singlet oxygen ((1)O2) under irradiated conditions and thus play a minor role in Fe(II) oxidation in the light. A kinetic model is developed that adequately describes all aspects of the experimental data obtained and which is capable of predicting Fe(II) oxidation rates and Fe(III) reduction rates in the presence of natural organic matter and light.

UV-induced transformation of four halobenzoquinones in drinking water

Halobenzoquinones (HBQs) are a group of emerging disinfection byproducts (DBPs) found in treated drinking water. Because the use of UV treatment for disinfection is becoming more widespread, it is important to understand how the HBQs may be removed or changed due to UV irradiation. Water samples containing four HBQs, 2,6-dichloro-1,4-benzoquinone (DCBQ), 2,3,6-trichloro-1,4-benzoquinone (TCBQ), 2,6-dichloro-3-methyl-1,4-benzoquinone (DCMBQ), and 2,6-dichloro-1,4-benzoquinone (DBBQ), were treated using a modified bench scale collimated beam device, mimicking UV treatment. Water samples before and after UV irradiation were analyzed for the parent compounds and products using a high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) method. As much as 90% of HBQs (0.25 nmol L(-1)) in both pure water and tap water were transformed to other products after UV254 irradiation at 1000 mJ cm(-2). The major products of the four HBQs were identified as 3-hydroxyl-2,6-dichloro-1,4-benzoquinone (OH-DCBQ) from DCBQ, 5-hydroxyl-2,6-dichloro-3-methyl-1,4-benzoquinone (OH-DCMBQ) from DCMBQ, 5-hydroxyl-2,3,6-trichloro-1,4-benzoquinone (OH-TCBQ) from TCBQ, and 3-hydroxyl-2,6-dibromo-1,4-benzoquinone (OH-DBBQ) from DBBQ. These four OH-HBQs were further modified to monohalogenated benzoquinones when the UV dose was higher than 200 mJ cm(-2). These results suggested possible pathways of UV-induced transformation of HBQs to other compounds. Under the UV dose commonly used in water treatment plants, it is likely that HBQs are partially converted to other halo-DBPs. The occurrence and toxicity of these mixed DBPs warrant further investigation to understand whether they pose a health risk.

Photochemical formation of hydroxyl radical from effluent organic matter

The photochemical formation of hydroxyl radical (HO•) from effluent organic matter (EfOM) was evaluated using three bulk wastewater samples collected at different treatment facilities under simulated sunlight. For the samples studied, the formation rates of HO•(R(HO•)) were obtained from the formation rate of phenol following the hydroxylation of benzene. The values of R(HO•) ranged from 2.3 to 3.8 × 10(-10) M s(-1) for the samples studied. The formation rate of HO• from nitrate photolysis (R(NO3)(HO•)) was determined to be 3.0 × 10(-7) M(HO)• M(NO3)(-1) s(-1). The HO• production rate from EfOM (R(EfOM)(HO•)) ranged from 0.76 to 1.3 × 10(-10) M s(-1). For the wastewater samples studied, R(EfOM)(HO•) varied from 1.5 to 2.4 × 10(-7) M(HO)• M(C)(-1) (s-1) on molarcarbon basis, which was close to HO• production from nitrate photolysis. The apparent quantum yield for the formation of HO• from nitrate (Φ(NO3-HO•)(a)) was determined as 0.010 ± 0.001 for the wavelength range 290-400 nm in ultrapure water. The apparent quantum yield for HO• formation in EfOM (Φ(EfOM-HO•)(a)) ranged from 6.1 to 9.8 × 10(-5), compared to 2.99 to 4.56 × 10(-5) for organic matter (OM) isolates. The results indicate that wastewater effluents could produce significant concentrations of HO•, as shown by potential higher nitrate levels and relatively higher quantum yields of HO• formation from EfOM.

Assessing the contribution of free hydroxyl radical in organic matter-sensitized photohydroxylation reactions

Photochemical formation of reactive oxygen species from dissolved organic matter (DOM) is incompletely understood, especially in the case of hydroxyl radical (•OH) production. Many studies have used various probes to detect photochemically produced •OH from DOM, but the fundamental reactions of these probes are not necessarily specific for free •OH and may also detect lower-energy hydroxylation agents. In this study, two tests were applied that have previously been used as a diagnostic for the presence of free •OH: methane quenching of •OH and hydroxybenzoic acid (hBZA) product yields. Upon application of these two tests to a set of five DOM isolates, it was found that methane quenching and the hBZA ratio results were not necessarily consistent. Overall, the results provide compelling evidence that all isolates studied photochemically produce free •OH. The hydroxylating acitivity of Elliot Soil Humic Acid and Pony Lake Fulvic Acid, however, also had a significant contribution from a photochemically generated hydroxylating agent that is lower in energy than free •OH. Catalase quenching experiments were conducted to assess whether hydrogen peroxide was the immediate precursor to hydroxyl in these systems. In all cases, catalase addition slowed photohydroxylation of terephthalate, but the contribution of hydrogen peroxide photolysis was determined to be less than 50%.

Electron transfer from aromatic amino acids to triplet quinones

The photoreduction of 1,4-benzoquinone, 1,4-naphthoquinone, 9,10-anthraquinone (AQ) and several methylated or halogenated derivatives in argon-saturated acetonitrile-water mixtures by indole, N-acetyltryptophan and N-acetyltyrosine was studied by time-resolved UV-vis spectroscopy using 20 ns UV laser pulses. The quinone triplet state is quenched by the aromatic amino acids and the rate constants are (1-5)x10(9)M(-1)s(-1). The semiquinone radical anion Q.(-) is the major observable transient after electron transfer from amino acids to the quinone triplet state. Termination of Q.(-) and amino acid derived radicals takes place in the mus-ms range. The effects of structure and other specific properties of quinones and amino acids are discussed. The radicals are subjects of intercept with oxygen, whereby hydrogen peroxide is eventually formed. The quantum yield of oxygen uptake Phi(-O2) as a measure of formation of hydrogen peroxide increases with increasing amino acid concentration, approaching Phi(-O2) for AQ in air-saturated solution.