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Hydrogen peroxide plays an important role in photochemical processes in aquatic environments. However, whether it can be transformed by photoexcited chromophoric dissolved organic matter (CDOM) remains unclear. Therefore, this study examined the photosensitized degradation of H 2 O 2 in CDOM-enriched solutions under simulated solar irradiation. Our results suggest that the presence of CDOM enhances the photodegradation rate of H 2 O 2 via the photosensitization process and · OH is generated stoichiometrically with H 2 O 2 attenuation. Experimental results with model photosensitizers indicate that one-electron reducing species of CDOM (CDOM · - ), not triplet CDOM, is the primary reactive species that reduces H 2 O 2 to yield · OH. By monitoring the variation of CDOM · - , the reaction rate constant of CDOM · - with H 2 O 2 was estimated to be 1.5-fold greater than that with O 2 . Furthermore, a wastewater effluent was exposed to simulated solar irradiation with the addition of H 2 O 2 , and the results demonstrated that the photodegradation of trace organic contaminants (TrOCs) was significantly enhanced by the increased · OH level. Overall, the current study provided new insights into the photochemical formation of · OH via the one-electron reduction of H 2 O 2 by CDOM · - . The solar irradiation of wastewater with H 2 O 2 enhancement could be a useful and economically beneficial advanced oxidation process for TrOC abatement.

Hydroxyl radicals ( • OH) are important reactive species that are photochemically generated through solar irradiation of chromophoric dissolved organic matter (CDOM) in surface waters. However, the spatial distribution within the complex three-dimensional structure of CDOM has not been examined. In this study, we used a series of hydrophobic chlorinated paraffins as chemical probes to elucidate the microheterogeneous distribution of • OH in illuminated CDOM solutions. The steady-state concentration of • OH inside the CDOM microphase is 210 ± 31-fold higher than the concentration in the aqueous phase. Our results suggest that the most photochemically generated • OH are confined into the CDOM microphase. Thus, illuminated CDOM behaves as a natural microreactor for • OH-based oxidations. By including intra-CDOM • OH, the quantum yield of • OH for CDOM solutions was estimated to be 2.2 ± 0.5 × 10 -3 , which is 2 orders of magnitude greater than previously thought. The elevated concentrations of photogenerated • OH within the CDOM microphase may improve the understanding of hydrophobic pollutant degradation in aqueous environments. Moreover, our results also suggest that • OH oxidation may play more important roles in the phototransformation of CDOM than previously expected.

Although it is known that the solar irradiation of chromophoric dissolved organic matter (CDOM) solutions generates H 2 O 2 , whether or not organic hydroperoxides (ROOHs) are photochemically formed remains unclear. This study employs high-performance liquid chromatography with the postcolumn enzymatic derivatization method to examine whether ROOHs can be formed in CDOM solutions under simulated solar irradiation. Methylhydroperoxide (MHP) is the only identified ROOH under our experimental conditions, and the quantum yields of MHP (Φ MHP ) vary from (1.09 ± 0.09) × 10 -6 to (4.95 ± 0.11) × 10 -6 in the tested CDOM solutions, including four reference natural organic matters and two effluent organic matters. The quantum yields of H 2 O 2 (Φ H 2 O 2 ) are simultaneously measured, and the ratios of Φ H 2 O 2 to Φ MHP range from 147 to 676. The formation of MHP is highly related to the presence of superoxide radical ions (O 2 •- ) and methyl radicals (CH 3 • ); therefore, a photoformation mechanism of MHP has been proposed. The photochemically generated CH 3 • reacts with O 2 to yield CH 3 OO • . Subsequently, CH 3 OO • is reduced to MHP by O 2 •- . Our results also suggest that the yield of CH 3 • to MHP under air-saturated conditions is 52% and increases to 98% under oxygen-saturated conditions. The decays of MHP and H 2 O 2 are very similar in terms of photodegradation, hydrolysis, Fenton, and photo-Fenton reactions. This study can be useful to understand the photochemical formation of organic peroxides in surface waters.

Dissolved black carbon (DBC) is an important component of dissolved organic matter pool; however, its photochemical properties are not fully understood. In this study, we determined the excited triplet-state quantum yields of DBC ( 3 DBC*) and 1 O 2 quantum yields (Φ 1 O 2 ) of six diverse DBCs using sorbic alcohol, 2,4,6-trimethylphenol (TMP), and furfuryl alcohol and compared the results with quantum yields of reference natural organic matters (NOMs). The average Φ 1 O 2 of six DBCs (4.2 ± 1.5%) was greater than that of terrestrial NOM (2.4 ± 0.3%) and comparable to autochthonous NOM (5.3 ± 0.2%). Using TMP as a probe for oxidizing triplets, DBC presented significantly higher apparent quantum yield coefficients for degrading TMP ( f TMP ) than the reference NOM, reflecting that the f TMP values of low-energy 3 DBC* were approximately 12-fold greater than those of low-energy 3 NOM*. The differences in the f TMP and Φ 1 O 2 trends among the DBCs indicated that the 3 DBC* responsible for these reactions may be from different sources. In addition, DBC was much more effective than NOM, on a carbon-normalized basis, during photodegradation of pharmaceutically active compounds. This result confirms that the presence of DBC can accelerate the photodegradation of contaminants that are susceptible to one-electron oxidation by triplets.

The carbonate radical (CO 3 •- ) is a strong oxidative radical that is generated via the reactions of HCO 3 - /CO 3 2- with hydroxyl radical (HO • ) or triplet states of dissolved organic matter ( 3 DOM ∗ ) in sunlit surface water. The bimolecular reaction rate constants of CO 3 •- with various DOM isolates ( [Formula: see text] ) were calculated as 15-239 (mg of C/L) -1 s -1 and were correlate to the bulk DOM properties, such as the content of phenolic moieties, the specific UV absorbance (SUVA), the E2/E3 value, and the fluorescence index (FI). The spectroscopic E2/E3 values was found to strongly correlated (R 2  = 0.93) with [Formula: see text] , and an empirical equation was established. Our results also demonstrate that CO 3 •- is involved in the photobleaching of dissolved organic matter (DOM) and in particular reacts with electron-donor moieties, leading to faster decay rates at long wavelengths of UV-vis absorption. Furthermore, a model was developed to calculate the steady-state concentrations of CO 3 •- during DOM photobleaching. These results allow us to estimate the reactivity of DOM with CO 3 •- and to evaluate the role of CO 3 •- in sunlit surface water. It will also allow a better assessment of the concentration and utilization of CO 3 •- during the application of advanced oxidation processes., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Buckminsterfullerenes (C 60 ) are widely used nanomaterials that are present in surface water. The combination of C 60 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 ( 1 O 2 ), hydroxyl radical (HO • )-like species, superoxide radicals (O 2 •- ), hydrogen peroxide (H 2 O 2 ), and triplet excited states ( 3 C 60 */ 3 HA*) in solutions containing both C 60 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 C 60 proved to have only a slight impact on the 1 O 2 formation from HA, C 60 played a key role in the inhibition of O 2 •- . The photochemical formation of H 2 O 2 followed the conservative mixing model due to the reaction of C 60 •- with HO 2 • /O 2 •- , and the biomolecular reaction rate constant has been measured as (7.4 ± 0.6) × 10 6 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 3 C 60 * with the TMP probe. In contrast, C 60 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 C 60 on aqueous environments.

The formation of reactive oxygen species (ROS) from effluent organic matter (EfOM) was investigated under simulated solar irradiation. In this study, EfOM was isolated into three different fractions based on hydrophobicity. The productivity of ROS in EfOM was measured and compared with that of natural organic matter (NOM) isolates, including Suwannee River humic acid/fulvic acid (SRHA/FA) and Pony Lake fulvic acid (PLFA). The hydrophilic (HPI) component had a greater quantum yield of 1O2 than those of the hydrophobic (HPO) and transphilic (TPI) fractions because the HPI contained peptides and proteins. Regarding O2•-, the phenolic moieties acted as electron donating species after photochemical excitation and therefore electron transfer to oxygen. A positive correlation was found between the phenolic concentrations and the steady state O2•-concentrations. H2O2 accumulated during the irradiation process from superoxide as precursor. Potentially, due to the presence of proteins or other organic species in the HPI fraction, the decay rates of H2O2 in the dark for both the effluent wastewater and the HPI fraction were significantly faster than the rates observed in the standard NOM isolates, the HPO and TPI fractions. Autochthonous NOM showed a higher •OH productivity than terrestrial NOM. The [•OH]ss was lowest in the HPI fraction due to the lack of humic fraction and existence of soluble microbial products (SMPs), which easily reacted with •OH. Overall, the HPO and TPI fractions were the major sources of superoxide, H2O2 and •OH under simulated solar irradiation. The HPI fraction dominated the production of 1O2 and acted as a sink for H2O2 and •OH.

In the past few years, the fate and transportation of pharmaceutically active compounds (PhACs) in aqueous environments have raised significant concerns among the public, scientists and regulatory groups. Photodegradation is an important removal process in surface waters. This review summarizes the last 10 years (2003-2013) of studies on the solar or solar-simulated photodegradation of PhACs in aqueous environments. The PhACs covered include: beta-blockers, antibiotics, non-steroidal anti-inflammatory drugs (NSAIDs), histamine H₂-receptor antagonists, lipid regulators, carbamazepine, steroid hormones, and X-ray contrast media compounds. Kinetic studies, degradation mechanisms and toxicity removal are the three major topics involved in this review. The quantum yield for the direct photolysis of PhACs and the bimolecular reaction rate constants of PhACs with reactive oxygen species (ROS), such as the ˙OH radical and singlet oxygen, are also summarized. This information is not only important to predict the PhAC photodegradation fate, but also is very useful for advanced treatment technologies, such as ozone or advanced oxidation processes.