Predicting apparent singlet oxygen quantum yields of dissolved black carbon and humic substances using spectroscopic indices

Photochemistry of plateau lake-derived dissolved organic matter: Reactive species generation and effects on 17β-estradiol photodegradation

Naturally strong ultraviolet irradiation at high altitudes causes photobleaching of plateau lake DOM (P-DOM) and affects its photochemical activity. However, the photoreactivity of P-DOM has remained unclear under natural photobleaching condition. Here, six P-DOM samples isolated from plateau lakes in Yunnan Province, China as well as two reference DOM as comparisons were used to explore the photogeneration of reactive species (RS) and their effects on 17β-estradiol photodegradation. Compared with SRHA/SRFA, P-DOM has lower aromaticity, average molecular weight, and electron-donating capacity. The quantum yields of triplet state P-DOM (3P-DOM*), 1O2, and ∙OH produced in P-DOM solutions were greatly higher than those of reference DOM. The RS quantum yields had positive linear correlations with E2/E3 and SR, whereas were negatively linear correlated with SUVA25. Radical quenching experiments showed that 3P-DOM* was the prominent RS for 17β-estradiol photodegradation, and its contribution exceeded 70% for each of P-DOM. 3P-DOM*-mediated photodegradation was mainly attributed to the electron-transfer reactions with an average second-order rate constant of 4.62 × 109 M-1s-1, indicating the strong photoreactivity towards 17β-estradiol. These findings demonstrate that P-DOM is an efficient photosensitizer for RS production, among which 3P-DOM* may play an important role in enhanced photodegradation for organic micropollutants in plateau lake enriched with DOM.

Photo-production of excited triplet-state of dissolved organic matters in inland freshwater and coastal seawater

The excited triplet-state of dissolved organic matter (3DOM*) is a major reactive intermediate in sunlit waters. Its quantum yield is important in understanding the fate of organic micropollutants. The degradation efficiency of its chemical probe, 2,4,6-trimeythlphenol (fTMP), is generally used as a proxy of the quantum yield. However, fTMP has been described and modelled only for freshwater systems. Therefore, this study quantified fTMP in inland freshwater and coastal seawater sampled in Japan by conducting steady-state photochemical experiments. Optical properties of water were then used to model fTMP. Results indicated that the inland freshwater DOM originated mainly from terrestrial sources, while the coastal seawater DOM were microbial-dominated. On average, inland freshwater exhibited lower fTMP (61.2 M-1) than coastal seawater (79.7 M-1) and the coastal seawater exhibited significant variations in the proportion of high-energy 3DOM* (> 250 kJ/mol). In addition, E2:E3 (ratio of absorbance at 254 to 365 nm) was positively correlated with fTMP of inland freshwater, coastal seawater, and the overall dataset. Catchment conditions such as forest coverage also influenced the production of 3DOM* and high-energy 3DOM* in inland freshwater. Furthermore, the developed models estimated fTMP based on the optical properties of both freshwater and seawater, providing valuable insights about 3DOM* photochemistry in the aquatic environment.

Singlet Oxygen Quantum Yields of Pyrogenic Dissolved Organic Matter from Lab-Prepared and Wildfire Chars

Wildfires or prescribed fires release pyrogenic dissolved organic matter (pyDOM) into the environment, which can photochemically produce singlet oxygen (1O2) in sun-lit surface waters. 1O2 quantum yields (ΦΔ) are well-studied for non-pyrogenic DOM, but little is understood about the 1O2 generation from pyDOM, especially the ΦΔ values from real wildfire samples and their wavelength dependence. In this study, time-resolved 1O2 phosphorescence was used to determine the wavelength-dependent ΦΔ values for pyDOM generated from wildfire char and a series of lab-prepared chars produced by combusting oak and pine wood. Wildfire and most lab-prepared pyDOM generally had similar ΦΔ values (2.1-2.7%) at 365 nm compared to the reference Suwannee River Natural Organic Matter (SRNOM) isolate (2.4%). Interestingly, pyDOM from the highest combustion temperature char was found to possess extremely low ΦΔ values compared to SRNOM and other pyDOM at all excitation wavelengths. In addition, it was revealed that the predicted steady-state concentration of 1O2 from pyDOM was similar to that from SRNOM, indicating that the addition of pyDOM from wood chars may not strongly impact surface water photochemistry.

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

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

Dissolved black carbon mediated photo-oxidation of arsenic(III) to arsenic(V) in water: The key role of triplet states

Arsenic is a common contaminant found in natural waters, and has raised significant environmental concerns due to its toxicity and carcinogenicity. In this study, we investigated the mediated photo-oxidation of arsenite (As(III)) under simulated sunlight by dissolved black carbon (DBC), an important dissolved organic matter (DOM) constituent released from black carbon. Five DBC were collected from the water extracts of black carbons that were derived by pyrolyzing different biomass (i.e., bamboo, rice, peanuts, corn, and sorghum stalks), and four well-studied dissolved humic substances (DHS) were selected for benchmarking. The presence of DBC (i.e., 5 mg C-1) significantly accelerated the photo-oxidation of As(III) to arsenate (As(V)), with the observed pseudo-first-order rate constant of reaction increased by 5∼11 times. Quenching experiments of photochemically produced reactive intermediates suggested that As(III) was mainly oxidized by triplet-excited DBC (3DBC*, contribution of 48%), singlet oxygen (1O2, 18%) and superoxide anions (O2•-, 28%) in sunlight-irradiated DBC solutions. The average apparent quantum yield of As(III) photo-oxidation for DBC was found to be more than 4 times higher in comparison with DHS. Such a strong mediation efficiency of DBC was due to its smaller molecular size and higher aromaticity than DHS, which facilitated the non-charge-transfer process to produce triplet-excited states and their sensitized 1O2. Consistently, DBC exhibited a higher apparent quantum yield and a longer lifetime of triplet states as compared with DHS. The results imply that DBC may play a previously unrecognized important role in the fate of arsenic in aquatic environments.

Prediction of Photochemical Properties of Dissolved Organic Matter Using Machine Learning

Apparent quantum yields (Φ) of photochemically produced reactive intermediates (PPRIs) formed by dissolved organic matter (DOM) are vital to element cycles and contaminant fates in surface water. Simultaneous determination of ΦPPRI values from numerous water samples through existing experimental methods is time consuming and ineffective. Herein, machine learning models were developed with a systematic data set including 1329 data points to predict the values of three ΦPPRIs (Φ3DOM*, Φ1O2, and Φ·OH) based on DOM spectral parameters, experimental conditions, and calculation parameters. The best predictive performances for Φ3DOM*, Φ1O2, and Φ·OH were achieved using the CatBoost model, which outperformed the traditional linear regression models. The significances of the wavelength range and spectral parameters on the three ΦPPRI predictions were revealed, suggesting that DOM with lower molecular weight, lower aromatic content, and a more autochthonous portion possessed higher ΦPPRIs. Chain models were constructed by adding the predicted Φ3DOM* as a new feature into the Φ1O2 and Φ·OH models, which consequently improved the predictive performance of Φ1O2 but worsened the Φ·OH prediction likely due to the complex formation pathways of ·OH. Overall, this study offered robust ΦPPRI prediction across interlaboratory differences and provided new insights into the relationship between PPRIs formation and DOM properties.

Intrinsic peroxidase-like activity of dissolved black carbon released from biochar

Dissolved black carbon (DBC) is an important constituent of the natural organic carbon pool, influencing the global carbon cycling and the fate processes of many pollutants. In this work, we discovered that DBC released from biochar has intrinsic peroxidase-like activity. DBC samples were derived from four biomass stocks, including corn, peanut, rice, and sorghum straws. All DBC samples catalyze H2O2 decomposition into hydroxyl radicals, as determined by the electron paramagnetic resonance and the molecular probe. Similar to enzymes that exhibit saturation kinetics, the steady-state reaction rates follow the Michaelis-Menten equation. The peroxidase-like activity of DBC is controlled by the ping-pong mechanism, as suggested by parallel Lineweaver-Burk plots. Its activity increases with temperature from 10 to 80 °C and has an optimum at pH 5. The peroxidase-like activity of DBC is positively correlated with its aromaticity as aromatics can stabilize the reactive intermediates. The active sites in DBC also involve oxygen-containing groups, as inferred by increased activity after the chemical reduction of carbonyls. The peroxidase-like activity of DBC has significant implications for biogeochemical processing of carbon and potential health and ecological impacts of black carbon. It also highlights the need to advance the understanding of the occurrence and role of organic catalysts in natural systems.

Water-soluble organic carbon (WSOC) from vegetation fire and its differences from WSOC in natural media: Spectral comparison and self-organizing maps (SOM) classification

Vegetation fire frequently occurs globally and produces two types of water-soluble organic carbon (WSOC) including black carbon WSOC (BC-WSOC) and smoke-WSOC, they will eventually enter the surface environment (soil and water) and participate in the eco-environmental processes on the earth surface. Exploring the unique features of BC-WSOC and smoke-WSOC is critical and fundamental for understanding their eco-environmental effects. Presently, their differences from the natural WSOC of soil and water remain unknown. This study produced various BC-WSOC and smoke-WSOC by simulating vegetation fire and used UV-vis, fluorescent EEM-PARAFAC, and fluorescent EEM-SOM to analyze their different features from natural WSOC of soil and water. The results showed that the maximum yield of smoke-WSOC reached about 6600 folds that of BC-WSOC after a vegetation fire event. The increasing burning temperature decreased the yield, molecular weight, polarity, and protein-like matters abundance of BC-WSOC and increased the aromaticity of BC-WSOC, but presented a negligible effect on the features of smoke-WSOC. Furthermore, compared with natural WSOC, BC-WSOC had a greater aromaticity, smaller molecular weight, and more humic-like matters, while smoke-WSOC had a lower aromaticity, smaller molecular size, higher polarity, and more protein-like matters. EEM-SOM analysis indicated that the ratio between the fluorescence intensity at Ex/Em: 275 nm/320 nm and the sum fluorescence intensity at Ex/Em: 275 nm/412 nm and Ex/Em: 310 nm/420 nm could effectively differentiate WSOC of different sources, following the order of smoke-WSOC (0.64-11.38) > water-WSOC and soil-WSOC (0.06-0.76) > BC-WSOC (0.0016-0.04). Hence, BC-WSOC and smoke-WSOC possibly directly alter the quantity, properties, and organic compositions of WSOC in soil and water. Owing to smoke-WSOC having far greater yield and bigger difference from natural WSOC than BC-WSOC, the eco-environmental effect of smoke-WSOC deposition should be given more attention after a vegetation fire.

Photolysis and photo-enhanced toxicity of three novel designed neonicotinoids: Impact of novel modifying groups

Three novel neonicotinoids (cycloxaprid, flupyradifurone and sulfoxaflor) were designed to reduce the biotoxicity for non-target organisms. These neonicotinoids were photolyzed under light radiation, but it was unclear for the photo-enhanced toxicity and influences of the novel modifying group of the three neonicotinoids. The photolysis and photo-enhanced toxicity experiments were performed for the three neonicotinoids, coupled with quantum chemistry calculation, the mechanisms of photolysis, photo-enhanced toxicity and the influences of novel modifying groups were analyzed. The results showed the photolysis pathways were enriched as compared with previous neonicotinoids due to the composition of modifying groups, singlet oxygen and hydroxyl participated the photolysis of cycloxaprid and flupyradifurone. All tested neonicotinoids exhibited photo-enhanced toxicity to Vibrio fischeri. Due to the difference of photolysis mechanism and toxicity to V. fischeri, the photo-enhanced toxicity curves showed diverse variation when histidine, tert-butanol or dissolved organic matters was in presence of the test solutions. The impact of novel modifying groups over photolysis and photo-enhanced toxicity were analyzed based on the comparison with previous neonicotinoids, theoretically predicted UV-Vis spectra and photo-physical/chemical property descriptors. The data showed the composition of novel modifying group increased the light absorption and photo-chemical activities for the three neonicotinoids.

Apparent quantum yield for photo-production of singlet oxygen in reservoirs and its relation to the water matrix

Man-made reservoirs are important for human daily lives and offer different functions, however they are contaminated due to anthropogenic activities. Dissolved organic matter (DOM) from each reservoir is unique in composition, which further determines its photo-reactivity. Thus, this study aimed to investigate the photo-reactivity of reservoir DOM in terms of the quantum yield for photo-production of singlet oxygen (Ф1O2). We sampled surface water of 50 reservoirs in Japan and determined their Ф1O2 using simulated sunlight together with bulk water analysis. Their Ф1O2 ranged from 1.46 × 10-2 to 6.21 × 10-2 (mean, 2.55 × 10-2), which was identical to those of lakes and rivers reported in the literature, but lower than those of wetland water and wastewater. High-energy triplet-state of DOM accounted for 59.4% of the 1O2 production in the reservoir water on average. Among the bulk water properties, the spectral slope of wavelength from 350 to 400 nm (S350-400) was statistically detected as the most important predictor for Ф1O2. Furthermore, the multiple linear regression model employed S350-400 and the biological index as predictors with no intercorrelations and reasonable accuracy (r2 = 0.86), while the random forest model showed a better accuracy (r2 = 0.90). Overall, these major findings are beneficial for understanding the photo-reactivity of reservoir waters.

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

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

Effect of adsorption on ferrihydrite on the photoreactivity of dissolved black carbon for photodegradation of sulfadiazine

The selective adsorption of dissolved black carbon (DBC) on inorganic minerals is a widespread geochemical process in the natural environment, which could change the chemical and optical properties of DBC. However, it remains unclear how selective adsorption affects the photoreactivity of DBC for photodegradation of organic pollutants. This paper was the first to investigate the effect of DBC adsorption on ferrihydrite at different Fe/C molar ratios (Fe/C molar ratios of 0, 7.50 and 11.25, and marked as DBC₀, DBC_7.50 and DBC_11.25) on the photoproduction of reactive intermediates generated from DBC and their interaction with sulfadiazine (SD). Results showed that UV absorbance, aromaticity, molecular weight and contents of phenolic antioxidants of DBC were significantly decreased after adsorption on ferrihydrite, and higher decrease was observed at higher Fe/C ratio. Photodegradation kinetics experiments showed that observed photodegradation rate constant of SD (k_obs) increased from 3.99 × 10^-5 s^-1 in DBC₀ to 5.69 × 10^-5 s^-1 in DBC_7.50 while decreased to 3.44 × 10^-5 s^-1 in DBC_11.25, in which ³DBC* played important roles and ¹O₂ played a minor role, while ·OH was not involved in the reaction. Meanwhile, the second-order reaction rate constant between ³DBC* and SD (k_{SD, 3DBC*}) increased from 0.84 × 10⁸ M^-1 s^-1 for DBC₀ to 2.53 × 10⁸ M^-1 s^-1 for DBC_7.50 while decreased to 0.90 × 10⁸ M^-1 s^-1 for DBC_11.25. The above results might be mainly attributed to the fact that the decrease of phenolic antioxidants in DBC weakened the back-reduction of ³DBC* and reactive intermediates of SD as the Fe/C ratio increased, while the decrease of quinones and ketones reduced the photoproduction of ³DBC*. The research revealed adsorption on ferrihydrite affected the photodegradation of SD by changing the reactivity of ³DBC*, which was helpful to understand the dynamic roles of DBC in the photodegradation of organic pollutants.

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

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

Relationship between photolysis mechanism and photo-enhanced toxicity to Vibrio Fischeri for neonicotinoids with cyano-amidine and nitroguanidine structures

Neonicotinoids are widely used pesticides that contaminate aquatic environments. Although these chemicals can be photolyzed under sunlight radiation, it is unclear for the relationship between photolysis mechanism and toxicity change in aquatic organisms. This study aims to determine the photo-enhanced toxicity of four neonicotinoids with different main structures (acetamiprid, and thiacloprid for cyano-amidine structure, imidacloprid and imidaclothiz for nitroguanidine). To Achieve the goal, photolysis kinetics, effect of dissolved organic matter (DOM) and reactive oxygen species (ROSs) scavengers on photolysis rates, photoproducts, and photo-enhanced toxicity to Vibrio fischeri were investigated for four neonicotinoids. The results showed direct photolysis plays a key role in the photo-degradation of imidacloprid and imidaclothiz (photolysis rate constants are 7.85 × 10^-3 and 6.48 × 10^-3 min^-1, respectively), while the photosensitization process of acetamiprid and thiacloprid was dominated by ·OH reactions and transformation (photolysis rate constants are 1.16 × 10^-4 and 1.21 × 10^-4 min^-1, respectively). All four neonicotinoid insecticides exerted photo-enhanced toxicity to Vibrio fischeri, indicating photolytic product(s) posed greater toxicity than their parent compounds. The addition of DOM and ROS scavengers influenced photo-chemical transformation rates of parent compounds and their intermediates, leading to diverse effects on photolysis rates and photo-enhanced toxicity for the four insecticides as a result of different photo-chemical transformation processes. Based upon the detection of chemical structures of intermediates and Gaussian calculations, we observed different photo-enhanced toxicity mechanisms for the four neonicotinoid insecticides. Molecular docking was used to analyze the toxicity mechanism of parent compounds and photolytic products. A theoretical model was subsequently employed to describe the variability of toxicity response to each of the four neonicotinoids.

Spectral characteristics of dissolved organic carbon (DOC) derived from biomass pyrolysis: Biochar-derived DOC versus smoke-derived DOC, and their differences from natural DOC

Biochar-derived dissolved organic carbon (BDOC) and smoke-derived dissolved organic carbon (SDOC) are two different biomass-pyrogenic DOCs. They inevitably enter soil and water, then potentially pose different impacts on the chemistry of these media. This study systemically investigated the emissions and spectral characteristics of BDOC and SDOC as well as their differences from natural DOC. The results showed that the emission of SDOC was 1-3 orders of magnitude greater than that of BDOC after biomass pyrolysis. UV-vis spectra indicated that BDOC had higher aromaticity and molecular weight as well as lower polarity than SDOC. The two-dimensional correlation infrared spectrum (2D-PCIS) matrix indicated that BDOC contained more chemical groups with stronger temperature-dependence than SDOC. Fluorescence EEM-PARAFAC analysis showed that BDOC was dominated by macromolecular humic-like substances, while SDOC was primarily composed of small molecules of aromatic protein/polyphenols-like compounds. The fluorescence indicators including humification index (HIX) (0.08-0.76) and biological index (BIX) (1.18-1.72) of SDOC were significantly different from those of BDOC (HIX: 1.64-12.68, and BIX: 0.17-1.62). The higher BIX and more small molecules of aromatic protein/polyphenols-like compounds indicated SDOC had potentially higher bioavailability and turnover rate in the environment than BDOC. Furthermore, the UV-vis spectral indicator (S_275-295) and fluorescence spectral indicators (HIX, and BIX) of BDOC were equivalent to those of natural DOC, whereas these indicators of SDOC were significantly different from those of natural DOC. This study demonstrated that BDOC and SDOC had significantly different components and properties and they might present different environmental behaviors and effects.

The binding characteristics of sediment-derived dissolved organic matter with ceftazidime: a microstructural and spectroscopic correlation study

This research focused on the characterization of sediment-derived dissolved organic matter (SDOM) extracted from sediment of Yellow River and the binding behaviors of ceftazidime (CAZ) with the presence of SDOM. The morphology, surface composition and structure of SDOM and the complexation between SDOM and CAZ in terms of component features, binding capacity and sequence were studied by multiple approaches. Results showed that SDOM was in situ autochthonous-dominated with a low weight-average molecular weight and aromaticity (the value of S_R was 2.523). The multiple morphological characteristics, high surface oxygen contents (53.49%) and more aliphatic (H/C = 1.91) of SDOM were further confirmed. Studies on SDOM-CAZ interaction suggested that the functional groups and chemical compositions of SDOM were susceptible to CAZ. In more detail, the aromatic protons and aliphatic protons of CAZ impacted significantly, and the binding between CAZ and SDOM might relate to noncovalent. The protein-like fractions were considered to be the primary participant with 49% fractions lost and the aromatics and amides as mainly active site interaction with CAZ. These findings have significant implications on the environmental fate of cephalosporin antibiotics and that of sediment-derived DOM.

Radical chemistry of dissolved black carbon under sunlight irradiation: quantum yield prediction and effects on sulfadiazine photodegradation

Dissolved black carbon (DBC) is regarded as an important part of the natural organic matter pool. However, it is unclear about DBC's photochemical activity and the relationships between reactive intermediates (RIs) and the molecular structure of DBC remain unclear. In this study, we investigate the photochemical formation ability of RIs and spectral parameters (E2/E3, S275-295) of DBC made from five types of plants at five pyrolysis temperatures. The results showed that there were good linear regressions between the RI quantum yields and the spectral parameters (E2/E3, S275-295), and this was indicative of the RI generation prediction from DBC under solar irradiation. The DBC-mediated photochemical experiment of sulfadiazine revealed that 3DBC* was the primary active species for the indirect photodegradation of sulfadiazine. Further studies indicated that a linear relationship was observed between the indirect photodegradation ability of sulfadiazine induced directly by 3DBC* at different pyrolysis temperatures and the 3DBC* quantum yields or E2/E3. These findings indicate that the simple models of the RI quantum yield as a function of spectral parameters can be used to evaluate the degradation of pollutants with known DBC spectral parameters.

Comparing Photoactivities of Dissolved Organic Matter Released from Rice Straw-Pyrolyzed Biochar and Composted Rice Straw

Here, we systematically compared the photoactivity and photobleaching behavior between dissolved black carbon (DBC) from rice straw biochar and leached dissolved organic carbon (LDOC) from rice straw compost using complementary techniques. The Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis showed that DBC was dominated by polycyclic aromatic (55.1%) and tannin-like molecules (24.1%), while LDOC was dominated by lignin-like (58.9%) and tannin-like molecules (19.7%). Under simulated sunlight conditions, DBC had much higher apparent quantum yields for ³DOM* and ¹O₂ but much lower apparent quantum yields for ^•OH than LDOC. After a 168 h irradiation, the total number of LDOC formulas identified by FT-ICR MS decreased by 40.1% with concurrent increases in O/C and H/C ratios and also decreases in double bond equivalence minus oxygen (DBE - O) and average molecular weight identified by gel permeation chromatography. However, despite the large decreases in UVA₂₅₄ and DOC, the total number of DBC formulas decreased only by 12.0% with nearly unchanged O/C ratio, DBE - O values, molecular weight distribution, and benzenepolycarboxylic aromatic condensation (BACon) index regardless of the decreased percentage of condensed aromatic carbon (ConAC %). Compared with LDOC, the photolysis of DBC was much less oxidative and destructive mainly via breakup of a small portion of the highly condensed aromatic rings, probably accompanied by photodecarboxylation.

Photochemistry of dissolved organic matter in water from the Pearl river (China): Seasonal patterns and predictive modelling

Photochemical properties of dissolved organic matter (DOM) vary widely in natural and engineered water systems due to the different dominant compositions. However, seasonal patterns of DOM photochemical properties in urban rivers remain unclear. In this study, two seasons (wet and dry) of water samples were collected from eleven sites throughout the Pearl River (China) to investigate the spatiotemporal variability of DOM optical and photochemical properties. The optical properties of DOM in the Pearl River were characterized by UV-vis and fluorescence spectroscopies, which showed the substantial decrease in absorption coefficient and fluorescence intensity and increase in absorbence ratio (E₂/E₃) and specific absorption coefficient (SUVA) from the wet to dry season. The photochemical properties in terms of the apparent quantum yields of ³DOM*, ¹O₂ and ·OH from DOM (Φ³_DOM*, Φ¹_O2 and Φ_{·OH, DOM}) under illumination also displayed a significant decrease from the wet to the dry season. Spearman's rank correlation analysis revealed the strongest relationships between Φ³_DOM*, Φ¹_O2 and Φ_{·OH, DOM} and the relative abundance of microbial humic-like component (C2%) derived from parallel factor analysis (PARAFAC). Partial least squares regression (PLSR) modelling exhibited an excellent prediction strength for steady-state concentrations of ¹O₂ ([¹O₂]_ss) and ·OH ([·OH]_ss) with adjusted R² values of 0.85 and 0.91, respectively, by using DOC concentration ([DOC]), optical properties, nitrate and nitrite concentrations as the response variables. In addition, the model identified that the F_max of humic-like component C4 (F_max-C4) was the most effective predictor amongst the used response variables. This study provides an approach to describe and predict the seasonal patterns of DOM photochemical properties in urbanized rivers, offering a good understanding of the formation mechanism of reactive species from river DOM.

Comparative analysis on the photolysis kinetics of four neonicotinoid pesticides and their photo-induced toxicity to Vibrio Fischeri: Pathway and toxic mechanism

Neonicotinoids are widely used pesticides all over the world and pose severe water pollution. Although they can be degraded via absorbing sunlight, few attentions have been paid to the environmental risks of their photolysis products. In this paper, the photo-toxicity was investigated for four neonicotinoids (dinotefuran, nitenpyram, thiamethoxam and clothianidin) based on a series of experiments (i.e., photolysis kinetics, radical scavenging, bioluminescent inhibition test to Vibrio Fischeri and intermediate identification) and in-silico calculation of photolysis pathway. The results show that direct photolysis dominates the photolysis of the four neonicotinoids under simulated sunlight radiation. The bioluminescent inhibition kinetics shows that all four neonicotinoids have photo-induced toxicity to V. fischeri, but with different light-induced responses. Scavenging radicals (·OH and ¹O₂) will decrease the photo-induced toxicity of all the four neonicotinoids, indicating radicals play important roles to the photo-chemical reactions of intermediates. Dissolved organic matters exhibit slightly shading effect to the photolysis rates of four parent compounds. However, the ROSs generated by DOM can accelerate the photo-chemical reactions of intermediates, leading to different photo-induced toxicity in present of DOM. According to the detected intermediates and Gaussian calculations, there are different photolysis pathways and mechanisms for the four neonicotinoids. The calculation for photo-sensitization reactions with ³O₂ indicates that both energy transfer reactions and electron transfer reactions can be produced under simulated sunlight radiation, which further consolidate that reactive oxygen species are involved in the photolysis process. A theoretical model has been developed to explain the toxicity variations of four neonicotinoids in different aqueous conditions.

Bioengineered biochar as smart candidate for resource recovery toward circular bio-economy: a review

Biochar's ability to mediate and facilitate microbial contamination degradation, as well as its carbon-sequestration potential, has sparked interest in recent years. The scope, possible advantages (economic and environmental), and future views are all evaluated in this review. We go over the many designed processes that are taking place and show why it is critical to look into biochar production for resource recovery and the role of bioengineered biochar in waste recycling. We concentrate on current breakthroughs in the fields of engineered biochar application techniques to systematically and sustainable technology. As a result, this paper describes the use of biomass for biochar production using various methods, as well as its use as an effective inclusion material to increase performance. The impact of biochar amendments on microbial colonisation, direct interspecies electron transfer, organic load minimization, and buffering maintenance is explored in detail. The majority of organic and inorganic (heavy metals) contaminants in the environment today are caused by human activities, such as mining and the use of chemical fertilizers and pesticides, which can be treated sustainably by using engineered biochar to promote the establishment of a sustainable engineered process by inducing the circular bioeconomy.

Estimation of the biogeochemical reactivities of dissolved organic matter from modified biochars using color

Modified biochar is widely used as a soil amendment in agricultural systems to improve crop yields and remove environmental pollutants. The water-soluble fraction of biochar, called biochar-derived dissolved organic matter (DOM_BC), is the most active biochar component. However, the correlation between the optical properties of DOM_BC and its biogeochemical activity remain unclear. In this study, one biochar and six modified derivatives were used to extract DOM_BC and characterize its optical properties. The biogeochemical reactivities of DOM_BC were determined using biodegradation, photodegradation, and electron-donating capacity assays. The results show that modification changes the biochar characteristics, leading to a variety of DOM_BC properties. The DOM_BC from modified biochars degrades more rapidly than the original biochar. On the other hand, modification reduces the redox functional groups in DOM_BC, resulting in a lower electron-donating capacity of DOM samples. However, the modifications did not seem to affect photodegradation. Not all spectral parameters provide information about the correlations between the DOM_BC properties and biogeochemical reactivity. However, two fundamental properties, that is, the specific UV absorbance at 254 nm (SUVA₂₅₄, showing aromaticity) and spectral slopes over the ranges of 275-295 nm of the UV absorbance (S_275-295, showing molecular weight), are the dominant factors affecting the biodegradation and electron-donating capacities of DOM_BC. In this study, a rapid and straightforward method is presented, which can be used to characterize DOM_BC and predict the reactivity of biochar that is used as an environmental amendment to minimize toxic organic compounds.

Photogeneration of Reactive Species from Biochar-Derived Dissolved Black Carbon for the Degradation of Amine and Phenolic Pollutants

Due to agricultural waste combustion and large-scale biochar application, biochar-derived dissolved black carbon (DBC) is largely released into surface waters. The photogeneration of reactive species (RS) from DBC plays an important role in organic pollutant degradation. However, the mechanistic interactions between RS and pollutants are poorly understood. Here, we investigated the formation of DBC triplet states (³DBC*), singlet oxygen (¹O₂), and hydroxyl radical (^•OH) in straw biochar-derived DBC solutions and photodegradation of typical pharmaceuticals and personal care products (PPCPs). Laser flash photolysis and electron spin resonance spectrometry showed that DBC exhibited higher RS quantum yields than some well-studied dissolved organic matter. The RS caused rapid degradation of atenolol, diphenhydramine, and propylparaben, selected as target PPCPs in this study. The ³DBC* contributed primarily to the oxidation of selected PPCPs via one-electron-transfer interaction, with average reaction rate constants of 1.15 × 10⁹, 1.41 × 10⁹, and 0.51 × 10⁹ M^-1 s^-1, respectively. ^•OH also participated in the degradation and accounted for approximately 2.7, 2.5, and 18.0% of the total removal of atenolol, diphenhydramine, and propylparaben, respectively. Moreover, the photodegradation products were identified using high-resolution mass spectrometry, which further confirmed the electron transfer and ^•OH oxidation mechanisms. These findings suggest that DBC from the combustion process of agricultural biomass can efficiently induce the photodegradation of organic pollutants under sunlight in aquatic environments.

Quantification of seasonal photo-induced formation of reactive intermediates in a municipal sewage lagoon upon sunlight exposure

Photochemically produced reactive oxygen species in wastewater lagoons upon sunlight exposure are important in the attenuation of emerging contaminants (ECs). The production of reactive radicals in wastewater lagoons depends on both environmental factors and the composition of effluent organic matter (EfOM) in the wastewater. Knowing the steady state concentrations of these reactive species produced in a particular lagoon wastewater is critical to the prediction of the persistence and attenuation of ECs in that sunlit wastewater treatment lagoon. This study quantified the formation of four photochemically produced reactive intermediates (PPRIs): hydroxyl radical, carbonate radical, singlet oxygen, and triplet excited state EfOM in 11 samples collected from a municipal wastewater lagoon over a full year. The temporal distribution of these key PPRIs in the lagoon under investigation was determined in relation to sunlight irradiance, wastewater composition and temperature. Greater sunlight intensity led to greater PPRI production over the year. Increasing wastewater temperature from 12 to 25 °C led to greater production of singlet oxygen, a moderate decrease in hydroxyl radical and increase in triplet excited state EfOM, and minimal impact on carbonate radical production. The optical properties of the lagoon wastewater of Napierian absorption coefficient (A₃₀₀) and E₂:E₃ ratio could be used as indicators of the formation of singlet oxygen (Pearson's r = 0.79) and triplet excited EfOM (Pearson's r = 0.76) produced upon solar irradiation. The concentration of carbonate radical formed was strongly correlated to the nitrate level in the wastewater (Pearson's r = 0.85). The findings could be used for modelling the seasonal sunlight-induced photolysis process of ECs during lagoon-based wastewater treatment, with a view to optimising the treatment process, predicting the efficacy of EC removal, and risk assessment of the treated water.

Singlet Oxygen Quantum Yields in Environmental Waters

Singlet oxygen (¹O₂) is a reactive oxygen species produced in sunlit waters via energy transfer from the triplet states of natural sensitizers. There has been an increasing interest in measuring apparent ¹O₂ quantum yields (Φ_Δ) of aquatic and atmospheric organic matter samples, driven in part by the fact that this parameter can be used for environmental fate modeling of organic contaminants and to advance our understanding of dissolved organic matter photophysics. However, the lack of reproducibility across research groups and publications remains a challenge that significantly limits the usability of literature data. In the first part of this review, we critically evaluate the experimental techniques that have been used to determine Φ_Δ values of natural organic matter, we identify and quantify sources of errors that potentially explain the large variability in the literature, and we provide general experimental recommendations for future studies. In the second part, we provide a qualitative overview of known Φ_Δ trends as a function of organic matter type, isolation and extraction procedures, bulk water chemistry parameters, molecular and spectroscopic organic matter features, chemical treatments, wavelength, season, and location. This review is supplemented with a comprehensive database of Φ_Δ values of environmental samples.

Monitoring dissolved organic matter in wastewater and drinking water treatments using spectroscopic analysis and ultra-high resolution mass spectrometry

Dissolved organic matter (DOM) plays a critical role in determining the quality of wastewater and the safety of drinking water. This is the first review to compare two types of popular DOM monitoring techniques, including absorption spectroscopy and fluorescence excitation-emission matrices (EEMs) coupled with parallel factor analysis (PARAFAC) vs. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), for the applications in wastewater and drinking water treatments. The optical techniques provide a series of indices for tracking the quantity and quality of chromophoric and fluorescent DOM, while FT-ICR-MS is capable of identifying thousands of DOM compounds in wastewater and drinking water at the molecule level. Both types of monitoring techniques are increasingly used in studying DOM in wastewater and drinking water treatments. They provide valuable insights into the variability of DOM composition in wastewater and drinking water. The complexity and diversity of DOM highlight the challenges for effective water treatments. Different effects of various treatment processes on DOM are also assessed, which indicates that the information on DOM composition and its removal is key to optimize the treatment processes. Considering notable progress in advanced treatment processes and novel materials for removing DOM, it is important to continuously utilize these powerful monitoring tools for assessing the responses of different DOM constituents to a series of treatment processes, which can achieve an effective removal of DOM and the quality of treated water.

Singlet oxygen production abilities of oxidated aromatic compounds in natural water

Singlet oxygen (¹O₂) is well known to be formed through energy transfer from excited state organic matters to O₂, playing an important role in the transformations of contaminants. However, the contribution of small oxidated aromatic compounds (OACs) to the production of ¹O₂ in surface water is unclear. In this study, 28 OACs were selected to investigate the correlations between their photochemical production abilities of ¹O₂ and molecular structures. Our results showed that the steady-state concentrations and quantum yields of ¹O₂ (Φ_1O2) generated by OACs were in the range of 7.0 × 10^-14-1.4 × 10^-12 M and 2.2 × 10^-4-4.7 × 10^-2, respectively, indicating that the photochemical production abilities of ¹O₂ by OACs varied greatly with types and positions of functional groups on the molecule. More importantly, the observed photochemical production of ¹O₂ was most notable in cases of molecules containing -OCH₃ group and benzoquinone. A good quantitative structure-property relationship model was established between ¹O₂ producing ability, energy of the lowest unoccupied molecular orbitals (E_LUMO) and the most positive net charge of hydrogen atoms (qH⁺) of OACs. In addition, the role of ¹O₂ produced by 2, 6-dimethoxy-1, 4-benzoquinone, the OAC with the highest Φ_1O2, in the photodegradation of organic contaminants was validated by the enhanced degradation of atorvastatin under simulated sunlight, suggesting that OACs ubiquitously existed in surface water may greatly affect the fate and ecological risks of organic contaminants.

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.

Phototransformation of biochar-derived dissolved organic matter and the effects on photodegradation of imidacloprid in aqueous solution under ultraviolet light

Dissolved organic matter (DOM) strongly influences the photodegradation of organic pollutants, varying depending on the structure of DOM. With the wide application of biochar, increasing amounts of DOM is released from biochar to the environment, which has different structural characteristics compared to natural DOM. In this study, DOM was derived from maize straw (MS) and pig manure (PM) and biochars by pyrolyzing MS and PM at 300 °C and 500 °C and the optical characteristics of DOM before and after phototransformation were explored via ultraviolet-visible spectroscopy and excitation-emission matrix fluorescence. Photodegradation of an insecticide, imidacloprid (IMI) in the presence of DOM was examined. The results showed that DOM derived from biochar obtained by pyrolyzing MS and PM mainly contained two identified fluorescent components and high pyrolysis temperature (500 °C) was associated with low molecular weight, small light-screening effects and great aromaticity of the DOM. After exposure to UV light, the aromaticity and molecular weight of the DOM declined due to phototransformation. Significant enhancement was observed in IMI photodegradation in the presence of biochar-derived DOM, and the enhancement was the greatest with DOM derived from pig manure biochar pyrolyzed at 500 °C. In addition to the light shielding effect, the ¹O₂ generated from DOM played an important role in the phototransformation of IMI and DOM. The loss of the nitro group and oxidation at the imidazolidine ring were the main photodegradation pathways for IMI. This study expands our understanding of the fate of biochar-derived DOM and its effects on the fate of coexisting organic pollutants.

Triplet Photochemistry of Dissolved Black Carbon and Its Effects on the Photochemical Formation of Reactive Oxygen Species

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 (³DBC*) and ¹O₂ quantum yields (Φ_¹O2) 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_¹O2 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 ³DBC* were approximately 12-fold greater than those of low-energy ³NOM*. The differences in the f_TMP and Φ_¹O2 trends among the DBCs indicated that the ³DBC* 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.

Development of a quantitative structure-activity relationship model for mechanistic interpretation and quantum yield prediction of singlet oxygen generation from dissolved organic matter

Singlet oxygen (¹O₂) is capable of degrading organic contaminants and inducing cell damage and inactivation of viruses. It is mainly generated through the interaction of dissolved oxygen with excited triplet states of dissolved organic matter (DOM) in natural waters. The present study aims at revealing the underlying mechanism of ¹O₂ generation and providing a potential tool for predicting the quantum yield of ¹O₂ (Φ_1O2) generation from DOM by constructing a quantitative structure-activity relationship (QSAR) model. The determined Φ_1O2 values for the selected DOM-analogs range from (0.54 ± 0.23) × 10^-2 to (62.03 ± 2.97) × 10^-2. A QSAR model was constructed and was proved to have satisfactory goodness-of-fit and robustness. The QSAR model was successfully used to predict the Φ_1O2 of Suwannee River fulvic acid. Mechanistic interpretation of the descriptors in the model showed that hydrophobicity, molecular complexity and the presence of carbonyl groups in DOM play crucial roles in the generation of ¹O₂ from DOM. The presence of other heteroatoms besides O, such as N and S, also affects the generation of ¹O₂. The results of this study provide valuable insights into the generation of ¹O₂ from DOM in sunlit natural waters.

Impact of the Three Gorges Dam on the Quality of Riverine Dissolved Organic Matter

The Three Gorges Dam (TGD) is the largest hydropower facility in the world, influencing the riverine hydrology and mass flux in the Yangtze River. Little is known about its impact on the riverine dissolved organic matter (DOM) quality. In this work, the water quality and DOM quality for water samples collected from the upstream and downstream sites of TGD were investigated. The presence of TGD significantly affects the quantity and quality of DOM but has no pronounced effect on nutrient concentrations. Upstream DOM had higher concentration but lower average molecular weight and aromaticity than the downstream DOM. The biological processes in the dam reservoir contribute significantly to upstream DOM. In the downstream sites, terrestrial DOM input raises the average molecular weight and aromaticity of the overall DOM pool. These results suggest that TGD will influence not only the mass flux of organic carbon but also its quality and lability, which has both environmental and ecological significance.

Dissolved Organic Matter Singlet Oxygen Quantum Yields: Evaluation Using Time-Resolved Singlet Oxygen Phosphorescence

Singlet oxygen (¹O₂) generation quantum yields from chromophoric dissolved organic matter (CDOM) have been reported for many samples over the past 4 decades. Yet even for standardized isolates such as those from the International Humic Substance Society (IHSS), wide-ranging values exist in the literature. In this manuscript, time-resolved ¹O₂ phosphorescence was used to determine the ¹O₂ quantum yields (Φ_Δ) of a variety of dissolved organic matter (DOM) isolates and natural waters. In general, the ¹O₂ quantum yield values in this study are in the middle, although below the median of the range of past reported values (e.g., for Suwannee River Natural Organic Matter IHSS isolate: 1.8% vs 0.23-2.89%). Notably, hydrophobic neutral fractions of DOM isolates were found to possess the highest ¹O₂ quantum yields, an interesting result given that these fractions are not retained in typical humic and fulvic acid isolation procedures that use XAD resins. The excitation wavelength dependence of ¹O₂ generation from CDOM was also examined, and an approximate linear decrease with longer excitation wavelength was observed. This work advances the understanding of CDOM photoprocesses, especially in relation to wavelength-dependent ¹O₂ production, which is valuable for assessing real-world environmental behavior.

Predictive role of spectral slope ratio towards 17α-ethynylestradiol photodegradation sensitized by humic acids

Humic acids (HAs) have been shown to dominate the photodegradation of steroid estrogens in natural waters. Nevertheless, how the photosensitizing ability of HAs relates to their structural and optical characteristics remains largely unknown. In this study, 17α-ethynylestradiol (EE2) was selected as a model compound to study to what extent easily-measurable characteristics of HAs might be used to predict their photosensitization potency. HAs were extracted from sediments of two different sources, and then subjected to structural and optical properties characterization using elemental analyzer, UV-vis spectroscopy and fluorescence spectroscopy. Photochemical experiments show that the HAs from the two sources can effectively meditate EE2 photodegradation. Although with drastically different structural and optical properties, the photosensitizing ability of these HAs towards EE2 can be well described by simple linear regressions using a spectroscopic index, the spectral slope ratio (S_R). This optical indicator is correlated with various physicochemical properties of HAs, including the molecular weight, lignin content, charge-transfer interaction potential, photobleaching extent and sources. No universal prediction model could be established for predicting EE2 photodegradation kinetics on the basis of S_R, but in specific waters S_R could be a powerful indictor for predicting the EE2 photodegradation sensitized by HAs.

Dissolved Mineral Ash Generated by Vegetation Fire Is Photoactive under the Solar Spectrum

Vegetation fire generates vast amounts of mineral ash annually that can be readily mobilized by water or wind erosion. Little is known about the photoactivity of dissolved mineral ash in aquatic systems and its ability to mediate redox reactions of environmental pollutants. This study reports that dissolved mineral ash derived from pyrolysis of biomass is photoactive under simulated sunlight, generating reactive oxygen species. It can mediate the photoreduction of hexavalent chromium (Cr(VI)) in the presence of electron donors; for example, phenols and dissolved organic matter, at pH 4.7. The reaction kinetics followed the Langmuir-Hinshelwood model, suggesting a heterogeneous photocatalytic reaction. The enhancement of reduction efficiency was linearly correlated with the one-electron reduction potential of phenols. The synergy between dissolved mineral ash and phenols is attributed to the inhibition of electron-hole recombination. The reduction rate decreases with increasing solution pH, owing to the decreased reduction potential and surface adsorption of Cr(VI). The silicon and silicon carbide components are most likely responsible for the photocatalytic activity of dissolved mineral ash. Our results suggest that dissolved mineral ash is a natural photocatalyst that can mediate redox reactions of pollutants in sunlit aquatic systems, playing an overlooked role in natural attenuation and aquatic photochemistry.

Dissolved Black Carbon as an Efficient Sensitizer in the Photochemical Transformation of 17β-Estradiol in Aqueous Solution

Dissolved black carbon (DBC) is an important component of the dissolved organic matter (DOM) pool. Nonetheless, little is known about its role in the photochemical processes of organic contaminants. This study investigated the effect of DBC on the phototransformation of 17β-estradiol in aqueous solutions under simulated sunlight. Four well-studied dissolved humic substances (DHS) were included as comparisons. DBC acted as a very effective sensitizer to facilitate the phototransformation of 17β-estradiol. The apparent quantum yield for 17β-estradiol phototransformation mediated by DBC was approximately six times higher than that by DHS at the same carbon concentration. Quenching experiments suggested that direct reaction with triplet-excited state DBC (³DBC*) was the predominant pathway of 17β-estradiol phototransformation. The higher mediation efficiency of DBC than DHS is likely due to the higher contents of aromatic groups and smaller molecular sizes, which facilitated the generation of ³DBC*. The apparent quantum yield of triplet-excited states production for DBC was 4-8 times higher than that for DHS. The results suggest that ³DBC* may have a considerable contribution to the overall photoreactivity of triplet-excited state DOM in aquatic systems. Our findings also imply that DBC can play an important role in the phototransformation of organic contaminants in the environments.