Direct Photolysis Rate Research Articles

Elucidating the photodegradation mechanism of octylisothiazolinone and dichlorooctylisothiazolinone in surface water: An in-depth comprehensive analysis

Octylisothiazolinone (OIT) and Dichlorooctylisothiazolinone (DCOIT), widely used antibacterial agents in coatings, have seen a sharp increase in use in response to the Coronavirus disease 2019 (Covid-19) pandemic, ultimately leading to their increase in the aquatic environment. However, their photodegradation process in surface water is still unclear. The purpose of this study is to investigate the photodegradation kinetics and mechanisms of OIT and DCOIT in natural water environments. Under simulated solar irradiation, they undergo direct photolysis in both natural freshwater and seawater mainly via their excited singlet states, while no self-sensitization photolysis was observed. The direct photolysis rate constants of OIT and DCOIT were 1.19 ± 0.07 and 0.57 ± 0.03 h−1, respectively. In addition, dissolved organic matter (DOM), NO3− and Cl− in natural waters did not contribute significantly to the photodegradation, and the light screening effect of DOM was identified as the main inhibiting factor. The photodegradation half-life of OIT was estimated to be 0.66 to 1.69 days, while the half-life of DCOIT was as high as 20.9 days during winter in surface water at 30°N latitude. Ring opening of the N-S bond and covalent bond breaking between CN are the main pathways for the photodegradation of OIT and DCOIT, which is verified by density-functional theory calculations. Ecological Structure Activity Relationships (ECOSAR) results indicate that OIT and DCOIT have “Very Toxic” biological toxicity, and the acute toxicity of their products is significantly reduced. It is noteworthy that the toxicity of the products of DCOIT is generally higher than that of OIT, and the chronic toxicity of most of the products is still above the “Toxic” level. Therefore, an in-depth understanding of the photodegradation mechanisms of OIT and DCOIT in aqueous environments is crucial for accurately assessing their ecological risks in natural water environments.

Dissolved black carbon mediated photo-transformation of tetrachlorantraniliprole: Kinetics, pathways, and adverse effects of the photoproduct

The combustion of agricultural and forestry residues, along with subsequent runoff, can lead to the production of a significant quantities of dissolved black carbon (DBC), which plays a pivotal role in the photo-transformation of organic pesticides. As a result, the photochemical activity of DBC and its associated photo-transformation mechanisms must elucidate. This study investigated the effect of DBC on tetrachlorantraniliprole (TCTP) photo-transformation, a new type of pesticide independently developed and widely used in China, using photochemical simulation experiments. The results revealed that the prepared DBC had a smaller molecular weight and higher aromaticity, generating 3DBC*, ·OH, and 1O2, with apparent quantum yields of 4.75 × 10−2, 4.35 × 10−5, and 4.01 × 10−2, respectively. 3DBC* was the dominant intermediate reactive responsible for accelerating TCTP photolysis by determining its direct photolysis rate constant as 0.278 min−1. The degradation pathway of TCTP mainly involved nucleophilic substitution, dechlorination, and cleavage of the C–N bond. Notably, TCTP can photo-generate the cross-condensation products only in the presence of DBC. Ecotoxicity and organ-specific toxicity analysis demonstrated reduced environmental risks compared to TCTP for both direct photoproducts, as well as those mediated by DBC. These findings highlighted how DBC altered the photo-transformation pathway leading to decreased environmental risk-associated amide pesticides.

Enhanced Direct Photolysis of Organic Micropollutants by Far-UVC Light at 222 nm from KrCl* Excilamps.

Krypton chloride (KrCl*) excilamps emitting at far-UVC 222 nm represent a promising technology for microbial disinfection and advanced oxidation of organic micropollutants (OMPs) in water treatment. However, direct photolysis rates and photochemical properties at 222 nm are largely unknown for common OMPs. In this study, we evaluated photolysis for 46 OMPs by a KrCl* excilamp and compared it with a low-pressure mercury UV lamp. Generally, OMP photolysis was greatly enhanced at 222 nm with fluence rate-normalized rate constants of 0.2-21.6 cm2·μEinstein-1, regardless of whether they feature higher or lower absorbance at 222 nm than at 254 nm. The photolysis rate constants and quantum yields were 10-100 and 1.1-47 times higher, respectively, than those at 254 nm for most OMPs. The enhanced photolysis at 222 nm was mainly caused by strong light absorbance for non-nitrogenous, aniline-like, and triazine OMPs, while notably higher quantum yield (4-47 times of that at 254 nm) occurred for nitrogenous OMPs. At 222 nm, humic acid can inhibit OMP photolysis by light screening and potentially by quenching intermediates, while nitrate/nitrite may contribute more than others to screen light. Overall, KrCl* excilamps are promising in achieving effective OMP photolysis and merit further research.

Photolysis mechanism of eleven insecticides under simulated sunlight irradiation: Kinetics, pathway and QSAR

Insecticides are widely used in crop protection against insects and frequently detected in aquatic environment. Photolysis kinetics are directly related with exposure assessment and risk assessment. However, the photolysis mechanism of neonicotinoid insecticides with different structures has not been studied and compared systematically in the literature. In this paper, the photolysis rate constants in water were determined for eleven insecticides under irradiation of simulated sunlight. At the same time, the photolysis mechanism and effect of dissolved organic matter (DOM) on their photolysis were studied. The results showed that photolysis rates of eleven insecticides vary in a large range. The photolysis rates of nitro-substituted neonicotinoids and butenolide insecticide are much faster than that of cyanoimino-substituted neonicotinoids and sulfoximine insecticide. The ROS scavenging activity assays reveal that direct photolysis dominates the degradation of seven insecticides and, on the other hand, self-sensitized photolysis dominates four insecticides. The shading-effect from DOM can reduce the direct photolysis rates, on the other hand, ROSs generated by triplet-state DOM (3DOM*) can also accelerate photolysis of insecticides. According to the photolytic products identified from HPLC-MS, these eleven insecticides have different photolysis pathways. Six insecticides are degraded from the removal of nitro group from their parent compounds and four insecticides are degraded through ·OH reaction or singlet oxygen (1O2) reaction. QSAR (quantitative structure-activity relationship) analysis showed that photolysis rate was directly related to the energy gap between the highest occupied molecular orbital to the lowest unfilled molecular orbital (Egap = ELUMO−EHOMO) and dipole moment (δ). These two descriptors reflect the chemical stability and reactivity of insecticides. The pathways developed from identified products and the molecular descriptors of QSAR models can well verify the photolysis mechanisms of eleven insecticides.

Photodegradation of enestroburin in water by simulated sunlight irradiation: Kinetics, isomerization, transformation products identification and toxicity assessment

Enestroburin is the first strobilurin fungicide developed by China and has been widely used to control fungal disease for 15 years. Investigation of its photolytic behaviour is essential for the comprehensive evaluation of its ecological risk. The effects of solution pH, humic acid (HA) and Fe(III) ions on photolysis were studied. The direct photolysis rates of enestroburin in the acidic solution (pH = 4) was faster than that in the basic (pH = 7) or neutral condition (pH = 9). HA and Fe3+ ions inhibited photolysis by the light screening effect. The photolysis of enestroburin was very fast due to the generation of photo-isomers. Seven isomeric products of enestroburin were observed using SFC-MS/MS, and the reaction mechanism for photo-induced isomers was proposed. The reaction occurred on three double bonds, including tautomerism of enol ether and oxonium and the triplet energy transfer of the CC and CN double bond. 12 transformation products (TPs) were identified by screening suspect compounds and non-target compounds, and one product (M-381) was synthesized for confirmation and quantification. A probable transformation mechanism was suggested based on the identified TPs and DFT calculations. The main transformation reactions included hydration, hydrolysis, oxidation, reduction and decarboxylation. Finally, the toxicities of the identified TPs and parent compound to aquatic organisms were predicted using ECOSAR software, and the toxicities of enestroburin and M-381 to daphnia magna were tested in the laboratory. The toxicity classification proposed by ECOSAR is reliable to a certain extent. Enestroburin and 2 TPs (M-313 and M-327) were classified as “very toxic”, which may pose a potential threat to aquatic ecosystems.

A numerical modeling framework for simulating the key in-stream fate processes of PAH decay in Muskeg River Watershed, Alberta, Canada

Most previous water quality studies oversimplified in-stream processes for modeling the fate and transport of critical organic contaminants, such as Polycyclic Aromatic Hydrocarbons (PAHs). Taking four selected PAHs as representative organic contaminants, we developed a numerical modeling framework using a Water Quality Analysis Simulation Program 8 (WASP8) and a well-established watershed model, i.e., Soil and Water Assessment Tool (SWAT) to: (1) address the influence of in-stream processes, including direct photolysis, volatilization, partitioning of PAHs to suspended solids, and DOC complexation processes on PAH concentrations; and (2) establish relationships between spatiotemporal distribution of environmental factors (e.g., ice coverage, water temperature, wind, and light attenuation), in-stream processes, and PAH concentrations at a watershed scale. Using calibrated SWAT and WASP8 models, we evaluated the impacts of seasonal changes in environmental factors on in-stream processes in the Muskeg River watershed, which is part of the Athabasca Oil Sands Region (AOSR), the third-largest crude oil reserves of the world in western Canada. Among four selected PAHs, simulation results suggest that Naphthalene primarily decay in the water through volatilization or direct photolysis. For Phenanthrene, Pyrene, and Chrysene, DOC complexation, volatilization, and direct photolysis all contribute to their decay in the water, with a strong dependence on seasonality. Model simulations indicated that direct photolysis and volatilization rates are meager in cold seasons, mainly due to low river temperature and ice coverage. However, these processes gradually resume when entering the warm season. In summary, the model simulation results suggest that critical in-stream processes such as direct photolysis, volatilization, and partitioning and their relationship with environmental factors should be considered when simulating the fate and transport of organic contaminants in the river systems. Our results also reveal that the relationship between environmental factors and fate processes affecting PAH concentrations can vary across a watershed and in different seasons.

UV direct photolysis of amisulbrom in buffer solutions: Kinetics, quantum yield, products identification, DFT, mechanism and predict toxicity

Direct photolysis is one of the most important abiotic transformations for pesticides in the aquatic environment. In this study, direct photolysis of amisulbrom was studied under low and medium pressure mercury lamps at pH 4.0, 7.0 and 9.0. The result shows that the direct photolysis rates of amisulbrom in the basic solution are 2.5–2.8 times faster than in the acidic (pH 4.0) or neutral (pH 7.0) solutions under low pressure mercury lamp, while the direct photolysis rate of amisulbrom is similar in different buffer solutions under medium pressure mercury lamp. The quantum yield of amisulbrom is determined in the wavelength range of 190–400 nm. Eight main photolysis products were identified using ultra-performance liquid chromatography quadrupole time-of-flight tandem mass spectrometry (UPLC-Q-TOF-MS/MS). Based on the identified photolysis products, the density functional theory (DFT) analysis, photolysis profiles and the possible photolysis pathway of amisulbrom are proposed. The energy gap of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for amisulbrom is 4.76 eV and it corresponds to a photon energy of 260.5 nm. The quantitative structure–activity relationship (QSAR) model is used to predict the toxicity of amisulbrom and its TPs. The Results show that TP-I (3-((6-fluoro-2-methyl-1H-indol-1-yl)sulfonyl)-1H-1,2,4-triazol-1-ium), TP-II (3-bromo- 6-fluoro-2-methyl-1H-indole), TP-III (1-((1H-1,2,4-triazol-3-yl)sulfonyl)-3-bromo-6-fluoro- 2-methyl-1H-indole) and TP-V (6-fluoro-2-methyl-1H-indole) are more toxic than their parent.

UV direct photolysis of halogenated disinfection byproducts: Experimental study and QSAR modeling

UV direct photolysis has been used as a promising process to remove halogenated disinfection byproducts (DBPs) generated in water. In this study, experimental studies and modeling approaches were applied to investigate the UV direct photolysis rate constants for 40 kinds of halogenated DBPs. The fluence-based pseudo-first-order rate constants for the removal of halogenated DBPs under UV photolysis spanned more than 2 orders of magnitude, with a range of (0.23–29.84) × 10−4 cm2 mJ−1. DBPs with higher number of halogenated substituents featured higher photolysis rate constants. The degradation efficiencies of DBPs were also affected by the species of halogen substituents, which followed the trend of iodo- > bromo- > chloro- DBPs. A quantitative structure-activity relationship (QSAR) model was established on the basis of the observed degradation rate constant values, which contained a quantum-chemical descriptor (ELUMO–EHOMO) and a molecular descriptor (Eta_C). The calculated parameters of the developed model indicated its good robustness and high reliability. The developed QSAR model can predict the degradation rate constants for DBPs within factors of 1/3 to 3. The model was validated using application domain and visualized in a Williams plot. The selected descriptors for QSAR model can explain the reaction mechanism for UV direct photolysis.

Degradation of imipramine by vacuum ultraviolet (VUV) system: Influencing parameters, mechanisms, and variation of acute toxicity

Degradation of imipramine (IMI) in the VUV system (VUV185 + UV254) was firstly evaluated in this study. Both HO• oxidation and UV254 direct photolysis accounted for IMI degradation. The quantum yields of UV254 direct photolysis of deprotonated and protonated IMI were 1.31×10−2 and 3.31×10−3, respectively, resulting in the higher degradation efficiency of IMI at basic condition. Increasing the initial IMI concentration lowered the degradation efficiency of IMI. While elevating reaction temperature significantly improved IMI degradation efficiency through the promotion of both the quantum yields of HO• and the UV254 direct photolysis rate. The apparent activation energy was calculated to be about 26.6 kJ mol−1. Negative-linear relationships between the kobs of IMI degradation and the concentrations of HCO3−/CO32−, NOM and Cl− were obtained. The degradation pathways were proposed that cleavage of side chain and hydroxylation of iminodibenzyl and methyl groups were considered as the initial steps for IMI degradation in the VUV system. Although some high toxic intermediate products would be produced, they can be further transformed to other lower toxic products. The good degradation efficiency of IMI under realistic water matrices further suggests that the VUV system would be a good method to degrade IMI in aquatic environment.

Effects of Ozone on the Photochemical and Photophysical Properties of Dissolved Organic Matter.

This study focused on the effects of ozonation on the photochemical and photophysical properties of dissolved organic matter (DOM). Upon ozonation, a decrease in DOM absorbance was observed in parallel with an increase in singlet oxygen (1O2) and fluorescence quantum yields (Φ1O2 and ΦF). The increase in Φ1O2 was attributed to the formation of quinone-like moieties during ozonation of the phenolic moieties of DOM, while the increase in ΦF can be explained by a significant decrease in the internal conversion rate of the first excited singlet state of the DOM (1DOM*). It is a consequence of an increase in the average energy of the first electronic transition (S1 → S0) that was assessed using the wavelength of maximum fluorescence emission (λF,max). Furthermore, ozonation did not affect the ratio of the apparent steady-state concentrations of excited triplet DOM (3DOM*) and 1O2, indicating that ozonation does not affect the efficiency of 1O2 production from 3DOM*. The consequences of these changes for the phototransformation rates of micropollutants in surface waters were examined using photochemical model calculations. The decrease in DOM absorbance caused by ozonation leads to an enhancement of direct photolysis rates due to the increased transparency of the water. Rates of indirect photooxidation induced by 1O2 and 3DOM* slightly decrease after ozonation.

Photochemical degradation of halogenated estrogens under natural solar irradiance.

Halogenated estrogens are thought to be moderately potent endocrine-disrupting compounds that are formed during chlorine-based wastewater disinfection processes and may represent a significant fraction of the total amount of estrogen delivered from wastewater treatment plants to receiving waters. Yet we lack key information about the photochemical degradation of halogenated estrogens, a process that has important implications for UV-based wastewater treatment and environmental fate modeling. To better understand halogenated estrogen degradation in aquatic environments, we studied the direct photolysis of 17β-estradiol (E2), 2-bromo-17β-estradiol (monoBrE2), 2,4-dibromo-17β-estradiol (diBrE2), and 2,4-dichloro-17β-estradiol (diClE2) as well as the indirect photolysis of diBrE2 under natural solar irradiance. We found that direct photolysis rate constants increased with halogenation as pKa values decreased and molar absorptivity spectra shifted toward higher wavelengths. Compared to E2, quantum yields were threefold larger for monoBrE2, but 15-32% smaller for the dihalogenated forms. The rate of diBrE2 (pKa ∼ 7.5) photolysis was strongly influenced by pH. At pH 7, diBrE2 degraded on minute time scales due to the large red-shifted molar absorptivity values and greater quantum yields of the phenolate form. Degradation rates were only slightly different in the presence of Suwannee River Humic Acid (5 mg L-1), and quenching experiments pointed to excited triplet state dissolved organic matter (3DOM*) as the dominant reactive intermediate responsible for the indirect photolysis of diBrE2. Overall, our data suggest that halogenated estrogens are particularly susceptible to photochemical degradation at environmentally relevant pH values.

Base-catalyzed hydrolysis and speciation-dependent photolysis of two cephalosporin antibiotics, ceftiofur and cefapirin

Lately, special attention has been given to veterinary cephalosporin antibiotics due to their broad activity spectrum and significant consumption. Indeed, the determination of hydrolytic and photolytic kinetics provides a better comprehension of the undesired persistence of cephalosporins in aqueous matrices. In this work, the two widely used veterinary antibiotics ceftiofur (CEF) and cefapirin (CEPA) showed high instability under alkaline conditions, degrading in few minutes at pH > 11. In buffered solutions at neutral pH and natural temperature (T = 22 ± 1 °C), both drugs presented moderate stability (t½ = 3 d, CEPA and 1.4 d, CEF). Our study also demonstrated that CEPA and CEF speciation did not significantly influence the direct photolysis rates. Using a simulated water disinfection set-up (λ = 254 nm), all ionic species of CEF and CEPA presented fast and similar pseudo-first order degradation rates, kapp 0.0095 ± 0.0004 and 0.0092 ± 0.001 cm2 mJ-1, respectively. Furthermore, using surface water in hydrolysis experiments, CEF demonstrated significant matrix-dependent stability with a half-life (t½ = 14.7 d) tenfold higher than in buffered solutions. In contrast, CEPA presented a very similar hydrolysis rate in river water (t½ = 4.2 d) and a subtle faster photo-degradation rate in this same matrix (kapp 0.0128 ± 0.001 cm2 mJ-1), highlighting the importance of disinfection radiation for cephalosporin depletion in aqueous environments.

A novel method for photo-oxidative degradation of diatrizoate in water via electromagnetic induction electrodeless lamp

In this study, an electromagnetic induction electrodeless lamp (EIEL) was first introduced into UV advanced oxidation processes (AOPs) for photodegradation of Diatrizoate (DTZ), which was the most persistent iodinated X-ray contrast medium (ICM), and traditional Hg lamps were taken as references. Direct photolysis rate of DTZ under EIEL irradiation was 1.34 times as that under Hg irradiation, but the electric energy consumption was 0.87 times. In this sense, the combination of EIEL and oxidants (O2, H2O2 and S2O82−(PS)) was further investigated. The remarkably increased photodegradation rates were observed in UV/PS system due to primary contribution rate of SO4− (62.5%) based on the results of radical concentrations and second-order rate constants of DTZ with SO4− and OH. Inorganic ions influencing the photodegradation process were investigated. The effect of natural organic materials (NOMs) in UV/PS system was studied based on contribution ratios of light screening effect and quenching. Transformation mechanisms of DTZ in UV/PS system included deiodination, intramolecular cyclization, decarboxylation, deacetylation and deamination, which were further confirmed by frontier electron density calculations. The study indicated that UV/PS with EIEL irradiation has the potential to remove pharmaceuticals in contaminated aquatic environments.

Development of Predictive Models for the Degradation of Halogenated Disinfection Byproducts during the UV/H2O2 Advanced Oxidation Process.

Previous research has demonstrated that the reverse osmosis and advanced oxidation processes (AOPs) used to purify municipal wastewater to potable quality have difficulty removing low molecular weight halogenated disinfection byproducts (DBPs) and industrial chemicals. Because of the wide range of chemical characteristics of these DBPs, this study developed methods to predict their degradation within the UV/H2O2 AOP via UV direct photolysis and hydroxyl radical (•OH) reaction, so that DBPs most likely to pass through the AOP could be predicted. Among 26 trihalomethanes, haloacetonitriles, haloacetaldehydes, halonitromethanes and haloacetamides, direct photolysis rate constants (254 nm) varied by ∼3 orders of magnitude, with rate constants increasing with Br and I substitution. Quantum yields varied little (0.12-0.59 mol/Einstein), such that rate constants were driven by the orders of magnitude variation in molar extinction coefficients. Quantum chemical calculations indicated a strong correlation between molar extinction coefficients and decreasing energy gaps between the highest occupied and lowest unoccupied orbitals (i.e., ELUMO-EHOMO). Rate constants for 37 trihalomethanes, haloacetonitriles, haloacetaldehydes, halonitromethanes, haloacetamides, and haloacetic acids with •OH measured by gamma radiolysis spanned 4 orders of magnitude. Based on these rate constants, a quantitative structure-reactivity relationship model (Group Contribution Method) was developed which predicted •OH rate constants for 5 additional halogenated compounds within a factor of 2. A kinetics model combining the molar extinction coefficients, quantum yields and •OH rate constants predicted experimental DBP loss in a lab-scale UV/H2O2 AOP well. Highlighting the difficulty associated with degrading these DBPs, at the 500-1000 mJ/cm2 UV fluence applied in potable reuse trains, 50% removal would be achieved generally only for compounds with several -Br or -I substituents, mostly due to higher molar extinction coefficients.

Direct photolysis of oxytetracycline: Influence of initial concentration, pH and temperature

Abstract Oxytetracycline (OTC) commonly undergoes direct photolysis as the main elimination pathway in surface waters. The photolysis of OTC was studied as a function of initial concentration, pH and temperature. The experiments were performed in the absence of oxygen to avoid oxygen induced indirect photolysis. The photolysis rate decreased when increasing the initial OTC concentration, and was accelerated as raising the temperature. pH was the key factor controlling the photolysis rate. By measuring the UV–vis absorption spectra of OTC under various pH conditions and fitting them with global analysis, four OTC species H3L＋, H2L±, HL−, and L2− (L represents OTC) were decomposed, meanwhile, the acid dissociation constant of OTC and the molar absorption coefficient of each species were resolved. Because apparent photolysis rate constants under varied light source conditions are often incomparable, therefore the direct photolysis quantum yield of OTC species were calculated by a mathematical approach which based on the molar absorption coefficients, the species composition and apparent kinetic rates of OTC under experimental pH conditions. The result shows the photolysis quantum yield of OTC followed the order L2－ > H2L± > HL－ > H3L＋. The direct photolysis rates of OTC were tightly correlated to the energy dissipation processes at the excited states, e.g., the excited states inter-/intramolecular proton transfer (ESIPT), and the molecular conformation transfer induced internal energy transmission between ground state and excited state. Fluorescence emission was not an important energy dissipation process. The results of this study provide the useful parameters for predicting the environmental fate of OTC in surface waters. And the approach for calculating the photolysis quantum yield is also available for other antibiotics with multiple pKa values.

Direct Photolysis Rates and Transformation Pathways of the Lampricides TFM and Niclosamide in Simulated Sunlight.

The lampricides 3-trifluoromethyl-4-nitrophenol (TFM) and 2',5-dichloro-4'-nitrosalicylanilide (niclosamide) are directly added to many tributaries of the Great Lakes that harbor the invasive parasitic sea lamprey. Despite their long history of use, the fate of lampricides is not well understood. This study evaluates the rate and pathway of direct photodegradation of both lampricides under simulated sunlight. The estimated half-lives of TFM range from 16.6 ± 0.2 h (pH 9) to 32.9 ± 1.0 h (pH 6), while the half-lives of niclosamide range from 8.88 ± 0.52 days (pH 6) to 382 ± 83 days (pH 9) assuming continuous irradiation over a water depth of 55 cm. Both compounds degrade to form a series of aromatic intermediates, simple organic acids, ring cleavage products, and inorganic ions. Experimental data were used to construct a kinetic model which demonstrates that the aromatic products of TFM undergo rapid photolysis and emphasizes that niclosamide degradation is the rate-limiting step to dehalogenation and mineralization of the lampricide. This study demonstrates that TFM photodegradation is likely to occur on the time scale of lampricide applications (2-5 days), while niclosamide, the less selective lampricide, will undergo minimal direct photodegradation during its passage to the Great Lakes.

Kinetics and modeling of sulfonamide antibiotic degradation in wastewater and human urine by UV/H2O2 and UV/PDS

Sulfonamide antibiotics have been frequently detected in the aquatic environment and are of emerging concern due to their adverse bio-effect and potential of inducing antibiotic resistance. This study investigated the degradation kinetics of sulfonamide antibiotics in synthetic wastewater and hydrolyzed human urine by low pressure (LP) UV, UV/H2O2 and UV/peroxydisulfate (PDS). Direct photolysis rates of sulfonamide antibiotics varied and depended on the structures. Sulfonamides with a five-membered heterocyclic group underwent faster direct photolysis. For indirect photolysis processes, second-order rate constants of sulfonamide antibiotics with hydroxyl radical, sulfate radical and carbonate radical were determined, which were (6.21–9.26) × 109, (0.77–16.1) × 1010 and (1.25–8.71) × 108 M−1 s−1, respectively. A dynamic model was applied and successfully predicted the degradation kinetics of sulfonamides in different water matrices. In synthetic wastewater, carbonate radical contributed to approximately 10% of the overall removal, whereas in synthetic hydrolyzed urine, carbonate radical was the dominant reactive species to degrade sulfonamides. Sulfonamide antibiotics were eliminated more efficiently in synthetic hydrolyzed urine than in synthetic wastewater and UV/PDS was more efficient than UV/H2O2 to degrade most sulfonamides. Energy evaluation showed that UV/PDS costs less energy than LPUV and UV/H2O2 under the experimental conditions applied in this study, particularly for sulfonamides whose indirect photolysis overweighed direct photolysis. By varying UV dose and oxidant dose, the UV/H2O2 process can be optimized to achieve higher efficiency than the UV/PDS process in synthetic wastewater.

Comparative evaluation of iodoacids removal by UV/persulfate and UV/H2O2 processes

To develop a cost-effective method for post-formation mitigation of iodinated disinfection by-products, degradation of iodoacids by UV, UV/PS (persulfate), and UV/H2O2 was extensively investigated in this study. UV direct photolysis of 4 iodoacids followed first-order kinetics with rate constants in the range of 2.43 × 10−4−3.02 × 10−3 cm2 kJ−1. The derived quantum yields (Ф254) of the 4 iodoacids range from 0.13 to 0.34, respectively. A quantitative structure-activity relationship (QSAR) model was subsequently established and applied to predict the direct photolysis rates of 6 other structurally similar iodoacids whose standards are commercially unavailable. At a UV dose of 140 mJ cm−2 which is typically applied for disinfection of drinking water, the removal percentages of 4 iodoacids were only between 3.35% and 34.7%. Thus, ICH2CO2H (IAA), the most photo-recalcitrant species, was selected as the target compound for removal in the UV/PS and UV/H2O2 processes. The IAA degradation rates decreased with increasing pH from 3 to 11 in both processes. Humic acid (HA) and HCO3− had inhibitory effects on IAA degradation in both processes. Cl− adversely affected the IAA degradation in the UV/PS process but had no effect in the UV/H2O2 process. Generally, in the deionized (DI) water, surface water, treated drinking water, and secondary effluent, UV/PS process is more effective than UV/H2O2 process for IAA removal, based on the same molar ratio of oxidant: IAA. SO4− generated in the UV/PS process yields a greater mineralization of IAA than HO in the UV/H2O2 process. IO3− was the predominant end-product in the UV/PS process, while I− was the major end-product in the UV/H2O2 process. The respective contributions of UV, HO, and SO4− for IAA removal in the UV/PS process were 7.8%, 14.7%, and 77.5%, respectively, at a specific condition (1.5 μM IAA, 60 μM oxidant, and pH 7). Compared to UV/H2O2 process, UV/PS was also observed as more cost-effective process based on the electrical energy per order (EE/O) and chemical cost.

Natural solar photolysis of total organic chlorine, bromine and iodine in water

Municipal wastewater has been increasingly used to augment drinking water supplies due to the growing water scarcity. Wastewater-derived disinfection byproducts (DBPs) may negatively affect the aquatic ecosystems and human health of downstream communities during water reuse. The objective of this research was to determine the degradation kinetics of total organic chlorine (TOCl), bromine (TOBr) and iodine (TOI) in water by natural sunlight irradiation. Outdoor solar photolysis experiments were performed to investigate photolytic degradation of the total organic halogen (TOX) formed by fulvic acid and real water and wastewater samples. The results showed that TOX degradation by sunlight irradiation followed the first-order kinetics with half-lives in the range of 2.6–10.7 h for different TOX compounds produced by fulvic acid. The TOX degradation rates were generally in the order of TOI > TOBr ≅ TOCl(NH2Cl) > TOCl(Cl2). High molecular weight TOX was more susceptible to solar photolysis than corresponding low molecular weight halogenated compounds. The nitrate and sulfite induced indirect TOX photolysis rates were less than 50% of the direct photolysis rates under the conditions of this study. Fulvic acid and turbidity in water reduced TOX photodegradation. These results contribute to a better understanding of the fate of chlorinated, brominated and iodinated DBPs in surface waters.

Can activated sludge treatments and advanced oxidation processes remove organophosphorus flame retardants?

This study aims to determine the occurrence of 10 OPFRs (including chlorinated, nonchlorinated alkyl and aryl compounds) in influent, effluent wastewaters and partitioning into sludge of 5 wastewater treatment plants (WWTP) in Catalonia (Spain). All target OPFRs were detected in the WWTPs influents, and the total concentration ranged from 3.67µgL−1 to 150µgL−1. During activated sludge treatment, most OPFRs were accumulated in the sludge at concentrations from 35.3 to 9980ngg−1dw. Chlorinated compounds tris(2-chloroethyl) phosphate (TCEP), tris(2-chloroisopropyl) phosphate (TCIPP) and tris(2,3-dichloropropyl) phosphate (TDCPP) were not removed by the conventional activated sludge treatment and they were released by the effluents at approximately the same inlet concentration. On the contrary, aryl compounds tris(methylphenyl) phosphate (TMPP) and 2-ethylhexyl diphenyl phosphate (EHDP) together with alkyl tris(2-ethylhexyl) phosphate (TEHP) were not detected in any of the effluents. Advanced oxidation processes (UV/H2O2 and O3) were applied to investigate the degradability of recalcitrant OPFRs in WWTP effluents. Those detected in the effluent sample (TCEP, TCIPP, TDCPP, tributyl phosphate (TNBP), tri-iso-butyl phosphate (TIBP) and tris(2-butoxyethyl) phosphate (TBOEP)) had very low direct UV-C photolysis rates. TBOEP, TNBP and TIBP were degraded by UV/H2O2 and O3. Chlorinated compounds TCEP, TDCPP and TCIPP were the most recalcitrant OPFR to the advanced oxidation processes applied. The study provides information on the partitioning and degradability pathways of OPFR within conventional activated sludge WWTPs.