Study of chlorothalonil photodegradation in natural waters and in the presence of humic substances

Photodegradation of chlorothalonil was studied in different natural waters (sea, river and lake) as well as in distilled water under natural and simulated solar irradiation. The effect of dissolved organic matter (DOM) such as humic and fulvic substances on the photodegradation rate of chlorothalonil was also studied in simulated sunlight. The presence of DOM enhanced the photodegradation of chlorothalonil with the exception of seawater. The kinetics were determined through gas chromatography electron capture detection (GC/ECD) and the photodegradation proceeds via pseudo-first-order reaction in all cases. Half-life ranged from 1 to 48 h. In natural and humic water chlorothalonil photodegradation gave rise to two different intermediates compared to distilled water demonstrating that the transformation of chlorothalonil depend on the constitution of the irradiated media and especially from DOM. The byproducts identified by GC/MS techniques were: chloro-1,3-dicyanobenzene, dichloro-1,3-dicyanobenzene, trichloro-1,3-dicyanobenzene and benzamide.     

The role of carbonate radical in limiting the persistence of sulfur-containing chemicals in sunlit natural waters

Carbonate radical (*CO3-) is a selective oxidant that may be important in limiting the persistence of a number of sulfur-containing compounds in sunlit natural waters. Thioanisole, dibenzothiophene (DBT), and fenthion were selected to investigate the degradation pathway initiated by *CO3-; electron-rich sulfur compounds are particularly reactive towards the *CO3-. Using HPLC, GC, GC-MS and LC-MS for structural confirmation, the major photodegradation products of thioanisole and DBT were the corresponding sulfoxides. The sulfoxide products were further oxidized through reaction with *CO3- to the corresponding sulfone derivatives. Fenthion showed a similar pathway with appearance of fenthion sulfoxide as the major product. The proposed mechanism involves abstraction of an electron on sulfur to form a radical cation, which is then oxidized by dissolved oxygen. Each of the sulfur probes were further investigated in a sunlight simulator under varying matrix conditions. The highest rate constants occurred in the *CO3- matrix, and the lowest occurred in a matrix of dissolved organic carbon (DOC) and bicarbonate. In synthetic and natural field water, thioanisole photodegraded faster than under direct photolysis, with half-lives of 75.1 and 85.8 min, respectively. Fenthion photodegraded more rapidly than thioanisole. DBT photodegraded rapidly in a *CO3- matrix with a half-life of 24.8 min, while the half-life of direct photolysis was 350 min. Photodegradation products of each compound were also investigated. Ultimately, *CO3- was found to contribute toward the photodegradation of sulfur-containing compounds in natural waters.     

Photodegradation of sulfonamide antibiotics in simulated and natural sunlight: Implications for their environmental fate

The photochemical fate of seven sulfonamides was investigated in matrices representative of natural water bodies under various light sources. Fundamental photolysis parameters such as molar absorption coefficient, quantum yield (QY) and first-order rate constants were determined. The photolysis decay rate was dependent on the protonation state of the molecule, pH of the water sample and dissolved organic matter. Natural organic matter was the most significant factor in the indirect photolysis of sulfonamides. Half-lives were in the range of minutes at 254 nm to days under natural sunlight. Under natural sunlight, all sulfonamides showed higher removal rates in natural waters implying that indirect photolysis is the predominant mechanism.     

Effect of short-term sunlight irradiation on absorbance spectra of chromophoric organic matter dissolved in coastal and riverine water

Samples of riverine and coastal, filtered (filter pore size 0.2 μm) water were exposed to short-term sunlight irradiation which reduced their absorbance in the UV and visible regions. Absorbance losses in coastal chromophoric dissolved organic matter (CDOM) were up to 10-fold smaller than those in riverine CDOM. Accompanying changes of absorbance spectra shapes (increased slope parameter) were, probably, a result of decrease of the mean molecular size of light absorbing organic matter. The potential of coastal CDOM to photodegradation was smaller and was exhausted during the course of a day-long experiment. A distinctive feature of spectral changes after sunlight exposure was a maximum absorbance decrease which appeared at 300 nm in riverine and at 280 nm in coastal water. That selective absorbance loss has been ascribed in both cases to the disappearance of chromophores of terrestrial origin which, in coastal water, had a lower mean molecular size (due to flocculation and/or prior photodegradation) but, nevertheless, retained their molecular properties.     

Effects of photodegradation of dissolved organic matter on the binding of benzo(a)pyrene

Dissolved organic matter (DOM) in natural waters can bind various organic pollutants, and the affinity of this binding is strongly influenced by the chemical characteristics of the DOM and water pH. This study examined the effects of photochemically induced alteration of the DOM's chemical properties and water pH on the binding of benzo(a)pyrene (BaP). Time- and pH-series of solar-simulated irradiations were performed on a natural water sample and aqueous DOM solutions prepared from aquatic and soil humic substances. The binding affinity of BaP, expressed as a partition coefficient of a compound to DOM, decreased substantially after the DOM samples were irradiated over environmentally relevant radiation doses and pH ranges. The lowering of the pH due to the photoproduction of acidic products often partly offsets the reduction of the binding affinity caused by direct photoalteration of the DOM's chemical structure. The decrease of the binding affinity, after correction for the photoinduced pH change, was positively correlated with the decrease in the molecular weight and the aromaticity of the DOM in the course of irradiation. Increasing O(2) abundance accelerated the decrease of the binding affinity as a result of enhanced DOM photodegradation. Visible light played a more important role in reducing the molecular weight and aromaticity of the DOM than in reducing the content of dissolved organic carbon (DOC) via photoremineralization while the reverse was true for UV radiation, indicating that photochemical reduction of the binding affinity may occur in natural waters at depths greater than UV radiation can reach. A decrease of the affinity of DOM for binding BaP will increase the free dissolved fraction of BaP and thus its availability and toxicity to aquatic organisms. The results from this study may have similar implications for organic pollutants other than BaP.     

水中四环素类化合物在不同光源下的光降解

研究了模拟太阳光和实际室外太阳光下四环素类化合物在水溶液中的光降解。结果表明,四环素类化合物(TCs)在模拟太阳光照射下均能发生直接光降解,且其降解速率随pH不同变化很大。四环素(TC)、土霉素(OTC)、金霉素(CTC)和多西环素(DTC)的光降解速率随溶液pH升高明显加快,而米诺环素(MTC)在水中的光降解速率随溶液pH升高略有下降。天然水体pH范围内,模拟太阳光照下四环素类化合物表观光降解速率常数在0.004～0.026 min-1范围内,降解半衰期在26～136 min范围内。在室外实际太阳光照下,四环素类化合物光降解反应速率与太阳光强成正比例关系。晴朗无云天气下,四环素类化合物降解半衰期在春冬季节较长、夏季节较短;四环素在东江实际水体中表观光降解速率很快,在冬季晴天天气下,其平均半衰期仅为8.2 min。     

Analysis of extinction in ultraviolet and visible spectra of water bodies of the Paraguay and Brazil wetlands

The extinction spectra in ultraviolet and visible radiation were analyzed using filtered and unfiltered water samples obtained in 11 open water bodies in the Neembucù (Paraguay) and Pantanal (Brazil) wetlands. The role of dissolved and suspended matter in the total extinction was analyzed between 260 nm and 700 nm. The chromophoric dissolved organic matter (CDOM) was the major component in extinction of considered ultraviolet radiation (260-400 nm). The differences in CDOM concentrations explained the main pattern of extinction of the ultraviolet radiation in the samples. Nevertheless, differences between the studied water bodies were found also to depend on the rate of photodegradation and photobleaching. The methodology developed in the present study was to distinguish "humic optic waters" according to quantity and quality of dissolved and suspended matter present. In the "humic optic water", the penetration of 10% of incident UV radiation and the photoactive layer are estimated. The influence of particulate matter increases in the total extinction of the wavelengths higher than 400 nm. The integral of the extinction curve of suspended matter in the visible wavelengths (400-700 nm) was found to relate with the total suspended solids and chlorophyll concentrations.     

Fate of pharmaceuticals--photodegradation by simulated solar UV-light

The fate of pharmaceuticals in surface waters under solar irradiation was investigated. Photodegradation of pharmaceuticals caused by sun irradiation may be of major significance in the natural elimination process. Based on a data compilation from the literature, the lipid lowering agent metabolite clofibric acid, the iodinated X-ray contrast media iomeprol, which contribute to the adsorbable organic halogen compounds, and the antiepileptic drug carbamazepine were selected. The irradiation experiments were carried out in batch experiments with simulated UV–sunlight. The photodegradation of the pharmaceuticals showed a pseudo-first-order kinetics. The objective of this investigation was to demonstrate that the extent of photoinduced degradation of pharmaceuticals can vary significantly for the different pharmaceuticals and it strongly depends on the water constituents present in solution. The influences of different initial pharmaceutical concentrations, the presence of other pharmaceuticals like carbamazepine or clofibric acid and the presence of natural organic matter on the photochemical degradation rate of pharmaceuticals in aqueous solutions were investigated. Analyses of the pharmaceuticals and their photodegradation products were carried out by high performance liquid chromatography with diode-array and fluorescence detection.     

Photodegradation of ethylenediaminetetraacetic acid (EDTA) and ethylenediamine disuccinic acid (EDDS) within natural UV radiation range.

The rate of photodegradation of two chelating agents, ethylenediaminetetraacetic acid (EDTA) and an isomeric mixture of ethylenediamine disuccinic acid (EDDS), was analysed in humic lake water and in distilled water using exposure to sunlight, and in the laboratory using lamps emitting UV radiation in the range 315-400 nm. Degradation was studied using Fe(III) complexes and sodium salts of chelates. Fe(III) complexes were illuminated at pH 3.1 and 6.5. The results demonstrated that the rate of photodegradation of Fe(III)-EDTA and Fe(III)-EDDS complexes seems to be pH dependent. In the laboratory experiments degradation occurred much faster when the original pH was 3.1 rather than 6.5. The photodegradation of the isomeric mixture of EDDS was markedly faster than the degradation of EDTA both in the laboratory and field experiments, and both in humic and distilled water. The results indicated that in natural waters photodegradation of EDDS is independent of initial speciation of EDDS, while degradation of EDTA is dependent on its existence as Fe(III)-EDTA species.     

Photosensitized degradation of Irgarol 1051 in water

Irgarol 1051 (2-methylthio-4-tert-butylamino-6-cyclopropylamino-s-triazine) is a herbicide analogue that is added to antifouling agents used on ships. Our former study on its degradation in sunlight suggested that unknown photosensitizers in natural waters accelerated the photodegradation to the degradation product, M1. In this study, the photodegradation of Irgarol in water was investigated in the presence of some photosensitizers. Test water containing Irgarol or M1, with or without photosensitizers, was irradiated with light from a UV-A fluorescent lamp for 48h. The concentrations of Irgarol and M1 in the test water were determined by HPLC after solid-phase extraction. M1 was more stable than Irgarol when irradiated in the presence of photosensitizers such as acetone, benzophenone, tryptophan, and rose bengal. Hydrogen peroxide (HP) accelerated the photodegradation of Irgarol, and the product M1 was degraded in the presence of more than 100mgl(-1) HP after 10h. Natural humic substances (NHS) also accelerated the photodegradation of Irgarol, but in this case, the product M1 persisted even when Irgarol was completely degraded. Photosensitized degradation of Irgarol by NHS may result in the accumulation of M1 in aquatic environments.     

太阳光对湖泊中有机污染物降解的研究进展

论述了太阳光对湖泊水体中有机污染物降解作用的研究,就其降解机理、动力学特征、作用对象及降解产物等作了逐一介绍.阐明了太阳光对生物降解湖泊水体中有机污染物具有协同作用,也概述了光降解作用受pH、溶解性有机物(DOM)、水深与水体运动、地理、水文、水质与气候等因素的影响.并对实验室模拟条件下的降解与自然条件下的降解进行对比,提出今后该领域的发展前景与研究方向.     

Water quality parameters controlling the photodegradation of two herbicides in surface waters of the Columbia Basin,Washington

The water quality parameters nitrate-nitrogen, dissolved organic carbon, and suspended solids were correlated with photodegradation rates of the herbicides atrazine and 2,4-D in samples collected from four sites in the Columbia River Basin, Washington, USA. Surface water samples were collected in May, July, and October 2010 and analyzed for the water quality parameters. Photolysis rates for the two herbicides in the surface water samples were then evaluated under a xenon arc lamp. Photolysis rates of atrazine and 2,4-D were similar with rate constants averaging 0.025 h⁻¹ for atrazine and 0.039 h⁻¹ for 2,4-D. Based on multiple regression analysis, nitrate-nitrogen was the primary predictor of photolysis for both atrazine and 2,4-D, with dissolved organic carbon also a predictor for some sites. However, at sites where suspended solids concentrations were elevated, photolysis rates of the two herbicides were controlled by the suspended solids concentration. The results of this research provide a basis for evaluating and predicting herbicide photolysis rates in shallow surface waters.     

UV-light induced mineralization of organic matter bound chlorine in Lake Bjän,Sweden--a laboratory study

Surface water and aqueous solutions of isolated organic matter from a humic rich lake in southern Sweden were exposed to artificial UV radiation to investigate the UV light induced influence on organic matter bound chlorine in natural systems. It was found that the photodegradation of organic matter bound chlorine was more pronounced than the photodegradation of organic carbon. After 120 h of irradiation of the isolated organic matter, only 35% of the initial organochlorine was still in the solution compared to about 70% of the dissolved organic carbon (DOC). A similar result was obtained for unfractionated surface water. Furthermore, our results indicate that the loss of organic chlorine was mainly due to a mineralization of organic chlorine into chloride ions. The total decrease of organic chlorine after 120 h was 32 microg Cl(org) l(-1), of which the major part disappeared in the initial irradiation phase. A similar increase was observed in the chloride concentration (34 microg Cl(-) l(-1)).     

Aerosolization of an anionic surfactant (LAS) and dissolved organic carbon (DOC) under laboratory conditions.

Aerosolization of natural salt and brackish waters under laboratory conditions is responsible for the transfer of synthetic surfactants such as linear alkylbenzene sulphonate (LAS) from water to the atmosphere. Excluded the lagoon sample which stands apart, on the average ca. 45% of the LAS present at concentrations between 0.1 and 4.1 microg l(-1) in the tested coastal and offshore marine and lagoon waters was transferred to the aerosol extract. The distribution of the individual LAS homologs in the aerosol is very similar to that in water, which indicates no preferential removal from the bulk water of any LAS homolog. The fraction of dissolved organic carbon (DOC) undergoing aerosolization under the same conditions for each tested sample was on an average ca. 3.3%, corresponding to ca. 5.6 mg l(-1). On the average, the enrichment factor of the sea samples, resulting from the changing of the LAS/DOC ratio before and after aerosolization, was ca. 20.     

Direct and indirect photolysis of two quinolinecarboxylic herbicides in aqueous systems

The photodegradation of two quinolinecarboxylic herbicides, 7-chloro-3-methylquinoline-8-carboxylic acid (QMe) and 3,7-dichloroquinoline-8-carboxylic acid (QCl), was studied in aqueous solution at different irradiation wavelengths. The effect of sunlight irradiation was investigated also in the presence of titanium dioxide (TiO₂). UV irradiation degraded rapidly QMe affording 7-chloro-3-methylquinoline (MeQ) through a decarboxylation reaction. The reaction rate was lower in the presence of dissolved organic carbon (DOC) because of the adsorption of the herbicide on the organic components. Instead, QCl was stable under both UV light and sunlight irradiation. The irradiation of QMe or QCl solutions with simulated sunlight in the presence of TiO₂ produced the complete mineralization of the two herbicides.     

Photosensitized degradation of bisphenol A involving reactive oxygen species in the presence of humic substances

The photodegradation of endocrine disrupter bisphenol A (BPA) in the presence of natural humic substances (HS) under simulated solar irradiation was studied. BPA underwent slow direct photolysis in neutral pure water, but rapid photosensitized degradation in four kinds of HS, following pseudo-first-order reaction. Reactive oxygen species (ROS) formed from HS were determined, including OH, (1)O(2) and H(2)O(2). The enhancement of BPA degradation by adding Fe(III) was primarily attributed to the oxidation of OH produced from photo-Fenton-like reaction. And the joint effects of HS and nitrate ions coexisting on BPA degradation appeared to depend on respective concentration levels. The effects of dissolved oxygen suggested that the energy transfer between excited state of SRFA and NOFA likely occurred, while the abstraction of phenolic hydrogen atom to reactive triplet state of NOHA possibly took place. Based on the structural analyses of main intermediates and degradation products of BPA detected by GC-MS, the possible photodegradation pathways were proposed, involving the alky cleavage, alkyl oxidation and OH addition. This study gave a better understanding for the photochemical transformation of BPA induced by ROS generated from natural water composition under sunlight irradiation.     

Environmental photodegradation of mefenamic acid

Pharmaceuticals and personal care products are an emerging class of environmental pollutants. Photolysis is expected to be a major loss process for many of these compounds in surface waters, including the common non-steroidal anti-inflammatory drug mefenamic acid. The direct photolysis solar quantum yield of mefenamic acid was observed to be 1.5+/-0.3x10(-4). Significant photosensitization was observed in solutions of Suwanee River fulvic acid and Mississippi River water, as well as for the model photosensitization compounds 3'-methoxyacetophenone, 2-acetonaphthone and perinaphthenone. Quenching, sparging and light-filtering experiments suggested a direct reaction of mefenamic acid with excited triplet-state dissolved organic matter as the major photosensitization process. The persistence of the model photosensitizer suggests that the photosensitization by perinaphthenone occurs either by triplet-energy transfer or an electron transfer followed by rapid regeneration of the sensitizer. Due to its low quantum yield, the loss of mefenamic acid in sunlit natural waters is expected to depend on both direct and indirect photodegradation processes.     

Degradation of lidocaine,tramadol, venlafaxine and the metabolites O-desmethyltramadol and O-desmethylvenlafaxine in surface waters

The photodegradation and biotic transformation of the pharmaceuticals lidocaine (LDC), tramadol (TRA) and venlafaxine (VEN), and of the metabolites O-desmethyltramadol (ODT) and O-desmethylvenlafaxine (ODV) in the aquatic environmental have been investigated. Photodegradation experiments were carried out using a medium pressure Hg lamp (laboratory experiments) and natural sunlight (field experiments). Degradation of the target compounds followed a first-order kinetic model. Rates of direct photodegradation (light absorption by the compounds itself) at pH 6.9 were very low for all of the target analytes (?0.0059 h^?1 using a Hg lamp and ?0.0027 h^?1 using natural sunlight), while rates of indirect photodegradation (degradation of the compounds through photosensitizers) in river water at pH 7.5 were approximately 59 (LDC), 5 (TRA), 8 (VEN), 15 (ODT) and 13 times (ODV) higher than the rates obtained from the experiments in ultrapure water. The accelerated photodegradation of the target compounds in natural water is attributed mainly to the formation of hydroxyl radicals through photochemical reactions. Biotic (microbial) degradation of the target compounds in surface water has been shown to occur at very low rates (?0.00029 h^?1). The half-life times determined from the field experiments were 31 (LDC), 73 (TRA), 51 (VEN), 21 (ODT) and 18 h (ODV) considering all possible mechanisms of degradation for the target compounds in river water (direct photodegradation, indirect photodegradation and biotic degradation).     

Different stabilities of multiwalled carbon nanotubes in fresh surface water samples

The stability of multiwalled carbon nanotubes (MWNTs) in particulate aggregates and surfactant-facilitated suspensions after being mixed into eight types of fresh surface water samples was investigated. MWNTs in particulate aggregates could not be stabilized in any of the water samples except for the one having relatively high content of dissolved organic matter with the aid of sonication. Sodium dodecyl benzenesulfonate (SDBS), polyethylene glycol octylphenyl ether (TX100) and cetyltrimethyl ammonium bromide (CTAB) were used to prepare MWNT suspensions. SDBS- and TX100-stabilized MWNTs could partly remain suspending after being mixed into the water samples, whereas CTAB-stabilized MWNTs readily sedimentated due to the surface charge neutralization by the surface water contained negatively-charged anions and colloids. This is a first systematic study examining the stability of carbon nanotubes in natural surface waters, the results from which will be useful for understanding the transport, fate and ecological effect of carbon nanotubes in the aqueous environment.     

Tetracycline photolysis in natural waters: loss of antibacterial activity

Previous work has shown that tetracycline undergoes direct photolysis in the presence of sunlight, with the decomposition rate highly dependent on conditions such as water hardness and pH. The purpose of this study was to examine the potential long-term significance of photoproducts formed when tetracycline undergoes photodegradation under a range of environmentally relevant conditions. Tetracycline was photolyzed in nine different natural and artificial water samples using simulated sunlight. The pH values of the samples ranged from 5 to 9. Total hardness values (combined Ca²⁺ and Mg²⁺ concentrations) varied from 30 to 450 ppm. Assays based on growth inhibition of two bacterial strains, Escherichia coli DH5α and Vibrio fischeri, were used to determine the antibacterial activity of tetracycline’s photoproducts in these water samples. In all tested conditions, it was determined that the photoproducts retain no significant antibacterial activity; all observed growth inhibition was attributable to residual tetracycline. This suggests that tetracycline photoproducts formed under a wide range of pH and water hardness conditions will not contribute to the selection of antibiotic-resistant bacteria in environmental systems.