| Steroid estrogens(SEs)discharged into the environment can act as endocrine disrupting chemicals when they enter the body of human and animal by environmental media and food chains.They can exert adverse health effects on the development,growth and reproduction of organisms by disordering the production,release,transport and metabolism of natural hormones.Among the SEs,17a-ethynylestradiol(EE2),a synthetic steroid estrogen,possesses the highest estrogenic potency and can exert adverse effects on ecosystems at a concentration of lower than 1.0 ng/L.However,EE2 has been widely detected out in the effluents of wastewater treatment plants,surface water and tap water of many countries during recent years.Thus,it is urgent not only to effectively manage the production and application of EE2 but also to accurately know its environmental behaviors,which can provide the theoretical basis for the ecological risk assessment and pollution controls establishment.Once discharged into surface waters,EE2 always undergoes different migration and transformation processes,among which photodegradation is known as one of the important environmental bebaviors.Natural organic matters(NOM)are ubiquitious and important phoytosensitizers and light-filters in surface waters.Thus,exploring the influence of NOM on the photodegradation of EE2 is crutial to know the laws of photochemical behaviors and the ecological risks of EE2 in sunlit natural waters.However,to date,the photodegradation mechanisms and estrogenic potency change of EE2 induced by NOM are unclear.Furthermore,the structures and properties of NOM are too complex,and the influence of NOM on the photochemical transformation of organic pollutants are usually different with the pollutants and the origins of NOM.Therefore,the photochemical behaviros of EE2 induced by NOM cannot be directly deduced from the presently reported conclusions.Herein,humic acids(HA),one of the main components of NOM,were selected as the representatives to study their influence on the photodegradation kinetics and mechanisms of EE2 by taking account of some important environmental factors.It was also explored that the estrogenic potency and phytotoxicity changes of EE2 induced by the irradiated HA solutions.Furthermore,the compositions,physicochemical properties and photoinductive reactivities of HA altered by the irradiation were characterized to study the photobleaching.Finally,the correlationships between the photodegradation rate constant of EE2 and the structural and optical index of HA were analyized to explore the photoinductive components and structures of HA in inducing EE2 photodegradation.Some key conclusions are drawn as follows:(1)The photodegradation kinetics of EE2 in pure and HA solutions can be well fitted with pseudo first-order model.In prue aqueous,EE2 was slowly photodegraded at a rate constant of about 0.0163 h-1,resulting from the direct and auto-sensitized photodegradation.In HA solutions(5.0 mgC/L),EE2 was mainly bound to HA by hydrogen bond,?-? and hydrophobic distribution interactions.The photodegrdation rate constant of EE2 in the HA solutions was significantly enhaced by about 4 times compared to that in pure aqueous,and the half-life of EE2 was decreased to lower than 10 hours.(2)The rapid photodegradation of EE2 in the HA solutions was attributed to the generated reactive species(RS),mainly HO· and ~3HA*.The global contribution ratio of the two RS to EE2 photodegradation was about 64%(34%of HO· and 30%of ~3HA*).The main reaction mechanism between HO· and EE2 was addition reaction,and the main reaction mechanisms between EE2 and ~3HA*were hydrogen and electron abstract.Although the concentration of ~1O2 in the illuminated HA solutions was determined to be three orders of magnitude higher than that of HO·,it cannot contribute significantly to EE2 degradation,with a global contribution of lower than 17%.The photogenerated H2O2 and O2·-cannot directly react with EE2,while they can be further transformed into HO·,subsequently reacting with EE2.Besides the photogenerated RS,the binding role of HA was also responsible for EE2 phtodegrdation in the HA solutions by promoting the reactions between the photogenerated RS and EE2.(3)The phenolic structure of EE2 was photosensitive,and the photodegradation of EE2 was mainly occurred on that ring in both pure aqueous and the HA solutions.Althought the photodegradation of EE2 can be effectively promoted by the HA,EE2 cannot be mineralized but transformed into other organic compounds without estrogenicity detected by yeasts and MCF-7 cells.However,the phytotoxicity of EE2 cannot be eliminated completely by photodegradation,and some degradation products can promote or inhibit the germination and growth of rice seed,wheat and ormosia.(4)Water environment factors can influence EE2 photodegradation in both pure aqueous and HA solutions,which obeys the laws displayed as follows.In both pure aqueous and HA solutions,the photodegradation rate of EE2 decreased with the increased EE2 concentration to a stable one,and it usually reached the minmum rate constant in the pH range of 6.5?8.5 and dramatically enhanced when pH exceeded 9.0.Dissolved oxygen and Fe(III)can promote the photodegradation of EE2 in both pure aqueous and HA solutions,while HCO3-exert no effects on that.Although ionic strength(IS)and Cl-have no influence on EE2 photodegradation in pure aqueous,IS and Cl-can promote and inhibit the photodegradation of EE2 in HA solutions,respectively.Nitrate could significantly promote the photodegrdation of EE2 in pure aqueous,but its role in promoting EE2 degradation could be inhibited by HA filtering the incident light and scavenging the photogenerated HO·.(5)Photobleaching can significantly alter the structural and physicochemical properties of HA.During the irradiation period,HA preferentially lost the ability of absorbing the long wavelength light and tranformed into low molecular weight chemicals,such as aliphatics and organic acids,which enhaced the bioavailability of HA.Although RS generation was inhibithed by photobleaching,EE2 photodegradation rate in the HA solutions photobleached 70 hours was still about 2.5 times higher than that in pure aqueous solutions.(6)The photochemical activity of HA is tightly related to its hydrophobility,polarity and molecular weight.The photoinductive activity of HA fractions with different hydrophobility followed the order of transphilics>hydropholics>hydrophilics.The capability of HA in photogenerating RS decreased with an increase in its polarity and molecular weight.According to the correlationship between EE2 photodegradation rates and the structural index of HA,the photochemical activity of HA in inducing EE2 degradation was predominated by the oxygen-containing groups,electron transfer compositions,aromaticity and molecular weight of HA.In a certain concentration range of HA,spectral slope ratio(S275-295/S350-400)can well reflect the capability of HA in inducing EE2 photodegradation,which could not be disturbed by the presence of low concentration of H2O2 and NaClO altering the structure and property of HA.The conclusions,on the one hand,can give us a better understand of EE2 photochemical behaviors in waters and the role of photochemical processes of HA in pollutants' geochemical behaviors.On the other hand,they can provide theoretical basis for the environmental fate prediction,ecological risk assessment,and control measure establishment of EE2 pollution. |
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