Aqueous multivariate photochemical reaction kinetics and pathways of ionizable dienestrol under simulated sunlight irradiation
Environmental estrogens(EEs),typical endocrine-disrupting chemicals,were often detected as new micropollutants in the aqueous environment.With great concern,photochemical transformation plays an important role in governing the persistence of the pollutants in sunlit surface waters.Among the EEs,dienestrol(Dien)widely exists in surface waters and may dissociate with different forms.This study selected Dien as a model compound and investigated the reaction kinetics of the three dissociated forms for apparent photolysis,as well as hydroxyl radicals(·OH)and singlet oxygen(1O2)photo-oxidation.The corresponding environmental persistence and contribution were evaluated.Furthermore,the products and pathways of the multivariate photochemical reactions were compared,and the toxicity changes were revealed during degradation process.It was observed that the apparent photolysis followed the pseudo-first-order kinetics under simulated sunlight irradiation(λ＞290 nm).The rate constant(k)was obviously dependent on the solution pH,which was attributed to the changes in dissociation forms(H2Dien0,HDien-and Dien2-)as well as corresponding differences in ki and quantum yields(Φi).The competition kinetic experiments further indicated that both·OH and 1O2 can degrade Dien,and the degradation rate first decreased and then sharply increased with the enhancement of pH.This was related to the diverse deprotonation degrees and photo-oxidation reactivities of the three dissociation forms.Based on the different reactivities,the integrated environmental phototransformation half-lives(t1/2,E)of Dien in surface water(pH=6-9)were evaluated,and the t1/2,E ranged from 1.84 to 9.16 h at 45°N latitude.Compared with the apparent photolysis and OH oxidation reactions,1O2 photo-oxidation contributed more to the photochemical decay of Dien in surface water.Furthermore,based on the photoproduct identification with HPLC-MS/MS,it was found that the three reactions involved different primary transformation intermediates and pathways.Apparent photolytic products were simple,occurring with mono-hydroxylation of Dien.The oxidation products via OH and 1O2 were relatively abundant,and the two reactions mainly involved poly-hydroxylation,as well as continuous oxidation of hydroxyl substitution sites,respectively.The toxicity tests with Vibrio fischeri revealed the photo-modified toxicity of Dien.The above results are of great significance for better understanding the multivariate photochemical fate and risk of the ionizable micropollutant in surface waters.
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