Study On Photochemical Reaction Characteristics Of Kaolin And Its Effect On Photodegradation Of EDCs And Involved Mechanisms

Kaolin and other turbidity-causing matters widely exist in natural waters.They can absorb light.These turbid substances may release active groups after absorbing light energy and promote the transfer and conversion of organic components in water.At the same time,with the increase of human activity,the content of endocrine disrupting chemicals(EDCs)in natural water is increasing year by year.In this study,bisphenol A and 17?-estradiol were used as representative substances,their mineralization and degradation in the ultraviolet radiation environment were evaluated to clarify its transfer and conversion characteristics.At the same time,the effects of pH,initial concentration of EDCs and the content of kaolin on the reaction process were investigated.In addition,the principle of photochemical reaction of EDCs in kaolin coexistence environment was proposed based on the fitting of reaction kinetics,the evaluation of physical and chemical properties of the interface and the detection of hydroxyl radicals.The main conclusions are as follows:(1)In the system without kaolin,the alkaline condition was not conducive to the photolysis of bisphenol A.With the decrease of pH,the removal rate increased.When the kaolin exists in the system,the remove efficiency of bisphenol A in alkaline environment(pH=9)was improved,it was higher than the remove efficiency in neutral environment.The removal efficiency of bisphenol A was inhinited under acidic environment,decreased by 3.0% to 6.0%.Unlike bisphenol A,the photodegradation rate of 17?-estradiol reached its highest in alkaline environment and its minimal value in neutral environment.The presence of kaolin had no significant effect on this trend.In addition,the removal efficiecy of 17?-estradiol was inhibited at any p H after the addition of kaolin.(2)The photodegradation process of bisphenol A conformed to the first order reaction kinetics,pH and the content of kaolin had no significant effect on this rule.With the increase of the initial concentration of bisphenol A,the photodegradation rate constant k decreased gradually,leading to an increase in half-life.Unlike bisphenol A,the photodegradation process of 17?-estradiol was only in line with the first-order reaction kinetics under strong alkaline condition.In the acidic and neutral environment,the photodegradation reactions were consistent with the first order reaction kinetics after 4.0 h of illumination.When its initial concentration is 8 mg/L,the photolysis reaction rate constant k was the largest,so the half-life period was minimal.Increasing or decreasing the 17?-estradiol content would decrease the value of k,then the half-life period incresed.The reason for the above differences may be related to the difference in physical and chemical properties of this two substances.(3)In the presence of kaolin coexisting system,acidic and neutral environments were not conducive to the formation of hydroxyl radicals.Combined with kaolin's scattering,shielding and absorption of light,the photochemical reaction processes of bisphenol A and 17?-estradiol were inhibited.Under the alkaline condition,the kaolin suspension could generate the highest content of the hydroxyl radical after photoexcitation.The enhancement of hydroxyl radicals on the oxidation of bisphenol A was greater than the adverse effects of kaolin on the scattering,shielding and absorption of light.For 17?-estradiol,the enhancement of hydroxyl radicals less than kaolin's scattering,shielding and absorption to light because of its relatively strong antioxidant capacity.(4)Further studies had shown that,the proportion of-OH structure on the kaolin surface decreased during the ultraviolet radiation process which results in the changes in the charge of kaolin and the formation of hydroxyl radicals in the system.In addition to the strong oxidation capacity of hydroxyl radicals,the micro-interface system formed by kaolin also had an adsorption function on EDCs,but its contribution was relatively low.In the case of bisphenol A,the removal rate(pH = 9.0)after 4 h of UV radiation was increased by 9.7% compared with the situation of without UV radiation.The hydroxyl radicals' contribution was about 6.2%,significantly higher than the kaolin adsorption contribution(3.5%)in the above process.In summary,the photolysis process of EDCs in natural environment was not only affected by the p H value of the solution and turbid substances,but also restricted by its own structure.At the same time,other coexisting substances such as Ca2+ and Mg2+ in the system would affect the degradation efficiency of EDCs by changing the aggregation state of kaolin in the system and promoting or inhibiting the formation of hydroxyl radicals.These findings and conclusions is significant for the understanding of the function of micro-interface system formed by kaolin and other turbidity-causing matters in water chemical reactions.In the meantime,it will provide new ideas for the study of the transfer and conversion of EDCs in natural water.