Reaction Kinetics And Degradation Mechanism Of Nature Estrogens(E2/E3)Chlorination In Water Distribution System

As an important kind of endocrine disrupting chemicals (EDCs), natural estrogens (17β-estradiol (E2) and estriol (E3)) have been confirmed to cause estrogenic effects on aquatic wildlife. More and more estrogens were detected in natural and urban water systems, but most of them cannot be removed by water treatment processes, even advanced oxidation process (AOPs), which brings a big threat to the urban water quality assurance and ecological security. Two typical estrogens, E2and E3in urban water system were selected. Influence factors, reaction kinetics and degradation mechanism of nature estrogens (E2, E3) during chlorination were studied in the pilot-scale water distribution system (WDS) in Zhejiang University.According to the experiment results, the kinetics of E2and E3chlorination in pure water is a second-order reaction. Influencing factors including pH values, temperature and the concentration of dissolved organic carbon (DOC) in water were studied. In these influencing factors, the effect of influencing factors follows as:pH> DOC> temperature. When the DOC concentration increases from1.0mg/L to3.0mg/L, the overall reaction rate constant of E2decreases from283.39M-1· s-1to166.94M-1· s-1.However, the kinetics of E2and E3chlorination in WDS is a first-order reaction. The degradation of E2and E3is effected by not only the water quality conditions, but also the operating temperature, velocity and the pipe materials. The concentration of free chlorine, pH values, temperature, the concentration of bromide ions and flow velocity increasing is beneficial to degradation of E2and E3. The factors influencing the degradation follows as:[HClO]f> T>[Br-]> pH>> pipe material≈velocity. The chlorination rates constants for E2and E3degradation under different temperatures were adjusted to an Arrhenius expression; and the activation energy for E2and E3were calculated to be25.87kJ/mol and26.05kJ/mol, respectively. Under different pipes, the degradation rates of E2and E3follow the order: stainless steel pipe> polyethylene pipe (PE)> ductile iron pipe.In addition, the chlorination of E2and E3in WDS were also affected by the pipe walls and chemical compounds of bulk water. Degradation rates of E2and E3under the optimal conditions in ductile iron pipe were studied. Results show that after60min reaction, E2and E3were degraded99.2%and98.3%in the pure water and74.8%and74.8%in the WDS, respectively. The results showed that the contribution of wall of pipes accounts for5.74%and6.60%; intermediate products in the early stage of the chlorine decay accounts for30.1%and39.6%; chemical compounds of bulk water, including organic matter, inorganic ions and chloride products, accounts for0.7%and12.3%to E2and E3degradation.In addition, in this study, Gas Chromatography-Mass Spectrometer (GC-MS) and Liquid Chromatography-Mass Spectrometer-Mass Spectrometer (LC-MS-MS) were used to identify the degradation products and to investigate the transformation and reaction mechanism of E2. The results showed that the E2chlorination in WDS can be divided into three stages:1)The first stage (0-2h) is mainly for hydroxide of C17and chlorine substitution in C2and C4; the chlorinated products2-chloro-E2、4-chloro-E2、2-chloro-E1、4-chloro-E1、2,4-dichlo-E1、2,4-dichlo-E2were found in this stage;2) The second stage (2-4h) is mainly for C10methylation and the breakdown between C10-C9and C16-C17bond, followed by the substitution and oxidation reactions;3) The third stage (4-12h) is mainly for further damage of the benzene ring. After the constantly chlorine substitution and oxidation reactions, E2could be destructed and degraded. The concentrations of THMs (Trihalomethanes) and HAAs (Halogenated Acetic Acids) increased rapidly at this stage, and bromine products could be generated because of the bromine ion in water.