Radiolytic Degradation Characteristics And Mechanisms Studies On Phthalate Esters Environmental Endocrine Disrupting Chemicals

Phthalate esters, as a kind of typical environmental endocrine disrupting chemicals, have aroused worldwide attention, and it is an significant issue to explore their effective degradation methods and mechanisms. The radiolytic degradation of phthalate esters was investigated by the combination of transient and stable technologies with dimethyl phthalate (DMP) and diethyl phthalate (DEP) as the model compounds. The property and kinetics characteristic of transient products was studied by laser flash photolysis. The reactions of DMP and DEP with ^·OH, e_aq^- , SO₄^-·, ^·N₃ and Br₂^-· were investigated by pulse radiolysis. Meanwhile, the degradation of DMP and DEP in aqueous solutions by electron beam irradiation was also studied with emphasis on the influence factors, intermediate products and degradation mechanisms.Laser flash photolysis showed that both DMP and DEP could produce their excited state under the excitation of the laser pulse of 266 nm. The respective self-quenching rate constants were 1.8×10⁵ s^-1 and 1.7×10⁵ s^-1, while the respective self-quenching rate constants were 2.4×10⁸ M^-1s^-1 and 2.7×10⁸ M^-1s^-1.Pulse radiolysis showed that both DMP and DEP could react with ^·OH, e_aq^- and SO₄^-· very rapidly, but neither of them could react with ^·N₃ and Br₂^-·. ^·OH mainly attack the benzene ring of DMP/DEP forming the OH-Adducts, while e_aq^- transfer to the ester group forming radical anions. DMP/DEP react with SO₄^-· forming the corresponding radical cations, and then the radical cations react with water forming OH-Adducts. The respective rate constants for the reaction of DMP and DEP with ^·OH in natural conditions were 3.4×10⁹ M^-1s^-1 and 2.3×10⁹ M^-1s^-1, in alkaline conditions were 3.7×10⁹ M^-1s^-1 and 3.1×10⁹ M^-1s^-1; that with e_aq^- in natural conditions were 1.6×10¹⁰ M^-1s^-1and 1.0×10¹⁰ M^-1s^-1, in alkaline conditions were 2.0×10¹⁰ M^-1s^-1 and 1.2×10¹⁰ M^-1s^-1; that with SO₄^-· were 1.5×10⁸ M^-1s^-1 and 1.0×10⁸ M^-1s^-1. The difference of the rate constants indicated that the pH value and molecular structure had influence on the rate constants for the above reactions. In the system of O₂-saturated, the OH-Adducts further react with O₂ forming the peroxyl radicals.Electron beam radiolysis indicated that the degradation rate of DMP/DEP in aqueous solutions by electron beam irradiation was different at different experiment conditions. Due to both ^·OH and e_aq^- have great roles in their radiolysis, the addition of radicals scavengers is not facilitate to degrade DMP/DEP. The reaction efficiencies of DMP and DEP react with ^·OH were 94% and 81%, respectively, and that with e_aq^- were 14% and 33%. DMP/DEP degradation was found to follow pseudo-first-order kinetics, G values and initial concentrations followed linear relationship. The degradation rate order of four radiolysis systems was ^·OH oxidative system without oxygen > ^·OH oxidative system with oxygen > oxidative-reductive system > e_aq^- reductive system, which demonstrated that the degradation effectiveness of ^·OH was better than e_aq^- and O₂ was favorable for DMP/DEP radiolytic degradation. The degradation products and mechanisms of DMP/DEP were different in four radiolysis systems. In oxidative-reductive system, ^·OH attacked radical anion forming monomethyl phthalate/monoethyl phthalate, and then undergoing a series of reactions the intermediates were completely mineralized. In ^·OH oxidative system without oxygen, OH-adducts formed hydroquinone and pyrocatechol through oxidation, and then underwent a further oxidation forming ring-opening products. These intermediate products were completely mineralized, ultimately. In ^·OH oxidative system with oxygen, peroxyl radicals were directly oxidized to non-aromatic acids, or formed aromatic intermediates which were further oxidized to non-aromatic acids. Finally, all these intermediates were mineralized into CO₂ and H2O. In e_aq^- reductive system, radical anions resulted from the reaction of with DMP/DEP producing phthalic acid via hydrolysis. The pH levels of DMP/DEP aqueous solutions after electron beam radiolysis decreased with the increasing of irradiation doses, and the pH order of four systems was e_aq^- reductive system > oxidative-reductive system > ^·OH oxidative system without oxygen > ^·OH oxidative system with oxygen. TOC removal lagged behind DMP/DEP removal in the oxidative systems, indicating that radiation technology alone was not efficient in terms of mineralization and the presence of O₂ facilitated the mineralization process.