Study On The Stable Carbon And Hydrogen Isotopic Characterstics Of Phthalate Esters During Degradation Processes

In this study,three homologues of phthalate esters?PAEs?including dimethyl phthalate?DMP?,diethyl phthalate?DEP?and dibutyl phthalate?DBP?were chosen as model compounds to establish the analytical methods of carbon and hydrogen isotope measurement.Subsequently,the degradation kinetic and variations in carbon and hydrogen isotopic compositions of DMP,DEP and DBP were investigated during different degradation processes.After that,the carbon and hydrogen isotope fractionations were determined for three PAEs and the correlations?A?of carbon and hydrogen isotope were characterized.Based on previous studies of degradation pathways and the tentatively identified intermediate products in the present study,apparent kinetic isotope effect?AKIE?of PAEs were investigated to provide more information on the degradation mechanisms.Finally,taking DBP as a case study,comprehensive summary of the obtained A values in eleven kinds of degradation processes were used to explore the potential of two-dimensional isotopic?C-H?characteristic model to differentiate and characterize transformation processes.The main results of this thesis are summarized as follows.1.The analysis technique for measuring carbon and hydrogen isotopic compositions of DMP,DEP and DBP was researched using gas chromatography-isotope ratio mass spectrometry?GC-IRMS?.The optimum ranges of signal intensity for carbon and hydrogen isotope measurement of three PAEs were established through linearity analysis.An elemental analysis-isotope ratio mass spectrometry?EA-IRMS?was used to validate the accuracy of GC-IRMS method.Therefore,the result provided technological supporting for subsequent carbon and hydrogen isotope measurements of PAEs during different degradation processes.2.The hydrolysis reactions of DMP,DEP and DBP under different pH conditions?pH 2,pH 7 and pH 10?could be described with the first-order kinetic equation and the hydrolysis rate constants were affected by pH values in the order of pH 2<pH 7<pH 10.Moreover,the rate constants under pH 7 and pH 10 were found to decrease with the increase of molecular weight in PAEs,i.e.,DMP>DEP>DBP.The carbon isotopic compositions of three PAEs were analyzed and carbon isotope fractionations were observed during hydrolysis reactions.The degree of carbon isotope fractionation was related to the chemical structure of PAEs and the degree at the same pH condition decreased along with the increase of carbon atoms in PAEs molecule due to isotope dilution effect.The calculated carbon apparent kinetic isotope effects?AKIEc?during hydrolysis reactions both accorded with the proposed ester hydrolytic pathway?C-O bond cleavage?,with the exception of DBP hydrolysis at pH 2.3.During the photodegradation of DMP,DEP and DBP induced by UV/H2O2,the degree of carbon isotope fractionation was observed to decrease with increasing molecular weight,whereas hydrogen isotope enrichment showed a trend to increase.Based on stable isotopic characteristics?carbon and hydrogen?and tentatively identified intermediate products using gas chromatography-mass spectrometry?GC-MS?,hydroxyl radical?HO^·?addition to the aromatic ring was assumed to be the main reaction pathway for DMP and DEP.The calculated AKIEc values for DMP,DEP and DBP in UV/H2O2 reaction were 1.029,1.028 and 1.015,respectively,which fell in the expected KIE range?1.01?1.03?of C-H bond cleavage.4.Significant carbon and hydrogen isotope enrichment were noticeable during the degradation of DMP,DEP and DBP in heat-activated persulfate oxidation?35??at pH 2 and pH 7.The variations in carbon and hydrogen isotopic values showed a well-fitted linear correlation?A?.Three PAEs both exhibited larger ? values in persulfate oxidation compared to UV/H2O2 reaction,which indicated more significant variations in hydrogen isotopic composition of DMP,DEP and DEP in persulfate oxidation.Thus,? values allowed to distinguish different oxidation reactions of PAEs.Moreover,study of radical quenching and compound specific isotope analysis were combined to explore the dominant radical species during persulfate oxidation of DEP.Sulfate radical(SO4^·-)was found to be the predominant radical at pH 2 and both SOa^·-and HO^· contributed to DEP degradation at pH 7.HO' was estimated to have a relative contribution of 21%?63%according to dual C-H isotope fractionation data,which provided a novel approach to improve the understanding of radical oxidation reactions.5.The aerobic biodegradation experiments of DMP and DEP were conducted with Rhodococcus opacus DSM 43250 and the carbon isotope fractionations were similar to those of hydrolysis reactions.AKIEc values were also in accordance with the proposed hydrolytic pathway of C-O bond cleavage.This result indicated that for hydrolytic reactions of DMP and DEP,similar degree of carbon isotope enrichment was obtained irrespective of hydrolysis conditions?pH value,abiotic and aerobic?and degradation kinetics;thus,carbon isotope ef'fect primarily indicated the type of bond cleavage?C-O?in hydrolytic reaction.The aerobic biodegradation of DBP showed smaller carbon isotope fractionation in comparison with neutral hydrolysis reaction.Moreover,the calculated AKIEc value was lower than the expected KIE range?1.03?1.09?of C-O bond cleavage and it was likely associated with the masking of intrinsic isotope effects in enzyme catalyzed reactions.6.Two-dimensional isotopic?C-H?characteristic model of DBP during degradation processes was established based on the ? values in hydrolysis reaction,UV/H2O2,persulfate oxidation and aerobic biodegradation.Moreover,direct photolysis and UV-activated persulfate oxidation of DBP were applied to validate the model.It demonstrated that the model allowed to characterize and distinguish different types of degradation processes?hydrolysis,photolysis and persulfate oxidation?and contributed to illustrate the degradation mechanisms.During the direct photolysis of DBP at pH 2,pH 7 and pH 10,similar degradation rate constants,carbon and hydrogen isotope fractionations were observed as well as undistinguishable ? values.Moreover,the direct photolysis and UV/H2O2 reaction exhibited a similar range of ? values?9±2?11±2?.Different activation approaches?UV-activated,heat-activated at pH 2 and pH 7?applied in persulfate oxidation of DBP had influence on the degradation kinetics but had no effect on the stable isotope fractionations?carbon and hydrogen?and ? values.This study provided the corresponding theoretical basis and technical support for applying compound specific isotope analysis to optimize the isotope fractionation database of PAEs and to evaluate the effectiveness and applicability of contaminated PAEs remediation engineering quantitatively.