Application Of Compound-specific Isotope Analysis In Degradation Of Phthalic Acid Esters

Phthalic acid esters (PAEs) are a class of persistent organic pollutants (POPs), which have been identified in diverse environmental samples including groundwater, soil, lake and marine sediments. There is a deep concern about their possible toxicity to human beings and other organisms, since some of them are considered as potential carcinogens, teratogens, mutagens and environmental endocrine disruptors (EEDs). To understand the environmental behavior and fate of PAEs in marine environment, it is necessary to study the degradation of PAEs under simulated conditions. In this dissertation, four PAEs (DMP, DEP, DBP and DOP) were chosen as model compounds to investigate their behavior of biodegradation and photochemical degradation as well as their fate in marine environment. The major achievements of the study are as following:(1) In-situ biodegradation of DEP by natural microbial community under oxic and anoxic sedimentary conditions was simulated by using a laboratory microcosm system with natural marine sediment overlying with natural seawater. The results indicated that DEP at relatively lower concentrations was degraded rapidly by natural microbial community in sediments under both oxic and anoxic conditions, which can be described with a first order kinetic equation. The rate constants for DEP degradation under oxic and anoxic conditions were 0.103 and 0.069 day^-1, respectively.Stable carbon isotopic fractionation produced in biodegradation of DEP under oxic and anoxic conditions was evaluated with compound-specific isotope analysis. A relatively pronounced (13)^C-enrichment with aδ(13)^C shift of 1.80±0.19‰(f=0.05) in residual DEP at advanced stage of biodegradation under oxic condition was observed, while a small (13)^C-enrichment was noted for biodegradation under anoxic condition, with aδ(13)^C shift of 0.74±0.28‰(f=0.08).(2) The biodegradation of three PAEs (DMP, DBP and DOP) by natural microbial community under oxic sedimentary condition and their isotope fractionation were compared by using a laboratory microcosm system with natural marine sediment overlying with natural seawater. The results showed that the degradation of the three tested phthalates followed a first-order kinetics, with rate constants of 0.0541, 0.0352 and 0.00731 day^-1 for DMP, DBP and DOP, respectively, indicating that the degradation rate of PAEs is a inverse function of the length of the alkyl side chain: the longer the side chain, the slower the rate is.(13)^C-enrichment for the residual PAEs was evaluated with compound-specific isotope analysis. A relatively evident (13)^C-enrichment, with maximumδ(13)^C shifts of△δ(13)^C_DMP= 2.05±0.21‰(f=0.17) and△δ(13)^C_DBP=1.92±0.23‰(f=0.08) in residual DMP and DBP, respectively, at advanced stages of biodegradation was observed. No significant (13)^C-enrichment occurred in residual DOP (△δ(13)^C_DOP=0.55±0.21‰, f=0.16) within the accuracy and reproducibility for GC-C-IRMS (±0.5‰). The experiment results indicated that the degree of isotopic fractionation in residual PAEs appeared to be related to the number of carbon atoms, which is in the order of DMP > DBP > DOP.(3) The photodegradation process of DMP, DBP and DOP in natural seawater under UV radiation (254nm) can be described well by a first-order kinetics, with rate constants of 0.02636, 0.1005 and 0.958 h^-1 for DMP, DBP and DOP, respectively. The photodegradation rate increased in the order of DMP < DBP < DOP, indicating that the photodegradation rate of PAEs is in a inverse trend of that for biodegradation under oxic condition described in part (2). The results of TOC measurement showed that PAEs could not be completely mineralized under UV radiation.The (13)^C-enrichment for remaining DMP, DBP and DOP at different stage of photodegradation under UV radiation was monitored by GC-C-IRMS. Pronounced (13)^C-enrichment, with maximumδ(13)^C shifts of△δ(13)^C_DMP=10.04±0.13‰(f=0.09),△δ(13)^C_DBP=7.4±0.09‰(f=0.06) and△δ(13)^C_DOP=2.9±0.17‰(f=0.25) in residual DMP, DBP and DOP, respectively, at advanced stages of photodegradation was clearly observed. The results also revealed that the degree of isotopic fractionation in residual PAEs under UV radiation is a inverse function of the number of carbon atoms in the PAE molecule, which is in the order of DMP > DBP > DOP. The KIEs of the three xPAEs during the course of photolysis were in the range of 1.0018¹.0045. In addition, (13)^C-enrichment for photodegradation of PAEs was more significant than that for biodegradation.(4) The mechanism for photodegradation of DEP in natural seawater was investigated by using a combination of intermediates detection and determination of isotopic fractionation.The photodegradation process of DEP in natural seawater under UV radiation fellows a first-order kinetics, with a rate constant of 0.028 h^-1. The result of TOC analysis revealed that photochemical process could not mineralize DEP thoroughly, which is similar to the conclusion mentioned in part (3).The primary degradation intermediates of DEP identified with GC-MS during the course of photolysis were mono-ethyl phthalate and phthalic anhydride. Based on this observation, a plausible mechanism was proposed for photodegradation of DEP. First, one C-O bond in the DEP molecule is broken, to form mono-ethyl phthalate, which is the initial rate-determining step. Then, mono-ethyl phthalate is degraded to phthalic anhydride.CSIA was used to further confirm the conclusion. The KIE and AKIE obtained, based on Rayleigh equation, were 1.005 and 1.037, respectively, which were in consistent with those reported in literatures for C-O single bond cleavage, supporting the above conclusion drawn from intermediates detection. CSIA combined with intermediates detection makes it more reliable to deduce the mechanism of DEP photolysis, which might be used as a reference for mechanism investigation in complicated environment systems.A profound (13)^C-enrichment, with aδ(13)^C shift of 11.92±0.13‰(f=0.02), in residual DEP molecule for photolysis in natural seawater under UV radiation was clearly an evidence of its degradation. Compared with biodegradation by natural microbial community under oxic sedimentary condition, the (13)^C fractionation for photolysis was much greater.