| Groundwater is one of the most important fresh water resource, which is30%of fresh water resource on the earth, and the even the only water-supply source in arid and semi-arid region. The rapid industrialization and urbanization has introduce severe organic contamination to the groundwater, by ways like the leak of crude oil from oil production or cracking of buried oil pipeline, the discharge of organic wastewater, the use of pesticides, waste leachate, etc. Recent studies show the volatile chlorinated hydrocarbon (VCHs, Volatile Chlorinated Hydrocarbons), especially trichloroethylene (TCE), tetrachloroethylene (PCE), dichloromethane (DCM) and carbon tetrachloride (CT), etc. are significant organic contaminants of high detectable rate in the groundwater in China. The environmental behavior and in-situ remediation of VCHs in underground environment are always the research hotspots in environmental science.In the underground aquifers, the convection of groundwater is the dominant driving force of the underground pollutants, whereas dispersion, diffusion, absorption, provision and volatilization of the groundwater may influence the concentration and migration rate of pollutants, and hence affect the distribution of VCHs in groundwater. Meanwhile, the attenuation of organic contaminants in groundwater may proceed along multi competitive degradation routines. These routines are sometimes of competitive, zymolytic and inorganic reaction, which depends on the molecular constitution and ambient condition of the chemicals. It is hardly to confirm why the concentration of pollutants vary. To assess the in-situ remediation of groundwater pollution, it is crucial to discriminate the dominant mechanism in compound and hence evaluate the conversion efficiency of the pollutants. The compound specific stable isotope analysis (CSIA) can be a powerful tool for these issues and the multi-dimensional-CSIA (MD-CSIA)technique is developed recently for more complicated problems. To study and describe the isotope effect in the conversion of the pollutants can provide basic for CSIA applications. To date, isotope composition characteristics and the isotope effect of the carbon and chlorine in VCHs have not been systematically studied. This study carried on carbon and chlorine isotopic analysis in cases of the reductive dechlorination, volatilization and adsorption, hence to quantify the attenuation process or degradation mechanism in these processes, especially to validate a quantitative assessment method of the in-situ remediation and its technical parameter.The contents, clue and methods of this theses are summarized as follows:1) The analysis techniques for measuring carbon and chlorine isotopes of the VCHs of the low concentration were firstly researched and the detection method were established. The quantification method were established using the head space-solid phase micro extraction-gas chromatography (HS-SPME-GC) as to monitoring the concentration variation of the VCHs in their conversion and adsorption processes. An online carbon isotope analysis technique for VCHs was developed using HS-SPME-GC/C-IRMS (head space-solid phase micro extraction-gas chromatography/combustion-isotope ratio mass spectrometry) as to detect the composition of carbon isotopes at different time during the conversion processes of VCHs. And this study also designed the off-line method for simultaneous determination of both carbon and chlorine isotopic composition of VCHs during its conversion, volatilization, and adsorption process.2) The isotopic composition of C and Cl in the VCHs from separate sources were measured using the established methods for C and Cl isotopic analysis. If the unique isotopic composition of C and Cl in the selected VCHs, so-called ’isotopic fingerprint characteristic’, were detected, these unique ’fingerprint’ can be the evidence to source apportionment. And the genesis of the difference of isotopic composition was studied. The feasibility of using ’isotope fingerprint’ for source apportionment of the source of groundwater pollution was discussed.3) Based on the established concentration and isotopic analysis techniques for VCHs, the controllable condition experiments were designed to research the C and Cl isotopic composition variation in different attenuation process of VCHs. If the isotopic fractionation was found significant in the convention, volatilization and absorption process of VCHs, the kinetic isotopic fractionation model would be set up to obtain the isotope fractionation factor. The direction and level of the isotopic fractionation can indicate theoretical and practical supporting information of C and Cl isotopic fractionation.4) The stability and feasibility of the isotopic enrichment factor are important to reveal the mechanism of the significant C and Cl isotopic effect. The factor should be examined because the environmental conditions and the physical chemical properties of the pollutants are complicated. If the isotopic enrichment factor varied, the mechanism of isotopic enrichment factor variation should be studied, especially to quantify the variation between computing and actual values of the isotopic factor caused by introducing varied isotopic enrichment factors.In this thesis, the analytical methods for C and Cl isotope are summarized as follows:The analytical error of the on-line method HS-SPME-GC/C-IRMS is±0.35%o (n=3); The method error of the off-line C and Cl isotopic analysis for pure solvent (chemicals) are±0.12‰(n=5) and±0.14‰(n=5) respectively; the method errors of C and Cl isotope analysis of the VCHs by the low-pressure vacuum extraction from water sample are±0.26‰(n=6),±0.15‰ (n=6). Based on the high-precision analytical technique, the feasibility of using stable C/Cl isotope analysis for pollution tracing, environmental behaviors and environment monitoring was systematically studied. Especially, this study apply the Rayleigh fractionation model and the kinetic isotope effect theories for systematically explaining the composition variation of C/Cl isotope in different attenuation process. The main results and conductions are as follows:Ⅰ. The isotopic fingerprint of C/Cl isotopes in VCHs was described. The genesis of C/Cl isotopic composition variation was revealed, and the potential process and the factors of isotopic composition variation.1)4batches of VCHs solvents produced by main plants in China were analyzed. The δ13C values range from-51.09‰to24.62‰,and δ3/Cl values from-1.23‰to6.73‰. The results indicated the C/Cl isotopic composition of different solvent is unique.2) the variation of carbon isotope between different VCHs are inherited from the difference between raw materials used by different factories. And the variation of Cl isotopic composition is on account of the different production process which cause varied fractionation of Cl isotopes.3) Using both the C and Cl isotopic composition of a sort of VCHs, as its isotopic fingerprint can be significantly unique. This way is called2-dimensional isotopic analysis which is a potential power tool in the future study of pollution source apportionment of VCHs in groundwater.4) Theoretically, the variation of C/Cl isotopic composition of VCHs in environment is caused by the isotopic effects in different processes of transport and conversion of the VCHs. This time the Rayleigh model is adapted to describe the isotope fractionation of the C/Cl isotopes in VCHs, as to study the kinetic isotope fractionation and relevant mechanism.Ⅱ. The attenuation characteristics and C/Cl isotope fractionation factors of VCHs in different processes were obtained. These results show the isotope fractionation can be an important tool for explaining the attenuation processes of the VCHs.1) the degradation process of TCE catalyzed by Vitamin B12can be generalized as3steps:a. TCE is complexed by vitamin B12; b. TCE is reduced by reactions like hydrolyzation, hydrogenolysis, etc., while the vitamin B12is oxidized; c. the oxidized vitamin B12is reduced by Ti(Ⅲ) that is oxidized to Ti(Ⅳ). The activation energy of vitamin B12as a reduction catalysis is tested56.7-41.2kJ·mol-1. The vitamin B12tunes down the activation energy of the reduction-dechlorination system, and highly efficiently push the reaction forward. Under suitable reaction condition, with a initial concentration of113mg·L-1,97%of TCE can be converted within10h. Hence, vitamin B12is a potential chemical for in-situ remediation. As the CSIA can be practically used in the study of environmental behaviour of the pollutants and the environmental monitoring, the CSIA of carbon isotope fractionation in process where vitamin B12catalyzing the TCE’s reduction-dechlorination was carried out. The results show the carbon isotopic composition of TCE was significantly changed during catalysed reduction process, and the δ13C become more positive when the more TCE was conversed. Significant carbon isotopic fractionation was observed during the reduction-dechlorination of TCE catalyzed by vitamin B12, and the isotope enrichment factor εC of carbon were from-14.0%o to-18.0%o. The C/Cl isotope composition in the conversion process were analyzed by time series based on the off-line isotope analysis technique. The isotope enrichment factor of bulk carbon is much lower than that of compound specific carbon isotope of TCE (εC,bulk=-1.3～-2.5‰, εCl,bulk=-1.6‰). The results displayed that the carbon isotope and chlorine isotope composition are positively correlated during the conversion process of TCE.2) The C/Cl isotopes of VCHs were observed significantly fractionated in the volatilization process. For TCE, the carbon isotope enrichment factor εC ranged from+0.28‰to+0.29‰, and the chlorine one εCl from-1.48‰to-1.18‰. For PCE, the carbon isotope enrichment factor εC ranged from+0.63‰to+0.50‰, and the chlorine one εCl was-1.00‰. When using the isotope fractionation as to quantifying the isotopic fractionation, the’musk effect’ of the isotope fractionation cannot be ignored which may occur in the volatilization process. However, studies show the kinetic volatilization process may cause the fraction direction of carbon isotope fractionation is opposite to that of chlorine one, and this trend is different from that of conversion. Therefore, this contradiction can be used to distinguish the volatilization and conversion process of VCHs.3) The batches experiments of adsorption showed the adsorption process of18to20mesh granular activated carbon (GAC) made from coconut and nutshell accord with the Langmuir model. The saturated extent of adsorption of GAC were obtained as follows:coconut GAC for TCE was3.16mg·g-1, coconut GAC for PCE was5.64mg/g, nutshell GAC for TCE was2.30mg·g-1, and nutshell GAC for PCE was4.30mg·g-1. The study showed the GAC have high capacity of adsorption for PCE than TCE, and the coconut GAC’s adsorption capacity is higher than nutshell GAC. The activated carbon is definitely one of the convenient materials for physical remediation of VCHs pollution. The results of C/Cl isotope analysis for adsorption process showed the C/Cl isotope composition was not significantly correlated to the remaining of VCHs which was not adsorbed, and thus the significant isotope fractionation may not occur in the adsorption process. Comparing with the isotope fractionation in the conversion or even volatilization process, the isotope fractionation in adsorption can be hardly observed. In the in-situ or chemical remediation, even if there is adsorption, using the isotope enrichment factor as to compute the conversion rate, the adsorption process may not significantly influence the isotope composition of the target compound within the detection error limits. However, the analysis of C/Cl isotope composition can be used for resolving the source apportionment of VCHs in the groundwater where if the adsorption is the dominant attenuation mechanism of VCHs.Ⅲ. the stability of isotope fractionation factor was analyzed, and the effects on the conversion efficiency quantified by the Rayleigh fractionation model from the variation of isotope enrichment factor (ε) were quantitatively evaluated.1) The C/Cl isotopes in VCHs were observed fractionated during the volatilization process of VCHs. The gas-liquid equilibrium experiments show in the temperature range from10to35℃, the average difference of isotope composition of TCE between gas and liquid phase△13Cvapor-lqiud was+0.49±0.16‰for carbon, and average△37Clvapor-lqiud was-0.82‰±0.08%o for chlorine; for PCE, average△13Cvapor-lqiud was+0.56±0.16‰, average△37Clvapor-lqiud was-0.44±0.08‰. Studies show in a gas-liquid equilibium system, the VCHs of gas phase get the heavy carbon isotope depleted. And the experiments of volatilization process show the heavy carbon isotope of VCHs trend to get depleted. Therefore, the controlling mechanism of the isotope fractionation in the volatilization process can be related to the kinetic isotope effect, and also the equilibrium isotope effect.2) Studies show the isotope fractionation in the gas-liquid equilibrium process can be effected by temperature, which effects the kinetic volatilization rate of VCHs and thus influence the relationship between isotope fractionation and C/Cl isotope composition variation in the volatilization process. For TCE, at the temperature20±1℃,εC=+0.28±0.02‰,εC=-1.48±0.04%o;at26±1℃,εC=+0.29±0.02%o, εCl=-1.18±0.02; and for PCE, at20±1℃,εC=+0.63±0.04%o,εCl=-1.00±0.02%o;at26±1℃,εC=+0.50±0.02‰,εC=-1.00±0.04. The results show even at a low temperature, the isotope fractionation, especially the chlorine isotope fractionation caused by the volatilization can be significant.3) Batches of TCE degradation experiments catalyzed by vitamin show the dose of vitamin B12, reaction temperature and pH value can be significantly influence the conversion efficiency of the TCE, and the effects accord with linearity. When the concentration of vitamin B12increased from43.1mg·L-1to215.5mg·L-1, the degradation rate increased from0.068±0.003h-1to0.355±0.011h-1, that is vitamin B12increased to ca.5times larger than the initial, while the reaction rate increased to ca.5times much as the initial. When the reaction temperature increased from20℃to40℃, the degradation rate of TCE increased from0.102±0.005h-1to0.361±0.013h-1. However, the results show that the significance of carbon isotope effect is not definitely related to the conversion rate of TCE, and the carbon isotope enrichment factors obtained from batch experiments did not vary significantly while the average is-15.9±0.2‰. There were two competitive degradation pathways in the committed reaction of TCE degradation catalysed by vitamin B12. The initial pH value of the solution was changed and thus the degradation pathway of different degradation was changed, so the carbon isotope enrichment factor was relevantly correlated with pH value, and hence εC=-14.0±0.4%o when pH=6.5, εC=-15.7±0.3‰when pH=8.0; and εC=-18.0±0.3‰when pH=9.0. The bulk carbon isotope enrichment factor was much lower than that of TCE, and εC,bulk=-1.3±0.1‰when pH=7.5, εC,bulk=-2.5±0.1‰when pH=9.0, and influenced by the reaction initials, the carbon isotope enrichment factor εC,bule varied significantly; whereas the chlorine isotope is influenced by the mass effect and thus the influence on εCl,bulk by pH value was not significant, all εCl,bulk=-1.6±0.1‰. The results indicate that the carbon isotope effect in the reduction-dechlorination catalyzed by vitamin B12is mainly influenced by the degradation pathway.4) The study used the Rayleigh fractionation model to emulate and compute the degradation degree (B). When the relative minimum and maximum of carbon isotope enrichment factor εC-14.0%o and-18.0%o are used for computing degradation B, the results come from these two factor values can be varied by more than10%. Therefore, selection of the proper isotope enrichment factor for field application, is the essential condition for getting accurate monitoring results. There are two innovations in this thesis:I. the kinetic isotope fractionation models in the reduction dechlorination catalysed by vitamin B12and volatilization process were set up, where the isotope fractionation factor was obtained and the stability of the isotope fractionation factor was verified; Ⅱ. the carbon isotope fractionation was mainly controlled by the vapour-liquid equlibrium mechanism, and the chlorine isotope fractionation was mainly controlled by a kinetic mechanism during evaporation process. |
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