地下水中挥发性氯代烃的碳氯稳定同位素组成特征及其衰减过程解析
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摘要
地下水约占地球上整个淡水资源的30%,是可供人类利用的重要淡水资源之一,在干旱、半干旱地区甚至成为唯一的供水水源。然而快速的工业化和城市化进程亦使地下水遭受到严重的有机污染,如石油开采过程中原油的泄漏、地下输油管道的破裂、有机废水的排放、农药的使用、垃圾渗滤液的淋滤等。近期研究成果表明,挥发性氯代烃(VCHs, Volatile chlorinated hydrocarbons)例如三氯乙烯(TCE)、四氯乙烯(PCE)、二氯甲烷(DCM)和四氯化碳(CT)等已成为我国地下水中一类检出率极高的重要的有机污染物。开展VCHs的地下环境行为及其原位修复研究,一直是环境科学领域的研究热点。
在地下含水层中,对流是驱动地下污染物运动的主要机制,而弥散、扩散、吸附、补给、挥发等过程也会对污染物的浓度和迁移速率产生影响,这些过程会导致VCHs在地下水中的分布发生改变。同时有机污染物在地下水中可能沿着多个竞争性的降解途径发生浓度的衰减,有时是竞争性的、酶解的和无机的反应途径,这取决于化合物的分子结构和环境条件。引起污染物产物浓度变化的原因往往不易肯定。对于地下水污染原位修复技术的评估中,关键即是辨析导致浓度衰减的主要机制,以有效评价转化效率。单体同位素分析技术(CSIA,Compound-specific Stable Isotope Analysis)则为此提供了一种有效的方法,特别是MD(多维)-CSIA技术的日趋完善和成熟。CSIA应用的关键问题和前提条件是研究并表征污染物迁移转化过程的同位素效应。目前,国内外关于VCHs碳、氯同位素组成特征和不同衰减过程的碳、氯同位素效应的研究尚未系统开展。因此,本研究开展了VCHs还原脱氯过程、挥发过程和吸附过程的碳、氯同位素分析研究,以期为运用CSIA、MD-CSIA技术识别与量化衰减过程(或降解机理),特别是为定量评价原位修复方法的有效性提供科学依据和技术参数。
论文主要研究内容、思路和方法如下：
(1)首先开展低浓度VCHs样品的碳、氯同位素的测试技术研究,重点是建立VCHs含量和碳、氯同位素测试技术。需要建立的含量分析技术为HS-SPME-GC(顶空-固相微萃取-气相色谱法),主要为分析转化和吸附过程中的VCH的含量变化。同位素分析技术包括碳同位素在线测试技术HS-SPME-GC/C-IRMS(顶空-固相微萃取-气相色谱／高温燃烧-同位素比值质谱仪),为测定转化过程的VCH的碳同位素组成；还拟建立碳、氯同位素线外分析技术,以同时测定转化过程VCH的碳、氯同位素组成,以及挥发及吸附过程的VCH的碳、氯同位素组成。(2)基于建立的碳、氯同位素分析技术,测定不同来源的VCHs的碳、氯同位素组成,若各VCH的碳、氯同位素组成具有各自独特的分布,即所谓的“同位素指纹特征”,则可为“同位素指纹特征”应用于源解析提供佐证。并分析同位素特征值的成因,进一步确定“同位素指纹特征”在地下水污染源源解析中实践的可行性。(3)基于建立的含量和同位素分析技术,进行控制性条件实验,探究VCHs在不同衰减过程中的碳、氯同位素组成变化。若转化过程、挥发过程和吸附过程中存在显著的同位素分馏,则建立降解过程的同位素动力学分馏模型,关键是获取同位素分馏系数。对比分析不同类型衰减过程同位素分馏的程度和方向,为碳、氯同位素分馏应用于衰减过程解析提供理论和实践依据。(4)由于环境条件或化合物物化性质对于不同过程的碳、氯同位素效应的影响十分复杂,基于建立的含量和同位素分析技术,探寻显著性碳、氯同位素效应的影响机制,关键是校验获得的同位素富集系数的稳定性和适用性。若同位素富集系数存在变异,则讨论同位素富集系数不稳定的影响因素及其影响机制,特别是量化变异的同位素富集系数在实地应用时可能导致的降解程度的拟合计算值与实际同位素系数拟合计算值的差异。
本文建立的HS-SPME-GC/C-IRMS在线碳同位素的分析误差为±0.35‰(n=3);纯试剂的碳、氯同位素线外分析技术的方法误差分别为±0.12%o(n=5),±0.14‰(n=5);水样的低压真空萃取的碳、氯同位素分析技术的方法误差分别为±0.26‰(n=6),±0.15‰(n=6)。基于建立的高精度分析技术,系统探讨了稳定碳、氯同位素分析在地下水污染物示踪、环境行为研究及环境监测中实践应用的可行性。特别是基于瑞利分馏模型和动力同位素效应的相关理论,对不同类别衰减过程的碳、氯同位素组成的变化特征进行了系统解析。得出以下主要结论：
(一)分析了VCHs的碳、氯“同位素指纹特征”,并揭示了碳、氯同位素组成差异的成因,指出导致同位素组成发生变化的潜在因素:
(1)对国内氯代烃主要生产厂商产出的四种VCHs试剂进行了分析测试,试剂的δ13C值在-51.09‰到-24.62%o之间,δ37Cl值在-1.23‰到6.73‰之间。研究结果表明,各类试剂的碳、氯同位素组成具有各自独特的分布。
(2)不同来源VCHs试剂碳同位素组成具有差异,主要是由于不同厂商使用的原料其碳同位素组成有差别,而造成氯同位素组成差异的原因主要是由于不同的生产工艺流程,使氯同位素在VCHs试剂生产过程中发生了不同程度和方向的分馏。
(3)联合碳、氯同位素,VCHs试剂的二维同位素特征具有更为明显的“同位素指纹”特征,与单一同位素相比,二维同位素组成分析在地下水VCHs源解析研究中应当具有更大的应用潜力。
(4)通过理论研究指出VCHs碳、氯同位素组成在赋存环境中发生变化的原因即是由于不同迁移转化过程的同位素效应导致,同时详细介绍了同位素分馏程度的表征—瑞利分馏模型,为后续不同过程的动力学分馏及其机理的分析研究提供理论基础。
(二)获取了不同过程的VCHs的衰减特征及其碳、氯同位素分馏系数,揭示了同位素分馏是衰减过程解析重要的工具:
(1)维生素B12催化降解TCE可以归纳为三步：TCE与维生素B12络合反应；通过水解、氢解等反应TCE被还原,维生素B12被氧化：Ti(Ⅲ)将氧化态的维生素B12还原而自身被氧化为Ti(Ⅳ)。实验获得了维生素B12催化降解TCE的活化能为56.7～41.2kJ·mol-1,维生素B12的投加大大降低了还原脱氯体系的活化能。维生素B12的出现不仅促使反应发生,而且体现了它的高效性。在合适的反应条件下,TCE理论初始浓度为113mg·L-1,10h内TCE的转化率即可达97%。维生素B12是一种原位化学修复的潜在可利用材料之一。鉴于CSIA技术在污染物环境行为和环境监测领域的实践应用意义,对维生素B12催化TCE还原脱氯过程的单体碳同位素分馏进行了研究。研究结果表明催化还原脱氯过程导致TCE的单体碳同位素组成变化明显,随着转化程度的增加,δ13C值逐渐偏正。维生素B12催化TCE还原脱氯过程导致TCE发生了显著的碳同位素分馏。碳同位素富集系数ε达-14.0‰～-18.0‰。并利用线外分析技术,对转化过程总得碳、氯同位素组成进行了时间序列的分析。总的碳同位素富集系数较单体TCE的碳同位素富集系数小的多,εC.bulk=-1.3～-2.5‰,εCl.bulk为-1.6‰。研究发现转化过程的TCE的碳、氯同位素组成呈正相关的变化趋势。
(2)通过挥发过程碳、氯同位素组成分析发现,VCHs的碳、氯同位素发生了相对明显的分馏。对于TCE,碳同位素富集系数为εc为+0.28‰～+0.29‰,氯同位素富集系数为εC1为-1.48‰～-1.18‰。对于PCE,碳同位素富集系数为εC为+0.63‰～+0.50‰,氯同位素富集系数为εCl为-1.00‰。在利用同位素分馏量化VCHs的转化效率时,挥发过程导致的同位素分馏产生的“假面效应”不容忽视。然而研究发现动力挥发过程导致碳同位素的分馏方向与氯同位素分馏方向相反,与已有的转化过程的相关变化趋势完全不同,因此这种相反的相关变化趋势可又来有效区分挥发过程和转化过程。
(3)通过吸附批实验研究发现,18～20目的椰壳GAC和果壳GAC对TCE和PCE的吸附过程均符合Langmuir模型。且获取了GAC的饱和吸附量：椰壳GAC的TCE饱和吸附量为3.16mg·g-1,椰壳GAC的PCE饱和吸附量为5.64mg/g果壳GAC的TCE饱和吸附量为2.30mg·g-1,果壳GAC的PCE饱和吸附量为4.30mg·g-1。研究结果表明GAC对PCE的吸附能力更强,且椰壳GAC的VCHs的吸附能力整体强于果壳GAC。活性炭确是VCHs污染物理修复技术中可供使用的便捷材料之一。通过吸附过程碳、氯同位素组成分析发现,碳、氯同位素组成与吸附实验的VCHs剩余份额之间没有显著的相关关系,吸附作用没有导致明显的同位素分馏。与转化过程甚至挥发过程相比,吸附作用导致的同位素组成的变化可忽略不计。原位生物或化学修复技术中,即使存在吸附过程,使用转化过程的同位素富集系数用来计量转化程度时,吸附作用导致的同位素组成的误差限内的变化不会造成显著影响。同时,对于已吸附作用为主要衰减机制的地下水的VCHs,碳、氯同位素组成分析仍可用来解析VCHs的污染来源。
(三)对同位素分馏系数的稳定行进行了分析,并定量评价了同位素富集系数(ε)的变异对瑞利分馏模型量化转化效率的影响：
(1)通过挥发过程碳、氯同位素组成分析发现,VCHs的碳、氯同位素发生了相对明显的分馏。气-液平衡实验研究得到温度为10～35℃,TCE气、液两相的碳同位素组成的差值△13Cvapor-lqiud的均值为+0.49±0.16‰,Δ37Clvapor-lqiud均值为-0.82‰±0.08‰；PCE的△13Cvapor-lqiud均值为±0.68±0.15 ,△37Clvapor-lqiud均值为-0.44±0.08‰。研究结果表明,气-液平衡体系中,气相VCHs亏损碳的重同位素。而挥发过程的同位素分析的实验结果表明,VCHs的碳同位素向着亏损碳的重同位素方向发生变化。因此,可以得出,控制挥发过程的同位素分馏的机制既包括动力学同位素效应,也包括平衡同位素效应。
(2)研究结果还表明气-液平衡过程的同位素分馏会受到温度的影响,而温度影响VCHs的动力挥发速率,从而影响挥发过程的同位素分馏和碳、氯同位素组成的相关变化趋势。对于TCE,在20±1℃的条件下,εC=+0.28±0.02‰、εCl=-1.48±0.04‰;在26±1℃的条件下,εC=+0.29±0.02‰、εCl=-1.18±0.02。对于PCE,在20±1℃的条件下,εC=+0.63±0.04‰、即εCl=-1.00±0.02‰;在26±1℃的条件下,εC=+0.50±0.02‰、εCl=-1.00±0.04.结果表明即使在较低的温度条件下,挥发过程导致的同位素分馏,尤其是氯的同位素分馏不容忽视。
(3)维生素催化降解TCE的批实验研究发现,维生素Biz投加量、反应温度和pH值对TCE的转化效率会产生显著影响,且影响都是线性。维生素B12浓度由43.1mg·L-1增加至215.5mg·L-1时,降解速率由0.068±0.003h-1增加为0.355±0.011h-1,即维生素B12增加了5倍,反应速率亦增加了约5倍。当反应温度由20℃增加至40℃时,TCE的降解速率由0.102±0.005h-1增加至0.361±0.013h-1。然而分析结果表明,碳同位素效应显著与否与TCE的转化速率没有一定的相关关系,批实验获得碳同位素富集系数εC并没有显著差异,均值为-15.9±0.2‰。维生素B12催化降解TCE时的限速反应存在两种竞争性的降解途径,由于溶液初始pH的变化使得各降解途径所占份额发生改变,因此碳同位素富集系数εC与pH值呈现一定的相关性,且pH=6.5时,εC=-14.0±0.4‰;pH=8.0时,εC=-15.7±0.3‰;pH=9.0时,εC=-18.0±0.3‰。总的碳同位素富集系数较单体TCE的碳同位素富集系数小的多,且pH=7.5时,εC.bulk=-1.3±0.1‰,pH=9.0时,εC.bulk=-2.5±0.1‰,受到反应初始条件的影响,εC,bulk变化较大;而氯同位素由于质量效应,εCl,hulk受到pH值的影响较小,均为εCl.bulk=-1.6±0.1‰。研究结果表明,维生素B12催化TCE还原脱氯过程的碳同位素效应主要受到降解途径的影响。
(4)利用瑞利分馏模型进行模拟计算降解程度(B)。当碳同位素富集系数εC相对最小值-14.0‰和相对最大值-18.0‰来计算降解程度B时,二者的最大差值可达10%以上。研究结果表明,选择合适的同位素富集系数进行场地应用,是保证监测结果准确性的必备条件之一。
本研究主要有两个创新点：①揭示了TCE在维生素B12催化还原脱氯过程中的碳、氯同位素分馏机理及其影响因素;②发现了TCE和PCE挥发过程的碳同位素分馏主要由气-液平衡机制控制,氯同位素分馏则由动力学机制控制。
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.
在地下含水层中,对流是驱动地下污染物运动的主要机制,而弥散、扩散、吸附、补给、挥发等过程也会对污染物的浓度和迁移速率产生影响,这些过程会导致VCHs在地下水中的分布发生改变。同时有机污染物在地下水中可能沿着多个竞争性的降解途径发生浓度的衰减,有时是竞争性的、酶解的和无机的反应途径,这取决于化合物的分子结构和环境条件。引起污染物产物浓度变化的原因往往不易肯定。对于地下水污染原位修复技术的评估中,关键即是辨析导致浓度衰减的主要机制,以有效评价转化效率。单体同位素分析技术(CSIA,Compound-specific Stable Isotope Analysis)则为此提供了一种有效的方法,特别是MD(多维)-CSIA技术的日趋完善和成熟。CSIA应用的关键问题和前提条件是研究并表征污染物迁移转化过程的同位素效应。目前,国内外关于VCHs碳、氯同位素组成特征和不同衰减过程的碳、氯同位素效应的研究尚未系统开展。因此,本研究开展了VCHs还原脱氯过程、挥发过程和吸附过程的碳、氯同位素分析研究,以期为运用CSIA、MD-CSIA技术识别与量化衰减过程(或降解机理),特别是为定量评价原位修复方法的有效性提供科学依据和技术参数。
论文主要研究内容、思路和方法如下：
(1)首先开展低浓度VCHs样品的碳、氯同位素的测试技术研究,重点是建立VCHs含量和碳、氯同位素测试技术。需要建立的含量分析技术为HS-SPME-GC(顶空-固相微萃取-气相色谱法),主要为分析转化和吸附过程中的VCH的含量变化。同位素分析技术包括碳同位素在线测试技术HS-SPME-GC/C-IRMS(顶空-固相微萃取-气相色谱／高温燃烧-同位素比值质谱仪),为测定转化过程的VCH的碳同位素组成；还拟建立碳、氯同位素线外分析技术,以同时测定转化过程VCH的碳、氯同位素组成,以及挥发及吸附过程的VCH的碳、氯同位素组成。(2)基于建立的碳、氯同位素分析技术,测定不同来源的VCHs的碳、氯同位素组成,若各VCH的碳、氯同位素组成具有各自独特的分布,即所谓的“同位素指纹特征”,则可为“同位素指纹特征”应用于源解析提供佐证。并分析同位素特征值的成因,进一步确定“同位素指纹特征”在地下水污染源源解析中实践的可行性。(3)基于建立的含量和同位素分析技术,进行控制性条件实验,探究VCHs在不同衰减过程中的碳、氯同位素组成变化。若转化过程、挥发过程和吸附过程中存在显著的同位素分馏,则建立降解过程的同位素动力学分馏模型,关键是获取同位素分馏系数。对比分析不同类型衰减过程同位素分馏的程度和方向,为碳、氯同位素分馏应用于衰减过程解析提供理论和实践依据。(4)由于环境条件或化合物物化性质对于不同过程的碳、氯同位素效应的影响十分复杂,基于建立的含量和同位素分析技术,探寻显著性碳、氯同位素效应的影响机制,关键是校验获得的同位素富集系数的稳定性和适用性。若同位素富集系数存在变异,则讨论同位素富集系数不稳定的影响因素及其影响机制,特别是量化变异的同位素富集系数在实地应用时可能导致的降解程度的拟合计算值与实际同位素系数拟合计算值的差异。
本文建立的HS-SPME-GC/C-IRMS在线碳同位素的分析误差为±0.35‰(n=3);纯试剂的碳、氯同位素线外分析技术的方法误差分别为±0.12%o(n=5),±0.14‰(n=5);水样的低压真空萃取的碳、氯同位素分析技术的方法误差分别为±0.26‰(n=6),±0.15‰(n=6)。基于建立的高精度分析技术,系统探讨了稳定碳、氯同位素分析在地下水污染物示踪、环境行为研究及环境监测中实践应用的可行性。特别是基于瑞利分馏模型和动力同位素效应的相关理论,对不同类别衰减过程的碳、氯同位素组成的变化特征进行了系统解析。得出以下主要结论：
(一)分析了VCHs的碳、氯“同位素指纹特征”,并揭示了碳、氯同位素组成差异的成因,指出导致同位素组成发生变化的潜在因素:
(1)对国内氯代烃主要生产厂商产出的四种VCHs试剂进行了分析测试,试剂的δ13C值在-51.09‰到-24.62%o之间,δ37Cl值在-1.23‰到6.73‰之间。研究结果表明,各类试剂的碳、氯同位素组成具有各自独特的分布。
(2)不同来源VCHs试剂碳同位素组成具有差异,主要是由于不同厂商使用的原料其碳同位素组成有差别,而造成氯同位素组成差异的原因主要是由于不同的生产工艺流程,使氯同位素在VCHs试剂生产过程中发生了不同程度和方向的分馏。
(3)联合碳、氯同位素,VCHs试剂的二维同位素特征具有更为明显的“同位素指纹”特征,与单一同位素相比,二维同位素组成分析在地下水VCHs源解析研究中应当具有更大的应用潜力。
(4)通过理论研究指出VCHs碳、氯同位素组成在赋存环境中发生变化的原因即是由于不同迁移转化过程的同位素效应导致,同时详细介绍了同位素分馏程度的表征—瑞利分馏模型,为后续不同过程的动力学分馏及其机理的分析研究提供理论基础。
(二)获取了不同过程的VCHs的衰减特征及其碳、氯同位素分馏系数,揭示了同位素分馏是衰减过程解析重要的工具:
(1)维生素B12催化降解TCE可以归纳为三步：TCE与维生素B12络合反应；通过水解、氢解等反应TCE被还原,维生素B12被氧化：Ti(Ⅲ)将氧化态的维生素B12还原而自身被氧化为Ti(Ⅳ)。实验获得了维生素B12催化降解TCE的活化能为56.7～41.2kJ·mol-1,维生素B12的投加大大降低了还原脱氯体系的活化能。维生素B12的出现不仅促使反应发生,而且体现了它的高效性。在合适的反应条件下,TCE理论初始浓度为113mg·L-1,10h内TCE的转化率即可达97%。维生素B12是一种原位化学修复的潜在可利用材料之一。鉴于CSIA技术在污染物环境行为和环境监测领域的实践应用意义,对维生素B12催化TCE还原脱氯过程的单体碳同位素分馏进行了研究。研究结果表明催化还原脱氯过程导致TCE的单体碳同位素组成变化明显,随着转化程度的增加,δ13C值逐渐偏正。维生素B12催化TCE还原脱氯过程导致TCE发生了显著的碳同位素分馏。碳同位素富集系数ε达-14.0‰～-18.0‰。并利用线外分析技术,对转化过程总得碳、氯同位素组成进行了时间序列的分析。总的碳同位素富集系数较单体TCE的碳同位素富集系数小的多,εC.bulk=-1.3～-2.5‰,εCl.bulk为-1.6‰。研究发现转化过程的TCE的碳、氯同位素组成呈正相关的变化趋势。
(2)通过挥发过程碳、氯同位素组成分析发现,VCHs的碳、氯同位素发生了相对明显的分馏。对于TCE,碳同位素富集系数为εc为+0.28‰～+0.29‰,氯同位素富集系数为εC1为-1.48‰～-1.18‰。对于PCE,碳同位素富集系数为εC为+0.63‰～+0.50‰,氯同位素富集系数为εCl为-1.00‰。在利用同位素分馏量化VCHs的转化效率时,挥发过程导致的同位素分馏产生的“假面效应”不容忽视。然而研究发现动力挥发过程导致碳同位素的分馏方向与氯同位素分馏方向相反,与已有的转化过程的相关变化趋势完全不同,因此这种相反的相关变化趋势可又来有效区分挥发过程和转化过程。
(3)通过吸附批实验研究发现,18～20目的椰壳GAC和果壳GAC对TCE和PCE的吸附过程均符合Langmuir模型。且获取了GAC的饱和吸附量：椰壳GAC的TCE饱和吸附量为3.16mg·g-1,椰壳GAC的PCE饱和吸附量为5.64mg/g果壳GAC的TCE饱和吸附量为2.30mg·g-1,果壳GAC的PCE饱和吸附量为4.30mg·g-1。研究结果表明GAC对PCE的吸附能力更强,且椰壳GAC的VCHs的吸附能力整体强于果壳GAC。活性炭确是VCHs污染物理修复技术中可供使用的便捷材料之一。通过吸附过程碳、氯同位素组成分析发现,碳、氯同位素组成与吸附实验的VCHs剩余份额之间没有显著的相关关系,吸附作用没有导致明显的同位素分馏。与转化过程甚至挥发过程相比,吸附作用导致的同位素组成的变化可忽略不计。原位生物或化学修复技术中,即使存在吸附过程,使用转化过程的同位素富集系数用来计量转化程度时,吸附作用导致的同位素组成的误差限内的变化不会造成显著影响。同时,对于已吸附作用为主要衰减机制的地下水的VCHs,碳、氯同位素组成分析仍可用来解析VCHs的污染来源。
(三)对同位素分馏系数的稳定行进行了分析,并定量评价了同位素富集系数(ε)的变异对瑞利分馏模型量化转化效率的影响：
(1)通过挥发过程碳、氯同位素组成分析发现,VCHs的碳、氯同位素发生了相对明显的分馏。气-液平衡实验研究得到温度为10～35℃,TCE气、液两相的碳同位素组成的差值△13Cvapor-lqiud的均值为+0.49±0.16‰,Δ37Clvapor-lqiud均值为-0.82‰±0.08‰；PCE的△13Cvapor-lqiud均值为±0.68±0.15 ,△37Clvapor-lqiud均值为-0.44±0.08‰。研究结果表明,气-液平衡体系中,气相VCHs亏损碳的重同位素。而挥发过程的同位素分析的实验结果表明,VCHs的碳同位素向着亏损碳的重同位素方向发生变化。因此,可以得出,控制挥发过程的同位素分馏的机制既包括动力学同位素效应,也包括平衡同位素效应。
(2)研究结果还表明气-液平衡过程的同位素分馏会受到温度的影响,而温度影响VCHs的动力挥发速率,从而影响挥发过程的同位素分馏和碳、氯同位素组成的相关变化趋势。对于TCE,在20±1℃的条件下,εC=+0.28±0.02‰、εCl=-1.48±0.04‰;在26±1℃的条件下,εC=+0.29±0.02‰、εCl=-1.18±0.02。对于PCE,在20±1℃的条件下,εC=+0.63±0.04‰、即εCl=-1.00±0.02‰;在26±1℃的条件下,εC=+0.50±0.02‰、εCl=-1.00±0.04.结果表明即使在较低的温度条件下,挥发过程导致的同位素分馏,尤其是氯的同位素分馏不容忽视。
(3)维生素催化降解TCE的批实验研究发现,维生素Biz投加量、反应温度和pH值对TCE的转化效率会产生显著影响,且影响都是线性。维生素B12浓度由43.1mg·L-1增加至215.5mg·L-1时,降解速率由0.068±0.003h-1增加为0.355±0.011h-1,即维生素B12增加了5倍,反应速率亦增加了约5倍。当反应温度由20℃增加至40℃时,TCE的降解速率由0.102±0.005h-1增加至0.361±0.013h-1。然而分析结果表明,碳同位素效应显著与否与TCE的转化速率没有一定的相关关系,批实验获得碳同位素富集系数εC并没有显著差异,均值为-15.9±0.2‰。维生素B12催化降解TCE时的限速反应存在两种竞争性的降解途径,由于溶液初始pH的变化使得各降解途径所占份额发生改变,因此碳同位素富集系数εC与pH值呈现一定的相关性,且pH=6.5时,εC=-14.0±0.4‰;pH=8.0时,εC=-15.7±0.3‰;pH=9.0时,εC=-18.0±0.3‰。总的碳同位素富集系数较单体TCE的碳同位素富集系数小的多,且pH=7.5时,εC.bulk=-1.3±0.1‰,pH=9.0时,εC.bulk=-2.5±0.1‰,受到反应初始条件的影响,εC,bulk变化较大;而氯同位素由于质量效应,εCl,hulk受到pH值的影响较小,均为εCl.bulk=-1.6±0.1‰。研究结果表明,维生素B12催化TCE还原脱氯过程的碳同位素效应主要受到降解途径的影响。
(4)利用瑞利分馏模型进行模拟计算降解程度(B)。当碳同位素富集系数εC相对最小值-14.0‰和相对最大值-18.0‰来计算降解程度B时,二者的最大差值可达10%以上。研究结果表明,选择合适的同位素富集系数进行场地应用,是保证监测结果准确性的必备条件之一。
本研究主要有两个创新点：①揭示了TCE在维生素B12催化还原脱氯过程中的碳、氯同位素分馏机理及其影响因素;②发现了TCE和PCE挥发过程的碳同位素分馏主要由气-液平衡机制控制,氯同位素分馏则由动力学机制控制。
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.
引文
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