Carbon And Chlorine Isotope Fractionation During Trichloroethene Degradation In Fenton-like Reaction And Its Environmental Significance

In recent decades, contamination of soil and groundwater systems caused by organic contaminants such as chlorinated hydrocarbons become increasingly prominent due to human activities. To research environmental behaviors and in situ remediation technologies of chlorinated hydrocarbons in subsurface, has being the research focus in the environment field.A key issue for in situ soil and groundwater remediation is how to accurately quantify the extent of contaminants degradation and to verify effectiveness and applicability of remediation methods. Isotope analysis is among the most promising tools for resolving the issue. A prerequisite and critical parameter for applying this tool in the field is to determine and characterize the magnitude and variability isotope fractionation (i.e. isotope enrichment factor ε) associated with specific degradation processes and to understand the factors that influence this parameter. In order to better play the role and value of isotope analysis in the field of environmental science, it is essential to constantly enrich the database for isotope fractionation associated with various transformation processes.Mineral-catalyzed Fenton-like reaction is a promising alternative for in situ soil and groundwater remediation. Naturally-occurring iron-bearing minerals, which are widespread in soils or aquifers, can catalyze Fenton-like reaction to degradation of recalcitrant organic contaminants without the addition of external soluble iron, and can also be used to build in situ permeable reactive barrier based on Fenton-like reaction. Meanwhile, the oxygen released from Fenton-like reaction during in situ remediation process, may also provide sufficient electron acceptor to enhance in situ aerobic biodegradation. The research about mineral-catalyzed Fenton-like reaction to degrade organic contaminants, not only greatly enriches the environmental mineralogy and advanced oxidation in situ remediation technology, but also promotes the crossing and infiltrating of disciplines in environment, chemistry, geology, and comprehensive utilization for mineral resources. To date, carbon and chlorine isotope fractionation associated with degradation of chlorinated hydrocarbons by Fenton-like reaction has not yet been studied.Thus, for the critical scientific question about how to accurately and effectively evaluate the in situ soil and groundwater remediation, this study is the first to investigate carbon and chlorine isotope fractionation during trichlorothene (TCE) degradation in Fenton-like reaction. It can provide the theory basis and technical parameters for applying isotope analysis to quantify and verify the effectiveness and applicability of in situ remediation implemented by Fenton-like reaction.The main objectives of this study were to characterize carbon and chlorine isotope fractionation associated with TCE degradation by Fenton-like reaction, and to reveal the mechanism and potential effect of factors for carbon and chlorine isotope fractionation during TCE degradation in Fenton-like reaction, and to evaluate the uncertainty for quantifying in situ degradation implemented by Fenton-like reaction using carbon and chlorine isotope analysis approach.The main research contents, ideas and methods are as follows:Firstly, analytical methods for TCE concentration and its carbon and chlorine isotope composition were established, which could provide technical means for the follow-up studies on carbon and chlorine isotope fractionation during TCE degradation in Fenton-like reaction. Secondly, the kinetics of TCE degradation by magnetite-catalyzed Fenton-like reaction were investigated, which were preliminary experiments for the follow-up studies on carbon and chlorine isotope fractionation during TCE degradation in Fenton-like reaction. Thirdly, applying carbon and chlorine isotope analysis techniques and based on isotope fractionation theory and combined with previous studies of carbon and chlorine isotope fractionation in other transformation processes, compound-specific carbon isotope fractionation and two-dimensional carbon and chlorine isotope fractionation associated with TCE degradation by Fenton-like reaction were characterized. After that, the potential effects of environmental factors (NO3-, SO42-, Cl", and humic acid) for carbon and chlorine isotope fractionation during TCE degradation in Fenton-like reaction were revealed. Finally, the uncertainty for quantifying in situ degradation implemented by Fenton-like reaction using the Rayleigh equation approach due to the influence of isotope composition and isotope enrichment factor was evaluated.The following conclusions can be drawn from the present study:(A) Carbon and chlorine isotope fractionation during TCE degradation in Fenton-like reaction were characterized, and the mechanism for carbon and chlorine isotope fractionation was revealed.(1) Significant carbon and chlorine isotope fractionation during TCE degradation in Fenton-like reaction were noticeable, with average carbon and chlorine enrichment factors of ec=-3.0±0.2%o and εCl=-1.0±0.1%o. Carbon and chlorine isotope fractionation for TCE degradation via Fenton-like reaction were substantially different from those observed for zerovalent iron and microbial reductive dechlorination of TCE. One major difference between Fenton-like reaction degradation and reductive dechlorination of TCE is the reaction mechanism for TCE degradation, which the cleavage of a carbon-carbon double bond (C=C) to a single carbon bond (C-C) as the rate-determining step associated with Fenton-like oxidation is different from the cleavage of C-Cl bond as the first step during microbial and abiotic reductive dechlorination(2) Carbon isotope fractionation associated with TCE degradation by magnetite-catalyzed Fenton-like reaction was significantly smaller than those reported for permanganate oxidation of TCE and aerobic mineralization of TCE by B. cepacia G4, although those reactions for abiotic and microbial oxidation of TCE have similar mechanisms involving breakage of a carbon-carbon double bond (C=C) as the rate-determining step. The similar carbon isotope fractionation observed for TCE degradation by Fenton reaction in homogeneous solution and magnetite-catalyzed Fenton-like reaction, suggests that the explanation of commitment to catalysis contribute to the small isotope fractionation observed during Fenton-like oxidation of TCE can be precluded. An alternate explanation for the large differences of ε values is likely the result of transition state structure differences between Fenton-like oxidation and other oxidation reactions (permanganate and G4oxidation). Further simulation of the transition state structure using computational methods is necessary to illuminate these relationships.(3) For carbon and chlorine isotope fractionation associated with TCE degradation by Fenton-like reaction, the average apparent kinetic isotope effects for carbon and chlorine were AKIEC=1.0030±0.0002and AKIECl=1.0010±0.0001, respectively. An observed small secondary chlorine isotope effect was consistent with the expected reaction mechanism involving breakage of a C=C bond to single carbon bond as the rate-determining step.(4) For the same element, secondary isotope effects are generally at least one order of magnitude smaller than primary isotope effects. It has been proposed that hydroxyl radicals react with organic compounds via three mechanisms:hydrogen atom abstraction, electron transfer, and addition to multiple bonds. For the hydrogen abstraction reaction, one C-H bond is broken in the rate-limiting step for TCE degradation by Fenton-like reaction. The AKIECl is1.003(or AKIECl-1=0.003), which is about a quarter of the primary chlorine isotope effect (KIECl=1.013, or KIECl-1=0.013). Thus, it is inconsistent with secondary isotope effects for chlorine, and proposed mechanism that hydroxyl radicals react with organic compounds via hydrogen atom abstraction can be precluded. For the other two mechanisms:electron-transfer and addition to the double C=C bond, the AKIECl is1.001(or AKIECl-1=0.001), and is about one thirteenth of the primary chlorine isotope effect (KIECl=1.013, or KIECl-1=0.013), which is consistent with secondary isotope effects for chlorine.(5) For carbon and chlorine isotope fractionation associated with TCE degradation by Fenton-like reaction, the relative change in carbon and chlorine isotope ratios (Λ=△δ37Cl/△δ13C) was calculated to be0.348±0.049, and approximatively equal to the ratio of chlorine and carbon isotope enrichment factors (εCl/εC=0.349±0.057). The A value for TCE degradation via Fenton-like reaction was substantially different from the εCl/εC value for reductive dechlorination of TCE by zerovalent iron. The differences in the relative changes in isotope ratios of carbon and chlorine (or the ratio of chlorine and carbon isotope enrichment factors) for different reaction mechanisms, implies that it can be applied for evaluating transformation processes. In order to better play the role and worth of the relative changes in isotope ratios of carbon and chlorine, it is essential to further investigate the relative changes in isotope ratios of carbon and chlorine and its potential effect factors for various transformation processes.(6) Environmental significance.①Significant carbon isotope fractionation effects were noticeable during TCE degradation in magnetite-catalyzed Fenton-like reaction. Thus, stable carbon and chlorine isotope analysis in combination with the Rayleigh equation approach, can be used to independently quantify the efficacy of in situ remediation implemented by Fenton-like reaction and authenticate each other.②The results demonstrate significant differences in isotope fractionation for Fenton-like oxidation and aerobic biological processes, which is a first step toward potentially distinguishing between Fenton-like oxidation versus aerobic biological processes. The previously reported ε value (ε=-18.2to-20.7%o) for microbial TCE transformation by B. cepacia G4was more negative than those for Fenton-like oxidation. Thus, field-derived ε values that are more negative than those for Fenton-like oxidation, may indicate the occurrence of aerobic biological processes at contaminated sites undergoing in situ remediation with Fenton-like reaction. The more negative field-derived ε values, the more aerobic biological processes contribute to the transformation of TCE. However, field-derived ε values that are less negative than those for Fenton-like oxidation are unable to determine whether there is the occurrence of aerobic biological processes such as B. cepacia OB3b with only carbon isotope analysis, because non-fractionating processes such as physical processes may also result in "dilution" or underestimation of field-derived εvalues at contaminated sites undergoing in situ remediation with Fenton-like reaction. In these cases, other lines of evidence should be considered, including hydrogeochemical conditions, biogeochemical indicators, multidimensional compound-specific isotope analysis, etc.③The two-dimensional carbon and chlorine isotope fractionation (εC,εCl, AKIEc, AKIECl, A) associated with TCE degradation by Fenton-like reaction was significant different from those observed for other attenuation processes. Thus, two-dimensional carbon and chlorine analysis can provide more exhaustive information for distinguishing between Fenton-like reaction versus other attenuation processes (or degradation mechanisms).(B) The potential effect of factors for carbon and chlorine isotope fractionation during TCE degradation in Fenton-like reaction was revealed.(1) The carbon enrichment factors (εC) were robust and reproducible, and relatively insensitive to different initial reactive conditions (magnetite/H2O2dosage and delivery method, TCE concentration, TCE/PCE co-contamination). There was no discernible relationship between carbon enrichment factors (εC) and apparent first-order rate constants (kobs), which indicates that variations of reaction rate do not directly affect carbon isotope fractionation for TCE degradation in Fenton-like reaction.(2) Enrichment factors for TCE degradation by Fenton-like reaction under various initial concentrations of nitrate ion (NO3-), sulfate ion (SO42-), and humic acid agreed closely, which indicates that NO3-, SO42-, and humic acid do not significantly affect carbon and chlorine isotope fractionation for TCE degradation in Fenton-like reaction.(3) An increase in the initial chloride ion (Cl-) concentration from0to2M caused the carbon isotope enrichment factors (εC) ranged from-3.0‰to-7.7‰, and the chlorine isotope enrichment factors (εCl) ranged from-0.6‰to-1.1‰for TCE degradation in Fenton-like reaction, respectively. There was a significant positive linear correlation between the Cl-concentrations and the εC values (R2=0.992). This indicates that Cl-significantly affected carbon isotope fractionation for TCE degradation in Fenton-like reaction. The effect of Cl-on isotope fractionation may be explained by the change in reaction mechanism due to the scavenging of Cl-for hydroxyl radicals (HO·) to generate inorganic radicals such as Cl2-, and carbon isotope fractionation for TCE degradation by Cl2-may be substantially larger than those observed for TCE degradation by HO-. Follow up studies are required to characterize the isotope fractionation associated with Cl2·-radicals degradation of TCE using a more appropriate method for generating inorganic radicals.(4) Environmental significance.Chloride ion (Cl-) significantly affected isotope fractionation for TCE degradation in Fenton-like reaction. Thus, for a quantitative assessment of in situ remediation using the Rayleigh equation approach, it is crucial and necessary to select an appropriate and representative isotope enrichment factors (ε) according to the Cl-concentration in fields. The isotope fractionation associated with TCE degradation by Fenton-like reaction was relatively insensitive to other factors (magnetite/H2O2dosage and delivery method, TCE concentration, TCE/PCE co-contamination, NO3-, SO42-, and humic acid), which can reduce the uncertainty associated with application of carbon isotope analysis for quantification of in situ remediation by Fenton-like reaction.(C) The uncertainty for quantifying in situ degradation implemented by Fenton-like reaction using carbon and chlorine isotope analysis was evaluated.(1) The uncertainty of degradation extent (B) introduced by analytical errors of isotope composition steadily decreases towards higher amounts of degradation. The uncertainty of degradation extent (B) introduced by the uncertainty in isotope enrichment factors firstly increases and then decreases with the increase of degradation extent (B).(2) If aerobic degradation of TCE by OB3b or G4occur simultaneously at contaminated sites undergoing in situ remediation implemented by Fenton-like reaction, the extent of degradation may be underestimated or overestimated using the Rayleigh equation approach with the carbon enrichment factor for TCE degradation in Fenton-like reaction. Therefore, for a quantitative assessment of in situ remediation using the Rayleigh equation approach, it is crucial and necessary to select an appropriate and representative isotope enrichment factors (ε) for specific degradation processes and to understand the factors that influence this parameter.The main innovations are as follows:To provide the theory basis and technical parameters for applying isotope analysis to quantify and verify the effectiveness and applicability of in situ remediation implemented by Fenton-like reaction, this study is the first to investigate carbon and chlorine isotope fractionation during trichlorothene (TCE) degradation in Fenton-like reaction:(1) The carbon and chlorine isotope fractionation for TCE degradation in Fenton-like reaction has been first determined and characterized.(2) The potential effects of reactive conditions and environmental factors for carbon and chlorine isotope fractionation during TCE degradation in Fenton-like reaction has been studied systematically, and the variation for the carbon isotope enrichment factor (εc) of TCE degradation by Fenton-like reaction due to Cl-effect was revealed.