Yunde Liu

Stable carbon isotope fractionation during trichloroethene degradation in magnetite-catalyzed Fenton-like reaction

Mineral-catalyzed Fenton-like oxidation of chlorinated ethylenes is an attractive technique for in situ soil and groundwater remediation. Stable carbon isotope enrichment factors associated with magnetite-catalyzed Fenton-like oxidation of trichloroethylene (TCE) have been determined, to study the possibility of applying stable carbon isotope analysis as a technique to assess the efficacy of remediation implemented by Fenton-like oxidation. The carbon enrichment factors (ε values) ranged from -2.7‰ to -3.6‰ with a mean value of -3.3±0.3‰, and only small differences were observed for different initial reactive conditions. The ε values were robust and reproducible, and were relatively insensitive to a number of environmental factors such as ratios of reactants and PCE co-contamination, which can reduce the uncertainty associated with application of isotope enrichment factors for quantification of in situ remediation by Fenton-like reaction. ε values for Fenton-like oxidation of TCE were intermediate in those previously reported for aerobic biological processes (ε=-1.1 to -20.7‰). Thus, field-derived ε values that are more negative than those for Fenton-like oxidation, may indicate the occurrence of aerobic biodegradation at contaminated sites undergoing in situ remediation with Fenton-like reaction. However, stable carbon isotope analysis is unable to determine whether there is the occurrence of biodegradation processes if field-derived ε values are less negative than those for Fenton-like oxidation.

A technique for carbon and chlorine isotope analyses of chlorinated aliphatic hydrocarbons in groundwater

Chlorinated aliphatic hydrocarbons (CAHs), significant contaminants in groundwater, can be characterized by stable isotopic compositions of carbon and chlorine. Previously published methods were of low analytical sensitivity or not ideal for natural samples with low concentrations of CAHs. This method is reported here to carry out simultaneously carbon and chlorine isotope analyses for micromolar concentrations of dissolved CAHs. It was executed by extracting and converting CAHs to carbon dioxide and methyl chloride (CH3Cl). Specially, a continuous-flow interface GasBench II was used to extract CH3Cl for online chlorine isotope analysis. As a result, it greatly enhances the efficiency for isotope analysis by eliminating procedures for offline CH3Cl preparation and separation. Sample size requirement was reduced to approximately 11 μmol chlorine. The standard deviation of δ13C and δ37Cl for both TCE solvents and water samples was better than 0.30‰ and 0.20‰ (1σ), respectively. Carbon and chlorine isotope analyses can be used as an important tool to study the sources of organic contaminants in groundwater and their behaviors in the aquifers. The method is applicable to manufacturers’ products as well as a sample from a polluted site in principle, which will be validated in our field studies.

Variability in carbon isotope fractionation of trichloroethene during degradation by persulfate activated with zero-valent iron: Effects of inorganic anions.

Stable carbon isotope analysis has the potential to be used for assessing the performance of in situ remediation of organic contaminants. Successful application of this isotope technique requires understanding the magnitude and variability in carbon isotope fractionation associated with the reactions under consideration. This study investigated the influence of inorganic anions (sulfate, bicarbonate, and chloride) on carbon isotope fractionation of trichloroethene (TCE) during its degradation by persulfate activated with zero-valent iron. The results demonstrated that the significant carbon isotope fractionation (enrichment factors ε ranging from -3.4±0.3 to -4.3±0.3‰) was independent on the zero-iron dosage, sulfate concentration, and bicarbonate concentration. However, the ε values (ranging from -7.0±0.4 to -13.6±1.2‰) were dependent on the chloride concentration, indicating that chloride could significantly affect carbon isotope fractionation during TCE degradation by persulfate activated with zero-valent iron. The dependence of ε values on chloride concentration, indicated that TCE degradation mechanisms may be different from the degradation mechanism caused by sulfate radical (SO4(-)). Ignoring the effect of chloride on ε value may cause numerous uncertainties in quantitative assessment of the performance of the in situ chemical oxidation (ISCO).

Carbon and chlorine isotope fractionation during Fenton-like degradation of trichloroethene.

Dual isotope approach has been proposed as a viable tool for characterizing and assessing in situ contaminant transformation, however, little data is currently available on its applicability to chlorinated ethenes. This study determined carbon and chlorine isotope fractionation during Fenton-like degradation of trichloroethene (TCE). Carbon and chlorine isotope enrichment factors were εC=-2.9 ± 0.3‰ and εCl=-0.9 ± 0.1‰, respectively. An observed small secondary chlorine isotope effect (AKIECl=1.001) was consistent with an initial transformation by adding hydroxyl radicals (OH) to CC bonds without cleavage of CCl bonds. The relative change in carbon and chlorine isotope ratios (Δ=Δδ(13)C/Δδ(37)Cl) was calculated to be 3.1 ± 0.2, approximately equal to the ratio of chlorine and carbon isotope enrichment factors (εC/εCl=3.2). The similarity of the Δ (or εC/εCl) values between Fenton-like degradation and microbial reductive dechlorination of TCE was observed, indicating that application of solely dual isotope approach may be limited in distinguishing the two transformation pathways.

Effects of inorganic anions on carbon isotope fractionation during Fenton-like degradation of trichloroethene

Understanding the magnitude and variability in isotope fractionation with respect to specific processes is crucial to the application of stable isotopic analysis as a tool to infer and quantify transformation processes. The variability of carbon isotope fractionation during Fenton-like degradation of trichloroethene (TCE) in the presence of different inorganic ions (nitrate, sulfate, and chloride), was investigated to evaluate the potential effects of inorganic anions on carbon isotope enrichment factor (ε value). A comparison of ε values obtained in deionized water, nitrate solution, and sulfate solution demonstrated that the ε values were identical and not affected by the presence of nitrate and sulfate. In the presence of chloride, however, the ε values (ranging from -6.3±0.8 to 10±1.3‰) were variable and depended on the chloride concentration, indicating that chloride could significantly affect carbon isotope fractionation during Fenton-like degradation of TCE. Thus, caution should be exercised in selecting appropriate ε values for the field application of stable isotope analysis, as various chloride concentrations may be present due to naturally present or introduced with pH adjustment and iron salts during Fenton-like remediation. Furthermore, the effects of chloride on carbon isotope fractionation may be able to provide new insights about reaction mechanisms of Fenton-like processes.

Roles of hydroxyl and sulfate radicals in degradation of trichloroethene by persulfate activated with Fe2+ and zero-valent iron: Insights from carbon isotope fractionation

Active species including hydroxyl (HO) and sulfate radicals (SO4-) play important roles in contaminant degradation during the persulfate based in-situ chemical oxidation (ISCO) process. The generation and contribution of active species are critical and can potentially be evaluated using compound specific isotope analysis (CSIA). However, the evaluation of stable isotope fractionation (or isotope enrichment factor ε values) for contaminants degraded by individual active species of concern is required but lacking. This study firstly determined the carbon isotope fractionation of trichloroethene (TCE) degradation by SO4·- with chemical probe methods to obtain ε values from -6.4±0.7 to -6.9±0.5‰. The ε values were significantly different from those reported for TCE degradation by HO, which could be used to identify the competing TCE degradation by HO and SO4-. Relying on the observed ε values and the extended Rayleigh-type equation, the contributions of SO4- and HO to TCE degradation were evaluated in persulfate activated by Fe0 or Fe(II). This study provides an illuminating idea to determine stable isotope fractionation for contaminant degradation by individual active species, which is crucial for the application of CSIA in relevant environments.

Investigation of Stable C and Cl Isotope Effects of Trichloroethene and Tetrachloroethylene during Evaporation at Different Temperatures

AbstratThere are variations of reported isotope enrichment factors of chlorinated organic contaminants in evaporation processes. Trichloroethene (TCE) and tetrachloroethylene (PCE) were chosen to study carbon and chlorine isotope effects during evaporation at different temperatures. Equilibrium vapor-liquid carbon and chlorine isotope effects experiments were also conducted. In the equilibrium liquid-vapor system, the 13C was enriched but 37Cl was depleted in the vapor phase, being consistent with previous results. For evaporation average carbon isotope enrichment factor ɛC were +0.28‰±0.01‰ for TCE and +0.56‰±0.09‰ for PCE at temperature from 20 to 26 °C. Meanwhile, average chlorine isotope enrichment factor ɛCl were −1.33‰±0.21‰ for TCE and −1.00‰±0.00‰ for PCE. The results indicate that during evaporation the equilibrium isotope effect attenuates the magnitude of carbon isotope fractionation whereas enhances the chlorine isotope effect. Isotope fractionation during evaporation is determined by both equilibrium and kinetic factors. Chlorine isotope fractionation is influenced by the evaporation rate which is linked to temperature. When using stable isotope to investigate the behavior of chlorinated organic contaminants in groundwater with slow biodegradation rate, the isotope fractionation resulted from evaporation should be taken into consideration. Furthermore, the environment conditions such as temperature are also factors to be considered.