Linnea J. Heraty

Carbon and chlorine isotope fractionation of chlorinated aliphatic hydrocarbons by evaporation

Two pure solvents (trichloroethene and dichloromethane) were evaporated at room temperature (24±1°C). The carbon and chlorine isotopic compositions of the residual solvents were measured as a function of the fraction remaining. Isotopic fractionation factors (α) derived from these experimental results are 1.00031±0.00004 for C and 0.99818±0.00022 for Cl in trichloroethene, and 1.00065±0.00002 for C and 0.99952±0.00006 for Cl in dichloromethane. The negative correlation between δ13C and δ37Cl values caused by evaporation of chlorinated aliphatic hydrocarbons can be used to distinguish evaporation from biodegradation in contaminated systems.

Isotopic fractionation of carbon and chlorine by microbial degradation of dichloromethane

Microbial culture experiments were performed at 22°C to investigate isotopic fractionation of carbon (C) and chlorine (Cl) during aerobic degradation of dichloromethane (DCM) by MC8b, a gram-negative methylotrophic organism closely related to the genera Methylobacterium or Ochrobactrum. Values of the isotopic fractionation factor, α, were estimated from the experimental results. These values are 0.9576±0.0015 for C and 0.9962±0.0003 for Cl, corresponding to kinetic isotope effects (KIE) of 1.0424 for C and 1.0038 for Cl. The C isotope effect accompanying aerobic degradation of DCM by MC8b is roughly twice that resulting from microbial oxidation of CH4. Further studies of this type, for a variety of chlorinated aliphatic hydrocarbons (CAHs) under a range of biogeochemical conditions, may find important applications in the characterization and remediation of sites contaminated with CAHs.

Stable chlorine and carbon isotopic compositions of selected semi-volatile organochlorine compounds

Abstract To assess whether the isotopic composition of semi-volatile organochlorine compounds (SVOCs) may be a useful tool, we measured the bulk δ37Cl and δ13C values of several pesticides and Aroclor mixtures from different suppliers. Overall, the δ37Cl and δ13C values ranged from −5.10 to +1.22‰ and −31.63 to −22.39‰, respectively. These values are narrower than the ranges observed previously for volatile organic contaminants (VOCs). In particular, the isotopic compositions of the Aroclor mixtures were very tightly constrained for both chlorine and carbon. We also observed that SVOCs synthesized from hexachlorocyclopentadiene had the most enriched δ37Cl values. These data provide a baseline for future work employing isotope ratios to study the environmental fate of SVOCs.

Isotopic Mapping of Groundwater Perchlorate Plumes

Analyses of stable isotope ratios of chlorine and oxygen in perchlorate can, in some cases, be used for mapping and source identification of groundwater perchlorate plumes. This is demonstrated here for large, intersecting perchlorate plumes in groundwater from a region having extensive groundwater perchlorate contamination and a large population dependent on groundwater resources. The region contains both synthetic perchlorate derived from rocket fuel manufacturing and testing activities and agricultural perchlorate derived predominantly from imported Chilean (Atacama) nitrate fertilizer, along with a likely component of indigenous natural background perchlorate from local wet and dry atmospheric deposition. Most samples within each plume reflect either a predominantly synthetic or a predominantly agricultural perchlorate source and there is apparently a minor contribution from the indigenous natural background perchlorate. The existence of isotopically distinct perchlorate plumes in this area is consistent with other lines of evidence, including groundwater levels and flow paths as well as the historical land use and areal distribution of potential perchlorate sources.

Fractionation of stable isotopes in perchlorate and nitrate during in situ biodegradation in a sandy aquifer

Environmental context. Perchlorate (ClO4–) and nitrate (NO3–) are common co-contaminants in groundwater, with both natural and anthropogenic sources. Each of these compounds is biodegradable, so in situ enhanced bioremediation is one alternative for treating them in groundwater. Because bacteria typically fractionate isotopes during biodegradation, stable isotope analysis is increasingly used to distinguish this process from transport or mixing-related decreases in contaminant concentrations. However, for this technique to be useful in the field to monitor bioremediation progress, isotope fractionation must be quantified under relevant environmental conditions. In the present study, we quantify the apparent in situ fractionation effects for stable isotopes in ClO4– (Cl and O) and NO3– (N and O) resulting from biodegradation in an aquifer. Abstract. An in situ experiment was performed in a shallow alluvial aquifer in Maryland to quantify the fractionation of stable isotopes in perchlorate (Cl and O) and nitrate (N and O) during biodegradation. An emulsified soybean oil substrate that was previously injected into this aquifer provided the electron donor necessary for biological perchlorate reduction and denitrification. During the field experiment, groundwater extracted from an upgradient well was pumped into an injection well located within the in situ oil barrier, and then groundwater samples were withdrawn for the next 30 h. After correction for dilution (using Br– as a conservative tracer of the injectate), perchlorate concentrations decreased by 78% and nitrate concentrations decreased by 82% during the initial 8.6 h after the injection. The observed ratio of fractionation effects of O and Cl isotopes in perchlorate (ϵ18O/ϵ37Cl) was 2.6, which is similar to that observed in the laboratory using pure cultures (2.5). Denitrification by indigenous bacteria fractionated O and N isotopes in nitrate at a ratio of ~0.8 (ϵ18O/ϵ15N), which is within the range of values reported previously for denitrification. However, the magnitudes of the individual apparent in situ isotope fractionation effects for perchlorate and nitrate were appreciably smaller than those reported in homogeneous closed systems (0.2 to 0.6 times), even after adjustment for dilution. These results indicate that (1) isotope fractionation factor ratios (ϵ18O/ϵ37Cl, ϵ18O/ϵ15N) derived from homogeneous laboratory systems (e.g. pure culture studies) can be used qualitatively to confirm the occurrence of in situ biodegradation of both perchlorate and nitrate, but (2) the magnitudes of the individual apparent ϵ values cannot be used quantitatively to estimate the in situ extent of biodegradation of either anion.

Stable chlorine intramolecular kinetic isotope effects from the abiotic dehydrochlorination of DDT

Intention, Goal, Scope, Background. Identifying different sources and following reaction pathways of chlorinated organic contaminants in the environment can be challenging, especially when only their concentrations are available. Compoundspecific stable chlorine measurements of some contaminants have recently been shown to provide additional information and an increased understanding of their biogeochemistry. These studies, however, have been generally limited to volatile molecules.ObjectiveHere, the stable chlorine isotope ratios of the semi-volatile pesticide, 1, 1, 1-trichloro-2, 2-bis(p-chlorophenyl)ethane (DDT) were investigated. Specifically, the intramolecular stable chlorine isotopic compositions of DDT and the kinetic isotope effect (KIE) for the abiotic dehydrochlorination of DDT to 2, 2-bis(p-chloro-phenyl)-1, 1-dichloroethene (DDE) were determined.MethodsSelective chemical oxidation of DDT to dichlorobenzo-phenone (DCBP) and analysis of each compound was used to calculate the stable chlorine isotope ratios of the alkyl and aromatic chlorines in DDT. To determine the KIE for dehydrochlorination, DDT was reacted in a basic solution to yield DDE at 52°C, 60°C, and 72°C for 3, 5, and 5 days, respectively.Results and DiscussionSignificant intramolecular stable chlorine isotopic differences were observed in one sample of DDT where the alkyl and aromatic δ37C1 values were −5.76 ± 0.45 and −2.21 ± 0.24%‰, respectively. Dehydrochlorination of DDT to DDE in basic solutions at 52, 60, and 70°C resulted in a substantial intramolecular KIE where the alkyl chlorines of DDE shifted by ∼3%‰ relative to the alkyl chlorines in DDT. However, no temperature dependence was observed. The KIE, calculated by an iterative program, was 1.009.ConclusionsIntramolecular differences in the stable chlorine isotope ratios were observed in DDT and this is the first such finding. Dehydrochlorination of DDT yields a measurable and distinct intramolecular stable chlorine KIE.Recommendation and outlookThe results of this study demonstrate the existence of significant intramolecular differences in chlorinated organic compounds. Many other chlorinated semi-volatile and volatile organic contaminants are synthesized from multiple sources of chlorine, and we recommend that similar studies be performed on many such molecules in order to attain a clear understanding of their intramolecular chlorine isotopic differences. The existence of a measurable KIE for the dehydrochlorination of DDT to DDE shows the potential strength of using isotopic measurements to investigate the biogeochemistry of these important compounds. For example, the isotopically depleted aqueous chloride produced by dehydrochlorination of DDT to DDE may be a useful tracer of these reactions in freshwater environments.

Extraction of chlorinated aliphatic hydrocarbons from groundwater at micromolar concentrations for isotopic analysis of chlorine.

A method is described for near-quantitative extraction of micromolar concentrations of chlorinated aliphatic hydrocarbons (CAHs) from water for determination of chlorine (Cl) isotope ratios. A low pressure, carrier-gas procedure of extraction was proven to be applicable to CH2Cl2, CCl4, C2H2Cl2, and C2HCl3. The pH of the water was adjusted with NaOH to prevent extraction of CO2 from air and/or dissolved inorganic carbonate species. Recoveries of CAH samples (approximately 15 mumol), added to and extracted from approximately 340 ml of water, averaged approximately 96%. Average changes in the delta 37Cl values of the CAHs, attributable to the extraction process, were -0.01 +/- 0.06@1000. Significant isotopic fractionation of Cl was measured during partial extraction of C2CHCl3 from water, indicating that near-quantitative extraction is required for reliable stable Cl isotope analysis of CAHs. This method is also suitable for the extraction of dissolved CAH for gas chromatography-combustion-isotope ratio mass spectrometric measurements of hydrogen and carbon.

Method for Purification of Krypton from Environmental Samples for Analysis of Radiokrypton Isotopes

Radiokrypton isotopes ((81)Kr and (85)Kr) are ideal tracers and chronometers of various environmental processes. Atom trap trace analysis (ATTA) is capable of determining the ultralow isotopic abundances of radiokryptons (<10(-12)) provided that 50 microL of pure Kr is available. The analysis by using ATTA of (81)Kr in naturally occurring gases of interest, e.g., dissolved gases in hydrological reservoirs, requires separation of parts-per-million (ppm) level Kr from chemically airlike bulk gas. A newly developed Kr purification system is based on conventional cryogenic distillation and gas chromatography to which continuous monitoring of gas effluent composition using a quadrupole mass spectrometer brings significant advantages. Simple cryogenic distillation is controlled based on the evolution of N2/Ar ratio that is relatively constant in naturally occurring, inorganic gas. Gas chromatographic separation of parts-per-million by volume (ppmv) level Kr from up to a few liters of bulk gas can be achieved by concentrating the Kr under the chromatographic tails of major components. The system described here is capable of extracting Kr of >98% purity from 5-125 L STP (standard temperature and pressure) of bulk gas with >90% yield within several hours. This system is generally useful for separation of microliter amounts of unreactive trace volatile compounds from large-volume gas samples.

Relating Carbon and Nitrogen Isotope Effects to Reaction Mechanisms during Aerobic or Anaerobic Degradation of RDX (Hexahydro-1,3,5-Trinitro-1,3,5-Triazine) by Pure Bacterial Cultures.

ABSTRACT Kinetic isotopic fractionation of carbon and nitrogen during RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) biodegradation was investigated with pure bacterial cultures under aerobic and anaerobic conditions. Relatively large bulk enrichments in 15N were observed during biodegradation of RDX via anaerobic ring cleavage (ε15N = −12.7‰ ± 0.8‰) and anaerobic nitro reduction (ε15N = −9.9‰ ± 0.7‰), in comparison to smaller effects during biodegradation via aerobic denitration (ε15N = −2.4‰ ± 0.2‰). 13C enrichment was negligible during aerobic RDX biodegradation (ε13C = −0.8‰ ± 0.5‰) but larger during anaerobic degradation (ε13C = −4.0‰ ± 0.8‰), with modest variability among genera. Dual-isotope ε13C/ε15N analyses indicated that the three biodegradation pathways could be distinguished isotopically from each other and from abiotic degradation mechanisms. Compared to the initial RDX bulk δ15N value of +9‰, δ15N values of the NO2 − released from RDX ranged from −7‰ to +2‰ during aerobic biodegradation and from −42‰ to −24‰ during anaerobic biodegradation. Numerical reaction models indicated that N isotope effects of NO2 − production were much larger than, but systematically related to, the bulk RDX N isotope effects with different bacteria. Apparent intrinsic ε15N-NO2 − values were consistent with an initial denitration pathway in the aerobic experiments and more complex processes of NO2 − formation associated with anaerobic ring cleavage. These results indicate the potential for isotopic analysis of residual RDX for the differentiation of degradation pathways and indicate that further efforts to examine the isotopic composition of potential RDX degradation products (e.g., NOx) in the environment are warranted. IMPORTANCE This work provides the first systematic evaluation of the isotopic fractionation of carbon and nitrogen in the organic explosive RDX during degradation by different pathways. It also provides data on the isotopic effects observed in the nitrite produced during RDX biodegradation. Both of these results could lead to better understanding of the fate of RDX in the environment and help improve monitoring and remediation technologies.