Stable isotope analyses of oxygen (18 O:17 O:16 O) and chlorine (37 Cl:35 Cl) in perchlorate: reference materials, calibrations, methods, and interferences

Source identification of PCBs in Antarctic air by compound-specific isotope analysis of chlorine (CSIA-Cl) using HRGC/HRMS

Occurrence of persistent organic pollutants (POPs) in the Polar Regions has received great concern in the past several decades due to their long-term adverse effect on biological health in such a fragile environment. However, there is still argument over their source and fate in these pristine areas. Here we attempted to use a novel approach (compound-specific isotope analysis of chlorine, CSIA-Cl) to identify the source of POPs in Antarctic air by comparison with the source area. The results showed that the relative isotope-ratio variation of Cl (δ³⁷Cl') values showed a large variation from - 137 to 9.04 ‰ in the gas-phase samples, and a significantly negative correlation (p < 0.01) was obtained against the logKoa values of PCBs. There were no significant correlations (p > 0.05) observed between the δ³⁷Cl' values and meteorological parameters except for PCB-28 which showed temperature dependence. By contrast, the δ³⁷Cl' values in the urban (Beijing) air ranged from - 12.8 to 2.03 ‰. The larger variation of δ³⁷Cl' in Antarctic air indicated evidently influence of long-range atmospheric transport (LRAT) on isotopologue fractionation of PCBs. This study may shed light on the application of CSIA-Cl for source identification of chlorinated POPs on a large scale.

Isotopic Tracing of Perchlorate Sources in the Environment

Perchlorate (ClO4−) is an emerging persistent pollutant that is ubiquitous in the environment at trace concentrations. Perchlorate ingestion poses a risk to human health because it interferes with thyroidal hormone production. The identification of perchlorate sources in groundwater is a primary concern. Chlorine and multi-oxygen isotopic tracing of perchlorate (δ37Cl, 36Cl/Cl, δ18O, and Δ17O) can provide a unique tool for identifying the origin and transport of perchlorate in groundwater. Along with the kinetic fractionation of chlorine and oxygen isotopes, the Δ17O value, 36Cl/Cl ratio, and ε18O/ε37Cl (the fractionation coefficient of oxygen and chlorine isotopes) are constant, potentially indicating the biodegradation of perchlorate, without disguising its source information. Therefore, comprehensive characterization of stable chlorine and poly-oxygen isotopes is expected to provide direct evidence for identifying the source of perchlorate in groundwater. However, further studies are needed to increase the amount of isotopic data of different perchlorate sources, to make the end-member model available to broader regions. It is critically important to understand the range of values and differences of isotopes among natural perchlorate sources and the perchlorate formation mechanisms.

Isotopic composition of natural and synthetic chlorate (δ18O, Δ17O, δ37Cl, 36Cl/Cl): Methods and initial results

Natural chlorate (ClO₃^-) is widely distributed in terrestrial and extraterrestrial environments. To improve understanding of the origins and distribution of ClO₃^-, we developed and tested methods to determine the multi-dimensional isotopic compositions (δ¹⁸O, Δ¹⁷O, δ³⁷Cl, ³⁶Cl/Cl) of ClO₃^- and then applied the methods to samples of natural nitrate-rich caliche-type salt deposits in the Atacama Desert, Chile, and Death Valley, USA. Tests with reagents and artificial mixed samples indicate stable-isotope ratios were minimally affected by the purification processes. Chlorate extracted from Atacama samples had δ¹⁸O = +7.0 to +11.1‰, Δ¹⁷O = +5.7 to +6.4‰, δ³⁷Cl = -1.4 to +1.3‰, and ³⁶Cl/Cl = 48 × 10^-15 to 104 × 10^-15. Chlorate from Death Valley samples had δ¹⁸O = -6.9 to +1.6‰, Δ¹⁷O = +0.4 to +2.6‰, δ³⁷Cl = +0.8 to +1.0‰, and ³⁶Cl/Cl = 14 × 10^-15 to 44 × 10^-15. Positive Δ¹⁷O values of natural ClO₃^- indicate that its production involved reaction with O₃, while its Cl isotopic composition is consistent with a tropospheric or near-surface source of Cl. The Δ¹⁷O and δ¹⁸O values of natural ClO₃^- are positively correlated, as are those of ClO₄^- and NO₃^- from the same localities, possibly indicating variation in the relative contributions of O₃ as a source of O in the formation of the oxyanions. Additional isotopic analyses of ClO₃^- could provide stronger constraints on its production mechanisms and/or post-formational alterations, with applications for environmental forensics, global biogeochemical cycling of Cl, and the origins of oxyanions detected on Mars.

Sources and behavior of perchlorate in a shallow Chalk aquifer under military (World War I) and agricultural influences

Perchlorate (ClO₄^ö) has been detected at concentrations of concern for human health on a large scale in groundwater used for drinking water supplies in NE France. Two sources are suspected: a military source related to World War I (WWI) and an agricultural source related to past use of Chilean nitrate fertilizers. The sources and behavior of ClO₄^ö have been studied in groundwater and rivers near the Reims city, by monitoring monthly the major ions and ClO₄- concentrations for two years (2017-2019), and by measuring the isotopic composition of ClO₄^ö and NO₃^ö in water samples. ClO₄^ö was detected throughout the study area with high concentrations (> 4 μg⋅L^-1) detected mainly downgradient of the Champagne Mounts, where large quantities of ammunition were used, stored and destroyed during and after WWI. A WWI military origin of ClO₄- is inferred from isotopic analysis and groundwater ages. Different tendencies of ClO₄- variation are observed and interpreted by a combination of ClO₄- concentrations, aquifer functioning and historical investigations, revealing major sources of ClO₄- (e.g., unexploded ordnance, ammunition destruction sites) and its transfer mechanisms in the aquifer. Finally, we show that concentrations of ClO₄^ö in groundwater seems unlikely to decrease in the short- to medium-term.

Environmental fluxes of perchlorate in rural catchments, Ontario, Canada

Quantitative information about fluxes of perchlorate in the environment is lacking. This study reports analyses of perchlorate in various environmental waters sampled from rural headwater catchments in the Thames River basin in southern Ontario (Canada) that provide evidence about the fluxes and fate of perchlorate in the environment. Concentrations in streams (16 to 1047 ng/L) were used to estimate exports from these rural catchments (228-1843 mg/(ha·year)), atmospheric deposition (1480 ± 230 mg/(ha·year)), as well as variable rates of microbial degradation of perchlorate, which appeared to be enhanced in catchments with higher percentages of wetlands. Groundwater data supported earlier evidence that degradation of perchlorate occurs in the subsurface under oxygen-depleted conditions. The stream data suggest that the rate of degradation varies strongly between catchments and ranges up to >1000 mg/(ha·year).

Worldwide occurrence and origin of perchlorate ion in waters: A review

Perchlorate (ClO₄^-) is a persistent water soluble oxyanion of growing environmental interest. Perchlorate contamination can be a health concern due to its ability to disrupt the use of iodine by the thyroid gland and the production of metabolic hormones. Its widespread presence in surface water and groundwater makes the aquatic environment a potential source of perchlorate exposure. However, the amount of published data on perchlorate origins and water contamination worldwide remains spatially limited. Here, we present an overview of research on perchlorate origins and occurrences in water, and the methodology to distinguish the different perchlorate sources based on isotope analysis. All published ranges of isotopic content in perchlorate from different sources are presented, including naturally occurring and man-made perchlorate source types, as well as the effects of isotope fractionation that accompanies biodegradation processes. An example of a case study in France is presented to emphasize the need for further research on this topic.

37Cl/35Cl isotope ratio analysis in perchlorate by ion chromatography/multi collector -ICPMS: Analytical performance and implication for biodegradation studies

In the present study we propose a new analytical method for ³⁷Cl/³⁵Cl analysis in perchlorate by Ion Chromatography(IC) coupled to Multicollector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS). The accuracy of the analytical method was validated by analysis of international perchlorate standard materials USGS-37 and USGS -38; analytical precision better than ±0.4‰ was achieved. ³⁷Cl/³⁵Cl isotope ratio analysis in perchlorate during laboratory biodegradation experiment with microbial cultures enriched from the contaminated soil in Israel resulted in isotope enrichment factor ε³⁷Cl = -13.3 ± 1‰, which falls in the range reported previously for perchlorate biodegradation by pure microbial cultures. The proposed analytical method may significantly simplify the procedure for isotope analysis of perchlorate which is currently applied in environmental studies.

Stable isotopic composition of perchlorate and nitrate accumulated in plants: Hydroponic experiments and field data

Natural perchlorate (ClO₄^-) in soil and groundwater exhibits a wide range in stable isotopic compositions (δ³⁷Cl, δ¹⁸O, and Δ¹⁷O), indicating that ClO₄^- may be formed through more than one pathway and/or undergoes post-depositional isotopic alteration. Plants are known to accumulate ClO₄^-, but little is known about their ability to alter its isotopic composition. We examined the potential for plants to alter the isotopic composition of ClO₄^- in hydroponic and field experiments conducted with snap beans (Phaseolus vulgaris L.). In hydroponic studies, anion ratios indicated that ClO₄^- was transported from solutions into plants similarly to NO₃^- but preferentially to Cl^- (4-fold). The ClO₄^- isotopic compositions of initial ClO₄^- reagents, final growth solutions, and aqueous extracts from plant tissues were essentially indistinguishable, indicating no significant isotope effects during ClO₄^- uptake or accumulation. The ClO₄^- isotopic composition of field-grown snap beans was also consistent with that of ClO₄^- in varying proportions from irrigation water and precipitation. NO₃^- uptake had little or no effect on NO₃^- isotopic compositions in hydroponic solutions. However, a large fractionation effect with an apparent ε (¹⁵N/¹⁸O) ratio of 1.05 was observed between NO₃^- in hydroponic solutions and leaf extracts, consistent with partial NO₃^- reduction during assimilation within plant tissue. We also explored the feasibility of evaluating sources of ClO₄^- in commercial produce, as illustrated by spinach, for which the ClO₄^- isotopic composition was similar to that of indigenous natural ClO₄^-. Our results indicate that some types of plants can accumulate and (presumably) release ClO₄^- to soil and groundwater without altering its isotopic characteristics. Concentrations and isotopic compositions of ClO₄^- and NO₃^- in plants may be useful for determining sources of fertilizers and sources of ClO₄^- in their growth environments and consequently in food supplies.