Isotopic Fractionation of Chlorinated Hydrocarbon Compounds During Sorption to Activated Carbon

Abstract

Activated carbon (AC) amendments are leading remediation efforts of groundwater systems containing organic contaminants. More recently, in situ AC treatment is being explored to sequester contaminants in place. However, evaluating the effectiveness of in situ AC amendments is difficult because the reduction in concentration of parent compounds alone cannot differentiate between degradative and adsorption processes. Compound-specific isotope analysis (CSIA) has emerged as an important tool in identifying subsurface (bio)transformation of compounds like chlorinated hydrocarbons (CHCs) during remediation processes that can reveal subsurface processes. In principle, (bio)chemical transformations induce isotopic fractionation in organic compounds, whereas processes like adsorption, diffusion, and advection do not affect isotope ratios. This study seeks to understand the degree to which multi-step sorption contributes to the overall change in the isotope ratios of compounds treated with AC. Frist, working standards for a set of chlorinated organic compounds were analyzed using GC-qMS to determine the analytical uncertainty in δ37Cl. When international or interlaboratory standards were available, working standards were calibrated to Standard Mean Ocean Chlorine, the international calibration standard for chlorine isotope measurements. Second, single and multi-step sorption batch experiments were conducted to understand kinetics and the extent of isotope fractionation during sorption to AC. All single-step batch experiments showed no trends in isotope enrichment nor depletion during sorption. However, in the multi-step experiments, perchloroethene (PCE), 1,2-dichlorobenzene (1,2-DCB) and 1,2,4-trichlorobenzene (1,2,4-TCB) produced cumulative shifts in δ37Cl exceeding their calculated analytical uncertainties. These observations are in agreement with the findings of Wanner et al. (2017), which found similar trends for another CHC, 1-2 dichloroethane in low permeable soils. This study provides a foundation for future research to analyze the total shift in isotope ratios due to small physical processes by remediation technology and low permeable material present in field scenarios. Lastly, this study continues to stress the importance of interlaboratory comparisons when reporting isotope data.