Stable carbon and hydrogen isotope fractionation of dissolved organic groundwater pollutants by equilibrium sorption

Linear free energy relationships (LFERs) were established which relate equilibrium vapor–liquid isotope effects to stable carbon and hydrogen isotope enrichment factors for equilibrium sorption to geosorbents. The LFERs were established for normal, cyclic or branched alkanes, monoaromatic hydrocarbons, and chloroethenes. These LFERs predict that isotopic light compounds sorb more strongly than their heavy counterparts. Defining fractionation as in classical literature by “heavy divided by light”, carbon enrichment factors for equilibrium sorption were derived which ranged from − 0.13 ± 0.04‰ (benzene) to − 0.52 ± 0.19‰ (trichloroethene at 5–15 °C). Hydrogen enrichment factors for sorption of 14 different compounds were between − 2.4 and − 9.2‰. For perdeuterated hydrocarbons the predicted enrichment factors ranged from − 19 ± 5.4‰ (benzene) to − 64 ± 30‰ (cyclohexane). Equilibrium sorption experiments with a soil and activated carbon as sorbents were performed in the laboratory for perdeuterocyclohexane and perdeuterotoluene. The measured D/H enrichments agreed with the LFER prediction for both compounds and both sorbents within the uncertainty estimate of the prediction. The results of this work suggest that equilibrium sorption does create only very small isotope shifts for 13C in groundwater pollutants in aquifers. It is also suggested that deuterium shifts are expected to be higher, especially for strongly sorbing pollutants.

► New theoretical predictions for stable isotope fractionation by sorption are developed.
► The predictions are experimentally validated for fully deuterated hydrocarbons.
► The sorption enrichment factors strengthen the assessment of the fate of pollutants in aquifers.