SORPTION MODELS FOR CHLOROFLUOROCARBONS (CFCs) WITH SUBSURFACE SEDIMENTS

Chlorofluorocarbons (CFCs: CFC-11, CFC-12, CFC-113) have been used to date groundwater recharge last 60 years. Conservative or linear sorption was generally assumed for CFC transport. Our recent study showed sorption potential of CFCs to thermally altered carbonaceous matter (TACM) (i.e. synthetic wood char). Sorption of organic chemicals to TACM is characterized by nonlinear sorption behavior with large capacity. Robust sorption models are required to replace the simplistic linear model for the interpretation and correction of CFC groundwater ages measured in TACM-containing subsurface environments.

Sorption batch techniques were used to determine the CFC sorption by three subsurface sediments, kerogen-rich Marcellus and Kettle Point shales, and Sarnia aquitard, at solubility-normalized concentrations (C_w/S) of 10^-7–1. Measured data were fitted to nine sorption models using ISOFIT program (http://www.groundwater.buffalo.edu/software/IsoFit/IsoFitMain.html). Four models (Freundlich, Langmuir, Brunauer-Emmett-Teller, and Generalized Langmuir-Freundlich) deviated from the data and the corrected Akaie Information Criterion (AICc) values reflected inadequate fits for all three samples. These results indicate that CFC sorption cannot be described by assuming continuous energy distributions on sorption sites, and monolayer or multilayer sorption on a homogeneous surface. The sorption models of Langmuir-Partitioning, Toth, Polanyi-Manes, and Polanyi-Manes with Partitioning provided better fits. The Langmuir-Partitioning model had relatively low AICc values for the shale samples that suggest two different sorption domains such as partitioning into general organic matter and adsorption onto TACM. Although the Polanyi-based models showed the lowest AICc, the adsorption potential versus adsorption volume of CFC-113 was not consistent with that of the other CFCs as assumed by Polanyi-Manes-Dubinin sorption theory. In general, the Toth model provided the best fit for all CFC solubility-normalized sorption isotherms with three subsurface samples. Robust sorption isotherm model for CFCs using the measured concentrations is useful to predict CFC retardation at low concentrations, because the concentrations of CFCs in groundwater are very low ranged from C_w/S = 10^-12–10^-9.