Modeling nonlinear sorption of trichloroethene in natural sorbents with kerogen
Munger, Zackary William
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In the subsurface, chlorinated solvent sorption to aquifer materials hinders remediation efforts and makes predicting the transport and fate of chlorinated solvents in groundwater a nontrivial enterprise. Due to heterogeneity in natural sorbents, there is a lack of clear evidence for a sorption model that can reliably simulate sorption behavior across broad aqueous sorbate concentrations on the basis of quantifiable sorbent characteristics. The goal of this study was to test if there is a sorption model that can be shown to be the best choice for simulating trichloroethene (TCE) sorption to natural sorbents and to link the sorption behavior to sorbent characteristics. Sorption isotherms were collected for four natural sorbents from Ontario and New York. The isotherms span an aqueous concentration range nearly five orders of magnitude. The sorbents contain type II kerogen, a common organic constituent in autochthonous marine sedimentary rocks and more recent allochthonous deposits, making them valid model sorbents. These sorption data were fit with 11 isotherm models common in the literature and founded on various partitioning, adsorption, pore-filling, and dual-mode mechanisms including Freundlich, Langmuir, Tóth, and Polanyi-type models. For all examined sorbents the Polanyi or Polanyi-partition models were ranked as the best fit based on the corrected Akaike Information Criterion, suggesting that pore-filling or combined pore-filling and linear partitioning mechanisms are valid for sorbents that contain kerogen. The estimated linear partitioning coefficients for the Polanyi-partition model were within a factor of four of that predicted on the basis of the fraction of organic carbon and an octanol-water partitioning linear free energy relationship. The sorption capacity term was constant within a factor of three when normalized by the fraction of organic carbon. The A and B fitting parameters remained fairly consistent among the studied sorbents. This study demonstrates that the single and dual-mode Polanyi-partition models are the most plausible to describe TCE sorption to sorbents with kerogen and are more accurate across broad aqueous concentrations than commonly used linear and Freundlich models. Model parameters are shown to be related to sorbent characteristics demonstrating the potential for estimating reliable model parameters from measurable sorbent properties.