Non-linear chlorinated-solvent sorption in four aquitards

Perchloroethene (PCE) sorption was determined over a wide concentration range ( 1–90,000 μg L−1, or 0.0007–60% of solubility) for four diverse natural clay-rich aquitards using a batch method. All of the samples have high content of clay-size materials, and were selected to represent a range of fraction organic carbon (foc) (0.1–3.83%) and smectite mineral content. Indexing to specific surface area (measured by EGME and N2(g)- BET) or foc could not explain the amount or variation of sorption measured. The sorption results were most adequately fit over the whole concentration range tested by nonlinear Freundlich isotherms with 1/n ranging from 0.72 to 0.95. Trichloroethene (TCE) sorption was also measured for one of the aquitard samples. Scaling by compound solubility resulted in superposition of the PCE and TCE isotherms measured for this sample, suggesting that the driving force for sorption was dominantly hydrophobic. Isotherm nonlinearity implies that aquitard mass storage will vary with concentration and that solute mobility will be concentration-dependent. For example, the PCE distribution factor, defined as bKd,app/η [=(bulk density)* (apparent Kd/(porosity)], predicted for the most nonlinear isotherm ranged from 60 to 3 over the measured concentration range. Isotherm nonlinearity also suggests that competitive sorption could be significant.