Evaluating competitive sorption mechanisms of volatile organic chemical mixtures in soils and sediments using polymers and zeolites

Volatile organic chemicals (VOCs) such as chlorinated compounds are among the most problematic groundwater contaminants. Prior research suggests that VOC mixtures compete for micropore sorption sites present in soils and sediments. It is hypothesized that the size and polarity of these micropores control competitive sorption and mass transfer rates of sorbate mixtures. To test this hypothesis, aqueous isotherms and desorption kinetic profiles for trichloroethylene (TCE) and tetrachloroethylene (PCE), either alone or in a mixture, were measured at 30°C for three natural solids, two polymers, and four zeolites. The polymers and zeolites were used as surrogates for the different sorption environments in natural solids. Isotherm results for natural solids and polymers suggest that competition between TCE and PCE in soils and sediments is controlled by competition for hydrophobic micropores in condensed organic matter, and isotherm results for zeolites indicate that smaller more hydrophobic micropores result in stronger competition. Desorption kinetic results show that in general diffusion of TCE is faster in the presence of PCE and vice versa, suggesting that the presence of each sorbate reduces the adsorption potential of the micropore sorption sites and hence increases the diffusion rates for both sorbates. These results support the hypothesis that competitive sorption and mass transfer rates of VOC mixtures in the subsurface are controlled by the properties of the micropore sorption sites in soils and sediments.