Sorption of perchloroethylene by surfactant-modified zeolite as controlled by surfactant loading

The sorption of nonpolar hydrophobic organic compounds by soil organic matter has long been attributed to a partitioning mechanism, with the sorption coefficient proportional to the fractional organic carbon content of the soil. However, deviations from this linear proportionality have been observed and reported in the literature by many authors. In our study a natural zeolite was modified with a cationic surfactant to achieve different fractional organic carbon contents and different surfactant molecule configurations on the surface. The sorption of perchloroethylene (PCE) by the surfactant-modified zeolite (SMZ) was found to be dependent on the bound surfactant molecule configuration as well as on the fractional organic carbon content. Below monolayer coverage by the surfactant, the PCE sorption coefficient on SMZ was proportional to the fractional organic carbon content. Above monolayer coverage, increasing fractional organic carbon content resulted in minimal further increase in the PCE sorption coefficient. The change in PCE sorption behavior was attributed to the structural differences between sorbed surfactant monolayers and bilayers. The surfactant surface configuration has a significant impact on the effective volume and density of the bound organic phase that is responsible for partitioning nonpolar organic compounds. The ratio of the organic carbon-based distribution coefficient (K-OC) for the monolayer versus that for the bilayer systems was 1.7, similar to the estimated bilayer to monolayer hydrocarbon density of 1.3. Our results reinforce the notion that the structure of natural organic matter as well as its quantity controls the sorption of nonpolar organics to soils and sediments.