Trichloroethylene Vapor Sorption onto Soil Minerals

Abstract

AbstractAdsorption of trichloroethylene (TCE) and water vapor onto five common soil minerals (montmorillonite, kaolinite, Fe oxide, silica, and Al oxide) and an organic phase (humic acid) was studied across a wide range of relative vapor pressures (0 to 90%). Dividing the amount of sorbate sorbed by the monolayer capacity for water and TCE (at 0% relative humidity [RH]) transforms this variable to values that are of the same magnitude. Normalizing the sorption data for monolayer capacity confirms that surface area serves as a good measure of the adsorptive capacity of the dry solids. The lack of complete convergence of the relative adsorption isotherms is attributed to specific sorbent‐sorbate interactions and the presence of micropores. The computed Brunauer, Emmet, and Teller (BET) surface areas, using TCE and water as sorbates, were different from the BET N2 surface area for a given sorbent. The differences in these surface areas may indicate sorbate packing arrangement for TCE and water that are different from the simple hexagonal close packing that is assumed for N2 molecules. Adsorption of TCE onto oven‐dried Al oxide was enhanced when the surface was coated with an organic phase (i.e., humic acid). Larger amounts of TCE were sorbed by humic acid at 80% RH than at 0% RH or under saturated aqueous conditions. Expansion of the humic acid due to hydration and exposure of internal surfaces/pore volume for sorption/condensation is proposed as an explanation for the higher uptake of TCE at high RH. In binary‐vapor (water plus TCE) sorption studies, isotherms for the total volume sorbed at different RH were coincident for porous solids such as Al and humic‐acid‐coated Al. This result strongly indicates that, at high TCE vapor pressures, vapor condensation plays an important role for porous sorbents in determining the amount of TCE sorbed.