Predicting volatile organic vapor sorption from soil specific surface area and texture

Vapor sorption of a volatile organic chemical (VOC) to soil minerals at low water contents can increase the total VOC sorption capacity of the soil by several orders of magnitude. Vapor sorption can subsequently cause high VOC fluxes out of the soil during periods with fluctuating low soil water contents. Models for predicting VOC sorption, including vapor sorption, as a function of soil water content are therefore valuable for evaluating contaminant volatilization and exposure risk at polluted sites. A model for estimating trichloroethylene (TCE) sorption parameters at low relative vapor pressure from ethylene glycol monoethyl ether (EGME)-measured soil specific surface area (SA) and soil clay and organic carbon content was developed based on TCE sorption data from 26 different porous media. Significant improvement in prediction accuracy compared with a previous model was achieved. Based on data for 13 soils, an organic carbon partitioning coefficient of K_oc = 0.60 f_oc cm³ g^-1 (R² = 0.97) for TCE is recommended for use in the new TCE sorption model. In case SA is not available, an expression for predicting SA from soil clay content is proposed. The clay-based TCE sorption model was tested against independent data for two Japanese soils (a sand and a clay loam) measured in this study by a gas chromatography-based soil microcolumn method. The sorption model accurately predicted TCE retardation (sorption) as a function of soil water content for both soils. Model calibration to sorption data for three different VOC's (benzene, TCE, and toluene) implies that the sorption model may also be applied to chemicals other than TCE. However, sorption measurements for more chemicals and soils at varying water contents are necessary to further generalize the VOC sorption model.