The mechanisms controlling desorption rates from soils and sediments were investigated by measuring TCE desorption kinetics from model solids, sediments, and soil under unsaturated conditions at 100% relative humidity. A new experimental methodology enabled measurement of desorption rates over more than 7 orders of magnitude and revealed that intraparticle pores of molecular dimensions may be responsible for the slow release of sorbed contaminants. Desorption kinetics proceeded on two distinct time scales, and all solids had both a fast and slowly released fraction. The amount of slow desorbing TCE was found to depend on the initial vapor concentration, but not in direct proportion. At TCE vapor concentrations near saturation, the fast fraction comprised the majority of the sorbed TCE and required less than 10 min for desorption, whereas the slow desorbing fraction was released over periods of months to years. Although model solids with uniform pore and particle sizes were used, a pore diffusion model was not adequate to describe both the fast and slow desorbing fractions. The measured physical properties of the solids were not useful for making a priori predictions and, in comparisons among the solids, did not correlate with the amount and the rate of slowly released TCE. The Freundlich isotherm exponents correlated with the fraction of slowly released TCE, but isotherm nonlinearity was not sufficient to account for the dual rate behavior.
ASJC Scopus subject areas
- Environmental Chemistry