| Abstract
| - Desorption profiles of trichloroethylene (TCE), tetrachloro-ethylene (PCE), and a TCE−PCE mixture were measuredfor three natural solids and four zeolites. Initial sorbedmass (Mi) in slow desorbing sites of natural solids andin micropores of zeolites were obtained from desorptionprofiles. In natural solids, Mi increases with recalcitrantorganic matter content. In zeolites, Mi generally increaseswith decreasing micropore width and increasing microporehydrophobicity, but the effect of hydrophobicity is stronger.In both natural solids and zeolites, competition betweenTCE and PCE causes Mi for each sorbate in the mixture tobe less than or similar to that for each sorbate alone.Zeolite results indicate that micropore width affects thiscompetition more than micropore hydrophobicity for the solidsexamined. Desorption in all solids was simulated withthe radial diffusion model, either alone or coupled withthe advection−dispersion equation when necessary; diffusionrate constants (D/R2) were obtained. In natural solids,mean values of D/R2 increase with decreasing recalcitrantorganic matter content. In zeolites, values of D/R2 generallyincrease with increasing micropore width, while theyare a weak function of hydrophobicity. In both natural solidsand zeolites, competition between TCE and PCE causesD/R2 for each sorbate in the mixture to generally be largerthan that for each sorbate alone. Zeolite results indicatethat the effects of competition on D/R2 generally decreasewith decreasing micropore width for the solids examined;a trend with micropore hydrophobicity is not apparent.For the three natural solids and four zeolites examined inthis study, the similar effects of competition betweenTCE and PCE on values of Mi and D/R2 and the overlappingrange of D/R2 values support the hypothesis that diffusionthrough hydrophobic micropores affects and may controlslow mass transfer processes in the recalcitrant organicmatter of natural solids. These results contribute to thefundamental understanding of slow mass transfer processesin natural solids, and they indicate that characterizationof micropore width and polarity may be necessary to predictorganic chemical transport and fate.
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