Counter-Diffusion of Isotopically Labeled Trichloroethylene in Silica Gel and Geosorbent Micropores: Column Results

Abstract

To investigate counter-diffusion in microporous sorbents, the rate of exchange between deuterated trichloroethylene (DTCE) in fast desorbing sites and nondeuterated TCE ({sup 1}HTCE) in slow desorbing sites was measured. Exchange rates were measured for a silica gel, a Santa Clara sediment, and a Livermore clay/silt fraction, all at 100% relative humidity and 30 C. Initially, solids were packed into stainless steel columns and incubated with {sup 1}HTCE for 1--3 weeks. After incubation, {sup 1}HTCE was replaced with DTCE in fast desorbing sites. Next, columns were capped, and DTCE was allowed to exchange with {sup 1}HTCE in slow desorbing sites for 1, 3, or 30 days. Elution profiles were then measured to determine the extent of exchange that occurred while the columns were capped. Results from experiments conducted with different exchange times support the hypothesis that slow sorption kinetics is controlled by diffusion in micropores. For the silica gel and the Santa Clara sediment, {sup 1}HTCE was incompletely exchanged with DTCE after 30 days. This indicates that the counter-diffusion rate of DTCE into {sup 1}HTCE-filled micropores is less than the diffusion rate of {sup 1}HTCE into micropores not filled with TCE.