TSD column results were consistent with a mechanism where slow diffusion rates are controlled by sorptive forces at hydrophobic micropore constrictions. The model attributes non-equilibrium sorption of moderately hydrophobic, volatile organic compounds to diffusion. The disparity of time scales of the two processes was used to approximate a boundary condition for the distributed diffusion equation, allowing it to be solved analytically. The slower diffusion process is attributed to activated diffusion through micropores. Advection and dispersion control TCE transport though the vapor phase in purged columns.
Diffusion controls the TCE transport in unswept columns.
For both swept and unswept columns, a fast and a slow desorbing fraction of the TCE were observed. The DDDM effectively simulated both of these fractions. For the fast fraction, the DDDM predicted desorption with no fitting parameters. Micropore capacity factors obtained by fitting the DDDM were used to predict the onset of slow desorption in unswept columns of the same solid. After incubation, HTCE was replaced with DTCE in fast desorbing sites.