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EFFECTS OF TEMPERATURE ON TRICHLOROETHYLENE DESORPTION FROM SILICA GEL AND NATURAL SEDIMENTS. 2. KINETICS. (R822626)
Citation:
Werth, C. J. AND M. Reinhard. EFFECTS OF TEMPERATURE ON TRICHLOROETHYLENE DESORPTION FROM SILICA GEL AND NATURAL SEDIMENTS. 2. KINETICS. (R822626). ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 31(3):697-703, (1997).
Description:
Isothermal desorption rates were measured at 15, 30, and 60 src="/ncer/pubs/images/deg.gif">C for trichloroethylene (TCE) on a silica gel,
an aquifer sediment, a soil, a sand fraction, and a clay and silt fraction, all
at 100% relative humidity. Temperature-stepped desorption (TSD) rates were
measured for these solids in columns prepared and equilibrated at 30 src="/ncer/pubs/images/deg.gif">C, but heated instantaneously to 60
src="/ncer/pubs/images/deg.gif">C after ~1000 min of slow desorption. Fast and
slow elution rates are observed for all solids. Modeling results for the fast
eluting fraction of TCE show that fast desorption is controlled by diffusion
through aqueous filled mesopores. Rates predicted from diffusion and
surface-barrier models are compared to slow isothermal and TSD rates. Diffusion
model fits are superior to surface-barrier model fits in all cases. Slow
diffusion coef ficients and a high activation energy calculated from silica gel
data (~34 kJ/mol) indicate that slow desorption is controlled by activated
diffusion in micropores. Initial amounts of slow desorbing TCE do not affect
these rates and are found to obey Polanyi's equation. The mass adsorbed in
non-Freundlich isotherm regions, where micropores are hypothesized to control
adsorption, is 10 times greater than the mass adsorbed at the onset of slow
desorption, sug gesting that these pores are undulating in nature. TSD column
results are consistent with a mechanism where slow diffusion rates are
controlled by sorptive forces at hydrophobic micropore constrictions.