This study describes the formation of slowly reversible sorbed fractions of various halogenated alkanes and alkenes in two surface soils. After the initial sorption equilibration period, the compounds were desorbed by one of two methods. The first used repetitive batch extraction with water. After 16 extractions of 24 to 72 h each, concentrations in the aqueous phase reached low values and a slow desorbing, residual fraction remained in the soil. The residual fraction in the soil was determined independently after extraction with hot acetone, whose efficacy was demonstrated. During desorption, apparent soil-water distribution coefficients increased progressively to as much as 200 times greater than equilibrium sorption coefficients, Kd, obtained separately from sorption isotherms. With increasing sorption equilibration time, the residual became greater in magnitude and less mobile. The second desorption method simulated desorption to infinite dilution over a 96 h period by using Tenax GC polymeric adsorbent beads included in the suspension as a sink for desorbed chemical. Control experiments proved the usefulness of Tenax and showed that desorption from the soil was rate limiting. All compounds studied formed slowly reversible fractions in the soils. This fraction amounted to several percent of the total sorbed from solution. The results indicate that formation of slowly reversible fractions is probably typical of nonpolar organic compounds, including those with weak sorbing tendencies.
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