Rate-limited desorption of volatile organic compounds from soils and implications for the remediation of a Louisiana superfund site

The rates of desorption of trichloroethylene (TCE) and 1,3-dichlorobenzene (DCB) from a silty soil at a Superfund site and a silty-clayey soil from an uncontaminated bottomland hardwood swamp in Baton Rouge, Louisiana were studied in laboratory batch systems. The effect of the age of soil contamination was studied using a laboratory-spiked soil incubated for 3 days, 3 months and 5 months. An empirical non-linear model was used to describe the bi-phasic nature of desorption with one fraction (labile) being released in relatively short periods of time (typically 24-100 hr) and a second fraction (non-labile or irreversible) being resistant to desorption. The non-linear model parameters, viz., the fraction of the chemical released rapidly (F), and the first order desorption rate coefficients, k1 and k2 respectively for the labile and slowly released fractions were determined by fitting the experimental data to the model. The data fit the model well as indicated by the high r2 values. The estimate of k1 was good. However, the values of k2 are known with less precision due to the limited duration of the experiment and number of samples taken at long times. In addition, desorption kinetics of 3 and 5-month old contaminated soils showed that progressively less amount of contaminant was available for facile desorption (lower F) compared to freshly contaminated soil. The labile fraction had desorption rate constants of the order of 10(-1) h(-1), whereas the slowly released fraction had rate constants of the order of 10(-4) h(-1) in accord with literature reported values for a variety of other compounds and soils. Possible mechanisms describing these rates and implications for the site clean up are discussed.