The purpose of this research was to characterize the rate limited release of polyaromatic hydrocarbons (PAHs) from soils to an aqueous phase. Experimental results were obtained for the aqueous release of naphthalene, C1-naphthalenes, C2-naphthalenes, fluorene, and phenanthrene from field soils obtained from manufactured gas plant sites. To characterize rate-limited release of compounds from soils, fixed-bed rate of release (ROR) experiments were used. A two-site equilibrium/rate-limited model was used to interpret the slow rate of release of a fraction of each compound initially in the soil. The model was used to analyze the experimental data and obtain the following key parameters: Kd, F, and k2 (day-1), where Kd is the equilibrium partition coefficient for contaminant, F is the fraction available for equilibrium release, and k2 is the mass transfer rate constant for the slowly released fraction. The Kd values were also independently measured in batch partitioning experiments.
It was observed that the partition coefficients determined from the fixed-bed experiments for most of the soils tested were very consistent with the batch partitioning results. This result is also consistent with the values of F that were generally observed, which indicated that large fractions of the various PAHs for these soils were readily available for transport to the aqueous phase.
Rate constants for the slowly released fractions, k2, were measured to be of the order of 0.001?0.01 day-1 for the five different PAHs in the various soils. Values within this range are significant with respect to establishing environmentally acceptable endpoints, since these values are lower or of the same magnitude as those for other loss processes that can occur in soil such as biodegradation.