Desorption of polycyclic aromatic hydrocarbons from soil in the presence of dissolved organic matter:: Effect of solution composition and aging

The effect of dissolved organic matter (DOM) on the desorption of polycyclic aromatic hydrocarbons (PAHs) from soil was studied with C-14-benzo[a]pyrene and C-14-pyrene in different soil-aqueous solution systems and after prolonged periods of aging (110 and 216 d). The desorption of C-14-benzo[a]pyrene and C-14-pyrene was strongly affected by the composition and properties of the aqueous phase. Desorption linearly increased with increasing DOM concentrations. Distribution coefficients (log K-OC') for the desorption of C-14-labeled PAHs were about 3.5 (pyrene) and 25 (benzo[a]pyrene) times lower in the presence of DOM. The enhancement of PAM desorption between various types of DOM is controlled by the molecular weight distribution of DOM. Dissolved organic matter with a high proportion of high molecular weight components (>14 000 dalton) has a high affinity for PAHs and thus is more effective in desorbing PAHs from soil. In addition, the desorption of PAHs from soil is controlled by the time elapsed since contamination. In the first 36 d following contamination with C-14-benzo[a]pyrene, a decrease of the PAM concentration that could be desorbed in the solution phase was observed. The desorbable fraction did not show any further changes for up to 183 d after contamination, suggesting a fraction of PAM that is not or very slowly desorbing from soil. The major proportion (>98%) of the desorbable PAM-fraction is apparently bound at sites that show a rate-limited release of PAHs. This is corroborated by the model discrimination conducted with three different models, which favors a two-site equilibrium-nonequilibrium model.