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 polycycli c aromatic hydrocarbons (PAHs) from soil was studied with C-14-benzo[a]pyre ne and C-14-pyrene in different soil-aqueous solution systems and after pro longed periods of aging (110 and 216 d). The desorption of C-14-benzo[a]pyr ene and C-14-pyrene was strongly affected by the composition and properties of the aqueous phase. Desorption linearly increased with increasing DOM co ncentrations. Distribution coefficients (log K-OC') for the desorption of C -14-labeled PAHs were about 3.5 (pyrene) and 25 (benzo[a]pyrene) times lowe r in the presence of DOM. The enhancement of PAM desorption between various types of DOM is controlled by the molecular weight distribution of DOM. Di ssolved organic matter with a high proportion of high molecular weight comp onents (>14 000 dalton) has a high affinity for PAHs and thus is more effec tive 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. Th e desorbable fraction did not show any further changes for up to 183 d afte r contamination, suggesting a fraction of PAM that is not or very slowly de sorbing from soil. The major proportion (>98%) of the desorbable PAM-fracti on is apparently bound at sites that show a rate-limited release of PAHs. T his is corroborated by the model discrimination conducted with three differ ent models, which favors a two-site equilibrium-nonequilibrium model.