Partitioning of polycyclic aromatic hydrocarbons (PAH) to water‐soluble soil organic matter

The mobility of polycyclic aromatic hydrocarbons (PAH) in soils can be influenced by the presence of dissolved organic matter. Partition coefficients of selected polycyclic aromatic hydrocarbons, ranging from 3‐ring to 6‐ring compounds, to water‐soluble soil organic matter (WSSOM) were determined. Partition coefficients were determined for WSSOM obtained from two soils under agricultural use and forest and for commercially available humic acid (Aldrich), taking advantage of a reversed phase (C₁₈) separation method. The WSSOM was characterised with regard to charge and hydrophilic/hydrophobic properties with a dissolved organic matter (DOM) fractionation method. No sorption to WSSOM was found for the tri‐ and tetracyclic PAH, whereas the penta‐ and hexacyclic PAH showed a significant binding to both types of WSSOM and to Aldrich humic acid. The affinity of penta‐ and hexacyclic PAH to WSSOM was considerably lower compared to the affinity to Aldrich humic acid. This is suggested to be due to the lower amount of hydrophobic fractions, c. 30%, in the natural WSSOM as compared to Aldrich humic acid. Effective partition coefficients (Koc_eff) for the sorption of PAH to bulk soil calculated from K_DOC and DOM in the naturally occurring concentration range were only 60–70% of the K_oc values in pure water. The impact of DOM on pollutant transport is further influenced by non‐equilibrium behaviour of PAH in soils and by sorption of DOM to the solid‐soil matrix. Several scenarios are described in which the effect of DOM on pollutant transport may become important.