Sorption of polycyclic aromatic hydrocarbons and polychlorinated biphenyls to soot and soot-like materials in the aqueous environment: mechanistic considerations

Recent studies have shown that sorption of polycyclic aromatic hydrocarbons (PAHs) in soot-water systems is exceptionally strong. As a consequence, soot may fully control the actual fate of PAHs in the aquatic environment. However, sorption has only been characterized for a limited number of PAHs to diesel soot, and the mechanism is poorly understood. In this paper, we present an extensive data set of sorbent-water distribution coefficients (K(S), n = 236) for a series of PAHs (both native and added) and polychlorinated biphenyls (PCBs) to five different types of soot and five soot-like materials. Both Ks values and physicochemical properties of the sorbents show large variation. In general, sorption is very strong, with K(S) values up to 10(10), showing the highest distribution coefficients on a mass basis ever reported. Sorption of in particular PAHs is often over 1000 times as strong as sorption to amorphous sedimentary organic carbon. The variation in K(S) values cannot be explained by "soot carbon fractions" or specific surface areas of the sorbents. Instead, values for native PAHs are mostly determined by the sorbates' molar volume, and values for added PAHs and PCBs are determined by the sorbents' average pore diameter. From differences in K(S) values between native and added PAH analogues, it can be deduced that generally more than 50% (with values up to 97%) of the native PAH concentration in soot is not available for distribution to the aqueous phase. We conclude that this is caused by physical entrapment of the chemicals within the solid matrix. Furthermore, most sorbents appear to preferentially sorb PCBs with planar configurations, a phenomenon most likely driven by sorption in molecular-sized pores. Pore sorption is also concluded to be the most important sorption mechanism for added PAHs together with pi-pi interaction processes with flat aromatic sorbent surfaces. Frequently observed, slowly desorbing, resistant contaminant fractions in sediments may very well be explained on the basis of these results.