Quantification of the soot-water distribution coefficient of PAHs provides mechanistic basis for enhanced sorption observations

There is an increasing recognition of the necessity to consider the heterogeneity of geosorbents, and in particular the condensed carbon facies fraction, to improve prediction of hydrophobic pollutant phase speciation. Field observations of much elevated organic-carbon normalized distribution coefficients (K-oc) of PAHs-relative to predictions from bulk organic-matter partitioning models-have been suggested to be explainable by sept sorption. To afford testing of this hypothesis, we here report on the soot-water distribution coefficients (K-sc) for a series of PAHs (naphthalene (NP), fluorene (FL), phenanthrene (PH), and pyrene (PY)) using diesel particulate matter (NIST standard reference material SRM-1650) as model soot sorbent. Specifically adapted batch and column experiments yielded average log K-sc values of 5.23, 5.40, 5.82, and 6.59 (batch) and 4.63, 6.03, 6.62, and 7.03 (column) for NP, FL, PH, and PY, respectively tall data in [L-w/kg(sc)]). The obtained values are 35-250 times higher than respective K-oc predictions and are considerably closer to theoretically estimated soot-water distribution coefficients. Our data are among the highest solid-water distribution coefficients of an environmentally relevant sorbent ever reported and lend direct empirical support of active soot sorption as a viable explanation to the enhanced PAH partitioning. Sorption kinetics on the hours-days time scale and similarity of external geometric and BET surface areas suggest that interaction sites are largely restricted to the outer surface of the soot. The constrained K-sc values facilitate prediction of speciation and bioavailable exposures of PAHs in aquatic and sedimentary environments.