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

There is an increasing recognition of the necessity to consider the heterog eneity of geosorbents, and in particular the condensed carbon facies fracti on, to improve prediction of hydrophobic pollutant phase speciation. Field observations of much elevated organic-carbon normalized distribution coeffi cients (K-oc) of PAHs-relative to predictions from bulk organic-matter part itioning models-have been suggested to be explainable by sept sorption. To afford testing of this hypothesis, we here report on the soot-water distrib ution 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. Specifica lly 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 obtai ned values are 35-250 times higher than respective K-oc predictions and are considerably closer to theoretically estimated soot-water distribution coe fficients. Our data are among the highest solid-water distribution coeffici ents of an environmentally relevant sorbent ever reported and lend direct e mpirical support of active soot sorption as a viable explanation to the enh anced PAH partitioning. Sorption kinetics on the hours-days time scale and similarity of external geometric and BET surface areas suggest that interac tion sites are largely restricted to the outer surface of the soot. The con strained K-sc values facilitate prediction of speciation and bioavailable e xposures of PAHs in aquatic and sedimentary environments.