Adsorption-Partitioning Uptake of Nine Low-Polarity Organic Chemicals on a Natural Sorbent

Abstract

Sorption of comparatively nonpolar organic chemicals by natural solids not only can be predominated by partitioning with organic matter but also can reflect a substantial contribution from adsorption at low relative concentration. Sorption of nine polycyclic aromatic hydrocarbons (PAHs) and chlorinated benzenes (CBs) was investigated on a subsurface aquitard through batch study, with results interpreted by a composite adsorption-partitioning model. For both PAHs and CBs, the low-concentration adsorption slope and the coefficient for partitioning each correlated well with Kow; however, PAHs consistently sorbed more strongly than CBs at given Kow. For all chemicals, adsorption contributions were only important at low relative concentration and could be successfully modeled by assuming either Langmuir-type or Polanyi-type isotherms. Isotherms for all liquid chemicals fell on a single isotherm when plotted on a Polanyi basis (adsorbed volume per mass of sorbent versus adsorption potential density), providing evidence that a pore-filling phenomenon is involved. Adsorbed volumes of solid chemicals were observed to be less than those of liquids at the same adsorption potential density, consistent with a reduction in packing efficiency and as previously reported for activated carbons. These results suggest that the adsorption contribution is from pore filling within microporous solids.