Detailed sorption isotherms of polar and apolar compounds in a high-organic soil.

Abstract

Sorption isotherms of 13 apolar liquids and solids and polar solids-six in unprecedented detail-are used to evaluate a polymer-based model for natural organic matter. While all isotherms are nonlinear, the "running" Freundlich exponent n varies markedly with concentration. The isotherms show linear-scale inflection consistent with the presence of flexible (deformable) porosity as predicted by the glassy polymer-based Extended Dual-Mode Model (EDMM). The EDMM assumes dissolution and hole-filling domains in the organic solid, with provision for sorbate-caused plasticization of the solid and "melting" of the holes. Features of the EDMM are illustrated for chlorinated benzenes in poly(vinyl chloride). The solutes fall into categories of "hard" (aliphatics and 2,4-dichlorophenol) and "soft" (chlorinated benzenes, 2-chloronitrobenzene) according to their ability to plasticize organic matter. Comparison of domain coefficients at infinite dilution reveals that organic solutes have a modestly greater affinity for holes than dissolution sites (by 0.1-0.6 log unit), as expected by the polymer model. Sorption of CHCl3 shows time-dependent hysteresis diminished at high concentrations by the plasticizing effect. Sorption of CHCl3 also shows a type of hysteresis for glassy solids known as the "conditioning effect" in which high loading of sorbate increases hole population upon its removal and thus leads to enhanced uptake and nonlinearity when a second sorption is performed. A Polanyi-based, fixed-pore filling model applied to the adsorption component of the isotherms gave widely variant volumetric pore capacity, contrary to its own stipulations, and could not explain the hysteresis.