Simplified link solvation model (LSM) for sorption in natural organic matter

The aim of this paper is to formalize a conceptual model for explaining solvent-assisted sorption of organic compounds by a natural organic matter (NOM) sorbent. A local sorption isotherm is developed to describe the penetration of solute and/or solvent molecules into noncovalent inter- and intramolecular contacts in the NOM structure, and sorbate interactions with fragments of the disrupted contact. A general sorption isotherm accounting for an exponential sorption site energy distribution is derived from the local isotherm and is then extended to account for sorption cooperativity. This new model successfully describes cooperative sorption behavior of pyridine from different solvents on NOM and reproduces an experimentally observed maximum in pyridine uptake from mixtures of an inert hydrocarbon and a polar solvent at an intermediate polar solvent activity. The model engenders a number of observations and predictions that have been borne out experimentally: (1) the exponent of the Freundlich model applied to the sorption isotherm of a compound is not affected by the particular solvent; (2) the solvent-assisted effect is not a function of solute activity; (3) the solvent effect will be similar for organic compounds that successfully compete with a solvent for the same kind of link fragment; (4) where a solute cannot successfully compete with a solvent for a given link fragment, no solvent-assisted sorption is expected. The strength of this model is its flexibility and its potential to account for both asymmetry of disruptable NOM links and diverse forces of sorbate-solvent interactions at fragments of disrupted contacts.