Sorption of hydrophobic chemicals from water: A hypothesis for the mechanism of the particle concentration effect

The phenomena of hydrophobic organic chemical sorption to particulate organic matter in aqueous systems are reviewed. It is speculated that the primary process is “loose sorption” in which an organic chemical reduces the total organic-water interfacial area and hence free energy by associating loosely with the natural organic surface, displacing the water from the surface and from part of the chemical. This process is easily reversible as a result of particle collisions, thus the apparent sorption partition coefficient decreases as the particle concentration and collision rate rise. Quantifying this speculated phenomenon using a mass transfer approach yields an expression for a variable partition coefficient which is in agreement with observations that a maximum of approximately half the sorbate present can be sorbed to particles. Some implications are discussed.