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The widely accepted dissolution (partition) model of sorption to soil organic matter (SOM) has been challenged by evidence that SOM has a non-uniform sorption potential. This study presents data supporting a previously sug gested alternative dual-mode model of sorption in which dissolution and hole-filling mechanisms take place concurrently, as in glassy organic polymers. The holes are postulated to be nanometer-size voids within the organic matrix that provide complexation sites. The main focus was on sorption of chlorobenzene, 1,2-dichlorobenzene, and 1,3-dichlorobenzene, but some experiments were carried out also on 2,4-dichlorophenol and the herbicide metolachlor. Sorption from water to high-organic soils, humic acid particles, and poly(vinyl chloride) is nonlinear, competitive, and predictably responsive to conditions that affect hole populations such as temperature and co-solvent addition. Sorption to a peat soil and its components became progressively nonlinear and competitive in the order humic acid, native peat, humin; this order reflects the increasing “glassy” i.e., rigid, condensed nature of organic matter according to modern concepts of humic structure. Gas adsorption isotherms (N 2 at 77 K and CO 2 at 273 K) reveal the presence of internal microvoids accessible only by diffusion through the solid phase. The degree of nonlinearity and competition correlate with the CO 2 -measured microvoid volumes of the sorbents. The hole-filling mech anism is more important for the kinetically slow fraction.
Xing et al. (Thu,) studied this question.