Dual-Mode Sorption of Low-Polarity Compounds in Glassy Poly(Vinyl Chloride) and Soil Organic Matter

Abstract

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, condensednature of organic matter according to modern concepts of humic structure. Gas adsorption isotherms (N2 at 77 K and CO2 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 CO2-measured microvoid volumes of the sorbents. The hole-filling mech anism is more important for the kinetically slow fraction.