Tuesday, November 14, 2006

Summary of Evidence Supporting the Glassy Polymer Sorption Model for Nonionic Organic Compounds in SOM.

Joseph Pignatello, Connecticut Agricultural Experiment, 123 Huntington St; PO Box 1106, New Haven, CT 06504-1106 and Baoshan Xing, Dept of Plant, Soil & Insect Science, Stockton Hall, Univ of Massachusetts, Amherst, MA 01003.

Results obtained primarily with NRI support has led to modification of the classic “partition” paradigm for sorption of nonionic compounds to soil organic matter (SOM). We postulate that SOM contains glassy domains that behave similarly to glassy synthetic polymers and natural polymers, such as lignin. Direct evidence for the glassy character of SOM has been obtained. The glassy state is a perpetual non-equilibrium state solid; as a consequence, it contains free volume in excess of the thermodynamic state.  The excess free volume exists as molecular-scale internal micropores, or holes, that serve as ‘adsorption’ sites. In a simplified picture, sorption may be regarded as a distribution of incoming molecules between these voids and the bulk phase ‘dissolution domain’ of SOM, and can be modeled by a dual-mode equation expressed as a combination of linear and Langmuir terms. The glassiness of SOM, and thus the population of holes, is dependent on temperature and the concentration of sorbate. The model rationalizes isotherm nonlinearity and competitive sorption observed for many soils and humic substances. The model also explains true hysteresis.

Handout (.pdf format, 238.0 kb)