Evaluation of the Glassy/Rubbery Model for Soil Organic Matter

The glassy/rubbery model for soil organic matter (SOM) has been proposed to explain nonlinear sorption uptake isotherms, site-specific bisolute competition, increased isotherm nonlinearity with increasing uptake time, and desorption hysteresis observed for nonspecifically interacting solutes. The glassy SOM phase has been considered the locus of this nonlinear sorption behavior and of greater sorption (increased Freundlich constant K f or Langmuir capacity term) as contrasted with the rubbery phase. The report of a glass transition in Aldrich humic acid has been considered the linchpin of this model. In the current analysis, a number of points are made, including (i) a glassy polymer phase does not necessarily result in nonlinear sorption behavior; (ii) Aldrich humic acid, kerogen, and coal are poor models for soil organic matter; and (iii) a number of different mechanisms may result in nonpartitioning-like sorption behavior, such as complex formation with an organic matter macromolecule, interactions at the surface of organic matter, or cross-linking of organic matter macromolecules resulting in a finite sorption domain.