Macromolecular characteristics of natural organic matter. 2. Sorption and desorption behavior

The effects of the relative macromolecular mobilities of natural organic matter (NOM) matrices on their sorption/desorption behavior with respect to phenanthrene are described. Sorption isotherm characteristics are found to correspond directly to the relative dominance of glassy and rubbery states, with nonlinear sorption being linked to the dominance of glassy regions within the natural and synthetic macromolecules examined. The Dual Reactive Domain Model (DRDM) developed in earlier studies provides an effective tool for identifying specific adsorption and absorption contributions to overall isotherm patterns. The Hysteresis Index (H.I.), also developed in earlier studies, is useful for quantifying differences between the sorption and desorption isotherms for each macromolecular sorbent. Confirming earlier observations with soils and sediments, a trend of increasing H.I. with decreasing oxygen/carbon (O/C) atomic ratio is generally observed for the NOMs investigated. Correlation of isotherm nonlinearity and H.I, with macromolecular mobility is hypothesized and tested, leading to a general conclusion that the extent of isotherm nonlinearity and the H.I. are related to increasing glass transition temperature (T-g) of the NOM. Macromolecular sorbents display little or no desorption hysteresis under experimental conditions at or very near their T-g, while sorbents that are clearly in their glassy state under those conditions manifest significant desorption hysteresis. This may relate in part to the fact that the times required for attainment of true sorption and/or desorption equilibria vary with the mobility and flexibility of macromolecular sorbent matrices, attaining only over extremely long periods of solute migration into and out of the less flexible glassy states of such matrices.