A distributed reactivity model for sorption by soils and sediments .8. Sorbent organic domains: Discovery of a humic acid glass transition and an argument for a polymer-based model

Analysis of a humic acid by differential scanning calorimetry has revealed the existence of a glass transition point. Glass transition temperatures, T-g, of water-wet and desiccator-dry specimens were found to range from 43 degrees C for water-wet humic acid to 62 degrees C for dry samples. Phenanthrene sorption isotherms for these and other natural and synthetic organic matrices having known glass transition temperatures were determined to exhibit linearity and nonlinearity corresponding respectively to the rubbery (expanded) and glassy (condensed) states of the sorbent. Invoking a limiting case of the distributed reactivity model based on polymer sorption theory, we explain the observed sorption behavior as comprised in each case by a linear phase-partitioning component and a Langmuir-like nonlinear adsorption component. We conclude that polymer sorption theory provides a useful context in which to assess sorption phenomena associated with sail and sediment organic matter, providing more accurate projections of contaminant behavior in environmental systems and better informed specifications of appropriate remediation measures.