Macromolecular characteristics of natural organic matter and their influence on sorption and desorption behavior of organic chemicals.

Abstract

This thesis provides a fundamental experimental investigation of sorption and desorption behavior of hydrophobic organic contaminants in natural and model systems under water-saturated conditions. It emphasizes the development of a more thorough understanding of the role organic components of soils and sediments have in dominating sorption behavior of pollutants, and how this organic matter, as a macromolecule, manifests sorption behavior characteristic of synthetic organic polymers. The experimental methodology employed in this study involved the use of five well-characterized model sorbents and eight less well-characterized natural sorbents in identical experimental conditions; drawing appropriate conclusions based on observations of the behavior of both systems. Sorbent characterization was accomplished through use of differential scanning calorimetry, differential thermal analysis, and solid-state $\sp{13}$C nuclear magnetic resonance. Determination of sorbent surface area and microporosity was accomplished through volumetric gas-phase sorption utilizing nitrogen, argon, krypton, and carbon dioxide as probe solutes. Aqueous-phase sorption studies included both long-term equilibrium and short-term nonequilibrium evaluation of phenanthrene sorption and desorption within twelve sorbents; each evaluated at three different temperatures. Important findings from the experimental portions of this work include the discovery of glass transitions in soil-derived organic matter, thus effectively linking polymer sorption theory to observed sorption behavior in natural organic matter systems. A polymer sorption theory-based Dual Reactive Domain Model (DRDM) was developed to explain combined partitioning and adsorption behavior. Evaluation of sorption isotherms at different temperatures revealed increased sorption isotherm linearity with increased temperature; indicating that increased macromolecular mobility, as measured by a larger partitioning component within DRDM, was primarily responsible for this sorption trend. Investigation of equilibrium desorption isotherms concluded that desorption hysteresis could be linked to three primary mechanisms: (i) failure to reach thermodynamic equilibrium; (ii) entrapment of sorbate within fixed microvoids present within glassy matrices; and (iii) inelastic stretching of the macromolecule during the sorption cycle. Nonequilibrium studies revealed bi-rate sorption behavior in both natural and model systems, and a two-domain rate of sorption model was applied to interpret the relative contributions of fast and slow sorbing sorbent domains to overall sorption behavior.