Thermodynamic properties of several soil- and sediment-derived natural organic materials

Improved understanding of the structure of soil- and sediment-derived organic matter is critical to elucidating the mechanisms that control the reactivity and transport of contaminants in the environment. This work focuses on an experimental investigation of thermodynamic properties that are a function of the macromolecular structure of natural organic matter (NOM). A suite of thermal analysis instruments were employed to quantify glass transition temperatures (Tg), constant-pressure specific heat capacities (Cp), and thermal expansion coefficients (α) of several International Humic Substances Society (IHSS) soil-, sediment-, and aquatic-derived NOMs. Thermal mechanical analysis (TMA) of selected NOMs identified Tgs between 36 and 72 °C, and αs ranging from 11 μm/m °C below the Tg to 242 μm/m °C above the Tg. Standard differential scanning calorimetry (DSC) and temperature-modulated differential scanning calorimetry (TMDSC) measurements provided additional evidence of glass transition behavior, including identification of multiple transition behavior in two aquatic samples. TMDSC also provided quantitative measures of Cp at 0 and 25 °C, ranging from 1.27 to 1.44 J/g °C. Results from TMA, DSC, and TMDSC analyses are consistent with glass transition theories for organic macromolecules, and the glass transition behavior of other NOM materials reported in previous studies. Discussion of the importance of quantifying these thermodynamic properties is presented in terms of improved physical and chemical characterization of NOM structures, and in terms of providing constraints to molecular simulation models of NOM structures.