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Abstract:

Improved understanding of the physical, chemical, and thermodynamic properties of soil and sediment organic matter (SOM) is crucial in elucidating sorption mechanisms of hydrophobic organic compounds (HOCs) in soils and sediments. In this study, several thermoanalytical techniques, including thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), temperature-modulated differential scanning calorimetry (TMDSC), and thermal mechanical analysis (TMA) were applied to 13 different organic materials (three woods, two humic acids, three kerogens, and five black carbons) representing a spectrum of diagenetic and/or thermal histories. Second-order thermal transition temperatures (Tt) were identified in most materials, where the highest observed Tt values (typically characterized as glass transition temperatures (Tg)) were shown to closely relate to chemical characteristics of the organic samples as influenced by diagenetic or thermal alteration. Results further suggest a positive correlation between glass transition temperature and a defined diagenetic/thermal index, where humic-based SOM (e.g., humic and fulvic acids) possess lower transition temperatures than more mature carbonaceous-based SOM (i.e., kerogens and black carbons). The observed higher thermal transition temperature of aliphatic-rich Green River shale kerogen (120 oC) relative to that of aromatic-rich humic acids suggests that a sole correlation of aromaticity to thermal transition temperature may be inappropriate.

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