Molecular Identification of Water-Extractable Organic Carbon from Thermally Heated Soils: C-13 Nuclear Magnetic Resonance (NMR) and Accurate Mass Analyses Find Benzene and Pyridine Carboxylic Acids

To simulate the effects of wildfire on the combustion process in soils and their potential to leach organic compounds into streams and groundwater, mineral soil samples were heated at temperatures of 150-550o C. Then, the soils were leached with deionized water, filtered, and analyzed by dissolved organic carbon. The water extract was concentrated by both XAD-8 and XAD-4 resins and analyzed by C-13 nuclear magnetic resonance (NMR) and liquid chromatography time-of-flight mass spectrometry. Approximately 15-20% of the water extractable organic carbon was identified as benzene dicarboxylic acids, tricarboxylic acids, and tetracarboxylic acid isomers, commonly called BPCAs. Also identified were isomers of pyridine dicarboxylic acids and tricarboxylic acids (PCAs). The conversion of soil organic carbon to BPCAs occurs at 250o C and reaches a maximum between 350o-450oC. At higher temperatures (>450oC), the BPCA concentrations decrease, suggesting decarboxylation and conversion to carbon dioxide and water. This...