Lipid constituents of peat humic acids and humin. Distinction from directly extractable bitumen components using TMAH and TEAAc thermochemolysis

Humic acids and humin from an acidic peat were investigated using solid state 13C NMR and pyrolysis with tetramethylammonium hydroxide (TMAH) and tetraethylammonium acetate (TEAAc). The degradation products were compared with the solvent extractable lipids. The latter appeared to originate mostly from plant material, whereas the products derived from the humic fractions showed a more complex structure, partly inherited from natural insoluble material. The major products of TMAH thermochemolysis were aromatic compounds derived from lignin moieties, hydrocarbons and fatty acid methyl esters (FAMEs) partly incorporated via ester or ether bonds. Aliphatic hydrocarbons were present as n-alkene/n-alkane doublets with a Gaussian distribution in the C17-C35 range. These unsaturated and saturated hydrocarbons arose from resistant biopolymers. Bifunctional aliphatic compounds were also incorporated into the humic structure, contributing to alkyl bridges. Thermochemolysis of humin produced more aliphatic structures than did humic acids. The application of TEAAc thermochemolysis allowed identification of a wide range of products initially retained in the humic macromolecules via non-covalent bonds, thereby indicating that weak bonds such as hydrogen bonds play a key role in the structure of humic substances. The association of thermochemolysis using TMAH and TEAAc thus allows covalently-bound fatty acids, trapped fatty acids and trapped fatty acid methyl esters present in the structure of humic acids or humin to be distinguished.