STRUCTURALLY BOUND LIPIDS IN PEAT HUMIC ACIDS

Abstract

Humic acids from highly decomposed peat were subjected to oxidation with alkaline cupric oxide (CuO) at 170°C (single oxidation). Oxidation products were isolated as three fractions, oxidized humic acids, fulvic acids and lipophilic compounds. Isolated oxidized humic acids were subsequently re-oxidized (sequential oxidation) under the same conditions, and released lipophilic compounds were isolated. Lipids released during single and sequential oxidations were determined using capillary gas chromatography and mass spectrometry. Identified compounds accounted for 3.3 and 0.12 wt.% of humic acid content, respectively. Lipid profiles of lipophilic fractions released during single and sequential oxidation were markedly different. Lipids released by single oxidation contained triterpenoids (83%), shorter-chain fatty acids (11%) and β-sitosterol (6%). In contrast, sequential oxidation released various diterpenoid acids (53%) of coniferous resin origin, various unsaturated C18 fatty acids (20%), and a series (C22-C34) of n-alkanes (20%). The consistent distribution of n-alkane fraction with a minor predominance of even-chain homologues (CPI 0.95), a high proportion (77%) of dehydroabietic acid and its degradation products, and a high preponderance of abietane over pimarane structures in the diterpenoid acid fraction all indicate substantial microbial activity within the humic acid matrix. Since polyfunctional compounds were predominant in monomeric lipids released from humic acids, the current data indicate that these compounds may potentially bind to humic acids at two or more points.