Peat samples from typical tropical peat swamp around Narathiwat Province, southern Thailand, were collected from various depths of peat profile (from surface to 210 cm depth), of which dates were established using an accelerator mass spectrometry from 0 to 2600 years BP. The samples were analysed to discuss changes in structural characteristics of lignin in peat samples during peat formation. The samples were fractionated based on pH adjustment into dilute alkali soluble (AL_sol) and insoluble (AL_insol) fractions. Only glucosyl and xylosyl residues were detected in AL_insol fraction, but no monosaccharide in AL_sol fraction. Lignin contents of AL_insol fraction of the peat samples ranged from 73.0 to 83.3%. Lignin was significantly modified during peat formation. The progress of condensation reactions, introduction of carboxyl groups and partially loss of methoxyl groups, cleavage of arylglycerol-β-aryl ether linkage, were confirmed by alkaline nitrobenzene oxidation, functional group analysis and analytical ozonation, respectively, due to the microbial activities.
Lignin of the peat samples was isolated by Björkman's procedure to discuss structural feature in detail using spectrometrical procedures as the first trial in the field of geochemistry. The signals around 6.6 and 6.9 ppm in ¹H-NMR were considerably weak compare with those of lignin from fresh wood, suggesting high condensation. The most peaks originating from lignin of fresh wood were also detected in the FTIR spectrum. The ionization difference (ΔEi) spectrum clearly showed the presence of unconjugated and conjugated phenolic hydroxyl groups. Thus, lignin in peat was modified significantly by microbial activities. It is expected that further modified fragments of lignin would be soluble in water and flow-out to ocean through rivers.