Structural features of lignohumic acids

• Humic-like substances can be prepared by thermal conversion of technical lignosulfonate.
• Resulting lignohumate contains humic acids (HA) whose properties and structural features are close to young soil HA.
• Hypothetical chemical structure of “average” lignohumate HA was proposed.

The composition and structure of humic acids isolated from lignohumate, which is produced by hydrolytic–oxidative conversion of technical lignosulfonates, were characterized by chemical and spectral methods (UV/VIS, FTIR, and 13C NMR spectroscopy). As comparative samples, humic acids (HA) were isolated also from lignite and organic horizon of mountain spruce forest soil. When compared with other HA studied, the lignohumate humic acids (LHHA) contained relatively few carboxyl groups, whose role is partly fulfilled by sulfonic acid groups. Distinctive 13C NMR signal of methoxyl group carbons, typical for lignin and related humic substances, was found at the shift of 55.9 ppm. Other alkoxy carbons were present in limited quantity, like the aliphatic carbons. Due to the low content of these carbon types, the LHHA has high aromaticity of 60.6%.Comparison with the natural HA has shown that lignohumate obtained by thermal processing of technical lignosulfonate can be regarded as an industrially produced analog of natural humic substances. Based on the chemical and spectral data evaluation, structural features of lignohumate humic acids were clarified and their hypothetical chemical structure proposed, which described typical “average” properties of the isolated fraction.

Average structural formula of lignohumate humic acid.Figure optionsDownload as PowerPoint slide