Free-radical condensation as a natural mechanism of the formation of humic acids

Evidence for the formation of fused aromatic ring structures in an organic soil profile in the early diagenesis

“…Numerous investigations of the stability of SOM have shown that the recalcitrancy of SOM increases with depth in the soil pro le [22][23][24] . While the formation of FAR has been studied intensively in the laboratory 9,12,18 , their formation in natural soil environments has not received su cient research attention. In this study, we investigated molecular-level changes in the organic matter of an organic soil pro le with a focus on FAR.…”

“…Higuchi et al 11 (2004) described oxidation reactions that involve hydroxylation and ring opening of lignin to produce unsaturated aliphatic acids, hydroxylated carboxylic acids and hydroxylated muconic acids and that synthesize FAR and CRAM structures through polymerization and condensation reactions. Microorganisms, enzymes and free radicals signi cantly affect FAR and CRAM formation 12 in the early diagenesis process. Prevailing research evidence suggest that hydroxyl radicals are a key factor in creating the chemical bonds that lead to cross-linkages and condensation reactions to form FAR 13 .…”

Formation of Char-Like, Fused-Ring Aromatic Structures from a Nonpyrogenic Pathway during Decomposition of Wheat Straw

“…16,17 In controlled environments, however, photochemical processing of dissolved organic matter in natural waters has been shown to produce BC-like fused-ring aromatic compounds, 19 and exposure of ligninderived organic matter to hydroxyl radicals originating from Fenton-type reactions also generated BC-like fused-ring compounds. 20 Knyazev et al 21 proposed that condensation of low-molecular mass compounds could be driven by reactions with free-radical hydroxyls, •OH, that are produced from photochemical reactions 22 and atmospheric deposition, 21,23 or generated by plants 24 and microorganisms. 25 Free-radical hydroxyls can both break and create chemical bonds, leading to cross-linking and condensation reactions, providing the possibility for the formation of fused-ring aromatic C structures.…”

“…For example, previous studies have suggested that some fused or condensed aromatic compounds in soil may be formed biotically or abiotically by condensation and polymerization during the humification process. − The formation of condensed and polymerized compounds from small building blocks (e.g., coniferyl alcohol, sinapyl alcohol, and p -hydroxycinnamyl) can best be explained by a pairing mechanism involving mesomeric free radicals, although there is no convincing proof of this hypothesis under soil conditions. , In controlled environments, however, photochemical processing of dissolved organic matter in natural waters has been shown to produce BC-like fused-ring aromatic compounds, and exposure of lignin-derived organic matter to hydroxyl radicals originating from Fenton-type reactions also generated BC-like fused-ring compounds . Knyazev et al proposed that condensation of low-molecular mass compounds could be driven by reactions with free-radical hydroxyls, ·OH, that are produced from photochemical reactions and atmospheric deposition, , or generated by plants and microorganisms . Free-radical hydroxyls can both break and create chemical bonds, leading to cross-linking and condensation reactions, providing the possibility for the formation of fused-ring aromatic C structures.…”

Transformation of soils in areas with active recreational activity on the Russian Black sea coast

“…The concentration of free radicals in humus substances determines the degree of biochemical activity of high-molecular compounds. According to modern concepts, radical particles are intermediate participants in a stepwise redox reaction of polycondensation (Knyazev et al, 2009). Both the bioclimatic conditions of the formation of the soil layer and the anthropogenic load on natural ecosystems lead to a change in paramagnetic activity.…”