Structural Components of Humic Acids as Determined by Chemical Modifications and Carbon‐13 NMR, Pyrolysis‐, and Thermochemolysis‐Gas Chromatography/Mass Spectrometry

Abstract

The chemical structure of humic acids (HAs) extracted from a grassland surface soil and peat were studied using bleaching (NaClO2 oxidation) and acid hydrolysis (6 M HCl) in combination with advanced analytical techniques: solid‐state 13C nuclear magnetic resonance (NMR), pyrolysis‐gas chromatography/mass spectrometry (GC/MS), and tetramethylammonium hydroxide (TMAH) thermochemolysis‐GC/MS. The purpose of the chemical treatments was to remove known structural fragments from the HA to study the building blocks and components of the macromolecule. Bleaching the peat HA resulted in an attack on the lignin structures leading to a significant reduction in the C‐substituted and O‐substituted aromatic C peaks (128 and 150 ppm) in the 13C NMR spectrum. However, the bleached soil HA still contained residual aromatic C, suggesting that part of its aromaticity had originated from aromatic structures resistant to bleaching, possibly black C (charcoal). The pyrolysates of the bleached HAs contained mainly alkanes and alkenes (C8 to C29), whereas TMAH thermochemolysis yielded a homologous series of long‐chain fatty acid methyl esters (FAMEs) (C8 to C32) and dicarboxylic acid dimethyl esters (DAMES). The data are comparable with those obtained from pyrolysis and thermochemolysis of plant cuticular materials, and therefore suggest that cuticular residues are an integral part of the HA macromolecule. The acid hydrolysis treatment removed esters, amides, carbohydrates, and some of the N‐containing compounds from the HAs. This study demonstrates the effectiveness of bleaching and hydrolysis treatments together with advanced analytical techniques for characterization of aliphatic, lignin‐derived (LG) and nonhydrolyzable N‐containing structures associated with the HA macromolecule.

Nuclear Magnetic Resonance Analysis of Natural Organic Matter

“…The major structures include cuticular components (species derived from the cuticular coating of plant leaves) peptide/proteins, lignin, and carbohydrate . This is consistent with other NMR studies that identify these components (Almendros et al, 1991a ;K ö gel -Knabner et al, 1992 ;Zech et al, 1992 ;Knicker and Ludemann, 1995 ;Rumpel et al, 1998 ;Fan et al, 2000 ;Haiber et al, 2001a ;Chefetz et al, 2002 ;Simpson, 2002 ;Simpson et al, 2002a ) in abundance in soil organic matter. This is logical considering that soluble NOM from soils is operationally defi ned and at any one time microbial and plant residues will be the major contributors to soil biomass (K ö gel -Knabner, 2002 ).…”

Comparative study for separation of atmospheric humic-like substance (HULIS) by ENVI-18, HLB, XAD-8 and DEAE sorbents: Elemental composition, FT-IR, 1H NMR and off-line thermochemolysis with tetramethylammonium hydroxide (TMAH)

“…The aromatic compounds identified in HULIS were remarkably similar to pyrolysis products of HAs, FAs and lignin (Martin et al, 1994(Martin et al, , 1995del Rio et al, 1998;Chefetz et al, 2002;Li et al, 2006;Fukushima et al, 2011). All HULIS pyrograms present two distinct peaks at 32.8 min and 34.5 min (Nos.…”

Humification and Humic Acid Composition of Suspended Soil in Oligotrophous Environments in South Vietnam

“…is procedure was used in previous studies for characterization of soils of various environments [29,30]. e HAs were demineralized by shaking overnight in 0.1 M HCl/0.3 M HF (solid/solution ratio 1 : 1) and then repeatedly washed with deionized water until pH 3 was reached; they were then freeze-dried [31][32][33].…”

“…Groups of structural compounds were identified using the following chemical shifts values: 190-170 ppm for the carboxyl group and amidic carbonyl; 170-150 ppm for the aromatic C of phenols and phenol esters; 150-135 ppm for the alkylaromatics; 135-108 ppm for the protonized aromatic carbon, bridgehead C; 108-70 ppm for the cellulose anomeric carbon and hemiacetal carbon; 70-50 ppm for the resonance region of C-H bonds, secondary alcohols, and other carbon atoms bound to oxygen; 50-32 ppm for the methyl group resonance region of aliphatic and aromatic ethyl ethers, amino acid carbons, and methyl esters of carboxylic groups; 32-27 ppm for the resonance region of quaternary carbon and CH carbons; 27-10 ppm for the resonance region of CH 2 alkyl structures in transconformation; and 10-0 ppm for the resonance region of alkyl methyls and CH 2 units [33][34][35][36][37].…”