[1H] and [13C]NMR studies on the importance of aromatic structures in fulvic and humic acids

doi:10.1016/0016-7037(79)90025-5

Geochimica et Cosmochimica Acta

Volume 43, Issue 11, November 1979, Pages 1771-1775

https://doi.org/10.1016/0016-7037(79)90025-5 Get rights and content

Abstract

To obtain information on the contribution of aromatic fragments to the chemical structure of humic substances, we carried out a study on the [¹H]NMR and [¹³C]NMR spectra of humic and fulvic acids and their oxidative degradation products extracted from an Andosol soil.

[¹H]NMR spectra of all organic fractions present considerable adsorption between 7.4 and 8.8 ppm, due to the presence of aromatic protons.

The percentages of aromatic protons in respect to the total amount of protons are as follows: FA 20%, HA 19%, degraded FA < 12%, degraded HA 14%. The values indicate that the contribution of aromatic structures to the humic substances is significant, also considering that they are highly substituted.

The degraded fractions contain smaller amounts of aromatic protons, because degradation causes the opening of the aromatic rings. Thus results obtained from the degradation do not seem to be reliable for defining the importance of aromatic structures in humic substances.

Also the [¹³C]NMR spectra show signals in the aromatic region which derive from unsubstituted carbon atoms, while signals originating from tertiary carbon atoms merge with the noise. We believe that, at present, [¹H]NMR spectroscopy is more suited for studying the role played by aromatic compounds in organic soil fractions.

References (26)

H. Lentz et al.
Proton resonance spectra of humic acids from the solum of a Podzol
Geoderma
(1977)
O.B. Maximov et al.
On permanganate oxidation of humic acids
Geoderma
(1977)
G. Ogner et al.
Permanganate oxidation of methylated fulvic and humic acids in chloroform
Geoderma
(1977)
P. Ruggiero et al.
Spectroscopic studies on soil organic fractions—II. IR and ¹H-NMR spectra of methylated and unmethylated fulvic acids
Geochim. Cosmochim. Acta
(1978)
O. Sciacovelli et al.
Spectroscopic studies on soil organic fractions—I. IR and NMR spectra
Soil Biol. Biochem.
(1977)
D.H. Stuermer et al.
Investigation of seawater and terrestrial humic substances with carbon-13 and proton nuclear magnetic resonance
Geochim. Cosmochim. Acta
(1976)
H.A. Anderson et al.
Possible relationship between soil fulvic acids and polymaleic acid
Nature
(1976)
D.H.R. Barton et al.
A new experimental approach to the humic acid problem
Nature
(1963)
E. Breitmaier et al.
¹³C-NMR Spektroskopie
(1977)
D. Grant
Chemical structure of humic substances
Nature
(1977)

M.H.B. Hayes et al.

Humic materials from an organic soil: A Comparison of extradants and of properties of extracts

Geoderma

(1974)

R.D. Haworth

The chemical nature of humic acid

Soil Sci.

(1971)

B. Lakatos et al.

Biopolymer-metal complex systems. V. Study of the proton magnetic spectrum of humus compounds

Agrokémia és Talajtan

(1977)

Cited by (34)

Copper-binding ligands in the NE Pacific
2018, Marine Chemistry
Citation Excerpt :
In estuarine waters, terrestrially-derived humic substances (HS) account for a major fraction of the available organic ligands for Cu complexation (Abualhaija et al., 2015; Muller and Batchelli, 2013) and play a key role transporting metals, particularly iron (Fe) to coastal and open ocean waters (Bundy et al., 2015; Laglera and van den Berg, 2009; Misumi et al., 2013). Terrestrial HS are derived from the relatively recent degradation of plant matter (Averett et al., 1994) and are highly aromatic (Ruggiero et al., 1979; Sohn and Weese, 1986), forming organic Cu complexes in seawater with log K'Cu2+L = 12–13 (Kogut and Voelker, 2001; Whitby and van den Berg, 2015). Humic substances are also expected to be important in metal speciation in open ocean waters (Heller et al., 2013; Kitayama et al., 2009), given that 5–25% of dissolved organic carbon (DOC) in the surface ocean is HS (Benner, 2002).
Copper distribution and speciation were determined at stations P4 and P26 along Line P as part of a GEOTRACES Process Study in the Northeast Pacific, at depths between 10 and 1400 m. Two ligand classes (L₁ and L₂) were detected at both stations: the stronger L₁ ligand pool with log K'_Cu2+L1 15.0–16.5 and the weaker L₂ ligand pool with log K'_Cu2+L2 11.6–13.6. The L₁ class bound on average 94% of dCu, with the ratio between L₁ and dCu constant and close to unity (1.15 = [L₁]:[dCu]). The concentrations of total ligands exceeded those of dCu at all depths, buffering Cu²⁺ concentrations ([Cu²⁺]) to femtomolar levels (i.e. pCu 14.1–15.7). Measurements using cathodic stripping voltammetry also identified natural copper-responsive peaks, which were attributed to thiourea- and glutathione-like thiols (TU and GSH, respectively), and Cu-binding humic substances (HS_Cu). Concentrations of TU, GSH and HS_Cu were determined by standard addition of model compounds in an attempt to identify Cu-binding ligands. HS_Cu concentrations were generally higher at P26 than at P4, consistent with a marine origin of the humic material. Overall, HS_Cu contributed to 1–27% of the total L concentration (L_T) and when combined with the two thiols contributed to up to 32% of L_T. This suggests other ligand types are responsible for the majority of dCu complexation in these waters, such as other thiols. Some potential candidates for detected, but unidentified, thiols are cysteine, 3-mercaptopropionic acid and 2-mercaptoethanol, all of which bind Cu. Significant correlation between the concentrations of TU-like thiols and L₁, along with the high log K'_Cu2+L1 values, tentatively suggest that the electrochemical TU-type peak could be part of a larger, unidentified, high-affinity Cu compound, such as a methanobactin or porphyrin, with a stronger binding capability than typical thiols. This could imply that chalkophores may play a greater role in oceanic dCu complexation than previously considered.
Chemical and spectroscopic properties of two fractions of soil organic matter obtained by electro-ultrafiltration
1992, Science of the Total Environment, The
A soil sample, collected from vertisol in southern Italy and containing 3.7% of organic carbon, was extracted and fractionated by electro-ultrafiltration using a relatively low electric field and negative pressure. Micropore filters (20 000 Da and 100 000 Da) were selected to obtain two fractions of soil organic matter.
E₄/E₆ ratios and analysis data were in agreement with values of fulvic and humic acids as reported in the literature. FT-IR spectra showed high absorption values in the region of carboxylic acid groups and medium intensity peaks in the region of frequencies ascribed to aliphatic chain absorptions. ¹H-NMR spectra showed a resolution of signals as good as those previously reported for organic fractions that had been chemically extracted.
Structural and behavioral characteristics of a commercial humic acid and natural dissolved aquatic organic matter
1990, Chemosphere
Dissolved organic matter (DOM) from two natural surface water sources and a commercial humic acid were analyzed using gel-permeation chromatography, high pressure reverse phase liquid chromatography, and ¹H-NMR spectroscopy. Results from the chromatographic studies show that the DOM of two natural waters consisted primarily of relatively low molecular weight, polar organic constituents, while large and relatively nonpolar macromolecules comprised a significant fraction of the commercial humic acid. The ¹H-NMR assays indicated that DOM from the two natural water samples was comprised of nonaromatic organic constituents, while the commercial humic acids tested contained both aromatic and aliphatic moieties. Based upon these composite results of the several different types of analysis employed, it is evident that the humic acid examined, and possibly others prepared in the same way, contain molecular structures which exhibit physical and chemical properties that do not reflect the true nature of DOM in real aquatic systems. Commercially available humic substances of this type may therefore not be suitable surrogates for naturally occurring DOM in laboratory investigations and analysis of geochemical and environmental transformation reactions.
The uniqueness of humic substances in each of soil, stream and marine environments
1990, Analytica Chimica Acta
Definitive compositional differences are shown to exist for both fulvic acids and humic acids from soil, stream and marine environments by five different methods (¹H and ¹³C NMR spectroscopy, ¹⁴C age and δ¹³C isotopic analyses, amino acid analyses and pyrolysis-mass spectrometry). Definitive differences are also found between fulvic acids and humic acids within each environment. These differences among humic substances from various sources are more readily discerned because the method employed for the isolation of humic substances from all environments excludes most of the non-humic components and results in more purified humic isolates from water and soils. The major compositional aspects of fulvic acids and humic acids which determine the observed characteristic differences in each environment are the amounts and composition of saccharide, phenolic, methoxyl, aromatic, hydrocarbon, amino acids and nitrogen moieties.
The chemical structure of highly aromatic humic acids in three volcanic ash soils as determined by dipolar dephasing NMR studies
1989, Geochimica et Cosmochimica Acta
Dipolar dephasing ¹³C NMR studies of three highly aromatic humic acids, one from a modern soil and two from paleosols, have permitted the determination of the degree of aromatic substitution. From these data and the normal solid-state ¹³C NMR data we have been able to develop a model for the average chemical structure of these humic acids that generally correlates well with permanganate oxidation data. The models depict these humic acids as benzene di- and tricarboxylic acids interconnected by biphenyl linkages. An increasing degree of substitution is observed with increasing geologic age. These structures may be characteristic of the resistant aromatic part of the “core” of humic substances that survives degradation.
Aliphatic structures in peat fulvic acids
1987, Science of the Total Environment, The
Fulvic acids, extracted from strongly humified milled peat, were studied through ¹³C-NMR measurements and cupric oxide oxidations. The ¹³C-NMR spectrum shows signals caused by the carboxylic carbons, aromatic or olefinic carbons, acetal carbons, aliphatic alcohol carbons and methylene and methyl carbons. In cupric oxide oxidation three fractions were obtained: an ethyl acetate soluble fraction, yielding 11 %, a base and acid soluble fraction (“oxidized fulvic acids”), yielding 25 % and a base soluble,acid insoluble fraction yielding 4 %.

View all citing articles on Scopus

View full text

[1H] and [13C]NMR studies on the importance of aromatic structures in fulvic and humic acids

Abstract

Geoderma

Geoderma

Geoderma

Geochim. Cosmochim. Acta

Soil Biol. Biochem.

Geochim. Cosmochim. Acta

Possible relationship between soil fulvic acids and polymaleic acid

Nature

A new experimental approach to the humic acid problem

Nature

13C-NMR Spektroskopie

Chemical structure of humic substances

Nature

Humic materials from an organic soil: A Comparison of extradants and of properties of extracts

Geoderma

The chemical nature of humic acid

Soil Sci.

Biopolymer-metal complex systems. V. Study of the proton magnetic spectrum of humus compounds

Agrokémia és Talajtan

[¹H] and [¹³C]NMR studies on the importance of aromatic structures in fulvic and humic acids

¹³C-NMR Spektroskopie