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

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.