Effects of Chemical Modifications and Extractants on the Carbon-13 NMR Spectra of Humic Materials
1984
Preston, C. M. | Schnitzer, M.
Carbon-13 NMR was used to study the effects of methylation, 6M HCl hydrolysis and 6M HCl hydrolysis followed by methylation on two humic acids (HA's) and one fulvic acid (FA), as well as the effect of extraction by 0.5M NaOH vs. 0.1M HCl on an FA. Spectra of methylated HA's and FA's indicate that almost all CO₂H groups are attached to aromatic structures, and that phenolic OH's are significant functional groups. These spectra also exhibit fine structure in the 127- to 131-ppm region, possibly due to benzenecarboxylic acid-type structures, which have been proposed as major building blocks of humic materials. Other sharp signals are observed in the 133- to 146-ppm region, due to C-substituted carbons in aromatic or heterocyclic rings. Carbon-13 NMR spectra of hydrolyzed HA's and FA indicate, in agreement with chemical analyses, that 6M HCl hydrolysis removes all amino acids, most carbohydrates and increases the aromatic C content (aromaticity) of the residues. Carbon-13 NMR spectra of FA extracted by 0.5M NaOH and 0.1M HCl are similar.
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