13C NMR spectra and elemental composition of fractions with different particle sizes from an Andosol humic acid

Elemental composition and solution C-13 NMR spectra for six humic acid fractions with different particle sizes (i.e. 3K, 10K, 30K, 100K, 300K, and 500K) which were prepared from a humic acid in an Umbric Andosol by successive gel permeation chromatography were determined. Elemental composition of the particle size fractions clearly differed from one another. Contents of nitrogen and hydrogen increased with the increase of the particle size. Small and middle particle size fractions (3K to 100K) showed low contents of hydrogen and nitrogen corresponding to those in typical Andosol humic acids, while the contents in the large fractions (300K and 500K) were distributed in the ranges of those in the other humic acids. As the particle size increased, the H/C ratio increased whereas the O/H decreased. In the H/C versus O/C diagram, the large particle size fractions (300K and 500K) were observed in the area around humic acids with a low humification degree. Variation of the chemical structural properties with particle size differences was assumed, based on the analysis of the H/C versus O/H and H/C versus O/C diagrams. The changes of the carbon species with particle size differences were examined based on the C-13 NMR spectra. As the particle size increased, the aliphatic C content increased whereas the carboxylic and aromatic C content decreased and, therefore, the aromaticity decreased. In addition, a negative correlation (r=-0.977) was found to be significant at 0.1% level between the H/C ratios and aromaticity. The peak strength around 30 ppm assigned to chains of methylene group increased remarkably with the increase of the particle size. It was suggested that the changes of the content of the aliphatic carbon with the decrease of the particle size may depend on the changes of the aliphatic chain length. In conclusion, these findings suggest that the Andosol humic acid molecule may be composed of humic acid components with long chains of aliphatic groups for the larger particle size fractions, and of their homologous series through the reduction of the length of the aliphatic long chain for the smaller particle size fractions. These assumptions were compatible with those made in a series of studies previously reported.