Characteristics of soil humic acid fractions separated by ultrafiltration

Abstract Different molecular weight fractions obtained by ultrafiltration of a humic acid (HA) extracted from a Mollisol were analyzed by chemical, infrared, electron microscopy, and isotachophoresis methods. The results showed that the different HA fractions did not differ substantially in characteristics from each other, although each fraction was composed of subfractions exhibiting different electromobilities. It appeared that fractionation of HA would yield molecular weight fractions as imposed by any exclusion limits used in the procedure. However, the elemental composition, infrared spectra, and electron micrographs showed that the fractions, separated by molecular weight limits, contained similar compounds. These results do not support the concept of humic acids as a heterogeneous mixture of compounds, but show instead that humic acids may possess a composition more homogeneous in nature than previously postulated.