Molecular size distribution of humic acids as affected by the ionic strength and the degree of humification

Abstract

Molecular size distribution of soil humic acids was investigated by permeation chromatography on porous silica (µBondagel) and porous glass (Controlled Pore Glass, CPG) which enabled rapid analysis. Humic acid molecule was fully expanded at low ionic strength at pH 7.5, but it shrank considerably with increasing ionic strength. This behavior of humic acid molecule was analogous to that of linear polyelectrolyte molecule. Molecular size of humic acids decreased with increasing RF value and carboxyl group content and with decreasing 41og K value (namely with the increasing degree of humification). This phenomenon was attributed to oxidative depolymerization and intramolecular condensation during the process of humification. Based on the fractionation experiments on CPG, the fraction with the highest RF value and the lowest Δlog K value had an intermediate molecular size among the fractions. The excluded large molecular size fraction contained both highly humified components and a large amount of non-colored components. After the fraction with the highest RF value was eluted, RF value decreased and Δlog K value increased steeply with increasing elution volume.