Characterization of humic materials by flow field-flow fractionation

Several humic materials are characterized by flow field-flow fractionation, including humic acids, a fulvic acid, and aqueous leachates from compost. Hydrophilic and hydrophobic fractions of a compost leachate were also examined. After characterizing molecular weight distributions, the effect of pH and salt concentration on hydrodynamic size is studied. In general, the hydrodynamic size decreases as the pH is lowered. However, humic acids form large aggregates below pH 5. Small amounts of sodium chloride have little effect on the size distributions. In contrast, a little calcium chloride reduces the hydrodynamic size of individual molecules while inducing the formation of oligomers, although severe aggregation is absent. With further additions of calcium chloride, the decrease in hydrodynamic size continues but oligomer formation subsides. Precise characterization of the unaggregated material is hindered by sample penetration through the channel membrane.