Effects of ionic strength and electrolyte composition on the aggregation of fractionated humic substances studied by flow field-flow fractionation

Abstract

A sample of humic substances (HS) from Aldrich Co. has been studied by flow field-flow fractionation. The effect of the injected mass on retention, comparable to that previously found for synthetic water-soluble polyelectrolytes, suggests that the components present in this sample as Na salts are highly charged in water. Charges are, however, screened when a simple electrolyte is added to the carrier liquid. The electrolyte also appears to induce molecular aggregation to an extent that depends on the type of the metal ion and on its concentration. Investigation of the effect of the solution ionic strength and composition on fractions separately collected from the flow FFF channel confirms the aggregating effect of the salt added to the solubilizing medium and reveals a stronger tendency to particle aggregation by the HS in the presence of divalent cations. This finding seems common to all the fractions except that containing particles of lower size that appear to respond only to the ionic strength. A comparative analysis of the ionic strength effect on some fractions and on the unfractionated sample seems to support the hypothesis that, in the latter, the aggregation process occurs through a non-size-specific pathway. Therefore, in a broadly-dispersed humic mixture in solution of simple electrolytes, the size distribution is likely to be determined by random interactions of species of different size.