Conductivity-Dependent Flow Field-Flow Fractionation of Fulvic and Humic Acid Aggregates

Fulvic (FAs) and humic acids (HAs) are chemically fascinating. In water, they have a strong propensity to aggregate, but this research reveals that tendency is regulated by ionic strength. In the environment, conductivity extremes occur naturally—freshwater to seawater—warranting consideration at low and high values. The flow field flow fractionation (flow FFF) of FAs and HAs is observed to be concentration dependent in low ionic strength solutions whereas the corresponding flow FFF fractograms in high ionic strength solutions are concentration independent. Dynamic light scattering (DLS) also reveals insight into the conductivity-dependent behavior of humic substances (HSs). Four particle size ranges for FAs and humic acid aggregates are examined: (1) <10 nm; (2) 10 nm–6 μm; (3) 6–100 μm; and (4) >100 μm. Representative components of the different size ranges are observed to dynamically coexist in solution. The character of the various aggregates observed—such as random-extended-coiled macromolecules, hydrogels, supramolecular, and micellar—as influenced by electrolytic conductivity, is discussed. The disaggregation/aggregation of HSs is proposed to be a dynamic equilibrium process for which the rate of aggregate formation is controlled by the electrolytic conductivity of the solution.