Abstract
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.
Highlights
Natural organic matter (NOM) is to the environmental sciences what proteins, nucleic acids, lipids, and carbohydrates are to the life sciences
In flow field-flow fractionation (FFF) conducted at high electrolytic conductivity, the time scale of the experiment is too short to allow for the formation of aggregates—the fractograms are concentration independent
The question that began as—Why is the flow field-flow fractionation of fulvic and humic acids concentration independent at high ionic strength and concentration dependent at low ionic strength?—led to investigation of the fundamental differences in the character of Humic substances (HSs) aggregates as ionic strength varies
Summary
Natural organic matter (NOM) is to the environmental sciences what proteins, nucleic acids, lipids, and carbohydrates are to the life sciences. All of these materials possess unique properties determined by their size, folding, and patterns at the nanoscale. Soil scientists have studied the chemical nature of HSs for two centuries, and their composition and conformation have been intensely researched. Because of their complex nature, the primary, secondary, tertiary, and quaternary molecular arrangements of the fulvic (FAs) and humic (HAs) comprising TOC/DOC still remains ill defined
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