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

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.

Geochemical and biological controls on the ecological relevance of total, dissolved, and colloidal forms of trace elements in large boreal rivers: review and case studies

“…The colloidal phase is comprised of an incredibly diverse range of organic and inorganic material, such as macromolecules, humic substances, oxyhydroxides, and small phyllosilicate clays (Buffle and Leppard 1995b;Buffle et al 1998;Hertkorn et al 2007;Aiken et al 2011). As dynamic and reversible aggregations of molecules and particles held together through noncovalent bonds, the modern conception of colloidal structure is supramolecular (Piccolo 2001 Esfahani et al 2015;Wells 2015). Hence, the size and structure of colloids are heavily influenced by changes in water chemistry such as pH, Eh, ionic strength, and dissolved organic matter (DOM) concentration and type, causing aggregation or dispersal of the constituents and associated TEs.…”

A Dynamic Light Scattering Approach for Detection of Nanomaterials in Tennessee River

“…We have combined this DLS approach with scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy (EDX) for understanding the presence, release, and transport of nanomaterials within this water body. There have been a few studies on understanding the aggregation and disaggregation dynamics of natural organic matter such as humic acid and fulvic acid in simulated environmental water samples from Suwanee River, another major river of Southern United States (Esfahani et al., 2015; Wells, 2015; Wells & Stretz, 2019). However, there is a gap in analytical information from an environmental nanoscience perspective for the Tennessee River, particularly for engineered nanomaterials released from industrial effluents, tourist activities, or wastewater.…”

Supramolecular Answers to the Organic Matter Controversy

“…This manuscript provides enhanced data analysis and interpretation of previously reported empirical data. Detailed experimental procedures for flow FFF (Wells, 2015), DLS measurements (Esfahani et al, 2015a), EEM fluorescence spectroscopy, and parallel factor (PARAFAC) data processing (Wells et al, 2017) are published elsewhere. These data are summarized in the supplemental material associated with this manuscript.…”