Title

Characterization of Humic Substances Under Varying Environmental Conditions and Investigations of Their Interactive Properties with Colloidal Hematite Using Flow Field-Flow Fractionation

Publication Date

4-1996

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geophysics

Department

Geosciences

Major Advisor

M. E. Schimpf

Abstract

Humic materials are known to accumulate in clay soil horizons. These humic substances are the result of the migration of biochemically degraded plant and animal matter from land surface. Soil horizons containing humic substances affect the transport of pollutants into groundwater. Humics are important in agriculture and are known to react with various pollutants in the environment. In this thesis, humic substances and their interactions with hematite soil particles are characterized using flow field-flow fractionation (FlFFF).

FlFFF, a relatively new separations technique, can be used to characterize a variety of environmental samples. FlFFF is a subtechnique of field-flow fractionation (FFF). In FFF, an applied field is oriented perpendicular to the longitudinal flow of analytes through a thin fluid channel where a parabolic velocity profile gives rise to separations. In FlFFF, a uniform flow field is applied over the channel area.

Detailed theoretical derivations of the basic equations of normal mode FFF leading to a comprehensive FlFFF theory are provided. This theoretical chapter connects the basic experimental design of the FlFFF system to the experimental methods and results.

Humic and fulvic acids, aqueous compost-leachates, the hydrophobic and hydrophilic fractions of a compost-leachate (obtained from adsorption chromatography), and hematite particles (a common soil constituent) are characterized in terms of molecular weight distributions (MWDs) and hydrodynamic diameter distributions (HDDs) using FlFFF. Humic acid/hematite and fulvic acid/hematite mixtures are analyzed using FlFFF in terms of their kinetics and adsorption phenomena. MWDs of water-soluble organic soil constituents were determined by their calibration with poly(styrene-sulfonate) standards. HDDs for these soil constituents are determined directly from FlFFF data. Hematite particle diameters were determined from calibration standards and directly from FlFFF data.

The data show that a compost-leachate, collected with a lysimeter, is similar to that obtained by leaching soil samples with deionized water in the laboratory. The MWDs of the water leachate samples contain higher molecular weight components than either the hydrophobic or the hydrophilic fraction. And the hydrophobic fraction exhibits higher molecular weight components than the hydrophilic fraction. The similar MWDs obtained from ultraviolet (UV) absorption and refractive index detection for each sample, coupled with the evidence that the hydrophobic fraction contains higher molecular weight components than the hydrophilic fraction, indicates that the high molecular weight components of the water leachate may be hydrophobic but are not necessarily aromatic. The hydrophobic and hydrophilic fractions of the source compost-leachate have lower peak molecular weights, Mps, and narrower MWDs than their source material. This is possibly due to the fact that the humic acids were removed from the water leachate before they were separated into the hydrophobic and hydrophilic fractions, or it may be that the hydrophobic and hydrophilic fractions interact to form at least part of the higher molecular weight components in the source water leachates.

Upon characterizing the MWDs and HDDs for humic materials, the effect of pH and salt concentration on hydrodynamic size are investigated for the compost- leachate samples and a standard fulvic acid sample. When the pH of the carrier liquid is lowered, hydrodynamic diameters decrease for the compost-leachate samples and the fulvic acid. This is probably due to the collapse of their polymeric branches. Humic acids are typically larger macromolecules and form aggregates below pH 5, but the hydrodynamic diameters of the humic leachate samples decrease as the pH is lowered. Additions of sodium chloride do not significantly affect size distributions but the addition of calcium chloride at low ionic strengths indicates a molecular reconformation to smaller diameters as well as the formation of aggregates (probably oligomers), although profound coagulation is not observed. As the ionic strength of calcium chloride is further increased, aggregation subsides, HDDs are shifted to lower size distributions, and there is significant sample loss (probably through the membrane). This supports the idea that collapsed molecular branches inhibit intermolecular interactions. Although the HDDs were successfully characterized, the analysis of collapsed and unaggregated humic molecules is limited by their penetration and subsequent loss through the channel membrane.

Studies of the interaction of two humic substances--a humic acid and a fulvic acid--with colloidal hematite particles in DI water show that adsorption of the humic substances onto the hematite particles occurs readily. These studies demonstrate the strong affinity for humic molecules to adsorb onto the hematite particles, and that humic/hematite mixtures are in a dynamic state of interactive equilibria. Adsorption phenomena may continue indefinitely, but the adsorption of the humic substances onto the hematite particles reaches an apparent state of equilibrium within hours. The interactions between the humic molecules and the hematite particles can be interpreted in terms of physical adsorption.

Shifted elution profiles of the adsorbed humics show physical desorption where the desorbed molecules are either irreversibly lost by the hematite and elute early from the channel or are sequentially desorbed and adsorbed (also leading to their early elutions). The degree of shifting of adsorbed elution profiles is a function of concentration and molecular species. The fulvic acid shifts are greater than those of the humic acid which is in agreement with its higher diffusivity. And secondary chemical equilibrium theory predicts the shifted profiles from sequential desorption and adsorption as a function of concentration.

Quantitative adsorption isotherms are established for the humic and fulvic acid samples by computing the mass of humic substance lost to adsorption onto the hematite. These isotherms suggest Langmuir-type behavior where initial monolayer adsorption proceeds quickly as a function of equilibrium concentration and then begins to slow as the hematite adsorbent surfaces are increasingly saturated. They also show that the hematite surfaces adsorb more humic acid mass than fulvic acid mass at all of the tested equilibrium concentrations.

Files over 30MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS