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Document Type

Dissertation - University Access Only

Award Date

2000

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry and Biochemistry

First Advisor

James A. Rice

Abstract

Humic materials are one of nature's most ubiquitous, and heterogeneous, classes of organic compounds. They are defined as the by-products of the decomposition of organic matter in the environment. Humic materials comprise a broad range of molecular sizes and contain a variety of organic functional groups such as the amine, alcohol, phenol, carboxylate, and carbonyl groups. Because of their heterogeneity, and the variety of acidic functional groups present within these compounds, the determination of the charge of humic materials becomes a complex and difficult task. Because they play a role in essentially every aspect of environmental organic geochemistry, an understanding and quantification of the charge-state of humic materials is important in gaining a better understanding of their environmental chemistry. Capillary electrophoresis (CE) is a useful tool for the separation of molecules based on their differential migration in an applied electric field. Because it is known that humic materials have carboxylic acids as their primary ionizable functional group, humic substances are considered polyelectrolytes and CE becomes a uniquely applicable tool for the study of their charge-state. This dissertation investigates the relationship between electrophoretic mobility and charge density (CD). Charge density is defined as the number of ionized functional groups divided the molecular weight raised to the 2/3 power. The development of an equation that allows for the accurate prediction of charge densities was developed and applied to the understanding of the charge-state of humic materials. Using well characterized model fulvic acid molecules, a linear relationship (R2 = 0.971) between electrophoretic mobility and charge density was established. Using electrophoretic mobility data to determine the charge density of humic materials and the previously established molecular weight of standard and reference fulvic acids the ionized functional group content was calculated under a variety of environmental conditions. The impact of various environmental conditions such as pH, ionic strength, multivalent metals and concentration induced aggregation were evaluated for their influence on the charge density of fulvic acid. It was determined that as pH increased the expected increase in molecular charge and thus charge density did not occur in every case. Using two different fulvic acids from three different environments (i.e. aquatic, soil, and peat) the trend in CD was found to increase for one sample and decrease for the other within each of the three environments. Ionic strength effects on CD was investigated using a neutral salt (KCl) and borate. With the addition of the neutral salt, the CD was found to increase as the ionic strength increases. Using 10 mM carbonate as the background electrolyte the CD decreased with the addition of borate to the 5.0 mM ionic strength level then increased. The divalent cations Ca2+, Cu2+, and Hg2+, and trivalent iron, were used to investigate the influence various metals have on the CD of fulvic acid. It was determined that the fulvic samples from the three different environments interacted differently with the various metals. The divalent mercury cation was determined to have the largest influence on the CD distribution. Concentration-induced aggregation had on the CD of fulvic acid was found to have no effect on CD. Significant increases in fulvic acid concentration caused an increase in the distribution of the range of CD's for the fulvic acid samples. Based on the findings of this research the quantity of ionized functional groups under environmental conditions can be better evaluated than traditional titrimetric means for the determination of carboxylate groups. The general understanding of how pH, ionic strength, and divalent metals influence the charge-state of fulvic acid can be evaluated in a unique way using CE as an analytical tool.

Library of Congress Subject Headings

Fulvic acids -- Structure
Capillary electrophoresis
Environmental chemistry

Format

application/pdf

Publisher

South Dakota State University

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