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Scott D. Kohl

Document Type

Dissertation - University Access Only

Award Date

1999

Degree Name

Doctor of Philosophy (PhD)

Department / School

Chemistry

Abstract

Natural organic matter (NOM) in general, and humic materials in particular, are of paramount importance in the fate and transtport of anthropogenic organic compounds (AOCs) in the environment. The organic material found in soil largely controls the movement of organic compounds through the soil and ultimately into the water table. This control is established through various reversible and nonreversible sorption reactions which slow down or even completely stop the movement of AOCs through the soil profile. Due to these and other important attributes of NOM, it has been the object of specific scientific study for at least 200 years. In spite of the efforts and advances made to the understanding of its chemical nature, knowledge of the specific properties and chemical structure of humic substances is surprisingly limited. This lack of knowledge is primarily due to the extreme complexity and diversity of the components which comprise humic materials making traditional investigation techniques difficult to apply. This dissertation is an attempt to look at the role lipids play in the sorption and binding processes of nonpolar organic compounds to NOM. Initially, polyaromatic hydrocarbon (PAH) and polychlorinated biphenyl (PCB) bound residues were formed and quantified using mineral soils. Between 18 and 88% of the applied PAH material and 8 and 30% of the applied PCB material was irreversibly bound to the soils after 14 day of incubation. The soil organic matter containing these bound residues was then fractionated into fulvic acid, humic acid, and humin. For the PAH studies, these bound residues were found associated with each fraction in the following proportions; fulvic acid had between 0 and 28%, humic acid between 1 and 35%, and humin between 50 and 95% of the bound PAH. For the PCB studies, these bound residues were found associated with each fraction in the following proportions; fulvic acid had between 4 and 28%, humic acid between 6 and 29%, and humin between 59 and 87% of the bound PCB. The humin containing bound residues was further fractionated into bound-humic acid, bound-lipids and the insoluble residue. For the PAH studies, these bound residues were found associated with each fraction on the following proportions: bound-humic acid had between 6 and 59%, bound-lipids between 33 and 90%, and the insoluble residue between 2 and 23% of the bound PAH. For the PCB studies, these bound residues were found associated with each fraction in the following proportions: bound-humic acid had between 5 and 55%, bound-lipids between 39 and 89%, and the insoluble residue between 0 and 9% of the bound PCB. A relative affinity scale was developed to further understand the chemical affinities of the bound residues for the individual fractions. This mass balance indicated that the bound-lipid fraction of humin organic matter has a very high affinity for the bound residues and has an important role in deterring the ultimate location of nonpolar contaminants placed in soils. The role of a second pool of natural lipids present in soil, the extractable lipids, was then investigated. This investigation proceeded through batch sorption isotherms with whole and lipid-extracted soils. The sorption of PAHs to soil from an aqueous solution was found to be nonlinear, suggesting site specific interactions between the PAH and organic matter in the soil. In addition, the removal of the extractable lipids caused an increase in the amount of material sorbed as well as resulted in increasing the nonlinear character of the isotherms. This suggested that natural lipids compete sorption sites present in the soils. These results are surprising because many previous investigations determined that nonpolar organic compounds do not sorb to soil organic matter at specific sites, but through general partitioning interactions with the bulk organic matter present. A final set of experiments was performed in which the sorption of hexafluorobenzene to peat organic matter was observed via solid-state NMR. These investigations reinforced the findings of the nonlinear sorption studies in that extractable- lipids do appear to compete for sorption sites. In addition, these investigations provided definitive evidence that dual-mode sorption does occur with nonpolar organic compounds and NOM. Kinetics experiments from this study demonstrated that the “fast” stage of the sorption process occurs almost completely within the first 24 hours of contact. Based on the findings of this research, this dissertation has shown that two natural lipid pools, the bound-and extractable-lipids are extremely important in the sorption and binding of hydrophobic contaminants to natural organic matter. Extractable lipids compete for natural sorption sites present in NOM. The Sorption process is complicated with at least two different pools of sites available for the uptake of nonpolar AOCs.

Library of Congress Subject Headings

Soils -- Organic content

Soil absorption and adsorption

Lipids

Format

application/pdf

Number of Pages

156

Publisher

South Dakota State University

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