Document Type

Thesis - University Access Only

Award Date

1997

Degree Name

Doctor of Philosophy (PhD)

Department / School

Chemistry and Biochemistry

Abstract

Clay mineral surfaces interact with natural organic matter in soils to form the humin fraction of humic materials. The surface organization of the organic components of humin was investigated using clay-organic model compounds, natural humic acid sorbed onto clay minerals, and humin samples extracted from four different soils. Three reference clay minerals; a kaolinite (KGa-2), a hectorite, (SHCa-1), and a Ca-montmorillonite (STx-1); chosen to be used as adsorbent in this study, were characterized in terms of specific surface area and porosity, surface charge, type and distribution of surface hydroxyls, and surface morphology using the concept of fractal geometry. Their surfaces, as studied by 2 9Si CPMAS NMR, were found to be dominated by single silanol groups, regardless of the state in which they are found. STx-1 was found to have more acidic hydroxyls than either hectorite or kaolinite and is therefore expected to be more reactive. The surfaces of these clays were found to be fractal with fractal dimensions close to 2.0, consistent with the common description of clay mineral surfaces as smooth and planar. Humin was isolated from four soil samples. Surface area, surface charge, porosity measurements, and fractal analysis of X-ray scattering data were used to characterize changes in the surface properties resulting from the selective removal of the various components of organic matter from humin. Organic matter was selectively removed from humin by Soxhlet extraction, disaggregation with the MIBK method and bromine oxidation. The surface fractal dimensions of the materials decreased while surface area increased and surface pore size decreased upon removal of organic matter. These results suggest that the mineral components of humin have smooth surfaces over length scales of ~ 1 to 15 nm, and that it is the organic matter coatings that are responsible for their surface roughness. It was hypothesized in the work that the structural organization of organic matter on a mineral surface is best conceptualized by the formation of an admicelle rather than the amorphous coiling of "polymer" chains. Results obtained by measuring the spin-spin relaxation time of adsorbed organic matter using solid-state 1 H NMR demonstrated that humin's organic architecture conforms to the admicelle arrangement. Based on the findings of this research, this dissertation has shown that the organic architecture of humin consists of amphiphilic molecules (partly hydrophilic and partly hydrophobic sharacters) that organize themselves on the surface of clay minerals in an admicellar manner. This model is consistent with many of the observed properties of humin, and humic materials in general. For instance, the admicelles create environment for lipids and other hydrophobic organic compounds. They acknowledge the diversity of the individual molecules that comprise humin.

Library of Congress Subject Headings

Humin
Soils -- Organic compound content

Format

application/pdf

Number of Pages

219

Publisher

South Dakota State University

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