Abstracts Category : Other

Relationship of the Fractal and Chemical Characteristics of Self-Assembled Natural Organic Matter

by Dennis K Gibson

Natural Organic Matter (NOM) is a heterogeneous mixture formed by the degradation of organic substances during early diagenesis in surficial environments. It has been shown that the interaction of the components that comprise this mixture has a significant impact on its microbial mineralization to CO2. The extent of NOM selfassembly is emerging as an important factor in understanding its role in the global geochemical carbon cycle, and it is beginning to appear that it may be more important than the chemical composition of a sample. The overall goal of this research is to establish NOMs self-assembled architecture and the factors that control it. NOM was extracted from The International Humic Substance Societys Leonardite, Pahokee peat, and Elliot soil bulk reference materials using an alkaline extraction procedure. The samples were size-fractionated using density-gradient ultracentrifugation at 20,000 rpm (46,377 g) for 90 hours utilizing a sucrose step-gradient. The gradient steps ranged in density from 1.06 to 1.27 g/cc. The resulting NOM density fractions were purified via the MIBK liquid-liquid partitioning technique. The carbon-type distributions of each whole NOM sample and density fraction was characterized by quantitative C DPMAS solidstate NMR and small-angle x-ray scattering (SAXS) at Argonne National Laboratorys Advanced Photon Source. Using the slope of the Porod region of the scattering curves, the materials were classified as either mass or surface fractals. It was found that Leonardite fractions exhibit a surface fractal mass distribution regardless of particle density. Peat and Elliott fractions display mass and surface fractal behavior depending on particle density. For these materials the least dense fractions yielded small molecules and mass fractals, while the more dense fractions yield surface fractals. To determine the chemical organization of the components in these particles, each fraction was swollen in deuterium oxide and analyzed via Comprehensive Multi-Phase NMR (CMP-NMR). Leonardite particles did not swell in the solvent. The Peat and Elliott fractions with mass fractal mass distributions (low density materials) swelled in the solvent while surface fractal (high density material) did not. This study shows that as the material proceeds through the diagentic pathway, assembled particles begin to form. The architecture of these particles can be described as having a structural organization similar to the Yen- Mullins model, which is defined as an aggregation of large aromatic molecules forming a clustered network. However, our data suggests that in NOM a shell of aliphatic hydrocarbons then surrounds the aromatic core. The younger materials (Elliott and Peat) contain a matrix of anomeric and aliphatic components, suggesting that all of the components of NOM do not contribute to the formation of assembled particles equally. Advisors/Committee Members: James A. Rice.