Alicyclic and Aromatic Carboxylic Acids in Soil Organic Matter: An Investigation of Potential Origin and Association with Plutonium Using Advanced Analytical Techniques

Abstract

Carboxylic acids are a defining component of soil organic matter, responsible for many of the physical and chemical properties, including metal-organic matter interactions, which govern its role as an important constituent of soils. However, there is a shortage of detailed molecular level information regarding orientation and structural arrangement of carboxylic acids within soil organic matter. This dissertation utilizes electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS) as well as solid-state and multi-dimensional nuclear magnetic resonance (NMR) to investigate the molecular formula composition within several organic matter sources and the primary structures that feature carboxylic acids. Soil organic matter is evaluated in two forms: as the alkali-soluble, acid-insoluble portion of organic matter (humic acids) from a collection of sources, as well as the alkali soluble soil organic material associated with high Fe and Pu metal concentrations at a contaminated munitions facility. Two predominant carboxyl-containing molecular assemblages are found to be common in a wide variety of soil humic acids. Along with lignin-like assemblages, these are carboxyl-containing aliphatic molecules (CCAM) and condensed aromatic molecules. The proportion of these groups relative to lignin-like compounds within samples and the percent of total carboxylic acid molecular formulas among samples are found to increase with increasing humification of the soil. Since CCAM and condensed aromatic molecules have previously been shown to be generated from oxidization of lignin, this represents renewed evidence for lignin as a major source of organic matter in soils. Lignin ring-opening and radical re-polymerization reactions have been proposed to form alicyclic CCAM and condensed aromatic molecules. Detailed evaluation of the aliphatic molecules using multi-dimensional NMR confirms the presence of ring structures, replete with carboxylic acids, heteroatom substitutions in the form of alcohols and ethers, as well as a variety of methyl group substituents. Additionally, condensed aromatic carboxylic acid molecular formulas, primarily those containing nitrogen, were found composed in organic matter with elevated metal ions Pu and Fe. Carboxylic acid oxygens in combination with nitrogen in aromatic structures are suspected to be partially responsible for the high metal affinity. Nitrogen-containing hydroxamate groups were also investigated for their potential to be incorporated into stable organic matter by testing the reaction between an amine-containing hydroxamate siderophore and the biopolymer cutin. While products of this reaction could not be confirmed, carboxylic acid functional groups are identified in this thesis as key molecular components contributing to Pu and Fe metal-binding attributes of organic matter, and potentially formed during the production of condensed aromatic and alicyclic compounds as a result of radical oxidation reactions of lignin.