Liquid chromatographic analysis of geological organic substances of industrial importance

Publication Type:
Thesis
Issue Date:
2005
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Soluble organic species called humic substances are important in the Bayer process due to their adverse effect on the industrial scale production of alumina from bauxite. During the Bayer process the bauxite is subjected to a high temperature caustic digestion using sodium hydroxide. Most of the organic matter associated with the bauxite (up to 0.3%) ends up in the liquor. The soluble organic species can accumulate in the process liquor as the caustic solution is recycled for the digestion of fresh bauxite after the precipitation of the aluminium hydroxide trihydrate. In this work the humic substances were extracted from the Bayer process liquor obtained from a refinery plant operation at Kwinana Alcoa, Western Australia. The whole fraction as well as sub fractions obtained from a continuous solvent extraction were characterised by elemental and ash analysis, infrared spectroscopy, nuclear magnetic resonance spectroscopy and gas chromatography/mass spectrometry. High-performance liquid chromatography was used to further investigate the composition and structure of Bayer humic substances. In this study a one-dimensional HPLC separation was developed for Bayer humic substances that achieved a level of separation previously unreported in the literature. The one- dimensional HPLC method separated the Bayer humic substances into compound classes. The analysis of solvent fractions allowed further assignment of the separation. Small molecules and three discrete clusters of macromolecules were observed that are believed to represent micellar like aggregates of different amounts of polar groups as supported by the results of the NMR, FTIR and GC/MS analyses. Within these clusters there was some degree of further resolution. Certain stable configurations of molecular weights that are controlled by polarity through intramolecular binding were observed which provided strong evidence for a supramolecular structure to humic material rather than the existence of random conformational material. To further enhance the one-dimensional separation, model compounds were studied to find the most appropriate reversed phase column for the separation of the type of compounds found in humic substances. Five new generation columns were studied with the Phenomenex Synergi polar-RP column found to offer the best performance in terms of separation. This column was later used in the development of the two-dimensional HPLC separation. Finally, a two-dimensional reversed phase HPLC separation was successfully developed for the separation of Bayer humic substances using novel methodology developed in our laboratories, which successfully resolved uniform band profiles that showed promise of being essentially pure individual components. With the aid of mass spectrometric analysis of three second dimensional bands, the results of the separation strongly supported a host guest model for these compounds. It was concluded that small molecules are held in some way in some supramolecular structure by larger molecules (host guest complexes). The results suggested that the lower molecular weight material is capable of holding small guests more than larger molecular weight material making the supposition that the micellar host guest model is more probable than a model where hosts hide within the guests.
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