STUDIES ON THE STRUCTURE AND REACTIONS OF HUMIC SUBSTANCES

Abstract

Humic materials were chlorinated with sodium hypochlorite in basic solutions. The varied production of chloroform obtained from these compounds led to an investigation on the structure of humic substances. High pressure hydrogenation of a humic acid utilizing molybdenum-sulfide as a catalyst resulted in only a 26% yield of non-gaseous products. Spectral analysis of the residue indicated it to be composed of a mixture of highly aliphatically substituted aromatic oils. Separation of the components was unsuccessful via conventional techniques. The apparent destruction of the molecule led to an investigation of intact humic substances utilizing nuclear magnetic resonance (NMR). Proton NMR analysis of underivatized humic acids was complicated by the presence of paramagnetic iron. Removal of the iron resulted in poorly defined spectra. ¹³C NMR analysis resulted in absorptions from all the known functional groups of humic acids. The availability of ash free humic and fulvic acids led to the use of basic solutions as solvents in obtaining NMR spectra. The free radical nature of these solutions resulted in a study of ortho-quinone precursors by ¹³C NMR. Broadening of lines, loss of intensity, and shifting or absence of absorptions was noted in the basic ¹³C NMR spectra of catechol, catechin, quercetin, rutin, tannic acid, and fulvic acid when compared to the corresponding neutral spectra. Electron spin resonance studies indicated the existence of a quinone-radical equilibrium in the model compounds. The drastic differences observed upon introduction of the radical perturbs interpretation of ¹³C-NMR spectra on basic solutions of humic substances. Solid-state ¹³C-NMR analysis on humic substances appeared as an auspicious alternative. Magic-angle spinning alone proved useless with all but the most molecularly mobile solids such as polyisoprene. Since hydroxyl functionally plays an important role in the chemical properties of humic substances, derivatization with subsequent ¹³C-NMR analysis of these functional groups was undertaken. A dimethylation procedure utilizing first diazomethane followed by methyl iodide and sodium hydride was found to give complete methylation. This derivatization with ¹³C-enriched methyl precursors labels hydroxyl groups, eliminates hydrogen bonding and enhances signal assignment in the ¹³C-NMR spectra. Estimation of functional group types can also be inferred from integral absorptions. Since this method is direct and nondegradative, ¹³C-NMR relaxation studies can be used to give information on molecular size. The long T₁ values determined on methylated fulvic and humic acids are not characteristic of high molecular weight compounds, but are more appropriate for an aggregate of small units. This procedure completely characterizes hydroxyl functionality in these amorphous molecules and can therefore be used to characterize a whole range of oxygen containing macromolecules such as coal, shale oil, and lignins.