Low-temperature ashing of humic and fulvic acid
De Kimpe, C.R. | Schnitzer, M.
Low temperature (< 100 degrees C) ashing (LTA) has been proposed for removing organic matter from mineral soil constituents under relatively mild conditions. However, little is known about the chemical reactions governing organic-matter removal by this method. This study was conducted to evaluate the nature of residual materials at various stages of the LTA process. The LTA procedure was applied to a humic acid extracted from a lignite and a fulvic acid extracted from a Spodosol Bh horizon. Residues were recovered at various stages of weight loss and characterized by chemical, infrared (IR), and 13C unclear magnetic resonance (NMR) analyses. Chemical analyses showed, on a moisture- and ash-free basis, relatively small decreases in C for both the humic and fulvic acid with increasing exposure to LTA. The humic acid, however, lost more than half of its N after 15 min of LTA, whereas the N content of the fulvic acid more than doubled after 3 min of LTA and tripled after 30 min of exposure. These increases in N appeared to be due to the oxidation of NH3 and other N forms to NO3, which was adsorbed on mineral components of the fulvic acid. The IR and 13C NMR spectra of the residues at different stages of LTA were similar, which suggested that LTA decomposed the organic matter homogeneously by a surface reaction. Possible uses of LTA include the removal of organic matter from soils without damaging mineral components, the preparation of soil samples of uniform mineral composition but with different organic-matter contents, and the partial degradation of soil organic matter in structural studies on these materials.
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