A Spectroscopic and Chemical Investigation on the De-ashing of a Humin
- C. M. Preston ,
- M. Schnitzer and
- J. A. Ripmeester
The application of 13C cross-polarization magic-angle spinning nuclear magnetic resonance (CPMAS NMR) to humins has been limited because low C and high Fe contents degrade spectral quality. This study investigated the effect of de-ashing with aqueous HCl/HF (1.16 M HCl and 2.88 M HF) on chemical properties and 13C CPMAS NMR spectra of a humin. With progressive de-ashing, the humin became partly soluble in 0.5 M NaOH, and its ash-free composition approached that of the humic acid fraction. The CPMAS NMR spectra showed increasing improvement in resolution and signal-to-noise ratio. The spectrum of the most extensively de-ashed sample (520 g C kg−1) was very similar to that of the humid acid extracted from the same soil. The de-ashing treatment caused variable changes in the distribution of N determined after acid hydrolysis, but little change in the molar distribution of amino acids. Some monosaccharides were lost, and there was a change in the monosaccharide composition, with glucose predominating (on a molar basis) in the most extensively de-ashed samples. It is unlikely that major structural changes were caused by the de-ashing, which provides a useful approach to the NMR investigation of humins.
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