Carbon-13 Nuclear Magnetic Resonance Spectra of Soil Humic Substances Extracted by Different Mechanisms
Piccolo, A. | Campanella, L. | Petronio, B. M.
Humic substances were extracted from an organic soil of Tuscany, Italy, with solutions operating by three different mechanisms: charge repulsion (NaOH under N₂), ligand exchange (Na₄P₂O₇ at pH = 7), and H-bond disruption (dipolar aprotic solvents in mixture with dilute HCl). Solution-state ¹³C-nuclear magnetic resonance (NMR) spectra were obtained for the unpurified and purified (HCl-HF treatment) extracts, revealing that humic matter obtained through the H-bond-disruption mechanism was different from materials extracted using the two other mechanisms. Extracts obtained using dipolar aprotic solvents (DMF, DMSO, acetone) showed better resolved spectra than those using NaOH and Na₄P₂O₇. This was due to a lower content of coextracted silicate materials and lower molecular weights. Spectra of humic substances extracted with acetone and DMF had a higher percentage of aliphatic C than did the spectrum of the extract with DMSO. The latter was, in general, more similar to the spectra of stronger extractants. The H-bond-disruption mechanism of dipolar aprotic solvents appeared to have extracted more unsubstituted aromatic rings than NaOH and Na₄P₂O₇, although percent of aromatic C was generally equivalent in the spectra of all extractants. Purification with a HCl-HF treatment decreased mainly the content of aliphatic, carbohydrate, and amino acid components of all extracts. Poor agreement between values for total acidity of extracts calculated by NMR and those given by chemical methods was also observed in this study. Contribution of the Istituto Sperimentale per lo Studio e la Difesa del Suolo.
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