Polarity-Based Sequential Extraction as a Simple Tool to Reveal the Structural Complexity of Humic Acids
2021
Enev, Vojtěch | Sedlacek, Petr | Kubíková, Leona | Sovová, Šárka | Doskočil, Leoš | Klučáková, Martina | Pekař, Miloslav
A sequential chemical extraction with a defined series of eluotropic organic solvents with an increasing polarity (trichloromethane < ethyl acetate < acetone < acetonitrile < n-propanol < methanol) was performed on peat-bog humic acid. Six organic fractions were obtained and subjected to a physicochemical characterization utilizing methods of structural and compositional analysis. Advanced spectroscopic techniques such as Attenuated Total Reflectance (ATR-FTIR), total luminescence, and liquid-state ¹³C NMR spectrometry were combined with elemental analysis of the organic fractions. In total, the procedure extracted about 57% (wt.) of the initial material; the individual fractions amounted from 1.1% to 19.7%. As expected, the apolar solvents preferentially released lipid-like components, while polar solvents provided organic fractions rich in oxygen-containing polar groups with structural parameters closer to the original humic material. The fraction extracted with acetonitrile shows distinct structural features with its lower aromaticity and high content of protein-like structural motifs. The last two—alcohol extracted—fractions show the higher content of carbohydrate residues and their specific (V-type) fluorescence suggests the presence of plant pigment residues. The extraction procedure is suggested for further studies as a simple but effective way to decrease the structural complexity of a humic material enabling its detail and more conclusive compositional characterization.
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