Polarity-Based Sequential Extraction as a Simple Tool to Reveal the Structural Complexity of Humic Acids

Abstract

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.

Characterization of Ultrafiltration-Fractionated Humic Acid Derived from Chinese Weathered Coal by Spectroscopic Techniques

“…The UV-Vis absorbance of HA fractions decreased as the wavelength increased (Figure S1). This is in accordance with previous literature that the spectra were typically broad and featureless [1,21,33]. The E 4 /E 6 ratio is commonly used to reflect the molecular weight and condensation degree of the humic macromolecule [25,26].…”

“…This phenomenon could be attributed to the discrepancy about the inherence theory of fractionation methods, and the acidification before pH-fractionation to some extent. Atomic ratios can be used to deduce the structural characteristics of humic acid [11,21]. The H/C atomic ratio is related to the degree of aromatic condensation.…”

“…During the HA characterization, researchers usually separate it into different fractions to reduce heterogeneity [6,13,[17][18][19][20][21], and this gradually becomes the essential starting point for detailed studies. In a prior study, we fractionated HA derived from weathered coal by adjusting the pH of extraction and found that there were some slight differences among HA fractions, especially in the acidic functional groups and the degree of protonation in aromatic structure [15].…”

Use of the pK Spectroscopy Method in the Study of Protolytic Properties of Humic Substances and Other Soil Polyelectrolytes

“…The maximum values of pK = 9.4-10.5 have hydroxyl groups of phenolic fragments of FAs. The presence of carboxyl and phenolic groups in these positions in the FA's carbon skeleton has been confirmed by many researchers [32,34].…”

Characteristics of Chinese Weathered Coal from Six Geographical Locations and Effects on Urease Activity Inhibition

“…The FTIR parameters in Table S2 were calculated using the data derived from the integrated areas for each peak derived from the semi-quantitative FTIR analysis to evaluate the chemical characteristics of the weathered coal samples [21,23,26,[28][29][30][31][32][33]. In addition, the apparent aromaticity (f a ) of the coal samples was used to characterize the aromatic carbon fraction.…”