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.
Highlights
Natural organic matter is an extremely complex pool of organic substances
The results of our work support the current view that humic acids still represent a complex mixture of separable fractions with specific structural and physicochemical properties
For the analyzed peat-bog humic acid, it was confirmed that the individual fractions, isolated by the proposed polarity-resolved fractionation technique, are mutually different concerning their origin, molecular weight, aromaticity, and the content and composition of heteroatomic functional groups
Summary
Natural organic matter (by which only the non-living part should be understood here) is an extremely complex pool of organic substances. Its non-aqueous part was and is studied after extraction by an alkaline agent from solid natural matrix-like soil, peat, or coal. There are attempts, increasing in number, to study the natural organic matter directly in its natural environment, i.e., without extraction [4], isolation of organics from natural matrix remains an important part of natural organic matter research, which is motivated mainly by efforts to reduce the inevitable structural complexity of the natural organic matter before subjecting it to the intended analyzes. Hayes [5] overviewed solvent systems used for the isolation of organic components from soils. He states that, in contrast to the traditional mixture of aqueous base and pyrophosphate, organic solvents are much less used, usually because of problems with recovering solutes. A useful concept for predicting solubility is the solubility parameter concept developed mainly by Hildebrand et al [6] and Hansen [7]
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