Chemical and spectroscopic characterization of humic acids isolated from different Slovak soil types

Abstract Humic acids isolated from six different Slovak soils have been characterized comparatively by using chemical methods and spectroscopic techniques including ultraviolet-visible (UV-VIS), 13C nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR), fluorescence, and electron spin resonance (ESR). Results from elemental and functional group analyses and spectroscopic data are generally well correlated. Humic acids from the Calcaro-Calcic Chernozem and Gleyic Phaeozem are characterized by a high degree of aromaticity, high content of carboxylic groups and polycondensed aromatic and conjugated structures, low fluorescence intensity with main peaks in the long-wavelength spectral region, and very high organic free-radical concentrations. Differently, HAs from the Dystric Planosol, Eutric Cambisol, Orthic Luvisol, and Stagno-Gleyic Cambisol exhibit low aromaticity and high aliphatic character, low degree of polycondensation, low content of carboxylic groups and organic free radicals, fluorescence in the intermediate- and low-wavelength regions, and the distinct presence of amide groups and polysaccharidic structures. These differences in chemical structure and functionalities suggest a higher degree of humification and higher chemical reactivity for the first two humic acids, with respect to the other four samples. The agronomic functions and environmental implications of the two groups of humic acids examined are, therefore, expected to be markedly different.