Spectral characterization of selected humic substances

Abstract

Current concern for soil quality has stimulated research on soil organic matter (OM). Humic substances (HS) of different origin were compared applying ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), “steady-state” fluorescence spectroscopy, and ¹³C nuclear magnetic resonance (¹³C NMR). Sodium humates samples were isolated from soil (Gleyic Luvisol), compost, and South-Moravian lignite from the mine Mír in Mikulčice. Sodium humates (SH) were extracted by a conventional procedure recommended by the International Humic Substances Society (IHSS). Results showed that the presence of O-containing functional groups (carbonyl in aldehydes and ketones, carboxyl in carboxylic acids, ester and ether groups) are in the order of compost > soil > lignohumate > lignite. Further, results of FTIR, fluorescence spectroscopy, and ¹³C NMR suggested that samples of sodium humates isolated from soil, compost, and lignite were a more polycondensed, oxidized, unsaturated, humified, and aromatic structure. On the other hand, commercial lignohumate (LH) had very simple structural components and wide molecular heterogeneity. Furthermore, a small molecular size and weight, low degree of aromatic polycondensation, low level of conjugated chromophores and fluorophores, and low humification degree were characteristic for commercial LH. It should be noted that the sample of commercial LH was characterized by ¹³C NMR analysis with a slightly higher value of aromaticity α in comparison with the sample of compost. The application of non-destructive analytical methods such as UV-VIS, FTIR, ¹³C NMR, and fluorescence spectroscopy help us to provide main characteristics of selected humic substances.