Spectroscopic characteristics of humic acids originated in soils and lignite

The aim of our work was to characterise the stability, humification degree, and principal classes of fluorophores in humic acids isolated from different matrices. Soil humic acids were isolated from arable soils and grassland that differ in the texture and moisture regimes (e.g. aquic; udic; and ustic moisture regimes). Basic soil characteristics, such as total organic carbon content, humus fractionation, cation exchange capacity, soil reaction, texture, and optical indexes were determined. The international standard method for humic acids (HA) isolation was used. Lignite represents a valuable organic substrate, with mineral inclusion situated on the transformation route from phytomass to a dehydrated, dehydrogenated, and deoxidised carbon type complex and water. One of the most attractive ways of non-energetic exploitation of lignite is humic substances source exploitation. It is known that humic acids isolated from lignite show typical bands known from other HA soil samples due to aromatic and various C-O structures. Spectroscopic characterisation has been a topic of great interest, chemical species being analysed with respect to the overall spectral characteristics of the system. Therefore UV-VIS, FTIR, and synchronous fluorescence spectroscopy (SFS) were applied in our study. The elemental composition and ash content in HA samples were determined. HA preparations were more hydrated in hydromorphic soils (Fluvi-Eutric Gleysol and Gleyic Stagnosol). The highest carbon content was found in lignite HA (57.5 weight %). Generally, carbon content was decreasing in the following order: Lignite HA > Haplic Chernozem HA > Fluvi-Eutric Gleysol HA > Haplic Luvisol HA > Gleyic Stagnosol HA > Eutric Cambisol HA. FTIR spectroscopy showed that the aromatic indexes varied from 0.61 to 0.73. HA were divided into two groups according to the aromatic and aliphatic compounds in their molecules. The highest aromatic degree and stability was found in lignite HA and Haplic Chernozem HA. Humic acids isolated from grassland and hydric soils contained more aliphatic and newly formed compounds. Synchronous fluorescence scan spectra identified aliphatic compounds in grassy and hydric soils at lower wave lengths. At higher wave lengths, identical fluorophores were detected. We registered five main peaks at: 467/487, 481/501, 492/512, 450/470, 339/359 (at Δλ = 20 nm). The peaks positions corresponded to the fluorescence behaviour of Elliot soil HA standard. Only lignite HA revealed another fluorescence peak at 492/512 nm. The peaks positions complied with the fluorescence behaviour of Leonardite standard HA. The relationships between the fluorescence indexes, colour indexes, aromatic indexes, humification degree, and elemental composition were evaluated by correlation analysis.