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 (HA) isolated from different matrices. Soil HA were isolated from arable soils and grassland differing 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 HA isolation was used. UV-VIS, FTIR, and synchronous fluorescence spectroscopy 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 C content was found in lignite HA (57.5 weight %). 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. The relationships between the fluorescence indexes, colour indexes, aromatic indexes, humification degree, and elemental composition were evaluated by correlation analysis.