Temperature Dependences of IR Spectral Bands of Humic Substances of Silicate-Based Soils

Abstract

Temperature-dependent IR spectroscopy capable of revealing specific changes in the band intensities, positions, and shape was used to increase the information on humic substances (HS) from soils. Temperature dependences of IR spectra of HS isolated from silicate-based soils differing in the contents and nature of organic matter, chernozem and soddy podzolic soil, were investigated by attenuated total internal reflection FTIR in the mid-IR region (4000–400 cm−1) in the air within the moderate temperature range of 25–215 °C (298–488 K) with a step of 2.5 °C. The dependences of shifts in frequency (wavenumber) corresponding to band maxima and integrated band intensities were built for all major bands. Observed reversible frequency shifts upon heating and cooling can be interpreted as forming structures in the dry state. The behavior of integrated intensities of bands assigned to C–H and C–C vibrations, characteristic vibrations of polyaromatic compounds, carboxylic acids, and carboxylates were shown, and similar behavior for the same group (symmetric and antisymmetric stretches) were revealed. Differences in the temperature trends in chernozem and soddy podzolic soils due to different structures (aliphatic and aromatic) and functional groups (carboxylic and carboxylate) are shown. The different behavior of the bands corresponding to carboxylic groups and skeletal vibrations differentiates soil types with different organic matter. The temperature trends of band maximum and intensity shifts are less prone to measurement conditions and may serve as qualitative parameters characterizing the composition of soil humic substances.