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.

Chernozem Land Use Differentiation by Temperature-Dependent IR Spectra

“…Changes in spectra with temperature are summed up in Figures S1 and S2 (Supplementary Materials). As for humic substances separated from brown coal [28] and the same chernozem soil [27], frequency shifts of band maxima were reversible upon heating to 215 • C; when heated and then cooled, the frequencies of all the bands were restored to their values at 25 • C without a hysteresis (Figure S5a, Supplementary Materials). However, it should be noted that this behavior is reliably attained only for bands assigned to quartz and other inorganic-matrix constituents due to the high intensities of these bands.…”

“…The three-dimensional processed spectra are given in Figure S1, and corresponding spectral series are shown in Figures S2 and S3 (quartz), and Figure S4 for heating and cooling conditions, respectively (Supplementary Materials). For processing and interpretation, the soil fraction spectra in the whole IR range studied of 4000-100 cm −1 were divided: the range of hydrogen bonds (4000-3100 cm −1 ), CH stretch (3100-2800 cm −1 ), SOM (2000-1270 cm −1 , extended compared to humic substances [27] to overtone bands), and SiO 2 overtone (1270-650 cm −1 ) and fundamental vibration ranges (650-100 cm −1 ). These subranges corresponded to the dominating type of functional, matrix, or SOM structural groups; the divisions were described previously [28,31,32,51].…”

“…However, vibrational spectroscopy is based on the chemical bonds in substances and compounds and their interconnection; thus, an external perturbation that changes these bonds provides a larger volume of information with high selectivity. Among the approaches in IR spectroscopy that employ these features are correlation IR spectroscopy [18][19][20][21][22] and temperature-based IR measurements [23][24][25][26][27]. Temperature trends of IR spectra expose shifts in band parameters (intensities and positions) that are connected with the specific character of groups, species, and structures of the sample under study [23].…”

“…Previously, we applied the temperature dependences of IR spectra to differentiating humic substances of various origins and characterizing bands of crystalline and amorphous silica and SOM [27,28]. However, along with the difficulties associated with the superposition of IR spectral bands intrinsic to any multicomponent object with complex composition, serious problems in the case of soils are associated with the overlapping of bands in significant spectral ranges, especially SOM, with bands of water, which are present in soils in various amounts and in all possible forms and states.…”

Humic Substances: Importance for Agriculture, Affinity and Interactions with Soil Amendments and Pollutants

“…Volkov, and O.B. Rogova used specific temperature-induced changes in the band intensities, positions, and shapes of IR peaks in their article on the "Temperature Dependences of IR Spectral Bands of Humic Substances of Silicate-Based Soils" to obtain information on HS structure [16]. Attenuated total internal reflection FTIR in the mid-IR region within 298-488 K was selected to reveal the similarities and differences of HS isolated from silicate-based soils and HS originating from chernozem and soddy podzolic soils; structures formed in the dry state, skeletal vibrations were affirmed, and the behavior of polyaromatic compounds, carboxylic acids, and carboxylates was observed.…”