Effects of cations and minerals on FTIR spectra of humic acid

Content and composition of soil organic matter (SOM) is linked to soil ecosystem services like nutrient storage. Information on the molecular composition of SOM in terms of functional groups (e.g. carboxyl; C=O, and carboxylate groups; COO^-) can be obtained by spectroscopic approaches like Fourier Transform Infrared (FTIR) spectroscopy. Complex interactions between SOM, metal cations as well as soil minerals affect dynamics and reactivity of SOM, and cause -at the molecular level-changes in the strength of the bonds between C and O. The latter affects the wavenumbers (WN) and intensities of COO^- absorption bands in FTIR spectra. Such changes may become challenging when using approaches like partial least square analysis or machine learning to interpret FTIR spectra of soil samples with respect to SOM content or soil properties, because such approaches  are based on fixed ranges of WN (from textbooks), or sets of single WN determined by statistical approaches.

The aim of current study is to enhance the mechanistic understanding of the interactions between SOM, cations and  minerals by studying the cation/mineral effects on spectral data of organic matter in terms of absorption intensity and WN of band maxima characteristic for C=O and COO^- groups compared to the spectral data of organic matter itself.

Humic acids (HA; as a model substance for SOM) as well as mixtures of HA with Fe³⁺ were prepared at a 4:1 (HA:Fe³⁺) stoichiometric ratio in absence or presence of finely ground minerals (quartz, illite, and montmorillonite; to simulate soil solid surfaces). The mixtures were freeze-dried, and characterized by FTIR spectroscopy using KBr technique. After smoothing, baseline correction, and “subtraction procedure” to minimize the bands typical for soil minerals that might overlap with of humic acid, the FTIR spectra were interpreted for the intensity of C=O and COO^- bands, both related to polysaccharide (COC) band intensity as a reference. The spectra of HA-Fe³⁺ mixtures compared to those of HA showed an increase in intensity (although the SOM amount did NOT change) and a shift in WN of COO^- band maxima (WN 1620-1550 cm^-1). For the HA-Fe³⁺-mineral mixtures, the changes in both, intensity and shift, were significantly higher compared to HA-Fe³⁺ mixture with the strongest effect for HA- Fe³⁺ - montmorillonite mixture. Compared to the COO^- band, effects on C=O band (WN 1750-1719 cm^-1) were weaker.

Comparing FTIR spectra of HA-Fe³⁺, and HA-Fe³⁺-mineral mixtures with that of HA shows that SOM-cation as well as SOM-cation-mineral interactions may affect the spectral data of OM. This finding suggests FTIR analysis to offer a possibility for reflecting on OM-cation/mineral interactions. However, the changes observed for spectral properties of HA-Fe³⁺ -mineral mixtures compared to those of HA suggest that SOM-cation/mineral interactions also becomes of relevance when interpreting FTIR spectra of soil samples with respect to soil properties because the automated approaches used to do so are mostly based on predefined WN ranges and assumes band intensities to reflect on the amount.