Spectral characterization of organic matter is affected by polyvalent cations interaction

Abstract

Functional groups (e.g. alcohol, hydroxyl, carboxyl etc.) of soil organic matter are responsible for soil properties like cation exchange capacity or wettability. Functional groups can be characterized by spectroscopic procedures like Fourier transform infrared (FTIR) spectroscopy since the infrared light can introduce vibration modes within the groups. Only the light of an energy similar to the bonding energy of the C=O bond, for example, causes the absorption bands of the functional groups to show up at typical wavenumber (WN) regions in FTIR spectra. However, when the C=O group interacts with cations, the bonding energy may change thereby affecting the WN region of the absorption band. This change in WN (following organic matter (OM)-cation interaction) may limit procedures of automated spectral interpretation, which are mostly based on fixed ranges -determined from textbooks, or sets of single wave numbers determined by statistical approaches.

Our study aims to quantify the effect of OM-cation interactions on the spectral features (intensity and WN region of C=O absorption bands) in FTIR spectra. To quantify the effect of OM-cation interactions on defined functional groups as far as possible, we study mixtures of polygalacturonic acid (PGA; as a model substance for soil organic matter) with different polyvalent cations (Ca²⁺, Fe^³+, Al^³+) at different concentrations.

PGA-cation mixtures with different cation concentrations were prepared, freeze dried, and characterized using FTIR spectroscopy (KBr technique). A proton-cation exchange at the carboxylic acid groups during PGA-Cation interaction gives rise to a COO- band in FTIR. Since the cation effect is found in an earlier study to be stronger for the COO- band (1620-1550 cm^-1) as compared to the C=O band, the interpretation of the FTIR spectra focuses on the COO- band. Compared to spectra of pure PGA, the spectra of all PGA-cation mixtures show a significant positive correlation between COO- band intensity and cation concentration. Additionally, a shift in the maximum of COO- band towards lower WN was observed for all cations, which depends on the kind of cations and increased with cation concentration.

Increase in intensity of the COO- band and the shift in WN region of COO- band maxima confirms changes in FTIR spectral features with cation addition and that those changes depend on the type of cation. The results suggest that type and concentration of cation should be considered when interpreting FTIR spectra of organic matter since both, change in intensity and shift in the WN of the band maxima, could restrict procedures of automated spectral interpretation, which mostly rely on fixed ranges (from textbooks), or sets of single wave numbers determined by statistical approaches. For a deeper understanding on the relation between OM-cation interactions, organic matter with increasing heterogeneity and complexity need to be studied.