Effects of Polyvalent Cations on FTIR Spectra of Organic Matter–Cation Associations

ABSTRACT Background The composition of soil organic matter (SOM) can be characterized by functional groups in the organic matter (OM). The carboxylic acid and carboxylate groups of OM show specific absorption bands in Fourier‐transform infrared (FTIR) spectra. However, cation–OM interactions may affect absorption bands and limit spectral interpretation based on pre‐defined wavenumbers (WN) ranges. Aims The aims are to quantify the effects of major soil cations on OM spectral properties in terms of intensity and WN of characteristic absorption band maxima in FTIR, and to find relations for systems of single cations with PGA which can be used for an improved spectral interpretation. Methods Mixtures of polygalacturonic acid (PGA) as a model substance for SOM with five cations (K + , Mg 2+ , Ca 2+ , Al 3+ , and Fe 3+ ) were prepared at different stoichiometric ratios and freeze‐dried. These mixtures were characterized by FTIR spectroscopy using the KBr technique. After smoothing and baseline correction, the spectral data were interpreted for the carboxylate (COO − ) band in FTIR resulting from asymmetric vibrations ( ν as COO − ) and normalized to polysaccharide (COC) band to obtain ν as COO − /COC ratio. Results The cation–PGA mixtures showed a linear increase in ν as COO − /COC ratio with cation concentration and a shift of the ν as COO − band maxima towards lower WNs. The effects on ν as COO − /COC ratios and shift were smallest for K + –PGA mixtures and largest for Al 3+ –PGA and Fe 3+ ‐PGA mixtures. Conclusions Data confirmed that the intensity and WN of ν as COO − band maxima in FTIR of cation–PGA mixtures changed with type and concentration of cations. Especially for the Al 3+ –PGA and Fe 3+ –PGA mixtures, the intensity and WN of ν as COO − band maxima changed linearly with cation concentration. Understanding these relations can help to improve interpretation of FTIR spectra of SOM. However, further research is needed to extend the results for more complex systems.