Depth profiling of clay-xanthan complexes using step-scan mid-infrared photoacoustic spectroscopy

Many soil micro-organisms produce extracellular polysaccharides (EPS), and xanthan is a high-molecular-weight natural EPS produced by the bacterium; former studies demonstrate that EPS are produced in soil and are closely associated with the surrounding clay particles. The formed clay-EPS complexes play an important role in soil biogeochemistry. In the present study, experimental clay-xanthan complexes were prepared as models for the soil/biota interface, and the interface layers were investigated using spectroscopic method. Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) was applied to examine interface layer of soil clay minerals (kaolin and montmorillonite) and xanthan; specifically, the step-scan function of FTIR-PAS technique was initially applied to in situ explore the characteristics of surface layers. Soil clay minerals and xanthan were characterized using FTIR-PAS spectra with excellent performance; the variances of depth profiling spectra of montmorillonite were higher than that of kaolin, and more xanthan information was observed in the depth profiling spectra of montmorillonite, which was specifically verified by the absorptions in the region of 600 to 1,200 cm(-1). More xanthan was adsorbed in the montmorillonite surface, which resulted in a thicker surface layer; moisture content clay-xanthan complexes (both absorbed in montmorillonite surface and combined with xanthan) increased. Xanthan was likely to significantly contribute to the water retention capability of clay-xanthan complex, but the contribution of kaolin-xanthan complex was less than that of montmorillonite-xanthan complex. The surface of montmorillonite was more hydrophilic than that of kaolin due to the absorption in 1,640 cm(-1); thus, montmorillonite was easier to interact with hydrophilic xanthan, more xanthan was adsorbed, and a much broader surface layer was observed through depth profiling PAS spectra (9.8 mu m vs. 3.8 mu m). Thicker surface layer in montmorillonite resulted in a stronger water retention capability and will promote the formation of much more complicated organomineral complexes.