Adsorption and characterization of exopolysaccharides from Rhizobium tropici on clay minerals

Abstract

Exopolysaccharides (EPS) are extracellular macromolecular carbohydrate polymers secreted by microorganisms. EPS significantly improves soil structure, fertility and decreases erosion. Montmorillonite and kaolinite are two major clay minerals in soil. Interaction of EPS with clay minerals is important for understanding mechanisms of soil microaggregate and maintaining soil nutrients. The objectives of this study were to investigate mechanisms of EPS from Rhizobium tropici interaction with these clays and their adsorption capacity. Results show that EPS adsorption on clays were better described with Langmuir than Freundlich models, but kinetics followed the parabolic diffusion model. This suggested that EPS adsorption occurred on both surface and interlayers of montmorillonite as controlled by diffusion processes. The maximum adsorption capacity of EPS on montmorillonite and kaolinite were 236.4 mg/g and 168.6 mg/g, respectively. After initial adsorption of EPS on montmorillonite, the d-spacing was slightly increased, but further increase in EPS adsorption significantly decreased the d-spacing. This indicated the partial entrance of EPS into interlayers of montmorillonite, but high EPS presence possibly dehydrated metal ions within interlayers, shrinking montmorillonite. Thus, the current study provides an insight understanding of mechanisms of EPS from Rhizobium tropici interaction with clay minerals.