Interaction of exopolysaccharide with clay minerals and their effects on U(VI) adsorption

Purpose The modification of clay minerals by exopolysaccharides (EPS) may significantly increase their adsorption capacity for heavy metals. Therefore, this study focused on the characterization of two typical clay minerals, montmorillonite, and kaolinite modified with EPS (produced by Rhizobium tropici), their impact on the adsorption of uranium, U(VI), and the influence of pH, sulfate, and phosphate. Materials and methods The characterization of clays, EPS, and clay-EPS composites were carried out using X-ray power diffraction (XRD), fourier transform infrared spectroscopy (FTIR), atomic force microscope (AFM), and scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) analysis. Results Results showed that EPS partially entered and dehydrated cations in the interlayers of montmorillonite, resulting in the decrease in interlayer d-spacing, which were confirmed by XRD and FTIR. Montmorillonite had better EPS adsorption capacity than kaolinite. The EPS-clay mineral composites significantly increased U(VI) adsorption capacity by 650% and 60% for montmorillonite and kaolinite, respectively. The adsorption capacity for U(VI) was ordered as montmorillonite-EPS > EPS > montmorillonite > kaolinite-EPS > kaolinite. Montmorillonite-EPS modification for U(VI) was maximized at pH 4, while the high pH such as pH 9 inhibited the U(VI) adsorption. Sulfate or phosphate has a negligible effect on the adsorption of UO22+ on montmorillonite-EPS. Conclusion The current study provides a new insight for the application of biopolymers in remediation of U(VI) contaminated area.