Strontium and Cesium Adsorption on Exopolysaccharide-Modified Clay Minerals

The modification of clay minerals by exopolysaccharides (EPS) may significantly increase their adsorption capacity for heavy metals. This study focused on the adsorption of EPS (produced by Rhizobium tropici)-modified montmorillonite (MMT) and kaolinite (KLT) for Cs and Sr and the influence of external factors (pH, sulfate, and phosphate). The characterization of the composites was carried out using X-ray diffraction (XRD), Fourier transform infrared (FTIR), atomic force microscopy (AFM), and scanning electron microscopy/energy-dispersive X-ray analysis. With EPS modification, the adsorption capacity of MMT for Cs and Sr reached 256 and 90.9 mg/g, respectively, which were significantly improved by 53.8 and 54.5% compared to MMT alone, respectively. The adsorption capacity of KLT for Sr improved by 10.7%. KLT did not adsorb Cs either before or after EPS modification. The adsorption isotherms for Sr on MMT, EPS-MMT, KLT, and EPS-KLT as well as Cs on MMT and EPS-MMT were better described with the Freundlich adsorption models, indicating a heterogeneous layered adsorption process. XRD, FTIR, and AFM analysis confirmed the interlayer reaction of Sr/Cs with EPS-MMT. The Sr amounts adsorbed on EPS-MMT composites increased significantly with increasing pH, while the pH influence was not obvious on Cs adsorption but still slightly increased at pH 7 and then dropped at pH 9. In the presence of 50 and 500 mg/L sulfate, the Sr amount absorbed decreased by 12.5, and 29.3%, respectively. On the contrary, there was a significant increase in Cs adsorption by 12.2 and 33.9%, respectively. In the presence of phosphate, a significant increase (64.5%) was observed for Cs adsorption under 50 mg/L phosphate loading, but 500 mg/L phosphate inhibited (65.8%) the adsorption. In contrast, there was no significant change of Sr adsorption under different phosphate concentrations. The current study would provide a new insight for the application of biopolymers in remediation of Sr- and Cs-contaminated areas.