Cheap and easily separate Cs(I) and Rb(I) from high salty solution by forming RbCs–MgHCF

Abstract

Rubidium and cesium are highly-priced and important rare alkali elements but they always co-exist with large amount of sodium and potassium in a complex system resulting very difficult and expensive to recover. Hexacyanoferrates (MHCFs) is one of the most efficient methods to separate Rb+ and Cs+ if we don't consider certain drawbacks such as too small particle size and too viscous liquids to be precipitated. In this study, a new kind of hexacyanoferrate, magnesium (II) hexacyanoferrate (RbCs–MgHCF) has been fabricated at room temperature and applied for the first time to recover Rb+ and Cs+ from high-salty solutions. Compared with traditional divalent transition elements, such as CuHCFs, ZnHCFs, NiHCFs, CoHCFs or FeHCFs, RbCs–MgHCF had excellent precipitation property and large particle size of about 300 nm. The structure, morphology and composition were characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Inductively coupled plasma (ICP), and Energy dispersive spectroscopy (EDS). The effects of various factors (divalent metal cations, surfactant, Mg2+/(Rb++Cs+) mole ratio, pH value and titration rate) on the recovery of Rb+ and Cs+ were explored in 50 mL batch experiments. Scale-up experiment were conducted in 6 and 18 L. Results identified that deposition ratio for Cs+ and Rb+ were 99% and 95% among pH ranges of 4.0–11.0. The composition of RbCs–MgHCF was confirmed to be Rb0.61 Cs1.20K0.17Mg1.01Fe(CN)6 by ICP and EDS and the synthesis mechanism of RbCs–MgHCF is proposed.