Selective sorption of rubidium by potassium cobalt hexacyanoferrate

Recovering economically valuable rubidium (Rb) from natural resources is challenged due to its low concentration and limited selectivity of extracting agents. Equilibrium and kinetic studies were conducted on the sorptive removal of Rb at low concentration (5 mg/L) using a commercial and a laboratory prepared potassium cobalt hexacyanoferrate (KCoFC). These laboratory and commercial KCoFCs exhibited similar characteristics in terms of chemical composition, surface morphology (scanning electron microscopy) and crystal structure (X-ray diffraction peaks). KCoFC exhibited a higher sorption capacity for Rb (Langmuir maximum sorption 96.2 mg/g) and cesium (Cs) (Langmuir maximum sorption 60.6 mg/g) compared to other metals such as lithium (Li), sodium (Na) and calcium (Ca) (sorption capacity < 2 mg/g). KCoFC sorption capacity for Rb was affected only when Cs was present at twice the concentration of Rb, while the influence of other metals (Li, Na, and Ca) was minimal even at high concentrations. High Rb sorption capacity was due to the exchange of Rb for K inside the crystal lattice and strong sorption on the sorbent surface. These were evident from the data on K release during Rb sorption and reduced negative zeta potential at the sorbent surface in the presence of Rb, respectively. Kinetic sorption of Rb was satisfactorily described by the pseudo-second order model with intraparticle diffusion and exchange of Rb with structural K acting as major rate limiting steps. Up to 74% desorption of Rb was achieved with 0.1 M KCl. Overall, the results established the superior selectivity of KCoFC for Rb sorption. (C) 2016 Elsevier B.V. All rights reserved.