Efficient extraction of Rb⁺ and Cs⁺ by a precipitation flotation process with ammonium phosphowolframate as precipitant

The properties of a precipitation flotation process for Rb+ and Cs+ extraction from the aqueous solutions are investigated using ammonium phosphowolframate (AWP) as the precipitating agent and cetyltrimethylammonium bromide (CTAB) as the collector and foaming agent in this research. Different parameters including precipitation reaction time, solution pH, ions concentrations, molar ratio of AWP to Rb+/Cs+, and surfactant dosage, were studied. The results show that the extraction efficiency can up to almost 100 % under the optimum experimental conditions as: Contact time at 30 min, mixing speed at 500 rpm, pH at 10, CTAB concentration at 0.15 mmol L-1, Rb+/Cs+ concentration at 1 mmol L-1, the molar ratio of AWP to Rb+/Cs+ at 1:1. The energy analysis of DFT calculation determined that the binding site of Rb+/Cs+ is a four-membered ring of phosphowolframate. IGM reveals the non-covalent interaction between Rb+/Cs+ and phosphowolframate, proving that there are electrostatic attraction and Van der Waals effect between Rb+/Cs+ and the bridge oxygen of the four-membered ring of phosphowolframate ion. The analysis of flotation mechanism reveals that electrostatic attraction is the main force in the flotation process. The results of this research prove that precipitation flotation method has great potential in Rb+ and Cs+ separation as a salt lake brine treatment technology.