An environment-friendly capacitive deionization recovery strategy for rare-earth (La/Nd/Ce/Pr) ions based on nickel foam

Rare earth (RE) is a non-renewable strategic resource widely used in many high-tech fields, it is crucial to develop clean and efficient separation technologies for RE elements. In this work, the RE ions (RE=La, Nd, Ce and Pr) in single or mixed simulated solution are recovered using nickel foam (NF) as an adsorptive electrode by capacitive deionization (CDI) technology. The adsorption capacity of NF from solution of the studied RE is amounted to 1347.7 mg g−1 for La(NO3)3, 841.9 mg g−1 for Nd(NO3)3, 810.7 mg g−1 for Ce(NO3)3 and 1082.0 mg g−1 for Pr(NO3)3 within 300 min, indicating the strong effect of an electric field force on the mobility of RE ions. The obtained products are designated as La(OH)3, CeO2, Pr(OH)3 and Nd(OH)3, respectively. Changes in pH and conductivity occur due to the adsorption of RE ions and water hydrolysis. The respective precipitates are deposited on the ribs and in the void of NF electrode, leading to an increase in reaction impedance and reducing the adsorption rate. By monitoring the adsorption, La, Ce, and Pr are indicated to follow the second order kinetics, while Nd follows the first order kinetics. Furthermore, the RE elements are simultaneously adsorbed from mixed solutions, and the electrode can be reused for several times without Ni corrosion. RE3+ ions can be recovered rapidly within a few minutes without any other chemical agents, making this an inexpensive and environment-friendly way to recover or enrich RE elements.