Selective pseudocapacitive immobilization of REE elements on carbon based electrodes

Current extraction technologies for rare earth elements at ambient conditions are reagent and energy intensive, giving rise to significant quantities of secondary waste streams. Exploration of the removal of common cations from aqueous streams has been undertaken with carbon materials and have shown promising results. However, those promising materials have not been explored extensively for rare earth element capture from aqueous process streams. In this study, a carbon electrode was investigated for the adsorption and immobilization of REE elements from aqueous solutions. Cyclic voltammetry studies of the carbon electrode displayed a pseudocapacitive behavior where it was verified that cation adsorption is accompanied by an electron transfer process. Preliminary tests showed that a current density of 89.1 mA. g⁻¹, allowed for the pseudocapacitive adsorption (PSA) of Nd³⁺ cation without the formation of rare earth hydroxides. Hence, the selective PSA of Nd³⁺ was verified in an electrolyte solution with equimolar concentrations of Nd³⁺, Mg²⁺, Li⁺, Na⁺ and K⁺, achieving separation factors of 8.6, 1.1, and 10.9 for Nd³⁺/Li⁺, Nd³⁺/Na⁺, and Nd³⁺/Mg²⁺, respectively. Analysis of the potential-time curves for the various cations suggests that storage of ions in the electrode involved pore-spacings rather than interlayer spacings and was corroborated by XRD analysis. Specific capacitance as high as >640 F.g⁻¹ for Nd³⁺ was also observed for the carbon electrode, with Faradaic efficiencies (FE) of >20 % for Nd³⁺ and Mg²⁺, and >9 % for Na⁺. Furthermore, adsorption capacity of >125 mg g⁻¹ after 4hrs of electrosorption was observed for Nd³⁺. In the presence of the Nd³⁺ electrolyte, the electrode achieved increasing storage of Nd³⁺ with >94 % retention of Nd³⁺ with a FE > 20 % over 6 cycles between loading and releases in 1 M KCl solution. These preliminary results provide information and some guidance on the selective recovery of rare earth elements, such as Nd, in aqueous streams in the presence of competitive cations employing the use of carbon-based materials.