Highly Efficient Electrocapacitive Deionization of Rare Earth Yb(III) by Carbon-Doped Nickel–Iron Sulfide Nanospheres

The recovery of rare earths from mining water is critical to the recycling of strategic resources. In this work, a novel electrode material (termed as NiS2/FeS2/C) was successfully fabricated using nickel–iron glycerols as precursors through annealing and sulfidation processes and investigated for capacitive deionization of rare earth Yb(III). The maximal electrosorption capacity of NiS2/FeS2/C was 132.8 mg g–1 toward a 500 mg L–1 ytterbium nitrate electrolyte under the optimal conditions, which was higher than that of the contrast sample (NiFe2O4/C, 80.7 mg g–1). Furthermore, the electrosorption process of NiS2/FeS2/C for Yb(III) was correlated well with the Langmuir isotherm model as well as the pseudo-first-order kinetic model. In addition, NiS2/FeS2/C still maintained a high electrosorption efficiency of 91.56% upon five continuous electrosorption–desorption cycles. And NiS2/FeS2/C also exhibited higher selectivity for Na(I) than the competitive ions (i.e., Yb, Ca) in the mixture solution. In addition, X-ray photoelectron spectroscopy (XPS) analysis revealed that the electrosorption mechanism involved Faradaic redox reactions that were facilitated by the nickel–iron bimetallic system, as well as the electric double-layer capacitance. Therefore, this study supplied a novel electrode material for the efficient electrocapacitive deionization of rare earth ions from aqueous solutions.