Rationally designed nanotrap structures for efficient separation of rare earth elements over a single step

Extracting rare earth elements (REEs) from wastewater is essential for the growth and an eco-friendly sustainable economy. However, it is a daunting challenge to separate individual rare earth elements by their subtle differences. To overcome this difficulty, we report a unique REE nanotrap that features dense uncoordinated carboxyl groups and triazole N atoms in a two-fold interpenetrated metal-organic framework (named NCU-1). Notably, the synergistic effect of suitable pore sizes and REE nanotraps in NCU-1 is highly responsive to the size variation of rare-earth ions and shows high selectivity toward light REE. As a proof of concept, Pr/Lu and Nd/Er are used as binary models, which give a high separation factor of SFPr/Lu = 796 and SFNd/Er = 273, demonstrating highly efficient separation over a single step. This ability achieves efficient and selective extraction and separation of REEs from mine tailings, establishing this platform as an important advance for sustainable obtaining high-purity REEs.

Stability Test.NCU-1 was immersed in different pH (1, 3, 5, 7, 9, 11, 13) solution for 12 h, respectively.The mixture was then filtered and washed with ultra-pure water till the supernatant became neutral and dried under vacuum at 60 °C.Then, the PXRD patterns were obtained.
Sorption Experiments.The solid-liquid ratio in all experiments was 1.0 g/L.The pH value of the solutions was adjusted to 4.50 with HNO 3 or NaOH aqueous solution.
The rare earth tailing samples were collected from the Ganzhou City, Jiangxi Province.
The concentrations of REE ions during all the experiments were detected by ICP-MS for extra-low concentrations.All the adsorption experiments were performed under ambient conditions.All experiments were conducted in triplicate, with average values displayed in the graphs.
where V is the volume of the treated solution (L), m is the amount of used adsorbent (g), C o is the initial concentration of REE ions (mg/L), and C e is the equilibrium concentration of REE ions (mg/L), respectively.q e (mg/g) is the equilibrium adsorption capacity.q m (mg/g) is the maximum sorption capacity, k L is a constant indirectly related to sorption capacity and energy of sorption (L/mg), which characterizes the affinity of the adsorbate with the adsorbent.k F and n are the Freundlich constants related to the sorption capacity and the sorption intensity, respectively.REE Ions Adsorption Kinetics Experiments.10 mg of NCU-1 was added into 10 mL aqueous solutions of the initial concentrations of Pr 3+ , Nd 3+ , Eu 3+ , Gd 3+ , Dy 3+ , Er 3+ , or Lu 3+ (5 mg/L).Under magnetic stirring, the resulting mixture was stirred for the desired contact time (5 min, 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, 120 min, and 240 min), then using 0.22 um nylon membrane filter for ICP-MS detection.
The capture percentage was calculated based on equation ( 4).Pseudo-first-order model and pseudo-second-order model are usually used for the sorption kinetics data fitting.The formulas were expressed in equations ( 5) and ( 6), respectively.

Capture%
where q e and q t are the adsorption capacity at equilibrium and time t, k 1 (g mg −1 min −1 ) and k 2 (g mg −1 min −1 ) are the rate constants of pseudo-first-order and pseudo-second-order, respectively.The pseudo-first-order linear plot can be obtained by plotting ln (q e -q t ) versus t, and the pseudo-second-order linear plot can be obtained by plotting t/q t against t.pH-dependent Adsorption Experiments.10 mg NCU-1 adsorption material was added to a 10 mL aqueous solution containing 5 mg/L REE ions (La-Lu) and 5 mg/L competing transition metal and alkali metal ions (Na + , Al 3+ , Ca 2+ , Fe 3+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Pb 2+ ) with varying pH (2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0), and the solution pH was adjusted as required using NaOH and HNO 3 .The mixtures were shaken at a rate of 120 rpm for 6 h, and separated with a 0.22 µm nylon membrane filter for ICP-MS analysis.The affinity and selectivity of NCU-1 to REE ions can be detected by the distribution coefficient (K d ), as illustrated in equations (7).
= ( 0 −  )    (7)   Recyclability Test.After one run of adsorption, the adsorbents were regenerated by treatment with the elution solution of HNO 3 (pH 3) solution and reused for another adsorption experiment.For 100 mg adsorbents, a 100 mL elution solution was used to elute the binding REE ions (Pr 3+ , Nd 3+ , Gd 3+ , or Dy 3+ ) for 6 h at room temperature.
The resulting suspension was filtered and washed with ultra-pure water till the supernatant became neutral.After being dried under vacuum, the resultant material was used for another adsorption experiment.It was found that after four consecutive cycles NCU-1 still showed excellent REE uptake.
Binary Lanthanide Separation by NCU-1.Pr/Er separation: 10 mg NCU-1 was charged into a 10 mL mixture solution containing Pr(NO 3 ) 3 ·6H 2 O (0.03 mmol), Er(NO 3 ) 3 ·6H 2 O (0.03 mmol).The mixtures were shaken at a rate of 120 rpm for 6 h, and separated with a 0.22 µm nylon membrane filter for ICP-MS analysis.The other five binary lanthanide separations of Pr/Dy, Pr/Lu, Nd/Er, Nd/Dy, and Eu/Lu were under the same procedure with Pr/Er binary separation.Separation factors (SF) are calculated using the following equation (8). 4 All the molar fraction (B%) value was calculated from the separation factor, as shown in equation (9). 5

SF = 𝐾
The Rare Earth Tailing and Experimental Breakthrough Studies.The natural rare earth tailing sample was collected from Ganzhou city, Jiangxi province, and filtered through a 0.22 µm filter to remove insoluble particles and microorganisms.10 mg adsorbent was dispersed into the treated natural rare earth tailing sample.The mixtures were shaken at a rate of 120 rpm for 6 h, and separated with a 0.22 µm nylon membrane filter for ICP-MS analysis.500 mg crystalline products of NCU-1 as the stationary phase were filled in a glass column, the inside diameter of the column is 9 mm, and the treated mine tailing was collected from Ganzhou city, as mobile phase under ambient pressure (Figure S9).
The solutions with metal ions were controlled to slowly pass through the column and collected at the bottom in a bottle for ICP-MS analysis.
Theoretical Calculations.First-principles calculations were carried out using density functional theory (DFT) with generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) implemented in the Vienna Ab-Initio Simulation Package (VASP). 6,7The valence electronic states were expanded on the basis of plane waves with the core-valence interaction represented using the projector augmented plane wave (PAW) approach 8 and a cutoff of 520 eV.A Γ-centered k-mesh of 1 × 1 × 1 was used for the surface calculations.Convergence is achieved when the forces acting on ions become smaller than 0.02 eV/Å.The adsorption energies of metal ions on NCU-1 were defined by the equation (10).
where E M* is the total energy of the Metal-adsorbed NCU-1; E NCU-1 is the energy of the NCU-1, and E M is the energy of isolated metal ions such as Al 3+ , Dy 3+ , and Pr 3+ .

Table 1|Single -
crystal X-ray structure refinement of NCU-1.Supplementary Table 2|Elemental composition of mineral leachates and NCU-1capture efficiency of rare earth in tailings collected from Ganzhou city.Supplementary Table5|Kd values of REE were captured by NCU-1 at various pH values from 2.0 to 6.0.