Selective Separation of Scandium in Acidic Water Using Carboxyl Functionalized Covalent Phosphonitrile Polymers

Scandium (Sc) is widely used due to its excellent properties such as high melting point, high boiling point, low density, and good stability. However, as a typical dispersed element, Sc usually exists as an associated mineral. Sc needs to be recovered from the production of ore residues and tailings by-products of other metals, the production of main products from Sc containing internal (external) deposits, or from industrial wastewater and waste residue. Among numerous technologies for separating Sc, the adsorption method exhibits promising prospects due to its advantages of simple operation and high recovery. Currently, several materials including silicon-based (such as SAB-15), biomaterials, and metal organic framework, have been used for the separation of Sc(III). Although these adsorbents exhibit good adsorption ability for Sc(III), the inherent structural drawbacks such as poor chemical stability and with only one type of functional group result in the poor selectivity and inferior adsorption capacity, thereby greatly hindering their practical applications. Considering that acid treatment is often required when processing raw materials such as Sc containing tailings, it is crucial to prepare adsorbents that can efficiently and selectively recover Sc(III) in acidic media.　　Covalent organic polymers are porous organic polymer materials connected by covalent bonds. Due to their adjustable chemical structure, tunable pore size, and easy functionalization, they can be custom-made according to the characteristics of target ions. As one type of alternative adsorbents, they exhibit promising prospects. The pore size matching effect is conducive to achieving the efficient adsorption of target ions in acidic systems. However, because the pore size of the vast majority of covalent organic polymers currently synthesized is much larger than those of metal ions, this method is generally applicable to larger sized ions such as hydrated uranium ions. The utilization of ion imprinting technology or the ingenious selection of monomers with size matching cavities with the target ion to prepare covalent organic polymers are effective for address these issues. However, the complex preparation process makes it difficult to scale up. For covalent organic polymers, modification with special functional groups is another effective strategy to improve their adsorption performance. To tackle the issue of limited binding ability caused by the single functional group, research has shown that the introduction of various functional groups into the porous skeleton structure can effectively enhance the selective binding ability to target ions by utilizing the synergistic effect between different groups.

To improve the adsorption performance of porous organic polymers for Sc(III) in acidic media by utilizing the synergistic effect of multiple functional groups.

Phloroglucinol (1mmol) and hexachlorocyclotriphosphazene (0.5mmol) were dissolved in 1mL of 1,4-dioxane, followed by the addition of 0.84mL of triethylamine. The mixture was transferred to a hydrothermal reactor (8mL) and reacted at 80℃ for 24h. The product was washed several times with water, ethanol and acetone, respectively. Finally, CPF-T was prepared by vacuum drying at 50℃ for 12h. CPF-T-COOH was prepared according to the same procedure by changing phloroglucinol with 2,4,6-trihydroxybenzoic acid.　　The infrared spectra of the CPF-T and CPF-T-COOH were collected by Thermo Scientific Nicolet iS20 Fourier transform infrared spectrometer (FT-IR) (Thermo, USA). The microstructure of the polymers was studied using SU8010 scanning electron microscope (SEM) (Hitachi, Japan). The thermogravimetric analysis (TGA) curve of the polymers was collected by the DTA7200 TGA instrument (Hitachi, Japan) under N₂ conditions (N₂ flow rate: 20mL/min; heating rate: 10K/min). The elemental information of the materials was analyzed using Thermo Scientific K-Alpha X-ray photoelectron spectrometer (XPS) (Thermo, USA). The N₂ adsorption desorption isotherm was measured at 77K using the Micromeritics APSP2460 4-station fully automatic specific surface area analyzer (Micromeritics, USA). The sample was vacuum degassed at 120℃ for 12h before measurement.　　Adsorption experiments were conducted at room temperature using a constant temperature oscillator (150r/min). The adsorbent dosage was 1g/L, and the experimental data was obtained as the average of three parallel experiments. After adsorption, the supernatant was collected by filtration with 0.45μm microporous membrane. Subsequently, it was tested by EXPEC 6000 ICP-OES (Hangzhou Puyu Technology Development Co., Ltd.), and the linear correlation coefficient (R²) of the standard curve equation was >0.999.

CPF-T and CPF-T-COOH were characterized by FT-IR, SEM, TGA and N₂ adsorption-desorption analysis. The appearance of P-O-Ar, P=N, and P-N stretching vibration peaks in the FT-IR diagram indicates the successful crosslinking of the organic monomers. In addition, the appearance of C=O, C-O and -OH in CPF-T-COOH indicates the successful modification of —COOH. TGA characterization indicates that the resulting materials have a good thermal stability within 150℃. Through SEM images, it can be observed that the micro morphologies of the two materials are different. CPF-T presents a relatively smooth spherical structure with some agglomerating, CPF-T-COOH exhibits an irregular, ant-like porous structure with a rough surface. The N₂ adsorption-desorption isotherms of both materials belong to type II and show a strong absorption in the range of P/P₀=0.8−1.0, indicating the presence of microporous and mesoporous structures. The specific surface area of CPF-T is 76.5m²/g, and total pore volume is 0.38cm³/g. The specific surface area of CPF-T-COOH is 2.61m²/g, and the total pore volume is 0.035cm³/g.　　The adsorption performance of CPF-T and CPF-T-COOH for Sc(III) was investigated through batch adsorption experiments, including the influence of solution pH, adsorption kinetics, adsorption isotherms and adsorption selectivity. The adsorption efficiencies of Sc(III) on the two materials exhibit different trends with respect to the solution pH. As the acidity of the solution increases, the adsorption efficiency of CPF-T for Sc(III) significantly decreases, and at pH<3, the adsorption rate is below 60%. However, within the range of solution pH=1-3, the adsorption efficiency of CPF-T-COOH for Sc(III) remains above 95%. The adsorption kinetics experiment shows that CPF-T reaches adsorption equilibrium within 120min, while CPF-T-COOH can reach adsorption equilibrium within 30min. In addition, in the adsorption isotherms experiment, the maximum adsorption capacity of CPF-T-COOH (64.6mg/g) for Sc(III) was higher than that of CPF-T (22.5mg/g). Finally, the adsorption selectivity was evaluated through coexisting ion experiments. Both materials have the ability to selectively adsorb Sc(III), with K_d values of 5.1×10²mL/g and 4.0×103mL/g for Sc(III), respectively. The adsorption performance of CPF-T-COOH is not only higher than that of CPF-T, but also has unique advantages compared to some reported adsorbents, including a wider pH range, fast adsorption equilibrium time, and higher K_f value. These results indicate that the synergistic effect of multiple adsorption sites is an effective strategy for improving adsorption affinity of adsorbents.　　The adsorption mechanism was studied through XPS analysis. In the high-resolution XPS spectrum of N1s of Sc(III) loaded sample, all the binding energies of P-NH-P(398.0eV), P=N-H(399.3eV) and P-NH₂(401.6eV) show an increase, shifting to 398.1eV, 399.6eV and 403.5eV, respectively. Meanwhile, in the spectra of O1s, it can also be observed that the binding energies of C=O(534.2eV), C-OH(533.2eV), C-O(532.0eV) and P-O(531.0eV) shift to 534.3eV, 533.3eV, 532.2eV and 531.3eV, respectively. These results indicate the existence of coordination between O/N and Sc(III). In addition, a new peak at 529.3eV corresponding to O-Sc bond appears in the spectrum of O1s for Sc(III) loading sample, and the peak area of C-OH significantly decreases. This is because the H atom in the hydroxyl group is replaced by Sc(III) through ion exchange, resulting in the formation of a new coordination bond with O.

Covalent phosphonitrile polymer featuring abundant carboxyl functional groups is successfully synthesized by the solvothermal method. Compared with CPF-T, the carboxyl-functionalized CPF-T-COOH exhibits a much stronger binding ability toward Sc(III), where the adsorption efficiency of Sc(III) in acidic media is greatly improved from ~60% to greater than 95%. In addition, its adsorption capacity is 3.5 times that of CPF-T. The result of the mechanism study reveals that the enhanced adsorption performance is attributed to the synergistic effect of multiple functional groups, providing an alternative route for the preparation of new materials with high adsorption performance for Sc(III) capture.