Ammoniating Covalent Organic Framework (COF) for High‐Performance and Selective Extraction of Toxic and Radioactive Uranium Ions

Abstract An ideal porous adsorbent toward uranium with not only large adsorption capacity and high selectivity but also broad applicability even under rigorous conditions is highly desirable but still extremely scarce. In this work, a porous adsorbent, namely [NH4]+[COF‐SO3 −], prepared by ammoniating a SO3H‐decorated covalent organic framework (COF) enables remarkable performance for uranium extraction. Relative to the pristine SO3H‐decorated COF (COF‐SO3H) with uranium adsorption capacity of 360 mg g−1, the ammoniated counterpart of [NH4]+[COF‐SO3 −] affords ultrahigh uranium uptake up to 851 mg g−1, creating a 2.4‐fold enhancement. Such a value is the highest among all reported porous adsorbents for uranium. Most importantly, a large distribution coefficient, K d U, up to 9.8 × 106 mL g−1 is observed, implying extremely strong affinity toward uranium. Consequently, [NH4]+[COF‐SO3 −] affords highly selective adsorption of uranium over a broad range of metal ions such as SU/Cs = 821, SU/Na = 277, and SU/Sr = 124, making it as effective uranium adsorbent from seawater, resulting in amazing uranium adsorption capacity of 17.8 mg g−1. Moreover, its excellent chemostability also make it an effective uranium adsorbent even under rigorous conditions (pH = 1, 8, and 3 m acidity).

Synthesis of COF-SO 3 H. 0.3 mmol (63mg) of 2,4,6-Triformylphloroglucinol, 0.45 mmol (84.7mg) 2,4,6-Triformylphloroglucinol was added into a Pyrex tube with 1.5 mL butyl alcohol and 1.5mL 1,2-dichlorobenzene. The mixture was sonicated for 20 min, followed by addition of 0.5 mL of 3 M aqueous acetic acid. After that, the tube was degassed by freeze-pump-thaw cycles for three times, sealed under vacuum and heated at 120 °C for 3 days. The reaction mixture was cooled to room temperature and washed with deionized water, dimethylacetamide and acetone. The resulting dark red powder was dried at 120 °C under vacuum for 12 hours. Materials and general methods. Reagents and solvents were commercially available (Alfa) and were used without further purification. X-ray powder diffraction were collected by a Bruker AXSD8 Discover powder diffractometer at 40 kV, 40 mA for Cu Kλ ( λ= 1.5406 Å). The simulated powder patterns were calculated by Mercury 1.4. Infrared Spectra (IR) were measured by a Bruker VERTEX70 spectrometer in the 500-4000 cm -1 region. The gas adsorption isotherms were collected on a Belsorp-max. Ultrahigh-purity-grade (>99.999%) N 2 gases were used during the adsorption measurement. SEM and EDS measurements were carried out using a Hitachi S-4800 microscope.

Synthesis of [NH 4 ] + [COF-SO
The analyses of concentrations of metal ions in the solution was carried out by ThermoFisher iCapQ ICP-MS and ThermoFisher iCap7600 ICP-OES instruments. Elemental analyses of C, H, N, and S were carried out on a German Elementary Vario EL III instrument. X-ray photoelectron spectra (XPS) were collected by Thermo Scientific ESCALAB 250 Xi spectrometer. The NMR in solution was carried out on Bruker VANCEIIIHD500. The NMR in solide was carried out on a Bruker 400MHz WB solid-state NMR spectrometer.
In pH-dependent experiments, the U solution with pH=3-7 was adjusted by HCl (1 M) and NaOH (1 M). The dose of adsorbent is 10 mg, while the U solution is 20 mL and the contact time is 96 h.
In isotherm experiments, the U solution with initial concentration of 50-600 ppm and pH=5 was used. The dose of adsorbent is 10 mg, while the U solution is 20 mL and the contact time is 96 h.
In kinetics experiments, the U solution with initial concentration of 200 ppm and pH=5 was used. The dose of adsorbent is 10 mg, while the U solution is 20 mL.
In determining affinity experiments, the U solution with initial concentration of 50 ppm and pH=5 was used. The dose of adsorbent is 10 mg, while the U solution is 20 mL.
In selective adsorption experiments, a binary mixed solution contains both U and other metal ions respectively with initial concentration of 50 ppm and pH=5 was used. The dose of adsorbent is 10 mg, while the solution is 20 mL.
In the U uptake under rigorous conditions, the U solution with initial concentration of 10 ppm and pH=1, 8, and 3 M HCl was used. The dose of adsorbent is 10 mg, while the solution is 20 mL.
For the samples after γ radiation (5 Gy) for eight days, the U adsorption experiments was carried out in the U solution with initial concentration of 50 ppm and pH=5. The dose of adsorbent is 10 mg, while the solution is 20 mL.
In equation (1), c 0 (mg/L) and c e (mg/L) are the initial concentration and equilibrium concentration of uranium in the solutions, respectively; V (mL) is the volume of testing solution and m (mg) is the amount of sorbent.
The adsorption kinetics was analyzed by simplified kinetic models such as the pseudo-first-order and pseudo-second-order, through the following two equations, ln( − ) = ln − 1 (2) = 1 2 × 2 + Where Qe (mg/g) and Qt (mg/g) are the quantity of the adsorbed U(VI) at equilibrium and at t time, respectively, and K 1 (min -1 )/K 2 [g/(mg•min)] is the pseudo-first/second-order sorption rate constant that is deduced from the slope of the plot of t/Qt versus t.
The isotherm data was fitted by Langmuir and Freundlich models via the following two equations, = + 1 (4) ln = ln + 1 ln Where Q e (mg/g) is the amount adsorbed at equilibrium and C e (mg/L) is the equilibrium concentration; Q m (mg/g) is the maximum adsorption amount; K L (L/mg) is an equilibrium constant related to the binding strength; n and K F (L/mg) are Freundlich constants which are indicators of the adsorption capacity and adsorption intensity, respectively.
The K d value and selectivity (S) is calculated from the following two equations, where the unit for K d value is mL/g.

DFT calculation.
A series of models were established for optimizing the initial structure of COF with -SO 3 H function groups in frameworks, as well as for simulating the adsorption behavior of pseudopotentials were adopted to describe the electron-ion interaction. All the structures were optimized aiming to the global energy minimum (Table S6), fully relaxed until the residual force convergence value on each tom being less 0.05 eV/Å. The Brillouin zone was sampled by 3 x 3 x 1 Gamma k-point mesh and the wave functions were expanded using a plane-wave basis set with kinetic energy cutoff of 500 eV. Spin-polarization was calculations with the lowest energy magnetic configurations were identified. All of the above structures were establised by Materials Studio.