Synthesis of propenone-linked covalent organic frameworks via Claisen-Schmidt reaction for photocatalytic removal of uranium

The type of reactions and the availability of monomers for the synthesis of sp2-c linked covalent organic frameworks (COFs) are considerably limited by the irreversibility of the C=C bond. Herein, inspired by the Claisen-Schmidt condensation reaction, two propenone-linked (C=C–C=O) COFs (named Py-DAB and PyN-DAB) are developed based on the base-catalyzed nucleophilic addition reaction of ketone-activated α-H with aromatic aldehydes. The introduction of propenone structure endows COFs with high crystallinity, excellent physicochemical stability, and intriguing optoelectronic properties. Benefitting from the rational design on the COFs skeleton, Py-DAB and PyN-DAB are applied to the extraction of radionuclide uranium. In particular, PyN-DAB shows excellent removal rates (>98%) in four uranium mine wastewater samples. We highlight that such a general strategy can provide a valuable avenue toward various functional porous crystalline materials.


Supplementary Notes.
Sodium hydroxide, potassium hydroxide, acetic acid, aqua fortis, ascorbic acid,  In general procedure, to obtain the uranium adsorption isotherms, 5 mg material was added into 35 mL solution of UO 2 2+ with different concentrations in a bottle, respectively. The mixture was sonication for 30 seconds and shaken for 24 h at ambient temperature to achieve sorption equilibrium fully. Then the solid sorbent was filtered through a 0.22 µm membrane filter, followed by measuring the remaining uranium concentration using ICP-MS. The adsorbed amount at equilibrium ( , mg g -1 ) was calculated by where V is the volume of the treated solution (L), m is the amount of used adsorbent (g), and C 0 and C e are the initial concentration and the final equilibrium concentration of uranium, respectively.
The equation of the Langmuir isotherm model was represented as following: where q m is the maximum adsorption when the adsorption reaches equilibrium, and k L is a constant characterized by the affinity of the adsorbate with the adsorbent. The value of C e /q e as the function of C e were plotted and fitted with a linear equation from which the q m and k L could be calculated according to the slope and intercept.
The Freundlich model is appropriate for multilayer sorption, which can be described as: where K F (mg 1−n L n /g) denotes the Freundlich sorption coefficient, and n expresses how favorable the sorption process is. K F and n are empirical coefficients.
In the kinetics studies, 5 mg of the sorbent was added into a 35 mL solution containing 600 ppm uranium and used 0.1M HNO 3 to adjust the pH of the solution to 4.0. The mixture was stirred for a series of contact times, and the filtrate was collected at different contact time. The adsorption capacity of uranium as a function of contact time was obtained to determine the kinetics curve.
Pseudo-first-order kinetic model was described as the following function: where q e represents the amount of U on the adsorbent under equilibrium, and k f (min -1 ) is pseudo-first-order adsorption rate constant.
Pseudo-second-order kinetic model was described as the following function: where q e represents the amount of U on the adsorbent under equilibrium, and k s (g mg -1 min -1 ) is pseudo-second-order adsorption rate constant.
The anti-interference performance test of UO 2 2+ adsorption from aqueous solution containing various metal ions was carried out at pH 4.0, the initial concentration of all competitive metal ions is 10 times of UO 2 2+ and the residual concentration in the supernatant of metal ions was determined inductively coupled plasma mass spectrometry (ICP-MS).

Recyclability test
After one run of adsorption, the adsorbents were regenerated by treatment with the elution solution of 0.1 M HNO 3 solution and reused for another adsorption experiment. For 5 mg adsorbents, a certain amount of elution solution was used to elute the binding uranium at room temperature. The elution efficiency (E, %) was determined by using Equation: 8 in where C e (mg L -1 ) is the uranium concentration in elution solution, V e (L) is the volume elution solution, C t (mg L -1 ) is the uranium concentration in the simulated nuclear industry wastewater after uranium adsorption, C0 (mg L -1 ) is the initial uranium concentration of the simulated nuclear industry wastewater, V t (L) is the volume of simulated nuclear industry wastewater used for adsorption. 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 five consecutive cycles still showed excellent uranium removal rate. The inhibition rate was calculated using the Equation:

Antibacterial activity assay
in where C a (CFU/mL) indicates the concentration of bacterial cultures treated with adsorbent and C i (CFU/mL) indicates the concentration of bacterial cultures without treatment. The simulated sunlight with a light density of 1 kW m -2 was used 9 to illuminate the adsorbents.

DFT calculations
The ground state geometry is optimized using DFT. All calculations are performed with the Gaussian 16 package (Rev. C.01) using the hybrid B3LYP functional and the 6-311G(d)/SDD basis set. Grimme's D3BJ dispersion correction was used to improve calculation accuracy. The D index formula for measuring the distance between holes and the center of mass of electrons: