Spatial Engineering Direct Cooperativity between Binding Sites for Uranium Sequestration

Abstract Preorganization is a basic design principle used by nature that allows for synergistic pathways to be expressed. Herein, a full account of the conceptual and experimental development from randomly distributed functionalities to a convergent arrangement that facilitates cooperative binding is given, thus conferring exceptional affinity toward the analyte of interest. The resulting material with chelating groups populated adjacently in a spatially locked manner displays up to two orders of magnitude improvement compared to a random and isolated manner using uranium sequestration as a model application. This adsorbent shows exceptional extraction efficiencies, capable of reducing the uranium concentration from 5 ppm to less than 1 ppb within 10 min, even though the system is permeated with high concentrations of competing ions. The efficiency is further supported by its ability to extract uranium from seawater with an uptake capability of 5.01 mg g−1, placing it among the highest‐capacity seawater uranium extraction materials described to date. The concept presented here uncovers a new paradigm in the design of efficient sorbent materials by manipulating the spatial distribution to amplify the cooperation of functions.


Synthesis of POP3-PO 3 H 2 (11):
The synthetic procedures are similar to that of POP2-PO 3 H 2 and POP1-PO 3 H 2 with the yield of each individual step higher than 95%.

Sorption Experiments
The aqueous solutions with different uranium concentrations were obtained by diluting the stock UO 2 (NO 3 ) 2 •6H 2 O solution with the proper amount of distilled water unless otherwise indicated. The pH values of the solutions were adjusted by HNO 3 or NaOH aqueous solution. The concentrations of uranium during all the experiments were detected by inductively coupled plasma-optical emission spectroscopy (ICP-OES) and inductively coupled plasma-mass spectrometry (ICP-MS) for extra-low uranium concentrations. All the adsorption experiments were performed at ambient conditions. K d value calculation. The distribution coefficient ( ) value as used for the determination of the affinity and selectivity of sorbents for UO 2 2+ , is given by the equation: where V is the volume of the treated solution (mL), m is the amount of adsorbent (g), is the initial concentration of uranium, and is the equilibrium concentration of uranium. In the present work, the K d values were measured in the presence of two equivalents of immobilized ligands in various adsorbents (2.3 mg, 2.2 mg, and 3.6 mg for POP1-PO 3 H, POP2-PO 3 H, and POP3-PO 3 H, respectively) used against one equivalent of uranyl in the corresponding amount of aqueous solutions (10 ppm, 100 mL). To guarantee the adsorptions reached equilibrium, an overnight stirring step was used.

Uranium sorption isotherms.
To obtain the uranium adsorption isotherms for various adsorbents, 5 mg of each was added into 10 mL aqueous solutions with different concentrations of uranium. Sorbent materials were suspended fully by brief sonication, and then the mixtures were stirred vigorously overnight, by which time it was assumed that adsorption equilibrium had been reached. The treated solutions were filtrated through a 0.45-μm membrane filter. The supernatant was analyzed using ICP analysis to determine the remaining uranium concentration. The adsorbed amount at equilibrium ( , mg g −1 ) was calculated by: where V is the volume of the treated solution (mL), m is the amount of used adsorbent (g), and and are the initial concentration and the final equilibrium concentration of uranium, respectively.
Uranium sorption kinetics from distilled water. Uranium aqueous solution (400 mL, 20 ppm) and adsorbents (5 mg) were added to an Erlenmeyer flask with a magnetic stir bar. The mixture was stirred at room temperature. At appropriate time intervals, aliquots (5 mL) were taken from the mixture, and the adsorbents were separated by a syringe filter (0.45-μm membrane filter). The uranium concentrations in the resulting solutions were analyzed by ICP-OES. The adsorption capacity at different intervals was calculated as follows: where V is the volume of the treated solution (mL) and m is the amount of used adsorbent (mg), and and are the initial concentration and the concentration of uranium at t (min), respectively.
Uranium sorption kinetics from potable water. Uranium aqueous solution (200 mL, 5 ppm), and adsorbents (5 mg) were added to an Erlenmeyer flask with a magnetic stir bar. The mixture was stirred at room temperature for 3 h. At appropriate time intervals, the aliquots (3 mL) were taken from the mixture, and the adsorbents were separated by a syringe filter (0.45 μm membrane filter). The uranium concentrations in the resulting solutions were analyzed by ICP-MS. The percentage removal of uranium was calculated as follows: 7 Selectivity tests. To evaluate the removal efficiency of these adsorbents towards uranium species in the presence of a large excess of competing ions, tests were performed using a distilled water sample (100 mL) containing uranium (ca. 5 ppm) and various ions (Cu 2+ , Fe 3+ , Co 2+ , Pb 2+ , Zn 2+ , La 3+ , Ce 3+ , Cs + , Sr 2+ , Mg 2+ , and Ca 2+ ) with nearly equal concentrations (ca. 100 ppm) at a phase ratio (V/m) of 100 mL g -1 . After being stirred at room temperature for a certain time, aliquots were taken from the mixture, and the adsorbents were separated by a syringe filter (0.45 μm membrane filter). The uranium concentrations in the resulting solutions were analyzed by ICP-MS.
Uranium sorption kinetics from simulated seawater. Simulated seawater (25.6 g L -1 NaCl and 0.198 g L -1 NaHCO 3 ) spiked with 15 ppm uranium (400 mL), and adsorbents (5 mg) were added to an Erlenmeyer flask with a magnetic stir bar. The mixture was stirred at room temperature, and at appropriate time intervals, aliquots (5 mL) were taken from the mixture, and the adsorbents were separated by syringe filter (0.45 μm membrane filter). The uranium concentrations in the resulting solutions were analyzed by ICP-OES.
Uranium removal kinetics from simulated seawater. Simulated seawater (25.6 g L -1 NaCl and 0.198 g L -1 NaHCO 3 ) spiked with 4056 ppb uranium (10 mL), and adsorbents (5 mg) were added to each 20 mL vial with a magnetic stir bar. The mixtures were stirred at room temperature. At appropriate time intervals, the adsorbents were separated by a syringe filter (0.45 μm membrane filter). The uranium concentrations in the resulting solutions were analyzed by ICP-MS.
Uranium enrichment from real seawater. Adsorbent material (5 mg) was immersed in a tank containing 5 gallons of seawater and shaken at 100 rpm at room temperature. After 56 days, the adsorbent was collected by filtration, washed with water, and dried at 80 °C under vacuum for 24 h. The amount of uranium enriched in the adsorbent was determined by ICP-MS analysis after being digested by aqua regia. 8

Materials and measurements.
Commercially available reagents were purchased in high purity and used without purification. 1 H NMR spectra were recorded on a Bruker Avance-400 (400 MHz) spectrometer. Chemical shifts are expressed in ppm downfield from TMS at =0 ppm, and J values are given in Hz. 13 C (100.5 MHz) cross-polarization magic-angle spinning (CP-MAS) NMR experiments were recorded on a Varian infinity plus 400 spectrometer equipped with a magicangle spin probe in a 4-mm ZrO 2 rotor. Nitrogen sorption isotherms at the temperature of liquid nitrogen were measured using Micromeritics ASAP 2020M and Tristar system. The samples were outgassed for 1000 min at 80 C before the measurements. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) mapping were performed on a Hitachi SU 8000. Transmission electron microscope (TEM) images were collected using a Hitachi HT-7700 or JEM-2100F field emission electron microscope (JEOL, Japan) with an acceleration voltage of 110 kV. XPS spectra were performed on a Thermo ESCALAB 250 with Al K irradiation at θ=90 for Xray sources, and the binding energies were calibrated using the C1s peak at 284.9 eV. IR spectra were recorded on a Nicolet Impact 410 FTIR spectrometer. ICP-OES was performed on a Perkin-Elmer Elan DRC II Quadrupole. ICP-MS was performed on a Perkin-Elmer Elan DRC II Quadrupole Inductively Coupled Plasma Mass Spectrometer.

Theoretical methods:
Quantum chemical calculations. Density functional theory (DFT) calculations were performed with the Gaussian 16, Revision B.01 program package [1] using the M06 [2] functional with the standard Stuttgart small-core (SSC) 1997 relativistic effective core potential (RECP) [3] and the associated contracted [8s/7p/6d/4f] basis set for uranium atom, along with the 6-311++G(d,p) basis set for the light atoms. Frequency calculations were performed at the B3LYP/SSC/6-31+G(d) [4] level to confirm that geometries (optimized at the same B3LYP/SSC/6-31+G(d) level) were minima on the potential energy surface. All thermal corrections to the Gibbs free energy were computed using the ideal gas molecular partition functions within the rigid-rotor quasiharmonic oscillator approximation. In this approximation, [5] vibrational frequencies lower than 30 cm -1 were raised to 30 cm -1 due to the breakdown of the harmonic oscillator approximation for low frequency modes. Using the gas-phase geometries, implicit solvent corrections were obtained at 298 K with the SMD [6] model as implemented in Gaussian 09 at the B3LYP/SSC/6-31+G(d) level of theory. The results are reported using the lowest energy complexes identified at the M06/SSC/6-311++G(d,p) level for a given stoichiometry and binding motif. The preference for using a combination of the M06 and the B3LYP functionals with the SMD solvation model was based on the results of our previous studies. [7,8] Ligand-UO 2 2+ interactions. Assessment of second-order stabilization energies (E (2) , kcal/mol) in the considered uranyl complexes was performed with the natural bond orbital (NBO) method 9 at M06/SSC/6-311++G(d,p) using commercial stand-alone NBO 6.0 program. [10] The donor-acceptor interaction energy (second-order stabilization energies (E (2) ) in the NBOs was estimated via second-order perturbation theory analysis of the Fock matrix. [9] For each donor orbital (i) and acceptor orbital (j), the stabilization energy E (2) associated with i→j delocalization is given by: , where o i is the donor orbital occupancy, is the Fock operator, and ε i and ε j are the orbital energies.
Thermodynamic analysis of complexation. Complexation free energies in aqueous solution, ΔG aq , and stability constants, log β, were calculated using the methodology described in our previous works on uranium complexes. More specifically, this approach utilizes a thermodynamic cycle scheme involving the calculation of the gas-phase free energies and the change in free energy upon transfer of 1 mole of a species from the gas to the aqueous phase under standardized conditions. Once the various free energy terms of the cycle are calculated using quantum chemical methods, then the change in free energy for the aqueous reaction (ΔG aq ) can be determined. The log β values are obtained from the relation of log β to ΔG aq by the following equation: The final log β values in the main text are reported after applying the corresponding regression equation (log β expt = 0.5693×log β calc ) [8] to the calculated log β. Furthermore, a correction factor of 2.2 log units has been applied to correct log β of uranyl complex with the more flexible ligand in complex 2 (main text, Figure 4b), because the calculations seem to underestimate entropy of freely rotatable bonds using a harmonic oscillator approximation. This was based on the predictions of log β values for the uranyl complexes with simple dicarboxylic acids having different lengths of alkyl chain connecting two carboxylate groups. As evident from Tables S5 and S6, our approach significantly overestimates the formation constant for the more flexible ligands (adipic and pimelic acids).

EXAFS:
The X-ray absorption data were collected at Beamline 1W1B at the Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS). Spectra were collected at the uranium L 3 -edge (17166 eV) in transmission mode. The X-ray white beam was monochromatized by a double crystal Si(111) monochromator and detuned by 20% to reduce the contribution of higher-order harmonics to below the level of noise. The K-edge of Yttrium foil (17038 eV) was used as the reference for energy calibration and measured simultaneously for all samples. All spectra were collected at room temperature.
Samples were centered on the beam and adjusted to find the most homogeneous location in the sample for data collection. The data were processed and analyzed using the Athena and Artemis programs of the IFEFFIT package. Reference foil data were aligned to the first zero-crossing of the second derivative of the normalized μ(E) data, which was subsequently calibrated to the literature E0 for the yttrium Kedge (17038 eV). Spectra were averaged in μ(E) prior to normalization. Conditions: Tests were performed using a distilled water sample (50 mL) containing uranium (5 ppm) and various ions (Cu 2+ , Fe 3+ , Co 2+ , Pb 2+ , Zn 2+ , La 3+ , Ce 3+ , Cs + , Sr 2+ , Mg 2+ , and Ca 2+ ) with equal concentrations (100 ppm) at a phase ratio (V/m) of 100 mL g -1 . The adsorbent was regenerated by using HNO 3 (1 M) as eluent.        37 Figure S19.   Figure 3a of the main text. The equilibrium adsorption isotherm data were well fitted with Langmuir model expressed as: ⁄ , where is the equilibrium concentration (mg L -1 ), the amount adsorbed (mg g -1 ), is for complete monolayer adsorption capacity (mg g -1 ), and is the equilibrium adsorption constant (L mg -1 ) and all the fits have R 2 values higher than 0.99. ) Ce (ppm) Figure S24. Adsorption curve of uranium versus contact time in aqueous solution over various adsorbents and the corresponding pseudo-second-order kinetic plot for the adsorption. The adsorption kinetic process was well-fitted with the pseudo second-order kinetic model expressed as: where k 2 (g mg -1 min -1 ) is the pseudo second-order rate constant of adsorption, q t (mg g -1 ) is the amount of uranium species adsorbed at time t (min), and q e (mg g -1 ) is the amount of uranium species and all the fits have R 2 values higher than 0.99. The value of the adsorption rate constant k 2 was determined to be 0.0724, 0.595, and 0.0182 g mg -1 min -1 for POP1-PO 3 H 2 , POP2-PO 3 H 2 , and POP3-PO 3 H 2 , respectively, indicating that POP2-PO 3 H 2 exhibits a higher adsorption rate in relation to POP1-PO 3 H 2 and POP3-PO 3 H 2 .     shown above. Consistent with previous single-crystal X-ray diffraction data for phosphoryl functional groups, our calculations show that the ligand binds the uranyl cation in a monodentate fashion, while bidentate coordination mode involving two phorphoryl oxygen atoms was found to be 12.6 kcal mol -1 less energetically stable (a). Given the presence of urea oxygen donor atom in phosphorylurea functionality, the ligand can in principle form chelate complexes with uranyl (b) by displacing two equatorial water molecules. Our stability constant calculations indicate that the formation of such 1:1 uranyl chelate complex (log β = 6.4) is more thermodynamically favorable compared to the monodentate binding motif (log β = 3.9). Therefore, the ligand binds uranyl through phosphoryl and urea oxygens in 1:1 complex. However, when the phosphorylurea groups are joined together to afford the formation of 2:1 ligand:uranyl complexes, monodentate uranyl binding was found to be more thermodynamically stable than the corresponding chelate binding mode ( Figure S28). This can be possibly explained by the enhanced stabilization of the monodentate binding in 2:1 complex due to the increased hydrogen bonding interactions between phosphorylurea groups and inner-sphere water molecules. In addition, two phosphorylurea functionalities in the linked form are constrained and seemingly cannot reorient to adopt the most favorable chelate binding configuration as compared to the free ligand.