Optimizing radionuclide sequestration in anion nanotraps with record pertechnetate sorption

The elimination of specific contaminants from competitors poses a significant challenge. Rather than relying on a single direct interaction, the cooperation of multiple functionalities is an emerging strategy for adsorbents design to achieve the required affinity. Here, we describe that the interaction with the target species can be altered by modifying the local environment of the direct contact site, as demonstrated by manipulating the affinity of pyridinium-based anion nanotraps toward pertechnetate. Systematic control of the substituent effect allows the resulting anion nanotraps to combine multiple features, overcoming the long-term challenge of TcO4− segregation under extreme conditions of super acidity and basicity, strong irradiation field, and high ionic strength. The top material exhibits the highest sorption capacity together with record-high extraction efficiencies after a single treatment from conditions relevant to the used nuclear fuel (Hanford tank wastes, 95%) and legacy nuclear wastes (Savannah River Sites, 80%) among materials reported thus far.


Supplementary Tables
Supplementary Table 1  All energies in kcal mol -1 The top rows show quantum-mechanical binding free energies for each complex relative to the dissociated molecules using a 1M standard state. The second set of rows tabulate the (RISM3D-KH) solvation free energies computed for the dissociated molecules and the very next set of rows gives the solvation free energies of the complexes. Rows labeled Gsolv give the part of the solvation free energy due only to solvation effects. The third set of rows gives the ion estimated binding constants for attachment of anions to free (+1 charged) monomers in solution. The final set of rows provide the free energy of ion exchange into the polymer phase.
After stirring at room temperature to achieve homogeneity, the mixture was transferred into a 20 mL autoclave and maintained at 100 C for 24 h. A white solid product (1.0 g, 100% yield) was obtained by extracting the DMF solvent with EtOH and drying in vacuum at 50 C for 12 h.
The mixture was then stirred and heated to 80 C for 48 h. The resulting powder was washed with ethanol and then exchanged with 1 M NaCl solution three times to afford the title product as a light yellow powder.

Synthesis of porous polymer constructed by 4-aminepyridine moieties (POP-pNH2Py):
3 (1.0 g) was dissolved in DMF (10 mL), followed by the addition of AIBN (25 mg). After stirring at room temperature to achieve homogeneity, the mixture was transferred into a 20 mL autoclave and maintained at 100 C for 24 h. A yellow solid product was obtained by extracting the DMF solvent with EtOH and drying in vacuum at 50 C for 12 h.

Synthesis of porous ion exchange material constructed by 4-amino-1-methylpyridinium chloride moieties (PQA-pNH2Py-Cl):
POP-pNH2Py (0.5 g) was swelled in acetonitrile (40 mL), followed by the addition of iodomethane (1 g). The mixture was then stirred and heated to 80 C for 48 h. The resulting powder was washed with ethanol and then exchanged with 1 M NaCl solution three times to afford the title product as a yellow powder.

Synthesis of porous polymer constructed by dimethylaminopyridine [POP-pN(Me)2Py]:
6 (1.0 g) was dissolved in DMF (10 mL), followed by the addition of free radical initiator azobisisobutyronitrile (AIBN, 0.025 g). After stirring at room temperature to achieve homogeneity, the mixture was transferred into a 20 mL autoclave and maintained at 100 C for 24 h. A yellow solid product was obtained by extracting the DMF solvent with EtOH and drying in vacuum at 50 C for 12 h.

Synthesis of porous ion exchange material constructed by 4-(dimethylamino)-1-methylpyridin-1-ium moieties [PQA-pN(Me)2Py-Cl]:
POP-pN(Me)2Py (0.5 g) was swelled in acetonitrile (40 mL), followed by the addition of iodomethane (1 g). The mixture was then stirred and heated to 80 C for 48 h. The resulting powder was washed with ethanol and then exchanged with 1 M NaCl solution three times to afford the title product as a yellow powder.

Sorption Experiments
Caution! Although Tc-99 is a low-energy β-emitter (t1/2 = 2.13×10 5 a), it still possesses significant health risks when inhaled or digested. Standard precautions and procedures for handling radioactive materials should be followed, and all Tc-99 studies were conducted in a licensed laboratory dedicated to radiological investigations. The aqueous solutions with different rhenium concentrations were obtained by diluting the stock KReO4 solution with the proper amount of distilled water unless otherwise indicated. The pH levels of the solutions were adjusted by HNO3 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 concentrations. All the adsorption experiments were performed at ambient conditions.

Rhenium sorption isotherms.
To obtain adsorption isotherms for PQA-Py-Cl, PQA-pNH2Py-Cl, and PQA-pN(Me)2Py-Cl, 5 mg of adsorbent was placed in 10 mL aqueous solutions of varying rhenium concentrations (25-800 ppm). The solutions were then stirred overnight to achieve equilibrium. The solutions were filtered through a 0.45 μm membrane filter and the filtrate was analyzed via ICP to determine the residual rhenium species concentrations. The amount adsorbed or ion-exchange capacity, qe (mg g -1 ), at equilibrium was calculated using Equation 1.
Where C0 and Ce are the initial and equilibrium concentrations, respectively, V is the volume of solution used (mL), and m is the mass of adsorbent (g). 16 mg of PQA-Py-Cl, PQA-pNH2Py-Cl, or PQA-pN(Me)2Py-Cl was added to an Erlenmeyer flask containing 400 mL of a 50 ppm rhenium aqueous solution. The mixture was then stirred. At increasing time intervals 3 mL aliquots were removed from the mixture, filtered through a 0.45 μm membrane filter, and the filtrate was analyzed by ICP for the remaining rhenium concentration. The adsorption capacity at different intervals was calculated using Equation 2.

Rhenium sorption kinetics.
(2) Adsorption capacity (mg/g) = ( 0 − ) × / where V is the volume of the treated solution (mL), m is the amount of used adsorbent (mg), and 0 and are the initial concentration and the concentration of rhenium at t (min), respectively.

Kd value calculation.
The distribution coefficient ( ) value as used for the determination of the affinity and selectivity of sorbents for ReO4 -, is given by Equation 3.
where V is the volume of the treated solution (mL), m is the amount of adsorbent (g), 0 is the initial concentration of rhenium, and is the equilibrium concentration of uranium. In the present work, 10 ppm rhenium aqueous solutions were treated by the various adsorbents overnight at a V/m ratio of 1000 mL g -1 .

S-35
Rhenium removal from groundwater. Rhenium spiked groundwater sample (50 mL, 1000 ppb) and adsorbents (10 mg) were added to an Erlenmeyer flask with a magnetic stir bar. The mixture was stirred at room temperature. At increasing time intervals 3 mL aliquots were removed from the mixture, filtered through a 0.45 μm membrane filter, and the filtrate was analyzed by ICP-MS for the remaining ReO4concentration. The percentage removal of rhenium species was calculated by Equation 4.

PQA-pN(Me)2Py-Cl recyclability test.
The rhenium species included PQA-pN(Me)2Py-Cl [Re@PQA-pN(Me)2Py-Cl, ca. 50 mg] was stirred in saturated NaCl aqueous solution (200 mL) for 12 h. The solid was collected by centrifugation and repeated ion exchange with NaCl aqueous solution was performed for another two times. After that, the solid was washed with deionized water until the filtrate was Clfree and the solid was dried under vacuum to produce PQA-pN(Me)2Py-Cl.
Radiation-resistance measurements. The β-ray and the γ-ray used was provided by an electron accelerator equipped with an electron beam (10 MeV) and a 60 Co irradiation source (92.42 PBq), respectively. A β-ray irradiation experiment was conducted by irradiating PQA-pN(Me)2Py-Cl at a dose rate of 50 kGy h -1 for three different doses (400, 800, and 1000 kGy). A γ-ray irradiation experiment was performed by irradiating PQA-pN(Me)2Py-Cl at a dose rate of 3.125 kGy h -1 for three different doses (400, 800, and 1000 kGy). The radiation-resistance of PQA-pN(Me)2Py-Cl was characterized by FT-IR spectroscopy and further checked by ReO4uptake capacity of the irradiated samples.

Technetium removal from distilled water.
TcO4spiked groundwater sample (10 mL, 14.3 ppm based on technetium species) and adsorbents (10 mg) were added to a glass vial with a magnetic stir bar. The mixture was stirred at room temperature. At increasing time intervals aliquots were taken out from the mixture and the remaining TcO4concentration was analyzed by a liquid scintillation counting (LSC) system (Perkin Elmer Quantulus 1220 reported protocol (Supplementary reference 17). A certain amount of PQA-pN(Me)2Py-Cl was added to 10 mL of the above simulated wastes at V/m values of 200 mL g -1 and 100 mL g -1 , respectively. After being stirred for 12 h, the suspension was separated with a 0.22 µm nylon membrane filter for LSC analysis.

Computational Methods
Since the polymer is cross-linked and not dissolved in water, we treat it as a separate phase which contains one anion binding site per monomer. The monomer formula weights are 181.07 g mol -1 , 196.08 g mol -1 , and 224.11 g mol -1 . The fraction of binding sites occupied by anion Xis computed as the ratio, = • / ,total , of polymer sites bound to Xto total polymer sites, ,total . Assuming the polymer phase occupies a negligible fraction of the total solution volume, the occupancy fraction is also related to the solution concentration of ions through mass balance. Mass balance states that the formal concentration of Xions is However, a more direct way to plot the isotherm would be to take the logarithm of Equation 7 and show log ([X -]/[Cl -]) as the independent variable.
Ion-binding free energies for each polymer were estimated from calculations on the truncated fragments shown in Supplementary Fig. 23. Quantum calculations were carried out using density functional theory with the M06 exchange and correlation functionals to compute the cluster formation free energy, S-37 (9) ∆ 0 = ln where each q is a partition functions of a gas-phase cluster, defined explicitly in Equations 12-14. A 6-311++G(2d,2p) basis was used for all atoms except Tc, which was treated with the def2-tzvp basis and associated empirical core potential. This combination of basis sets and functionals was found previously to reproduce the experimental structure of the pertechnetate anion. 18.19 All our calculations used a "fine" grid setting which targets an energy accuracy of 10 -7 Hartreein this case by using Mura-Knowles radial quadrature with 60-141 points per atom and Lebedev angular quadrature with 590-974 points per atom before pruning.
Solvation free energies were computed using the 3D RISM method with the Kovalenko-Hirata closure 20 as implemented by Case and coworkers. 21 We also applied the pressure correction of Supplementary reference 22 as implemented by Misin and coworkers. 23 Solvent interaction Lennard-Jones parameters for all atoms were computed from MMFF94 nonbonded parameters between each atom and water oxygen. These were calibrated so that the parameters would be computed correctly by the Lorentz-Berthelot combination rule employed by the 3D RISM model with its SPC water model. These are summarized later as relative solvation free energies for the complexation reaction, Since water that hydrogen bonds to the side-chain amine may affect the distribution of charge in the ring strongly in the b structure (Fig 4, main text), we also computed the binding energies for c structure. For each anion, its complex with structure c was prepared by copying the position of the water in energy-minimized c into the minimized b-anion structure in the same position and orientation relative to the NH2 group. We did not reoptimize these complexes, since re-optimization would result in water positions partially hydrogen bonded to the anions, rather than directly affecting the NH2 group as intended.
Supplementary Table 4 shows the individual component energies used in the calculation, including quantum-mechanical gas-phase binding constants (G 0 ) and the free energies of hydration in pure water. Note that the solvation free energy difference from Clto TcO4is 52 kJ mol -, following the Born trend in Supplementary reference 24. The final three rows tabulate the free energies of anion exchange. The free energy of exchange relative to Cl -(corresponding to Equation 6) is a difference between two half-reactions, (11) ∆ exch = ∆ 0 ( + − ) + ∆∆ solv S + + X -(aq) ∆ exch → S + . X - The trends show that all structures are selective for TcO4over NO3and HSO4 -. We did not compare directly to SO4 2because the difference in net charge would introduce long-range correction terms,