Mechanism unravelling for ultrafast and selective <sup>99</sup>TcO<sub>4</sub><sup>−</sup> uptake by a radiation-resistant cationic covalent organic framework: a combined radiological experiment and molecular dynamics simulation study
Tc is one of the most problematic fission products in the nuclear fuel cycle owing to its large inventory in used nuclear fuel, long half-life, potential radiation hazard, high environmental mobility of its major species 99 TcO 4 À , and its redox-active nature. Ideally, 99 TcO 4 À should be removed at the first stage, when the used fuel rods are dissolved in highly concentrated nitric acid solution, which can substantially reduce its interference with the solvent extraction process through catalytic redox reactions with the key actinides and diminish the chance of discharge into the environment as the volatile species during the waste vitrification process. However, this task cannot be achieved by any of the reported anion-scavenging materials including traditional polymeric anion-exchange resins, inorganic cationic framework materials, and recently developed cationic metal-organic framework materials, because they either are not stable under the extreme conditions of the combined high acidity and strong radiation field or do not possess the required uptake selectivity towards 99 TcO 4 À in the presence of a huge excess of competing anions such as NO 3 À and SO 4 2À . Herein, we present the first study of 99 TcO 4 À removal under extreme conditions by a two-dimensional conjugated cationic covalent organic framework material, SCU-COF-1. This material exhibits ultrahigh acid stability, great resistance towards both large-dose b and g irradiation and unprecedented 99 TcO 4 À uptake capabilities including extremely fast sorption kinetics (sorption equilibrium can be reached within 1 min), ultrahigh uptake capacity (702.4 mg g À1 for the surrogate ReO 4 À at a slightly elevated temperature), and good anion-exchange selectivity towards 99 TcO 4 À . These excellent features endow SCU-COF-1 with the practical capabilities of separating 99 TcO 4 À from both simulant highly acidic fuel reprocessing solutions (3 M nitric acid) and low-activity waste streams at the US legacy nuclear site. The anion-exchange mechanism and the 99 TcO 4 À uptake selectivity are further demonstrated and clearly visualized by the molecular dynamics simulation investigations.
Introduction<!>À<!>Results and discussion<!>À<!>Conclusion<!>Security declaration<!>Reagents and characterizations<!>Synthesis of SCU-COF-1<!>Batch experiments<!>Sorption isotherm experiments<!>Anion-exchange selectivity experiments<!>Computational methods<!>Conflicts of interest
<p>Technetium is the lightest element in the periodic table that is comprised of only unstable isotopes such as 97 Tc, 98 Tc, 99 Tc, 99m Tc, and 101 Tc. 1 Among these, 99m Tc has been extensively utilized in medical imaging and diagnosis, 2 while 99 Tc represents a signicant issue in the nuclear fuel cycle and the environmental system at the legacy nuclear sites. 3 It is estimated that 1990 kg of 99 Tc had been generated from 1943 to 1987 since the nuclear weapon production at Hanford, Washington State, a legacy nuclear site in the United States. 4 With a high ssion yield of 6.06%, about 21 kg of 99 Tc is produced in a 1 GWe reactor each year. 5 According to the year-end total net electrical capacity determined by International Atomic Energy Agency (IAEA), 6 the estimated accumulation of 99 Tc from 1998 to 2017 is 154 539 kg, and this number would continue to rapidly increase due to active nuclear power production and the fast development of nuclear power plants in several countries, especially China. 99 Tc is an extremely long-lived (t 1/2 ¼ 2.13 Â 10 5 a) b-emitting radionuclide (E ¼ 294 keV) that is both chemical toxic and a radiation hazard. 4,7 The inhalation of vapor or dust contaminated with 99 Tc can pose a signicant cancer risk, 5 while 99 Tc can accumulate in the mammary tissue and thyroid aer digestion. 8 99 Tc is predominately present in the nuclear fuel cycle and in aqueous environments as the pertechnetate anion ( 99 TcO 4 À ).</p><p>Unlike its congener MnO 4 À , 99 TcO 4 À is not a strong oxidant, giving rise to its high stability under a variety of environmental conditions. The high symmetry (T d ) and low charge density of 99 TcO 4 À lead to its non-complexing nature and extremely high solubility (11.3 M at 20 C). 9 Therefore, 99 TcO 4 À can facilely migrate in the natural water system and its transportation velocity is not affected by the majority of natural materials. Currently, vitrication followed by geological disposal has been established to be the only acceptable strategy for treating the high levels of nuclear waste in several countries. 10 However, this strategy is not applicable to 99 Tc because it would easily escape from the vitrication glass as the volatile 99 Tc 2 O 7 . 11 This issue along with improper waste management further results in the identied contamination of 99 Tc in the groundwater at a variety of legacy nuclear sites, with 99 Tc being the major radioactivity contributor in the vadose zone. 12 Another signicant issue for 99 Tc is its catalytic redox capability with uranium, neptunium, and plutonium during used fuel reprocessing (e.g. the plutonium-uranium redox extraction (PUREX) process), hindering the precise valence state control of these key actinides, even when 99 Tc is present in very low concentrations. 13 Therefore, it would be highly desirable for 99 TcO 4 À to be rst separated when used nuclear fuel rods are dissolved in concentrated nitric acid prior to the PUREX process. This would be benecial not only for the effective extraction of actinides but also in the elimination of 99 Tc discharge into the environment during the following waste disposal processes. Up to now, there have only been a handful of materials, including inorganic cationic framework materials, 14 polymeric networks and anion-exchange resins, 15 as well as cationic metal-organic frameworks (MOFs), 16 reported to exhibit TcO 4 À separation capability from nuclear waste solutions. Among these, crystalline inorganic cationic framework materials, such as layered double hydroxides (LDHs), 14h,i Y 2 (OH) 5 Cl, 14f and NDTB-1 14a suffer from the clear demerits of poor selectivity, low capacity and slow uptake kinetics. Especially, poor selectivity makes these materials inapplicable because, either in nuclear waste solutions or natural water systems, coexisting anions such as NO 3 À , SO 4 2À , CO 3 2À , and PO 4 3À that can induce anionexchange competition with TcO 4 À are all present in 100-to 10 000-fold of excess. Commercially available polymeric anionexchange resins show a notable advantage in 99 TcO 4 À uptake selectivity, but their poor radiation resistance impedes their practical applications in the radiological eld especially in used fuel repossessing, where strong radiation elds (high uxes of a, g, b, and neutron irradiations) are present at extremely high dose rates. It was reported that the 99 TcO 4 À uptake capacity would gradually decrease as the receiving radiation doses increase. 15f,17 This critical issue is further amplied given that the 99 TcO 4 À uptake kinetics by these materials are rather slow, leading to a signicantly higher dosage received during the anion-exchange process. The recently developed systems based on cationic MOFs do offer solutions to the aforementioned drawbacks and nd extensive application potentials in the remediation of 99 TcO 4 À from contaminated water systems under relatively neutral pHs. 16 However, none of these cationic MOFs can survive the highly acidic conditions (e.g. 3 M nitric acid) that are required for used fuel reprocessing.</p><p>Covalent organic frameworks (COFs) are a class of emerging crystalline polymers with periodic structures and well-dened nanopores. 18 Importantly, all these structural features are predictable and highly tunable and can be tailored for specic applications, similar to the MOF system. Different from the common COFs with neutral skeletons, ionic COFs (iCOFs) are a subgroup of COFs with charged linkages in the framework or attachments in the side chain, which would promote particular properties such as selective ion-exchange, compared with neutral COFs. 19 We initially proposed that the system of iCOFs may overcome the challenge for TcO 4</p><p>À separation under the combined extreme conditions of high acidity, strong radiation eld, and high ionic strength for several reasons. First, compared with the majority of MOF materials, COFs exhibit clear elevations in the stabilities under different types of extreme conditions such as high acidity, alkalinity, and salinity, owing to the increased robustness of the covalent bond-based linkages for building the structure. 18d,e,g Second, many COFs are highly conjugated and can substantially stabilize the radicalbased intermediates generated by radiation, a merit not possessed by traditional polymeric anion-exchange resins. This may lead to excellent radiation resistance for these COF materials, one of the most important prerequisites for nuclear applications. Third, the porous nature and ordered channels may aid in the rapid transportation of 99 TcO 4 À in the COF structure, which is advantageous for accelerating the anionexchange process, in comparison to amorphous polymeric materials. Finally, the local hydrophobic environment in the COFs is expected to enhance the uptake selectivity for relatively hydrophobic 99 TcO 4</p><!><p>, compared to coexisting anions with higher charge densities, a design rule that has already been applied in selective polymeric anion-exchange resins. 15a Recently, two 3D cationic COF materials 3D-ICOF-1 and 3D-ICOF-2 were tested for the removal of MnO 4</p><p>À under neutral conditions by Qiu et al.,19f which initially suggests the promise of 99 TcO 4 À remediation using these types of materials, although the capability may be overestimated owing to the partial chemical reduction of MnO 4 À during the sorption experiment.</p><p>Despite that, to the best of our knowledge, there has been no study on the separation and remediation of 99 TcO 4 À by COFs through the direct radiological test. iCOFs are in general very challenging to build because the charge repulsion between the adjacent building blocks impedes their ordered connection, oen leading to poor crystallinity or an amorphous structure. 19g Herein, we provide a new strategy to constitute cationic COFs with good crystallinity based on positively charged linkers and neutral nodes via irreversible tautomerism. The synthesized two-dimensional viologen-based iCOF (denominated as SCU-COF-1) displays an unusual ABC-staggered stacking model and unprecedented 99 TcO 4</p><p>À uptake capability including large uptake capability, fast sorption kinetics, high anion-exchange selectivity towards 99 TcO 4 À , great acid stability, and decent radiation resistance. These excellent features endow SCU-COF-1 with the promising ability to remove 99 TcO 4 À from 3 M nitric acid solution, a key step for potentially establishing a new process aiming at solving the "technetium issue" in the nuclear fuel cycle.</p><!><p>SCU-COF-1 is synthesized from an aminated viologen and commercially available 2,4,6-triformylphloroglucinol (Tp). Aer acid-catalyzed solvothermal synthesis and tautomerization from the enol form to the keto form, 20 the dark brown crystalline product is obtained (Fig. 1a). The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of SCU-COF-1 show the layered structure of the material (Fig. S1 and S2 †).</p><p>The powder X-ray diffraction (PXRD) pattern of SCU-COF-1 (Fig. 1e) identies an ordered structure, where a strong low angle peak at 2q ¼ 4.02 is observed. Two other prominent peaks at 8.23 and 25.59 were also identied. The peak at 4.02 corresponds to the (100) plane of the structure. The broad feature at 8.23 is recognized as a superposition of peaks at 7.85 and 8.60 , which are attributed to the (110) and ( 200) planes, respectively. Additionally, the presence of another broad peak at 25.59 , which should be assigned to the (001) plane, suggests highly ordered p-p stacking existing between the adjacent layers and the good planarity of the 2D layers. 21 To obtain the simulated crystal structure of SCU-COF-1, theoretical simulations were conducted using Materials Studio. Three stacking models were generated and optimized: the eclipsed stacking (AA-stacking) model, the AB-staggered stacking model, and the ABC-staggered stacking model (see ESI † for details). The simulated PXRD patterns corresponding to the three different models are shown in Fig. 1e for comparison. The experimental pattern matches well with the simulated pattern derived from the unique ABC-stacking model. The features of stacking are shown in the top and side views of SCU-COF-1 (Fig. 1d). In order to distinguish the three distinct layers more clearly, we marked out layer A, layer B, and layer C with different colours in a periodic unit cell, respectively (Fig. 1b). Aer DFT optimization and Pawley renement with good agreement Overall, the spatial structure of SCU-COF-1 can be described as parts of two reacted ligands being connected sequentially into a at cationic 2D layer with a large hexagonal channel ($4.34 nm). These cationic layers are further stacked via p-p interaction in the ABC-staggered manner, which in fact minimizes the charge repulsion between the adjacent layers. A series of shrunken triangular channels ($1.44 nm) along the c-axis can be still observed aer the stacking. We further simulated the electrostatic potential (ESP) distribution of the cationic layer (Fig. 1c). The results show that the regions close to the N atoms in the pyridine rings provide the most positive ESP (blue areas in the hexatomic ring), which are appropriate for accommodating the charge-balancing Cl À anion, whose existence can be identied through EDS analysis (Fig. S3 †). In addition, the positive charges are delocalized on the pyridine ring, which is expected to play a leading role in the anion-exchange process.</p><p>The porosity of SCU-COF-1 is probed by the nitrogen gas adsorption isotherm measured at 77 K (Fig. 2a, red dots and lines). The calculated Brunauer-Emmett-Teller (BET) surface area of 22.98 m 2 g À1 indicates moderate porosity, which partially originates from the staggered stacking of layers that may hinder the efficient transportation of nitrogen in the structure. A similar phenomenon was observed in other 2D COF materials with the staggered stacking. 19h,j,21 On the other hand, the presence of charge-balancing Cl À in the pore may further block the entrance of guest gas molecules. However, this relatively small surface area is not an issue for rapid anionexchange as discussed below. In addition, the nonlocal density functional theory (NLDFT) analysis gives an average pore width of 1.37 nm, which is in good agreement with the simulated crystal structure of SCU-COF-1 (Fig. 2b).</p><p>To elevate the thermal stability of SCU-COF-1, thermogravimetric analysis (TGA) was measured from 25 C to 900 C in N 2 atmosphere. The weight loss before 100 C could be assigned to the loss of lattice water, and SCU-COF-1 was thermally stable up to 225 C (Fig. S4 †). The chemical stabilities of SCU-COF-1 were investigated by dispersing fresh samples in various solvents or acids including H 2 O, DMF, THF, aqueous HCl (1 M), aqueous HNO 3 (1 M and 3 M), and aqueous NaOH (1 M) solutions for 48 h. In various solvents (H 2 O, DMF, THF) or 1 M HCl, the PXRD patterns were almost identical to that of the original sample, indicating that SCU-COF-1 can retain its crystalline structure in these solvents. Even in 1 M HNO 3 and 3 M HNO 3 , the crystallinity of SCU-COF-1 slightly decreased but maintained the ordered structure (Fig. 2d). The unchanged characteristic vibrational peaks in the IR spectra (Fig. 2c) and a BET surface area of 18.25 m 2 g À1 (Fig. 2a, blue dots and lines) for samples treated with 3 M HNO 3 further underscored the acid stabilities of SCU-COF-1. Note that the slightly reduced surface area may partially originate from the larger ionic radii of NO 3 À exchanged into the structure compared with the original Cl À , which may further block the transportation pathway of gas molecules. SCU-COF-1 is unstable in highly basic solutions (1 M NaOH). This can be explained by the presence of the cationic pyridine hexatomic rings, which are easily attacked by the hydroxyl as the nucleophile in solution. 22 More impressively, the highly conjugated structure of SCU-COF-1 protects the functional cationic pyridinium rings from high doses (200 kGy, 400 kGy, and 600 kGy) of b-irradiation provided by an electron accelerator (1.5  MeV, dose rate of 2250 kGy h À1 ) or g-irradiation from a 60 Co source (92.42 PBq, dose rate of 3.125 kGy h À1 ). The PXRD patterns of the irradiated samples conrm that SCU-COF-1 fully retains its crystallinity aer irradiation (Fig. 2e and S5 †). In addition, the nearly unchanged IR spectra (Fig. 2c) and BET surface areas aer being exposed to b-(20.01 m 2 g À1 ) or g-(19.00 m 2 g À1 ) irradiations (Fig. 2a, green and purple dots) of SCU-COF-1 synergistically demonstrate its excellent radiation resistance. Moreover, the radiation resistance can be further conrmed by the anion-exchange capacity experiment using the irradiated sample (see discussion below). In the Fourier-transform infrared (FT-IR) spectrum, the strong peaks at 1580 and 1263 cm À1 originate from the stretching of C]C and C-N, respectively, conrming the existence of the cis-keto form (Fig. 2c). Furthermore, the 13 C nuclear magnetic resonance (NMR) spectrum exhibits four characteristic peaks. The signal at $185 ppm, which is assigned as the C atoms of ketone, demonstrates the formation of the cis-keto structure of SCU-COF-1 (Fig. 2f). 20b</p><p>To investigate the sorption kinetics of SCU-COF-1 towards 99 TcO 4</p><p>À or ReO 4 À as the non-radioactive surrogate of 99 TcO 4 À owing to their almost identical charge densities and therefore similar anion-exchange properties, 4 20 mg of solid materials was added into 20 mL of aqueous solution containing 28 ppm ReO 4 À or 14 ppm 99 TcO 4 À at 300 K. As shown in Fig. 3a, the concentrations of 99 TcO 4 À as a function of contact time were measured by its characteristic peak at 290 nm in the UV-vis spectra. In addition, the kinetics curve can be also conrmed by measuring the time-dependent radioactivity of the solution through a liquid scintillation counting (LSC) technique (Fig. 3b). The results show that SCU-COF-1 can almost quantitatively remove 99 TcO 4 À from solution within an extremely short of time of $30 s, which is, in fact, the shortest time required for experimental operation. Therefore, it is difficult to establish a sorption kinetics model, as the rst data point has already reached the sorption equilibrium. The results of ReO 4 À removal kinetics experiment are identical with those of 99 TcO 4 À (Fig. 3c).</p><p>This ultrafast anion-exchange kinetics sets SCU-COF-1 as the record-holding material along with the recently reported SCU-CPN-1 and is signicantly faster than those of other types of anion-exchange materials (Table S1 †). For inorganic cationic framework materials and commercial polymeric anionexchange resins, the anion-exchange equilibrium would require at least hours (up to days) to establish. For example, the rst purely inorganic 3D cationic framework NDTB- We propose that this excellent feature is a result of ordered channels in the structure of SCU-COF-1 that allows for the efficient transportation of guest anions and the hydrophobic pore that leads to the fast exchange-out of relatively hydrophilic Cl À back into the aqueous solution (more profound discussions are provided in the section of the sorption mechanism). The advantage in anionexchange kinetics is the benet of the rapid remediation of anionic radionuclide contamination during an emergency event as well as reducing the received radiation dosage when dealing with highly radioactive systems such as used fuel reprocessing to minimize the radiation-induced degradation of materials. Moreover, the distribution coefficient (K d ) of SCU-COF-1 calculated from the ReO 4 À sorption kinetics experiment is 3.89 Â 10 5 mL g À1 , which is higher than most reported materials (Table S2 †), exhibiting a promising removal depth for the remediation task. FT-IR (Fig. 2c) and EDS mapping measurements (Fig. S3 †) also conrm the successful substitution of Cl À by ReO 4 À in SCU-COF-1. The sorption isotherm experiment was carried out at 300 K to gain insight into the anion-exchange capacity of SCU-COF-1 for trapping ReO 4 À / 99 TcO 4 À . From the curve, the saturated sorption capacity calculated based on the Langmuir model is 367.6 mg ReO 4 À per g sorbents (Fig. 3d and Table S3 †). This value is notably lower than the theoretic sorption capacity (872.7 mg g À1 ) calculated by assuming Cl À anions are completely substituted, suggesting the presence of a kinetic barrier for the complete anion-exchange, which can be conrmed by the theory study discussed below. We therefore checked the sorption capacity at an elevated temperature of 373 K. Upon heating and reuxing for 24 h, the maximum ReO 4 À uptake capacity reached 702.4 mg g À1 , as the actual experimental value, being quite close to the theoretic value. This sets SCU-COF-1 as a signicant anion-exchange material with one of the highest reported exchange capacities. In comparison with inorganic materials such as Yb 3 O(OH) 6 Cl (48.6 mg g À1 ), 14d,16c NDTB-1 (49.4 mg g À1 ) 14a,16c and LDH (130.2 mg g À1 ), 14h,15a polymeric anion-exchange resins such as Purolite A532E (446 mg g À1 ), 16a,c and cationic MOFs including UiO-66-NH 3 + (159 mg g À1 ), 16e SCU-100 (541 mg g À1 ), 16c and SCU-101 (247 mg g À1 ), 16b SCU-COF-1 exhibits a notably larger sorption capacity towards ReO 4 À .</p><p>Since anions such as NO selectivity for ReO 4 À removal (Fig. 3e). In competing with different anions, SCU-COF-1 exhibits excellent uptake selectivity, and the removal percentages remain in the range of 85% to 99%. Even in the presence of a high concentration of NO 3 À (NO 3 À : ReO 4 À ¼ 100 : 1, molar ratio), the removal percentage of ReO 4 À still reached 60% (Fig. 3f). The high selectivity for ReO 4 À is likely owing to the hydrophobic nature of the COF skeleton, which provides a remarkable affinity for relatively hydrophobic anions with lower charge densities. 15c Some commercial resins, for example SuperLig-639, 24 with a high uptake capacity towards 99 S4 †), which contains far higher amounts of NO 3 À , NO 2 À and Cl À (at least 300-fold in excess for each anion) than 99 TcO 4</p><!><p>. The results show that SCU-COF-1 can remove 20.9% and 62.8% of 99 TcO 4</p><p>À at a solid/liquid ratio of 1 and 5, respectively (Table S5 †).</p><p>To probe the uptake capability of SCU-COF-1 in acidic and alkaline conditions, solutions containing 400 ppm ReO 4 À in various pH values were used in the experiments. The uptake capacities of SCU-COF-1 were almost unchanged through a wide pH range from 4 to 10 (Fig. 4a). More signicantly, adsorption experiments in extreme conditions were conducted to estimate its potential application in highly acidic conditions that is relevant to used fuel reprocessing. We further prepared a 1 M Under the conditions of 3 M HNO 3 , SCU-COF-1 can still remove 52.3% of ReO 4 À at a solid-liquid ratio of 1 : 40 in the solution (Fig. 4c). These signicant results indicate the SCU-COF-1 are indeed feasible for 99 Tc separation at the rst stage of used fuel reprocessing prior to the PUREX process. Aer treatment in different doses of bor g-irradiation, the uptake for ReO 4</p><p>À by SCU-COF-1 is uninuenced. The removal capacity at room temperature for ReO 4 À was completely unchanged aer irradiation (Fig. 4d and e). In comparison, two commercial resins Purolite A532E and A530E have reductions of 9% and 18% of sorption capacities aer exposure to 600 kGy birradiation (or g-irradiation), respectively. 15c Additionally, SCU-COF-1 is also a reversible anion-exchange material for multiple sorption/desorption trials. More than 95% of ReO 4 À could be desorbed back to solution aer treatment with 3 M NaCl solution at 300 K (Table S6 †). Even aer 4 cycles of sorption/ desorption, the sorption performance of SCU-COF-1 was not affected (Fig. 4f). More impressively, the COF can keep almost the same regeneration property aer removing ReO 4 À in 3 M HNO 3 (Fig. S6 †), which can signicantly reduce the cost for practical applications.</p><p>We further explored the adsorption dynamics of 99 TcO 4 À anions into the structure of SCU-COF-1 and the underlying uptake mechanism using all-atom molecular dynamics (MD) simulations (Fig. 5a). Impressively, aer placing the solid structure of SCU-COF-1 into the aqueous 99 TcO 4 À solution, a large number of 99 TcO 4 À anions initially diffused in aqueous solution were rapidly sorbed into the interior of SCU-COF-1. A video revealing the anion-exchange process is provided as a ESI le. † During this process, many Cl À anions originally dwelling in the pores of SCU-COF-1 were desorbed from the solid structure (Fig. 5b, also see the Movie S1 in ESI † for more intuitive illustration). To quantitatively describe this dynamics process, the adsorption rate of 99 TcO 4 À anions from the external solution environment (Fig. 5c red curve) and the residence rate of original Cl À anions in SCU-COF-1 (Fig. 5c green curve) along with the simulation time were calculated. As shown in Fig. 5c, a very clear crossover occurs at $26 ns. Aer this time point, the amount of 99 TcO 4 À anions entering SCU-COF-1 exceeds the amount of Cl À anions that still remain in SCU-COF-1. Aer $40 ns, a dynamic equilibrium is reached, i.e., the 99 TcO 4 À adsorption rate is nearly stabilized at a constant value of (60.6 AE 2.3)%. Meanwhile, the reserving rate of Cl À anions is nearly unchanged at (34.1 AE 2.0)%. In addition, another control simulation in pure water demonstrates that in the absence of 99 TcO 4</p><p>, nearly (87.2 AE 1.4)% of Cl À anions could be retained in SCU-COF-1 throughout the full simulation period (Fig. 5c blue curve). This suggests that the sorption of 99 TcO 4</p><p>À anions initiates and further complicates the following anion-exchange process.</p><p>To uncover the driving forces for the anion-exchange process, the time evolution of the nonbonded interaction energies of COF-99 TcO 4 À , COF-Cl À , and 99 TcO 4 À -Cl À are calculated and analyzed, and they were further deconvoluted into the contributions from the electrostatic and van der Waals (vdW) interactions as well (Fig. 5d). During the main 99 TcO 4</p><p>À uptake process (<40 ns; aer 40 ns, simulations are converged), the nonbonded interaction energy between 99 TcO 4 À anions and SCU-COF-1 sharply decreases by $13 000 kJ mol À1 (decreases from 0 to À13 000 kJ mol À1 ; Fig. 5d  suggest that the strong direct nonbonded interaction (particularly the electrostatic interactions) between 99 TcO 4 À anions and SCU-COF-1 intensively drives this vigorous 99 TcO 4 À uptake process. During this process, the 99 TcO 4 À anions gained considerable energy, which can overcompensate for the energetic penalty induced by the desorption of Cl À anions. Moreover, the direct electrostatic repulsive interaction between 99 TcO 4 À anions and Cl À anions further promotes the desorption of Cl À anions. To gain a more microscopic insight into the substituting procedure of Cl À anions by 99 TcO 4 À anions, we further compared the binding free energy of a single 99 TcO 4 À anion to the binding site that is most energetic favorable for the Cl À anion. This binding site is determined by the analysis of the Cl À anions residing probability distribution in the control simulation (for a better sampling). It was demonstrated that Cl À ions prefer to be located near the pyridine ring on the inner wall of SCU-COF-1 (Fig. 6a), where the most positive charges are distributed (Fig. 1f). The potential of mean force (PMF) data  demonstrates that the binding free energies are À9.9 kJ mol À1 and À13.7 kJ mol À1 for the Cl À anion and 99 TcO 4</p><p>À anion, respectively, indicating that the 99 TcO 4 À anion can successfully outcompete the Cl À anion in binding behavior (Fig. 6b). In other words, when one Cl À anion (at it most energetic favorable binding site) is exchanged with one 99 TcO 4 À anion, it can gain 3.8 kJ mol À1 of energy total, which can be attributed to the preferential binding of 99 TcO 4 À anions to SCU-COF-1 over Cl À anions. Therefore, an entire picture for the anion-exchange process can be revealed: on one hand, the much stronger binding free energy between the 99 TcO 4 À anion and SCU-COF-1 leads to successful substitution of Cl À by taking over the anion binding site in SCU-COF-1 as time progresses; on the other hand, the occupation of the 99 TcO 4 À anion signicantly weakens the direct interactions between the SCU-COF-1 and Cl À anion and gradually excludes Cl À anions out of the SCU-COF-1 due to the direct electrostatic repulsive interactions between the 99 TcO 4 À anion and Cl À anion.</p><!><p>In summary, we herein present the rst study on 99 TcO 4 À remediation and separation using a cationic COF material (SCU-COF-1), showing promises in material stabilities under the extreme conditions of concentrated acidic solutions and strong radiation elds and uptake capabilities including sorption kinetics, capacity, selectivity, and reversibility. Another attractive advantage for the cationic COF material is the meeting of the CHON-only rule, 25 which gives rise to potential combustibility that may be required for certain applications during nuclear waste management. The combination of these advantages cannot be achieved by the reported anion-exchange materials such as inorganic cationic framework materials, polymeric anion-exchange resins, and cationic MOF materials.</p><p>Given that these merits are now integrated into a single material, the possibility for solving the "technetium issue" in the nuclear fuel cycle becomes visible. Furthermore, the overcoming of other challenges of radionuclide partitioning during nuclear waste disposal such as trivalent lanthanide/actinide separation and caesium strontium uptake using highly robust and designable COF materials can be expected in the future.</p><!><p>Caution! 99 Tc is a b-emitter with a long half-life (t 1/2 ¼ 2.13 Â 10 5 years) and presents a signicant health risk once inhaled or digested. All experiments involving 99 Tc must be carried out in a licensed laboratory that is dedicated to radiological studies.</p><!><p>4,4-Bipyridine and 1-chloro-2,4-dinitrobenzene were provided by Aladdin Industrial Co., Ltd. 1,4-Diaminobenzene and 2,4,6triformylphloroglucinol (Tp) were purchased from Adamas Reagent Co., Ltd and Beijing HWRK Chem Co., Ltd, respectively. All solvents were purchased from Chinasun Specialty Products Co., LTD. [4,4 0 -Bipyridine]-1,1 0 -diium dichloride (BDB) and viologen-NH 2 was synthesized according to the reference. 26 NaReO 4 , HCl, NaOH, and HNO 3 were analytically pure and used. 99 TcO 4 À stock solutions were prepared by dissolving a desired amount of NH 4 99 TcO 4 (99%) in deionized water. The 1 H and 13 C nuclear magnetic resonance (NMR) spectra were acquired on Avance III HD-400 and Abance III WB-400 instruments, respectively. Fourier-transform infrared spectroscopy (FT-IR) spectra in the range of 4000-400 cm À1 region were carried out on a Thermo Nicolet iS50 spectrometer. Synthesis of viologen-NH 2 BDB (500 mg, 0.89 mmol) and 1,4-diaminobenzene (289 mg, 2.67 mmol) were dissolved in 250 mL of absolute ethanol in a ask. The mixture was next reuxed for 72 h. The reaction process was followed by thin-layer chromatography (TLC). The solution was then concentrated to 100 mL using a rotary evaporator. The mixture was poured into 750 mL of THF, and aer 2.5 h the dark brown solid was collected by suction ltration and washed with THF (10 mL Â 5). The precipitate was dried in  26 (d, 4H, 4.0 Hz), 8.68 (d,   4H, 2.0 Hz), 7.60 (d, 4H, 8.0 Hz), 6.05 (d, 4H, 4.0 Hz) (Scheme S1 and Fig. S8 †).</p><!><p>To a 10 mL Pyrex tube containing viologen-NH 2 (18.51 mg, 0.045 mmol) and Tp (6.30 mg, 0.03 mmol), a mixture of 1.9 mL of o-DCB, 0.1 mL of BuOH, and 0.2 mL of acetic-acid catalyst (19 : 1 : 2, by vol.) were added. Then, the tube was degassed by three-pump-thaw for three cycles. The sealed tube was heated at 120 C for 6 d (Fig. 1a). The precipitate was collected aer ltration and washed with THF (5 mL Â 3) and absolute methanol (5 mL Â 5) to ensure the removal of the residual oligomers and unreacted ligands. The dark brown SCU-COF-1 was gained aer drying in vacuum at 40 C for 5 h.</p><!><p>The sorption experiments for ReO 4 À and 99 TcO 4 À were rst performed at 300 K according to the batch sorption method with a solid/liquid ratio of 1 g L À1 . Typically, 5 mg of solid material (SCU-COF-1) was added to a stock solution of ReO 4 -À / 99 TcO 4 À and shaken on an automatic shaker. Aer a specic contact time, the mixture was separated a 0.22 mm nylon membrane lter. The concentrations of ReO</p><!><p>The sorption isotherm studies for ReO 4 À were performed using a series of stock solutions containing various ReO 4 À concentrations (28 to 1000 ppm) with the same ratio of solid-liquid. The resulting mixture was shaken for 14 h to reach the adsorption equilibrium (24 h for samples in heating sorption experiments) and then separated by a 0.22 mm nylon membrane lter. To overcome the kinetic barrier for the complete exchange process, an additional sorption experiment at an elevated temperature was conducted by adding 5 mg of SCU-COF-1 to a 5 mL solution containing 2000 ppm ReO</p><!><p>The effect of NO  Stream) were conducted in a similar method with determined conditions according to reported protocol. 14a 5 mg of COF materials were added to a 5 mL prepared solution (solid/liquid ¼ 1), and 10 mg of materials were added to a 2 mL prepared solution (solid/liquid ¼ 5). Aer being shaken for 14 h to ensure reaching equilibrium, the mixture was separated by a 0.22 mm nylon membrane lter for ICP analysis. . Aer being shaken for 14 h, the suspension was separated by a 0.22 mm nylon membrane lter for ICP analysis.</p><p>Regeneration study 130 mg of SCU-COF-1 material sorbed with 130 mL of 28 ppm ReO 4 À (at pH ¼ 7) was immersed into a 1 M NaCl solution, followed by washing with water. Multiple sorption/desorption trials of up to 4 runs were performed either in a pH 7 water solution or in a 3 M nitric acid solution.</p><!><p>The SCU-COF-1 model membrane was parallelly stacked by 15 unit cells (sheets) that were obtained from our rst-principle calculations based on the density functional theory (DFT) (Fig. S11 †), with an initial inter-sheet distance of 0.35 nm. Each sheet consists of 336 carbon atoms, 48 nitrogen atoms, 24 oxygen atoms, 240 hydrogen atoms, and 24 anions Cl À , which gives a total of 672 atoms. The partial charges for COF atoms were obtained from DFT calculations on the unit cell of COF. The Lennard-Jones (L-J) parameters for the COF atoms were obtained from all-atom optimized potentials for liquid simulations (OPLS-AA) force eld (FF). 27 The Lorentz-Berthelot combining rules were used for calculating cross L-J interaction parameters. Many theoretical simulation results have demonstrated good agreement with those in experimental studies. 28 The reasonable accuracy of OPLS-AA FF was also veried in characterizing microscopic insights for the adsorption or diffusion of a wide range of contaminants in various porous materials. 28a,b,29 The FF parameters of the 99 TcO 4 À anions were taken from a previous simulation study by 28d in which the hydration free energies, hydrated radii, and diffusion coefficients of 99 TcO anions, 360 NH 4 + (for neutralizing the simulation system), and 65 646 water molecules, thus giving a total of 210 618 atoms. As a control, the COF was also solvated into a pure water box with a size of (8.71 nm Â 7.54 nm Â 17.61 nm) containing 70 078 water molecules, which gave a total of 220 314 atoms, and served as the control system to check the residence of Cl À . All MD simulations were performed with the GROMACS soware package 30 (version 4.6.7). VMD 31 soware (version 1.9.3) was applied for trajectory visualization and analysis. The SPC/E water model 32 was used for water. In the production run, a time step of 2.0 fs was used, and the data were collected every 10 ps. For each system, three independent trajectories of 50 ns were generated, which resulted in a total aggregate simulation time of 300 ns. All production runs were under isothermalisochoric (NVT) ensemble; the temperature was maintained at 300 K using a y-rescale thermostat. 33 The particle-mesh Ewald (PME) method 34 was used for long-range electrostatic interactions. A cutoff distance of 1.2 nm was used for the calculation of van der Waals (vdW) interactions. All solute bonds between heavy atoms and hydrogens were constrained with the LINCS algorithm. 35 Periodic boundary conditions were applied in all directions, and the COF were frozen throughout the simulation process. We used umbrella-sampling simulations to investigate the potential of mean force (PMF) of a single 99 TcO 4</p><p>À and Cl À .</p><p>The reaction path for the PMF calculation was dened by pulling center of mass of the anion ( 99 TcO 4 À or Cl À ) from the most favourable binding site of Cl À along the upper surface of COF membrane to the bulk water (from $0.0 nm to $1.2 nm of the COF upper surface) with a harmonic force of 2000 kJ mol À1 nm À2 . The reaction coordinate was divided into 12 windows, with each window simulated for 3 ns and thus accumulating a total simulation of 36 ns. The weighted histogram analysis method 36 (WHAM) was applied to calculate the free energy. All PMF proles were normalized to zero at a distance of 1.2 nm.</p><!><p>The authors declare no competing nancial interests.</p>