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. 2021 Aug 13;7(8):1441–1450. doi: 10.1021/acscentsci.1c00847

Task-Specific Tailored Cationic Polymeric Network with High Base-Resistance for Unprecedented 99TcO4– Cleanup from Alkaline Nuclear Waste

Jie Li 1, Baoyu Li 1, Nannan Shen 1, Lixi Chen 1, Qi Guo 1, Long Chen 1, Linwei He 1, Xing Dai 1, Zhifang Chai 1, Shuao Wang 1,*
PMCID: PMC8393213  PMID: 34471688

Abstract

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Direct removal of 99TcO4– from alkaline nuclear waste is desirable because of the nuclear waste management and environmental protection relevant to nuclear energy but is yet to be achieved given that combined features of decent base-resistance and high uptake selectivity toward anions with low charge density have not been integrated into a single anion-exchange material. Herein, we proposed a strategy overcoming these challenges by rationally modifying the imidazolium unit of a cationic polymeric network (SCU-CPN-4) with bulky alkyl groups avoiding its ring-opening reaction induced by OH– because of the steric hindrance effect. This significantly improves not only the base-resistance but also the affinity toward TcO4– as a result of enhanced hydrophobicity, compared to other existing anion-exchange materials. More importantly, SCU-CPN-4 exhibits record high uptake selectivity, fast sorption kinetics, sufficient robustness, and promising reusability for removing 99TcO4– from the simulated high-level waste stream at the U.S. Savannah River Site, a typical alkaline nuclear waste, in both batch experiment and dynamic column separation test for the first time.

Short abstract

A steric-protected cationic polymeric network with excellent base-stability is designed to efficiently remove 99TcO4− from highly alkaline nuclear wastes in a dynamic column separation experiment.

Introduction

The advanced nuclear fuel cycle has been considered a critical requirement for the sustainable development of nuclear energy. To date, PUREX (plutonium uranium reduction extraction) process is the most used technology that has been employed for used fuel reprocessing in commercial plants. However, it still suffers from some drawbacks, for example, the flammability of the organic solvents, the occurrence of radiation-induced solvent degradation, high construction costs, and production of considerable amounts of radioactive wastes.1,2 Therefore, new reprocessing approaches that enable the separation of fission products from actinides in a more environment-friendly and cost-effective manner should be further investigated. A new conceptual process (carbonate extraction, CARBEX) that aims to reprocess used nuclear fuel using high-alkaline carbonate media with oxidizing agents (i.e., H2O2) is considered as an alternative way of PUREX in nuclear fuel management.3−5 Additionally, unlike the acidic stream in PUREX process, the alkaline nature of CARBEX process makes it a good choice for the management of alkaline high-level radioactive waste (HLW). On the other hand, it has been estimated that ∼18 000 m3 of alkaline HLW are stored in Mayak Production Association1 and millions of gallons of alkaline HLW are stored in Hanford Site, Washington State, and Savannah River Site (SRS), South Carolina, U.S.A.6 most of which are leftover by the cold war and are still stored in underground tanks awaiting pretreatment and safe disposal.7,8 A crucial challenge to conquer is how to efficiently separate fission products under highly alkaline conditions.

99Tc mainly presents as a soluble pertechnetate anion (99TcO4–) in aerobic conditions because of its noncomplexing nature and low charge density.7,8 This makes the depth removal of 99TcO4– difficult by the precipitation method. In addition, 99TcO4– can easily migrate into the environment via groundwater during long-term storage.9−11 Moreover, the volatile nature of Tc(VII) complexes brings higher risk of leakage during waste vitrification. In fact, Tc-99 has been leaked to the subsurface environment of several HLW storage sites, resulting in serious contamination of underground water,11 seawater,12,13 and rivers.14,15 Thus, it is highly desirable to seek an effective strategy for 99TcO4– removal from highly alkaline conditions. However, this task still represents a challenge given the harsh conditions of strong ionizing fields (β, γ, neutron irradiations, and so forth), high salinity, and super alkalinity in alkaline nuclear wastes.

Up to now, a series of cationic materials have been evaluated for 99TcO4– removal.16−21 Cationic inorganic materials, such as layered double hydroxides (LDHs),22 Yb3O(OH)6Cl,23 and NDTB-1,24,25 have the advantages of easy preparation and low costs but possess low capture capacity and poor selectivity toward 99TcO4–, which hinders their applications in actual alkaline nuclear waste streams since the concentrations of coexisting competing anions (NO3–, SO42–, CO32–, and OH–) are several orders of magnitude higher than 99TcO4–. Commercially available resins (Puroilte A530E and A532E) exhibit excellent sorption selectivity toward 99TcO4–,26 but the slow sorption kinetics and poor radiation resistance are unavoidable issues. Cationic metal–organic frameworks (MOFs) with exchangeable anions have been widely investigated in the capture of 99TcO4– recently by virtue of their high radiation resistance and excellent sorption selectivity,27−31 but the relatively weak coordination bonds perform poorly under highly alkaline conditions, thus resulting in the collapse of the cationic framework. Therefore, there is a crucial need for the development of alkaline-stable sorbent for 99TcO4– removal in super alkaline nuclear wastes.

The recently developed cationic polymeric networks (CPNs) with cationic functional quaternary ammonium groups (QAs) have been proposed as a promising material for TcO4– remediation in HLWs.32−35 The versatile functionalization of CPNs enables task-specific optimization at the molecular level to design adsorbents for envisioned functions.36,37 With ordered porosity and long-range frameworks, CPNs share similar virtues of MOFs including fast sorption kinetics and excellent sorption selectivity. More importantly, compared with MOFs, the strong covalent bonds and the nature of no heavy metal ions render CPNs a clear superiority in hydrolytic stability and sorption capacity. In addition, large conjugated modules in CPNs stabilize radical intermediates generated during the irradiation, resulting in superior anti-irradiation stability over traditional anion-exchange resins. Those superior properties make CPNs the most promising materials for the management of acidic HLWs. However, CPNs also suffer from poor alkaline stability since the cationic QAs can be easily destroyed by OH–.38−40 The rational design of CPNs conjugating with high alkaline resistance is therefore a key issue.

Inspired by the recent work that installing adjacent bulky groups at C2, C4, and C5 position of imidazolium moiety can shield the nucleophilic attack from OH–,41−43 we envision that this approach matches the goal of improving the alkaline resistance of CPNs and can be extended to the field of alkaline nuclear waste management. Along with this idea, a new alkaline-stable imidazolium-based CPN, namely SCU-CPN-4 (SCU = Soochow University), was constructed by the installation of bulky groups at N1, C2, N3, C4, and C5 position of imidazolium. This elaborately tailored SCU-CPN-4 not only reserves all the virtues of reported cationic CPNs in 99TcO4– removal including excellent radiation resistance, fast sorption kinetics, high sorption capacity, and excellent selectivity but also substantially overcomes the disadvantage of poor alkaline stability, meeting all the qualifications required in remediation of 99TcO4– from the actual alkaline nuclear waste.

Results and Discussion

To improve the alkaline stability of the sorbent for 99TcO4– removal in alkaline HLW streams, an alkaline-stable cationic imidazole group was designed through introducing several bulky groups to N1, C2, N3, C4, and C5 positions of imidazole to enhance alkaline resistance by steric hindrance protection (Figure 1a and Figure S1). To improve the radiation stability, conjugated phenyl groups are used to substitute the H atoms on C2, C4, and C5 positions. Besides, the total substitution around imidazolium parts by bulky groups also highly increases the overall hydrophobicity of CPN, which is beneficial to the excellent selectivity to 99TcO4– with relatively low hydration energy. As shown in Figure 1b, a 2,4,5-trisubstituted imidazole-based polymeric network (SCU-PN) was constructed by the polycondensation of 1,3,5-tris(p-formylphenyl)benzene (TFP) with 1,4-bisbenzil based on a mechanism of multicomponent reaction of imidazole.44 Subsequent deprotonation and quaternization reactions with methyl iodide were carried out at imidazole moieties successively to introduce ion-exchange sites, yielding the tailored 1,2,3,4,5-pentasubstituted cationic polymeric network (SCU-CPN-4-I). To further circumvent the perniciousness of I– to the environment, ion exchange was performed by immersing SCU-CPN-4-I powders into saturated sodium chloride solution three times to completely swap out toxic I– by Cl–, finally generating SCU-CPN-4-Cl (SCU-CPN-4). As probed by scanning electron microscope (SEM) and transmission electron microscope (TEM), no noticeable morphological changes occurred after the anion-exchange process, and SCU-CPN-4 features a uniform spherical morphology with the size of ∼100 nm (Figure S2). TEM-EDS mapping also confirms the nearly total exchange of I– by Cl– (Figure S6). The chemical structure SCU-PN and SUC-CPN-4 was elaborated by solid-state 13C NMR spectroscopy. As shown in Figure S3, the characteristic peaks of carbonyl 13C in 1,4-bisbenzil and the aldehyde 13C in TFP located in the range of 180.0–200.0 ppm disappear after the polycondensation between TFP and 1,4-bisbenzil, accompanied by the formation of the characteristic peak of carbon bonded to the nitrogen atom of imidazole at 146 ppm,44 confirming the successful construction of SCU-PN. The deprotonation process from SCU-PN to SCU-MPN was verified by the appearance of a new peak at 34.8 ppm that belongs to the methyl group. The increased relative intensity of methyl peak at SCU-CPN-4 demonstrates the occurrence of the quaternization reaction of SCU-MPN with CH3I. In addition, the peak at 970 cm–1 in Fourier transform infrared (FT-IR) spectrum corresponds to the characteristic peak of imidazole species (Figure S4),45 further indicating the successful construction of SCU-PN. After deprotonation and quaternization reaction with methyl iodide (CH3I), the characteristic peak of imidazole species in the FT-IR spectrum changed from 970 to 1017 cm–1, attributing to the decrease of conjugation induced by the introduction of a methyl species.

Figure 1.

Figure 1

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(a) Illustration of designing the cationic key structural fragment of a task-specific CPNs. (b) Synthesis route of SCU-CPN-4 and SCU-CPN-4-Re.

The initial sorption experiment was executed by immersing 20 mg of SCU-CPN-4 into 20 mL of aqueous solutions containing 0.15 mmol/L of 99TcO4– or ReO4– (nonradioactive surrogate of 99TcO4–). The effect of contact time of 99TcO4– with SCU-CPN-4 has been evaluated by monitoring the time-dependent radioactivity through liquid scintillation counting (LSC) of the mixed solution. The kinetics data depicted in Figure 2a show that the 99TcO4– could be rapidly adsorbed by SCU-CPN-4 and the sorption equilibrium was reached in 1 min with the removal rate as high as 99%. We assume that the fast exchange kinetics may derive from its smaller particle size, high positive charge density, and the increased hydrophobicity induced by the total substitution of imidazolium part. Remarkably, the sorption kinetics of SCU-CPN-4 is comparable to the record-holding SCU-CPN-133 and much more rapid than most of the reported sorbents (Table S1). For instance, the commercial anion-exchange resins, Purolite A532E and A530E, were reported to take as long as 120 min to reach sorption equilibrium,26 the removal rate of inorganic material NDTB-1 was only 72% after 36 h of sorption24,25 even for the crystalline cationic MOFs SCU-10346 and SCU-101,31 which own ordered pore channels, the sorption equilibrium was reached in at least 5–10 min. This significant advantage makes SCU-CPN-4 more attractive for emergency management of high-level radioactive waste, as it can provide a rapid emergency response to reduce the potential risk caused by accidental leakage of 99TcO4–. Given the high radioactivity of 99TcO4– required for the following batch anion-exchange experiments, nonradioactive ReO4– was used as a surrogate for chemical behavior. Additionally, the feasibility was also confirmed by the sorption kinetics study performed on ReO4–, which is almost identical to that of 99TcO4– (Figure 2a). The sorption kinetics of SCU-CPN-4 at a low solid/liquid ratio was carried out to further quantify the uptake kinetics. As Figure S5a shows, it takes about 25 min to reach the sorption equilibrium at a solid/liquid ratio of 0.2 g/L. Besides, the sorption data can be well fitted by the Pseudo-second-order model (Figure S5 and Table S2).

Figure 2.

Figure 2

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(a) Sorption kinetics of TcO4–/ReO4– by SCU-CPN-4. Condition: [Tc/Re]initial = 0.15 mmol/L and msorbent/Vsolution = 1 g/L. (b) Sorption isotherm of SCU-CPN-4 for ReO4– uptake. Condition: msorbent/Vsolution = 1 g/L and contact time = 2 h. (c) Effect of excess competing NO3– anions on ReO4– uptake by SCU-CPN-4. Condition: [Re]initial = 28 ppm, msorbent/Vsolution = 1 g/L and contact time = 2 h. (d) Comparison of the selectivity toward ReO4– by various sorbents in the presence of 100-fold excess of NO3–. (e) Effect of excess competing SO42– anions on ReO4– uptake by SCU-CPN-4. Condition: [Re]initial = 14 ppm, msorbent/Vsolution = 1 g/L and contact time = 2 h. (f) Comparison of the selectivity toward ReO4– by various sorbents in the presence of 1000-fold excess of SO42–.

On account of the large inventory of 99TcO4– in used nuclear wastes, sorption capacity is one of the most important parameters to evaluate the performance of adsorbents. The higher removal capacity is of great significance to increasing removal efficiency and subsequently reducing the generation of secondary waste. The isothermal sorption experiments were carried out to evaluate sorption capacity by exposing SCU-CPN-4 to ReO4– solutions with different initial concentrations. As Figure 2b and Table S3 shows, the sorption capacity as a function of equilibrium concentration is well fitted with the Langmuir model (R2 > 0.98), indicating a monolayer sorption mechanism. The maximum sorption capacity is calculated to be 437 mg/g, which is much higher than most of the reported sorbents including NDTB-1 (49.4 mg/g),27 LDHs (130.2 mg/g),27 NZVI/rGOs (85.77 mg/g),47 ZBC (25.92 mg/g),48 4-ATR resin (354 mg/g),49 D318 resin (351 mg/g),50 SCU-101 (217 mg/g),31 SCU-102 (291 mg/g),29 and SCU-103 (318 mg/g)46 (Table S4). Besides, the total exchange of Cl– by ReO4– is also demonstrated by TEM-EDS mapping (Figure S6).

Given the presence of a huge excess of competing anions in HLW streams, especially for NO3– and SO42–, which generally exist in excess of 100–6000 folds, we then evaluated the sorption selectivity of SCU-CPN-4 for ReO4–. As shown in Figure 2c, the removal efficiencies of SCU-CPN-4 toward ReO4– are negligibly influenced at the molar ratios of 1 and 10 (NO3–/ReO4–). Even the amount of NO3– is in a 100-fold excess, the removal percentage of ReO4– is still as high as 91.5%, which is notably superior to most of the reported sorbents such as SCU-101 (54.4%),31 SCU-100 (73%),27 SCU-CPN-2 (79%),36 and SCU-103 (88%)46 (Figure 2d). Impressively, for the SO42– anion, which is highly competitive due to its high charge density, the removal rates of ReO4– remain at a high quantitative value of 98.7% even though the amount of SO42– is in a 1000-fold excess (Figure 2e). As comparison, the removal rates by SCU-CPN-2,36 SCU-CPN-1,33 SCU-101,31 and SCU-10346 at the same condition are only 29.7%, 70%, 83% and 89.2%, respectively (Figure 2f). In addition, the Kd value of SCU-CPN-4 is as high as 1.5 × 107 mL/g, significantly higher than those of other reported sorbents including NDTB-1(652),27 LDHs (262),27 Purolite A520E (7.6 × 105),26 SCU-102 (5.6 × 105),29 and SCU-CPN-1 (6.2 × 105)33 (Table S5). Such excellent selectivity originates from the total substitution around imidazolium parts by bulky groups, which highly increases the overall hydrophobicity of SCU-CPN-4 and therefore the enhancement of the affinity for less hydrated 99TcO4–/ReO4–. These extraordinary characteristics make SCU-CPN-4 a potential candidate for 99TcO4– remediation in HLW streams with high ionic strengths.

Considering the strong ionizing field (β, γ, and neutron irradiations, and so forth) in HLW streams, the radiation resistance of sorbents is critically required for practical applications. Very impressively, the FT-IR spectra of SCU-CPN-4 after 100 and 200 kGy of β-radiation are almost identical to that of the pristine sample. In addition, ion exchange experiments also suggest no decrease in the ReO4– uptake capacity of SCU-CPN-4 after 100 and 200 kGy of β- or γ-irradiation (Figure 3a,b). The conjugated structure plays an important role in maintaining radiation stability by protecting the key imidazolium moieties from being degraded. As a useful comparison, the commercial resin Purolite A530E resin has been reported to exhibit excellent sorption performance for TcO4–, however, the poor radiation stability leading to a gradual decrease in sorption capacity with the increase of exposed radiation dosage is the main drawback that limiting its practical application.33 These results demonstrate that SCU-CPN-4 possesses a distinct advantage in the long term treatment of HLW streams. To validate the alkaline resistance of SCU-CPN-4, we first assessed the sorption property under different pH values. As shown in Figure 3c, a negligible decrease in the removal efficiency was observed within a wide pH range of 2 to 12, Even under the condition of 1 M NaOH solution, SCU-CPN-4 can still remove more than 98% of ReO4– at a solid–liquid ratio of 1 g/L. More impressively, SCU-CPN-4 is fully reusable under such an alkaline environment (Figure 3d). Even after six cycles of sorption/desorption, the removal efficiency of SCU-CPN-4 toward ReO4– still retains a high value of 98%. We further evaluated ReO4– sorption performance of SCU-CPN-4 after being exposed to 1 M NaOH solution for 24 h. As depicted in Figure 3e,f, the FT-IR spectrum of alkali-treated SCU-CPN-4 remains almost identical to that of the original samples and the sorption capacity is not affected. In comparison, the sorption capacities of SCU-CPN-1 and SCU-CPN-2 without steric protection decrease from 953 to 350 mg/g and from 1308 to 429 mg/g, respectively. Even for SCU-103 which was reported to be alkaline-stable and can remove TcO4– from alkaline conditions effectively within a short time of contact,46 the sorption capacity undergoes a decline from 328 to 73 mg/g under the same condition. It can thus be concluded that SCU-CPN-4 exhibits significantly enhanced alkaline resistance, compared with all existing anion-exchange materials. Such imposing virtue derives from the complete substitution around the key imidazolium moieties by bulk groups where the large steric hindrance can block the nucleophilic attack by OH–, avoiding the occurrence of ring-opening reactions on cationic imidazolium parts. This point can be further verified by comparing the sorption performance of SCU-CPN-1 and SCU-CPN-2 without steric protection. The significant advantage of high alkaline resistance endows SCU-CPN-4 with great practicability for 99TcO4– separation in alkaline nuclear waste inventory.

Figure 3.

Figure 3

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(a) FT-IR spectra of SCU-CPN-4 before and after being irradiated by β- or γ-rays. (b) ReO4– uptake capacity of SCU-CPN-4 before and after being irradiated by β- or γ-rays. Condition: [Re]initial = 500, msorbent/Vsolution = 1 g/L and contact time = 2 h. (c) Effect of pH on ReO4– uptake by SCU-CPN-4. Condition: [Re]initial = 28 ppm, msorbent/Vsolution = 1 g/L and contact time = 2 h. (d) Reversibility of SCU-CPN-4 under the condition of 1 M NaOH. Condition: [Re]initial = 28 ppm, msorbent/Vsolution = 1 g/L and contact time = 2 h. (d) FT-IR spectra of SCU-CPN-4 before and after being exposed to 1 M NaOH solution for 24 h. (f) Comparison of the sorption capacity of ReO4– by various sorbents before and after being exposed to 1 M NaOH solution for 24 h. Condition: msorbent/Vsolution = 1 g/L.

Given this, the column sorption investigations, which are more relevant to practical applications, were further performed on SCU-CPN-4 under extremely alkaline conditions. 100 mg of SCU-CPN-4 was filled in a plastic column with 1 M NaOH solution containing 28 ppm of Re(VII). As shown in Figure 4a, nearly 500 mL of this solution passed through the column with ReO4– completely removed (removal rate of ∼100%), indicating the outstanding removal efficiency toward ReO4– of this column. More impressively, the column can be regenerated by eluting with 2 M NaCl solution at a flow rate of 5 mL/min. The total column sorption capacity remains basically unchanged (∼248 mg/g) for all four runs of sorption/desorption. In comparison, the removal rates of ReO4– for SCU-CPN-1 (Figure 4c) and SCU-CPN-2 (Figure 4d) decrease rapidly during the dynamic sorption process with maximum removal rates of 54% and 32%, respectively. Besides, SCU-103 shows a maximum removal rate of only 76% and the removal rate gradually decreases as the contacted volume increases under the same column sorption condition (Figure 4e). These obvious differences again corroborate the enhanced stability and selectivity of SCU-CPN-4 compared to other anion-exchange materials, highly desirable for the actual TcO4– separation from alkaline nuclear waste streams.

Figure 4.

Figure 4

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(a) Dynamic sorption column analysis of SCU-CPN-4. Blue: sorption process, [Re]initial = 28 ppm, msorbent = 100 mg, flow rate = 2 mL/min. Red: desorption process, C0(NaCl) = 2 mol/L and flow rate = 5 mL/min. (b) Photo of the auto solid-phase extraction system (SepathsUP4). (c) Dynamic sorption column analysis of SCU-103, (d) SCU-CPN-1, and (e) SCU-CPN-2. Condition: [Re]initial = 28 ppm, msorbent = 100 mg and flow rate = 2 mL/min. (f) Comparison of removal of ReO4– by various sorbents under the condition of simulated Hanford waste. (g) Reversibility of SCU-CPN-4 under the condition of simulated SRS waste. Condition: msorbent/Vsolution = 40 g/L and contact time = 2 h. (h) Dynamic sorption column analysis of SCU-CPN-4 under the condition of simulated SRS HLW waste stream. Condition: msorbent = 300 mg and flow rate = 1.35 mL/min.

TcO4– uptake ability of SCU-CPN-4 from simulated Hanford Low Activity Waste (LAW) Melter Recycle Stream and Savannah River Site (SRS) HLW Stream was also evaluated. The Hanford LAW stream contains a sufficient excess of NO3– (5.96 × 10–2 M), NO2– (3.03 × 10–3 M), and Cl– (6.93 × 10–2 M) in addition to 1.94 × 10–4 M TcO4– (Table S6). Significantly, more than 97.4% of TcO4– could be removed by SCU-CPN-4 at a solid/liquid ratio of 5 g/L (Figure 4f and Table S8), which is significantly superior to those of SCU-CPN-2 (67%),36 SBN (13%),29 and SCU-CPN-1 (90%)27 (Figure 4f). Besides, the removal of TcO4– for SCU-10129 and SCU-10229 is 75% and 95% at a solid/liquid ratio of 10 g/L, respectively, notably lower than that of SCU-CPN-4 at a solid/liquid ratio of 5 g/L. SRS HLW waste is the super alkaline and extreme competitive stream (Table S7), of which the amounts of OH–, NO3–, SO42–, and NO2– are in 16 788-, 32 819-, 6576-, and 1691-fold excess compared with TcO4–, respectively. Astonishingly, the removal of TcO4– by SCU-CPN-4 in simulated SRS waste is as high as 94.3% at a solid/liquid ratio of 20 g/L (Table S8), much higher than that of SCU-103 (82%).46 More impressively, the removal efficiency of ReO4– remains unchanged even after four sorption/desorption cycles in simulated SRS waste streams (Figure 4g). This result suggests the splendid structural stability as well as excellent sorption selectivity of SCU-CPN-4, making the application of SCU-CPN-4 more potential and cost-saving.

For the first time, we performed column sorption tests on SCU-CPN-4 using simulated SRS HLW stream. As shown in Figure 4h, the ion-exchange chromatographic column filled with 300 mg of SCU-CPN-4 could remove nearly all the ReO4– from the first 40 mL of SRS HLW waste solution, initially exhibiting the high removal efficiency toward ReO4–. These virtues demonstrate the powerful application potentials of SCU-CPN-4 in the direct removal of TcO4– from highly alkaline wastes like SRS waste.

The sorption mechanism can be well investigated by TEM-EDS, FT-IR spectra, and XPS analysis. As probed by TEM-EDS mapping (Figure S6), the signal of Cl– disappears with the concomitant appearance of the ReO4– in SCU-CPN-4-Re, intuitively revealing the complete ion-exchange process during the sorption. The new peaks at 896 cm–1 in FT-IR spectrum SCU-CPN-4-Re correspond to the characteristic peaks of Re–O ν3 asymmetric stretch, indicating ReO4– was adsorbed into SCU-CPN-4 after the sorption process (Figure 5a). In XPS analysis, the new peaks for Re 4f, Re 4d, and Re 4p arise in SCU-CPN-4-Re at 46, 265, and 452 eV accompanied by the disappearance of Cl 2p peak (197 eV), further indicating the ion-exchange mechanism (Figure 5c). Furthermore, the Re 4f core-level spectra in SCU-CPN-4-Re was in accord with that of ReO4–. This indicated that the Re species remain unchanged during the ion-exchange process (Figure 5d).

Figure 5.

Figure 5

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(a) FT-IR spectra of SCU-CPN-4 and SCU-CPN-4-Re. (b) Solid-state 13C NMR spectrum of SCU-CPN-4 and SCU-CPN-4-Re. (c) XPS survey spectra of SCU-CPN-4 and SCU-CPN-4-Re. (d) XPS analysis of Re 4f from SCU-CPN-4-Re.

DFT calculations were performed to investigate the mechanisms of the remarkable alkaline-stability of SCU-CPN-4 and the superior separation capability of SCU-CPN-4 toward TcO4–. Two typical fragments of SCU-CPN-4 and SCU-CPN-1 that contain their foremost structural features are selected as the theoretical models (abbreviated as M+ and M*+, respectively). First, we studied the electrostatic potential (ESP) of M+ (up) and M*+ (down), as shown in Figure 6a. It can be seen that the electron density van der Waals surface near the imidazole rings shows a relatively concentrated positive ESP distribution. The ESP maxima near the imidazole ring of M+ and M*+ fragments are labeled by green dots and their corresponding ESP values are listed in Figure S7. It shows that the ESP maxima of M+ are all smaller than M*+. The ESP reduction of M+ can be attributed to the compensation of the electron density of the formally positive charged imidazole ring by the electron-donating benzene and methyl groups. This can be seen as one major effect of the enhancement of alkaline-stability of SCU-CPN-4 since the smaller ESP of M+ reduces the long-range electrostatic attraction between the positively charged imidazole ring and the negative charged OH– anion. Second, the calculated solvation energies (ΔGsolv, −36.69 and −40.85 kcal/mol for M+ and M*+) shows the installation of the bulky groups can effectively improve the hydrophobicity of M+ fragment. This result can be expected since both benzene and methyl are typical hydrophobic functional groups. We believe that the hydrophobicity enhancement of M+ is another important factor to enhance the alkaline-stability of SCU-CPN-4, since the OH– anion is extremely hydrophilic (calculated value of −370.7 kcal/mol).51 Third, the geometric fluctuations of M+ fragment were studied by means of molecular dynamics (MD) simulation. Five hundred M+ structures extracted from MD simulation trajectory are superimposed and presented in Figure 6b. It shows that the imidazole ring can be effectively encapsulated by the benzene and methyl groups since thermal motion indicates that the benzene and methyl groups can provide a certain degree of additional physical protection for the central imidazole ring. This can further assist the alkaline stability of SCU-CPN-4 because of the steric effect. In short, we conclude that the cooperation of the ESP reduction, the hydrophobicity enhancement, and the additional steric effect effectively enhance the alkaline stability of SCU-CPN-4.

Figure 6.

Figure 6

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(a) ESP distribution on the electron density surface (isodensity = 0.001 au) of the M+ and M*+ fragments. Green balls represent the ESP maxima near the imidazole ring of M+ and M*+ fragments. ΔG represents the solvation energy. (b) The superimposed structures of M+ fragment during 2000 fs trajectory. The structures are extracted every 4 fs. (c) Optimized structures of M+NO3–, M+SO42–, and M+TcO4–. ΔH represents the enthalpy change.

Batch experiments manifest that SCU-CPN-4 exhibits excellent sorption selectivity for TcO4– under the coexistence of NO3– or SO42–. To reveal the basis for this excellent selectivity, structures of M+NO3–, M+SO42–, and M+TcO4– and their corresponding ΔH values were optimized. As Figure 6c shows, all the negatively charged anions adsorbed on the positively charged imidazole ring. Despite showing quite similar sorption sites on M+, their corresponding ΔH values are distinctly different. The calculated ΔH values for M+TcO4– is −15.08 kcal/mol being 2 and 10 times higher than those for M+NO3–, (−8.44 kcal/mol) and M+SO42– (−1.50 kcal/mol), respectively. This indicates that TcO4– is more energetically favorable than NO3– and SO42– to bind to the M+ fragment. This is mainly attributed to TcO4– which is relatively more hydrophobic than other anions. The hydration-free energy of TcO4– is −251 kJ/mol, much lower than that of NO3– (−306 kJ/mol) and SO42– (−1090 kJ/mol).52 This leads to high ΔH values for M+TcO4– and the exceptional selectivity of SCU-CPN-4 for TcO4– uptake.

Conclusions

In summary, an alkaline-stable cationic polymeric network material (SCU-CPN-4) was rationally designed by the installation of superhydrophobic steric hindrance at the key imidazolium moiety for alkaline nuclear waste management. The elaborately tailored SCU-CPN-4 enjoys the merits of high alkaline resistance that benefits from the bulky steric hindrance as well as the excellent selectivity toward TcO4– as a result of enhanced hydrophobicity. Our study demonstrates the unexceptionable sorption properties of SCU-CPN-4 including fast sorption kinetics, high sorption capacity, excellent sorption selectivity, full reusability, and splendid sorption performances under highly alkaline aqueous solution. Moreover, the dynamic sorption performance under alkaline SRS HLW Stream reported for the first time, coupled with the splendid alkaline-resistance ability and full reusability, endow the SCU-CPN-4 with the feasibility for practical use. On the basis of these remarkable features, the newly designed steric-protected CPN holds vast potential in efficient removal of TcO4– from alkaline nuclear waste, offering a new clue to address the long-term challenges in the CARBEX process and alkaline nuclear waste management.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21790374, 21825601, 21806117, 21906116, and 22006108), Postdoctoral Science Foundation of China (2021M692346 and BX2021206), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the National Key R&D Program of China (2018YFB1900203).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscentsci.1c00847.

  • Experimental details including synthesis procedure, sorption procedure, and calculation methods; tables including sorption results, composition of Hanford Low Activity Waste Melter Recycle Stream and composition of Savannah River Site High-Level Waste Stream; figures including NMR spectra, FT-IR spectra, and SEM spectra (PDF)

Author Contributions

† J.L. and B.L. contributed equally to this paper.

The authors declare no competing financial interest.

Supplementary Material

oc1c00847_si_001.pdf (408.4KB, pdf)

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Supplementary Materials

oc1c00847_si_001.pdf (408.4KB, pdf)

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