Separation and Remediation of 99TcO4â from Aqueous Solutions

Perspective Cite This: Chem. Mater. 2019, 31, 3863−3877

Separation and Remediation of

99

pubs.acs.org/cm

TcO4− from Aqueous Solutions§

Chengliang Xiao,† Afshin Khayambashi,‡ and Shuao Wang*,‡ †

College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China

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‡

ABSTRACT: Technetium-99 (99Tc) is one of the most-problematic radioisotopes in used nuclear fuel owing to its intrinsic features of a high fission yield, long half-life, high environmental mobility, volatile nature during waste vitrification, and its redox interface capability with actinides during used fuel repossessing. The selective separation of pertechnetate (TcO4−) from legacy nuclear waste and contaminated natural water is therefore highly desirable but still a significant challenge, because the conditions of a strong radiation field, high ionic strength, high acidity/ alkalinity, and large amounts of competing anions are often involved in these systems. Until now, there are a handful of functional materials that can efficiently remove TcO4− from nuclear waste solutions with high uptake capacities, fast kinetics, and good selectivity, but room still remains to further improve our capabilities for controlling the contamination/ separation of TcO4−. In this Perspective, we discuss the current state of the art TcO4− separation materials including precipitation agents, reducing materials, ion-exchange resins, inorganic cationic frameworks, cationic metal−organic frameworks (MOFs), and cationic polymeric networks (CPNs) materials. The intriguing separation mechanisms of these materials for TcO4− are also disclosed, which may hopefully shed light on further development in this field. plutonium−uranium redox extraction (PUREX), TcO4− is one of the adverse interferents that can coextract with uranium, neptunium, and plutonium, affecting the purification of these fissile materials. In addition, as a result of the catalytic redox reactions of 99Tc, it is difficult to control the valence state of key actinides in reprocessing procedures. Therefore, it would be highly beneficial to eliminate 99Tc in an initial stage before the PUREX process for these aforementioned reasons and to avoid 99 Tc discharge into the environment. For instance, during the nuclear waste vitrification process, the resulting Tc(VII) compounds show a high probability of leakage due to their volatile tendencies. However, the separation of TcO4− from used nuclear fuels is a significant challenge, because this waste represents one of the most-extreme conditions on Earth with a high acidity (3 M HNO3), high ionic strength (large amounts of metal and nonmetal ions), and strong ionizing radiation field (β-, γ-, and neutron radiation). In addition, the tank wastes at US legacy nuclear sites, which are mostly represented by Hanford and Savannah River Sites, contain a large inventory of 99Tc. For example, it is estimated that ∼2000 kg of 99Tc is currently stored within the Hanford waste tanks.3 As a result of either planned or unplanned discharges, high concentrations of 99Tc can be found in the vadose zone and groundwater. These radioactive wastes are composed of large amounts of competing anions, such as

1. INTRODUCTION Currently, nuclear energy represents one of the most-important components in energy generation, although it faces several key obstacles for going even further on a larger scale worldwide. One major issue is the lack of efficient, reliable, and economical strategies to treat and dispose of used nuclear fuel. Another serious concern is nuclear safety management, because large amounts of radioactive wastes have been released into the environment as a result of nuclear accidents/improper waste management over the past several decades. To better develop nuclear energy, used nuclear fuel from power plants in addition to the legacy nuclear waste from military activity and radiologically contaminated water from nuclear accidents must be properly handled. In used nuclear fuel and high-level radioactive waste, technetium-99 (99Tc), which is a β-emitter with a long half-life of 2.13 × 105 years, is one of the most-problematic radionuclides.1 With a high fission yield of ∼6%, 21 kg of 99Tc is annually produced by a 1 GWe-scale nuclear reactor, leading to a large inventory of ∼155 tons accumulated over the past 20 years all over the world.2 99Tc predominately exists in the form of the pertechnetate anion (TcO4−) in both the nuclear fuel cycle and environments under oxic conditions. TcO4− is highly soluble and non/weakly complexing, resulting in fast migration within an environment when not retarded by most minerals and natural organic substances. When processing used nuclear fuels via

Received: January 26, 2019 Revised: May 18, 2019 Published: May 22, 2019

§

This Perspective is part of the Up-and-Coming series. © 2019 American Chemical Society

3863

DOI: 10.1021/acs.chemmater.9b00329 Chem. Mater. 2019, 31, 3863−3877

Perspective

Chemistry of Materials Table 1. Ionic Radii (R, nm) and Standard Gibbs Energies of Hydration (ΔGh, kJ/mol) R ΔGh

ClO4−

TcO4−

I−

NO3−

Br−

Cl−

OH−

F−

CrO42−

SO42−

CO32−

PO43−

0.225 −214

0.250 −251

0.220 −275

0.200 −306

0.196 −321

0.181 −347

0.152 −439

0.126 −472

0.229 −958

0.218 −1090

0.189 −1315

0.230 −2773

SO42−, NO3−, NO2−, Cl−, OH−, etc., resulting in a significant challenge to selectively remove TcO4−.4 Notably, the environmental issue of 99Tc at the legacy nuclear sites is basically unsolved despite the significant amount of effort directed at this problem over the past several decades. The goal of selective and efficient separation of 99Tc from wastes cannot be reached without a sophisticated understanding of the basic physical−chemical properties of 99Tc. Because 99Tc mainly exists as pertechnetate (TcO4−), an anionic species, cationic materials with a positive net charge such as quaternary ammonium salts, anion-exchange resins, layered double hydroxides (LDH), cationic metal−organic frameworks (MOFs), and cationic polymeric networks (CPNs) can act as potential candidates to uptake TcO4− mostly through anionexchange processes. In addition, TcO4− is a large, monovalent anion, resulting in an overall low charge density that is therefore relatively hydrophobic, compared to coexisting anions. The standard Gibbs energy of hydration of TcO4− at −251 kJ/mol is very low compared to those of SO42− and PO43− (−1090 and −2773 kJ/mol, respectively)5 (Table 1). According to the Hofmeister effects, the transportation of hydrophobic anions from aqueous solution to organic solvents or hydrophobic solid materials has priority.5 However, for hydrophilic anionexchange materials mostly represented by cationic hydroxide/ oxide frameworks, the anions sorbed onto such materials do not require dehydration in most situations. The selectivity is simply determined by the electrostatic interactions between anions and the frameworks. In this case, those anions with a high negative charge density may be preferentially captured by those hydrophilic materials. In contrast, dehydration is a highly energy-consuming process for the hydrophilic anions. Because the hydration energy is relatively low for TcO4−, it does not have to overcome a high energy barrier for dehydration. Taking advantage of this, one of the best strategies to enhance the uptake selectivity of hydrophobic TcO4− is to incorporate more lipophilic groups into anion-exchange materials. Considering that TcO4− is an oxo-anion containing four oxygen atoms in a high-symmetry tetrahedral configuration, the introduction of complementary hydrogen-bonding interactions will also boost its binding capability and selectivity.6 On the other hand, technetium can be converted to sparingly soluble Tc(IV) species under anoxic conditions, and this is known as an effective strategy for 99Tc immobilization by using reducing materials such as zerovalent iron (ZVI), magnetite, pyrite, mackinawite, sulfidated ZVI, and Sn(II)-bearing materials.7 This review will provide a comprehensive summary of technetium separation from radioactive wastes by precipitating agents, reducing materials, ion-exchange resins, and purely inorganic cationic frameworks. The selective sequestration of TcO4− by cationic MOFs and CPN materials and their intriguing separation mechanisms will also be discussed.

Table 2. Solubility and Solubility Product of Pertechnetate Precipitates precipitate NH4ReO4 Me4NTcO4 Et4NTcO4 Pr4NTcO4 Bu4NTcO4 Bu4NTcO4 TPPy−ReO4 TPPy−TcO4 TPPy−TcO4

conditions H2O H2O H2O H2O H2O 1 M HNO3 pH = 11.6 pH = 6.91 pH = 2.22

solubility, M 0.274 0.13 1.2 × 10−2 7.9 × 10−2 4.2 × 10−3 8.1 × 10−3 1.6 × 10−5 1.6 × 10−5 1.5 × 10−5

Ksp

ref. −2

7.5 × 10 1.7 × 10−2 1.4 × 10−4 6.2 × 10−5 1.8 × 10−5 6.6 × 10−5 1.4 × 10−10 2.6 × 10−10 2.2 × 10−10

8 9 9 10 9 9 11 12 12

firmly bonded with the precipitating agents through electrostatic interactions, forming new compounds with low solubility. Here, reduction-induced precipitation is not the scope of this section and will be discussed in the next section. Precipitating-agentcontaining metals may be unsuitable for immobilizing TcO4−, because the metals could contaminate the final Tc compounds. Soluble quaternary ammonium salts hold exceptional precipitation capability toward TcO4−, and NH4TcO4 is the simplest precipitate. However, the high solubility of NH4TcO4 in aqueous solution results in an inefficient removal of TcO4− from waste at low concentrations.8 By increasing the chain length of the quaternary ammonium salts, the solubility of the resulting compounds obviously decreases.10 The solubility of Me4NTcO4 in water was measured to be 0.13 M, whereas that of Bu4NTcO4 decreased to 4.2 × 10−3 M. Despite this, the solubility of pertechnetate-based quaternary ammonium salts is still high. Further increasing the chain length will make the quaternary ammonium salts more lipophilic, which is not suitable for precipitating agents. In addition, an increase in acidity in aqueous solution will also boost the solubility of the pertechnetate salts. Alternatively, 1,2,4,6-tetraphenylpyridinium acetate (TPPA) has been introduced as a promising precipitating agent for TcO4−, and TPPA can be synthesized in high yield by reacting 1,3,5-triphenyl-2-penten-1,5-dione with aniline in glacial acetic acid. It was found that TPPA favors large anions, such as ClO4− and ReO4−, and can quantitatively precipitate them from aqueous solution. In 1.0 M aqueous ammonia, the solubility product (Ksp) of TPPA with ReO4− was measured to be 1.35 × 10−10, representing the most-insoluble compound among all perrhenate materials.11 Direct precipitation of TcO4− with TPPA has not yet been reported, although the results from using another similar salt, 1,2,4,6-tetraphenylpyridinium tetrafluoroborate (TPPy−BF4), have.12 The TPPy−TcO4 compound formed has almost the same Ksp as the perrhenate surrogate. However, the precipitating agent, TPPy−BF4, is not fully soluble in water and requires an acetone/water solvent mixture to precipitate TcO4−, making this method environmentally unfriendly and operationally inflexible. On the basis of the crystal structure of TPPy−TcO4 (Figure 1a), TcO4− is attached to TPPy not only by electrostatic interactions but also by hydrogen bonding, which suggests that bis(guanidinium) ligands may act as TcO4− precipitating agents. Recently,

2. PERTECHNETATE SEPARATION MATERIALS 2.1. Precipitating Agents. The precipitation process is a facile and efficient method to separate TcO4− from radioactive waste that does not require any organic solvents, representing a green and low-energy input technology. Generally, TcO4− is 3864


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Chemistry of Materials

other Fe(II)-based materials,22−25 have been widely investigated as technetium reductants. In most of these cases, soluble TcO4− is reduced to an insoluble TcO2·nH2O precipitate to decrease its migration capability, but the reduced Tc(IV) species is highly susceptible to reoxidation and can be subsequently released. This problem can be overcome by employing sulfide materials (i.e., pyrite, mackinawite, tetrahedrite, pyrrhotite, stibnite, KMS2, and sulfidated ZVI).7,26−30 Although Tc(VII) can be reduced to Tc(IV), the speciation of the generated Tc(IV) shifts from TcO2 to TcS2-like or Tc2S7 analogue phases, which show lower solubility and better resistance to oxidation. For example, Fan et al.15 reported that sulfidated nano ZVI could greatly augment the removal kinetics and efficiency of TcO4− (Figure 2a). Such an efficient sequestration process can be achieved by yielding a sparingly soluble Tc sulfide phase (TcS2) instead of TcO2· nH2O. After TcO4− was exposed to this sulfidated material, the reoxidation rate of technetium decreases with an increase in the S/Fe ratio (Figure 2b), which further confirms that TcS2 is more recalcitrant to oxidation than TcO2·nH2O.26 Layered potassium metal sulfide materials are also capable of reducing Tc(VII) to Tc(IV).28 The removal percentage of TcO4− can reach up to 98% under optimal conditions. The postulated uptake mechanism is that TcO4− enters into the interlayer of KMS-2 and is reduced by the sulfide, yielding an insoluble Tc2S7 phase. Sn(II)-based materials have also been investigated to immobilize TcO4− by reduction. The reduction potential for Sn(IV)/Sn(II) is 0.384 V, which is much lower than that of Tc(VII)/Tc(IV) (0.738 V), so TcO4− can be readily reduced by Sn(II). This reduction reaction has long been recognized in the radiopharmaceutical industry. However, Sn(II)-based materials had not been evaluated for technetium removal from radioactive wastes until a nanoporous Sn(II/IV) phosphate material was synthesized in 2006, and this material could remove >95% of redox-sensitive ions from aqueous solutions with a distribution coefficient for TcO4− as high as 9.0 × 104 mL/g.31 Recently, Asmussen et al.32 developed another Sn(II)-based material called Sn(II)-apatite and obtained a record high distribution coefficient of 3.7 × 107 mL/g for TcO4− removal. Such excellent removal performance of Sn(II)-apatite might be attributed to an exchange reaction of TcO4− with the phosphate in apatite followed by a reduction reaction by Sn(II). However, Sn(II)-

Figure 1. (a) Crystal structure of TPPy−TcO4;12 (b) hydrogen bonding of the sulfate−water clusters with BBIG (1,4-benzenebis(iminoguanidinium)) ligands. Reprinted with permission from ref 14. Copyright 2016 Wiley VCH. (c) Hydrogen bonding of sulfate− water clusters with GBAH (glyoxal bis(amidiniumhydrazone)) ligands. Reprinted with permission from ref 13. Copyright 2015 Wiley VCH.

Custelcean et al.13,14 reported two bis(guanidinium) ligands for efficient crystallization separation of SO42− from aqueous solutions by hydrogen-bonding-assisted electrostatic interactions (Figure 1b,c). Considering that TcO4− has an almost identical tetrahedral geometry as SO42−, it is possible to precipitate TcO 4 − using these promising ligands. The preliminary results have confirmed this hypothesis, and relevant investigations are ongoing in our group. 2.2. Reducing Materials. Under oxic conditions, technetium is predominately present as the Tc(VII) anion, TcO4−. In anoxic environments, technetium can be reduced into the form of Tc(IV), an insoluble TcO2·nH2O precipitate. Thus, the reduction of technetium from soluble Tc(VII) to insoluble Tc(IV) is considered an efficient strategy for technetium remediation and immobilization. Low-valent iron-bearing materials, such as zerovalent iron (ZVI),15,16 magnetite,17,18 supported ZVI,19,20 Fe(II)-sorbed Al oxides/hydroxides,21 and

Figure 2. (a) TcO4− reduction kinetics by sulfidated ZVI materials with an increase in the S/Fe ratios. Reprinted with permission from ref 15. Copyright 2013 American Chemical Society. (b) Aqueous Tc redissolution kinetics of sulfidated ZVI materials after exposure to TcO4−. Reprinted with permission from ref 26. Copyright 2014 American Chemical Society. 3865


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The separation of TcO4− by ion-exchange resins was first reported in 1963.36 The resin that is used in Hanford technetium-containing alkaline tank wastes is IRA-400 (Figure 4), which is a traditional anion-exchange resin with a

apatite only worked very well in a near-neutral pH and low-ionicstrength environment. When the Sn(II)-apatite was contacted with alkaline solutions such as tank waste from the Hanford site, the removal efficiency of TcO4− significantly decreased. In addition, the competition with Cr(VI) from the wastes would increase the amount of Sn(II)-apatite used. Very recently, Levitskaia et al.33 overcame these issues by using a Sn(II/IV) phosphate supported on a polycrystalline aluminophosphate matrix. This material could not only reduce TcO4− to Tc(IV) with a high loading capacity but also incorporate Tc(IV) into the Sn(IV)-containing inert polycrystalline matrix. The distribution coefficient of TcO4− was measured to be 1.3 × 104 mL/g in simulated alkaline waste. Even in the presence of 33 mM Cr(VI), the Kd value can still reach up to 2.2 × 103 mL/g. These unprecedented properties set Sn(II/IV) aluminophosphate as one of the best-performing reductive sorbents for TcO4− developed to date. Burton-Pye et al.34,35 reported another similar strategy of simultaneous reduction and incorporation. They employed a polyoxometalate material, α2-[P2W17O61]10−, that could effectively photoreduce TcO4− to a low-valent state in 2-propanol using both sunlight and UV irradiation and incorporate it into the polyoxometalate framework to form the stable TcVO(α2-P2W17O61)7− species (Figure 3). In the system

Figure 4. Functional groups of various TcO4−-selective resins.

trimethylammonium functional group. A total of 90 000 L of alkaline waste was treated using 1520 L of IRA-401 resin, and as a result, 70% of the 99Tc (∼1.1 kg) was recovered. However, the selectivity was adversely affected by the high concentration of competing anions, such as Cl−, NO3−, SO42−, and CO32−. To enhance the transfer of TcO4− from solution to solid resins, larger hydrophobic chains (ethyl, n-butyl, n-hexyl, etc.) were introduced into the trialkyl ammonium groups. Meanwhile, some alkyl groups with shorter chain lengths were preserved in the resins to improve the ion-exchange kinetics. These bifunctional anion-exchange resins have overcome the demerits of poor exchange capacity and kinetics that most anionexchange resins containing only one type of large exchange site possess.37,38 Commercial Purolite A532E and 530E resins (Figure 4) belong to this type and have been proven to be good scavengers for TcO4−/ReO4− from radioactive wastes.39 Purolite A530 resin is currently in use at the 200 West Pump and Treat Plant to remove TcO4− and has also the potential to tie-up TcO4− from a supernate simulant for tank 241-AN-105 at Hanford with 6.5 M sodium. Recently, we systematically investigated batch experiments for ReO4− sorption by Purolite A532E and 530E resins.39 It was found that these two resins are able to remove ReO4− from aqueous solutions within 150 min and with maximum exchange capacities of 707 and 446 mg/g toward ReO4− for Purolite A530E and A532E, respectively. The sorption properties of these two introduced resins are independent of pH, and both exhibit excellent selectivity for the removal of ReO4− in the presence of large excesses of NO3− and SO42−. The ReO4− removal from the Hanford low-level waste melter off-gas scrubber simulant stream by these two resins reaches up to 90 and 80% for Purolite A530E and A532E, respectively. However, the strong binding ability between these resins and TcO4−/ReO4− makes elution difficult, limiting their use for multiple cycles.

Figure 3. Schematic illustration of the photoreduction of TcO4−/ ReO4− by a polyoxometalate material and their corresponding structures and mechanisms. Reprinted with permission from ref 34. Copyright 2011 American Chemical Society.

introduced by Burton-Pye et al., TcO4− is first reduced to a TcIV intermediate species and then is gradually converted to TcVO(α2-P2W17O61)7−. This mechanism is different from that of its surrogate ReO4−, which is rapidly reduced to ReVO species and incorporated into the (α2-P2W17O61)10− defects. Thus, this finding should be taken into account when using ReO4− as a surrogate of TcO4− when a redox reaction is involved. 2.3. Ion-Exchange Resins. Ion-exchange resins are insoluble materials with abundant pores and are normally fabricated by cross-linking styrene and divinylbenzene. The functional groups are then grafted onto a polymer substrate by one-pot polymerization or postmodification to synthesize cation- or anion-exchange resins. The anion-exchange resins are generally composed of some amino groups with a positive charge. The charge balanced anions (Cl−, OH−, etc.) can be freely replaced by other more favorable anionic species through an ion-exchange process. Compared to precipitation and reduction techniques, ion exchange is an efficient method for selectively sequestering TcO4− at low concentrations. 3866


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3867

It has been comprehensively investigated to remove 99Tc from Hanford tank waste simulants and actual tank waste supernate from laboratory-scale to pilotscale. When testing the Hanford Tank AW-101, the removal of 99Tc was >99.94% (DF ≈ 1700), and 99% of 99Tc could be eluted with deionized water. It is stable under radiation but not suitable for removing nonpertechnetate species.

TcO4− can be selectively retained in a high concentration of water-structuring anions, and this has been verified using Hanford tank wastes. It is unstable after exposure to a high radiation dosage.

This resin shows excellent removal efficiency for TcO4− in a wide pH range but needs a 1.0 M NaOH/1.0 M ethylenediamine/0.0050 M SnCl2 or an 8 M HNO3 solution for elution, and only 70% of 99Tc is recovered.

macroporous nonelutable nonelutable bifunctional trialkyl ammonium

SuperLig 639

ABEC resins

a mixture of 75% strong baseexchange (RHCH3+) and 25% weak base-exchange sites (RNH+) monomethylated PEGs on the elutable chloromethylated polystyrene− divinylbenzene support crown ether groups grafted onto a elutable polystyrene support

gel nonelutable bifunctional trialkyl ammonium

Purolite A532E Purolite A530E ReillexHPQ

elutable trimethylammonium IRA-400

type functional groups resins

Table 3. Summary of Ion-Exchange Resins for TcO4− Removal

−

general description

Reillex-HPQ3,40−45 is an alternative resin for the efficient removal of TcO4−. This resin was initially invented by Los Alamos National Laboratory and Reilly Industries, Inc. as a plutonium sorbent. The resin is composed of a mixture of 75% strong base-exchange (RNCH3+) and 25% weak base-exchange sites (RNH+) (Figure 4). Ashley and Marsh et al.3,40−43,45 thoroughly investigated the sorption behavior of Reillex-HPQ in a wide pH range and confirmed its excellent removal efficiency for TcO4−. The distribution coefficients of TcO4− varied from 11.5 mL/g at 10 M HNO3 to 1066 mL/g for the alkaline Hanford waste tank simulant, 101-SY. Although Reillex-HPQ showed promising capability to remove TcO4− both in batch and column tests, the elution process was arduous, especially because this process either needs a 1.0 M NaOH/1.0 M ethylenediamine/0.0050 M SnCl2 or an 8 M HNO3 solution for elution and only 70% of 99Tc was recovered, impacting both economic and safety viability. Compared to nonelutable resins such as Purolite A530E/ 532E and Reillex-HPQ, elutable resins have more advantages in process flow sheet development and economic cost. Aqueous biphasic extraction chromatography (ABEC) series materials46−51 are one such elutable resin for capturing TcO4− that can be simply regenerated by water. ABEC resins were fabricated by grafting monomethylated PEGs with different molecular weights onto a chloromethylated polystyrene−1% divinylbenzene support. It was found that TcO4− could be selectively retained within these resins when a high concentration of waterstructuring anions (SO42−, CO32−, OH−, etc.) is used as a mobile phase, behaving like an aqueous biphasic system (ABS). The removal capability of these resins toward TcO4− increases when increasing the concentration of water-structuring salts and anions with large negative solvation Gibbs energies. Moreover, updated ABEC resins conducive for large-scale chromatographic applications have also been synthesized. These updated resins have a size of 50−100 mesh with 99.94% (DF ≈ 1700), and 99% of 99Tc could be eluted with less than 15 bed volumes of deionized water at 65 °C. Although SuperLig 639 resin is relatively expensive, such simple regeneration by water allows the resin to be reused

It is the first ion-exchange resin for TcO4 removal. A total of 90 000 L of alkaline waste was treated using 1520 L of IRA-401 resin, and 70% of the 99Tc (∼1.1 kg) was recovered. However, the selectivity is adversely affected by the high concentration of competing anions. Purolite A530 resin is currently in use at the 200 West Pump and Treat Plant to remove TcO4−. The maximum exchange capacities toward ReO4− are 707 and 446 mg/g for Purolite A530E and A532E, respectively. The ReO4− removal reaches up to 90 and 80% for Purolite A530E and A532E, respectively, from the Hanford low-level waste. However, elution is difficult, and the sorption kinetics are relatively slow.



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Figure 5. Structures of purely inorganic cationic frameworks: (a) LDHs, (b) LRHs, (c) Yb3O(OH)6Cl, and (d) NDTB-1.

anion considerably improves the sorption capability for TcO4−. (3) The crystallinity of LDH materials could increase the sorption capability. (4) Considering that large amounts of CO2 will replace the nitrate sites, the Ni−Al−CO3 LDH with a Ni/Al ratio of 3:1 should be the best candidate for trapping TcO4− for real practical applications. (5) The sorption of TcO4− onto M(II)−M(III)−CO3 LDHs is dominated by the edge sites of the LDH layers. However, the distribution coefficients of TcO4− sorption by LDH materials are not as high. Designing new inorganic cationic materials to improve the sorption capabilities is still highly desirable. Fogg66−69 and Geng et al.70−72 prepared a new family of cationic layered rare-earth hydroxides (LRHs), RE2(OH)5(NO3)·nH2O, and RE2(OH)5Cl·nH2O (RE = Y, Sm−Lu) (Figure 5b). Employing similar synthesis routes, Fogg et al.73 also obtained the rare case of a three-dimensional (3D) purely inorganic cationic framework, Yb3O(OH)6Cl (Figure 5c). Initial investigations have confirmed that the nitrate and chloride anions in these structures could be exchanged with common inorganic (e.g., sulfate) and organic dicarboxylate anions. Wang et al.4,74,75 synthesized an interesting 3D cationic thorium borate ([ThB5O6(OH)6][BO(OH)2]·2.5H2O, NDTB1, Figure 5d) material using a boric acid flux reaction. In this structure, thorium is coordinated with BO3 triangles and BO4 tetrahedra, resulting in a cationic supertetrahedral framework full of cavities with a size of 9.4 × 7.8 Å. The disordered negative H2BO3− ions in these voids can be exchanged with anions. The anion-exchange experiments confirmed that NDTB-1 could efficiently sequester TcO4− from aqueous solution (Figure 6a). The maximum exchange capacity and distribution coefficient are reported as 162.2 mg/g and 1.0534 × 104 mL/g, respectively. Furthermore, the exchange selectivity has been evaluated, and the order entering into NDTB-1 is as follows: Cl− < I− ≈ ClO4− ≈ NO3− < H2PO4− < HSeO3− ≈ TcO4− ≈ ReO4− ≈ IO3− < H2AsO4− < SO42− < SeO42− < PO43− (Figure 6b). Additionally, the NDTB-1 material was tested using a simulated Hanford LAW melter recycle stream and had a 99Tc removal efficiency of

for multiple cycles. Furthermore, the resin showed no degradation at an irradiation dosage of less than 1 × 108 Rads, which indicated that the SuperLig 639 resin could be very suitable for 99Tc removal after the 137Cs removal process. However, one should bear in mind that SuperLig 639 resin is specifically designed for removing 99Tc in its +7 oxidation state. The removal performance would significantly decrease when waste contains some nonpertechnetate species. 2.4. Purely Inorganic Cationic Frameworks. Compared to purely inorganic anionic framework materials (zeolites, montmorillonite, etc.), natural cationic framework materials are less known and limited to only hydrotalite. A large amount of materials called layered double hydroxides (LDHs, Figure 5a) have been developed based on the hydrotalite-like characteristics. LDHs58−62 consist of positively charged brucite-type layers with the charge balanced by anions in the interlayer spaces. Generally, LDHs can be described by the formula [M2+1−xM3+x(OH)2]x+(Am−)x/m·nH2O, where M2+ and M3+ represent divalent (Mg2+, Zn2+, Co2+, Mn2+, Ni2+, and Ca2+) and trivalent (Al3+, Fe3+, and Cr3+) cations, and Am− is the interlayer charge-balancing anion (Cl−, NO3−, CO32−, and SO42−). The anions in the interlayers can facilely exchange with other more favorable anionic species.60,62 The excellent sorption capability of ReO4−/TcO4− onto LDHs was first investigated using calcined Mg6Al2O9.63,64 This LDH exhibited almost identical sorption behaviors toward ReO4− and TcO4− because of their similarity in size and geometry. The sorption of TcO4− onto LDHs is a typical stepwise ion-exchange process, in which one OH− of the LDH is first exchanged by TcO4−, forming the Mg6Al2(OH)17(TcO4) (s) phase. With an increase in the TcO4− concentration, two TcO4− anions are incorporated into the LDH bulk material. Wang et al.65 established a general structure−property relationship between TcO4− sorption and different LDH materials. The main findings are summarized as follows: (1) Ni−Al LDH with a Ni/Al ratio of 3:1 shows the maximum sorption capacity among all tested LDHs. (2) The LDH-bearing nitrate as an interlayer 3868


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and channels of NDTB-1 not only accommodate TcO4− but that most of TcO4− is trapped within the cavities. Very recently, we compared the sorption capacity of ReO4− on all purely inorganic cationic framework materials and found that although many efforts have been made to explore new inorganic cationic materials, LDHs are still the best. Additionally, all purely inorganic cationic materials show relatively poor selectivity, low capacity, and instability at low pH in applications for TcO4− separation.76 2.5. Metal−Organic Frameworks (MOFs). Metal−organic frameworks (MOFs) are assembled by functional organic linkers and metal ions/clusters and are an emerging class of porous materials. Their high specific surface areas, tunable pore size and shape, and facile functionalization endow MOFs with superior properties relative to those of traditional porous materials in applications of catalysis, gas storage, separation, sensing, and biomedicine.77−89 Recently, the utilization of MOFs for the separation of radionuclides has expanded considerately,83,90,91 although among them, cationic MOFs have been relatively less investigated. Generally, cationic frameworks are built through the strong coordination of soft neutral organic ligands with transition metal ions. The hydrogen atoms in the organic ligands play an essential role in forming a dense hydrogen-bonding network, which is beneficial for strongly sequestering anions. Although large amounts of cationic MOFs have been constructed for capturing anionic contaminants,92−97 few investigations have been directly conducted with radioactive TcO4−. As a result of the similar ionic size and chemical properties of ReO4− and TcO4−, ReO4− is usually considered to be a good surrogate for TcO4− in the study of its sorption process. Previously, small cationic metal−organic complexes have been reported to sense or immobilize TcO4−/ReO4−, but they all lack extended structures, resulting in high solubility in aqueous solution.98,99 Fei et al.100 first reported a cationic layered [Ag2(4,4′-bipy)2(O3SCH2CH2SO3)·4H2O] (SLUG-

Figure 6. TcO4− exchange kinetics (a) and selectivity (b) by NDTB-1. Reprinted with permission from ref 4. Copyright 2012 Wiley VCH.

44.8% after 4 h, indicating that NDTB-1 is a good TcO4− scavenger. 99Tc-MAS NMR spectra elucidated that the cavities

Figure 7. (a) Sorption properties of SCU-100 toward ReO4− as a function of contact time, (b) comparison of the sorption capacity of ReO4− by SCU100 and other inorganic cationic sorbents, and (c) schematic of the sorption mechanism between ReO4− and SCU-100. Reprinted with permission from ref 76. Copyright 2017 American Chemical Society. 3869


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Chemistry of Materials 21) MOF material for the efficient removal of ReO4−. The sorption process represented a new paradigm for anion separation entitled single-crystal-to-single-crystal transformation, giving a high sorption capacity of 602 mg/g for ReO4−. However, the ethanedisulfonate anions would exchange and may be problematic in vitrification of radioactive waste. Another 2D copper-based tripodal MOF has been synthesized with a similar ReO4− trapping mechanism.101 The cationic tricopper [(CuCl)3L]3+ [L = N(CH2−o-C6H4CH2N(CH2py)2)3] layer is linked by Cu(μ-Cl)2Cu and structurally transformed to covalent Cu3(μ3-ReO4) linkers after anion exchange. Banerjee et al. prepared a water-stable cationic zirconium-based MOF containing positive organic amine groups to remove ReO4− from aqueous solution, but there is still much room for improvement in the sorption kinetics, capacity, and selectivity. Recently, a hydrolytically stable three-dimensional cationic MOF material, SCU-100 (SCU = Soochow University), was assembled through a tetrakis[4-(1-imidazol-yl)-phenyl]methane ligand and two-coordinate Ag+ cations with open metal sites.76 SCU-100 contains large amounts of onedimensional channels with a size of 6.9 × 6.9 Å, which are filled with disordered NO3−. This MOF material is quite stable in a wide pH range and when exposed to high ionizing radiation. SCU-100 is the first MOF material that was directly tested with 99 TcO4− to determine its sorption properties. The batch experimental results showed that it can rapidly remove all the TcO4− from water within 30 min (Figure 7a). The saturated uptake capacity of SCU-100 toward ReO4− is 553 mg/g (Figure 7b). Impressively, the removal efficiency of TcO4− by the SCU100 sorbent in the presence of a large excess of competitive anions (NO3−, SO42−, CO32−, and PO43−) is minimally affected. Even in the complicated simulated Hanford low-level waste, SCU-100 is still able to capture nearly 87% of TcO4−. With the help of the single-crystal X-ray diffraction (SC-XRD) technique, the sorption mechanism was demonstrated to be a single-crystalto-single-crystal transformation process (Figure 7c). It was found that the 8-fold interpenetrated framework in the original SCU-100 is transformed to a 4-fold interpenetrated framework, and the disordered NO3− anions are totally exchanged with ReO4− anions. The open Ag+ sites play an important role in the capture of ReO4− by forming strong Ag−O−Re bonds. Furthermore, the ReO4− anions are surrounded by an extremely dense hydrogen-bonding network, which is another essential factor in determining the excellent sorption properties for ReO4−/TcO4−. However, as a result of the single-crystal-to-single-crystal transformation mechanism, large single crystals of SCU-100 would transform into small crystallites after the ion-exchange process, making it inappropriate for industrial column chromatographic separation. We then designed another robust 3D nickel-based MOF, SCU-101, using the same ligand.102 This material can anion exchange without a framework structure transformation, therefore overcoming the aforementioned issue. The overall structure of SCU-101 is a common porous cationic extended framework, where each nickel ion is coordinated by four neutral tipm ligands and one oxalate, affording a high positive charge density on the framework (Figure 8a,b). Disordered nitrate anions fill in the pores to balance charge and can be facilely exchanged. The sequestration equilibrium is reached within 10 min, apparently faster than that of commercial ion-exchange resins and other scavengers. The distribution coefficient and the maximum uptake capacity for ReO4− are 7.5 × 105 mL/g and 217 mg/g, respectively. Impressively, SCU-101

Figure 8. (a) TcO4− trapped within A-type channels in SCU-101, (b) hydrogen bonding between the framework of SCU-101 and TcO4−, (c) electrostatic potential (ESP) distribution of partial frameworks, and (d) optimized structure, by DFT calculations, of TcO4− trapped in the framework of SCU-101. Reprinted with permission from ref 102. Copyright 2017 American Chemical Society.

can selectively sequester TcO4− from an aqueous solution containing large amounts of NO3− and SO42−, as even the presence of 6000 times excess of SO42− would not considerably affect the removal of TcO4−. The sorption mechanism has been unraveled by the SC-XRD technique. In TcO4−-sorbed SCU101, TcO4− anions prefer to occupy the channels of A rather than either B or C (Figure 8c). A more in-depth examination of the single-crystal structure of TcO4−-sorbed SCU-101 revealed that TcO4− anions are trapped in a dense hydrogen-bonding network constructed with H atoms from phenyl and imidazolyl groups (Figure 8d). Furthermore, the DFT calculation shows that the binding energy is −20.42 kcal/mol, which is thermodynamically favorable. This is the first case of a singlecrystal structure containing TcO4− trapped in a porous sorbent material. In addition, we also reported a one-dimensional cationic MOF material, Ag(4,4′-bipyridine)NO3 (SBN),103 which demonstrates remarkably efficient removal of ReO4−. The uptake capacity of SBN reaches as high as 786 mg/g for ReO4−, which is the largest value among all reported cationic MOF materials. Furthermore, once captured, ReO4− is immobilized in an extremely insoluble solid (Figure 9), and no ReO4− can be exchanged even in the presence of 1000 times of nitrate anions. From this perspective, SBN can be considered a potential waste form for the direct immobilization of TcO4−. Both single-crystal structure and DFT calculations revealed that the ReO4−/TcO4− anions in the resulting compound are strongly trapped within the crystal lattice by multiple hydrogen and Ag−O−Re bonds. The open Ag+ sites play an important role in determining the selectivity of SBN toward ReO4−/TcO4−, which sheds light on designing similar highly efficient cationic MOFs for capturing TcO4−. Additionally, our endeavor in the design of cationic frameworks for TcO4− sequestration has been extended to actinide-based MOF materials, such as SCU-6, SCU-7, and SCU-8.104,105 Although the sorption properties of these materials are not comparable to those of the SCU-10X (X = 0,1,...) series, the assembly of such cationic extended frameworks is particularly insightful. For example, SCU-8 was synthesized through a solvothermal reaction between (1,1′-biphenyl)-3,4′,53870


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Figure 9. Sequestration mechanism between ReO4− and SBN. Reprinted with permission from ref 103. Copyright 2017 American Chemical Society.

Figure 10. Crystal structure depictions of SCU-8. Reprinted with permission from ref 105 under a Creative Commons CC-BY license.

tricarboxylic acid and Th(NO3)4·6H2O in an ionic liquid, tetramethylguanidine chloride. In the structure of SCU-8, three Th atoms are linked by a μ3-O and six μ2-O from −COOH groups, forming a cationic [Th3(COO)9O(H2O)3.78]+ building unit (Figure 10a,b). These units are further bridged by nine carboxylate groups from the ligands, forming a 3D cationic extended framework with 1D hexagonal tubular channels of 22 × 22 Å (Figures 10c,d). Highly disordered chloride ions are filled in the channels to balance charge and could be facilely exchanged, which was confirmed by ReO4− removal experiments. SCU-8 represents a rare example of a hydrolytically stable, mesoporous 3D cationic MOF with superb anionexchange capabilities. Very recently, Farha et al.106 investigated the capture of ReO4−/TcO4− by NU-1000. Although the NU-1000 is not a

cationic framework, it still shows a high uptake capacity of 210 mg/g for ReO4−, and sorption equilibrium is achieved in only 5 min. Structure refinement indicates that the zirconium cluster in the structure of the MOF plays a crucial role in the sequestration of ReO4−/TcO4−. There are two respective binding modes in the pores of NU-1000, including chelating and nonchelating perrhenate (Figure 11). The hydroxyl (−OH) and water (−OH2) of the Zr nodes are replaced by ReO4− through a ligand exchange mechanism. More specifically, one −OH and one −OH2 group each are replaced by ReO4− via a chelating mode; whereas another single −OH group is exchanged by ReO4− in a nonchelating manner. 2.6. Cationic Polymeric Networks (CPNs). The polymeric networks formed by repeating organic monomers have been extensively explored in the field of environmental-related 3871


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Figure 11. Crystal structure of NU-1000 after ReO4− is sorbed. Reprinted with permission from ref 106. Copyright 2018 American Chemical Society.

Figure 12. (a) Synthesis route of SCU-CPN-1 and its anion-exchange applications, (b) SEM image of SCU-CPN-1-Br, and (c) EDS mapping of SCUCPN-1-Br, SCU-CPN-1-Cl, and SCU-CPN-1-Re. Reprinted with permission from ref 109 under a Creative Commons CC-BY license.

treatment.107 These materials have many excellent properties, such as controllable synthesis and structure, adjustable pore size, and easy functionalization. Furthermore, these cationic polymeric networks disclose excellent water stability under extreme pH conditions, which is a significant advantage compared to most of the MOFs. Moreover, large conjugated fragments in the polymeric networks can enhance the radiation resistance capability because of the effective stabilization of the radiation-induced radical intermediates. Such merit is critical for used fuel reprocessing and nuclear accident emergencies. Lee et al. synthesized a 2D porous cationic polymeric framework (2D CPN) by a cyclotrimerization cross-linking reaction with a nitrile-functionalized task-specific ionic liquid at 400 °C, which had a moderate ReO4− exchange capability.108

Recently, we reported a novel noncrystalline cationic polymeric network, SCU-CPN-1 (Figure 12), to efficiently trap TcO4−.109 SCU-CPN-1 is obtained from the quaternization reaction between 1,4-bis(bromomethyl)benzene (BBB) and 1,1,2,2tetrakis(4-(imidazolyl-4-yl)-phenyl)ethene (TIPE) under microwave irradiation conditions. It possesses unprecedented TcO4− sorption properties in the following aspects: the highest sorption capacities (990 mg/g for ReO4−), the fastest sorption kinetics (uptake equilibrium can be reached within seconds), the most-promising removal capability in highly acidic media, and amazing water stability and radiation-resistant properties among all the reported materials to date. This novel adsorbent leads to efficient TcO4− separation from three different types of simulated nuclear waste solutions with extremely high ionic 3872


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Figure 13. (a) ESP distributions, (b) optimized sorption complexes of M+ReO4− (M+TcO4−) and M+NO3−, and (c) molecular orbital interaction diagrams of M+ReO4−. Reprinted with permission from ref 109 under a Creative Commons CC-BY license.

Savannah River sites. SuperLig 639 and Purolite A530E resins can be considered as a baseline for developing new TcO4− separation materials. Note that the key issues for these organic anion-exchange resins are their poor radiation resistance and relatively slow uptake kinetics that amplify the former drawback. Although the precipitation technique represents the simplest method to separate TcO4−, the current precipitating agents cannot make TcO4− insoluble enough to discharge. The reducing materials are promising for converting soluble TcO4− to insoluble Tc(IV) phases and incorporating them into bulky materials. These materials may be favorable for treating TcO4− wastes relevant to real environments. With respect to purely inorganic cationic frameworks, the discovery of these extremely rare types of materials is meaningful for chemical science, but they still exhibit low uptake capacity and poor selectivity for TcO4−. Cationic MOFs and CPNs are the most-promising and emerging materials for efficient removal of TcO4− with the fastest uptake kinetics, highest capacity, exceptional selectivity and are even robust under high radiation conditions. To further enhance the separation properties of TcO4−, some rules may aid in designing such types of new materials: (1) using small neutral molecules as linking agents that can increase the sorption capacity; (2) making the cationic framework as hydrophobic as possible, which will enhance the selectivity; (3) introducing open metal sites to improve the selectivity; and (4) avoiding single-crystal-to-single-crystal transformation during ion exchange, which may destroy the large crystals and increase the column pressure during chromatographic separation. However, in-depth studies to increase the stability of MOFs in highly acidic solutions and CPNs in highly alkaline solutions are still urgently desirable.

strength, such as the Hanford low-activity waste (LAW) melter recycle stream, spent fuel reprocessing solution in 3 M nitric acid, and contaminated groundwater and vadose zone pore water in the subsurface at the Hanford nuclear reservation. Additionally, this material is entirely recyclable for sorption/ desorption cycles, making it extremely appealing for the partitioning of high-level liquid wastes and emergency remediation tasks. The excellent TcO4− sorption capability of the SCU-CPN-1 cationic polymeric framework was further explained by synchrotron radiation X-ray absorption spectroscopy, solidstate NMR, and DFT analyses on anion coordination and bonding. The obtained results showed that this cationic framework attracted anions through strong electrostatic interactions. Moreover, the molecular orbital analyses showed that the π-bond of the imidazole group hybridizes with the lone pair electrons of three O atoms in ReO4−, constructing relatively strong p−π-interactions in the form of the face-to-face stacking geometry observed (Figure 13). In contrast, only one O atom of NO3− can hybridize with the π-bond of the imidazole ring. It can be concluded that the cationic imidazole ring plays a very important role in determining the selectivity toward TcO4−/ ReO4− over NO3− by SCU-CPN-1, which sheds light on further design of anion-exchange materials.

3. CONCLUSION AND OUTLOOK This Perspective discussed different methods for separation of TcO4− from radioactive waste, specifically methods that use precipitating agents, reducing materials, ion-exchange resins, purely inorganic cationic frameworks, cationic metal−organic frameworks (MOFs), and cationic polymeric networks (CPNs) materials. Comparatively, anion-exchange resins hold a definite advantage for large-scale manufacturing and are available in an engineered form suitable for column chromatographic separation. Several ion-exchange resins have been tested in the treatment of TcO4−-containing tank wastes at Hanford and

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. (S.W.) 3873


Perspective

Chemistry of Materials ORCID

HPQ anion exchange resin from Hanford and Melton Valley tank waste simulants and sodium hydroxide sodium nitrate solutions. Solvent Extr. Ion Exch. 1998, 16 (3), 843. (4) Wang, S. A.; Yu, P.; Purse, B. A.; Orta, M. J.; Diwu, J.; Casey, W. H.; Phillips, B. L.; Alekseev, E. V.; Depmeier, W.; Hobbs, D. T.; et al. Selectivity, Kinetics, and Efficiency of Reversible Anion Exchange with TcO4− in a Supertetrahedral Cationic Framework. Adv. Funct. Mater. 2012, 22 (11), 2241. (5) Custelcean, R.; Moyer, B. A. Anion separation with metal-organic frameworks. Eur. J. Inorg. Chem. 2007, 2007, 1321. (6) Katayev, E. A.; Kolesnikov, G. V.; Sessler, J. L. Molecular recognition of pertechnetate and perrhenate. Chem. Soc. Rev. 2009, 38 (6), 1572. (7) Pearce, C. I.; Icenhower, J. P.; Asmussen, R. M.; Tratnyek, P. G.; Rosso, K. M.; Lukens, W. W.; Qafoku, N. P. Technetium Stabilization in Low-Solubility Sulfide Phases: A Review. ACS Earth Space Chem. 2018, 2 (6), 532. (8) Johnson, D. A. Thermochemistry of Ammonium and Rubidium Perrhenates, and the Effect of Hydrogen-Bonding on the Solubilities of Ammonium-Salts. J. Chem. Soc., Dalton Trans. 1990, 11, 3301. (9) German, K. E.; Krjuchkov, S. V.; Belyaeva, L. I.; Spitsyn, V. I. Ion Association in Tetraalkylammonium Pertechnetate Solutions. J. Radioanal. Nucl. Chem. 1988, 121 (2), 515. (10) German, K. E.; Grigoriev, M. S.; Den Auwer, C.; Maruk, A. Y.; Obruchnikova, Y. A. Structure and solubility of tetrapropylammonium pertechnetate and perrhenate. Russ. J. Inorg. Chem. 2013, 58 (6), 691. (11) Chadwick, T. C. 1,2,4,6-Tetraphenylpyridinium Acetate as a New Anion Precipitant. Anal. Chem. 1976, 48 (8), 1201. (12) Mausolf, E.; Droessler, J.; Poineau, F.; Hartmann, T.; Czerwinski, K. Tetraphenylpyridinium pertechnetate: a promising salt for the immobilization of technetium. Radiochim. Acta 2012, 100 (5), 325. (13) Custelcean, R.; Williams, N. J.; Seipp, C. A. Aqueous Sulfate Separation by Crystallization of Sulfate-Water Clusters. Angew. Chem., Int. Ed. 2015, 54 (36), 10525. (14) Custelcean, R.; Williams, N. J.; Seipp, C. A.; Ivanov, A. S.; Bryantsev, V. S. Aqueous Sulfate Separation by Sequestration of [(SO4)2(H2O)4]4‑ Clusters within Highly Insoluble Imine-Linked BisGuanidinium Crystals. Chem. - Eur. J. 2016, 22 (6), 1997. (15) Fan, D. M.; Anitori, R. P.; Tebo, B. M.; Tratnyek, P. G.; Lezama Pacheco, J. S.; Kukkadapu, R. K.; Engelhard, M. H.; Bowden, M. E.; Kovarik, L.; Arey, B. W. Reductive Sequestration of Pertechnetate 99 TcO4− by Nano Zerovalent Iron (nZVI) Transformed by Abiotic Sulfide. Environ. Sci. Technol. 2013, 47 (10), 5302. (16) Zou, Y.; Wang, X.; Khan, A.; Wang, P.; Liu, Y.; Alsaedi, A.; Hayat, T.; Wang, X. Environmental Remediation and Application of Nanoscale Zero-Valent Iron and Its Composites for the Removal of Heavy Metal Ions: A Review. Environ. Sci. Technol. 2016, 50 (14), 7290. (17) Marshall, T. A.; Morris, K.; Law, G. T.; Mosselmans, J. F.; Bots, P.; Parry, S. A.; Shaw, S. Incorporation and retention of 99Tc(IV) in magnetite under high pH conditions. Environ. Sci. Technol. 2014, 48 (20), 11853. (18) Yalcintas, E.; Scheinost, A. C.; Gaona, X.; Altmaier, M. Systematic XAS study on the reduction and uptake of Tc by magnetite and mackinawite. Dalton Trans 2016, 45 (44), 17874. (19) Darab, J. G.; Amonette, A. B.; Burke, D. S. D.; Orr, R. D.; Ponder, S. M.; Schrick, B.; Mallouk, T. E.; Lukens, W. W.; Caulder, D. L.; Shuh, D. K. Removal of pertechnetate from simulated nuclear waste streams using supported zerovalent iron. Chem. Mater. 2007, 19 (23), 5703. (20) Li, J.; Chen, C. L.; Zhang, R.; Wang, X. K. Reductive immobilization of Re(VII) by graphene modified nanoscale zerovalent iron particles using a plasma technique. Sci. China: Chem. 2016, 59 (1), 150. (21) Peretyazhko, T.; Zachara, J. M.; Heald, S. M.; Kukkadapu, R. K.; Liu, C.; Plymale, A. E.; Resch, C. T. Reduction of Tc(VII) by Fe(II) sorbed on Al (hydr)oxides. Environ. Sci. Technol. 2008, 42 (15), 5499. (22) Cui, D. Q.; Eriksen, T. E. Reduction of pertechnetate in solution by heterogeneous electron transfer from Fe(II)-containing geological material. Environ. Sci. Technol. 1996, 30 (7), 2263.

Chengliang Xiao: 0000-0001-5081-2398 Shuao Wang: 0000-0002-1526-1102 Notes

The authors declare no competing financial interest. Biographies Prof. Chengliang Xiao obtained his Ph.D. at Zhejiang University with Prof. Anyun Zhang in 2011 and then joined Purolite (China) Co., Ltd., as an R&D staff to design new ion-exchange resins. In 2012, he returned to an academic career as a postdoctoral fellow with Prof. Zhifang Chai at the Institute of High Energy Physics, CAS. In 2014, he moved to Soochow University as an associate professor at the State Key Laboratory of Radiation Medicine and Protection. During 2016− 2017, he visited Northwestern University and collaborated with Prof. Mercouri G. Kanatzidis in exploring new layered metal sulfides as ionexchangers. Currently, he leads a group at the College of Chemical and Biological Engineering, Zhejiang University, and focuses on chemical separation relevant to spent fuel reprocessing, environmental remediation, and resource recycling. Dr. Afshin Khayambashi received his Ph.D. in nuclear science and technology from Shanghai Jiao Tong University in 2018 under the supervision of Prof. Yuezhou Wei. He worked on actinide separation using novel silica-polymer-based chelating adsorbents during his Ph.D. research. He is currently a postdoctoral research fellow in Prof. Shuao Wang’s group at Soochow University. His current research focuses on synthesizing new covalent organic framework materials for actinide separation. Prof. Shuao Wang received his B.S. from the University of Science and Technology of China (2007) and Ph.D. from the University of Notre Dame (2012). After conducting postdoctoral research at Lawrence Berkeley National Lab and the University of California, Berkeley, he became a professor and the director of the Center of Nuclear Environmental Chemistry at Soochow University, China (2013). Prof. Wang has published more than 140 journal articles, with an H-index of 36. He is the recipient of the Young Investigator Award from the American Chemical Society (2012), the Young Chemist Award from the Chinese Chemical Society (2016), and the National Science Fund for Distinguished Young Scholars from the National Science Foundation of China (2018). The research of his group involves solid-state chemistry, materials chemistry, separation chemistry, and environmental chemistry of the key radionuclides in the nuclear fuel cycle.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21790374, 21825601, U1732112, 21876124), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Fundamental Research Funds for the Central Universities (2019QNA4047). We would like to thank Dr. Mark A. Silver for his advice in the presentation of this publication.

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REFERENCES

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