Separation and Remediation of 99TcO4â from Aqueous Solutions

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Perspective

Separation and Remediation of 99TcO4- from Aqueous Solutions Chengliang Xiao, Afshin Khayambashi, and Shuao Wang Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 25, 2019

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Chemistry of Materials

Separation and Remediation of 99TcO4- from Aqueous Solutions Chengliang Xiaoa, Afshin Khayambashib, Shuao Wangb* a College

of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China

b 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

Abstract Technetium-99 (99Tc) is one of the most problematic radioisotopes in used nuclear fuel owing to its intrinsic features of 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 article, 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.

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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 the lack of efficient, reliable, and economical strategies to treat and dispose of used nuclear fuel. Another serious concern is nuclear safety management since 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 A2

!!

with a long half-life of 2.13 × 105 years, is one of 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 twenty 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 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, due to 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 99Tc 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 high acidity (3 M HNO3), high ionic strength (large amounts of metal and nonmetal ions), and strong ionizing radiation field 0A H and 2


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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 ~2,000 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 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. Table 1 Ionic radii (R, nm) and standard Gibbs energies of hydration (IGh, kJ/mol). ClO4-

TcO4-

I-

NO3-

Br-

Cl-

OH-

F-

CrO42-

SO42-

CO32-

PO43-

R

0.225

0.250

0.220

0.200

0.196

0.181

0.152

0.126

0.229

0.218

0.189

0.230

IGh

-214

-251

-275

-306

-321

-347

-439

-472

-958

-109

-131

-277

0

5

3

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. Since 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 anion-exchange 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- (-1,090 and -2,773 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 anion-exchange 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 3



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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 zero-valent 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.

2. PERTECHNETATE SEPARATION MATERIALS 2.1. Precipitating Agents Table 2 The solubility and solubility product of pertechnetate precipitates. Precipitate

Conditions

Solubility, M

Ksp

Ref.

NH4ReO4

H 2O

0.274

7.5 × 10-2

8

Me4NTcO4

H 2O

0.13

1.7 × 10-2

9

Et4NTcO4

H 2O

1.2 × 10-2

1.4 × 10-4

9

Pr4NTcO4

H 2O

7.9 × 10-2

6.2 × 10-5

10

Bu4NTcO4

H 2O

4.2 × 10-3

1.8 × 10-5

9

Bu4NTcO4

1 M HNO3

8.1 × 10-3

6.6 × 10-5

9

TPPy-ReO4

pH = 11.6

1.6 × 10-5

1.4 × 10-10

11

TPPy-TcO4

pH = 6.91

1.6 × 10-5

2.6 × 10-10

12

4


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TPPy-TcO4

pH = 2.22

1.5 × 10-5

2.2 × 10-10

12

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 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 agents containing metals may be unsuitable for immobilizing TcO4because the metals could contaminate the final Tc compounds. Soluble quaternary ammonium salts hold exceptional precipitation capability towards 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, while 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. (b)

(a)

(c)

Figure 1. (a) Crystal structure of TPPy-TcO4;12 (b) hydrogen bonding of the 5



sulfate-water clusters with BBIG (1,4-benzene-bis(iminoguanidinium)) ligands. Reprinted with permission from ref 14. Copyright 2016 Wiley VCH; and (c) hydrogen bonding of sulfate-water clusters with GBAH (glyoxal bis(amidiniumhydrazone)) ligands. Reprinted with permission from ref 13. Copyright 2015 Wiley VCH. 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. Based on 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, 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 (Figures 1b and 1c). Considering that TcO4- has an almost identical tetrahedral geometry as SO42-, it is possible to precipitate TcO4- using these promising ligands. The preliminary results have confirmed this hypothesis and relevant investigations are ongoing in our group.

2.2. Reducing Materials

6


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(a)

(b)

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. 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 zero-valent iron (ZVI)15,16, magnetite17,18, supported ZVI19,20, Fe(II)-sorbed Al oxides/hydroxides21, and other Fe(II)-based materials22-25, have been widely investigated as technetium reductants. In most of these cases, soluble TcO4is 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, KMS-2, 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 7



achieved by yielding a sparingly soluble Tc sulfide phase (TcS2) instead of TcO2·nH2O. After exposing TcO4- 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/g31. 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)-apatite only worked very well in a near-neutral pH and low ionic strength 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 could 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 8


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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, O2-[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 TcV;0O2-P2W17O61)7- species (Figure 3). In the system introduced by Burton-Pye et al., TcO4- is first reduced to a TcIV intermediate species and then is gradually converted to TcV;0O2-P2W17O61)7-. This mechanism is different from that of its surrogate ReO4-, which is rapidly reduced to ReVO species and incorporated into the 0O2-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.

O2-[P2W17O61]10-

TcV/ReVO(O2-P2 W17O61)7-

TcO4-: O2-[P2W17O61]10- + 2 eO2-[P2W17O61]12- + TcVIIO4Tc/P containing intermediate

O2-[P2W17O61]12Tc/P containing intermediate TcVO(O2-P2W17O61)7-

step 1 step 2 Fast step 3 Slow

ReO4-: O2-[P2W17O61]10- + 2 eO2-[P2W17O61]12- + ReVIIO4ReVIIO4-: O2-[P2W17O61]12-

O2-[P2W17O61]12ReVIIO4-: O2-[P2W17O61]12ReVO(O2-P2W17O61)7-

step 1 step 2 Fast step 3 Slow

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.

2.3. Ion-Exchange Resins Ion-exchange resins are insoluble materials with abundant pores and are normally fabricated by cross-linking styrene and di-vinylbenzene. The functional groups are then 9



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grafted onto a polymer substrate by one-pot polymerization or post-modification 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. H 3C

CH3

H 3C N+

N+ H 3C

ReillexTM-HPQ

CH3

H 3C

CH3

H

N+

N+

O

Purolite A520E

IRA-401

O

O Hexyl

Hexyl N+

CH3

H 3C

O

O O

N+

Hexyl

O

CH3

(CH2)n O

Purolite A530E/A532E

SuperLig@ 639

Figure 4. The functional groups of various TcO4--selective resins. 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 trimethylammonium functional group. A total of 90,000 L of alkaline waste was treated using 1,520 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 10


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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 anion-exchange 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 towards 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. ReillexTM-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 ReillexTM-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 1,066 mL/g for the alkaline Hanford waste tank simulant, 101-SY. Although ReillexTM-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. 11



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

Functional groups

Type

General description

IRA-400

trimethylammonium

elutable

It is the first ion-exchange resin for TcO4- removal A total of 90,000 L of alkaline waste was treated using 1,520 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 A532E

bifunctional

trialkyl

ammonium Purolite A530E

bifunctional

trialkyl

ammonium

gel

Purolite A530 resin is currently in use at the 200 West Pump

non-elutable

and Treat Plant to remove TcO4-. The maximum exchange

macroporous

capacities towards ReO4- are 707 and 446 mg/g for Purolite

non-elutable

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.

ReillexTM-HPQ

a mixture of 75% strong

non-elutable

base-exchange + 3

(RHCH ) weak

This resin shows excellent removal efficiency for TcO4- in a wide pH range, but needs a 1.0 M NaOH/1.0 M

and

25%

ethylenediamine/0.0050 M SnCl2 or an 8 M HNO3 solution for elution and only 70% of 99Tc is recovered.

base-exchange +

sites (RNH ) ABEC resins

monomethylated PEGs

elutable

on the chloromethylated

water-structuring anions and this has been verified using

polystyrene-divinylbenz

Hanford tank wastes. It is unstable after exposure to a high

ene support SuperLig@ 639

TcO4- can be selectively retained in a high concentration of

crown grafted

radiation dosage.

ether

groups

onto

polystyrene support

a

elutable

It has been comprehensively investigated to remove 99Tc from Hanford tank waste simulants and actual tank waste supernate from laboratory-scale to pilot-scale. When testing the Hanford Tank AW-101, the removal of ~1,700), and 99% of

99

Tc was > 99.94% (DF =

99

Tc could be eluted with deionized

water. It is stable under radiation but not suitable for removing non-pertechnetate species.

Compared to non-elutable resins such as Purolite A530E/532E and ReillexTM-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 water-structuring anions (SO42-, CO32-, OH-, etc.) is used as a mobile phase, behaving like an aqueous biphasic system 12


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(ABS). The removal capability of these resins towards 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 < 10% swelling and have shown efficient TcO4- uptake from a 4 M NaOH solution with a volume distribution ratio of 160. ABEC resins have also been verified to effectively remove and concentrate TcO4- from Hanford tank wastes. One main concern in the application of ABEC resins is that water-destructuring anions (NO3- or NO2-) could lower the uptake of TcO4- by the resins. Additionally, the PEG chains in the ABEC resins will breakdown to shorter chain lengths after exposure to a high radiation dosage. Another elutable resin, SuperLig@ 63952-57, provided by IBC Advanced Technologies (American Forks, UT), has been comprehensively investigated to remove 99Tc from Hanford tank waste simulants and actual tank waste supernate from laboratory-scale to pilot-scale. This resin is composed of functional crown ether groups grafted onto a polystyrene support (Figure 4). Taking advantage of the supramolecular hosting capability of crown ethers for K+/Na+/H+ guests to form positive complexes, TcO4- could be selectively bound by the resin as a counter anion. The batch experiments indicated that the SuperLig@ 639 resin is exceptionally selective for TcO4- from highly alkaline waste solutions obtained from the Hanford Site. The distribution coefficients were in the range of 287 - 886 mL/g with more than 80%

99Tc

removal achieved. When testing Hanford Tank AW-101 using column

chromatography, the removal of 99Tc was > 99.94% (DF = ~1,700) 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 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

non-pertechnetate species. 13


waste

contains

some


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, consisted 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/mY %2O, 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 towards ReO4- and TcO4- due to 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 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.

14


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Figure 5. Structures of purely inorganic cationic frameworks, (a) LDHs, (b) LRHs, (c) Yb3O(OH)6Cl, and (d) NDTB-1. 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)5 Y %2O (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, NDTB-1, 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- < PO4315



(Figure 6b). Additionally, the NDTB-1 material was tested using a simulated Hanford LAW melter recycle stream and had a 99Tc removal efficiency of 44.8% after four hours, indicating that NDTB-1 is a good TcO4- scavenger. 99Tc-MAS-NMR spectra elucidated that the cavities and channels of NDTB-1 not only accommodate TcO4-, but that most of TcO4- is trapped within the cavities.

(a)

(b)

Figure 6. TcO4- exchange kinetics (a) and selectivity (b) by NDTB-1. Reprinted with permission from ref 4. Copyright 2012 Wiley VCH. 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 16


Page 16 of 36

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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-. Due to 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

firstly

reported

a

cationic

layered

[Ag2(4,4’-bipy)2(O3SCH2CH2SO3)·4H2O] (SLUG-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( -Cl)2Cu and structurally transformed to covalent Cu3( 3-ReO4) linkers after anion exchange. Banerjee et al. prepared a 17



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 one-dimensional channels with a size of 6.9 Å × 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 99TcO4- 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 towards ReO4- is 553 mg/g (Figure 7b). Impressively, the removal efficiency of TcO4- by the SCU-100 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-crystal-to-single-crystal transformation process (Figure 7c). It was found that the eight-fold interpenetrated framework in the original SCU-100 is transformed to a four-fold interpenetrated framework, and the disordered NO3anions 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-.

18


Page 18 of 36

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Page 20 of 36

solution containing large amounts of NO3- and SO42-, as even the presence of 6,000 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 SCU-101, 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 TcO4anions 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 single crystal structure containing TcO4- trapped in a porous sorbent material.

(a)

(b)

(c)

(d)

0.06 -0.06

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 of TcO4- trapped in the framework of SCU-101 by DFT calculations. Reprinted with permission from ref 102. Copyright 2017 American Chemical Society. 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 20


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and six

2-O

from -COOH groups, forming a cationic [Th3(COO)9O(H2O)3.78]+ building unit

(Figures 10a and 10b). 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 Å × 22 Å (Figures 10c and 10d). 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 anion-exchange capabilities.

Figure 10. Crystal structure depictions of SCU-8. Reprinted with permission from ref 105 under a Creative Commons CC-BY license. 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 five minutes. 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 non-chelating perrhenate (Figure 11). The hydroxyl (-OH) 22


Page 22 of 36

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and water (-OH2) of the Zr nodes are replaced by ReO4- through a ligand exchange mechanism. More specifically, one -OH and -OH2 groups are replaced by ReO4- via a chelating mode; while another single -OH group is exchanged by ReO4- in a non-chelating manner.

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

2.6. Cationic Polymeric Networks (CPNs) The polymeric networks formed by repeating organic monomers have been extensively explored in the field of environmental-related 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 accidents emergencies. 23



Page 24 of 36

Lee et al. synthesized a 2D porous cationic polymeric framework (2D CPNs) by a cyclotrimerization cross-linking reaction with a nitrile-functionalized task-specific ionic liquid at 400 oC, which had a moderate ReO4- exchange capability.108 Recently, we reported a novel non-crystalline 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 1,1,2,2-tetrakis(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 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.

Figure 12. (a) Synthesis route of SCU-CPN-1 and its anion-exchange applications, (b) SEM 24


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image of SCU-CPN-1-Br, and (c) EDS mapping of SCU-CPN-1-Br, SCU-CPN-1-Cl, and SCU-CPN-1-Re. Reprinted with permission from ref 109 under a Creative Commons CC-BY license. The excellent TcO4- sorption capability of the SCU-CPN-1 cationic polymeric framework was further explained by synchrotron radiation X-ray absorption spectroscopy, solid-state 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

bond of the imidazole group

hybridizes with the lone pair electrons of three O atoms in ReO4-, constructing relatively strong p- 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

bond of the imidazole ring. It

can be concluded that the cationic imidazole ring plays a very important role in determining the selectivity towards TcO4-/ReO4- over NO3- by SCU-CPN-1, which sheds light on further design of anion-exchange materials.

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. 25



Page 26 of 36

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 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 TcO4wastes 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 it 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 26


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highly alkaline solutions are still urgently desirable.

ACKNOWLEDGEMENTS 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.

AUTHOR INFORMATION Corresponding Author * Shuao Wang. E-mail: [email protected] 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 ion-exchangers. 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 27



actinide separation using novel silica polymer-based chelating adsorbents during his Ph.D. research. He is currently a postdoctoral research fellow in Professor 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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