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Efficient and Selective Uptake of TcO4- by a Cationic Metal-Organic Framework Material with Open Ag+ Sites Daopeng Sheng, Lin Zhu, Chao Xu, Chengliang Xiao, Yanlong Wang, Yaxing Wang, Lanhua Chen, Juan Diwu, Jing Chen, Zhifang Chai, Thomas E. Albrecht-Schmitt, and Shuao Wang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 17 Feb 2017 Downloaded from http://pubs.acs.org on February 17, 2017

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Environmental Science & Technology

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Efficient and Selective Uptake of TcO4- by a Cationic Metal-Organic Framework

2

Material with Open Ag+ Sites

3

DAOPENG SHENG†,

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WANG†, ‡, YAXING WANG†, ‡,, LANHUA CHEN†, ‡,, JUAN DIWU†, ‡,, JING CHEN#, ZHIFANG

5

CHAI†, ‡, THOMAS E. ALBRECHT-SCHMITT§, AND SHUAO WANG†, ‡, *

6

†

7

Suzhou, P. R. China

8

‡

9

215123, Suzhou, P. R. China

‡, ¶

, LIN ZHU†,

‡, ¶

, CHAO XU#, CHENGLIANG XIAO†,

‡, *

, YANLONG

School for Radiological and Interdisciplinary Sciences (RAD-X), Soochow University, 215123,

Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions,

10

#

11

Technology, Tsinghua University, Beijing 100084, China

12

§

13

Florida 32306, United States

14

*

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[email protected] (SHUAO WANG); Tel: +86-512-65883945; Fax: +86-512-65883945.

16

¶

Nuclear Chemistry and Chemical Engineering Division, Institute of Nuclear and New Energy

Department of Chemistry and Biochemistry, Florida State University, 95 Chieftain Way, Tallahassee,

Corresponding

authors.

Email:

[email protected]

These two authors contributed equally.

1 ACS Paragon Plus Environment

(CHENGLIANG

XIAO);


99

Tc is one of the most problematic radioisotopes in used nuclear fuel

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ABSTRACT:

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owing to its combined features of high fission yield, long half-life, and high

19

environmental mobility. There are only a handful of functional materials that can remove

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TcO4- anion from aqueous solution and identifying for new, stable materials with high

21

anion-exchange capacities, fast kinetics, and good selectivity remains a challenge. We

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report here an 8-fold interpenetrated three-dimensional cationic metal-organic framework

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material, SCU-100, which is assembled from a tetradentate neutral nitrogen-donor ligand

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and two-coordinate Ag+ cations as potential open metal sites. The structure also contains

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a series of 1D channels filled with unbound nitrate anions. SCU-100 maintains its

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crystallinity in aqueous solution over a wide pH range from 1 to 13 and exhibits excellent

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β and γ radiation-resistance. Initial anion exchange studies show that SCU-100 is able to

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both quantitatively and rapidly remove TcO4- from water within 30 min. The exchange

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capacity for the surrogate ReO4- reaches up to 541 mg/g and the distribution coefficient

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Kd is up to 1.9×105 mL/g, which are significantly higher than all previously tested

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inorganic anion sorbent materials. More importantly, SCU-100 can selectively capture

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TcO4- in presence of large excess of competitive anions (NO3-, SO42-, CO32-, and PO43-)

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and remove as much as 87% of TcO4- from the Hanford low-level waste melter off-gas

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scrubber simulant stream within 2 hours. The sorption mechanism is well elucidated by

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single crystal X-ray diffraction, showing that the sorbed ReO4- anion is able to selectively

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coordinate to the open Ag+ sites forming Ag-O-Re bonds and a series of hydrogen bonds.

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This further leads to a single-crystal-to-single-crystal transformation from an 8-fold 2 ACS Paragon Plus Environment

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interpenetrated framework with disordered nitrate anions to a 4-fold interpenetrated

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framework with fully ordered ReO4- anions. This work represents a practical case of

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TcO4- removal by a MOF material and demonstrates the promise of using this type of

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material as a scavenger for treating anionic radioactive contaminants during the nuclear

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waste partitioning and remediation processes.



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INTRODUCTION During the past seventy years beginning with the utilization of the first nuclear 99

45

reactor, it is estimated that about 400 metric tonnes of

Tc has been produced by the

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fission of 235U or 239Pu,1 which is mostly awaiting final disposition. This becomes one of

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major environmental concerns because 99Tc can generate the greatest radiation dose in the

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vadose zone of a waste repository for an extremely long period of times (β emitter with a

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half-life of 2.13×105 years). It primarily exists in +7 oxidation state as the pertechnetate

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anion, TcO4- under oxidative, neutral or even slightly reducing conditions. The

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non-complexing nature, high water solubility, and great stability of TcO4- lead to its

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extremely high mobility in the environment as it can be transported in the subsurface at

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nearly the same velocity as the groundwater.2 Significant challenges are also present for

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handling 99Tc during the nuclear waste vitrification process because of the generation of

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volatile compounds such as Tc2O7. It would be highly beneficial that 99Tc can be initially

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removed from high level waste stream prior to the vitrification. In addition, significant

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amounts of 99Tc has already been released into the environment through weapon testing,

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spent nuclear fuel reprocessing, and nuclear accidents. Therefore, it would be critical and

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urgent for developing new techniques and materials for rapid and efficient uptake of

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TcO4- for both waste partitioning and contaminant remediation purposes. 3

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Ion exchange is an efficient method to remove TcO4-, because it is a relatively simple,

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safe, and low cost process, particularly suitable for removing TcO4- at low levels and

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point-of-use applications. Organic polymer based ion exchange resins including IRA-401, 4 ACS Paragon Plus Environment

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Purolite A520E, Reillex HPQ, and SuperLig-639TM are commercial products designed

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towards the removal of TcO4- from the nuclear waste solution.4-9 Though some of them

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exhibited excellent removal efficiency of TcO4-, the uptake kinetics are generally quite

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slow, rendering them not ideal for remediating technetium spills. In addition, They may

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lose capacity when exposed to highly alkaline solutions or high radiation doses.10

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Alternatively, inorganic cationic materials show some promise in this regard. Layered

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double hydroxides (LDH, e.g. [AlxMg1-x(OH)2]x+) consist of positively charged

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brucite-type layers with the charge balanced by anions in the interlayer spaces and can be

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used as anion exchange materials.11 However, there are still major drawbacks for LDHs

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to treat real wastes containing TcO4-, such as poor selectivity, low capacity, and requiring

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calcination before usage. We reported the first three dimensional (3D) inorganic cationic

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extended framework materials, [ThB5O6(OH)6][BO(OH)2]·2.5H2O (NDTB-1),12-14 which

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could selectively remove TcO4- from nuclear waste streams in presence of large excess of

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competing anions. However, the ion exchange kinetics is slow. In 36 h, less than 72% of

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TcO4‒ was removed from the aqueous solution with an initial Tc concentration of ca. 30

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ppm. In addition, the radioactive nature of thorium in NDTB-1 would significantly limit

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its further applications.

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Metal-organic frameworks (MOFs) built by metal ions/clusters and organic linkers

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are an emerging class of porous materials.15-21 The superior properties of high specific

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surface areas, tunable pore size and shape, and facile functionalization endow them with

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many advantages compared to the traditional porous materials in applications of gas 5 ACS Paragon Plus Environment


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storage, separation, sensing, catalysis, and biomedicine.22-28 The potential applications of

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MOFs in the nuclear fuel cycle (e.g. uranium extraction,29-30 I2 sorption,31-32 and Kr/Xe

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capture.33-34) were intensively investigated recently. Cationic MOFs represent a less

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investigated subclass that are constructed by positively charged frameworks and

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weakly/non- coordinated anions in the pore or interlayer spaces.35-38 One classic strategy

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to design cationic MOFs is through strong coordination between neutral nitrogen-donor

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ligands and soft transition metal ions.39-41 The charge-balancing anions accommodated in

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the pores can be exchanged with other anionic pollutants, such as Cr2O72-, CrO42-, ClO4-,

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MnO4- and anionic dyes.42-52 Until now, examples of water-stable cationic MOFs with

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real anion exchange applications are still quite scarce. Fu et al.51 and Li et al.50 prepared

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several water-stable cationic MOFs with large nanotubular channels, FIR-53, FIR-54, and

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[Ag2(btr)2]·2ClO4·3H2O, for fast and efficient Cr2O72- uptake. Desai et al.52 designed a

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water-stable cationic MOF [{Ni2(L)3(SO4)(H2O)3}(SO4)x(DMF)]n built from Ni2+ and

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tris(4-(1H-imidazolyl)amine (L) for trapping both Cr2O72- and MnO4-. Although MnO4-

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was investigated as a surrogate for TcO4-, the exchange efficiency is limited. For real

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applications, the SO42- anion exchanged out is problematic in the waste vitrification

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process. Similarly, ethanedisulfonate anion is released for cases of cationic inorganic

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layered [Ag2(4,4’-bipy)2(O3SCH2CH2SO3)·4H2O] (SLUG-21)53-54 and copper hydroxide

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ethanedisulfonate (SLUG-26)55-56 materials tested for the removal of ReO4- and MnO4-.

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Furthermore, these two cationic layered materials have relatively high solubility in

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aqueous solution.57 Recently, Banerjee et al.58-59 reported a cationic zirconium-based 6 ACS Paragon Plus Environment

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MOF and a functional porous aromatic framework (PAF) for the removal of ReO4- from

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aqueous solution. Nevertheless, until now there is almost no report on a MOF-based

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material that is directly tested for TcO4- removal as indicated by a recent review.44 Design

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of new hydrolytically and radiolytically stable cationic MOFs for selectively capturing

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TcO4- from nuclear waste solution or contaminated water system remains highly

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desirable.

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We report here the synthesis and the crystal structure of an extremely rare case of

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microporous

8-fold

interpenetrated

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[Ag2(tipm)]·2NO3·1.5H2O

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tetrakis[4-(1-imidazol-yl)phenyl]methane (Figure 1a), constructed by a tetradentate

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neutral nitrogen ligand and two-coordinate Ag+ site with nitrate as the weakly bound

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charge-balancing anion. This material possesses very good hydrolytic stability hydrolytic

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stability and radiation resistance towards high dose β and γ radiation that are required for

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TcO4- removal from nuclear waste solutions. Comprehensive investigations on the TcO4-

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uptake by SCU-100 including the sorption kinetics and uptake selectivity of TcO4- were

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conducted while the exchange capacity and desorption were investigated using the

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surrogate ReO4-. We also performed the TcO4- uptake test from a simulated Hanford LAW

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melter recycle stream solution and demonstrate the promising potential applications of

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SCU-100 for efficient removal of anionic radioisotopes from nuclear waste solutions or

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contaminated natural water systems. The sorption mechanism was elucidated by the

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single crystal structure of ReO4- sorbed SCU-100 material, which confirms that the ReO4-

(SCU-100,

SCU

cationic =

Soochow


framework University,

material tipm

=


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is trapped through the combination of coordination to the open Ag+ site and hydrogen

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bonds. This unique anion exchange mechanism leads to an expected anti-Hofmeister bias

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based exchange selectivity as even large excess of highly charged anions such as sulfate

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and phosphate are not able to affect the uptake of ReO4- by SCU-100.

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MATERIALS AND METHODS

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Synthesis of SCU-100 and SCU-100-Re. SCU-100 suitable for X-ray analysis can

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be synthesized solvothermally by reacting tipm (0.029 g, 0.05 mmol) with AgNO3 (0.017

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g, 0.10 mmol) in the mixture of CH3CN (0.5 mL) and deionized water (2 mL) in a 10 mL

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autoclave. The autoclave was sealed and heated to 90°C in for 4 days and then cooled

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down to room temperature at a rate of 1.25 °C h-1. The product was washed with ethanol

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before being dried in air at room temperature. The single crystals of ReO4- sorbed

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SCU-100 material (denoted as SCU-100-Re) were obtained by simply soaking 10 mg of

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SCU-100 crystals in 10 mL of NaReO4 solution (10 mM) for 12 hours. The crystals were

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filtered and washed with deionized water.

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Anion Exchange Studies. All the experiments were conducted at 25 oC using the

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batch sorption method. The solid/liquid ratio performed in all batch experiments was 1

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g/L. In a typical experiment, 10 mg of SCU-100 was added into 10 mL of aqueous

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solution containing certain contents of TcO4- or ReO4-. The resulting mixture was stirred

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for a desired contact time and separated with a 0.22 µm nylon membrane filter. The

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concentrations of ReO4- in aqueous solution were determined by an inductively coupled 8 ACS Paragon Plus Environment

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plasma mass spectrometry (ICP-MS, Thermo Scientific). The distribution coefficient Kd

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was calculated using the equation of Kd = [(C0 - Ce)V/Ce]/m, where C0 and Ce are the

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initial and equilibrium concentration of ReO4-, V is the volume of solution, m is the mass

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of SCU-100 solid. After anion exchange, the compounds were washed with deionized

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water several times and air-dried, and then the compounds were characterized by FT-IR

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spectroscopy, PXRD, and SEM-EDX.

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Exchange Kinetics Studies of SCU-100. 50 mg of SCU-100 material was added

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into 50 mL of a solution containing 28 ppm TcO4- (detailed studies for other inorganic

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materials are described in the supporting information). The uptake kinetics experiment

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was then repeated by using the same molar amount of ReO4- as the surrogate for TcO4-

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for comparison. The resulting mixture was stirred for a desired contact time by magnetic

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stirrer. UV-vis spectra were acquired to probe the concentration of TcO4- as a function of

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time using the absorption peak at 290 nm. In addition, 99Tc activity was also determined

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by liquid scintillation counting (LSC).

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Sorption Isotherm Experiments. The sorption isotherm experiments of SCU-100,

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Mg-Al LDH, NDTB-1, Y2(OH)5Cl, and Yb3O(OH)6Cl sorbents towards ReO4- were

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determined by varying the initial ReO4- concentration ranging from 10 to 500 mg/L. In a

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typical experiment, 10 mg of sorbents was added into 10 mL of aqueous solution

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containing certain concentration of ReO4-. The resulting mixture was stirred for 12 hours

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to ensure the equilibrium was reached and then separated using a 0.22 µm nylon

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membrane filter. The concentrations of ReO4- in aqueous solution were determined by 9 ACS Paragon Plus Environment


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ICP-MS.

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Exchange Reversibility Studies. The SCU-100 material sorbed with 30 ppm of

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ReO4- was added into a desorption solution containing 1 M NaNO3. The resulting mixture

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was stirred for 12 hours, and then the compound was collected by filtration and washed

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with deionized water. The obtained compound was characterized using PXRD and FT-IR

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spectroscopy.

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Anion Competition Studies. The effect of NO3- was performed by adding 0.15 mM,

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0.75 mM, 1.5 mM, 3 mM, or 15 mM NaNO3 solutions respectively into a 0.15 mM ReO4-

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solution. The competing effect of other anions including SO42-, CO32-, and PO43- were

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initially performed by adding 0.5 mM Na2SO4, Na2CO3, or NaH2PO4 solutions

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receptively into a 0.5 mM ReO4- solution. SCU-100 solid was added in the above solution,

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respectively. The ReO4- sorption capacity of SCU-100 in the presence of different

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concentrations of SO42- was further studied and the details are provided in Table S5. The

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concentrations of ReO4- after sorption in aqueous solution were determined by ICP-MS.

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Exchange Experiments with Simulated Hanford LAW Melter Recycle Stream

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and SCU-100. A simulated Hanford LAW Melter Recycle Stream was prepared

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according to a reported protocol12 and the molar concentration of the anions and molar

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ratio of each anion to that of TcO4- are provided in Table 1. Measured quantities of the

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simulated Hanford recycle stream were pipetted into Erlenmeyer flask containing a

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premeasured quantity of SCU-100 to provide the phase ratios of 1 g/L and 5 g/L. The

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Erlenmeyer flask were placed on magnetic stirrer platform and stirred for 12 h at ambient 10 ACS Paragon Plus Environment

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temperature. A blank test to ensure

Tc was not removed by sorption onto the

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Erlenmeyer flask or filter media or by precipitation during the 12 h test period. The

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suspension was separated with a 0.22 µm nylon membrane filter and the filtrate was

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collected in a clean polyethylene sample bottle.

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scintillation counting.

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Tc activity was determined by liquid

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Physical Property Measurements. Powder X-ray diffraction (PXRD) patterns were

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collected from 5° to 50° with a step of 0.02 on a Bruker D8 Advance diffractometer with

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Cu Kα radiation (λ=1.54056 Å) and a Lynxeye one-dimensional detector. The FT-IR was

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recorded in the range of 4000-400 cm-1 on a Thermo Nicolet iS50 spectrometer.

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Thermogravimetric analysis was carried out on a NETZSCH STA 449F3 instrument in

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the range of 30-800 °C under a nitrogen flow at a heating rate of 10 °C/ min. SEM and

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Energy-dispersive spectroscopy (EDS) images and mapping were recorded on a FEI

201

Quanta 200FEG scanning electron microscope (SEM) with the energy of the electron

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beam being 20 keV. Absorption spectra of TcO4- were carried out on a Cary 6000i

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spectrophotometer (Agilent Inc.) from 200 to 400 nm with an interval of 0.1 nm. The

204

concentration of

205

(PerkinElmer Inc.).

99

TcO4- in solution was also checked by an ultra-low level LSC

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Hydrolytic Stability Measurements. Hydrolytic stability measurements for

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SCU-100 was studied by soaking the samples in HNO3 or NaOH of different pH and

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shaked vigorously in an oscillator for 12 h. The PXRD results demonstrate that SCU-100

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is stable in aqueous solutions within pH range from 1 to 13. After immersed into aqueous 11 ACS Paragon Plus Environment


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solutions with different pH values ranging from 1 to 13, the solids were re-collected and

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the ReO4- sorption experiment were further measured to identify the stability of

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SCU-100.

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β and γ Radiation Resistance Measurements. β irradiation experiment was

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conducted using electron beams (1.2 MeV) provided by an electron accelerator. SCU-100

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was irradiated at a dose rate of 20 kGy/h for two different doses (80 and 200 kGy),

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respectively. γ irradiation experiment was conducted using a

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(2.22×1015 Bq). SCU-100 was irradiated at a dose rate of 1.2 kGy/h for two different

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doses (100 and 200 kGy), respectively. The PXRD patterns for the irradiated samples

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match well with the originated sample, which further confirm the excellent radiation

220

resistance of SCU-100.

60

Co irradiation source

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X-ray Crystallography Studies. Data collection was performed on a Bruker

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D8-Venture diffractometer with a Turbo X-ray Source (Mo–Kα radiation, λ = 0.71073 Å)

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adopting the direct drive rotating anode technique and a CMOS detector at room

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temperature. The data frames were collected using the program APEX 2 and processed

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using the program SAINT routine in APEX 2. The structures were solved by direct

226

methods and refined by the full-matrix least squares on F2 using the SHELXTL-97

227

program.60

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RESULTS AND DISCUSSION Synthesis and Structure of SCU-100. SCU-100 was synthesized solvothermally by 12 ACS Paragon Plus Environment

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reacting tipm ligand (Figure 1a) with AgNO3 in the mixture of CH3CN and water (v/v:

231

1/4) at 90oC for 4 days. It crystallizes in the tetragonal space group P42/nbc as

232

light-yellow blocks (Table S1 and Figure S1). Single-crystal X-ray diffraction reveals that

233

this compound consists of a 3D cationic extended framework with nitrate as

234

charge-balancing anions in the pores. Each Ag atom is coordinated by two tipm ligands

235

with an average Ag-N bond distance of 2.094(2) Å (Figure 1b). The bond angle of

236

N-Ag-N is 175.7(2)o, affording an approximately linear silver coordination. The low

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coordination number of Ag+ provides noticeably open space for further coordination by

238

additional ligands, which is the key structural feature for the selective TcO4-/ReO4- uptake

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as discussed below. This turns out to also account for the relatively high thermal

240

parameter of the Ag site (see the cif file), which contributes significantly to the overall

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residual factor of the structure solution of SCU-100. Each tipm ligand binds to four Ag

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atoms to construct a single 3D cationic framework (Figure S2a and Figure 1c). The

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self-assembly between long ligand and low-coordinate metal cation is a common strategy

244

for the construction of highly interpenetrating MOF structures.61 In SCU-100, 8

245

independent but symmetry-related sets of these networks are further entangled together to

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afford a rare case of 8-fold interpenetrated structure while each single network contains

247

large voids with a size of 48 × 32 Å (Figure S2a). Such To clearly display the 8-fold

248

interpenetration, the simplified topological structure is shown in Figure S2b. Although

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the complicate interpenetration significantly reduces the volume of void spaces, a series

250

of 1D channels with a size of 6.9 × 6.9 Å can be still observed along c axis (Figure S3 13 ACS Paragon Plus Environment


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and Figure 1c), which is occupied by the disordered nitrate anions and solvent molecules.

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More importantly, the interpenetration results in a significantly enhanced positive charge

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density in space for the main framework. This is responsible for the strong capability of

254

anion exchange of SCU-100 at the first place. In addition, the experimental powder X-ray

255

diffraction (PXRD) pattern (Figure S6) is consistent with that of the calculated one,

256

confirming the phase purity.

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Hydrolytical and Radiolytical Stability. Thermogravimetric analysis (TGA, Figure

258

S7) shows that SCU-100 is stable up to 300 oC. Significantly, SCU-100 exhibit great

259

hydrolytic stability in aqueous solutions as the PXRD patterns remain almost identical

260

after immersed into aqueous solutions over a wide pH range from 1 to 13 (Figure 2). In

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addition, no structural and crystal degradation are observed for SCU-100 even under 200

262

kGy 60Co γ irradiation or 200 kGy β irradiation (1.2 MeV) (Figure 2). Such an excellent

263

radiation resistance was recently reported for a polycatenated uranyl organic framework

264

material,62 which may also originate from the complex 8-fold interpenetrated architecture,

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providing necessary radiolytic stability for the removal of TcO4- from high level nuclear

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waste solutions. In addition, the radiolytic stability was also confirmed by anion

267

exchange experiments. As shown in Figure S12 and Figure S13, the sorption percentage

268

of ReO4- remained almost unchanged after β and γ irradiation and after immersed in

269

aqueous solutions with different pH values as compared with the original SCU-100

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sample.

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Anion Exchange Properties of SCU-100. To check the anion exchange properties 14 ACS Paragon Plus Environment

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of cationic MOFs, Cr2O72-, CrO42-, ReO4-, or MnO4- are often used as anionic

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targets/surrogates.50-55 In this work, SCU-100 was directly tested with TcO4- initially. As

274

shown in Figure 3a, the TcO4- concentration in solution is rapidly reduced to 76.0% and

275

95.2% of the original sample after 5 min and 30 min, respectively, indicating the kinetics

276

of removing TcO4- is extremely fast. This is significant given that at least several hours

277

are required before reaching anion exchange equilibrium for typical commercial resins.

278

For examples, the exchange equilibrium of Cr2O72- by [Ag2(btr)2]·2ClO4·3H2O is reached

279

after at least a day.50 SLUG-21 can only remove 64% of MnO4- from the aqueous solution

280

after 24 h and the equilibrium can still not be reached at that time.53 As for the TcO4-

281

removal, we have reported the first purely inorganic 3D cationic framework, NDTB-1,

282

which can only remove 72% of TcO4- in 36 h under the same condition.12 We have also

283

conducted ReO4- uptake experiments between SCU-100 and state of the art anion

284

exchange resins under the same condition as a useful comparison. As shown in Figure 3c,

285

the sorption kinetic of SCU-100 is clearly faster than those of commercial resins (A532E

286

and A530E) that are designed for the removal of ClO4- and TcO4-. This should be partially

287

attributed to the regular and ordered one-dimensional channels in crystalline SCU-100

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that can allow for more efficient transport and delivery of ReO4-/TcO4- compared to the

289

amorphous anion exchange resins containing randomly distributed pores. In addition, the

290

relatively smaller particle size of SCU-100 (~ 200 um across) compared to the resin

291

beads (~ 650 nm diameter) also contributes to the observed sorption kinetics difference.

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The advantage in ion exchange kinetics hold by SCU-100 is particularly important in 15 ACS Paragon Plus Environment


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treating radioactive wastes containing TcO4-. The ultrafast removal kinetics would

294

significantly decrease the contact time between the sorbents and radioactive solution,

295

lowering the magnitude of damage induced by radiation and the chance of the releasing

296

risk on an accidental occasion.

297

In addition, the concentration of TcO4- in solution was also checked by an ultra-low

298

level LSC. Using these data, it is confirmed that about 80.7% and 99.5% of TcO4- are

299

removed after 5 min and 30 min, which is consistent with the results obtained from

300

UV-vis spectra. It should be noted that this superior anion uptake capability may originate

301

from the significantly enhanced positive charge density induced by the 8-fold

302

interpenetration. To check if ReO4- is a good non-radioactive surrogate for TcO4- in the

303

ion exchange experiments, we performed the uptake kinetics experiment for ReO4- as a

304

comparison under the same condition. The results show that TcO4- and ReO4- are almost

305

identical (Figure 3b), indicating ReO4- is indeed an ideal model mimicking TcO4- in cold

306

experiments.

307

After ion exchange, the absorbance between 200-240 nm in the UV-vis spectra

308

increases correspondingly, which is attributed to the exchanging out of nitrate anions into

309

the solution. In the FT-IR spectra (Figure S8), the arise of a new peak at 896 cm-1 and

310

decrease of the peak intensity at 1332 cm-1 for the ReO4- loaded SCU-100 material

311

confirms the anion exchange process. Additionally, the EDX mapping profiles (Figure S9)

312

directly show that ReO4- is successfully exchanged into the whole crystals of SCU-100.

313

To obtain the ion exchange capacity of TcO4- by SCU-100, we performed the 16 ACS Paragon Plus Environment

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sorption isotherm experiments using ReO4- as the surrogate. Other four inorganic cationic

315

materials, Mg-Al LDH,63 NDTB-1,13 Y2(OH)5Cl,64 and Yb3O(OH)6Cl65 were also

316

synthesized to evaluate their sorption properties towards ReO4- for comparison (details

317

for these materials are described in the supporting information). When the molar ratio of

318

SCU-100/ ReO4- is 1:2.5, SCU-100 can remove 60% of ReO4- from the solution while 75%

319

of nitrate anions in SCU-100 are exchanged out. When the molar ratio of SCU-100/

320

ReO4- is decreased to 1:10, 100% of nitrate anions in SCU-100 can be quantitatively

321

replaced by ReO4- shown by the ICP analysis on the ReO4- sorbed sample, giving a large

322

exchange capacity of 541 mg ReO4-/ g SCU-100 material. This is significantly higher

323

than those for LDH, NDTB-1, Y2(OH)5Cl, and Yb3O(OH)6Cl (see details in Figures 3d

324

and S14 and Table S2). In addition, comparing with MOF and PAF materials, the overall

325

capacity of ReO4- by SCU-100 is higher than that of UiO-66-NH3+ (159 mg/g) and PAF-1

326

(420 mg/g), but lower than that observed for SLUG-21 (602 mg/g).53,58-59

327

Selectivity and Reversibility. For high level nuclear waste solutions, the

328

concentration of nitrate ion is very high. Therefore, the competing effect of nitrate during

329

anion exchange is critical when dealing with real wastes containing TcO4-. The competing

330

ion exchange experiments of ReO4- (0.15 mM) by SCU-100 were conducted in presence

331

of different amounts of NO3-. As shown in Figure 4a and Table S4, the removal

332

percentage of ReO4- is 99.5% when n = 1 (n is the molar ratio between NO3- and ReO4-).

333

When increasing the amounts of NO3-, the removal efficiency slowly decreases. Under

334

the condition of n = 20, the removal percentage of ReO4- is still higher than 90%, and the 17 ACS Paragon Plus Environment


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removal percentage of ReO4- still reaches 73% when n = 100, indicating that SCU-100

336

exhibits good selectivity towards ReO4- even with large excess of nitrate anions.

337

For certain types of nuclear waste solutions, the concentration of SO42- can be very

338

high up to 6000 times in excess compared to that of TcO4-. Generally, SO42- with higher

339

charge density often successfully outcompetes with TcO4- during the exchange process

340

into inorganic anion sorbent materials.12 We therefore checked the removal selectivity of

341

ReO4- in the presence of various amounts of SO42-. Impressively as shown in Table S5,

342

the removal percentage of ReO4- is almost unaffected by the concentration of SO42-. Even

343

when the concentration of SO42- is 6000 times in excess, the removal percentage of ReO4-

344

is as high as 97%. Furthermore, the removal selectivity between ReO4-, CO32-, SO42-, and

345

PO43- was also investigated and the results illustrate that SCU-100 can completely

346

remove ReO4- in the presence of other anions but exhibits nearly no uptake capability

347

towards those anions with higher charge densities (Table S6). This also contrast sharply

348

with the case of UiO-66-NH3+ under the same conditions, where sulfate and phosphate

349

anions effectively depress ReO4- uptake.58 The underlying reason for this significant

350

ReO4- uptake selectivity is well elucidated in the mechanism session below.

351

To check if SCU-100 is a reversible anion exchange material, a desorption solution

352

containing 1 M NaNO3 was used to elute the SCU-100 material sorbed with 30 ppm of

353

ReO4-. It was found that more than 86% of ReO4- could be exchanged back to the

354

solution, indicating the material can be regenerated. The FT-IR spectra also demonstrate

355

that the loaded SCU-100 material can retransform back to the original material after 18 ACS Paragon Plus Environment

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eluting (Figure S17).

357

TcO4- Removal from a Hanford LAW Melter Off-Gas Scrubber Solution. We

358

prepared a simulated Hanford LAW melter recycle stream to investigate the removal of

359

TcO4- for real applications. As listed in Table 1, the simulated solution is composed of

360

five competitive anions, NO3-, NO2-, Cl-, SO42-, and CO32-, in addition to TcO4-. Among

361

them, the concentrations of NO3-, NO2-, and Cl- are all 300 times higher than that of TcO4-,

362

resulting in a huge challenge to selectively remove TcO4-. Impressively, SCU-100 could

363

still capture as much as 59.3% of TcO4- from the stream when adding 10 mg of SCU-100

364

into 10 mL of the simulated solution (solid/liquid ratio = 1 mg/mL) (Figure 4b). When

365

increasing the solid/liquid ratio to 5 mg/mL, as high as 87% of TcO4- was removed by

366

SCU-100 within just 2 h (Figure 4b). By sharp contrast, NDTB-1 can only remove 13.0%

367

of TcO4- in 4 h under the same condition.12

368

Sorption Mechanism. For a detailed investigation of sorption mechanisms of ReO4-

369

on SCU-100, we have fortunately obtained the single crystals (Figures 5a and 5b) and

370

solved the crystal structure of SCU-100-Re with a formula of [Ag2(tipm)]·2ReO4·nH2O

371

after complete anion exchange of ReO4-. Interestingly, this anion exchange process is

372

accompanied by a single-crystal-to-single-crystal structural transformation, which is

373

initially probed by the obvious change of PXRD patterns from SCU-100 to SCU-100-Re

374

materials (Figure S10). As discussed in the crystal structure of SCU-100, the open

375

two-coordinate Ag+ cation provide available binding sites for further coordination of

376

ReO4-. The crystal structure of SCU-100-Re indeed illustrates the side-on coordination of 19 ACS Paragon Plus Environment


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two ReO4- anions to each Ag+ cation (Figure 5c) with the Ag-O(Re) distances ranging

378

from 2.74 to 2.81 Å, forming a new four-coordinate Ag+ site with a much smaller thermal

379

parameter in comparison with that in the SCU-100 structure solution. The ReO4- anions

380

are effectively immobilized in the structure as each ReO4- anion is tightly bound by two

381

Ag+ cations and multiple hydrogen atoms through a dense hydrogen bond network

382

(Figure 5d and Table S7). The overall structure topology transforms from an 8-fold

383

interpenetrated framework for SCU-100 to a 4-fold interpenetrated framework after anion

384

exchange (Figure S4a, b), as the result of occupation of ReO4- anion in the void spaces of

385

1D channels in the original material (Figure 5a). These comprehensive structural

386

information provides insights into the uptake selectivity for ReO4- by SCU-100 material.

387

First, similar Ag-O-Re bonds are not observed between Ag+ and NO3- in the structure of

388

SCU-100, giving rise to the disorder of NO3-; this well reflects the uptake selectivity for

389

ReO4- over NO3-. Second, the relatively soft Ag+ would preferentially bind to relatively

390

softer anions with smaller charge densities driven by the Pearson hard and soft acid and

391

base theory; therefore, SO42- and PO43- are not able to effectively coordinate to Ag+ in the

392

structure, leading to their low capabilities to be captured. This is further confirmed by the

393

additional experiment of soaking SCU-100 crystals in the concentrated SO42- and PO43-

394

solutions, where no structural transformation is observed shown by the PXRD data

395

(Figure S11). This selectivity trend follows the anti-Hofmeister bias observed in several

396

traditional organic anion-exchange resin material, which is thought to originate from their

397

hydrophobic nature.66 It seems that the capability of selectively binding of soft anions by 20 ACS Paragon Plus Environment

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open Ag+ sites enables a further amplification of the anti-Hofmeister bias based

399

selectivity.

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The long-term radiotoxic 99Tc are of great concern in the environment and spent fuel

401

reprocessing. Cationic inorganic or polymer based materials are widely investigated to

402

remove TcO4- from aqueous solution, but previous results showed that the inorganic ion

403

exchangers held poor stability, low capacity, or slow kinetic, etc, which significantly

404

limits their applications in the real technetium spills situations. The organic polymer

405

based IX exchange resins work well for removing TcO4- from groundwater but their poor

406

radiation resistances are disadvantages while the sorption kinetics are relatively slow.

407

Cationic MOFs have been rarely investigated for TcO4- removal by positively charged

408

frameworks and weakly/non-coordinated anions in the pore or interlayer spaces. These

409

results directly demonstrate the potential of a stable microporous 3D cationic MOF,

410

SCU-100, in the removal of TcO4- from water. The 8-fold interpenetration in SCU-100

411

endows this material with excellent hydrolytic stability and radiation resistance. In

412

addition, the positive charge density is significantly enhanced because of the 8-fold

413

interpenetration, which gives rise to stronger interaction between TcO4- anion and the

414

cationic framework and a compatible open space for accommodating TcO4- anion. The

415

open Ag+ sites in the structure allow for selective binding of TcO4-/ReO4- during the

416

single-crystal-to-single-crystal structural transformation based anion exchange process.

417

These features further lead to a combination of extremely fast kinetics, high capacity, and

418

excellent selectivity for TcO4-/ReO4- uptake, showing a clear advance over all reported 21 ACS Paragon Plus Environment


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419

inorganic cationic materials. The potential application is also demonstrated by the

420

selective removal of TcO4- from a simulated nuclear waste solution in presence of large

421

excess of NO3-, NO2-, and Cl- as well as anions with higher charge density such as SO42-

422

and PO43-. This work provides a practical case of TcO4- removal by a MOF material and

423

sheds light on the design and development of crystalline hybrid materials for applications

424

in the field of environmental radiochemistry.

425

426

Supporting Information

427

Experimental preparation and characterization of ligand and SCU-100, the sorption

428

isotherm models, PXRD and TGA data of SCU-100, characterization of sorption and

429

desorption materials, etc. This information is available free of charge via the Internet at

430

http://pubs.acs.org.

431

432

Corresponding Authors

433

*Email:

434

(SHUAO WANG); Tel: +86-512-65883945; Fax: +86-512-65883945.

435

Notes

436

The authors declare no competing financial interest.

ASSOCIATED CONTENT

AUTHOR INFORMATION

[email protected]

(CHENGLIANG

XIAO);


[email protected]

Page 23 of 34


437

438

This work was supported by grants from the National Natural Science Foundation of

439

China (21422704, U1532259, 11605118), the Science Foundation of Jiangsu Province

440

(BK20150313, BK20140007), the State Key Laboratory of Pollution Control and

441

Resource Reuse Foundation (PCRRF16003), a Project Funded by the Priority Academic

442

Program Development of Jiangsu Higher Education Institutions (PAPD) and Jiangsu

443

Provincial Key Laboratory of Radiation Medicine and Protection, "Young Thousand

444

Talented Program" in China. TEA-S is supported by the U.S. Department of Energy,

445

Office of Science, Office of Basic Energy Sciences, Heavy Elements Chemistry Program,

446

under Award Number DE-FG02-13ER16414.

447

References

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599 600

Figure 1. (a) Chemical structure of tipm ligand. (b) Crystal structure asymmetric unit of

601

SCU-100. (c) Perspective packing structure of SCU-100 viewed along c axis. Atom

602

colors: Ag = red, N = green, C = light blue, Re = olive.

603


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200 kGy γ

Intensity (a.u.)

200 kGy β pH 13 pH 11 pH 9 pH 7 pH 5 pH 3 pH 1 As synthesized

5

10

15

20

25

30

35

40

45

50

2θ (degree)

604 605

Figure 2. Powder X-ray diffraction (PXRD) patterns of SCU-100 after immersed in

606

aqueous solutions with different pH values ranging from 1 to 13 and after β and γ

607

irradiation.

608



(a) 1.2

TcO4-

0.6 0.4 0.2 0.0 200

250

300

350

100 80 60

SCU-100-ReO 4

40

SCU-100-TcO 4

(c) (c)

-

-

20 0 0

400

Wavelength (nm)

200

400

600

800 1000 1200

Contact time (min) (d) 400 (d)

100 80

-

SCU-100-ReO4-

60

A532E-ReO4-

40

q (mg Re/g/gsorbent) q (mg ReO sorbent) 4

Absorbance

0.8

Removal Percentage (%)

(b) 600 min 420 min 240 min 120 min 60 min 30 min 10 min 5 min 0 min

1.0

Removal Percentage (%)

Page 30 of 34

A530E-ReO4-

20 0

350 300

SCU-100 Mg-Al LDH NDTB-1 Y2(OH)5Cl Yb3O(OH)6Cl

250 200 150 100 50 0

0

20

40

60

80

100

120

140

160

0

50

100

150

200

Ceq (mg/L)

Contact time (min) 609 610

Figure 3. (a) UV-vis absorption spectra of aqueous TcO4- solution during the anion

611

exchange with SCU-100. (b) Removal percentage of TcO4- and ReO4- by SCU-100 as a

612

function of contact time. (c) Comparison of the sorption kinetics of ReO4- by SCU-100,

613

A532E, and A530E. (d) Sorption isotherms of ReO4- by cationic SCU-100 compared with

614

Mg-Al LDH, NDTB-1, Y2(OH)5Cl, and Yb3O(OH)6Cl materials.

615


Page 31 of 34


(a)

Removal rate (%)

100 80 60 40 20 0

0:1

10:1

5:1

1:1

20:1

100:1

-

-

Molar ratio between NO3 and ReO4 3

Removal percentage (%)

(b)100

1h SCU-100

4

20 h 2h SCU-100 SCU-100

80

1h SCU-100 60 40

4h NDTB-1

20 0 1

5

5

5

5

Phase ratio (mg/mL)

616 617

Figure 4. (a) Effect of competing nitrate ions on the anion exchange of ReO4- by

618

SCU-100. (b) TcO4- Removal from a Simulated Hanford Waste by SCU-100 and

619

NDTB-1.

620



621 622

Figure 5. (a) Perspective packing structure of SCU-100-Re viewed along c axis. (b) One

623

single network crystal structure depictions of SCU-100-Re. (c) Crystal structure

624

asymmetric unit of SCU-100-Re. (d) A view of hydrogen bond networks in SCU-100-Re.

625

Atom colors: Ag = red, N = green, C = light blue, Re = olive.

626


Page 32 of 34

Page 33 of 34

627


Table 1. Composition of Hanford LAW melter off-gas scrubber solution. Anion

Concentration, mol/L

Anion: TcO4-, molar ratio

TcO4-

1.94×10-4

1.0

NO3-

6.07×10-2

314

Cl-

6.39×10-2

330

NO2-

1.69×10-1

873

SO42-

6.64×10-6

0.0343

CO32-

4.30×10-5

0.222

628 629



630

TOC

631

632


Page 34 of 34

Efficient and Selective Uptake of TcO4â by a ... - ACS Publications

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