Article pubs.acs.org/cm Highly Selective and Rapid Uptake of Radionuclide Cesium Based on Robust Zeolitic Chalcogenide via Stepwise Ion-Exchange Strategy † † † † † † † Huajun Yang, Min Luo, Li Luo, Hongxiang Wang, Dandan Hu, Jian Lin, Xiang Wang, ‡ ‡ § ⊥ † Yanlong Wang, Shuao Wang, Xianhui Bu,*, Pingyun Feng,*, and Tao Wu*, † College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, China ‡ School for Radiological and Interdisciplinary Sciences (RAD-X), Soochow University, Jiangsu 215123, China § Department of Chemistry and Biochemistry, California State University Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90840, United States ⊥ Department of Chemistry, University of California, Riverside, Riverside, California 92521, United States *S Supporting Information ABSTRACT: The safe use of nuclear energy requires the development of advanced adsorbent technology to address environment damage from nuclear waste or accidental release of radionuclides. Recently developed amine-directed chalcogenide frameworks have intrinsic advantages as ion-exchange materials to capture radionuclides, because their exceptionally negative framework charge can lead to high cation-uptake capacity and their 3-D multidimensional intersecting channel promotes rapid ion diffusion and offer a unique kinetic advantage. Prior to this work, however, such advantages could not be realized because organic cations in the as-synthesized materials are sluggish during ion exchange. Here we report an ingenious approach on the activation of amine- directed zeolitic chalcogenides through a stepwise ion-exchange strategy and their use in cesium adsorption. The activated porous chalcogenide exhibits highly enhanced cesium uptake compared to the pristine form and is comparable to the best metal chalcogenide sorbents so far. Further ion-exchange experiments in the presence of competing ions confirm the high selectivity for cesium ions. Excellent removal performance has also been observed in real water samples, highlighting the importance of stepwise ion exchange strategy to activate amine-directed chalcogenide framework for 137Cs+ removal. ■ INTRODUCTION studied, but they tend to have poor performance, in addition to 17 + As an efficient and low-carbon power generation method, low chemical, radiolytic, and thermal stability. Recently, Cs − − nuclear power plays a critical role in meeting the increasing uptake in SO3H -decorated MIL-101 was reported, widening energy needs. However, nuclear wastes and reactor accidents the types of materials with potential applications in nuclear 18 could result in the leak of radionuclides into environments, energy applications. which is a key reason limiting more widespread use of nuclear Recently, Kanatzidis et al. reported the capture of radio- energy.1,2 Among various radioactive nuclides, 137Cs+ is the nuclides by metal sulfides with high selectivity and under most hazardous due to its high fission yield (6.09%), long half- variable pH.17 The selectivity by sulfides is related to the − life (∼30 years), and high solubility.3 5 When accidentally relative softness of radionuclides (e.g., Cs+,Sr2+) compared to released to the sea or ground, it must be decontaminated smaller alkali or alkaline earth ions, which is different from that 137 + immediately for public safety. The Cs ions also need to be of oxide sorbents where the hydration radius and Gibbs energy recycled effectively from nuclear waste solutions in the 137 + of dehydration of the exchanged ions play an important role in reprocessing plants. Therefore, for Cs cleanup, high 19−23 + the ion exchange process. For example, layered metal selectivity for Cs in the presence of relatively high sulfide KMS-1 has preference for Sr2+ in highly alkaline concentration of competing cations (Na+,K+,Ca2+,Mg2+), solutions and also is among the most effective sorbents for fast kinetics, and commercial availability are desired in large- 2+ 24−26 6 UO removal. Other layered metal sulfides, such as scale application. 2 Various sorbents for 137Cs+ selective uptake have been KMS-2, KTS-3, and FJSM-SnS, have also been studied for 27−29 fi fi studied. The early materials are inorganic oxides such as radionuclide capture. In addition, sul de-modi ed sorb- − − crystalline silicotitanate (CST),7 9 zeolites,10 12 silicododeca- ents, such as layered double hydroxides intercalated by molybdate,13 vanadosilicate (SGU-45),14 and clays.15 Such sorbents are inexpensive but can suffer from slow kinetics.14 Received: October 7, 2016 Furthermore, their Cs+ selectivity can decrease significantly Revised: November 13, 2016 with high salt concentrations.16 Organic resins have also been Published: November 16, 2016 © 2016 American Chemical Society 8774 DOI: 10.1021/acs.chemmater.6b04273 Chem. Mater. 2016, 28, 8774−8780 Chemistry of Materials Article fi 2− 2+ polysul des or MoS4 , have been studied for removal of UO2 ppm, H2O, Aladdin), and potassium standard solution (1000 ppm, 30 fi as well as heavy metal ions. H2O, Macklin) were used as received without any further puri cation. x− 34 Syntheses. The syntheses of UCR-20-GaGeS (GaxGe4−xS8 ), For ion-exchange applications, maximizing the concentration · 29 FJSM-SnS ((Me2NH2)4/3(Me3NH)2/3Sn3S7 1.25H2O), GaSbS-1 of exchangeable cations is of critical importance for the process · 33 ffi ([(CH3)2NH2]2Ga2Sb2S7 H2O), and KTS-3 (K2xSn4−xS8−x, x = e ciency. The concentration of cations in traditional oxide- − 28 3+ 4+ 0.65 1) are according to the literature methods. based zeolites is determined by the framework Al /Si molar Stepwise Ion-Exchange Strategy. ’ The stepwise ion-exchange ratio which has a maximum value of 1 due to the Löwenstein s strategy is composed of two steps. ̈ ’ rule (as in NaAlSiO4). Surprisingly, the Lowenstein s rule is not Step 1: The protonated amines (PA) were first exchanged out by obeyed in zeolite-type metal chalcogenides so that M3+/M4+ “soft” Cs+. Typically, 500 mg of pristine RWY is soaked in 100 mL of ratio can be significantly great than 1.31 This motivates us to CsCl aqueous solution (1 M) in glass vial, which was sealed and initiate investigating the ion-exchange applications of such subsequently put in an 85 °C oven. The CsCl solution was refreshed materials, because we expect that the large negative charge of once during treatment. After 48 h, the crystals (named as Cs@RWY) framework and the associated high concentration of exchange- were washed by deionized water and ethanol for several times, and dried in vacuum. The process can be described as follows: able cations will lead to a record-high cation exchange capacity. 85° C In addition, unlike low-dimensional materials, these zeolite-type Cs+ +⎯→ PA@RWY⎯⎯⎯ Cs@RWY + PA+ chalocgenides have 3-D multidimensional, and mutually intersecting channels that could greatly facilitate ion diffusion (PA= protonated amines) (1) and ion exchange kinetics. Step 2: 100 mg of Cs@RWY was dipped into 100 mL of KCl At the early stage of this study, we encountered a major solution (2 M) at room temperature. The solution was refreshed twice obstacle to unlock the aforementioned intrinsic advantages of with a total time of around 48 h to remove the Cs+ in the channels of zeolite-type chalcogenides. Specifically, the as-synthesized RWY completely. Subsequently, the samples (named as K@RWY) materials typically contain bulky protonated amines in the were washed by deionized water and ethanol, and dried in a vacuum channels and their ion exchange process is quite slug- oven. The ion-exchange process was according to − gish.29,32 34 + RT + Herein we designed a two-step ion-exchange strategy to K +⎯→Cs@RWY⎯+ K@RWY Cs (2) address this issue (Scheme 1). In this work, we selected a highly Ion-Exchange Experiments. Typically, 10 mg of K@RWY was added into 10 mL of aqueous solution containing different amounts of + Scheme 1. Stepwise Ion-Exchange Strategy To Prepare standard Cs solution (1000 ppm, H2O). The mixture was kept High-Performance Potassium Form of Porous Sulfide shaking for 15−18 h at room temperature. And then the polycrystal- (RWY) for Efficient Cs+ Capture line materials with darker color were isolated by filtration and washed by water and ethanol for several times. The filtrate was collected for ICP-MS or ICP-AES measurements. All the experiments were carried out by batch method. To determine the sorption isotherm, the initial concentrations Ci of Cs+ was set in the range of 1−500 ppm at room temperature with V:m = 1000 and a total contact time of 15 h. In pH−dependent experiments, the pH of the solution was adjusted either by HCl solution or NaOH solution. The pH value was measured by a PHS-3C pH meter at the beginning of the ion exchange experiments. The regeneration process is similar to the step 2 of the stepwise ion exchange strategy. For the ion-exchange experiments in potable water and seawater, generally, 10 or 40 mg of K@RWY was added into 10 mL of aqueous solution in a glass vial. The solution was intentionally contaminated by diluting the standard Cs+ solution. The weight ratios of the standard solution were smaller than 1/100 to maintain the competitive ionic stable and porous amine-directed zeolitic chalcogenide frame- concentrations of real water samples. The mixture was kept shaking at work, namely UCR-20 (zeolite type code: RWY).34,35 We room temperature for 15−18 h and then was filtered. The filtrate was demonstrated that the protonated amines located in its analyzed by ICP-MS. ffi ffi channels can be exchanged completely into “hard” alkali ions Distribution coe cient Kd, signifying the a nity and selectivity for + − through the stepwise ion-exchange strategy. Interestingly, the Cs , is calculated by the equation: Kd =[(Ci Ce)/Ce](V/m) where fi + K+-exchanged RWY (K@RWY) can rapidly capture Cs+ with Ci and Ce are the initial and nal concentrations of Cs , V is the volume of the tested solution, and m is weight of the sorbents.