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Rapid adsorption and removal of sulfur mustard with zeolitic imidazolate frameworks ZIF-8 and ZIF-67
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Rapid adsorption and removal of sulfur mustard with zeolitic imidazolate frameworks ZIF-8 and ZIF-67 Ye-Rim Son a, Sam Gon Ryu b, Hyun Sung Kim a, * a b
Department of Chemistry, Pukyong National University, Busan, 48513, South Korea Agency for Defense Development, Yuseong P.O. Box 35, Daejeon, 34186, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: ZIF-8 ZIF-67 Mustard gas Adsorption Hydrolysis Chemical warfare
This study applies two isostructural zeolitic imidazolate frameworks (ZIF-8 and ZIF-67) to rapidly adsorb and remove sulfur mustard (HD), a chemical warfare agent. Because HD is extremely toxic, some of the studies were conducted using an HD simulant, 2-chloroethyl ethyl sulfide (CEES), to understand the effect of solvent polarity on adsorption. Further, CEES and real HD were subsequently adsorbed and removed from aqueous solutions using ZIF-8 or ZIF-67. The adsorption abilities of ZIF-8 and ZIF-67 positively correlated with the polarity of the solvent. In addition, 97% of CEES (2.5 mg in 1 mL) was rapidly adsorbed by ZIF-8 and ZIF-67 within 1 min at 25 � C in a 9:1 (v/v) water/ethanol solution. ZIF-8 and ZIF-67 were also successfully fabricated on cotton, which was removed more than 95% of contaminants from substrates contaminated with the HD simulant. We believe that this work will encourage the development of ZIFs for chemical warfare defense.
1. Introduction Sulfur mustard (bis-(2-chloroethyl)sulfide), also known as HD, is an irritant and vesicant, i.e., a blistering agent, which is used as a chemical warfare agent (CWA) and causes serious blistering of the skin and mu cous membranes on contact [1–3]. HD primarily damages organs that come into immediate contact with either the liquid or vapor phase of HD. HD has two relatively nonselective electrophilic centers. It initially reacts with water and glutathione, and the sulfur oxidizes, thus forming many metabolites [3]. In particular, HD forms a cyclic sulfonium cation by neighboring substitution and promptly alkylates DNA, thus breaking DNA strands and causing cell death [4]. Owing to the presence of the highly electrophilic cyclic sulfonium ion, HD binds to a variety of cellular macromolecules. In general, decontamination is a process employed to remove chemical warfare agents from people and equipment that they come into contact with such that the contamination level is reduced to a harmless level. Decontamination may be accomplished by detoxification via chemical reaction or the removal of chemical agents by physical means. To diminish the toxicity of HD through a chemical reaction, its con version into the cyclic sulfonium cation should be prevented. Thus far, efforts to develop effective detoxifying catalysts for HD have focused on modifying the electrophilic center (-S-) via partial oxidation or swift
elimination of the Cl functional group via hydrolysis (Scheme 1) [5]. To date, numerous effective detoxifying catalysts for HD have been devel oped, but new alternatives that can improve the removal efficiency of these decontaminants significantly are required; they should be smaller, lighter, more efficient, and less expensive than the current options. For instance, the Farr group has developed effective detoxification systems based on metal–organic frameworks (MOFs) for the photo catalytic partial oxidation of HD using visible light [6,7]. However, these photocatalytic systems require a specific light source for stimu lating detoxification, thus limiting their applicability in real-life situa tions requiring fast detoxification. Regarding the safety of intermediate products, rapid capture and detoxification through hydrolysis in the liquid phase under ambient conditions have been demonstrated using Agþ exchanged zeolites [8,9] and modified MOFs [10,11]. However, catalytic or chemical detoxification products from HD are not guaran teed to be completely safe to human beings. Therefore, a rapid adsorbent as a physical means to decontaminate HD that does not cause additional safety concerns for people or the environment is urgently required. Some HD adsorbents based on high surface area materials such as activated carbon [12–14], metal oxide nanoparticles [15–17], zeolites [8,9,18–20], and MOFs [21–25] have been reported. In particular, zeolitic imidazolate frameworks (ZIFs) are a novel class of MOFs with interesting properties, which are composed of metal ions and imidazole
* Corresponding author. E-mail address: [email protected] (H.S. Kim). https://doi.org/10.1016/j.micromeso.2019.109819 Received 2 August 2019; Received in revised form 15 October 2019; Accepted 22 October 2019 Available online 23 October 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Ye-Rim Son, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109819
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Scheme 1. Schematic illustration of CEES hydrolysis and adsorption by ZIF-8 and ZIF-67.
types of ligands [26,27]. Owing to their numerous structures, tunable pore sizes, chemical stability, and high surface areas, ZIFs have received considerable attention for use in diverse applications. The special structures and specific features of ZIFs also make them excellent ad sorbents for adsorbing molecules ranging from small molecules such as CO2 to large toxic molecules [28–33]. Herein, we report the application of zeolitic imidazolate frameworks (ZIFs) as novel adsorbents for rapidly removing HD and its simulant 2-chloroethyl ethyl sulfide (CEES) from aqueous media. ZIF-8 and ZIF-67 have formula M(2-methylimidazole)2 (M ¼ Co and Zn, respectively) and a sodalite-related type of zeolite structure containing narrow six-membered ring pore windows (3.4 Å) and much larger inner pores (11.4 Å) [26,27]. The adsorption kinetics, thermodynamics, and regeneration of the ZIF-8 and ZIF-67 adsorbents for removing CEES and HD from an aqueous solution were studied in detail.
2.3. Test for removal of CEES To investigate how effectively ZIF-67 and ZIF-8 adsorb CEES, CEES (2.5 mg) was injected into sealed vials, each containing a water/ethanol mixture solution (1 mL) with various volume ratios, namely, pure ethanol, 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, and 9:1, dispersed with ZIF particles (20 mg). The vials were allowed to stand for 1 min at 20 � C for adsorption. The water/ethanol solutions were employed because of the poor solubility of CEES and HD in pure water. After the capturing pro cess, the solution was filtered, and the supernatant was diluted with acetonitrile. Finally, the amount of removed CEES was estimated by analyzing the supernatant solution using gas chromatography (GC). 2.4. Reusability of ZIF-67 The reusability of ZIF-67 and ZIF-8 as adsorbents of HD or CEES was investigated in a water/ethanol mixture (9:1, v/v). After the first adsorption process, ZIF-67 and ZIF-8 were separated by centrifugation at 10,000 rpm for 10 min, and the ZIFs obtained were regenerated through repeated washing with pure acetonitrile (10 mL) and centrifugation to desorb the CEES from ZIFs until no CEES were detected in the super natant solution by GC analysis. For the second cycle, the adsorption process was conducted again under identical conditions. Specifically, CEES (2.5 mg) was injected into sealed vials containing regenerated ZIF particles (20 mg) dispersed in 1 mL of a water/ethanol mixture (9:1, v/ v). The adsorption capability was evaluated again via GC analysis of the supernatant solution. This procedure was repeated five times to confirm the reusability of the ZIFs.
2. Experimental section 2.1. Materials 2-chloroethyl ethyl sulfide (CEES, 99.5%, TCI), ethyl-2-hydroxyethyl sulfide (98%, Acros) zinc nitrate hexahydrate (99%, Zn(NO3)2⋅6H2O, Alfa), Cobalt nitrate hexahydrate (Co(NO3)2⋅6H2O, Alfa), 2-methylimi dazole (97%, C4H6N2, Alfa), N,N-Dimethylformamide(99.5%, SAM CHUN), acetonitrile (99.8%, SAMCHUN), methanol (99.8%, DUKSAN), ethanol (99.9%, SAMCHUN), toluene (99.5%, SAMCHUN), and 3-amino propyl trimethoxysilane (96%, TCI) were used as received. 2.2. Preparation of ZIF-67 and ZIF-8 crystals
2.5. Test for real agent, HD
ZIF-67 and ZIF-8 crystals were synthesized using the same procedure except for the metal ion source followed a previously reported method [34,35]. For ZIF-67, first, 0.9 g (3.1 mmol) of Co(NO3)2⋅6H2O and 1.0 g (12.2 mmol) of 2-methylimidazole were dissolved separately in Erlen meyer flasks containing methanol (50 mL each). The Co2þ solution was then poured into the 2-methylimidazole solution and stirred gently with a magnetic bar at room temperature for 12 h. After purple ZIF-67 crys tals formed, the solution was centrifuged at 8000 rpm for 10 min to facilitate separation, and the crystals were washed with methanol five times. For ZIF-8, the same procedure was followed using 1.0 g (3.1 mmol) of Zn(NO3)2⋅6H2O as the Zn2þ source instead of Co (NO3)2⋅6H2O, resulting in white crystalline precipitates.
The removal of HD, a real CWA, was studied by separately sus pending 20 mg of each ZIF-67 and ZIF-8 sample in 0.5 mL of 9:1 and 5:5 water/ethanol solutions. Then, 1.5 μL of cyclohexanol (internal stan dard) and 2.5 mg of HD were added to the suspension. The mixture was agitated for 1 min using a vortex mixer and centrifuged before being injected into the GC instrument. Small doses of CWAs are known to be lethal if inhaled or in contact with the skin. Experiments should be performed only by trained personnel in adequate facilities. 2.6. Modification of ZIF-8 and ZIF-67 on cotton Prepared cotton squares (2 cm � 2 cm) were treated with O2 plasma to remove unknown organic contaminants and to produce hydroxyl 2
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Fig. 1. (a) XRD patterns, (b) SEM images, and (c) N2 sorption isotherms at 77 K of pristine ZIF-8 and ZIF-67 as indicated.
groups. The cleaned cotton squares were then introduced into a twoneck round-bottom flask (100 mL), which was then evacuated under vacuum below 10 2 torr for 2 h and charged with high-purity argon. Dry toluene (50 mL) was added to the flask and sonicated for 5 min to immerse the cotton in the toluene. 3-Aminopropyl trimethoxysilane (0.5 mL), which is a surface-silylating agent, was added and then refluxed for 6 h with continuous stirring. The reactor was then cooled to room temperature and washed with fresh toluene to completely remove extra surface-silylating agent. After washing, the surface-modified cot ton squares were dried under vacuum for 20 min. Next, the aminefunctionalized cotton squares were separately added to vials contain ing zinc acetate (10 mM) or cobalt acetate (10 mM) ethanolic solutions and then refluxed for 1 h. The metal-ion-coordinated cotton squares were then washed with copious amounts of fresh ethanol. Five pieces of each type of cotton were then placed into vials containing 1,4-methyli midiazole (0.06 mol, 0.06 g), dissolved in methanol (5 mL), and mildly stirred for 6 h. For ZIF-67, 1.484 g of Co(NO3)2⋅6H2O was first dissolved in 50 mL of methanol (solution A), and separately, 3.278 g of 1,4-meth ylimidiazole was dissolved in 50 mL of methanol, (solution B). The treated pieces of cotton were immersed into solution A, and then, so lution B was slowly poured into solution A under stirring at room tem perature (25 � 3 � C) for 6 h at 200 rpm. The cotton samples were then separated from the colloidal dispersion and washed with MeOH three times. Finally, the obtained products were dried in air for subsequent characterization.
3. Results and discussion 3.1. Characterization of ZIF-8 and ZIF-67 The structural purity of the white (ZIF-8) and purple (ZIF-67) extracted precipitates was identified via XRD. The diffraction patterns exhibit sharp peaks that agree well with those in the simulated patterns of the sodalite structures of ZIF-8 and ZIF-67 (Fig. 1a) [26]. No addi tional peaks corresponding to impurities were observed. Both ZIF-8 and ZIF-67 crystals exhibit a rhombic dodecahedral shape with sharp edges and smooth faces of ~0.5 μm and ~2 μm, respectively, which are also observed in the high-resolution SEM images shown Fig. 1b. To assess the porosity of the ZIF structures, N2 gas adsorption measurements were performed at 77 K, as shown in Fig. 1c, revealing a Type-I isotherm. The Brunauer Emmett Teller (BET) surface areas of ZIF-8 and ZIF-67 are calculated to be 1887 and 2126 m2 g 1, respectively, both of which agree with the values reported in the literature [26,27]. 3.2. Evaluation of the adsorption of HD simulant to ZIF-8 and ZIF-67 Owing to the high toxicity of HD, the HD simulant CEES is often employed to safely perform experiments. We evaluated the ability of ZIF-8 and ZIF-67 to adsorb CEES from a liquid phase (i.e., mixed water and ethanol solutions). ZIF-8 and ZIF-67 are isostructural but contain different central metal ions (i.e., Zn2þ and Co2þ, respectively). An important structural feature of these two ZIF materials is that they possess large pores (11.6 Å in diameter) connected through small ap ertures (3.4 Å across). These apertures are large enough for CEES mol ecules to access the pores. Because the inner pores exhibit strong hydrophobicity, the hydrophobic CEES molecules in polar media are expected to selectively adsorb onto the inside of the pores. The adsorption capability of ZIF-8 and ZIF-67 for CEES was evalu ated in various polar media (i.e., different water and ethanol mixtures). In this study, CEES is typically added to a solution with a certain amount of dispersed ZIF particles, followed by adsorption for a specific period of time, and the supernatant is then quantitatively analyzed with GC. CEES removal yields are derived from the decrease in CEES after adsorption compared to the initial amount of CEES detected by GC. However, less CEES does not necessarily mean it was removed by adsorption because CEES simultaneously hydrolyzes. According to previous reports, in water/ethanol solutions CEES hydrolyzes into ethyl 2-hydroxyethyl sulfide (EHES) and ethoxyethyl sulfide (EOES) while liberating HCl into the aqueous solution (Scheme 1) [5]. Thus, to determine the extent of hydrolysis, the hydrolysis of CEES (2.5 mg) was observed in water/ethanol mixtures (1 mL) of different
2.7. Instrumentation A GC instrument (Claus 500, PerkinElmer Instruments) equipped with a flame ionization detector (FID) and a capillary polar column (Elite-624, PerkinElmer) was used to quantitatively determine the adsorption amounts and hydrolysis yields for CEES and HD. X-ray diffraction (XRD) patterns to identify ZIF samples were obtained using an X-ray diffractometer (PHILIPS/X’Pert-MPD System) using Ni-filtered monochromatic Cu Kα radiation. Scanning electron microscopy (SEM) images of ZIF-67 and ZIF-8 samples were recorded using field-emission (FE)-SEM (MIRA 3 LMH In-Beam Detector, TESCAN) at an acceleration voltage of 40 kV. N2 adsorption–desorption isotherms were measured using a BELSORP-max (BEL, JAPAN) at 77 K. Low-pressure gas sorption isotherms were collected on a BELSORP-max. Prior to the measure ments, ZIF-67 and ZIF-8 samples were evacuated under vacuum (~20 mTorr) at 200 � C for 12 h, and their surface area, total pore volume, and micropore volume were determined from the N2 adsorption isotherm at 77 K. A microbalance (Mettler Toledo MT5 Microbalance) was utilized to measure amount of fabricated ZIFs on the cotton. 3
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Fig. 2. Total removal yield (solid dots), hydrolysis, and adsorption selectivity (blue and orange bars, respectively) of CEES in the presence of (a) ZIF-8 and (b) ZIF-67 as a function of the volume fraction of ethanol in the solvent mixture at 25 � C for 10 min (CEES: 2.5 mg, adsorbent samples: 20 mg). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
ratios for 10 min at 20 � C without any adsorbents. These hydrolysis yields are displayed as gray bars in ESM-S1 (Electronic Supplementary Material (ESM)). As the water content increases from 10% to 90%, the hydrolysis yield increases from 30% to 70%. In addition, as evidenced by the chemical equation for the hydrolysis of CEES, EHES is the hydro lyzed product, but hydrogen chloride (HCl) is a byproduct, which is a harmful substance that should also be removed. Therefore, the hydro lysis process in a simple mixed solvent solution provides limited safety as a detoxification method because it produces another toxic substance. As the degree of hydrolysis increases, the pH of the hydrolysis solution decreases. In the case of the 9:1 (v/v) water/ethanol mixture, the pH drops from neutral to ~2. This pH decrease demonstrates that the water ratio strongly affects the degree of hydrolysis and should be considered when using these ZIFs for adsorption. To determine the actual adsorption removal yield of ZIF materials, the contribution of the hydrolysis reaction to the total CEES removal from a solution must therefore be considered. Thus, the hydrolysis products (e.g., EHES and EOES) should be analyzed quantitatively. The actual amount of CEES adsorbed can then be calculated by subtracting the hydrolytic contribution from the total amount of CEES removed. The removal yields of CEES in the presence of ZIF-8 and ZIF-67 can be attributed to both hydrolysis and adsorption to varying degrees in mixed water/ethanol solutions with different ratios (10–100%) at 20 � C for
1 min (Fig. 2). When the water content was >50%, more than 90% (specifically, 10% via hydrolysis and 80% via adsorption) of the initial amount of CEES (2.5 mg) was removed from the mixed water/ethanol solution within 1 min. Meanwhile, in the 90% water solution, the removal yield of CEES by adsorption approaches 100%, as supported by GC traces corresponding to CEES and two hydrolysis products, EHES and EOES, which dramatically disappear. Furthermore, the pH of the solvent did not significantly change after adsorption in the 9:1 (v/v) water/ethanol solution, which remained at a neutral pH of 7. Importantly, no GC trace appeared for the starting material, which indicates that all CEES rapidly and completely adsorbed onto ZIF in the 90% water solution. This demonstrates that the removal of CEES using ZIF can be attributed to the high polarity of the media. Meanwhile, as control experiments, we evaluated the adsorption of hydrolysis products EHES and EOES in the ZIF-8 and ZIF-67 pores, and the results revealed poor adsorption prop erties toward EHES (ESM-S2). This indicates that hydrolysis did not occur and that the removal process is due solely to adsorption to ZIF in the 9:1 (v/v) water/ethanol solution. This is possible because CEES adsorb onto ZIF-8 and ZIF-67 much faster than it hydrolyzes in aqueous media. This finding suggests that ZIF materials in aqueous media can be safely used to rapidly remove toxic CEES or HD. We additionally eval uated the removal efficiency of condition in the presence of hydrolysis
Fig. 3. Total removal yield for CEES in the presence of ZIF-8 and ZIF-67 as a function of (a) reaction time and (b) temperature in water/ethanol (1 mL, 9:1, v/v) at 25 � C for 5 min (CEES: 2.5 mg, adsorbent samples: 20 mg). 4
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Fig. 4. Adsorption isotherm and model fittings of CEES on (a) ZIF-8 and (b) ZIF-67 (initial CEES concentration 10–370 mg/L) in water/ethanol (1 mL, 9:1, v/v) at 25 � C for 5 min.
products such as introducing ZIF-8/-67 into contaminated CEES or HD into solutions with various compositions (from 10 to 50% in an ethanolwater mixture), in which CEES and its hydrolysis product coexist. The CEES existing in the solvent was completely removed by ZIF-8/-67. This finding demonstrated that ZIF-8 and ZIF-67 are sufficiently capable of adsorption, even in the presence of CEES hydrolysis products or after some hydrolysis process. When ZIF-8 and ZIF-67 samples were extracted with acetonitrile after adsorption, all samples recovered 95–100% of their initial capac ity, as determined via GC. This finding indicates that ZIF-8 and ZIF-67 directly adsorb and retain CEES molecules without any chemical re actions within the pore. Together with the rapid adsorption discussed above (1 min), this finding demonstrates the high potential of these ZIFs for removing HD from aqueous solutions. Fig. 3a shows the adsorption profiles of ZIF-8 and ZIF-67 at 20 � C; specifically, the decrease in the amount of CEES is plotted as a function of time. Nearly 100% of CEES is captured at 20 � C in approximately 1 min, and this amount remains constant for 60 min after adsorption (only the first 20 min are shown in Fig. 3a). The experiment was extended to 60 min although 100% adsorption was achieved in 1 min to demonstrate that once CEES adsorbs onto ZIF-8/-67, it never desorbs and returns to the solution. For instance, the quantitative analysis of the tested aqueous solutions did not show any indication of leaching even after keeping the ZIF-8/-67 powder in the aqueous solvent for 60 min. This result indicates that ZIF-8/-67 have a very strong affinity for CEES, suggesting that its adsorption from aqueous solvents is an irreversible process. We also conducted additional experiments to determine the optimum quantity of ZIF-67 to be used as the adsorbent; 2.5, 5, 10, 15, and 20 mg of adsorbents were employed for the removal of CEES via adsorption from 1 mL of an ethanol-water solvent (10/90, v/v%). Based on the correlation between the removal yield via adsorption and the adsorbent dosage shown in ESM-S3, the removal yield could be improved by increasing the amount of the adsorbent from 2.5 to 10 mg. For adsorbent quantities exceeding 10 mg, very small improvements in removal yield were achieved. However, when the dosage was increased further to 20 mg, the removal yield almost remained constant. In addi tion, to further demonstrate the suitability of ZIF-8 and ZIF-67 for removing CEES, the effect of temperature was investigated (Fig. 3b). Experiments were conducted at 5, 10, 20, and 30 � C for 1 min each using samples containing CEES (2.5 mg), ZIF-8 or ZIF-67 (20 mg), and mixed water/ethanol solution (9:1 (v/v)). Evidently, temperature did not significantly affect the adsorption, as the removal yield remained >95%, regardless of temperature. Furthermore, as the hydrolysis rate is strongly dependent on the water content, we investigated the half-lives of hydrolysis under various
water contents from 10% to 50% without using the adsorbent. The ob tained half-lives of hydrolysis are tabulated in ESM-S4. Interestingly, although the hydrolysis occurred rapidly under the condition of 10% water, with the half-life being a few seconds, after the addition of CEES or HD into 10% water containing ZIF-67, the adsorption process was dominant due to the much faster adsorption rate compared to the hy drolysis rate. The hydrolysis of CEES in an aqueous solution is well known to be an equilibrium reaction. Therefore, the concentration of CEES affects the equilibrium of hydrolysis. We tested whether the removal yield of ZIF67 for CEES via adsorption changes depending on the amount of CEES. As shown in ESM-S5, by examining the adsorption performance at various amount of CEES from 0.5 to 2.5 mg, we confirmed that ~100% adsorption occurred, as assessed by the weak GC peaks detected (less than 5% the initial amount) corresponding to CEES and its hydrolysis product. As shown, the similar kinetic profiles clearly suggest that ZIF8/-67 removes almost all the CEES molecules in 1 min even at very low concentrations i.e., 0.5 and 1.0 mg mL 1. These results indicate that ZIF-8/-67 is a suitable material for the adsorption of CEES over a wide concentration range. 3.3. Adsorption isotherms To further characterize the CEES adsorption properties of ZIF-8 and ZIF-67, adsorption isotherms of CEES in ZIF nanopores at 25 � C in 9:1 (v/v) water/ethanol were obtained using different initial concentrations of CEES ranging from 1.0 g L 1 to 25.0 g L 1. As shown in Fig. 4, the adsorption capacity of ZIF-8, i.e., CEES removal excluding hydrolysis, was quantitatively analyzed as a function of the initial CEES concen tration. Clearly, adsorption increased with the increasing CEES concentration. Analytical isotherm equations, such as those of Langmuir and Freundlich, are widely adopted to model adsorption data. Therefore, both of these models were chosen to investigate the adsorption equi librium isotherms of CEES on ZIF-8 and ZIF-67. The Langmuir isotherm and Freundlich model are expressed as linear equations as follows [36,37]: Langmuir model :
Ce 1 Ce ¼ þ qe KL qm qm
Freundlich model : qe ¼ kCe1=n where qe (mg⋅g 1) represents the equilibrium capacity of ZIF-8 or ZIF-67 to adsorb CEES, Ce (mg⋅L 1) is the concentration of CEES, qm (mg⋅g 1) is the monolayer capacity for CEES adsorption, KL (L⋅mg 1) is the 5
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Fig. 5. Reusability of ZIF-8 and ZIF-67 for adsorbing CEES. Reaction condi tions: water/ethanol (1 mL, 9:1, v/v) at 25 � C for 10 min (CEES: 2.5 mg, adsorbent samples: 20 mg).
Fig. 6. Removal yield, hydrolysis, and adsorption yield for CEES using various adsorbents in water/ethanol (1 mL, 9:1, v/v) at 25 � C for 10 min (CEES: 2.5 mg, adsorbent samples: 20 mg).
Langmuir adsorption constant, k is the Freundlich adsorption constant, and 1/n is the heterogeneity factor related to the adsorption intensity of the adsorbent. The Langmuir model fitted the experimental data better and exhibited a higher correlation coefficient (>0.98 for both ZIF-8 and ZIF-67) than the Freundlich model (Fig. 4). The maximum adsorption capacity of ZIF-8 and ZIF-67 for CEES are 456.61 mg g 1 and 463.30 mg g 1, respectively, according to the Lang muir model. Additionally, the maximum adsorption capacity of ZIF-67 for CEES versus various ethanol fractions in the aqueous solution is displayed in ESM-S6. ZIFs are formed via the self-assembly of the central transition metal ions and imidazolate linkers through tetrahedral coor dination. In this study, CEES adsorbed onto ZIF-8/-67 did not react with the central transition metal ions such as Co2þ and Zn2þ. The adsorption properties of ZIF for CEES are attributed to specific hydro phobic–hydrophobic interactions between the hydrophobic surface of ZIF-8/-67 owing to methyl imidazole ligands and CEES. After adsorp tion, the original species were easily recovered through a desorption process using a pure organic solvent, as described in the manuscript. This indicated that the adsorption process originated from physisorption and not chemisorption. Consequently, the adsorption properties of ZIF are attributed to specific hydrophobic–hydrophobic interactions be tween the hydrophobic surface of ZIF-8/-67 and CEES. The complexa tion of Co2þ and Zn2þ with CEES does not occur. The Langmuir model is suitable to explain the hydrophobic–hydrophobic interactions between the hydrophobic surface of ZIF-8/-67 and CEES. Overall, ZIF-8 and ZIF-67 showed similar tendencies in their adsorption performance and characteristics. Therefore, it is concluded that the central metal ion does not have much influence on the adsorption properties of the these isostructural ZIFs. Thus, adsorption is affected more by the structure and hydrophobicity of the pores than by this ion.
95% of their initial adsorption capacity, even after five trials (Fig. 5). The XRD patterns and SEM images of the ZIF particles reveal no dif ferences between the pristine and regenerated samples, indicating their textural stability and confirming their structural stability as shown in ESM-S7. The high adsorption capacity, fast kinetics, and easy regener ation and reuse of ZIF particles make them suitable candidates for largescale HD detoxification processes. 3.5. Comparison of the adsorption ability To demonstrate the advantages of ZIF-8 and ZIF-67 for adsorbing CEES, the CEES adsorption capability of other representative adsorbent materials, such as zeolites X, Y, A, and ZSM-5; silica and activated car bon; and another ZIF (ZIF-7) were measured, and the results were compared with those of ZIF-8 and ZIF-67 under identical adsorption conditions. As shown in Fig. 6, the total extent of CEES removal followed the order: ZIF-8 and ZIF-67 > ZIF-7 > activated carbon > ZSM5 > zeolite X > zeolite Y > zeolite A. Importantly, the degrees of CEES adsorption excluding hydrolysis followed the order: ZIF-8 and ZIF67 > activated carbon > ZSM-5 > ZIF-7 > silica > zeolite X > zeolite Y > zeolite A. Owing to the polarity of the pores of zeolites X and Y, the adsorption affinity of their nanopores toward relatively nonpolar molecules such as CEES should be low. Zeolite A did not adsorb any CEES because of the strong hydrophilicity of its small pore. ZSM-5 is a zeolite with threedimensional channel containing two types of channels, straight and si nusoidal interconnecting channels. Further, ZSM-5 with a high Si/Al ratio of ~200 has good adsorption capability for CEES (72% removal) due to its strong hydrophobicity and suitable pore size. However, the removal yield through adsorption is still lower than that of ZIF-8/-67. This indicates that the exceedingly higher CEES-capturing abilities of ZIF-8/-67 primarily arise from their cage structure with high accessible area (1887 and 2126 m2 g 1 for ZIF-8 and ZIF-67, respectively) than that of ZSM-5 with straight and sinusoidal channels (accessible area: 453 m2 g 1). Activated carbon is included in this comparison because it has been considered a strong candidate for HD removal. However, it adsorbed only 50% of CEES under our conditions, while 30% was hy drolyzed. Although activated carbon is known to have excellent adsorption properties, the hydrolysis reaction was not prevented, thus making its adsorption capacity somewhat lower than that of ZIF mate rials because of its slower adsorption kinetics.
3.4. Reusability For practical applications of an adsorbent, it should be easy to regenerate and reuse. In this study, simply washing the ZIFs with acetonitrile safely extracted the CEES adsorbed inside the ZIF pores because of the high solubility of CEES in acetonitrile and the affinity between ZIF pores and acetonitrile. The CEES-adsorbed ZIF was stirred in pure acetonitrile for 10 min and subsequently centrifuged, and this process was repeated several times with fresh acetonitrile to completely extract the CEES. The regenerated ZIFs were then used and regenerated three consecutive times. Interestingly, ZIF particles retained more than 6
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with amine functional groups. For instance, in the case of ZIF-67 grown on cotton, the Co2þ ion coordinates to the modified amine group. A successive treatment with imidazole forms a Co2þ–imidazole complex on the cotton surface, which is constrained to implement direct hydro thermal treatment. The color of the ZIF-67-coated cotton changed remarkably from white to deep purple after growth. Fig. 8 displays an SEM image of pristine cotton possessing a clean, smooth surface, whereas the ZIF-67/cotton composite exhibits a rough surface covered with ZIF-67. Specifically, the ZIF-67/cotton composite displays welldispersed ZIF-67 nanocrystals grown on the cotton surface via primary growth. The total weight of ZIF-67 crystals bound to the cotton is determined to be 312 μg cm2 on average from the difference in the initial weight of each of the five pieces of pristine cotton (2.5 cm � 2.5 cm) and the weight of the ZIF-67-coated cotton; the weight was measured on a microbalance. To gain insights into the stability of ZIF-8/-67 on cotton, it was vigorously stirred for 12 h in ethanol, water, and ethanol-water (10/90%) to induce the detachment of the ZIF crystals bound to the cotton. Less than10% weight loss of the ZIF crystals was observed after the detachment tests. To demonstrate that the composites are functional for practical use, a simple proof-of-concept experiment was conducted, as illustrated in (Fig. 9). CEES (2.5 mg) was spread on two glass slides and then sepa rately covered with ZIF-8 and ZIF-67/cotton composites soaked with the water/ethanol (9:1, v/v) solution for 1 min. To confirm the nature of each residual chemical left in the composite and on the glass substrate, more 9:1 (v/v) water/ethanol solution was introduced to wash the composites, and their respective supernatant solutions were analyzed using GC in the aforementioned manner. The GC spectrum of the wash solutions did not show any peaks for CEES or other hydrolysis products related to the CEES anion, and nearly all of the product (over 99%) was retrieved by the pure organic solvent washes. Further, ZIF-8 and ZIF-67 showed the same effects. To evaluate the functionality of the ZIFfunctionalized cotton, pure, untreated cotton was tested in the same way. After covering the CEES-contaminated glass slide for 1 min, 20% of the initial amount of CEES remained in both its original and hydrolyzed forms on the glass substrate, while the remaining 80% was absorbed onto the cotton as CEES and its hydrolyzed form. This proof-of-concept experiment suggests that the prepared ZIF-8 and ZIF-67/cotton composites offer the advantage of being portable and amenable for use in protective gear, such as masks, suits, gloves, and
Fig. 7. Total removal yield for real HD in the presence of ZIF-8 and ZIF-67 as a function of the reaction time. Reaction conditions: water/ethanol (1 mL, 9:1 or 5:5 v/v) at 25 � C. (HD: 2.5 mg, adsorbent samples: 20 mg).
3.6. Real CWA Next, the practical performance of ZIF-8 and ZIF-67 was evaluated for removing real HD. The extent of HD captured by ZIF-8 and ZIF-67 was evaluated under adsorption conditions identical to those used for CEES. Fig. 7 shows that both ZIF-8 and ZIF-67 removed 100% of HD from the water/ethanol solution (9:1, v/v) within 1 min. Interestingly, the adsorption of HD by ZIF-8 and ZIF-67 in the 5:5 (v/v) water/ethanol mixture was also excellent (reaching 90% in 1 min), similar to CEES without hydrolysis. Further, no products corresponding to the hydro lysis of HD were observed using GC. This indicates that ZIF-8 and ZIF-67 can remove the real CWA in the field. 3.7. Cotton composites We also prepared ZIF-8 and ZIF-67 composites via site-directed growth using cotton as a structural scaffold. To induce ZIF-8 or ZIF-67 nucleation on the cotton substrate, the cotton surface was modified
Fig. 8. (a) Digital photographs, (b) SEM images, and (c) XRD patterns of pristine cotton and ZIF-8 and ZIF-67 grown on cotton. 7
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Fig. 9. (a) Illustration of the proof-of-concept experiment using ZIF-8- and ZIF-67-coated cotton. (b) Plot of the CEES remaining on the glass and extracted from each cotton sample.
cleaning mats for a variety of protective or remedial applications. [3]
4. Conclusions
[4]
In summary, we demonstrated the feasibility of ZIF-8 and ZIF-67 for rapidly adsorbing and removing CEES and HD from an aqueous solution. The fast adsorption kinetics, good solvent stability, high adsorption capacity, regeneration ability, and easy fabrication on textiles such as cotton make ZIF-8 and ZIF-67 promising as practical adsorbents for removing CEES and HD, a CWA, from aqueous solutions. Further, they demonstrate high potential for promptly removing HD on the battlefield, which will motivate more extensive investigations using ZIF-based materials for chemical warfare defense.
[5] [6] [7]
[8]
Declaration of competing interest
[9]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[10]
Acknowledgements
[11]
This work was supported by Defense Acquisition Program Adminis tration and Agency for Defense Development (ADD) under Contract No. UD170035ID. S. G. Ryu thank ADD for support under Project No. 912412201.
[12] [13]
Appendix A. Supplementary data
[14]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109819.
[15] [16]
References
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Rapid adsorption and removal of sulfur mustard with zeolitic imidazolate frameworks ZIF-8 and ZIF-67 Ye-Rim Son a, Sam Gon Ryu b, Hyun Sung Kim a, * a b
Department of Chemistry, Pukyong National University, Busan, 48513, South Korea Agency for Defense Development, Yuseong P.O. Box 35, Daejeon, 34186, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: ZIF-8 ZIF-67 Mustard gas Adsorption Hydrolysis Chemical warfare
This study applies two isostructural zeolitic imidazolate frameworks (ZIF-8 and ZIF-67) to rapidly adsorb and remove sulfur mustard (HD), a chemical warfare agent. Because HD is extremely toxic, some of the studies were conducted using an HD simulant, 2-chloroethyl ethyl sulfide (CEES), to understand the effect of solvent polarity on adsorption. Further, CEES and real HD were subsequently adsorbed and removed from aqueous solutions using ZIF-8 or ZIF-67. The adsorption abilities of ZIF-8 and ZIF-67 positively correlated with the polarity of the solvent. In addition, 97% of CEES (2.5 mg in 1 mL) was rapidly adsorbed by ZIF-8 and ZIF-67 within 1 min at 25 � C in a 9:1 (v/v) water/ethanol solution. ZIF-8 and ZIF-67 were also successfully fabricated on cotton, which was removed more than 95% of contaminants from substrates contaminated with the HD simulant. We believe that this work will encourage the development of ZIFs for chemical warfare defense.
1. Introduction Sulfur mustard (bis-(2-chloroethyl)sulfide), also known as HD, is an irritant and vesicant, i.e., a blistering agent, which is used as a chemical warfare agent (CWA) and causes serious blistering of the skin and mu cous membranes on contact [1–3]. HD primarily damages organs that come into immediate contact with either the liquid or vapor phase of HD. HD has two relatively nonselective electrophilic centers. It initially reacts with water and glutathione, and the sulfur oxidizes, thus forming many metabolites [3]. In particular, HD forms a cyclic sulfonium cation by neighboring substitution and promptly alkylates DNA, thus breaking DNA strands and causing cell death [4]. Owing to the presence of the highly electrophilic cyclic sulfonium ion, HD binds to a variety of cellular macromolecules. In general, decontamination is a process employed to remove chemical warfare agents from people and equipment that they come into contact with such that the contamination level is reduced to a harmless level. Decontamination may be accomplished by detoxification via chemical reaction or the removal of chemical agents by physical means. To diminish the toxicity of HD through a chemical reaction, its con version into the cyclic sulfonium cation should be prevented. Thus far, efforts to develop effective detoxifying catalysts for HD have focused on modifying the electrophilic center (-S-) via partial oxidation or swift
elimination of the Cl functional group via hydrolysis (Scheme 1) [5]. To date, numerous effective detoxifying catalysts for HD have been devel oped, but new alternatives that can improve the removal efficiency of these decontaminants significantly are required; they should be smaller, lighter, more efficient, and less expensive than the current options. For instance, the Farr group has developed effective detoxification systems based on metal–organic frameworks (MOFs) for the photo catalytic partial oxidation of HD using visible light [6,7]. However, these photocatalytic systems require a specific light source for stimu lating detoxification, thus limiting their applicability in real-life situa tions requiring fast detoxification. Regarding the safety of intermediate products, rapid capture and detoxification through hydrolysis in the liquid phase under ambient conditions have been demonstrated using Agþ exchanged zeolites [8,9] and modified MOFs [10,11]. However, catalytic or chemical detoxification products from HD are not guaran teed to be completely safe to human beings. Therefore, a rapid adsorbent as a physical means to decontaminate HD that does not cause additional safety concerns for people or the environment is urgently required. Some HD adsorbents based on high surface area materials such as activated carbon [12–14], metal oxide nanoparticles [15–17], zeolites [8,9,18–20], and MOFs [21–25] have been reported. In particular, zeolitic imidazolate frameworks (ZIFs) are a novel class of MOFs with interesting properties, which are composed of metal ions and imidazole
* Corresponding author. E-mail address: [email protected] (H.S. Kim). https://doi.org/10.1016/j.micromeso.2019.109819 Received 2 August 2019; Received in revised form 15 October 2019; Accepted 22 October 2019 Available online 23 October 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Ye-Rim Son, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109819
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Scheme 1. Schematic illustration of CEES hydrolysis and adsorption by ZIF-8 and ZIF-67.
types of ligands [26,27]. Owing to their numerous structures, tunable pore sizes, chemical stability, and high surface areas, ZIFs have received considerable attention for use in diverse applications. The special structures and specific features of ZIFs also make them excellent ad sorbents for adsorbing molecules ranging from small molecules such as CO2 to large toxic molecules [28–33]. Herein, we report the application of zeolitic imidazolate frameworks (ZIFs) as novel adsorbents for rapidly removing HD and its simulant 2-chloroethyl ethyl sulfide (CEES) from aqueous media. ZIF-8 and ZIF-67 have formula M(2-methylimidazole)2 (M ¼ Co and Zn, respectively) and a sodalite-related type of zeolite structure containing narrow six-membered ring pore windows (3.4 Å) and much larger inner pores (11.4 Å) [26,27]. The adsorption kinetics, thermodynamics, and regeneration of the ZIF-8 and ZIF-67 adsorbents for removing CEES and HD from an aqueous solution were studied in detail.
2.3. Test for removal of CEES To investigate how effectively ZIF-67 and ZIF-8 adsorb CEES, CEES (2.5 mg) was injected into sealed vials, each containing a water/ethanol mixture solution (1 mL) with various volume ratios, namely, pure ethanol, 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, and 9:1, dispersed with ZIF particles (20 mg). The vials were allowed to stand for 1 min at 20 � C for adsorption. The water/ethanol solutions were employed because of the poor solubility of CEES and HD in pure water. After the capturing pro cess, the solution was filtered, and the supernatant was diluted with acetonitrile. Finally, the amount of removed CEES was estimated by analyzing the supernatant solution using gas chromatography (GC). 2.4. Reusability of ZIF-67 The reusability of ZIF-67 and ZIF-8 as adsorbents of HD or CEES was investigated in a water/ethanol mixture (9:1, v/v). After the first adsorption process, ZIF-67 and ZIF-8 were separated by centrifugation at 10,000 rpm for 10 min, and the ZIFs obtained were regenerated through repeated washing with pure acetonitrile (10 mL) and centrifugation to desorb the CEES from ZIFs until no CEES were detected in the super natant solution by GC analysis. For the second cycle, the adsorption process was conducted again under identical conditions. Specifically, CEES (2.5 mg) was injected into sealed vials containing regenerated ZIF particles (20 mg) dispersed in 1 mL of a water/ethanol mixture (9:1, v/ v). The adsorption capability was evaluated again via GC analysis of the supernatant solution. This procedure was repeated five times to confirm the reusability of the ZIFs.
2. Experimental section 2.1. Materials 2-chloroethyl ethyl sulfide (CEES, 99.5%, TCI), ethyl-2-hydroxyethyl sulfide (98%, Acros) zinc nitrate hexahydrate (99%, Zn(NO3)2⋅6H2O, Alfa), Cobalt nitrate hexahydrate (Co(NO3)2⋅6H2O, Alfa), 2-methylimi dazole (97%, C4H6N2, Alfa), N,N-Dimethylformamide(99.5%, SAM CHUN), acetonitrile (99.8%, SAMCHUN), methanol (99.8%, DUKSAN), ethanol (99.9%, SAMCHUN), toluene (99.5%, SAMCHUN), and 3-amino propyl trimethoxysilane (96%, TCI) were used as received. 2.2. Preparation of ZIF-67 and ZIF-8 crystals
2.5. Test for real agent, HD
ZIF-67 and ZIF-8 crystals were synthesized using the same procedure except for the metal ion source followed a previously reported method [34,35]. For ZIF-67, first, 0.9 g (3.1 mmol) of Co(NO3)2⋅6H2O and 1.0 g (12.2 mmol) of 2-methylimidazole were dissolved separately in Erlen meyer flasks containing methanol (50 mL each). The Co2þ solution was then poured into the 2-methylimidazole solution and stirred gently with a magnetic bar at room temperature for 12 h. After purple ZIF-67 crys tals formed, the solution was centrifuged at 8000 rpm for 10 min to facilitate separation, and the crystals were washed with methanol five times. For ZIF-8, the same procedure was followed using 1.0 g (3.1 mmol) of Zn(NO3)2⋅6H2O as the Zn2þ source instead of Co (NO3)2⋅6H2O, resulting in white crystalline precipitates.
The removal of HD, a real CWA, was studied by separately sus pending 20 mg of each ZIF-67 and ZIF-8 sample in 0.5 mL of 9:1 and 5:5 water/ethanol solutions. Then, 1.5 μL of cyclohexanol (internal stan dard) and 2.5 mg of HD were added to the suspension. The mixture was agitated for 1 min using a vortex mixer and centrifuged before being injected into the GC instrument. Small doses of CWAs are known to be lethal if inhaled or in contact with the skin. Experiments should be performed only by trained personnel in adequate facilities. 2.6. Modification of ZIF-8 and ZIF-67 on cotton Prepared cotton squares (2 cm � 2 cm) were treated with O2 plasma to remove unknown organic contaminants and to produce hydroxyl 2
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Fig. 1. (a) XRD patterns, (b) SEM images, and (c) N2 sorption isotherms at 77 K of pristine ZIF-8 and ZIF-67 as indicated.
groups. The cleaned cotton squares were then introduced into a twoneck round-bottom flask (100 mL), which was then evacuated under vacuum below 10 2 torr for 2 h and charged with high-purity argon. Dry toluene (50 mL) was added to the flask and sonicated for 5 min to immerse the cotton in the toluene. 3-Aminopropyl trimethoxysilane (0.5 mL), which is a surface-silylating agent, was added and then refluxed for 6 h with continuous stirring. The reactor was then cooled to room temperature and washed with fresh toluene to completely remove extra surface-silylating agent. After washing, the surface-modified cot ton squares were dried under vacuum for 20 min. Next, the aminefunctionalized cotton squares were separately added to vials contain ing zinc acetate (10 mM) or cobalt acetate (10 mM) ethanolic solutions and then refluxed for 1 h. The metal-ion-coordinated cotton squares were then washed with copious amounts of fresh ethanol. Five pieces of each type of cotton were then placed into vials containing 1,4-methyli midiazole (0.06 mol, 0.06 g), dissolved in methanol (5 mL), and mildly stirred for 6 h. For ZIF-67, 1.484 g of Co(NO3)2⋅6H2O was first dissolved in 50 mL of methanol (solution A), and separately, 3.278 g of 1,4-meth ylimidiazole was dissolved in 50 mL of methanol, (solution B). The treated pieces of cotton were immersed into solution A, and then, so lution B was slowly poured into solution A under stirring at room tem perature (25 � 3 � C) for 6 h at 200 rpm. The cotton samples were then separated from the colloidal dispersion and washed with MeOH three times. Finally, the obtained products were dried in air for subsequent characterization.
3. Results and discussion 3.1. Characterization of ZIF-8 and ZIF-67 The structural purity of the white (ZIF-8) and purple (ZIF-67) extracted precipitates was identified via XRD. The diffraction patterns exhibit sharp peaks that agree well with those in the simulated patterns of the sodalite structures of ZIF-8 and ZIF-67 (Fig. 1a) [26]. No addi tional peaks corresponding to impurities were observed. Both ZIF-8 and ZIF-67 crystals exhibit a rhombic dodecahedral shape with sharp edges and smooth faces of ~0.5 μm and ~2 μm, respectively, which are also observed in the high-resolution SEM images shown Fig. 1b. To assess the porosity of the ZIF structures, N2 gas adsorption measurements were performed at 77 K, as shown in Fig. 1c, revealing a Type-I isotherm. The Brunauer Emmett Teller (BET) surface areas of ZIF-8 and ZIF-67 are calculated to be 1887 and 2126 m2 g 1, respectively, both of which agree with the values reported in the literature [26,27]. 3.2. Evaluation of the adsorption of HD simulant to ZIF-8 and ZIF-67 Owing to the high toxicity of HD, the HD simulant CEES is often employed to safely perform experiments. We evaluated the ability of ZIF-8 and ZIF-67 to adsorb CEES from a liquid phase (i.e., mixed water and ethanol solutions). ZIF-8 and ZIF-67 are isostructural but contain different central metal ions (i.e., Zn2þ and Co2þ, respectively). An important structural feature of these two ZIF materials is that they possess large pores (11.6 Å in diameter) connected through small ap ertures (3.4 Å across). These apertures are large enough for CEES mol ecules to access the pores. Because the inner pores exhibit strong hydrophobicity, the hydrophobic CEES molecules in polar media are expected to selectively adsorb onto the inside of the pores. The adsorption capability of ZIF-8 and ZIF-67 for CEES was evalu ated in various polar media (i.e., different water and ethanol mixtures). In this study, CEES is typically added to a solution with a certain amount of dispersed ZIF particles, followed by adsorption for a specific period of time, and the supernatant is then quantitatively analyzed with GC. CEES removal yields are derived from the decrease in CEES after adsorption compared to the initial amount of CEES detected by GC. However, less CEES does not necessarily mean it was removed by adsorption because CEES simultaneously hydrolyzes. According to previous reports, in water/ethanol solutions CEES hydrolyzes into ethyl 2-hydroxyethyl sulfide (EHES) and ethoxyethyl sulfide (EOES) while liberating HCl into the aqueous solution (Scheme 1) [5]. Thus, to determine the extent of hydrolysis, the hydrolysis of CEES (2.5 mg) was observed in water/ethanol mixtures (1 mL) of different
2.7. Instrumentation A GC instrument (Claus 500, PerkinElmer Instruments) equipped with a flame ionization detector (FID) and a capillary polar column (Elite-624, PerkinElmer) was used to quantitatively determine the adsorption amounts and hydrolysis yields for CEES and HD. X-ray diffraction (XRD) patterns to identify ZIF samples were obtained using an X-ray diffractometer (PHILIPS/X’Pert-MPD System) using Ni-filtered monochromatic Cu Kα radiation. Scanning electron microscopy (SEM) images of ZIF-67 and ZIF-8 samples were recorded using field-emission (FE)-SEM (MIRA 3 LMH In-Beam Detector, TESCAN) at an acceleration voltage of 40 kV. N2 adsorption–desorption isotherms were measured using a BELSORP-max (BEL, JAPAN) at 77 K. Low-pressure gas sorption isotherms were collected on a BELSORP-max. Prior to the measure ments, ZIF-67 and ZIF-8 samples were evacuated under vacuum (~20 mTorr) at 200 � C for 12 h, and their surface area, total pore volume, and micropore volume were determined from the N2 adsorption isotherm at 77 K. A microbalance (Mettler Toledo MT5 Microbalance) was utilized to measure amount of fabricated ZIFs on the cotton. 3
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Fig. 2. Total removal yield (solid dots), hydrolysis, and adsorption selectivity (blue and orange bars, respectively) of CEES in the presence of (a) ZIF-8 and (b) ZIF-67 as a function of the volume fraction of ethanol in the solvent mixture at 25 � C for 10 min (CEES: 2.5 mg, adsorbent samples: 20 mg). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
ratios for 10 min at 20 � C without any adsorbents. These hydrolysis yields are displayed as gray bars in ESM-S1 (Electronic Supplementary Material (ESM)). As the water content increases from 10% to 90%, the hydrolysis yield increases from 30% to 70%. In addition, as evidenced by the chemical equation for the hydrolysis of CEES, EHES is the hydro lyzed product, but hydrogen chloride (HCl) is a byproduct, which is a harmful substance that should also be removed. Therefore, the hydro lysis process in a simple mixed solvent solution provides limited safety as a detoxification method because it produces another toxic substance. As the degree of hydrolysis increases, the pH of the hydrolysis solution decreases. In the case of the 9:1 (v/v) water/ethanol mixture, the pH drops from neutral to ~2. This pH decrease demonstrates that the water ratio strongly affects the degree of hydrolysis and should be considered when using these ZIFs for adsorption. To determine the actual adsorption removal yield of ZIF materials, the contribution of the hydrolysis reaction to the total CEES removal from a solution must therefore be considered. Thus, the hydrolysis products (e.g., EHES and EOES) should be analyzed quantitatively. The actual amount of CEES adsorbed can then be calculated by subtracting the hydrolytic contribution from the total amount of CEES removed. The removal yields of CEES in the presence of ZIF-8 and ZIF-67 can be attributed to both hydrolysis and adsorption to varying degrees in mixed water/ethanol solutions with different ratios (10–100%) at 20 � C for
1 min (Fig. 2). When the water content was >50%, more than 90% (specifically, 10% via hydrolysis and 80% via adsorption) of the initial amount of CEES (2.5 mg) was removed from the mixed water/ethanol solution within 1 min. Meanwhile, in the 90% water solution, the removal yield of CEES by adsorption approaches 100%, as supported by GC traces corresponding to CEES and two hydrolysis products, EHES and EOES, which dramatically disappear. Furthermore, the pH of the solvent did not significantly change after adsorption in the 9:1 (v/v) water/ethanol solution, which remained at a neutral pH of 7. Importantly, no GC trace appeared for the starting material, which indicates that all CEES rapidly and completely adsorbed onto ZIF in the 90% water solution. This demonstrates that the removal of CEES using ZIF can be attributed to the high polarity of the media. Meanwhile, as control experiments, we evaluated the adsorption of hydrolysis products EHES and EOES in the ZIF-8 and ZIF-67 pores, and the results revealed poor adsorption prop erties toward EHES (ESM-S2). This indicates that hydrolysis did not occur and that the removal process is due solely to adsorption to ZIF in the 9:1 (v/v) water/ethanol solution. This is possible because CEES adsorb onto ZIF-8 and ZIF-67 much faster than it hydrolyzes in aqueous media. This finding suggests that ZIF materials in aqueous media can be safely used to rapidly remove toxic CEES or HD. We additionally eval uated the removal efficiency of condition in the presence of hydrolysis
Fig. 3. Total removal yield for CEES in the presence of ZIF-8 and ZIF-67 as a function of (a) reaction time and (b) temperature in water/ethanol (1 mL, 9:1, v/v) at 25 � C for 5 min (CEES: 2.5 mg, adsorbent samples: 20 mg). 4
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Fig. 4. Adsorption isotherm and model fittings of CEES on (a) ZIF-8 and (b) ZIF-67 (initial CEES concentration 10–370 mg/L) in water/ethanol (1 mL, 9:1, v/v) at 25 � C for 5 min.
products such as introducing ZIF-8/-67 into contaminated CEES or HD into solutions with various compositions (from 10 to 50% in an ethanolwater mixture), in which CEES and its hydrolysis product coexist. The CEES existing in the solvent was completely removed by ZIF-8/-67. This finding demonstrated that ZIF-8 and ZIF-67 are sufficiently capable of adsorption, even in the presence of CEES hydrolysis products or after some hydrolysis process. When ZIF-8 and ZIF-67 samples were extracted with acetonitrile after adsorption, all samples recovered 95–100% of their initial capac ity, as determined via GC. This finding indicates that ZIF-8 and ZIF-67 directly adsorb and retain CEES molecules without any chemical re actions within the pore. Together with the rapid adsorption discussed above (1 min), this finding demonstrates the high potential of these ZIFs for removing HD from aqueous solutions. Fig. 3a shows the adsorption profiles of ZIF-8 and ZIF-67 at 20 � C; specifically, the decrease in the amount of CEES is plotted as a function of time. Nearly 100% of CEES is captured at 20 � C in approximately 1 min, and this amount remains constant for 60 min after adsorption (only the first 20 min are shown in Fig. 3a). The experiment was extended to 60 min although 100% adsorption was achieved in 1 min to demonstrate that once CEES adsorbs onto ZIF-8/-67, it never desorbs and returns to the solution. For instance, the quantitative analysis of the tested aqueous solutions did not show any indication of leaching even after keeping the ZIF-8/-67 powder in the aqueous solvent for 60 min. This result indicates that ZIF-8/-67 have a very strong affinity for CEES, suggesting that its adsorption from aqueous solvents is an irreversible process. We also conducted additional experiments to determine the optimum quantity of ZIF-67 to be used as the adsorbent; 2.5, 5, 10, 15, and 20 mg of adsorbents were employed for the removal of CEES via adsorption from 1 mL of an ethanol-water solvent (10/90, v/v%). Based on the correlation between the removal yield via adsorption and the adsorbent dosage shown in ESM-S3, the removal yield could be improved by increasing the amount of the adsorbent from 2.5 to 10 mg. For adsorbent quantities exceeding 10 mg, very small improvements in removal yield were achieved. However, when the dosage was increased further to 20 mg, the removal yield almost remained constant. In addi tion, to further demonstrate the suitability of ZIF-8 and ZIF-67 for removing CEES, the effect of temperature was investigated (Fig. 3b). Experiments were conducted at 5, 10, 20, and 30 � C for 1 min each using samples containing CEES (2.5 mg), ZIF-8 or ZIF-67 (20 mg), and mixed water/ethanol solution (9:1 (v/v)). Evidently, temperature did not significantly affect the adsorption, as the removal yield remained >95%, regardless of temperature. Furthermore, as the hydrolysis rate is strongly dependent on the water content, we investigated the half-lives of hydrolysis under various
water contents from 10% to 50% without using the adsorbent. The ob tained half-lives of hydrolysis are tabulated in ESM-S4. Interestingly, although the hydrolysis occurred rapidly under the condition of 10% water, with the half-life being a few seconds, after the addition of CEES or HD into 10% water containing ZIF-67, the adsorption process was dominant due to the much faster adsorption rate compared to the hy drolysis rate. The hydrolysis of CEES in an aqueous solution is well known to be an equilibrium reaction. Therefore, the concentration of CEES affects the equilibrium of hydrolysis. We tested whether the removal yield of ZIF67 for CEES via adsorption changes depending on the amount of CEES. As shown in ESM-S5, by examining the adsorption performance at various amount of CEES from 0.5 to 2.5 mg, we confirmed that ~100% adsorption occurred, as assessed by the weak GC peaks detected (less than 5% the initial amount) corresponding to CEES and its hydrolysis product. As shown, the similar kinetic profiles clearly suggest that ZIF8/-67 removes almost all the CEES molecules in 1 min even at very low concentrations i.e., 0.5 and 1.0 mg mL 1. These results indicate that ZIF-8/-67 is a suitable material for the adsorption of CEES over a wide concentration range. 3.3. Adsorption isotherms To further characterize the CEES adsorption properties of ZIF-8 and ZIF-67, adsorption isotherms of CEES in ZIF nanopores at 25 � C in 9:1 (v/v) water/ethanol were obtained using different initial concentrations of CEES ranging from 1.0 g L 1 to 25.0 g L 1. As shown in Fig. 4, the adsorption capacity of ZIF-8, i.e., CEES removal excluding hydrolysis, was quantitatively analyzed as a function of the initial CEES concen tration. Clearly, adsorption increased with the increasing CEES concentration. Analytical isotherm equations, such as those of Langmuir and Freundlich, are widely adopted to model adsorption data. Therefore, both of these models were chosen to investigate the adsorption equi librium isotherms of CEES on ZIF-8 and ZIF-67. The Langmuir isotherm and Freundlich model are expressed as linear equations as follows [36,37]: Langmuir model :
Ce 1 Ce ¼ þ qe KL qm qm
Freundlich model : qe ¼ kCe1=n where qe (mg⋅g 1) represents the equilibrium capacity of ZIF-8 or ZIF-67 to adsorb CEES, Ce (mg⋅L 1) is the concentration of CEES, qm (mg⋅g 1) is the monolayer capacity for CEES adsorption, KL (L⋅mg 1) is the 5
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Fig. 5. Reusability of ZIF-8 and ZIF-67 for adsorbing CEES. Reaction condi tions: water/ethanol (1 mL, 9:1, v/v) at 25 � C for 10 min (CEES: 2.5 mg, adsorbent samples: 20 mg).
Fig. 6. Removal yield, hydrolysis, and adsorption yield for CEES using various adsorbents in water/ethanol (1 mL, 9:1, v/v) at 25 � C for 10 min (CEES: 2.5 mg, adsorbent samples: 20 mg).
Langmuir adsorption constant, k is the Freundlich adsorption constant, and 1/n is the heterogeneity factor related to the adsorption intensity of the adsorbent. The Langmuir model fitted the experimental data better and exhibited a higher correlation coefficient (>0.98 for both ZIF-8 and ZIF-67) than the Freundlich model (Fig. 4). The maximum adsorption capacity of ZIF-8 and ZIF-67 for CEES are 456.61 mg g 1 and 463.30 mg g 1, respectively, according to the Lang muir model. Additionally, the maximum adsorption capacity of ZIF-67 for CEES versus various ethanol fractions in the aqueous solution is displayed in ESM-S6. ZIFs are formed via the self-assembly of the central transition metal ions and imidazolate linkers through tetrahedral coor dination. In this study, CEES adsorbed onto ZIF-8/-67 did not react with the central transition metal ions such as Co2þ and Zn2þ. The adsorption properties of ZIF for CEES are attributed to specific hydro phobic–hydrophobic interactions between the hydrophobic surface of ZIF-8/-67 owing to methyl imidazole ligands and CEES. After adsorp tion, the original species were easily recovered through a desorption process using a pure organic solvent, as described in the manuscript. This indicated that the adsorption process originated from physisorption and not chemisorption. Consequently, the adsorption properties of ZIF are attributed to specific hydrophobic–hydrophobic interactions be tween the hydrophobic surface of ZIF-8/-67 and CEES. The complexa tion of Co2þ and Zn2þ with CEES does not occur. The Langmuir model is suitable to explain the hydrophobic–hydrophobic interactions between the hydrophobic surface of ZIF-8/-67 and CEES. Overall, ZIF-8 and ZIF-67 showed similar tendencies in their adsorption performance and characteristics. Therefore, it is concluded that the central metal ion does not have much influence on the adsorption properties of the these isostructural ZIFs. Thus, adsorption is affected more by the structure and hydrophobicity of the pores than by this ion.
95% of their initial adsorption capacity, even after five trials (Fig. 5). The XRD patterns and SEM images of the ZIF particles reveal no dif ferences between the pristine and regenerated samples, indicating their textural stability and confirming their structural stability as shown in ESM-S7. The high adsorption capacity, fast kinetics, and easy regener ation and reuse of ZIF particles make them suitable candidates for largescale HD detoxification processes. 3.5. Comparison of the adsorption ability To demonstrate the advantages of ZIF-8 and ZIF-67 for adsorbing CEES, the CEES adsorption capability of other representative adsorbent materials, such as zeolites X, Y, A, and ZSM-5; silica and activated car bon; and another ZIF (ZIF-7) were measured, and the results were compared with those of ZIF-8 and ZIF-67 under identical adsorption conditions. As shown in Fig. 6, the total extent of CEES removal followed the order: ZIF-8 and ZIF-67 > ZIF-7 > activated carbon > ZSM5 > zeolite X > zeolite Y > zeolite A. Importantly, the degrees of CEES adsorption excluding hydrolysis followed the order: ZIF-8 and ZIF67 > activated carbon > ZSM-5 > ZIF-7 > silica > zeolite X > zeolite Y > zeolite A. Owing to the polarity of the pores of zeolites X and Y, the adsorption affinity of their nanopores toward relatively nonpolar molecules such as CEES should be low. Zeolite A did not adsorb any CEES because of the strong hydrophilicity of its small pore. ZSM-5 is a zeolite with threedimensional channel containing two types of channels, straight and si nusoidal interconnecting channels. Further, ZSM-5 with a high Si/Al ratio of ~200 has good adsorption capability for CEES (72% removal) due to its strong hydrophobicity and suitable pore size. However, the removal yield through adsorption is still lower than that of ZIF-8/-67. This indicates that the exceedingly higher CEES-capturing abilities of ZIF-8/-67 primarily arise from their cage structure with high accessible area (1887 and 2126 m2 g 1 for ZIF-8 and ZIF-67, respectively) than that of ZSM-5 with straight and sinusoidal channels (accessible area: 453 m2 g 1). Activated carbon is included in this comparison because it has been considered a strong candidate for HD removal. However, it adsorbed only 50% of CEES under our conditions, while 30% was hy drolyzed. Although activated carbon is known to have excellent adsorption properties, the hydrolysis reaction was not prevented, thus making its adsorption capacity somewhat lower than that of ZIF mate rials because of its slower adsorption kinetics.
3.4. Reusability For practical applications of an adsorbent, it should be easy to regenerate and reuse. In this study, simply washing the ZIFs with acetonitrile safely extracted the CEES adsorbed inside the ZIF pores because of the high solubility of CEES in acetonitrile and the affinity between ZIF pores and acetonitrile. The CEES-adsorbed ZIF was stirred in pure acetonitrile for 10 min and subsequently centrifuged, and this process was repeated several times with fresh acetonitrile to completely extract the CEES. The regenerated ZIFs were then used and regenerated three consecutive times. Interestingly, ZIF particles retained more than 6
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with amine functional groups. For instance, in the case of ZIF-67 grown on cotton, the Co2þ ion coordinates to the modified amine group. A successive treatment with imidazole forms a Co2þ–imidazole complex on the cotton surface, which is constrained to implement direct hydro thermal treatment. The color of the ZIF-67-coated cotton changed remarkably from white to deep purple after growth. Fig. 8 displays an SEM image of pristine cotton possessing a clean, smooth surface, whereas the ZIF-67/cotton composite exhibits a rough surface covered with ZIF-67. Specifically, the ZIF-67/cotton composite displays welldispersed ZIF-67 nanocrystals grown on the cotton surface via primary growth. The total weight of ZIF-67 crystals bound to the cotton is determined to be 312 μg cm2 on average from the difference in the initial weight of each of the five pieces of pristine cotton (2.5 cm � 2.5 cm) and the weight of the ZIF-67-coated cotton; the weight was measured on a microbalance. To gain insights into the stability of ZIF-8/-67 on cotton, it was vigorously stirred for 12 h in ethanol, water, and ethanol-water (10/90%) to induce the detachment of the ZIF crystals bound to the cotton. Less than10% weight loss of the ZIF crystals was observed after the detachment tests. To demonstrate that the composites are functional for practical use, a simple proof-of-concept experiment was conducted, as illustrated in (Fig. 9). CEES (2.5 mg) was spread on two glass slides and then sepa rately covered with ZIF-8 and ZIF-67/cotton composites soaked with the water/ethanol (9:1, v/v) solution for 1 min. To confirm the nature of each residual chemical left in the composite and on the glass substrate, more 9:1 (v/v) water/ethanol solution was introduced to wash the composites, and their respective supernatant solutions were analyzed using GC in the aforementioned manner. The GC spectrum of the wash solutions did not show any peaks for CEES or other hydrolysis products related to the CEES anion, and nearly all of the product (over 99%) was retrieved by the pure organic solvent washes. Further, ZIF-8 and ZIF-67 showed the same effects. To evaluate the functionality of the ZIFfunctionalized cotton, pure, untreated cotton was tested in the same way. After covering the CEES-contaminated glass slide for 1 min, 20% of the initial amount of CEES remained in both its original and hydrolyzed forms on the glass substrate, while the remaining 80% was absorbed onto the cotton as CEES and its hydrolyzed form. This proof-of-concept experiment suggests that the prepared ZIF-8 and ZIF-67/cotton composites offer the advantage of being portable and amenable for use in protective gear, such as masks, suits, gloves, and
Fig. 7. Total removal yield for real HD in the presence of ZIF-8 and ZIF-67 as a function of the reaction time. Reaction conditions: water/ethanol (1 mL, 9:1 or 5:5 v/v) at 25 � C. (HD: 2.5 mg, adsorbent samples: 20 mg).
3.6. Real CWA Next, the practical performance of ZIF-8 and ZIF-67 was evaluated for removing real HD. The extent of HD captured by ZIF-8 and ZIF-67 was evaluated under adsorption conditions identical to those used for CEES. Fig. 7 shows that both ZIF-8 and ZIF-67 removed 100% of HD from the water/ethanol solution (9:1, v/v) within 1 min. Interestingly, the adsorption of HD by ZIF-8 and ZIF-67 in the 5:5 (v/v) water/ethanol mixture was also excellent (reaching 90% in 1 min), similar to CEES without hydrolysis. Further, no products corresponding to the hydro lysis of HD were observed using GC. This indicates that ZIF-8 and ZIF-67 can remove the real CWA in the field. 3.7. Cotton composites We also prepared ZIF-8 and ZIF-67 composites via site-directed growth using cotton as a structural scaffold. To induce ZIF-8 or ZIF-67 nucleation on the cotton substrate, the cotton surface was modified
Fig. 8. (a) Digital photographs, (b) SEM images, and (c) XRD patterns of pristine cotton and ZIF-8 and ZIF-67 grown on cotton. 7
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Fig. 9. (a) Illustration of the proof-of-concept experiment using ZIF-8- and ZIF-67-coated cotton. (b) Plot of the CEES remaining on the glass and extracted from each cotton sample.
cleaning mats for a variety of protective or remedial applications. [3]
4. Conclusions
[4]
In summary, we demonstrated the feasibility of ZIF-8 and ZIF-67 for rapidly adsorbing and removing CEES and HD from an aqueous solution. The fast adsorption kinetics, good solvent stability, high adsorption capacity, regeneration ability, and easy fabrication on textiles such as cotton make ZIF-8 and ZIF-67 promising as practical adsorbents for removing CEES and HD, a CWA, from aqueous solutions. Further, they demonstrate high potential for promptly removing HD on the battlefield, which will motivate more extensive investigations using ZIF-based materials for chemical warfare defense.
[5] [6] [7]
[8]
Declaration of competing interest
[9]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[10]
Acknowledgements
[11]
This work was supported by Defense Acquisition Program Adminis tration and Agency for Defense Development (ADD) under Contract No. UD170035ID. S. G. Ryu thank ADD for support under Project No. 912412201.
[12] [13]
Appendix A. Supplementary data
[14]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109819.
[15] [16]
References
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