Highly Porous Covalent Triazine Frameworks for Reversible Iodine

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A Highly Porous Covalent Triazine Frameworks for Reversible Iodine Capture and Efficient Removal of Dye Qin Jiang, Hongliang Huang, Yuanzhe Tang, Yuxi Zhang, and Chongli Zhong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02866 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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Industrial & Engineering Chemistry Research

A Highly Porous Covalent Triazine Frameworks for Reversible Iodine Capture and Efficient Removal of Dye Qin Jiang1, Hongliang Huang*2, Yuanzhe Tang1, Yuxi Zhang1, Chongli Zhong*1,2,3

1.State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

2.State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China

3.Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing 100029, China

E-mail address: [email protected], [email protected]

KEY WORDS: covalent triazine-based framework; iodine capture; Rhodamine B; adsorption; water treatment

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Porous organic frameworks may be a kind of promising material to address environmental issues such as removing the radioactive vapor wastes in fission as well as coping with organic pollutants of wastewater, due to their excellent porous character and remarkable stability. In this report, a covalent triazine-based framework (CTF-CTTD) with the hierarchically porous structure and large pore volume was synthesized via the nitrile trimerization. The CTF-CTTD shows an outstanding adsorption ability for iodine with the uptake value of 387 % due to their abundant porosity, triazine units as well as π-π conjugated structures, which is one of the highest values reported so far for iodine uptake values of solid porous adsorbents. Furthermore, iodine captured in CTF-CTTD can be also released easily from the pore of materials because of the highly porous structure. In addition, CTF-CTTD also presents an excellent adsorptive performance for removing Rhodamine B (RhB) with adsorption capacity of 684.9 mg g-1. These results obtained may provide a guidance for the design of adsorbent with high specific surface area and large pore volume as well as the hierarchically porous structure in capturing of radioactive iodine and removing RhB from wastewater.

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INTRODUCTION With the rapidly growing needs of current worldwide energy and the increasingly serious global warming, nuclear energy, as a non-carbon-emitting power source, become one of the most prominent and feasible alternative sources to minimize greenhouse gas emissions among the alternative energies.1, 2 However, an urgent issue along with nuclear energy is the nuclear waste pollution especially volatile radionuclide fission products, such as 129I, 131I, 3H, 85Kr, 99Tc and 235U.3-9 Radioactive iodine poses a major long-term risk due to its long half-lives (1.57×107 years for 129I and 8 days for 131

I), high volatility and adverse damage to the environment and human metabolic

processes. Therefore, it’s urgent to find effective means to capture and storage volatile radionuclide iodine for safe utilization of nuclear energy. Meanwhile, water pollution, especially organic dye wastewater, is also a matter of great concern due to the potential toxic, carcinogenic and nonbiodegradable nature of dye. Up to now, various porous materials, including inorganic porous materials10, active carbon 11-14, and metal-organic frameworks (MOFs)15-22, have been used for the capture of iodine or removal of organic dye. Based on the previous studies, in terms of adsorption process, the affinity of host to iodine molecules or organic pollutants, large pore volume and even hierarchically porous structure of the porous materials are all important to obtaining a high adsorption capacity of iodine and organic pollutants23-26. Therefore, it is essential to design the structure of absorbents with special pore character and adsorption sites. At present, porous organic frameworks (POFs), constructed from wellfunctionalized organic molecular building blocks, become a promising candidates for

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the practical applications for capturing iodine and organic pollutants due to chemical/thermal stability and structural diversities, including porous aromatic frameworks26,

27

, conjugated microporous polymers25,

28-32

, covalent organic

frameworks33-35 and so on. Covalent triazine frameworks (CTFs), as a kind of typical POFs, were synthesized through the trimerzation reaction of aromatic nitriles. Over the past few years, CTFs have exhibited potentiality in a variety of applications such as gas storage36-38, catalysis39-41, energy storage42-44 and so on. But similar study has not been reported for the capture of iodine on CTFs so far. In this work, we designed a rational building block to construct a novel conjugated amorphous CTF (CTF-CTTD) via the nitrile trimerization. The methyl group in the CTTD monomer can make the benzene rings of monomer perpendicular to each other. As expected, CTF-CTTD exhibits an excellent iodine adsorption capacity of 387 % and exceptional recyclability through a chemical and physical mechanism. In addition, this covalent triazine-based framework also shows an outstanding adsorptive performance for the removal of Rhodamine B (RhB) with a saturated adsorption capacity of 684.9 mg g-1.

MATERIALS AND METHODS Synthesis of CTTD. The synthetic procedure of CTTD is shown in Scheme 1. In a twonecked round bottom flask, PPh3 (1316 mg, 5.02 mmol) and Pd(OAc)2 (226 mg, 1.004 mmol) were added in 160 mL degassed dioxane at N2 atmosphere. The mixture was stirred until the colour of the solution became yellow, which indicated the complete formation of the Pd(0) catalyst. Then 1,3,5-triiodo-2,4,6-trimethylbenzene (5000 mg,

4


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10.04 mmol), (4-cyanophenyl) boronic acid (7380 mg, 50.22 mmol) and K2CO3 (13880 mg, 100.42 mmol) were added and the reaction mixture was stirred for 72 h at reflux. The resulting mixture was cooled to room temperature and then H2O (200 mL) was added and the suspension was filtered. The residue was then extracted with dichloromethane. The combined organic phase was dried over MgSO4 and evaporated under reduced pressure to give the orange residue solid. Then the orange residue was purified through the column chromatography with mixture of EtOAc and petroleum ether as eluent. Recrystallisation of the residue from mixture of EtOAc and petroleum ether

gave

5'-(4-cyanophenyl)-2',4',6'-trimethyl-[1,1':3',1''-terphenyl]-4,4''-

dicarbonitrile (CTTD) as a white solid.

Scheme 1. Synthetic procedure of CTTD

Synthesis of CTF-CTTD. CTTD (847 mg, 2 mmol) and ZnCl2 (818 mg, 6 mmol) were placed into a quartz glass ampoule under N2 atmosphere. The ampoules were evacuated, sealed and then heated to the desired temperature (400 or 500 °C) in 100 min, and kept the terminal temperature for 40 h. After being cooled down to room temperature, the black block of salt and product were then ground, washed, and stirred with distilled 5



water for 5 h to remove most of ZnCl2. After that, the black powder was subsequently stirred for 20 h in diluted HCl to remove the residual ZnCl2. After being filtered, the resulting black powder was washed thoroughly with water, dichloromethane and acetone to remove the monomers and HCl. CTF-CTTD was obtained as a black powder and dried in vacuum at 120 °C for 12 h. The yield is higher than 90 %. Characterization. The powder X-ray diffraction (PXRD) patterns were taken on a BRUKER D8-Focus Bragg-Brentano X-ray Powder Diffractometer equipped with a Cu sealed tube (λ = 1.54178 Å) at room temperature. Typical data acquisition times were 20 min for a 3°-50° 2θ scan using a step size of 0.02°. Fourier transform infrared spectroscopy (FT-IR) was performed by Nicolet 6700 FTIR spectrophotometer. The pressed slice was made with powder consisting of samples and KBr by a tablet machine and the spectroscopy data were measured from 4000 to 400 cm-1 with a resolution of 4 cm-1. The Brunauer-Emmett-Teller (BET) surface areas and pore structures of samples were measured on Autosorb-IQ-MP (Quantachrome Instruments) at 77 K. Thermogravimetric analysis (TGA) data were recorded by using a TGA-50 (SHIMADZU) thermogravimetric analyzer with a heating rate of 10 °C min-1 under air and N2 atmosphere. X-ray photoemission spectroscopy (XPS) was obtained from a Thermo Fisher ESCALAB 250. The morphologies of the CTFs were investigated by using a Hitachi S-4700 scanning electron microscope (SEM) with an accelerating voltage of 20.0 kV. Transmission electron microscope (TEM) images were taken by a JEM-1200EX microscope operated at 120 kV. Elemental analysis was characterized on a Vairo EL elementar analysensysteme. The Zn content was analyzed by inductive

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coupled plasma on 7700 series ICP-MS (Agilent technologies). The CTFs were digested with concentrated HNO3 at 150 ºC for 10 h. After cooled to room temperature, the solutions were diluted with deionized water and measured by ICP. Bruker AV 300 NMR spectrometer (300 MHz) was used for 13C NMR spectra. 1H NMR spectra was recorded on a Bruker AV-600 (600 MHz) at 25 °C with CDCl3 as solvent. Iodine capture. The iodine vapor uptake capacities of CTF-CTTD were evaluated by gravimetric measurements. The CTF-CTTD powder (10 mg) was transferred into a non-capped glass vial which was accurately pre-weighted. Then the vial was located in another sealed vessel with excess iodine pellets kept at the bottom. The vessel was heated at 348 K for a given time under ambient pressure. After certain time intervals, the vessel was naturally cooled down to room temperature and then the vial was taken out and weighted. The iodine uptake of CTF-CTTD samples was calculated according the following equation: (m 2 - m1 ) / m1  100 wt %

Where m1 and m2 are the mass of CTF-CTTD samples before and after iodine uptake. Furthermore, the capture of iodine in solution was also investigated by CTF-CTTD. The solution of iodine/hexane with concentration of 100 mg L-1 was acquired by dissolving the weighted iodine in hexane. The CTF-CTTD powder (10 mg) was soaked in iodine/hexane solution (10 mL) and then the mixture was stirred for some time. After adsorption, the solid samples were separated from solution by filtering with Nylon 66 organic membrane ( The pore size of the membrane is 0.22 μm, and the membrane is

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from Tianjin Jinteng Experimental Equipment Co., Ltd. of China. ) and the filtrates were analyzed by using a double beam model PERSEE TU-1901 UV–vis spectrophotometer ( Beijing Purkinje General Instrument Co., Ltd., China ). Adsorption of RhB. The batch experiments for the adsorption of RhB aqueous solutions by CTF-CTTD were carried out in a thermostatic vibration shaker at 303 K and 150 rpm for a certain time in 20 mL glass vessel. Typically, 10 mg adsorbent was added to 10 mL solution containing RhB. After adsorption, the adsorbent was removed from the solution by centrifugation and then the concentration of residual RhB in the supernatant solution was measured by using UV–vis spectrophotometer at the maximum absorbance wavelength of 554 nm.

RESULTS AND DISCUSSION

ºC 400-500 ℃

CTTD

CTF-CTTD

Scheme 2. The synthesis of CTF-CTTD. Synthesis and characterization of CTF-CTTD. The monomer CTTD was synthesized by a threefold Suzuki coupling and demonstrated by

1

H NMR

characterization (Figure S1). 1H NMR (CDCl3, 600 MHz): 7.75 (6H, d)， 7.33 (6H, 8


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d) ， 1.64 (9H, s). As reported previously, most of CTFs can be synthesized by ionothermal synthesis at high temperatures(≥400 °C),45,

46

catalytic reaction with

strong Brønsted acid such as CF3SO3H at room temperature,47 or the condensation reaction of aldehydes and amidines under a mild condition,41 but the second one is not appropriate for acid-sensitive monomers and may result in non-layered structures with low porosity and the last one is not suitable to the monomer CTTD. Hence, the framework CTF-CTTD was synthesized by ionothermal polymerization among nitrile monomers in molten ZnCl2 (as shown in Scheme 2), which acted as both a solvent and a Lewis acid catalyst in the trimerization of nitrile monomers. The CTF-CTTDs synthesized at 400 and 500 °C were denoted as “CTF-CTTD-400” and “CTF-CTTD500”, respectively. To confirm the successful formation of the triazine rings, the FR-IR, 13

C-NMR and XPS spectra of the samples were conducted. The infrared spectra of

CTF-CTTD and CTTD (Figure 1a) shows a significant decrease of the intensity of the characteristic carbonitrile stretching band at 2227 cm-1 while the strong bands for the triazine units at 1570 (C-N stretching mode of the triazine ring) and 1377 cm-1 (in-plane triazine ring stretching vibrations) appear,46, 48, 49 indicating the existence of the triazine rings.36, 47, 50 The solid-state

13

C cross-polarization/ magic-angle spinning (CP/MAS)

NMR spectrum (Figure 1b) also unambiguously verifies the formation of the triazine rings in CTF-CTTD, taking CTF-CTTD-400 as an example, where the chemical shift at 170.8 ppm can be assigned to the sp2 carbons from triazine rings, and the chemical shift at 133.8 ppm corresponds to aromatic carbons. The formation of the triazine rings was also confirmed by XPS. The N 1s XPS spectra of CTF-CTTD includes two peaks

9



(Figure S2), where the peak at 398.9 eV corresponds to the nitrogen atoms in the triazine units, and the signal at 400.35 eV may be assigned to terminal nitrile groups and pyrrolic nitrogen from the decomposition of samples.36, 49 The triazine units in CTF-CTTD possess a comprehensive quality of rich nitrogen content and inherent π-π conjugated structures, which offer rich active sites for the guest molecule adsorption.36, 51

(a) Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(b)

CTF-CTTD

* *

1570 1377

CTTD

CTF-CTTD-400

CTF-CTTD-500 2227

3600

3000

2400

1800

1200

300

600

-1

250

200

Wavenumber (cm )

Figure 1. (a) FT-IR spectra of CTTD and CTF-CTTD; (b)

13

150

/ppm

100

50

0

C CP-MAS solid state

NMR spectra of CTF-CTTD-400 and CTF-CTTD-500.

The pore structure of the CTF-CTTD was characterized by N2 adsorptiondesorption experiment at 77 K. As shown in Figure 2a, the isotherms of CTF-CTTD prepared at different temperatures exhibit type IV sorption isotherms and hysteresis loops, which indicates that mesopores exist in the materials. The isotherms reveal a sharp nitrogen gas uptake in the low pressure region (P/P0 < 0.01), implying the presence of micropores. Pore size distributions of samples were calculated by nonlocal 10


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density functional theory (NL-DFT) based on N2 adsorption isotherms (Figure 2b). For CTF-CTTD synthesized at 400 °C, an apparent peak at about 3.8 nm shows that there are mainly mesopores in the framework, while weak peaks at about 1.4 nm and 1.7 nm demonstrate the existence of micropores. All of them are in agreement with the shape of the N2 adsorption and desorption isotherms. The ratio of micropore to mesopore are 0.89 and 0.63 for CTF-CTTD-400 and CTF-CTTD-500, respectively, which indicates that there are mainly mesopores in the structure. These pore characteristics indicate that CTF-CTTD is a kind of hierarchically porous material with micropores and mesopores, and the material possesses a high BET surface area of 1684 m2 g-1 and a large pore volume of 1.44 cm3 g-1, as shown in Table 1. All results imply that the CTF-CTTD may own a fast adsorption speed and a great adsorption capacity during the adsorption process of hazardous waste. The powder XRD patterns of CTF-CTTD are shown in Figure S3, the XRD patterns of two samples show a weak and characteristic broad peak like most amorphous polymers, impling that they are amorphous in nature.36,

41, 47

Scanning electron microscopy (SEM) image (Figure 3a) shows an irregular lump morphology with smooth surface. The TEM picture (Figure 3b) exhibits that the edge is a little bright and the middle is dark.36, 41, 52-55 The thermogravimetric analysis (TGA) graph of CTF-CTTD under air atmosphere is exhibited in Fig. S4, which indicates that CTF-CTTD is thermally stable up to 520 °C. Elemental analysis of CTF-CTTD suggests that N contents are significantly lower than theoretical values due to the partial decomposition of triazine rings as well as CN group elimination in the frameworks, as shown in Table 1.56 In addition, the overall C, N, and H contents of materials are lower

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3

(dV/dD)(cm3 g-1 nm-1)

-1

N2 uptake at 77 K(cm g )

1 2 3 4 than theoretical values because of residual water adsorbed.36, 38 Hence, the N contents 5 6 significantly decrease, while the carbon contents are close to the theoretical values, 7 8 9 which has also been observed previously.46, 57 ICP analysis results manifests 0.66 and 10 11 0.58 wt % Zn contents for CTF-CTTD-400 and CTF-CTTD-500, respectively, which 12 13 14 is low and reasonable. 36 15 16 17 18 (a) 1000 (b) CTF-CTTD-400 19 CTF-CTTD-400 CTF-CTTD-500 CTF-CTTD-500 20 800 21 22 600 23 24 25 400 26 27 200 28 29 30 0 0.0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 31 P/P0 Pore width(nm) 32 33 34 Figure 2. (a) Nitrogen adsorption and desorption isotherms of CTF-CTTDs measured 35 36 at 77 K; (b) Pore size distributions of CTF-CTTDs, calculated according to desorption 37 38 39 data by DFT. 40 41 42 43 44 Table 1. Pore Characteristics and Elemental Analysis of CTF-CTTDs 45 46 temperature SBETa Vtb Vmicroc % found % theory d 47 compound V micro/Vmeso °C m2 g-1 cm3 g-1 cm3 g-1 C H N C H N 48 49 CTF-CTTD-400 400 1684 1.44 0.68 0.89 87.82 3.37 1.48 85.08 5.00 9.92 50 CTF-CTTD-500 500 1334 1.40 0.54 0.63 90.09 2.17 1.28 85.08 5.00 9.92 51 a SBET is the BET surface area calculated from N2 adsorption isotherms using the BET equation. 52 b 53 Vt is the total pore volume determined by using the adsorption branch of N2 isotherm at P/P0 = 0.99. 54 c Vmicro is the micropore volume calculated by subtracting Vmeso from Vt. 55 d Vmeso is the mesopore volume obtained from the Barrett-Joyner-Halenda (BJH) cumulative specific 56 adsorption volume of pores of 1.70-300.00 nm in diameter. 57 58 59 60 12


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2.0 µm

40 µm

Figure 3. (a) SEM and (b) TEM pictures of CTF-CTTD

Iodine capture and release. Absorbents with electron-rich aromatic networks and heterocyclic units, high specific surface and large pore volume provide approaches to increase the uptake amount of iodine.27, 30 Therefore, it was expected that the designed CTF-CTTD, through a combination of the excellent porous character and electronic properties, would be an ideal option for iodine enrichment. To explore the performance of iodine capture, the CTF-CTTD powder was exposed to nonradioactive iodine vapor in a closed system at 348 K and ambient pressure, and the iodine uptake was measured at different time intervals, as shown in Figure 4a,b. The results demonstrate that the iodine uptake increased gradually over the first 25 h, and little change in the amount of iodine captured was observed after 48 hours, indicating the system was basically saturated. The equilibrium adsorption capacity was measured to be 357 wt% (3.57 g g1

) and 387 wt% (3.87 g g-1) for CTF-CTTD-400 and CTF-CTTD-500. To the best of

our knowledge, this is one of the highest values reported so far for iodine capacity values of solid porous adsorbents like MOFs, zeolites, POPs and CMPs, such as ZIF-8 (1.2 g g-1)58, PAF-25 (2.6 g g-1)27, and CMPN-3 (2.08 g g-1)29 , and can rank the top five

13



among these 70 materials (Table S2). With a view of the capture of volatile iodine from previous studies, the combination of the strong affinity of host to iodine and porous features plays a crucial role in the increase of iodine uptake.28, 59 In this work, the comprehensive effect of electron rich triazine unit and aromatic networks with conjugated π-electrons, hierarchically porous structure as well as large pore volume may be the important factor for the high iodine uptake capacity of the synthesized CTFCTTD. First, the triazine unit with lone pair electrons has potential to increase the iodine uptake capacity through enhancing the interaction between adsorbents and iodine.28, 30, 60 Second, hierarchically porous structure and high specific surface area would lead to an increase in the iodine uptake capacity.23, 25, 29 Third, previous studies suggest that phenyl ring in the structure may cause forceful interaction with iodine.26, 30, 60

The XPS spectra were recorded to characterize the existence of iodine captured in

CTF-CTTD. As seen in Figure 5a, the iodine spectrum displays two split peaks located at 630.1 and 618.6 eV, which are assigned to the I3d3 and I3d5 orbitals of iodine molecules,23, 61, 62 respectively, indicating that it exists as molecule I2.

400

3.5

350

3.0

300

2.5

250 CTF-CTTD-400 CTF-CTTD-500

2.0

200

1.5

150

1.0

100

0.5

50

0.0

0

10

20

30

40

50

60

70

0

80

(c) 100

I2 uptake(%)

(b) 4.0

Weight (%)

(a)

I2 uptake(g g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

loss of I2

60 I2@CTF-CTTD CTF-CTTD 40

Time(h)

100

200

300

400

500

Temperature (C)

Figure 4. (a) Schematic representation of the iodine vapor experiment; (b) Gravimetric

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changes of iodine capture capacity of CTF-CTTDs as a function of time at 348 K and ambient pressure; (c) TGA trace under N2 atmosphere of I2@CTF-CTTD prepared at 348 K.

640

635

630

CTF-CTTD

625

620

615

Binding energy(eV)

700

600

500

400

300

I2@CTF-CTTD

200

100

640

600

0

Intensity(a.u.)

I3d3

Intensity(a.u.)

CTF-CTTD

(b)

I3d

I2@CTF-CTTD

Intensity(a.u.)

(a) Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

I3 d I3 d 3

635 630 625 620 Binding energy(eV)

200

615

0

Binding energy (eV)

Binding energy (eV)

Figure 5. (a) XPS spectrum of CTF-CTTD after iodine vapor capture; (b) XPS spectrum of CTF-CTTD after iodine adsorption in hexane.

The TGA graph of the I2-loaded CTF-CTTD (named as I2@CTF-CTTD) exhibits a broad mass loss step from 70 to 350 °C (Figure 4c), which is attributed to the release of I2. Besides heating,63 the trapped iodine can be released by immersing in organic solvents

23, 27, 64, 65

. Here, we preformed the iodine release research by the means of

soaking in ethanol. The curve of controlled release of iodine at different time intervals is shown in Figure 6a. The color of the solution changed from light orange to dark brown quickly with time (Figure 6c), which suggested that iodine could be quickly released from the pore of materials. The phenomenon may be attributed to the pore characteristic of high specific surface area and large pore volume as well as the 15



hierarchically porous structure.66 The samples regenerated were reused for the next round of iodine vapor adsorption cycle to test the regeneration of CTF-CTTD. As shown in Figure 6b, CTF-CTTD exhibits almost the same capacity over four cycles. Moreover, after being reused four times, the N2 adsorption isotherms of CTF-CTTD before I2 capture and after desorption were shown in Fig. S8. CTF-CTTD still maintains a large pore volume of 1.0 cm3 g-1, indicating that the porous structure of CTF-CTTD is stable in the process of iodine capture. All these results indicate that the CTF-CTTD can be used as a kind of recyclable absorbent for iodine capture.

84

350

82 80

100

80

300 60

250 200

40

78

Iodine uptake Recycling percentage

150

20

76 0

20

40

100

60

1

2

T(min)

(c)

0h

3

Recycling percentage (%)

(b) 400 I2 uptake (%)

(a) 86

wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

Cycle time

0.5 h

3h

6h

12 h

24 h

Figure 6. (a) The controlled release of iodine of I2@CTF-CTTD prepared at 348 K; (b) Recycling percentage of CTF-CTTD; (c) Photographs of the iodine release progress of I2@CTF-CTTD over time, which was soaking in ethanol.

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In addition, CTF-CTTD can also be applied as absorbents for iodine capture in organic solution. When the CTF-CTTD (10 mg) was added into iodine/hexane solution (100 mg L-1, 10 mL) in a small sealed vial at 303 K, the purple solution gradually faded to pale red and finally to colorless with adsorption process being prolonged. The adsorption amount increased quickly within 10 min (Figure 7a), and then a slow increase was observed after 9 h. The concentration of the residual iodine was analyzed by UV/Vis spectroscopy, as shown in Figure 7b. The highest removal efficiency was found to be up to 81 % (84.04 mg g-1) for iodine solution of 100 mg L-1. The correlation coefficient R2 values of pseudo-second-order kinetic model is 0.999, which is obviously closer to 1.0 than that of pseudo-first-order kinetic model (0.896) in Table S1. This illuminates that the adsorption kinetic data can be well described by pseudo-secondorder kinetic model. To research the form of iodine adsorbed in CTF-CTTD, the XPS spectrum was also analyzed. The iodine existed as molecule as evidenced by the valence of zero loading on CTF-CTTD (Figure 5b).

(a) 90

(b) 0.6

-1

Iodine adsorption qe(mg g )

80

0.5

70

before

after

60 50

Absorption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.3 0.2

2 min 5 min 10 min 30 min 60 min 180 min 360 min 540 min 720 min 1440 min

40

0.1 30 0

200

400

600

0.0 440 460 480 500 520 540 560 580 600

800 1000 1200 1400

Wavelength(nm)

Time(min) 17



Figure 7. (a) Kinetic studies of iodine adsorption by CTF-CTTD in hexane solutions; (b) UV-Vis spectra of iodine in hexane at different time intervals after adsorption.

Adsorption of RhB. In view of the structural property of CTF-CTTD, it may show potential advantages for removing pollutants from wastewater apart from the capture of iodine. Therefore, as an example, we studied the adsorption behavior of the RhB adsorbed by CTF-CTTD. There was little difference between the equilibrium adsorption capacity of CTF-CTTD-400 and CTF-CTTD-500 for RhB in the solution of 800 mg L-1 at 303 K (Figure S9), hence CTF-CTTD-400, as an example, was taken to further explore the adsorption behavior for RhB. To investigate the adsorption kinetics and the mechanism of the adsorption process, the adsorption experiments about contact time toward RhB were conducted at relatively high initial concentrations of 200, 400 and 500 mg L-1. The experimental data were fitted with the pseudo-first-order and pseudo-second-order kinetics models. The corresponding kinetic data were fit linearly with the two kinetic models listed in Table S3, which shows that pseudo-second-order kinetics model fits better than pseudo-firstorder kinetics model with the correlation coefficient R2 for the adsorption of RhB on CTF-CTTD being as high as 1.0. Additionally, the Qe values found in the second-order kinetics model are closer to the experimental Qe values for all the concentrations studied, when compared with the first-order kinetics model. Moreover, the values of rate constant k2 decrease with the increase of initial dye concentration, which indicates that the adsorption process of RhB is faster in the lower concentration. These results verify

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that the pseudo-second-order kinetic model can explain the adsorption process of RhB adsorption by CTF-CTTD, implying that chemisorption may be the rate-limiting step in the adsorption process. 90 % of dyes were absorbed within the first 15 min at the initial concentration of 200 mg L-1. The fact of the fast removal of RhB may be attributed to the hierarchically porous structure of the materials. Furthermore, 99.97 % of dyes in the solution were removed with the presence of CTF-CTTD after equilibrium (Figure 8a). As we know, the adsorption equilibrium isotherm is used to explain the diffusion of adsorbates molecules and adsorbents at equilibrium. The Langmuir isotherm model illustrates the assumption of adsorption homogeneity, such as monolayer surface coverage, equally available adsorption sites, and no interaction between adsorbed species, while the Freundlich isotherm model is based on the heterogeneous surfaces and used to describe the multilayer adsorption process. The Langmuir isotherm model and Freundlich isotherm model are used to analyze the adsorption data, calculate the adsorption capacities and describe the sorption behavior of CTF-CTTD towards RhB.

(b) 750

(a) 100

600

80

after 12 h

-1

before

Qe (mg g )

Removal efficiency(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40

450

300

20 150

0 0

15

30

45

60

0

75

300

600

900 -1

Time(h)

Ce (mg L )

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1200

1500


Figure 8. (a) Removal efficiency of RhB in the RhB solution of 200 mg L-1 with the presence of CTF-CTTD over time; (b) Equilibrium isotherms for the adsorption of RhB on CTF-CTTD at 303 K.

The equilibrium isotherms for the adsorption of RhB on CTF-CTTD at 303 K is shown in Figure 8b. Table S4 summarizes the fitting results of Langmuir and Freundlich models for RhB adsorption on CTF-CTTD. It can be found that the linearity of the Langmuir model (R2 = 0.9995) is significantly better than that of the Freundlich model (R2 = 0.8643). Therefore, the Langmuir model is more appropriate than the Freundlich model to describe the adsorption behavior of RhB on CTF-CTTD, which means the adsorption of CTF-CTTD towards RhB is a monolayer adsorption reaction. The maximum adsorption capacity of CTF-CTTD was 684.9 mg g-1 at the temperature of 303 K. What’s more, the removal percentage of RhB can reach 90 % at a relatively high initial concentration of 600 mg L-1, and the removal efficiency is higher than 99 % within the initial concentration of 400 mg L-1. All of these values indicate that CTFCTTD is a good adsorbent for RhB removal from aqueous solutions.

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RhB uptake capacity (mg g-1)

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600 500 400 300 200 100 0

0

1

2

3

4

Cycle times Figure 9. Regeneration of CTF-CTTD for RhB adsorption.

Regeneration capacity of adsorbents decides the costs of operations and plays a crucial role in the practical application. CTF-CTTD can be easily regenerated after washing with ethanol for several times. Figure 9 illustrates the regeneration behavior of CTF-CTTD and it shows that CTF-CTTD maintains almost the same adsorption capacity and retains 98% of initial RhB adsorption capacity after four cycles, indicating that the prepared adsorbent may be a potential candidate for industrial applications. The N2 adsorption and desorption isotherms of CTF-CTTD before RhB removal and after desorption were similar, as shown in Fig. S11, which indicates that CTF-CTTD remains integral and stable during the experiment. The zeta potential of CTF-CTTD was about -14.8 mV, which infers that material surface is with static negative charge. And RhB is a cationic dye. After interaction with RhB, the zeta potential of CTF-CTTD increased

21



to 2.06 mV (as shown in Table S5). indicating electrostatic interactions between CTFCTTD and RhB.

CONCLUSIONS In summary, a covalent triazine-based framework was designed and synthesized by using stereoscopic monomer CTTD. CTF-CTTD possesses a high BET surface areas of 1684 m2 g-1, a large pore volume of 1.44 cm3 g-1, and hierarchical pores. Taking advantage of the electron rich triazine unit and aromatic networks as well as a hierarchical porous character, a high iodine adsorption capacity of 387 % was obtained in our experimental condition. Moreover, the iodine absorbed by samples can be easily and quickly released in ethanol and a little loss of iodine adsorption capacity is observed after 4 times cycles. In addition, CTF-CTTD also shows an excellent adsorptive performance for the removal of RhB with adsorption capacity of 684.9 mg g-1 with a fast adsorption rate. Besides, the adsorbents can be easily regenerated and reused in RhB adsorption. All results obtained from this work may provide a guidance for the design of CTFs with high specific surface area and large pore volume as well as the hierarchically porous structure in capturing of radioactive iodine and removing pollutants from wastewater, which may be of importance for world-threatening pollution issues.

ASSOCIATION CONTEBT Supporting information Synthetic procedures; 1H NMR, XPS and PXRD patterns; more UV-Vis absorption spectra and fluorescence spectra; more information of pseudo-first-order and pseudo-

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second-order kinetic models; more information of Langmuir and Freundlich isotherm model; summary of iodine absorption and RhB absorption

AUTHOR INFORMATION Corresponding Authors *E-mail address: [email protected]. *E-mail address: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the Natural Science Foundation of China (NO. 21536001 and 21606007), National Key Projects for Fundamental Research and Development of China (2016YFB0600901).

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