Synthesis of microporous covalent phosphazene-based frameworks

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Applications of Polymer, Composite, and Coating Materials

Synthesis of microporous covalent phosphazenebased frameworks for selective separation of uranium in highly acidic media based on size-matching effect Meicheng Zhang, Yang Li, Chiyao Bai, Xinghua Guo, Jun Han, Sheng Hu, Hongquan Jiang, Wang Tan, Shoujian Li, and Lijian Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06842 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Applied Materials & Interfaces

Synthesis of Microporous Covalent Phosphazenebased Frameworks for Selective Separation of Uranium in Highly Acidic Media Based on Sizematching Effect Meicheng Zhang,a Yang Li,a Chiyao Bai,b Xinghua Guo,a Jun Han,c Sheng Hu,c Hongquan Jiang,a Wang Tan,a Shoujian Li,*, a and Lijian Ma*, a a

College of Chemistry, Sichuan University, Key Laboratory of Radiation Physics &

Technology, Ministry of Education, Chengdu, 610064, P. R. China b

c

Chengdu New Radiomedicine Technology CO. LTD., Chengdu, 610207, P.R. China

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang,

621900, P. R. China

KEYWORDS: highly acidic media, a wide acidic range, separation of uranium, size-matching, radiation stability

ACS Paragon Plus Environment

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ABSTRACT

Based on high stability of phosphorus-oxygen linkage, we constructed two microporous covalent phosphazene-based frameworks (CPFs), for the first time, by choosing hexachlorocyclotriphosphazene (HCCP) as core unit and polyhydroxy aromatic compounds (hydroquinone or phloroglucinol) as monomers, named CPF-D and CPF-T respectively. Characterization studies by using FT-IR, NMR, TGA and

60

Co γ-ray irradiation etc. demonstrated that both of the CPF

materials have excellent acid and radiation stability, relatively higher thermal stability. The results of batch adsorption experiments show that CPF-T is significantly more capable of sorbing uranium than CPF-D. In a pure uranium system with higher acidity (pH 1), the uranium sorption amount of CPF-T can reach up to 140 mg g-1. Distinctively, in mixed-metal solution with 12 coexisting cations, CPF-T shows relatively stable and excellent uranium adsorption capability over a wide range of acidity (pH 4~3 M HNO3), and the difference in uranium sorption amounts is less than 30 % with the maximum of 0.26 mmol g-1 at pH 4 and the minimum of 0.20 mmol g-1 at 3 M HNO3, which is far superior to that of the conventional solid phase extractant (SPE) materials previously reported. The research results suggested that the sorption model based on the speculated mechanism of size matching plus hydrogen bond network has played a dominant role in the process of uranium adsorption. The proposed strategy for the one-pot fabrication of an acid-resistant microporous framework materials by bridging the aromatic monomers via P-O bonds provides an alternative approach for design and synthesis of new SPE materials with sizematching function desired for effective separation of uranium or other valuable metals from highly acidic environments.


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1. Introduction As is well-known, uranium is an important resource for nuclear fuels. But the dilemma faced in the field of nuclear energy at present is that natural uranium is less-abundant and non-renewable on the Earth’s crust,1 and meanwhile lower utilization rate of uranium in nuclear reactors causes the spent fuel still containing a large number of unreacted uranium (up to 95% with about 94.3% of

238

U and about 0.8% of

(containing > 99% of

235

U).2 In addition, although the radioactivity of natural uranium

238

U with a half-life of 4.5×109 a) can be almost ignored, but as a heavy

metal, it has strong chemical and biological toxicity, which might bring potential hazards to human health and ecological environment.3-4 Therefore, extraction of the uranium from spent fuel can not only recycle the uranium resource, but also protect the human health and ecological environment. However, it is still a challenge for the separation and recovery of key nuclides like uranium, due to the extreme environments (high acidity, strong radiation, high salinity, etc.) in spent fuel reprocessing.5-6 As a facile, efficient and scalable separation method, Solid Phase Extraction (SPE) has attracted more and more attention in the application of separation and recovery of uranium in recent years.7 The SPE materials widely studied currently include inorganic mineral materials,8-9 biomass materials,10-11 polymer materials,12-18 carbon materials,19-24 etc. However, for the vast majority of reported SPE materials, the matrix and functional groups are linked mostly by C-N, C-O, Si-O bonds, etc. Although some of these materials showed good, even excellent capability in uranium sorption under weak acidic conditions (pH 2~7), but all of them have their own inherent shortcomings:


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(1) The matrix materials, such as hydrothermal carbon,21, 23-24 some mesoporous silica,25-27 and some supramolecular,28 COF,29 MOF30 and polymer/resin materials13-15 would be unstable or even collapse in dilute acid solutions (usually pH < 2); (2) The design flaws in distribution of the active grafting sites on the surface of SPE materials with almost no consideration of geometric factor for metal-ligand coordination made the bonding interaction of these materials with uranyl ions relying mostly on the single functional groups (such as carboxyl, hydroxyl, amino and amidoxime group) during uranium adsorption, which could lead to finite uranium capturing ability of these materials; (3) Also in weakly acidic medium (generally pH < 2) the functional groups would reduce or lose the coordination ability due to protonation, as is well-known. All of these still remains a great challenge for the application of the SPE materials in highly acidic environment. As one of the most important liquid-liquid extractants in nuclear fuel cycle, the tributylphosphate (TBP) has been used over 60 years in PUREX process for the separation of key nuclides in spent fuel reprocessing.31 The P-O double bond (bond energy: 585 kJ mol-1) and single bond (bond energy: 410 kJ mol-1) in TBP are very stable with high tolerance to the extreme environment mentioned above. Besides having higher bond energy, the P-O bond can also have enhanced chemical stability due to the d-p π back-donation effect, and furthermore, the P-O bond could form a p-π conjugate system if it links with adjacent aromatic-ring monomers, which could enable the framework formed more stable. Thus, for the purpose of (1) getting the framework structures more stable than the conventional SPE materials and (2) using nanoscale ion-sieving for selective separation of uranium instead of


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acid-nonresistant ligand-metal interaction, hexachoro-cyclotriphosphazene (HCCP) was chosen as the core skeleton, and two polyhydroxy monomers, hydroquinone and phloroglucinol, as the linkers to synthesize two microporous covalent phosphazene-based frameworks (CPFs) linked by P-O bonds with different pore size, and named CPF-D and CPF-T respectively. The structure, physico-chemical characteristics, and especially, acid, radiation and thermal stability of the two CPFs were examined. The adsorption behaviors on uranium of these two microporous materials in uranium solution as well as simulated nuclear industry effluent samples were systematically investigated and compared, and finally the adsorption mechanism was explored. 2. Experimental Section 2.1. Reagents The hydroquinone, phloroglucinol, HCCP, metal nitrates and oxides used in this study were supplied by Aladdin Chemistry Co., Ltd. (China). Dioxane, acetone, ethanol, sodium hydroxide and nitric acid were purchased from Chengdu Kelong Chemical Reagent Factory. All reagents were analytical and guaranteed reagents and used without purification. 2.2. Characterization methods Fourier Transform Infrared (FT-IR) spectra were inspected by NEXUS 670 spectrometer from 400 to 4000 cm-1 with the KBr-pellet technique. The morphology of materials was obtained with a Scanning Electron Microscope (SEM, JSM-7500F, Japan ) operated at 5-18 kV accelerating voltage with a field emission electron source. Thermal Gravimetric Analysis (TGA, SDT Q600) meter was used to measure the thermal stability under N2 protection (N2 flow rate: 20 mL min-1, heating rate: 10 K min-1). Bruker spectrometer (400 MHz) was employed to inspect solid-state Nuclear Magnetic Resonance (NMR) spectra for 13C and 31P. X-Ray Photoelectron Spectroscopy


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(XPS, XSAM800) was used to analyze the elemental chemical species on the surface of materials equipped with a monochromatic Al Kα X-ray source (1361 eV). The initial and equilibrium concentrations of uranium and other metal ions were determined by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES, Optima 8000, PerkinElmer, USA) and the concentrations of standard curve are 0-90 mg L-1 (r2 > 0.999). The N2 adsorptiondesorption isotherms were measured by Micromeritics ASAP 2460 at 77 K. The pore-size distributions were calculated using Barrett–Joyner–Halenda method and Horvath-Kawazoe model. BET surface areas were obtained via automatic calculation of the built-in software by BET equation. The potentiometric titrations were performed by a G20 Compact Titrator (Mettler Toledo, Switzerland) using NaOH as the titrant. The contact angles were recorded with a drop shape analysis system (JC200D3, Beijing kangguang instrument co. LTD, Beijing, China) equipped with a video camera. 2.3. Synthesis of CPF-D and CPF-T


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Scheme 1. Schematic illustration of the synthesis of CPF-D and CPF-T. The synthesis procedure is shown in Scheme 1. 1.5 mmol hydroquinone (or 1.0 mmol phloroglucinol) and 0.5 mmol HCCP were dissolved in 1 mL dioxane to form transparent solution in an 8 mL glass pressure bottle. Then 0.84 mL triethylamine was added and the bottle was sealed and heated at 80°C for twenty-four hours. After the reaction mixture was cooled down naturally, the generated powder was filtered off and washed several times using deionized water, EtOH and acetone sequentially until the filtrate was neutral and colorless. Then the powder was dried overnight in a vacuum oven at 50°C. The resulting materials were denoted as CPF-D and CPF-T and their yields were about 60% and 65%, respectively. 2.4. Batch experiments of uranium adsorption 10 ± 0.2 mg CPF-D or CPF-T and 10 ± 0.1 mL of pure uranium solution or multi-ion solution were added into a 13 mL plastic centrifuge tube. The tube was shaken for a specified time period (t, h) at room temperature. Then the solution was centrifuged at 5000 rpm and the supernatant was separated to determine the concentrations of the metal ions before and after sorption by ICPOES. All plastic centrifuge tubes and glassware were soaked in 10.0 wt% HNO3 solution for twelve hour to remove any metal impurities before the experiment. All tests were performed in duplicates. Adsorption amount qe (mg g-1) is calculated with the formula 1:

qe =

(c0 − ce ) × V w

(1)

Where c0 is the initial concentration of metal ion (mg L-1 or mmol L-1), and ce is the equilibrium concentration of metal ion (mg L-1 or mmol L-1). w is the weight of sorbent (g) and V is the volume of solution (L).


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A specific term, uranium-selectivity (SU), is introduced to describe the potency and degree of the selectivity of sorbents to uranium to evaluate the effect of the competitive cations on the uranium sorption in a multi-ion solution29, 32:

SU =

qe-U × 100% qe-tol

(2)

Where qe-U is the sorption amount of uranium (mmol g-1) and qe-tol is the total sorption amount of the all cations (mmol g-1) from the multi-ion solution. The sorption amount of each metal ion (mmol g-1) is calculated by the formula 1. Each data point in the figures about the adsorption represents the average of 2 measurements from duplicate samples and the error bars represent standard deviation calculated from duplication experiments. 3. Results and discussions The as-prepared covalent phosphazene-based frameworks CPF-D and CPF-T were characterized with FT-IR, NMR and SEM. The adsorption behaviors of the two materials towards uranium were investigated by batch sorption experiments. 3.1. Characterizations 3.1.1 FT-IR and SSNMR


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Figure 1. FT-IR spectra of HCCP, hydroquinone, phloroglucinol, CPF-D and CPF-T. The FT-IR spectra of the monomers and the products are displayed in Figure 1. The peaks located at 1241 cm-1 of CPF-D and 1234 cm-1 of CPF-T belong to the P=N stretching vibration. The peaks around 881 cm-1 of CPF-D and 862 cm-1 of CPF-T are ascribed to the P-N stretching vibration. And the peaks observed at 539 and 563 cm-1 belong to the unreacted P-Cl bonds. Especially, the emerging middle peaks at 962 and 1020 cm-1 are assigned to the stretching vibration of Ar-O-P.33

Figure 2. 13C (a) and 31P (b) NMR of CPF-D and CPF-T.


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The solid-state 13C NMR spectra in Figure 2a exhibit distinct signals at about 144 and 152 ppm, corresponding to the carbons of C-O bonds in CPF-D and CPF-T, respectively. The peak around 108 ppm of CPF-D and 95 ppm of CPF-T can be assigned to the signal of the carbons of C-H in benzene ring. The peaks at 147 and 112 ppm of CPF-D should be attributed to the carbons of COH and C-H of unreacted hydroquinone (13C NMR spectrum of hydroquinone as seen in Figure S1). The peaks at 155 and 97 ppm of CPF-T are from unreacted phloroglucinol (Figure S2) as well. The solid-state

31

P NMR spectra (Figure 2b) exhibit signals at 23 and 24 ppm,

corresponding to the phosphorus of P-O bonds in CPF-D and CPF-T, respectively. 3.1.2. SEM

Figure 3. SEM images of CPF-D (a) and CPF-T (b). The surface micromorphology of CPF-D and CPF-T characterized by SEM are shown in Figure 3. It can be observed that CPF-D exhibits hollow tube-shaped structure with some cracked ends. While CPF-T shows a smooth and regular spherical morphology. From Mapping (Figure S3) of SEM, it is seen that the C, N, O, P and Cl elements uniformly distribute on the surface of CPF-D and CPF-T. 3.1.3. Nitrogen adsorption-desorption isotherms


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Figure 4. N2 adsorption-desorption isotherms and pore-size distributions of CPF-D (a and b) and CPF-T (c and d) (insert: the micropore-size distributions calculated by Horvath-Kawazoe model). From the N2 adsorption-desorption isotherm (Figure 4a, 4c), a steep rise can be found at the low relative pressure (P/P0), indicating that both CPF-D and CPF-T belong to microporous materials. The pore-size distribution curve of CPF-D (Figure 4b) shows that CPF-D has two main pore diameters of 1.82 and 0.55 nm, which are close to the pore sizes of theoretical structure (1.66 and 0.34 nm). CPF-T has two diameters of 1.26 and 0.34 nm (Figure 4d), which are much close to the theoretical pore sizes (0.95 and 0.34 nm). The corresponding pores in the structures simulated by Materials Studio software are shown in Figure S4. The BET surface area of CPF-D calculated from nitrogen adsorption data is 278.9 m2 g-1 and the total pore volume is 1.26 cm3 g1

. While, the BET surface area of CPF-T is 654.1 m2 g-1 and the total pore volume is 0.33 cm3 g-

1

. The simulation structure of CPF-D and CPF-T can be seen in Scheme.1. In contrast, the

surface area of CPF-T is larger but the pore volume is smaller, which may be due to that CPF-T contains more pore channels and smaller pore diameter than CPF-D at the same mass. The nitrogen-adsorption isotherms for CPF-D and CPF-T are also simulated by Materials Studio


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software and shown in Figure S5. According to the theoretical adsorption curves, the BET surface areas of CPF-D and CPF-T are 1002.9 and 818.1 m2 g-1 respectively, much bigger than the experimental values. The difference between experimental and theoretical values may be attributed to the influence of the disordered stack of the structural units in the experiments, and the residual solvents and other small molecules in holes of the samples that cannot be completely eliminated. 3.1.4. TGA

Figure 5. TGA curves of CPF-D and CPF-T. Thermal stabilities of the two materials were studied by TGA. The TGA thermograms of both CPF-D and CPF-T (Figure 5) show four main stages of weight loss. The loss of adsorbed water and residual solvent causes the first stage below 150°C. The second stage of weight losses below 440°C are about 21% for CPF-D and 27% for CPF-T, respectively, which could be attributed to the collapse of the phosphazene ring.34 The third stage occurred from 440 to 550°C (about 9% and 7% for the two materials separately) may be due to the breakage of P-O bond. In the fourth


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stage, the weight losses of CPF-D and CPF-T above 550°C may be caused by carbonization of the remaining benzene ring structure in frameworks. TGA curves show that the as-prepared materials are thermally stable up to 150°C and have the potential to be used in spent fuel reprocessing. 3.1.5. Potentiometric titration The potentiometric acid-base titration was used to determine the content of acid groups on the surface of the materials, and the titration data were processed using the Gran function (seen in supporting information Section 1). The Hs values of CPF-D and CPF-T, the total concentration of surface acidic groups per solid weight, are shown in Table 1 and Figure S6. The results indicate that the concentration of acid functional groups (-OH) on the surface of CPF-T is greater than that of CPF-D. Table 1. Values of Veb1, Veb2 and HS obtained from linear regression of the Gran plots. Veb material

Veb1

Hs (mmol g-1) Veb2

CPF-D

1.032

1.659

2.61

CPF-T

1.313

2.100

3.36

3.1.6. Hydrophobic property In order to investigate the hydrophobic property of the materials, the water contact angles of the dried samples after tabletting was tested with Contact Angle Meter. The water-drop profiles for CPF-D and CPF-T taken respectively at different time were recorded in Figure 6. When water drops were dropped on the materials, both of the materials were fully wetted soon and CPF-D


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was completely wetted within 2s and CPF-T was instantaneously fully wetted. The results indicate that both materials belong to super-hydrophilic materials and CPF-T has a better superhydrophilicity.

Figure 6. the water-drop profiles for CPF-D and CPF-T at different times.

3.1.7. Acid stability


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Figure 7. FT-IR spectra of CPF-D and CPF-T before and after soaking in 0.1 M, 1 M, 2 M and 3 M HNO3. In order to investigate the acid stability, CPF-D and CPF-T were soaked in different concentrations of HNO3 solution for 24 h. According to FT-IR spectra of CPF-D and CPF-T shown in Figure 7, no significant change of the main peaks before and after the acid treatment is observed, which preliminarily indicates that both the materials have good acid stabilities. CPF-T after soaking were also used to adsorb uranium under the same condition and their adsorption capacities are shown in Table S1. It is observed that the sorption amount of CPF-T is slightly affected by acidity. Based on the results, it is speculated that CPF-T still can be used for adsorbing uranium after the soaking treatment in HNO3 solution, which provides support for its practical application in strong acid environments. 3.1.8. Irradiation Stability


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Figure 8. FT-IR spectra of CPF-D and CPF-T before and after irradiation (irra-air CPF: sample irradiated in air, irra-acid CPF: sample irradiated in HNO3 solution). The irradiation stabilities of the materials were also investigated using GAMMATOR M-38-2 (USA) irradiator (60Co source, γ-ray). The samples of CPF-D and CPF-T were irradiated both in air and in an aqueous solution of 0.1 M HNO3 with radiation dose of 101.85 kGy, and then characterized by FT-IR spectroscopy (Figure 8). It is clear that the FT-IR spectra of the two materials did not change significantly after irradiation, which means that CPF-D and CPF-T have good irradiation stabilities. Interestingly, the sorption amount of irradiated CPF-T was still up to 40 mg g-1 in 3 M HNO3 solution which was much close to its original value before irradiation (48 mg g-1), indicating that the irradiation has little effect on uranium adsorption of CPF-T. 3.2. Adsorption experiments 3.2.1. Effect of acidity In order to investigate the sorption performance of CPF-D and CPF-T for uranium, the effect of the solution acidity was researched at first (Figure 9). From the species distribution of uranium under different pH conditions (Figure S7), it can be found that the concentration of (UO2)2(OH)22+ increases gradually when pH > 4, which could cause the appearance of precipitate of uranium. Therefore, the appropriate pH range for adsorption experiments was confirmed to be less than 4. When the pH changed from 4 to 1, the sorption amount of CPF-D decreased obviously because the functional groups on CPF-D were increasingly protonated with the increasing acidity, which led to the gradual loss of its coordination ability to uranium. Distinctively, the sorption amount of CPF-T was not significantly affected by the pH change from 4 to 1. The sorption amount of CPF-D decreased from 12 to 2 mg g-1 when the solution


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acidity was adjusted from 0.1 to 3 M HNO3, which suggests that CPF-D tends to lose most of its sorption ability for uranium under high acidity. The sorption amount of CPF-T was 88.5 mg g-1 at 0.1 M HNO3, and more inspirationally, it still retained 48 mg g-1 under high acidic condition of 3 M HNO3. In addition, when the initial concentration of uranium is around 1.0 mmol L-1, the sorption amounts of CPF-T are 180.3, 137.1 and 55.2 mg L-1 respectively in pH 4, pH 1 and 3 M HNO3 solution.

Figure 9. Effect of acidity on the uranium sorption onto CPF-D and CPF-T (c0 ≈ 0.45 mmol L1

, t =12 h, V = 10 mL, w = 10 mg, T = 298 K).

Generally, the uranium adsorption capacities of most SPE materials reported before decrease obviously with the decreasing pH value from 5 to 2, and they are likely to lose all of their adsorption abilities for uranium when pH < 2. Quite distinctively, CPF-T exhibited relatively stable and excellent uranium adsorption capacity in the testing system with large acid range (pH 4 to 3 M HNO3). 3.2.2. Effect of sorption time and kinetic studies


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Figure 10. Effect of contact time on the uranium adsorption onto CPF-D and CPF-T (pH = 1, c0 ≈ 0.45 mmol L -1, V = 10 mL, w = 10 mg, T = 298 K). The sorption kinetics of the two materials were investigated by the study of the uranium adsorption at different times at pH 1. As seen in Figure 10, the adsorption of CPF-D reached equilibrium within 1 h. While it took CPF-T about 4 h to reach a primary balance and about 80% of the total amount of sorption occurred within 1.25 h. Several dynamic models were used to examine the controlling mechanism of adsorption process, including pseudo-first-order model, pseudo-second-order model and intra-particle diffusion model .25 The equations of these models are shown as follows:

ln( qe − qt ) = ln qe − k1t

(3)

t 1 1 = + t qt k2 qe 2 qe

(4)

qt = kint t 0.5 + C

(5)


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where qt represents the sorption amount of uranium (mmol g -1) at any time t. qe represents the equilibrium sorption amount of uranium (mmol g-1). k1 (min-1) is the kinetic constant for pseudofirst-order model and k2 (g mg-1 min-1) is the kinetic constant for pseudo-second-order model. kint (mg g-1 min-1/2) is the intra-particle diffusion rate constant. C (mmol g-1) is the intercept proportional to the thickness of boundary layer. The experimental data of CPF-D and CPF-T are fitted by these three models. The corresponding constant values are included in Table 2 (the fitted linear forms as seen in Figure S7). The correlation coefficient r2 of CPF-D in pseudo-first-order model, pseudo-second-order model and intra-particle diffusion model is 0.8989, 0.9923 and 0.6249, respectively. The r2 of CPF-T in the three models is 0.7340, 0.9997 and 0.5312, respectively. The results suggest that the r2 value for pseudo-second-order model is very high (r2 > 0.99), and the theory values of equilibrium sorption amount (CPF-D 0.088 mmol g-1, CPF-T 0.366 mmol g-1) are very close to the experiment values (CPF-D 0.064 mmol g-1, CPF-T 0.359 mmol g-1). Therefore, it is believed that the uranium adsorption kinetics of CPF-D and CPF-T belong to pseudo-second-order process and chemisorption could be the rate-controlling step. Table 2. Simulation parameters of CPF-D and CPF-T in different kinetic models. Model

Pseudo-first order model

Parameter

CPF-D

CPF-T

qe (mmol g-1)

0.064

0.359

k1 (min-1)

0.0059

0.0027

qe, cal (mmol g-1)

0.0342

0.1065

r2

0.8989

0.7340

k2 (gmmol-1 min-1)

-3.88

0.0977


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Pseudo-second order model Intraparticle Diffusion

Page 20 of 39

qe, cal (mmol g-1)

0.088

0.366

r2

0.9923

0.9997

kint (mmolg-1min-0.5)

0.0015

0.0053

C (mmol g-1)

0.0265

0.1774

r2

0.6249

0.5312

3.2.3. Effect of initial concentration of uranium and isotherm studies of CPF-T Through the above experiments, we found that the adsorption performance of CPF-T was much better than that of CPF-D. Therefore, the uranium adsorption behavior of CPF-T was further investigated in the following experiments. As can be seen in Figure 11, when the initial uranium concentration c0 was less than 230 mg L-1, the sorption amounts of CPF-T were rising with the increase of c0. When c0 further increased to greater than 230 mg L-1, the sorption amounts of CPF-T reached and remained around 140 mg g-1 and didn’t increase any longer with the increasing c0, from which the saturated adsorption amount of CPF-T towards uranium is determined to be 140 mg g-1 at pH 1. Herein, Langmuir, Freundlich and Dubinin-Radushkevich (D-R) models for describing the adsorption of solid to liquid interface are used to analyze the experiment data.32


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Figure 11. Effect of initial concentration for uranium adsorption on CPF-T (pH = 1, V = 10 mL, w = 10 mg, t = 12 h, T = 298 K). Langmuir isotherm assumes the sorption process is monolayer adsorption occurred at specific homogenous site and its linear expression is given as followed in the formula 6:

ce c 1 = + e qe bL qL qL

(6)

Where qe is the equilibrium sorption amount (mmol g-1). ce is the equilibrium concentration of uranium (mmol L-1). qL is the Langmuir saturated monolayer sorption amount (mmol L-1). The Freundlich model is an empirical equation based on an exponential distribution of sorption sites, characteristic of heterogeneous surface, and the linear form is given in the formula 7:

ln qe = ln K F +

1 ln ce nF

(7)


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Where KF [mmol1-1/n·L1/n/g] is the Freundlich constant related to the adsorption capacity of the adsorbent and nF (unitless) is a constant related to the adsorption intensity. The Dubinin-Radushkevich isotherm is a semi-empirical equation where the adsorption follows a pore-filling mechanism. The linear form of D-R isotherm model is given in the formula 8:

ln qe = ln qDR − βε 2

(8)

Where qDR (mmol g-1) is the D-R saturated sorption amount and β (mol2 J-2) is the D-R constant. ε (J2 mol-2) is the Polanyi potential related to the equilibrium concentration and calculated with the formula 9:

ε = RT ln(1 +

1 ) ce

(9)

Where R is the gas constant (8.314 J mol-1 K-1) and T is the absolute temperature (K). The sorption energy EDR (kJ mol-1) is obtained using the following the formula 10:

EDR =

1 2β

(10)

The parameters of Langmuir, Freundlich and D-R models of CPF-T are listed in Table 3 (the fitted linear forms as seen in Figure S9). Table 3. The value of parameters for Langmuir, Freundlich, and D-R models of CPF-T.


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Model

Langmuir

Freundlich

Parameter

Value

qm( mmol g-1)

0.591

qL (mmol g-1)

0.594

bL (L mmol-1)

49.529

r2

0.999

nF

5.330

KF [mmol g-1(L mmol-1)1/n]

0.621

r2

0.846

qDR (mmol g-1)

0.600

β (mol2 J-2) DubininRadushkevich r2 EDR (kJ mol-1)

7E-09 0.899 8.451

The theoretical maximum sorption amount of Langmuir model and D-R model are 0.594 mmol g-1 and 0.600 mmol g-1, respectively, which are very close to the experimental value (0.591 mmol g-1), indicating that these two models could better describe the adsorption process. On the comparison of these three models, the r2 of Langmuir model is the highest (r2 > 0.99) indicating that the adsorption process is more likely to be monolayer adsorption, and the adsorption sites on the surface of the materials are homogeneous (consistent with the SEM Mapping image of CPFT). The EDR positive value of the D-R model indicates that the adsorption process is endothermic. And the EDR value (8.451 kJ mol-1) is within the range of 8-16 kJ mol-1, indicating that the


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chemical adsorption process is considered to be favorable. The conclusion is also consistent with the dynamic studies. The essential characteristics of Langmuir isotherm can be expressed by separation factor RL from the formula 11:

RL =

1 1+ b Lc0

(11)

The value of RL reflect the shape of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0). As shown in Figure 12, RL values are between 0 and 1 in different initial concentration, indicating that the adsorption can be carried out successfully under current experimental condition.

Figure 12. Separation factor RL of Langmuir model in different initial concentration c0 (pH = 1, V = 10 mL, w = 10 mg, t = 12 h, T = 298 K).


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The results of adsorption kinetics and isotherms suggest that the uranium adsorption onto CPF-T could be a chemisorption process decided by rate-determining step, and the adsorption is supposed to be monolayer. That is to say, the adsorption capabilities of the materials are rising with the increasing amount of the active sites on the surface. 3.2.4. Selective sorption of CPF-T towards uranium

Figure 13. Effect of acidity on the uranium sorption amount and uranium selectivity onto CPF-T (inset) in multi-ion solutions (c0,all ion ≈ 0.45mmol L-1, V = 10 mL, w = 10 mg, t = 12 h, T = 298K). Actually, it is always a hotspot and difficulty in field of separation science to adsorb uranium efficiently and selectively from the spent fuel solution containing various metal ions. In this research, the selective separation of uranium by CPF-T was discussed in simulated nuclear industry effluent which contains 11 competitive metal cations, including representative alkaline earth metal ions, transition metal ions and five typical lanthanide nuclides (La3+, Sm3+, Nd3+,


25


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Gd3+, and Ce3+) besides UO22+ under different acidities (Figure 13). These competitive ions usually have strong coordination abilities with the functional groups on the sorbents. Over a wide acidity range from pH 4 to 3 M HNO3, although the sorption amounts of other competitive ions reduced significantly, the change of uranium sorption amounts (Table S2) was only within 30% where the maximum was 0.26 mmol L-1 at pH 4 and the minimum was 0.20 mmol L-1 at 3 M HNO3, which resulted in a sharp increase of the uranium selectivity SU. In particular, the sorption amount was still up to 47.6 mg g-1 (0.20 mmol g-1) in acidity condition of 3 M HNO3 which is the common acidity of typical spent fuel, and the selectivity SU was 72% at this point. With the increase of solution acidity, the N-, O- functional groups at the edge of the material framework would be more protonated. Therefore, the binding capacity of CPF-T for other ions except uranyl would be reduced, and the corresponding adsorption amount would also be decreased, in which the adsorption of divalent main group and transition metal ions were more influenced by acidity than that of trivalent lanthanide ions. The results show that CPF-T has obvious advantages in sorption of uranium in high acidity condition over most reported SPE materials which lost the uranium adsorption abilities when pH < 2, especially in the selective adsorption aspect. 3.2.5. Possible mechanism of uranium sorption onto CPF-T


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Figure 14. The XPS spectra of CPF-T before and after sorption of uranium: the typical XPS survey spectrum (a) and O 1s (c), N 1s (d) and U 4f (b) high-resolution spectra. TGA curves of CPF-T before and after adsorption of uranium at pH 1 (e). XPS was used to characterize the changes of bonding environment and the surface chemical composition of CPF-T in detail before and after the adsorption at pH 1 (Figure 14). A new strong U 4f peak is observed distinctly after sorption in XPS spectra (Figure 14a and 14b), revealing the definite adsorption of uranium onto CPF-T. Comparing the electron binding energy of each element, there are no shifts for other elements except O and N. The high-resolution O1s spectra in Figure 14c show two chemical species of oxygen in CPF-T. The peak at around 531.3 eV can be ascribed to the synthetic P-O bond and the species at 533.2 eV can be attributed to the unreacted -OH. The electron binding energies of these two species increase by 0.2 and 0.37 eV respectively after the sorption of uranium. From the high-resolution N 1s spectra (Figure 14d), it can be found that there are three types of nitrogen species in CPF-T. The peak at 397.96 eV can be attributed to P=N-P, while peaks at 399.26 and 401.74 eV belong to the P-NH-P and P-NH2 formed by phosphazene isomerization, respectively. And the electron binding energy of all the


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three species of nitrogen increase about 0.3 eV after the sorption of uranium. To further verify the above fact, we also tested the XPS spectra of CPF-T after sorption in 3 M HNO3. The result is almost the same as that described above, that is, the electron binding energies of N and O increase distinctly (Figure S10). Therefore, it can be concluded from XPS analysis that CPF-T has certain interaction with uranyl ion even in high acidic condition. According to the strategy and research target of this study, CPF-D and CPF-T were designed to have similar structures and elements, and their N and O atoms can effectively coordinate with metal ion, which are embodied as the adsorption behavior of CPF-D and CPF-T for uranium at usually low acidity. But when the acidity increases continually, functional groups on the materials will lose their coordination abilities due to the continually strengthening protonation effect, resulting in the loss of uranium adsorption abilities of the materials ultimately. However, in this study we were surprised to find that the CPF-T exhibited relatively stable and prominent capability for selective adsorption of uranium over a wide acidity range (pH 4 to 3 M HNO3). Especially in 3 M HNO3, the sorption amount of CPF-T was still up to 47.6 mg g-1 with the uranium selectivity of 72%, while CPF-D almost lost its total adsorption ability for uranium under this condition (Figure 9). Comparing the selectivity SU of CPF-D and CPF-T in the competitive system (Figure S11), it was found that the SU of CPF-D gradually declined while the SU of CPF-T increased extraordinarily with the increasing acidity. To our knowledge, most of SPE materials generally will lose their adsorption abilities for uranium gradually along with the increasing acidity of solution because of the protonation of coordination atoms. In order to clarify this unusual phenomenon, the existing form of the target uranyl ion and the geometrical structure and pore size of the SPE materials were taken into consideration.


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Through lots of investigations on the adsorbate uranyl ion, it was found that uranyl is dominantly coordinated with five water molecules to form hydrate, [UO2(H2O)52+], in aqueous solution35-42. To verify the existing form of uranium on CPF-T, we calculated the weight increment ∆wH(%) of the coordinated water molecules after sorption from the following formula:

Δ =

×

+ [ ] ×

× 100% 12

Where m is the weight of CPF-T before adsorption and it can be set to 1 g when calculated. [ ] is the weight of hydrated uranyl ion ([UO2(H2O)5]2+) and is the weight of

water molecules coordinated with uranyl ion in CPF-T which can be obtained from formula 13 and 14:

[ ] =

=

× [ ]! 13 "

× 5 × 14 "

Where qe is uranium sorption amount (mg g-1) and it is measured to be 85.5 mg g-1 at pH 1 (Figure 9). Mr is molecular weight and Ar is atomic weight. The result shows the theoretical value of ∆wH is 2.9% if the adsorbed uranium on CPF-T is [UO 2(H2O)5]

2+

. To calculate the experimental value of the coordinated water increment ∆wH, the

CPF-T sample after sorption at pH 1 was washed thoroughly by distilled water and the TG analysis was performed after drying at 100°C for twenty-four hours. Comparing the TGA curves of CPF-T before and after uranium adsorption from 100 to 200°C of which the weight loss should be attributed to coordinated water instead of hygroscopic moisture, the experimental


29


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value of ∆wH is 2.7%, much similar with the theoretical value, which suggests the possible existing form of uranium in the material is [UO2(H2O)5]2+.

Scheme 2. Possible sorption mechanism of CPF-T (the bonding mode of hydrogen bonds is simulated using the Materials Studio software). On the other hand, the angle between adjacent hydroxyl groups on phloroglucinol monomer used for the synthesis of CPF-T (120°) is smaller than that on hydroquinone monomer used for the synthesis of CPF-D (180°), which provides CPF-T with more compact structural skeleton with pore size of 1.26 nm, smaller than that of CPF-D (1.82 nm). Since the theoretical pore of CPF-T is 0.95 nm, the diameter of CPF-T is considered to be 0.95-1.26 nm. Compared with the diameter of the hydrated uranyl ion (0.59-0.66 nm),37-42 the pore size of CPF-T is supposed to be more suitable for matching. The position of hydrated uranyl ions [UO2(H2O)5]2+ in CPF-T framework were simulated in Scheme 2 and it can be found that the pore diameter of CPF-T matches well with that of the hydrated uranyl ion. Therefore, we speculate that the coordinated water molecules in the hydrated uranyl ion may mediate the binding effect of uranyl ion with CPF-T through a dense and stable hydrogen bond network with N and O atoms (hydrogen bonding


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acceptors) in the frameworks,43 which is also reflected in the increased thermal stability of CPFT (Figure S12). While, CPF-D does not own this effect due to its larger pore size, which leads to the loss of its adsorption ability under strong acidity. This size-matching effect based on the geometrical environment with honeycomb-like structure would effectively weaken the protonation of functional groups, and actually enhance the selective separation ability of CPF-T towards uranium. 4. Conclusion In the current work, we designed and constructed two microporous covalent phosphazene-based frameworks with uranyl ion-sieving effect by linking phosphazene rings and phenyl rings via PO bond. Experimental results show that both as-synthesized CPFs have excellent acid and radiation stability and relatively good thermostability. Especially, under different acidic conditions, the change of uranium sorption amounts in the acid range of pH 4~3 M HNO3 is only within 30%, very different from most existing uranium adsorbents reported so far. CPF-T displays relatively stable and prominent capability for selective separation of uranium over a wide acidity range (pH 4 to 3 M HNO3), which breaks through the limitation of the conventional SPE materials in their applications to uranium separation from highly acidic environments. Table 4. Comparison of adsorption amounts for various uranium sorbents in different acidity* qmax (mg g-1) Sorbent

ref pH ≥ 4

pH 1

3 M HNO3

BImPhP(O)(OH)/SiO2NH2

176 (pH 7)

-

-

44

MIL-101(Cr)

200 (pH 4.5)

-

-

45

FJSM-SnS

338.43 (pH 4)

-

-

46


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Agarose-Chitosan

435 (pH 5.5)

47

PAA

320 (pH 4)

20

-

48

ImP(O)(OH)2/SiO2

618 (pH 4)

-

-

25

MPCOF

214 (pH 4.5)

98

-

29

ND-AO

212 (pH 4.5)

-

-

32

COF-HBI

211 (pH 4.5)

2

-

49

CPF-T

180.3 ± 1.2 (pH 4)

140.4 ± 0.5

55.2 ± 1.0

This work

* in pure uranium solution The uranium sorption capabilities of some representative SPE materials25, 29, 32, 44-49 have been compared with that of the CPF-T by tabulating the adsorption amounts of the materials at different acidic condition in pure uranium solution and the results of the comparison are shown in Table 4. The results show that the performance of CPF-T in pH 1, even in 3 M HNO3 solution, is far superior to that of other SPE materials which rely normally on coordination interaction of functional groups on them. CPF-T is designed to have more compact structural skeleton with pore size of 0.95-1.26 nm. Compared with that of CPF-D, the smaller pore of the hexagonal rings in CPF-T matches well with the diameter of hydrated uranyl ion (0.59-0.66 nm), and meanwhile, might provide a geometrical environment more suitable for the water molecules on uranyl hydrate to form hydrogen bond with the O and N donor atoms in the hexagonal pores of CPF-T. In short, by applying the above strategy, we obtained a new SPE material (CPF-T) with high physical and chemical stability, especially acid stability, and moreover the selective separation ability for uranium was significantly maintained under all the acidic conditions employed in this work, particularly in highly acidic environments, attributed to the nanoscale ion-sieving effect


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based on the geometrical environment of the functional hexagonal pore in the CPF-T frameworks. The results from batch sorption experiments show that as a new SPE material, CPFT has superior comprehensive performance quite distinctively different from the conventional solid phase extractants previously reported. Furthermore, we employed one pot process to integrate the covalent linkage of the selected monomers and additional functionalization into a whole for the synthesis of the CPF materials instead of the traditional methods by using grafting modification. This generalized synthesis route has many advantages such as simple procedure, facile operation, mild conditions, timesaving, low-cost and so on. Meanwhile, the strategy also provides an alternative approach to design and synthesize new SPE materials with high physicochemical stability and high separation efficiency for other valuable metal ions. ASSOCIATED CONTENT Supporting Information The sorption amounts of CPF-T before and after soaking in acid, the sorption amounts and selectivities of uranium onto CPF-T in different acidity in multi-ion solutions.13C-NMR spectra of hydroquinone and phloroglucinol, SEM Mapping of CPF-D and CPF-T, Schematic representation of two kinds of pores in the structures of CPF-D and CPF-T, Simulated N2 adsorption isotherms (77 K) of CPF-D and CPF-T by Materials Studio software, Gran plots of CPF-D, CPF-T and blank, the species distribution of uranium under different pH conditions, kinetic models for the uranium sorption onto CPF-D and CPF-T, isotherm models for the uranium sorption onto CPF-T, XPS spectra of CPF-T after sorption in 3 M HNO3, effect of pH on the uranium selectivity of CPF-D and CPF-T, TGA curves of CPF-T before and after sorption of uranium at pH 1, potentiometric titration of CPF-D and CPF-T and the Gran function (PDF) AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected] (S.L.). * E-mail: [email protected]. (L.M.). Tel.: +86-28-85412329. Fax: +86-28-85412907 ORCID Shoujian Li: 0000-0002-3311-8131 Lijian Ma: 0000-0002-6317-6287 Author Contributions The manuscript was written by the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support from the Science Challenge Project TZ2016004 and the National Natural Science Foundation of China (Grants 21771128, 21671140,11575122 and 11475120). We would like to thank the Analytical & Testing Center of Sichuan University for structured illumination microscopy work. REFERENCES (1) Murray, R.; Holbert, K. E. Nuclear Energy: An Introduction to the Concepts, Systems, and Applications of Nuclear Processes. Elsevier, 2014. (2) Smith, S. M. National Geochemical Database Reformatted Data from the National Uranium


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Synthesis and Application as a Selective Solid-Phase Extractant for Separation of Uranium. J. Colloid Interface Sci. 2015, 437, 211–218.


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Synthesis of microporous covalent phosphazene-based frameworks

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