“Stereoscopic” 2D super-microporous phosphazene-based covalent organic framework: Design, synthesis and selective sorption towards uranium at high acidic condition

Accepted Manuscript Title: “Stereoscopic” 2D super-microporous phosphazene-based covalent organic framework: design, synthesis and selective sorption towards uranium at high acidic condition Author: Shuang Zhang Xiaosheng Zhao Bo Li Chiyao Bai Yang Li Lei Wang Rui Wen Meicheng Zhang Lijian Ma Shoujian Li PII: DOI: Reference:

S0304-3894(16)30372-7 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.04.031 HAZMAT 17639

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

5-12-2015 19-3-2016 13-4-2016

Please cite this article as: Shuang Zhang, Xiaosheng Zhao, Bo Li, Chiyao Bai, Yang Li, Lei Wang, Rui Wen, Meicheng Zhang, Lijian Ma, Shoujian Li, “Stereoscopic” 2D super-microporous phosphazene-based covalent organic framework: design, synthesis and selective sorption towards uranium at high acidic condition, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.04.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

“Stereoscopic”

2D

super-microporous

phosphazene-based

covalent organic framework: design, synthesis and selective sorption towards uranium at high acidic condition Shuang Zhang, Xiaosheng Zhao, Bo Li, Chiyao Bai, Yang Li, Lei Wang, Rui Wen, Meicheng Zhang, Lijian Ma,* Shoujian Li* College of Chemistry, Sichuan University, Key Laboratory of Radiation Physics & Technology, Ministry of Education, Chengdu, 610064, P. R. China. * Corresponding author at: College of Chemistry, Sichuan University, № 29 Wangjiang Road, Chengdu, 610064, P. R. China Tel.: +86 28 85412329 Fax: +86 28 85412907 E-mail address: [email protected] (S. Li)

1  

Graphical Abstract

 

Highlights： 

Phosphorus element was first introduced into covalent organic frameworks (COFs).



Monomer in C3-like spatial configuration was first used to construct COF materials.



A new 2D super-microporous phosphazene-based sorbent (MPCOF) was synthesized.



Separation of U (VI) by MPCOF at high acidic media (up to 1M HNO3) was achieved.



Selectivity for U (VI) separation from multi-ion solution can reach unreported 92%.

 

2  

Abstract So far, only five primary elements (C, H, O, N and B) and two types of spatial configuration (C2-C4, C6 and Td) are reported to build the monomers for synthesis of covalent organic frameworks (COFs), which have partially limited the route selection for accessing COFs with new topological structure and novel properties. Here, we reported the design and synthesis of a new “stereoscopic” 2D super-microporous phosphazene-based

covalent

organic

framework

(MPCOF)

by

using

hexachorocyclotriphosphazene (a P-containing monomer in a C3-like spatial configuration) and p-phenylenediamine (a linker). The as-synthesized MPCOF shows high crystallinity, relatively high heat and acid stability and distinctive supermicroporous structure with narrow pore-size distributions ranging from 1.0–2.1 nm. The results of batch sorption experiments with a multi-ion solution containing 12 coexisting cations show that in the pH range of 1–2.5, MPCOF exhibits excellent separation efficiency for uranium with adsorption capacity more than 71 mg/g and selectivity up to record-breaking 92%, and furthermore, an unreported sorption capacity (> 50 mg/g) and selectivity (> 60%) were obtained under strong acidic condition (1 M HNO3). Studies on sorption mechanism indicate that the uranium separation by MPCOF in acidic solution is realized mainly through both intra-particle diffusion and size-sieving effect.

Keywords Hexachlorocyclotriphosphazene;

Solvethermal 3

 

synthesis;

Covalent

organic

frameworks; Uranium; Separation    

1.

Introduction Covalent organic frameworks (COFs) are a class of novel multidimensional

functional materials, which are completely composed of light-weight elements and formed by building blocks covalent-linked via coupling reaction such as boronic acidbased coupling [1], Schiff-base reaction [2], trimerization of nitriles [3] and the carboncarbon covalent bond formation [4], etc. It has attracted wide concern and exhibited great potential for application in catalysis [5], gas storage (such as CO2 capture) [6] and optoelectronic devices [7]. The key of the design of COFs is to select suitable building block and appropriate coupling reaction, from which we can construct the topological structure with desired morphology, porous features and reactive properties [8]. Hereinto, spatial configuration of the building block has an important influence on the topological structure of COFs. The spatial configurations used nowadays for COFs has been reported mostly limited at two classes: 1) the C2-C4 and C6 symmetrical monomers used to build 2D COFs, such as 2,6-naphthalene dicarbonitrile [9], 1,3,5triethynylbenzene [10], porphyrins [11], hexaphenylbenzene [12], etc.; 2) the Td symmetrical monomer for 3D COFs, such as tetra-substitution products of methane [13], silicane [14] and adamantine [15, 16]. The finite change of spatial configuration

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for building blocks restricts the structure diversity and range of application of COFs [12, 17]. Furthermore, the component elements of building blocks are mainly limited to C, H, O, N and B at present [17-19]. Other elements, like P and S atoms with the bigger atomic radius and weaker electronegativity, have not yet been used in the mainstream of building blocks because it might be comparatively difficult to form the “stable and structurally symmetrical” building monomers for the construction of COFs. Nevertheless, comparing with the analogous N and O, P and S have more abundant change of valence states which could drive more structural diversity of building blocks with more probable spatial configuration. Moreover, the bonding nature of phosphorus or sulfur could be more favorable to satisfying the reversible reaction principle for the design and synthesis of COFs [17]. Based on this, we try to introduce P-containing building monomers combining with different selectable reaction to construct new COFs with various structure and performance for the purpose of significantly expanding the range of research and application of COFs. Actually, the “stable and structural symmetrical building blocks” can also be found in P-containing compounds. For example, phosphazene [(PNCl2)n] possesses outstanding structural stability and reactivity, and commonly is used in preparation of polymer [20, 21], biological materials [22-24] and optoelectronic materials [25]. In general, there are two major types of Phosphazene: cyclophosphazene and linear phosphazene [26]. Among them, trimeric hexachlorocyclotriphosphazene (HCCP) [27, 28] possesses benzene-like rigid, symmetrical structure and strong chemical stability 5  

owing to its stable six-membered-ring system. By using HCCP as a building block, we can choose not only C3-like spatial configuration monomer to synthesis 2D COFs, but also D3h spatial configuration monomer for construction of 3D COFs. And also the facile substitution of P-Cl bonds with amine and phenolic monomer allows ready preparation of functional materials [29-32]. This offers a chemical basis for construction of COFs from HCCP. On the other hand, with the increase of energy consumption and environment requirement, nuclear energy, as a safe, reliable, clean and sustainable energy, receives widespread concern in the world. A great quantity of actinide elements and fission products are generated in the spent fuel by nuclear fission. So the separation and recovery of key nuclide uranium from various nuclear industry effluents has attracted increasing attention in recent years. However, the application of many excellent extractants, especially solid phase extractants, in this field was limited to a large extent by the normal acid environment (1-3 M HNO3) [33] of the nuclear industry effluents, which has so far remained the bottleneck of the efficient separation and recovery of uranium. Generally speaking, the extraction of uranium by existing solid phase extraction (SPE) materials from multi-cation system in different acidic conditions could be classified into three types: (1) in mild acidic condition (pH 2.5-5.0 or over), some have certain capability for separation of uranium, and some even behave excellently [34-40]; (2) in higher acidic condition (pH 1-2.5), for most of the SPE sorbents, the uranium separation capacity could be brought down, and many of them could even begin to break down or degrade [41-43]; (3) in the strong acidic condition (1-3 M 6  

HNO3), an overwhelming majority would be decomposed, and even if a few stable ones do survive, the high acidity would totally disabled them from capturing uranium and other co-existing cation species because of protonation of ligands and/or other effects. Up to now, only a few materials have been reported to have the ability of adsorbing uranium from acidic pure uranium solution [44-47]. Therefore, it is meaningful to develop new SPE materials for efficiently selective separation of uranium from high acidic media. Above-mentioned benzene-like phosphazene ring provides a preferred selection for the researchers in this area. In this work, in order to realize selective separation of uranium under acidic environment, we proposed a new approach to synthesize a phosphazene-based COF material by utilizing P-containing monomer HCCP in C3-like spatial configuration as the node. HCCP and р-phenylenediamine (PDA) were employed as starting materials, and a new “stereoscopic” 2D COF material was synthesized via solvothermal reaction in tetrahydrofuran (THF). The new COF material was expected to possess supermicroporous structure and simultaneously almost no active coordinating sites or functional groups for the desired application mentioned above, especially in acidic media. The components and structural properties of the products were characterized in detail. Batch sorption experiments were conducted to assess the performance of the product on selective separation of uranium in various conditions, and the related adsorption mechanism was also explored.

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2.

Experimental section

2.1. Regents p-Phenylenediamine (PDA) and all metal oxides and nitrates used in this research were purchased from Aladdin Chemistry Co., Ltd. (China). The synthesis of hexachlorocyclotriphosphazene (HCCP) was preformed according to a literature method [48]. Tetrahydrofuran (THF), ethanol, acetone, NaOH and HNO3 were purchased from Chengdu Forest Science & Technology Development Co., Ltd. All chemicals used were of analytical grade or better and were used without further purification in addition to tetrahydrofuran with dehydration in advance.

2.2. Characterizations Fourier transform infrared (FT-IR) spectra were obtained from a Perkin Elmer IR843 spectrometer (USA). The powder X-ray diffraction (XRD) patterns of products were obtained using a DX-1000 X-ray diffractometer (Dandong, China). The morphology analysis was examined by scanning electron microscopy (SEM, FEI Company, Oregon, USA) and transmission electron microscopy (TEM, FEI Company, Tecnai G2 F20, USA). Thermal stability was inspected using a thermogravimetric analysis (TGA, SDT Q600, USA). The specific surface area and pore structure information were measured by using a nitrogen adsorption-desorption analysis (Micromeritics 3FLEX, USA). Elemental chemical states on the surface of materials were measured by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI, USA). Inductively coupled plasma atomic emission spectroscopy (ICP-AES, 8  

Optima8000, Perkin Elmer, USA) was used to analyze the initial and equilibrium concentration of U (VI) and other metal ions in solution.

2.3. Synthesis of super-microporous phosphazene-based covalent organic framework (MPCOF) The synthesis procedure is illustrated in Scheme 1. Typically, HCCP (0.5 mmol) and PDA (1.5 mmol) were dissolved in dehydration THF (15 mL) to form light purple suspension. Then the suspension was transferred to a 25 mL Teflon-lined stainless steel autoclave which was then sealed and heated in an oven. The reaction temperature reached 393 K in 1 h and maintained for 24 h. The product was cooled down to room temperature naturally. The resulting purple grey powders were filtered off and washed alternatively with deionized water, ethanol and acetone until the filtrate was colorless and nearly neutral, then 323 K drying in vacuum overnight. The resulting purple grey material was denoted as MPCOF.

2.4. Batch sorption studies 10 mg MPCOF was added into a 50 mL Erlenmeyer flasks along with 25 mL of either pure uranium (VI) solution or multi-ion test solution, a simulated nuclear industrial effluent solution, containing 12 co-existing cations (Table S1) with a designed metal ion concentration at a given pH. Simulated nuclear industrial effluent samples were prepared mainly based on the composition of a typical nuclear power 9  

reactor effluent [49]. More detailed information on the preparation of stock solution can be found in supplementary Section S1.The flasks were shaken for specified time (t, min) at desired temperatures (T, K). Then the sorption system was centrifuged, and the concentrations of metal ions in the supernatant, before and after sorption, were determined by ICP-AES. All glassware was soaked in 10.0 wt% HNO3 solution for 12 h before use to remove any metal impurities which might be sorbed on the walls of glassware. All tests were carried out at least in duplicates. Sorption amount qe (mg/g) was calculated by Equ. (1): qe 

(c0  ce )  V w

(1)

Where c0 and ce are the initial concentration and equilibrium concentration of metal ion (mg/L), respectively. V is the volume of the testing solution (L), and w is the amount of sorbent (g). In order to evaluate the effect of the competing cations in a multi-ion solution on the uranium selectivity of a uranium-sorbing material, a specific term, uraniumselectivity (SU), was introduced to describe the potency and degree of the selectivity of sorbents to uranium [50]:

SU 

qe U  100% qe tol

(2)

Where qe-U is the amount of uranium sorbed (mmol/g) and qe-tol is amount of all cations sorbed (mmol/g) from the multi-ion solution.

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3.

Results and discussions

In this work, the super-microporous phosphazene-based covalent organic framework (MPCOF) was first designed and synthesized by using inorganic benzenelike P, N-containing HCCP and coupling-agents PDA. As shown in Scheme 1, structurally, the possible “stereoscopic” 2D framework of the as-synthesized MPCOF could be constructed based on the basic structural units of hexagonal honeycomb-like host lamellar structure and the six spindle-shaped rings around the flank of each hexagonal honeycomb-like ring. The larger honeycomb-like pores structure, serving as public passage, could allow free flow of metal ions within, into and out of the framework, and the smaller spindle-shaped rings, serving as ion-sieve, might recognize and capture the targeted uranyl ion.

3.1. Characterization 3.1.1. FT-IR and PXRD Fig. 1 shows the FT-IR spectra of MPCOF. The common sorption bands at about 3432 cm-1 and the peak of 1630 cm-1 are assigned to the N─H stretching and bending vibration. The peak at 1515 cm-1 belongs to the benzene-ring. The peaks at 1384 cm-1 and 1263 cm-1 are assigned to the P═N─P stretching. The peak at 1084 cm-1 is assigned to the P─NH─P stretching. 540 cm-1 belongs to strong shaking of P─Cl. Especially, the middle peak of 970 cm-1 belongs to the P─NH─C stretching vibration [31]. The successful designing bonding strategy is proved. 11  

As show in Fig. 2, compared with peaks of raw materials, the sample PXRD spectra shows two sharp peaks at 8.9° and 18.0°. And the diffraction bands of 22°-30° break into three peaks. Two weak peaks appear at 34° and 40°. The PXRD demonstrated the long-range topological structure in MPCOF.

3.1.2. SEM and TEM SEM images of the pristine MPCOF displayed in Fig. S1a. After 30 s ultrasonic dispersion in water, the SEM image of MPCOF shows clearly layered packing structure (Fig. 3 and Fig. S1b-c). And TEM image of MPCOF also shows a clear large sheet layer (Fig. S1d). The possible formation process of microcosmic morphology of MPCOF is described in supplementary Section S2.

3.1.3. N2 adsorption-desorption isotherm N2 adsorption-desorption isotherm was examined and showed in Fig. 4a. The specific surface area of MPCOF is approximately 27.2 m2/g. The total pore volume of MPCOF was calculated to be 0.0771 cm3/g. The pore size distribution between 1.0-2.1 nm for MPCOF can be observed in Fig. 4b, which is among the size distribution range of super-microporous between 1.0-2.0 nm [51]. The measured pores size range accords with the pores size calculated theoretically for honeycomb-like host pores structure (1.8 nm, see Scheme 1).

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3.1.4. TGA-DTG The results of TGA and differential thermogravimetric (DTG) are shown in Fig. 5. The TGA thermograms clearly show four main processes occurred during thermal degradation. The first stage (weight loss ~3%) below 160°C is attributed to desorption of small amounts of water and residual solvent THF. The second stage is from 160°C to 370°C, the weight loss ~11% can be observed, which could be attributed to decomposition of oligomers and the breakdown of the framework because of the bond breaking between P, N-containing six-member ring and amino group. Then the temperature gets slightly higher to a narrow range of 390°C, and a rapid weight loss process can be observed, which could be attributed to deamination of the phenylamino group. As the temperature continues to rise up to 520°C, the aromatic structure begin to carbonization and the slowly lose weight. Finally as the temperature further goes higher, the phosphazene ring would decompose.

3.1.5. Acid stability studies The experimental details are given in supplementary Section S3. The XRD patterns of the soaked MPCOF samples are shown in Fig. 6. For the sample soaked in the HNO3 solution of pH 1.5 after 1 w, the XRD pattern (Fig. 6b) was nearly unchanged compared with that of the unsoaked MPCOF (Fig. 6a). And after 5 h soaking in 1 M HNO3 solution, the XRD peaks of the sample (Fig. 6c) are only slightly weakened, which 13  

suggests that MPCOF can remain relatively stable in the strong acidic condition. The above results provide a foundation for the as-synthesized sorbent to separate uranium from strong acidic environment.

3.1.6. XPS The related information regarding the surface chemical bonding environment of the materials before and after (denoted as MPCOF-U) sorption of uranium is obtained using XPS analysis (Fig. 7). By comparing the survey spectra of MPCOF and MPCOFU in Fig. 7a, along with the high resolution XPS spectrum of U4f for MPCOF-U (Fig. 7b), it is clear that the uranium is indeed adsorbed onto MPCOF. The high resolution spectra of N1s for MPCOF and MPCOF-U (Fig. 7c-d) show that there exist three chemical species of nitrogen in both samples. The first species, appearing at about 397.6 eV, can be assigned to phosphazene-type nitrogen (P═N─P). While the second one, around 399.2 eV, may be attributed to a P─NH─C, the bonded secondary nitrogen of MPCOF. Finally, the third one at about 401.3 eV is reasonably due to P─NH2 group and/or the presence of protonation-type nitrogen (N+-type). Herein the P─NH2 group could be derived from the phosphazene-phosphazane isomerization [52] (Scheme S1a), and N+-type nitrogen might be formed from protonation of secondary amine or phosphazene-ring nitrogen (Scheme S1b) by HCl generated during the synthesis of MPCOF. The electron-donating ability of N atom would be decreased because of protonation, which could lower the probability of chemical interaction between 14  

MPCOF and the metal-ion encountered. It is notable that almost no chemical shift is observed for the binding energy of the three types of nitrogen species before and after uranium sorption except the conspicuous change in peak intensity of protonation-type nitrogen at about 401.3 eV. The results demonstrate that the nitrogen species of MPCOF have not involved in uranium sorption via chemical interaction. Similar case could also reference to phosphorus atom in MPCOF as show in Fig. S2.

3.2. Sorption experiments 3.2.1. Effect of pH The results are shown in Fig. 8. It is observed that the uranium sorption capacity (~0.40-0.46 mmol/g) does not have significant change with pH change from 1.0 to 3.0. But with the increasing of pH from 3.0 to 4.5, the U (VI) sorption capacity increases rapidly. The distinctive sorption behavior is essentially different from conventional chemisorption behavior of ligand-based solid sorption materials. According to the distribution of U (VI) species (Fig. 9) calculated using CHEMSPEC (C++) program [53] under different pH conditions, when pH ≤ 3, almost all U (VI) exists in a single UO22+ species, but when pH > 3, UO22+ ion will begin to hydrolysis, and consequently, the larger dimer {[(UO2)2(OH)]3+ and [(UO2)2(OH)2]2+} and even trimer {[(UO2)3(OH)4]2+, [(UO2)3(OH)5]+ and [(UO2)3(OH)7]-} of UO22+ will appear, and the contents of these multimers will also increase with increasing pH. This might be a real reason for the significant increase in the sorption amount of uranium onto MPCOF after 15  

the pH > 3. The results show that the uranium sorption is predominantly related to the types of uranyl species present at different pH. Moreover, we further investigate the capability of MPCOF for selective sorption of uranium from a multi-ion solution containing 12 nuclides under high acidic condition of 1 M HNO3 (see section 3.2.4.).

3.2.2. Effect of contact time and kinetic studies The effect of contact time of U (VI) sorption by MPCOF is shown in Fig. 10. The sorption amount of U (VI) increased slowly with increasing contact time. The sorption process can be roughly divided into three stages: 0-8 h for a relatively fast sorption, 896 h for a relatively slow sorption and after 96 h for a very slow sorption. It would even take three weeks or more to come close to the sorption equilibrium. Three different kinetic models, namely pseudo-first-order model, pseudo-second-order model and Weber-Morris intra-particle diffusion model [50], were employed to evaluate the mechanism of the rate controlling process (for more details, please see supplementary Section S4). Comparing the three correlation coefficients (R2) in Table S2, it is clear that the intra-particle diffusion model could describe the sorption process better than the other two models. The lower value of kint (0.003 mmol/g·min1/2) indicated that the intraparticle diffusion rate for uranyl ion into MPCOF is very slow, and the lower C value (0.113 mmol/g) declared that the intra-particle diffusion should be the rate-determining step of the sorption process [54]. 16  

3.2.3. Effect of initial uranium concentration and isotherm studies The sorption isotherm of uranium is presented in Fig. 11. The results showed that the sorption amount of uranium onto MPCOF increased with increasing equilibrium uranium concentration. The maximum sorption capacity was found to be 0.52 mmol/g under the current experimental conditions. The sorption data were fitted using the three types of frequently used isotherms, namely Langmuir, Freundlich and Dubinin-radushkevich (D-R) isotherm [55, 56] to further understand the sorption performance of MPCOF toward U (for more details, please see supplementary Section S5). On the comparison of the R2 values given in Table S3, we could be concluded that Langmuir equation represents a better fit to the experimental data. In addition, the calculated value of EDR was 7.278 kJ/mol. The magnitude of EDR is useful for estimating the type of sorption: If this value is between 8-16 kJ/mol, the sorption can be explained by chemical sorption [50]. But the value of EDR found in this study is within the energy range for physical sorption (EDR < 8 kJ/mol) [57], which is in agreement with the results from the above kinetic studies.

3.2.4. Selective sorption of MPCOF towards uranium

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The selective sorption of U (VI) on the as-synthesized MPCOF was investigated by using batch sorption experiments in simulated nuclear industry effluent containing 12 co-existing ions including UO22+ ion at different pH. The results are shown in Fig. 12. For uranium, the selectivity can be observed up to 76% at pH 4.5 with the sorption amount of 169 mg/g (0.71 mmol/g); especially, at high acidic condition (pH 1.5-2.0), the sorption amount can reach more than 95 mg/g (0.40 mmol/g) and the selectivity can reach up to unprecedented 92% or more. Based on above results, we further investigated the performance of MPCOF for separation of U (VI) at strong acidic condition (1 M HNO3). The experiment results in Fig. 13 show that the uranium sorption capacity reaches about 57 mg/g (0.24 mmol/g) and uranium selectivity is up to 64%. According our knowledge, no other uranium solid sorption materials have been reported to reach such an effect so far.

3.2.5. Possible sorption mechanism of uranium onto MPCOF As is well known, chemisorption and/or physisorption are the general principles for solid materials to be applied in the separation of metal ions. Chemisorption is easily influenced by external factors, especially acidity of medium. For example, at higher acidity, the protonation of surface functional groups would generally weaken the sorption ability of the material used. But physisorption could be barely affected by medium acidity. According to the design idea of this study, following factors were taken into account: (1) From the composition point of view, almost no specific functional 18  

groups or active sites on MPCOF could cause effective coordination with metal ions. MPCOF should not possess the condition/ability for chemically sorbing uranium. However, this would benefit selective separation of U (VI) from low pH or high acidity solution system. (2) From the topological structure point of view, the possible “stereoscopic” 2D framework of MCOPF could be constructed based on the basic structural units of honeycomb-like host lamellar structure and the six spindle-shaped rings around the flank of each hexagonal honeycomb-like ring. The 2D framework possesses narrow microporous size distribution in the range of 1.0-2.1 nm. The supermicroporous structure could provide passageways for metal ions getting in and out. Meanwhile, the spindle-shaped rings with much smaller pore size might restrict the movement of hydrated uranyl ion diffused into the framework. On the contrary, other hydrated cations with smaller size than hydrated uranyl ion could go in and out of the framework more freely (see Scheme 2). In fact, for the batch experiments in multi-ion system, the comparison result of sorption amount of the co-existing ions (Table S4) shows that the sorption amounts of the all metal ions except uranium have almost no changes with different pH and contact time, which verifies the above speculation. These suggest that the architectural features of MPCOF provides structure basis for realizing separation of uranyl by the way of physical sorption from acidic environment. From the more experiment data, we found that, when pH ≤ 3.5 with other conditions being equal, the sorption amount of MPCOF towards uranyl ion in pure uranium solution is similar with that in simulated nuclear industrial effluent (Table 1). The fact indicated that other co-existing ions do not cause obvious interference with the 19  

uranium sorption of MPCOF under the experimental conditions. That may be why the selectivity of MPCOF towards uranium can reach up to unprecedented 92% at pH 1.52.0. The sorption behavior of MPCOF is significantly different with that of the reported SPE materials, the sorption capacity of which would drop down sharply with the decline of pH value [58-61]. Furthermore, XPS analysis shows no chemical interaction between uranium and MPCOF. And isotherm studies reveal a lower sorption energy (EDR = 7.278 kJ/mol). These results suggest the sorption of uranium by MPCOF is realized dominantly by physical interaction. Taking N2 adsorption/desorption measurement into account, the lower specific area and total pore volume of the as-synthesized MPCOF lead to a very slow adsorption process. That is only a limited number of uranyl ion and/or other coexistent cations would be allowed to diffuse into MPCOF through the super-micropores at a time, which means a long period of accumulation must be need to reach a higher or saturated sorption capacity. On the other hand, the results of the kinetic studies indicated the limitation to the movement of the hydrated uranyl ion in the framework of the 2D MPCOF mainly ascribes the topological features of the material as mentioned above. Here, intra-particle diffusion might be a major rate-determining step in the slow sorption process. In light of the above discussion, we suggest that in the range of acidity pH 3 to 1.0 M-acid used in the batch sorption experiments, especially in multi-ion solution, the possible mechanism of the selective sorption of U (VI) by MPCOF would be 20  

physisorption, that is, the ion-sieving effect on the co-existing cations with different size and charges caused by super-microporous topological structure and the smaller super-micropore volume of the as-synthesized COF material (Scheme 2), which has partly achieved our design requirement.

4.

Conclusions In this research, a new P-containing monomer HCCP in C3-like spatial

configuration was successfully introduced to design and synthesize a novel “stereoscopic” 2D super-microporous COF material. Meaningfully, almost no active coordinating groups or active sites on MPCOF could cause chemical bonding with uranium and other metal ions. Moreover, MPCOF possesses the typical advantages of COFs: fairly good crystallinity, relatively high heat and acid stability. These features provide a structural basis for MPCOF to selective separation and recovery of uranium by using the size sieving effect of the super-microporous material under acidic condition. The XPS analyses prove the non-chemical interaction between uranium and MPCOF during separation process. The results of batch sorption experiments show that MPCOF not only has higher separation efficiency of uranium with adsorption capacity of 0.71 mmol/g and selectivity of 76% at weak acidic condition (pH 4.5), but also exhibits extraordinary ability to highly selective adsorption of uranium with unprecedented selectivity of more than 90% at relatively high acidic condition (pH ≤ 2.0). Even under strong acidic condition of 1 M HNO3, MPCOF still shows an unreported high practical ability in selective separation of uranium. The performance of MPCOF for selective separation of uranium under acidic environment would be 21  

superior to all the SPE materials reported so far. Compared with majority SPE materials for separating uranium by using the coordination interaction of ligand or function group, the separation mechanism proposed in this work would not be influenced markedly by solution acidity. Thus this material could have its application potential in real nuclear industry effluents with usually acidic environment. Meanwhile, the strategy in this research also provides an alternative approach to the bottleneck problem, ubiquitous poor performance or breakdown under highly acidic conditions, for almost all SPE materials at present. It is worth mentioning that kinetic rate is one of the important issues of attracting extensive attention in practical industrial applications. So it will also be one of the focuses of our next research works to improve the sorption kinetic rate via modifying the structure of the sorbent. In general, the strategy in this study may open up new avenues in designing and synthesizing COFs with new topology, distinctive properties and extensive application prospect.

Acknowledgments The financial support from the National Natural Science Foundation of China (Grants 21171122, 21271132, 11475120, J1103315 and J1210004) is gratefully acknowledged.

Appendix A. supplementary data Supplementary data associated with this article can be found, in the online version. 22  

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Scheme and Figure Captions Fig. 1 FT-IR spectra of MPCOF. Fig. 2 PXRD patterns of MPCOF, HCCP and PDA. Fig. 3 SEM images of the as-synthesized MPCOF with 30 s ultrasonic treatment. Fig. 4 The N2 adsorption-desorption isotherm and pore-size distribution of MPCOF. Fig. 5 The TGA and DTG curve of MPCOF. Fig. 6 XRD of MPCOF before (a) and after (b, pH 1.5 with 1 w; c, 1 M HNO3 with 5 h) soaking in different concentration nitric acid solutions over different time. Fig. 7 The XPS spectra of MCOFP and MPCOF-U. (a) The typical XPS survey spectrum of MPCOF and MPCOF-U; and the high resolution XPS spectra of (b) U4f for MPCOF-U and N1s for (c) MPCOF and (d) MPCOF-U. Fig. 8 Effect of pH on the sorption of U (VI) onto MPCOF. (c0 = 0.45 mmol/L, w = 10 mg, V = 25 mL, t = 1 w, T = 298 K) Fig. 9 Distribution of U (VI) species in aqueous solution with a total concentration of 0.45 mmol/L and pH values ranging from 0 to 6. Calculated by using a CHEMSPEC (C++) program. Fig. 10 The effect of contact time for the sorption of U (VI) onto MPCOF. (c0 = 0.45 mmol/g, w = 10 mg, V = 25 mL, pH = 1.5, T = 298 K)

31  

Fig. 11 The effect of initial concentration for the sorption of U (VI) onto MPCOF. (w = 10 mg, V = 25 mL, pH = 1.5, t = 1 w, T = 298 K) Fig. 12 Effect of pH on the sorption capacity and selectivity (inset) of MPCOF towards U (VI) in simulated nuclear industry effluent. (c0 = 0.45 mmol/L for all cations, w = 10 mg, V = 25 mL, t = 1 w, T = 298 K) Fig. 13 Sorption capacity and selectivity for sorption of each metal ions onto MPCOF in strong acidic multi-ion system. (c0 = 0.45 mmol/L for all cations, w = 10 mg, V = 25 mL, t = 5 h, T = 298 K, 1 M HNO3)

Scheme 1. Schematic illustration of the preparation of MPCOF. Scheme 2. The sorption modes for uranium onto MPCOF.

32  

Fig. 1

33  

Fig. 2

34  

Fig. 3

35  

Fig. 4

36  

Fig. 5

37  

Fig. 6

38  

Fig. 7

39  

Fig. 8

40  

Fig. 9

41  

Fig. 10

42  

Fig. 11

43  

Fig. 12

44  

Fig. 13

 

 

45  

Scheme 1.

46  

Scheme 2.

 

 

47  

Tables Table 1. Comparison of sorption capacity (mmol/g) for U (VI) sorption onto MPCOF in pure uranium and multi-ion system at different pH. (c0 = 0.45 mmol/L for all cations, w = 10 mg, V = 25 mL, t = 1 w, T = 298 K) pH

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Sorption capacity Pure-U

0.41 0.46 0.41 0.43 0.46 0.66 0.76 0.84

Multi-ions 0.30 0.41 0.40 0.38 0.42 0.62 0.63 0.71  

48