Stable Covalent Organic Frameworks as Efficient Adsorbents for High

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Stable Covalent Organic Frameworks as Efficient Adsorbents for High and Selective Removal of Aryl-Organophosphorus Flame Retardant from Water Wei Wang, Shubo Deng, Lu Ren, Danyang Li, Wenjing Wang, Mohammadtaghi Vakili, Bin Wang, Jun Huang, Yujue Wang, and Gang Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06229 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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ACS Applied Materials & Interfaces

Graphical abstract

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

Stable Covalent Organic Frameworks as Efficient Adsorbents for High and Selective Removal of Aryl-Organophosphorus Flame Retardant from Water Wei Wanga,b, Shubo Deng a,*, Lu Ren a, Danyang Li a, Wenjing Wang a, Mohammadtaghi Vakili a, Bin Wang a, Jun Huang a, Yujue Wang a, Gang Yu a

a

School of Environment, Beijing Key Laboratory for Emerging Organic Contaminants Control,

State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC), Tsinghua University, Beijing 100084, China b

State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xi’ning, Qinghai

Province 810016, China

1


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Abstract A critical challenge in environmental remediation is the design of adsorbents with proper pore size for the removal of organic pollutants. Three covalent organic frameworks (COFs) with different pore sizes were successfully prepared by a room temperature solution-suspension method, and used to remove the typical aryl-organophosphorus flame retardant (triphenyl phosphate (TPhP)) from aqueous solution. The prepared COFs showed strong acid resistance and thermal stability. The 1,3,5-triformylphloroglucinol (TFP) reacted with benzidine (BD) (COF2) exhibited the highest sorption capacity for TPhP, followed by the reaction of TFP and 4,4''-Diamino-p-terphenyl (DT) (COF3), and the reaction of TFP and p-phenylenediamine (PDA) (COF1). Their adsorption equilibriums were achieved within 12 h, and COFs with larger pore size have higher initial sorption rate but longer time to reach sorption equilibrium. According to the Langmuir fitting, the maximum sorption capacities of three COFs for TPhP were 86.1, 387.2 and 371.2 mg/g, respectively. The density functional theory (DFT) calculation verified that COF1 with small pore size prevent TPhP molecules from entering the pores resulting in extremely low sorption capacity, while relatively too large pore size (COF3) will decrease the sorption energy, which is also not conducive to the adsorption of TPhP. Moreover, the prepared COFs can selectively adsorb TPhP in the presence of coexisting compounds and had high removal of TPhP from actual municipal wastewater, showing promising application potential for selective removal of micro-pollutants from water by precisely controlling the COFs structure.

Keywords: Covalent organic frameworks; triphenyl phosphate; organophosphorus flame retardant; adsorption mechanism; selective adsorption 2



1. Introduction Adsorption has been proved to be effective method for the removal of micropollutants from water and wastewater.1 Traditional porous adsorbents such as activated carbon and resin usually have limited adsorption capacity for targeted compounds in real wastewater due to the competitive adsorption and slow pore diffusion.2 Covalent organic frameworks (COFs) have emerged as porous crystalline materials with inherent porosity and periodic molecular ordering structure,3 which are mainly composed of light elements (C, H, O and N) by covalent bonding. Compared with their similar materials like metal organic frameworks (MOFs), COFs overcome the unsatisfactory stability of most MOFs in water due to their instability coordination bonds between the metal ions and organic ligands.4 Moreover, COFs exhibit great thermal stability, high internal surface area as well as the elaborate control of geometry,5 showing great potential in various applications such as gas sorption and separation, catalysis, energy storage, etc.6-7 Up to now, few papers about the application of COFs in environmental issues have been published. For example, COFs show high removal efficiency for mercury from aqueous solution.7-8 Therefore, it is attractive to prepare novel COFs with proper structure for the effective removal of targeted pollutants from water. Synthetic methods of COFs have been developed under solvothermal conditions,9 such as microwave heating,10 ionothermal,11 and sonochemical synthesis.12 Harsh conditions such as high temperature/pressure and long reaction time are necessary in traditional reactions. Besides, the crystallinity and porosity of COFs prepared by those synthetic methods are unstable,3 and most of these methods need to follow complex processes. Recently, the room temperature batch approach 3


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has been developed and proven as a facile synthesis for COFs, with simple device and operation under ambient conditions.3,13 It has been proven that 2D COFs prepared by room temperature batch synthesis exhibited good porosity and crystallinity.3 Considering these facts on synthesis, it is therefore practicable to utilize room temperature batch synthesis to prepare COFs for high removal of organic contaminants. Organophosphate flame retardants (OPFRs) as organic compounds are added into manufactured materials, such as varnishes, plastics, electronics equipment, wood, and glues,14-15 aiding in prevention or slow ignition. Since polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) are restricted in Europe and the United States,15-16 as their substitute, OPFRs production and utilization have greatly increased. The worldwide consumption of OPFRs has increased from 16% in 2013 to approximately 18% in 2016.17 Triphenyl phosphate (TPhP) as a typical OPFRs is widely used as a flame retardant and plasticizer in variety of products.18 TPhP is proved to be bioaccumulative, toxic to aquatic ecosystems, also related to developmental toxicity and neurotoxicity,19-20 which is stable in water with the longest half-life in OPFRs, about 5.5 year under neutral conditions.21 TPhP has been found in wastewater, oceans, river water, groundwater and drinking water all over the world.21-22 It is reported that the maximum level of TPhP in surface water was 14000 ng/L in influent samples in Norway in 2007.19 Therefore, it is necessary to remove them before being discharged into aquatic environments. The objectives of this study are to develop efficient COFs as promising adsorbents for the removal of TPhP as a typical OPFR, and to elucidate the adsorption behavior and mechanism. The COFs were synthesized by room temperature solution-suspension approach, and the material 4



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pore size was controlled to achieve the selective adsorption of TPhP in aqueous solution. The three COFs were characterized to clarify the relationship between textual structure and TPhP adsorption. Additionally, the potential adsorption mechanism was studied through DFT calculations. Finally, the adsorbent regeneration and the efficient removal of TPhP from real wastewater by three COFs were investigated. 2. Materials and methods 2.1. Chemicals and materials Triphenyl phosphate (TPhP), acetic acid, anhydrous tetrahydrofuran and anhydrous ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Dioxane, p-phenylenediamine (PDA), benzidine (BD) and 4,4''-Diamino-p-terphenyl (DT) were purchased from

Shanghai

Aladdin

Biochemical

Technology

Co.,

Ltd.

(Shanghai,

China).

1,3,5-Triformylphloroglucinol (TFP) was obtained from Zhengzhou Anmschem Co., Ltd. (Zhengzhou, China). Other chemicals are analytical reagent or HPLC grade. 2.2. COFs preparation COF-1, COF-2 and COF3 were synthesized via the Schiff-base reactions and the irreversible enolto-keto tautomerization3,

23

between 1,3,5-triformylphloroglucinol (TFP) and linkers

(p-phenylenediamine (PDA), benzidine (BD) or 4,4''-Diamino-p-terphenyl (DT)). Firstly, 63 mg of 1,3,5-triformylphloroglucinol (TFP) was dispersed into 1.5 mL dioxane in an 8-mL glass vial, followed by sonicating for 8 min to get a homogenous suspension. Secondly, p-phenylenediamine (PDA) was dispersed into 1 mL dioxane solution and added into the suspension prepared before, and the mixture was sonicated for 1 min to get fully contact. Thirdly, 0.1 mL acetic acid was slowly added, and the vial was sealed for 3 days at 25 oC without stirring. The red powder (COF1) 5


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was obtained by centrifugation, and washed with N,N-dimethyllformamide (DMF) and ethanol to remove unreacted TFP and PDA, and finally dried under vacuum at 80 °C for 48 h. To prepare COFs with large pore size, COF2 and COF3 were synthesized via the above-mentioned method but changing PDA to BD and DT, respectively, and the obtained COF2 and COF3 were orange powder and yellow powder, respectively. 2.3. Characterization of COFs The thermal properties of the COFs were obtained by a thermogravimeter (Mettler Toledo, TGA/DSC 1 Stare, Switzerland) and all the measurements were carried out under an oxidative (air) atmosphere and protective gas (N2) at a heating rate of 10 °C/min over a temperature range of 25-800 °C. The specific surface areas of the COFs were tested by nitrogen adsorption at 77 K using a gas adsorption instrument (Autosorb iQ, Quantachrome Corp., US). All COFs samples were degassed at 120 °C for 6 h in vacuum before determination. The elemental compositions of the prepared materials were measured by an elemental analyzer (EA3000, Italy). An X-ray diffractometer (XRD, Rigaku S2, Japan) was used to measure the XRD patterns. A Thermo Nicolet NEXUS Fourier transform infrared (FTIR) spectrometer at 25 oC was used to test the FTIR spectra. The surface morphologies of the COFs were observed by a scanning electron microscopy (SEM, JSM-6460LV, JEOL, Japan). 2.4. Adsorption experiments Adsorption experiments were conducted with 2.0 mg/L adsorbent in 300 mL flask containing 250 mL of TPhP solution. The mixture solution was shaken in an orbital shaker at 170 rpm and 27 °C for 24 h. The preliminary equilibrium experimental results indicated that the sorption of

6



TPhP on the flask was negligible. Adsorption kinetics were conducted with 2.0 mg/L adsorbent in 500 mL of 1.9 mg/L TPhP solution at pH 7.5. Effect of pH on the sorption of TPhP was studied at pH ranging from 2.0 to 8.0 with the initial TPhP concentration of 1.9 mg/L, and no pH adjustment was conducted during the sorption process. The experiments of sorption isotherm were conducted with initial TPhP concentrations from 0.1 mg/L to ~1.95 mg/L at pH 7.5, and the adsorbent dose was 2.0 mg/L. Five kinds of commercial adsorbents including coal-based activated carbon (AC-1), shell-based activated carbon (AC-2), XAD7hP resin (non-ion exchange resin), IRA96 resin (anion exchange resin) and 732 resins (cation exchange resin) were used to carry out the comparison adsorption experiments with COF2, and 2 mg of different adsorbents was added into 200 mL of 1.9 mg/L TPhP solution. The surface areas of AC-1, AC-2 and XAD7hP were 323, 656 and 450 m2/g, respectively, and the exchange capacity of IRA96 and 732 was 4.7 and 4.5 mmol/g, respectively. To evaluate the selectivity of the prepared COFs, triphenylphosphine oxide (TPPO), carbamazepine (CBZ), diclofenac acid (DF), caffeine (CAF), perfluorobutyric acid (PFBA) and perfluorooctane sulphonate (PFOS) were selected as coexisting compounds. The concentrations of TPhP, TPPO, CBZ, DF, CAF, PFBA and PFOS were all 5 µmol/L in the multi-solute experiments, the same concentration as TPhP in the single-solute experiment. In the adsorption experiment for the application of COFs in real wastewater treatment, the municipal wastewater from Qinghe wastewater treatment plant (Beijing, China) was taken from the influent (pH=7.2; COD=183.67 mg/L; SS=184.9 mg/L), and the suspended matters were removed by filtration with a 0.45 µm membrane. The wastewater was used as the solution for the preparation of TPhP-contained solution, the adsorption experiments were conducted with 0.05 7


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g/L COFs in 1.9 mg/L TPhP solution at 27 °C for 24 h. 2.5. Regeneration and reuse experiments After the sorption experiments were conducted with 0.05 g/L COFs in 1.9 mg/L TPhP solution at 27 °C for 24 h, the COFs were separated by a nylon filter and used in the regeneration experiments. Thereafter, COFs were regenerated by adding 1 mg of adsorbent in 8 mL ethanol and shaking for 12 h at 170 rpm and 27 °C. After the regeneration, the regenerated COFs were used in the next sorption experiment. Four sorption-regeneration cycles were carried out to evaluate the recyclability of three COFs. 2.6. Computational method To explore the interaction mechanism of TPhP with COF1, COF2 or COF3, the plane-wave basis Vienna Ab-initio Simulation Package (VASP, v. 5.3.5) code was used to perform geometry optimization and static total energy calculations.24-25 The density functional theory (DFT) employing projector augmented wave (PAW)26 method with the exchange-correlation functional of generalized gradient approximation GGA-PBE27 was used for all calculations. The kinetic energy cutoff for the plane-wave basis set was set to be 450 eV.28 The vacuum space was set as 15 Å for the three COFs-TPhP sorption systems, which was enough to distinguish the interaction among periodic images. The self-consistent field iterations were regarded converged when the total energy change was less than 10−5 eV. A Monkhorst–Pack k-point mesh of 1 × 1 × 1 was used to perform geometry optimizations. Three 2 × 2 supercells with 144, 216 and 288 carbon atoms for COF1, COF2 and COF3, respectively, were built with the unit cell parameters of a=b= 22.37290 Å for COF1; a=b= 29.81100±0.12050 Å for COF2 and a=b= 37.19465±0.09725 Å for COF3. The sorption energy (Esorption) was calculated as Esorption=ECOFs+ETPhP- ECOFs-TPhP, 8



where ECOFs, ETPhP, ECOFs-TPhP are the total energies of the adsorbent (COF1, COF2 or COF3), adsorbate (TPhP) and combined system (COFs-TPhP), respectively. The positive Esorption value means that the sorption is exothermic and the COFs-TPhP composite system is stable.29 2.7. Analytical methods Before determination, the samples were filtered by a 0.45 µm nylon membrane. In order to eliminate the influence of membrane adsorption, 0.5 mL was collected after 7 mL of the filtrate was discarded. The concentrations of TPhP were determined using a LC-10ADvp HPLC with UV-Vis detector (Shimadzu, Japan) and a TC-C18 column (4.6 mm×250 mm i.d., particle size of 5 µm) from Agilent Technologies (USA), and its maximum absorption wavelength is 204 nm. The mobile phase was 70% methanol (v/v) at a flow rate of 1.0 mL/min. The sample volume injected was 40 µL. The total analysis time of a sample was about 13 min (Figure S1). 3. Results and discussion 3.1. COFs preparation and characterization Briefly, TFP is considered as the knot, and PDA, BD and DT are the linkers for the preparation of COF1, COF2 and COF3 in the presence of acetic acid catalyst. The three COFs were obtained with the isolated yield of over 80%. The formation of COF1, COF2 and COF3 were first confirmed by FTIR spectra (Figure 1). The COF2 exhibited complete disappearance of the N−H stretching band (3374, 3305 and 3203 cm−1), C-H stretching (2896 cm−1) and the substitution benzene ring stretching (709 cm−1) compared with its monomers (Figure S2), and the peaks in these three bands of COF1 and COF3 were also significantly decreased or disappeared. The broadening stretching vibration band of three COFs at 1617 and 1583 cm−1 was assigned to the C=O and C=C bands, repectively,30 and the typical peaks for C−N (1280 cm−1) stretching 9


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vibration of three COFs still remained. These results indicated the imine condensation reaction occurrence and the transformation from amorphous networks to ordered crystallization structure,31 as shown in Figure 2. The morphologies of the COFs were examined by SEM (Figure 3). The structure of COF1 is like fluffy pellets, with a large quantity of nanofibers on their surfaces (Figure 3a). When changing the linker of PDA to BD (COF2), the reformed pellet seemed to be thinned with the peculiar structure similar to rose (Figure 3b). When the linker DT was used to prepare COF3, the thin covalent organic nanosheets were observed (Figure 3c).

COF3

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


C=O (1617)

C=C

C-N(1280)

(1583)

COF2 COF1

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm )

Figure 1. FTIR spectra of COF1, COF2 and COF3

Figure 2. Synthetic scheme of COF1, COF2 and COF3 adsorbents using three different linkers for the regulation of pore sizes

10



(a)

(b)

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(c)

Figure 3. SEM images of COF1 (a), COF2 (b), and COF3 (c) The powder XRD of COF1, COF2 and COF3 showed the most intense peak (2θ) arising from the (100) at 1.22°, 1.18° and 0.71° respectively, together with some relatively weaker diffraction peaks at 6.7° (200), 10.38° (210), 15.08° (220) and 26.68° (001) for COF1, 3.5° (110) and 26.02° (001) for COF2, and 19.74° (001) for COF3 (Figure 4). The (001) facet is due to the π−π stacking, and the corresponding distances between COF layers for COF1, COF2 and COF3 were 3.3 Å, 3.4 Å and 4.5 Å, respectively. The results indicate that the structural ordering tendency of 2D layers to the 3D structure is more obvious for the shorter linker, which matched well with the morphologies observed by SEM. However, the most intense peak (100) shifted much in comparison with the reported references23,32. The previous study suggested that the isoreticulation in COFs caused the larger crystal lattice, resulting in the shifts

33

. In addition, the simplest

method of room-temperature solvent synthesis always creates many defects on the surface of COFs 34, leading to certain shifts of peak positions. The optimized geometrical structures by DFT calculations showed that the pore diameters were about 1.81 nm, 2.57 nm and 3.34 nm for COF1, COF2 and COF3, respectively. Moreover, elemental analysis corroborated well with the theoretical values of infinite 2D layer (Table S1), and the little high content of oxygen element was due to the immobilized water generated during the synthesis process of COFs, which was confirmed by TGA analysis (Figure 5a). N2 sorption was carried out at 77 K to evaluate the 11


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architectural rigidity and permanent porosity of the three COFs (Figure S3). The sorption curve was found to be a typical type I isotherm. The Brunauer−Emmett−Teller (BET) surface area was calculated to be 166.0 m2/g for COF1, 283.6 m2/g for COF2 and 109.5 m2/g for COF3. The pore size distribution of the three COFs is shown in Figure S4. It is notable that the pore size increased from COF1 to COF3, while all the pore sizes of the three COFs were smaller than the theoretical values, indicating that the reaction precursors and solvents still remained in the pores and the possibly incomplete irreversible tautomerism. Moreover, the surface area of COF1 was much lower than that room temperature synthesized 3. The protonation−deprotonation of diamines on the linkers in acidic solution is a reversible reaction

35-36

, which is in favor of sustaining the

reversibility of the entire COFs formation reaction by competing with the imination reaction and slowing down the overall reaction. It is confirmed that the distance between the hydrogen atom on the diamines and the oxygen atom on the acid significantly affected the surface area of COFs, and shorter distance would cause lower surface area 35. In our study, more acetic acid was added into the reaction system compared with previous research

3

. The relatively too high

concentrations of hydrogen ions in the reaction system will break the reversible reaction of diamines on the linkers, and higher protonation of the diamines will short the distance between the diamines and the acid, resulting in lower surface area. In addition, there is one more tedious steps in the sorbent activation process by multiple solvent exchange (methanol) 3, and this step will fully replace the reaction precursors and solvents remained in the pores of the COFs.

12



Figure 4. XRD patterns and geometric structures of COF1 (a), COF2 (b) and COF3 (c) 3.2. Stability of COFs To evaluate the thermal property of the COFs, thermogravimetric analysis (TGA) was used to investigate their thermal stability. All COFs exhibited high thermal stability up to 350 °C (Figure 5a), and the gradual weight losses were above 360 °C, indicating the decomposition and combustion of the frameworks. The stability of prepared adsorbents in acidic solution is also important for real application. The acid resistance stability of the three COFs was confirmed by the retention of the peak positions in XRD curves after the treatment in 8 M HCl for 6 days (Figure 5b). The high stability of the COFs under these harsh conditions is due to the irreversible phenomenon of the enol-to-keto tautomerism, which forms only C−N bond.37

13


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(b) COF1 (Pristine) COF1 in 8M HCl

Intensity (a.u.)

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COF2 (Pristine) COF2 in 8M HCl COF3 (Pristine) COF3 in 8M HCl

0

5

10 15 20 2θ (Degree)

25

30

Figure 5. TGA profiles of COF1, COF2 and COF3 (a) as well as their XRD curves before and after HCl treatment (b) 3.3. Adsorption of TPhP on three COFs Figure 6a shows the adsorption kinetics of TPhP on three COFs, and their sorption equilibriums were achieved after 12 h. The pseudo-second-order model was used to fit the kinetic data (Figure 6a), which can be fitted well according to the correlation coefficient (R2) (Table S2). The initial sorption rate (v0) of TPhP on COFs followed the order of COF1

Stable Covalent Organic Frameworks as Efficient Adsorbents for High

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