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ZIF-8 nanocomposite membranes
Journal Pre-proofs Removal of iodine from aqueous solution by PVDF/ZIF-8 nanocomposite membranes Xing Long, Ya-Shuo Chen, Qian Zheng, Xing-Xiao Xie, Hao Tang, Li-Ping Jiang, Juan-Tao Jiang, Jian-Hua Qiu PII: DOI: Reference:
S1383-5866(19)33893-6 https://doi.org/10.1016/j.seppur.2019.116488 SEPPUR 116488
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
29 August 2019 24 December 2019 24 December 2019
Please cite this article as: X. Long, Y-S. Chen, Q. Zheng, X-X. Xie, H. Tang, L-P. Jiang, J-T. Jiang, J-H. Qiu, Removal of iodine from aqueous solution by PVDF/ZIF-8 nanocomposite membranes, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116488
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Removal of iodine from aqueous solution by PVDF/ZIF-8 nanocomposite membranes Xing Long,† Ya-Shuo Chen,† Qian Zheng, Xing-Xiao Xie, Hao Tang, Li-Ping Jiang, Juan-Tao Jiang*, Jian-Hua Qiu*
School of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guangxi Key Laboratory of Low Carbon Energy Materials, Guilin 541004, P. R. China
ABSTRACT: Metal-organic frameworks (MOFs) are capable in capture and storage of iodine ions with high efficiency, and polymer/MOF nanocomposite membrane has the potential to be used for efficient removal of iodine from radioactive wastewater. In this work, we have reported a PVDF/ZIF-8 nanocomposite adsorption membrane prepared by contra-diffusion method. The combination of surface graft of poly(4-vinylpyridien) brushes and optimal addition of sodium formate into ligand contributes to the formation of a well-intergrown and continuous ZIF-8 layer on the metal ions side surface of PVDF-g-P4VP membrane. In the batch adsorption, the PVDF/ZIF-8 nanocomposite membrane exhibits high affinity for iodine following a pseudo-second-order kinetics and fitting the Freundlich isotherm model with a maximum adsorption capacity of 73.33 mg/g. In the dynamic adsorption, the PVDF/ZIF-8 nanocomposite membrane exhibits a high flux of 66.19 L/(m2•h•MPa)
1
and an iodine removal efficiency of 73% until 180 min. Initial iodine concentration has a little influence on the iodine removal efficiency, but solution pH shows a significant effect. The iodine removal efficiency can be maintained around 92% in a weakly alkaline medium of pH=8. The PVDF/ZIF-8 nanocomposite membrane possesses an excellent reusability with an iodine removal efficiency of 73.4% after five cycles.
KEYWORDS: PVDF/ZIF-8 nanocomposite membranes; Iodine removal; 4-vinypyridine; Dynamic adsorption; Contra-diffusion
1. INTRODUCTION Radioactive iodine is one of the most toxic radionuclides presented in radioactive waste due to its high fission yield, volatility, environmental mobility, and significant radiological hazards. Among the iodine isotopes,
129
I stands out due to its extremely
long half-life of 1.57×107 years, which is detrimental to the environment, and 131I has a short half-life of 8.02 days, which can increase the risk of thyroid cancer with its accumulation in thyroid gland of human body [1]. Radioiodine produced in nuclear facilities is often released in off-gas stream [2]. Due to the slight solubility of iodine in water, radioiodine contaminations are often detected in surface water, seawater and groundwater after the Fukushima nuclear power plant accident [3]. Except the nuclear facilities, radioiodine is also employed in the treatment of thyroid cancer, and as result radioactive wastewater is discharged by numerous medical research institutes [4]. Therefore, radioiodine wastewater has to be treated including volume reduction and 2
radioiodine capture to ensure the safe discharge into the environment. There are many methods for radioactive wastewater treatment, such as chemical precipitation, sedimentation, thermal evaporation, ion exchange, and membrane permeation [5]. Membrane separation technology shows advantages in high decontamination factor, large volume reduction and low energy consumption and has been widely applied in removing radionuclides from radioactive wastewater, such as reverse osmosis (RO) and ultrafiltration (UF) [6-9]. RO membrane process performs a favorable rejection against trace nuclides and has been proved to be an effective technology in the treatment of low level radioactive wastewaters (LLRWs) in nuclear power plants, but suffers from relative high energy consumption [10]. Composite membrane comprised of UF membrane and chemical additive, such as adsorbent, coagulant and complexing agent, has been developed as an alternative to the RO membrane treatment of LLRWs [11]. Adsorption membrane characterized with specific functional groups on the surface of membrane, which can bind the nuclide by ion exchange or surface complexation, is a typical composite membrane and can remove target nuclides (e.g., Co2+, Sr2+, Cs+) from LLRWs effectively with higher water permeation than RO membrane [12,13]. Recently, metal-organic frameworks (MOFs) have shown potential as good adsorbents for iodine capture with high capacity, fast uptake, and good reusability [14]. ZIF-8 is a typical class of MOF material that is composed of imidazolate linkers and metal ions in zeolite topologies. It is selected for iodine adsorption due to its large surface area (1810 m2/g) and suitable pore aperture (3.4 Å). Yuan has investigated the 3
iodine adsorption of ZIFs in a multi-component system systematically and proved that ZIF-8 was the efficient and economical adsorbent with high diversity for iodine [15]. Similar conclusion was also confirmed by Bennett on the iodine adsorption and retention properties of ZIF-4, ZIF-8, ZIF-69 and ZIF-mnIm [16]. Nenoff’s group has suggested that iodine adsorption of ZIF-8 was related to the favorable interaction between iodine and 2-methylimidazole linker in the framework and found that 5.4 iodine molecules could be captured by each cage of ZIF-8 [17]. In a follow-up study, the strong chemisorption of iodine with ZIF-8 cage was identified by aqueous solution calorimetry, which is driven by the ideal confinement conditions within ZIF-8 cages, and it was found that ZIF-8 could bind iodine 4 time more strongly than activated carbon [18]. However, the further practical application of ZIFs materials to remove radioiodine in water still faces several hurdles. Firstly, a large mass of radioelement-contaminated solid adsorbents are generated during the desalination process. Secondly, nanomaterials used as adsorbents tend to aggregate easily under some conditions, which will discount the natural and advantageous physicochemical properties of ZIFs. polymer/ZIF-8 nanocomposite membrane, which shows superiorities in improving robustness and enhancing deliverability and also in development as a continuous flow-through unit for the repetitive treatment of iodine wastewater, seems to be a feasible solution and attracts attention of researchers [19]. Recently, some novel methodologies have been reported for preparing polymer/ZIF-8 nanocomposite membrane. Lei et al. have reported a novel layer-by-layer fabrication to prepare a 4
polyamide (PA)/ZIF-8 nanocomposite membrane with a multilayer structure by growing ZIF-8 nanoparticles interlayer on a porous surface of a polymer membrane through in situ growth and then coating it with an ultrathin PA layer through interfacial polymerization [20]. Stark et al. have proposed a membrane synthesis method by restricting ZIF-8 growth in the pores of poly(ether sulfone) (PES) pores by exploiting directed ZnO seed-nanoparticles to prepare PES/ZIF-8 composite membranes [21]. Although the in situ growth is a simple method allowing the simultaneous nucleation and crystal growth of ZIF-8, it is not effective in preparing continuous ZIF-8 layer due to the limited heterogeneous nucleation sites on polymer membrane. In addition, Wang’s group reported a contra-diffusion synthesis method for the intergrowth of ZIF-8 layer on a flexible nylon substrate [22]. This method depends on the surface properties and porous structure of support, and the ZIF-8 film can grow on both sides of support or within porous channels of support [23]. Poly(vinylidene fluoride) (PVDF) is characterized with high mechanical strength, thermal stability, chemical resistance and high hydrophobicity, and is widely used as a membrane material in various membrane separation processes. Khataee’s group has modified the PVDF membranes with different size of ZIF-8 nanocrystals to prepare PVDF/ZIF-8 mixed matrix membranes and found that the modified membranes showed higher water flux compared to the unmodified PVDF owing to their higher porosity
and
selective
pore
size
of
ZIF-8
nanoparticles
[24].
It is commonly recognized that surface modification of PVDF membranes by organic molecular, such as (3-aminopropyl)triethoxysilane, polydopamine, and ethylenediami 5
ne, is an effective method to solve the interfacial binding problem between continuous ZIF-8 film and PVDF membrane and to promote the heterogeneous nucleation of ZIF -8 crystals on its surface [25-27]. Polymer brushes were reported can be used as macromolecular primers to effectively promote the heterogeneous nucleation and crystalline growth of ZIF-8 thin films [28]. In theory, the use of polymer brushes to modify PVDF membrane substrate can also promote the heteromorphic nucleation and growth of ZIF-8 on the PVDF membrane. In our previous work the porous PVDF membrane was first in-situ grafted polymerization of 4-vinylpyridine (4VP) using SI-AGET ATRP, and the pyridine functional groups of poly(4-vinylpyridine) (P4VP) can be a driving factor for maintaining a high concentration of zinc ions selectively near the surface of PVDF support membrane [29]. When combined with the manipulation of the sodium formate to ligand ratios, this approach can lead to continuous ZIF-8 layer on the PVDF support membrane. Therefore, combination of ZIF-8 crystals and P4VP polymer brushes modified PVDF constituting PVDF/ZIF-8 nanocomposite membrane can be predicted to show a positive effect on the iodine ions removal from aqueous solution. In this work, we report a simple and effective method for preparation of well-intergrowth and continues ZIF-8 layer with controllable location on a PVDF substrate based on P4VP polymer brushes modified PVDF support membrane in conjunction with contra-diffusion method. The morphologies of PVDF/ZIF-8 nanocomposite membrane are characterized, and the corresponding iodine removal behavior from iodine aqueous solution including batch adsorption and dynamic 6
adsorption are evaluated. Adsorption kinetics, mechanical properties, regeneration and reusability of the PVDF/ZIF-8 nanocomposite membrane are investigated.
Scheme 1. (A) Preparation of PVDF/ZIF-8 nanocomposite membrane and (B) Schematic illustration
of iodine removal
from
water using
PVDF/ZIF-8
nanocomposite membrane. 2. EXPERIMENTAL SECTION 2.1. Reagents and materials PVDF membranes (average pore size of 0.22 μm) were purchased from Merck Millipore. 4VP (96%) was purchased from Alfa Aesar and distilled under negative pressure over NaOH pellets and then stored at -18 °C under Ar. LiOH•H2O (98%), NaBH4
(98%),
diisobutylaluminium
hydride
(DIBAL-H,
1M
in
hexane),
N,N,N',N',N"-pentamethyldiethylenetriamine (PMDETA, 99%), 2-bromoisobutyryl bromide (98%), ethyl 2-bromobutyrate (99%), CuBr2 (99.95%), ascorbic acid (99.99%), Zn(NO3)2•6H2O (99%), 2-methylimidazole (mIm, 99%) and sodium 7
formate (99%) were purchased from Aladdin Chemical Co., Ltd, China and used without further purification. Iodine (99.8%) and potassium iodine (KI, 99.9%) were purchased from Xi-Long Scientific Co., Ltd, China. Bovine serum albumin (BSA, Mw: 67 kDa) was obtained from Aladdin reagent Co., Ltd. (Shanghai, China). All the other reagents and organic solvents, unless under specific illumination, were purchased from Sinopharm Chemical Reagent Co., Ltd. and used without any further purification. Deionized (DI) water was purified by Milli-Q integral water purification system with a resistivity of 18.2 MΩ/cm. 2.2. Preparation of PVDF-g-P4VP membranes and PVDF/ZIF-8 nanocomposite membranes Due to the nonpolarity of PVDF which cannot provide the heterogeneous nucleation sites for the well-intergrowth of ZIF-8 adsorbent layer, P4VP brushes were grafted firstly onto PVDF membrane via surface-initiated atom transfer radical polymerization with activators generated by electron transfer (SI-AGET ATRP) method. The pyridine groups on P4VP side chains can coordinate to the free zinc ions and provide a large number of nucleation sites. Subsequently, the modified PVDF membrane denoted as PVDF-g-P4VP was employed as the support for the growth of ZIF-8 layer via contra-diffusion method following a previous report, and well-controlled crystal growth in the vicinity of the membrane surface can be obtained with slow diffusion of ligand in contra-diffusion process. Typically, the PVDF-g-P4VP membrane was mounted on a homemade setup, where the zinc acetate solution and mIm solution were separated by the supporting membrane. Zinc acetate 8
solution was prepared by dissolving 0.7348 g Zn(NO3)2•6H2O in 100 mL methanol, and mIm solution was prepared by adding 1.6223 g mIm and 0.9022 g sodium formate in 100 mL methanol. After crystallization of ZIF-8 on the surface of PVDF-g-P4VP membrane for a predetermined time at room temperature, the nanocomposite membranes denoted as PVDF/ZIF-8 were taken out and rinsed with deionized water several times, and finally, were dried in ambient conditions for 12 h. 2.3. Measurement of the adsorption kinetics The batch adsorption experiments were conducted in glass vials containing 100 mL iodine solution with an iodine concentration of 0.25 mmol/L and 8.55 cm2 PVDF/ZIF-8 nanocomposite membranes to evaluate the adsorption capacity for iodine removal from aqueous solution. The glass vials were kept on shaker for 33 h at room temperature. Then the PVDF/ZIF-8 nanocomposite membrane samples were taken out and the concentration of residual iodine in the solution was determined using UV-vis spectrophotometer. For ZIF-8 crystals adsorption experiments, ZIF-8 particles were outgassed under vacuum at 120 °C for 12 h before the iodine ions adsorption and at a certain time point, a small amount of mixture was taken out and filtered through a 0.45 μm porous membrane for UV-vis measurement. The adsorption capacity (qe, mg/g) was calculated as Eq. (1): qe
(c0 ce ) V M
(1)
Where c0 and ce are the initial and equilibrium iodine concentration of aqueous solution respectively, and V is the volume of iodine solution, and M is the weight of dried membrane. 9
2.4. Dynamic adsorption with iodine aqueous solution of PVDF/ZIF-8 nanocomposite membranes The dynamic adsorption experiments were conducted with a dead-end stirred cell (Stirred Cell 8200, Millipore, USA). Iodine solution with an initial concentration of 0.25 mmol/L was used to evaluate the iodine capture capacity of PVDF/ZIF-8 nanocomposite membrane in a dynamic adsorption with a feed pressure of 40 KPa. The iodine solution is 250 mL and the effective area of the PVDF/ZIF-8 nanocomposite membrane is 8.55 cm2. The filtration system was equilibrated with deionized water prior to the experiment. The filtrate was collected and its weight was determined using an electronic balance. The iodine concentration in filtrate was determined by a UV-vis spectrophotometer. The capture capacity for iodine of the membrane was calculated from the change in iodine concentration before and after dynamic adsorption. The water permeation flux (J, L/(m2•h •MPa) of the PVDF/ZIF-8 nanocomposite membrane was calculated as Eq. (2): J
V At P
(2)
Where V is the permeation amount collected under operating pressure ΔP on a time scale t and A is the effective area of the membrane. The iodine removal efficiency (R, %) was calculated as Eq. (3):
R ( 1
cp cf
) 1 0 0 %
(3)
Where cp and cf represent the iodine concentration of filtrate and feed solution, respectively. 2.5. Iodine release and recycling test of PVDF/ZIF-8 nanocomposite membrane 10
The saturated PVDF/ZIF-8 nanocomposite membranes from the iodine adsorption experiments were further used in the iodine release and regeneration studies. I2@PVDF/ZIF-8 nanocomposite membranes were immersed in 10 mL of ethanol (EtOH) at room temperature. To ensure the completion of iodine release, the samples were left in EtOH for 24 h. The iodine adsorption efficiency of regenerated PVDF/ZIF-8 membrane was acquired using dynamic adsorption approach described before. Samples were analyzed for iodine concentration using UV-vis spectroscopy for all experiments. 2.6. Antifouling test A 1.0 g/L protein solution was prepared by dissolving BSA in PBS (phosphate buffer solution, pH=7.2-7.4). In the antifouling test, after DI water was first used as the feed solution of permeation and then displaced by the 1.0 g/L BSA solution, and the Jp was recorded until a steady flux. After protein solution permeation, the membrane was washed with DI water to remove the reversible adsorbed protein. Then, the DI water permeation for the washed membrane was performed again. The flux recover ratios (FRw) of PVDF/ZIF-8 nanocomposite membrane was obtained as Eq. (4): FRw
J w2 100% J w1
(4)
where Jw1 and Jw2 are the steady pure water flux through membrane before and after protein solution permeation, respectively. The permeation flux was resulted from both reversible and irreversible protein fouling, which were defined as Eq. (5) and Eq. (6):
11
Rr Rir
J w2 J p
100%
(5)
J w1 J w 2 100% J w1
(6)
J w1
where Rr and Rir denote the reversible and irreversible iodine removal efficiency, respectively. 2.7. Characterization The surface chemical compositions of the membranes were characterized through ATR-FTIR spectroscopy (Excalibur 3100, Varian), each spectrum was captured by 32 averaged scans at a resolution of 4 cm-1. The membrane morphology was examined by a field-emission scanning electron microscope (SEM, FEI Quanta 200 FEG) with 10 kV accelerating voltage on gold sputter-coated samples. Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku D/Max-IIIc using Cu-Ka radiation. The iodine absorbance was recorded using a UV-vis spectrophotometer at 288 nm (UV-2700, Shimadzu). X-ray photoelectron spectra were obtained on an ESCALAB 250Xi spectrometer with a monochromic X-ray source and the charging shift was corrected by the binding energy of C 1s at 284.6 eV. Mechanical properties of the pristine PVDF, PVDF/ZIF-8 nanocomposite membrane were analyzed by a tensile tense machine (INSTRON 5569, UK). The testing speed was set at a loading velocity of 5 mm/min at ambient temperature. All samples were prepared with a width of 5 mm and a length of 30 mm. Each test was repeated for three times. The quantitative surface roughness analysis of the PVDF/ZIF-8 nanocomposite membrane was measured using Atomic-force microscopy (AFM, Dimension Icon, Bruker) with the 12
spring constant of 0.1 N/m through the contact mode in dry air. All the membrane samples were dried for 12 h in vacuum before the AFM analysis. Contact angle of water droplets on the samples were measured by contact angle analyzer (OCA-15EC, Dataphysics). DI water was deposited on the surface of dried membrane, and images were obtained with water spreading over the membrane. The reported contact angle here was the average value of three locations in one sample. 3. RESULTS AND DISCUSSION 3.1. Fabrication and characterization of PVDF/ZIF-8 nanocomposite membranes The pyridine functional groups on PVDF-g-P4VP membrane were confirmed by ATR-FITR spectra (Fig. S1) and XPS spectra (Fig. S2). ATR-FTIR spectra show the additional peaks at 1596 cm-1, 1556 cm-1, 1494 cm-1, and 1413 cm-1, which are attributed to the vC=C and vC=N stretching at the pyridine ring of grafted 4VP. XPS spectra also show an additional N 1s signal around 398.7 eV, which indicates the successful graft of 4VP onto the PVDF membrane surface. SEM images (Fig. S3) show obvious decrease in the pore size at the top surface of PVDF membrane after graft with P4VP. After the modification of PVDF membrane, ZIF-8 layer can be formed on the PVDF-g-P4VP support membrane by contra-diffusion method. Unlike the conventional contra-diffusion method where the crystals grow on the both side of support, it is interesting to observe that the ZIF-8 crystals grow only on the metal ions surface side of PVDF-g-P4VP membrane in our experiment as verified in the XRD pattern (Fig. 1a). SEM images (Fig. 1b and Fig. 1d) further confirm that a dense and 13
continuous ZIF-8 layer of rhombic dodecahedron morphology with a thickness about 5 μm is formed on the metal ions side of PVDF-g-P4VP membrane. At the same time, no ZIF-8 crystal particles are found on the surface of mIm ligand side of PVDF-g-P4VP membrane (Fig. 1c). In a typical contra-diffusion method, it is difficult to control precisely the position of heterogeneous nucleation and growth of ZIF-8 crystals on the support. Different to the growth of high-quality ZIF-8 layer only on the surface side of ethylenediamine pretreated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) substrate by Shamsaei [30], P4VP polymer brushes were grafted onto the both side of PVDF membrane in our experiment and therefore the pyridine groups on both side of the surface of PVDF membrane are not the main factor controlling the growth position of ZIF-8. Due to the high mIm concentration gradient in mIm side, mIm can slowly diffuse through the PVDF-g-P4VP membrane and then react with zinc ions rapidly, resulting in the enrichment on the other surface side of membrane grafted by P4VP and in-situ growth of ZIF-8 crystals on the zinc nitrate side [31,32]. Therefore, the Zn/mIm molar ratio and solution concentration of reactants on both sides of PVDF-g-P4VP membrane are maybe the key factors determining the growth position of ZIF-8 layer.
14
Fig.1. XRD pattern (a) and SEM images (metal ions side (b), mIm side (c), and Zn2+ side cross-section (d)) of the PVDF/ZIF-8 nanocomposite membrane prepared without sodium formate at room temperature for 24 h. Furthermore, the mechanical properties including tensile strength, Young’s module and breaking elongation of the pristine PVDF and PVDF/ZIF-8 nanocomposite membranes are showed in Table 1. It is obvious that the tensile strength and breaking elongation of the PVDF/ZIF-8 nanocomposite membrane decrease with the growth of ZIF-8 layer on it. The maximum tensile strength and breaking elongation of PVDF/ZIF-8 nanocomposite membranes are 6.40 MPa and 20%, respectively. This confirms that the formation of ZIF-8 layer on the PVDF membrane has reduced the mechanical properties of the PVDF composite membrane resulting from the increase in brittleness and decrease in flexibility of the PVDF membrane by continuous and dense rigid ZIF-8 layer. Table 1. The mechanical properties of the PVDF and PVDF/ZIF-8 nanocomposite 15
membranes prepared by contra-diffusion method. Membrane samples Pristine PVDF membrane PVDF/ZIF-8 nanocomposite membrane
Tensile strength (Mpa)
Young’s module (Mpa)
Breaking elongation (%)
9.37 6.40
10.50 32.41
89 20
In the process of contra-diffusion synthesis of ZIF-8 layer, the crystallization reaction and diffusion occur simultaneously. Theoretically, the crystallization reaction should be faster than the diffusion so that the heterogeneous nucleation and growth can happen before the metal ions are depleted from the support [33]. When the ZIF-8 layer is prepared on the PVDF support membrane by contra-diffusion method, it is an effective method to control the uniformity and continuity of ZIF-8 layer by adjusting the synthesis time. Recently, Khataee’s group reported that a uniform and continuous layer of ZIF-8 nanocrystals were successfully formed on the PVDF support surface by increasing synthesis time to 5 h [34]. However, in this study, we mainly studied the effect of changing the molar ratio of ligand to sodium formate on the ligand solution side on the microstructure of ZIF-8 layer on PVDF membrane. In fact, it was reported that sodium formate (SF) could enhance the crystallization reaction rate leading to the formation of ZIF-8 layer [35-37], and here the effect of the molar ratio of ligand to sodium formate on the ZIF-8 layer microstructure formed on the PVDF-g-P4VP membrane is investigated. Fig. 2 displays the SEM images of the ZIF-8 layer prepared at different molar ratio of ligand to sodium formate. As shown, poorly intergrown layer composed of rhombic decahedron particles covers the Zn side surface but few particles can be observed on the ligand side surface when Zn/mIm/SF 16
molar ratio is 1:8:0.672 (Fig. 2a, 2b and 2c). However, well-intergrown and continuous ZIF-8 layer is formed on the Zn side when Zn/mIm/SF molar ratio increases to 1:8:1.344 (Fig. 2d, 2e and 2f). Further increasing Zn/mIm/SF molar ratio to 1:8:2.688, ZIF-8 layer becomes uneven and the thickness is about 3 μm on the Zn side surface of membrane. In addition, the XRD peak intensities of the ZIF-8 on Zn side are stronger than that on mIm solution side, which confirms the former observation that ZIF-8 layer is mainly formed on the metal ions side surface of PVDF-g-P4VP membrane. These results indicate that the addition of sodium formate to the ligand solution can effectively control the microstructure of the ZIF-8 layer when the Zn/mIm molar ratio is constant. With the increasing addition of sodium formate, ZIF-8 layer becomes more continuous and well-intergrown. The weakly alkaline sodium formate is a proton accepter/scavenger and is expected to play a critical role in deprotonating the bridging mIm ligand and facilitating the heterogeneous nucleation as well as the intergrowth of ZIF-8 crystals on support [38,39]. The addition of sodium formate will increase the solution pH value and decrease the protons concentration in solution, which can shift the equilibrium and thereby drive the deprotonation of mIm ligands. In the meantime, P4VP (pKa=4.7) is a weakly alkaline substance with pH sensitive property. When the PVDF-g-P4VP membrane equilibrate in a weakly alkaline solution, the deprotonated pyridine groups on the P4VP side chain can react with Zn ions, and in turn promote Zn ions to maintain in the vicinity of the metal ions side surface of PVDF-g-P4VP membrane with a relatively high concentration. Because the coordination reaction rate of ZIF-8 17
is faster than the diffusion rate of mIm ligand, it will lead to the fast formation of ZIF-8 nuclei and subsequent crystal growth on the Zn side surface of membrane, and finally the well-intergrown and continuous ZIF-8 layer.
Fig. 2. SEM images of ZIF-8 layer prepared with Zn/mIm/SF molar ratios of 1:8:0.672 (a,b,c), 1:8:1.344 (d,e,f), and 1:8:2.688 (g,h,i) at room temperature for 24 h. (a)
(b )
Zn/mIm/SF=1:8:2.688
Zn/mIm/SF=1:8:1.344
Intensity (a..)
Intensity (a..)
Zn/mIm/SF=1:8:2.688
Zn/mIm/SF=1:8:1.344
Zn/mIm/SF=1:8:0.672
Zn/mIm/SF=1:8:0.672
Simulated ZIF-8
Simulated ZIF-8 5
10
15
20
25
30
5
10
15
20
25
30
2 theta (degree)
2 theta (degree)
Fig. 3. XRD patterns of ZIF-8 layer prepared on PVDF-g-P4VP membranes with
18
varying Zn/mIm/SF molar ratios: The Zn side (a) and the mIm solution side (b). 3.2. Batch adsorption of PVDF/ZIF-8 nanocomposite membranes The potential application of PVDF/ZIF-8 nanocomposite membrane for iodine adsorption from aqueous solution was investigated. The iodine adsorption kinetics by the PVDF/ZIF-8 nanocomposite membrane is shown in Fig. 4. It can be seen that the saturation adsorption capacity of iodine on PVDF/ZIF-8 nanocomposite membrane is 73.33 mg/g. The initial fast adsorption is mainly ascribed to the quick uptake of iodine ions by the ZIF-8 layer located at the surface of PVDF/ZIF-8 nanocomposite membrane. The subsequent slow adsorption is attributed to the slow diffusion of iodine ions through the mesopores of the ZIF-8 particles. The interaction between iodine ions and PVDF/ZIF-8 nanocomposite membrane can be estimated by the adsorption kinetics. Langmuir first-order and pseudo-second-order models are used to investigate the adsorption kinetics and the corresponding kinetic data and kinetic parameters are illustrated in Fig. 5 and Table 2, respectively. It can be seen that the pseudo-second-order kinetic model (R2>0.970) is suitable for the iodine adsorption process with PVDF/ZIF-8 nanocomposite membrane. Compared with the adsorption saturation of ZIF-8 crystals in 300 min, the iodine adsorption of PVDF/ZIF-8 nanocomposite membrane reaches saturation up to 1500 min, indicating that the iodide ions adsorption rate in aqueous solution by PVDF/ZIF-8 nanocomposite membrane is significantly slower than that of ZIF-8 crystals. It is also can been seen that the saturated adsorption capacity of ZIF-8 crystals and PVDF/ZIF-8 nanocomposite membrane is 81.80 mg/g, and 73.33 mg/g under the same original 19
adsorbent mass, respectively. The above experimental results indicate that, for the static iodine adsorption, ZIF-8 crystals formed on the PVDF membrane will significantly decrease the adsorption rate with a slightly decreased iodine adsorption capacity compared with the ZIF-8 crystals. (a)
80
(b) 80
70 60
qe (mg/g)
qe (mg/g)
60
50 40 30 20
40
20
10 0
0 0
500
1000
1500
0
2000
50
100
150
200
250
300
Time (min)
Time (min)
Fig. 4. Effect of adsorption time on the iodine ions adsorption of PVDF/ZIF-8 nanocomposite membrane (a) and ZIF-8 crystals (b) in a batch system.
b 30
a
25 20
1
t/qt
log(qe-qt)
2
15 10
0
5 -1
0 0
800
1600
0
500
1000
1500
2000
Time (min)
Time (min)
Fig. 5. The linear fitted curves of Langmuir first-order model (a) and pseudo-second-order kinetics model (b) of PVDF/ZIF-8 nanocomposite membrane. Table 2. Kinetic parameters of iodine ions adsorption by PVDF/ZIF-8 nanocomposite membrane from aqueous solutions. qe,exp (mg/g) 73.36
Lagergren-first-order
Pseudo-second-order
qe,cal (mg/g)
k1 (min-1)
R2
qe,cal (mg/g)
k2 (min g/mg)
R2
106.89
0.003
0.932
95.60
0.00002
0.970
20
In order to explain the mechanism of iodine adsorption with PVDF/ZIF-8 nanocomposite membrane, the adsorption equilibrium isotherm was studied. In this study, the iodine ions adsorption equilibrium data of the PVDF/ZIF-8 nanocomposite membranes are fitted by Langmuir and Freundlich isotherm models, and the results are presented in Fig. 6 and Table 3. Taking the correlation coefficient as the criterion of fitting degree, the Freundlich isotherm model shows better correlation (R2=0.987) than the Langmuir isotherm model and therefore Freundlich isotherm model can represent the iodine adsorption process with PVDF/ZIF-8 nanocomposite membrane. (b)
(a)
6.4
0.07 6.2
0.06
lnqe
Ce/qe
6.0
0.05
5.8
0.04
5.6
0.03
5.4
5
10
15
20
25
30
35
2.0
40
2.4
2.8
3.2
3.6
lnCe
Ce(mg/L)
Fig. 6. Langmuir isotherm plot (a) and Freundlich isotherm plot (b) for iodine ions adsorption of PVDF/ZIF-8 nanocomposite membrane. Table 3. Langmuir and Freundlich isotherm parameters for PVDF/ZIF-8 nanocomposite membrane. Langmuir isotherm parameters
Freundlich isotherm parameters
qmax (mg/g)
KL (mL/mg)
R2
KF (mg/g)
n
R2
79.365
0.550
0.942
88.352
2.0
0.987
3.3. Dynamic adsorption with iodine aqueous solution of PVDF/ZIF-8 nanocomposite membranes 21
To further investigate the separation capability of PVDF/ZIF-8 nanocomposite membranes for iodine removal from aqueous solution in a more practical continuous operation way, the dynamic adsorption of PVDF/ZIF-8 nanocomposite membranes was conducted at the operating pressure of 0.04 MPa. As shown in Fig. 7a, with the dynamic adsorption processing to 180 min, the water permeation through the PVDF/ZIF-8 nanocomposite membrane decreases from 86.57 to 66.19 L/(m2•h•MPa) and the iodine ions removal efficiency increases from 53% to 73%, which can be ascribed to the decreasing pore size of ZIF-8 crystals and increasing thickness of membrane. In addition, it is interesting to note that the ZIF-8 layer on the surface of nanocomposite membrane seems to be disappeared after the dynamic adsorption, which is confirmed by the XRD characterization of PVDF/ZIF-8 nanocomposite membrane as show in Fig. 7c. The surface morphology of PVDF/ZIF-8 nanocomposite membrane before and after the dynamic adsorption are also shown in Fig. 7b and Fig. 7d, respectively. In order to confirm the chemical state of adsorbed iodine in PVDF/ZIF-8 nanocomposite membrane, the corresponding XPS spectrum was obtained. As seen in Fig. 7f, the iodine spectrum displays two split peaks located at 630.6 and 618.8 eV, which are assigned to the I 3d3/2 and I 3d5/2 orbitals of iodine molecules, respectively. In addition, the I 3d5/2 peak shows two peaks at 618.8 and 619.3 eV, which are attributed to the I3- and I5- anions, respectively [40]. It should be noted that both I3- anions and neutral I2 could form polyiodide. During the ultrafiltration process, the iodine aqueous solution permeates through the ZIF-8 layer and the ZIF-8 crystals are gradually decomposing by water molecules under pressure, 22
which finally leads to the irreversible collapse of ZIF-8 crystalline framework. The possible mechanism for PVDF/ZIF-8 nanocomposite membrane with iodine removal is most likely due to the formation of a charge transfer interaction between iodine molecules and the electron-rich ZIF-8.
Fig. 7. Dynamic adsorption performance of the PVDF/ZIF-8 nanocomposite membrane (a), SEM image of PVDF/ZIF-8 nanocomposite membrane before dynamic 23
adsorption (b), XRD patterns of PVDF/ZIF-8 nanocomposite membrane before and after dynamic adsorption (c), SEM image of PVDF/ZIF-8 nanocomposite membrane after dynamic adsorption (d), XPS survey spectrum of PVDF/ZIF-8 nanocomposite after dynamic adsorption (e), XPS spectrum of I 3d after dynamic adsorption (f). It was reported that ZIF-8 crystal was sensitive to the solution pH. Wu et al. have reported that ZIF-8 crystal is relatively stable in the solution pH range of 5.0-12.0, but will gradually disintegrate when the solution pH is less than 4.0 [41]. Therefore, the effect of the initial iodine concentration as well as solution pH on the iodine ions removal performance of PVDF/ZIF-8 nanocomposite membrane need to be further studied. As shown in Fig. 8a, as the initial iodine ions concentration of aqueous solution increases from 0.061 to 0.2374 mmol/L, the permeation flux remains substantially stable from 36.78 to 35.65 L/(m2•h•MPa), while the iodine removal efficiency decreases from 79.65% to 72.88%. As shown in Fig. 8b, the iodine ions removal efficiency of PVDF/ZIF-8 nanocomposite membrane mainly maintains around 92% at pH=8 but rapidly decreases from 49.67% to 6.38% at pH=3. When ZIF-8 crystals are exposed in acidic aqueous solution, hydrated protons will attack ZIF-8 framework structure and the mIm linkers become protonated, which in turn breaks the Zn-mIm bonds and carves those external crystal surfaces with the high density of Zn-mIm bonds [42]. Therefore, the irreversible destruction of continuous and dense ZIF-8 layer in nanocomposite membrane weakens the iodine removal ability. Besides, the high iodine removal performance of PVDF/ZIF-8 nanocomposite membrane at high pH can be explained by the relationship between pH and zeta 24
potential of ZIF-8. The point of zero charge (pHzpc) of ZIF-8 is approximately occurred at pH=9.3 [43], and this implies that the charge of ZIF-8 crystal is positive at pH range of 5-10 [44]. At pH=8, iodine mainly exists as I2 and I3- with negative charge in solution and the electrostatic attraction between iodine ions and ZIF-8 becomes a critical factor in the removal mechanism, which maintains the removal efficiency at a high level. On the other hand, ZIF-8 crystals are easily decomposed in acidic solution. After filtration of PVDF/ZIF-8 nanocomposite membrane with strong acid iodine solution, the ZIF-8 layer on the surface of PVDF supporting membrane completely disappeared, indicating that ZIF-8 crystals have been completely decomposed by acidic aqueous solution.
70
60
60
50
50
40
40
30
30
20
20
10
10
0
0
0.1217
0.1824
-2
90
Removal efficiency (%)
70
0.061
(b) 100
80
Removal efficiency (%)
P e rm e a n ce R e m o va l e fficie n cy
80
-1
-1
Permeance (Lm h MPa )
(a)
80 pH=8 pH=3
70 60 50 40 30 20 10 0
0.2374
0
C0(mmol/L)
50
100
150
200
250
Time (min)
Fig. 8. Effect of initial iodine concentration (a) and solution pH (b) on the iodine removal efficiency of PVDF/ZIF-8 nanocomposite membranes. 3.4. Regeneration and reusability of PVDF/ZIF-8 nanocomposite membrane The regeneration and reusability are important parameters for a membrane in practical application and are studied here. The encapsulated iodine could be removed from the ZIF-8 framework with the immersion of I2@PVDF/ZIF-8 nanocomposite membrane in organic solvents. In this study, it is found that once the I2@PVDF/ZIF-8 25
nanocomposite membrane was soaked in fresh EtOH at room temperature, the color of EtOH solution would deepen from colorlessness to light yellow quickly with time, which clearly indicates that iodine could be easily desorbed from the nanocomposite membranes. The absorbance of the desorbed iodine solution with time from I2@PVDF/ZIF-8 nanocomposite membrane at room temperature was recorded using UV-vis as shown in Fig. 9a and the corresponding iodine desorption percentage based on the calibration curve of iodine concentration in EtOH was shown in Fig.9b. It is can been seen that the desorbed iodine amount from I2@PVDF/ZIF-8 nanocomposite membrane increases almost linearly with time within 120 min and the highest iodine desorption percentage is 37.72 wt.% at 240 min on the basis of the total captured iodine amount by PVDF/ZIF-8 nanocomposite membrane.
(b) 40 Release Percentage (%)
(a) 1.8
Absorbance
1.6
1.4
1.2
1.0
35
30
25
20 0.8 0
50
100
150
200
250
0
50
100
150
200
250
Time (min)
Time (min)
Fig. 9. Iodine desorption of I2@PVDF/ZIF-8 nanocomposite membranes in EtOH: absorbance of desorbed iodine solution (a) and iodine release percentage (b). The regenerated PVDF/ZIF-8 nanocomposite membranes were further reused for the next round of iodine dynamic adsorption to test its reusability. Fig. 10 a shows the
26
recycled adsorption ability of the PVDF/ZIF-8 nanocomposite membrane. It is can been seen that the iodine removal efficiency still retains 73.4% after five cycles. The dead-end dynamic adsorption was further employed to investigate the antifouling performance of PVDF/ZIF-8 nanocomposite membranes including permeance flux, Rir and Rr. Fig. 10b shows the time-dependent permeation flux of PVDF/ZIF-8 nanocomposite membranes with the feed of water and protein in sequence. The FRw of PVDF/ZIF-8 nanocomposite membrane can be calculated and is equal to 82.51%, which indicates that the PVDF/ZIF-8 nanocomposite membrane can still possess a relative high permeation flux after the protein contamination. In addition, the Rir and Rr of the total fouling is 17.49% and 4.82%, respectively. The multiple factors can be responsible for this phenomenon, including the high hydrophilicity (the water contact is 58.52° as shown in Fig. 10c) and the smooth surface (Rq=0.994 nm as shown in Fig. 10d) of PVDF/ZIF-8 nanocomposite membranes.
27
Fig. 10. Recycled adsorption ability of PVDF/ZIF-8 nanocomposite membrane for dynamic adsorption of iodine solution (a), Permeation flux of PVDF/ZIF-8 nanocomposite membranes with the feed of water and protein in sequence (b), Water contact of the PVDF/ZIF-8 nanocomposite membrane (c), AFM image of the PVDF/ZIF-8 nanocomposite membrane (d). 4. CONCLUSIONS In summary, the work demonstrated a simple and effective method including contra-diffusion method in conjunction with P4VP grafted modification to prepare a well-intergrown and continuous PVDF/ZIF-8 nanocomposite membrane with controllable location. The ZIF-8 crystals grow only on the metal ions side surface of PVDF-g-P4VP membrane and the optimum Zn/mIm/SF molar ratio is 1:8:1.344. The maximum iodine adsorption capacity of PVDF/ZIF-8 nanocomposite membrane is 73.33 mg/g in the batch adsorption and the removal efficiency is high as 73% till to 180 min in the dynamic iodine adsorption. The iodine removal efficiency of PVDF/ZIF-8 nanocomposite membrane slightly decreases with the increase of initial iodine concentration. Weakly alkaline solution has positive effect on the iodine removal of PVDF/ZIF-8 nanocomposite membrane, around 92% at pH=8, while acidic solution shows the opposite. About 37.72 wt.% of adsorbed iodine in PVDF/ZIF-8 nanocomposite membrane can be desorbed until 240 min. The iodine removal efficiency of PVDF/ZIF-8 nanocomposite membrane is still high as 73.4% after five cycles. 5. ASSOCIATED CONTENT 28
Supporting Information ATR-FTIR spectrum of PVDF-g-P4VP membrane; XPS spectrum of PVDF-g-P4VP membrane; SEM images of surface morphology and cross section of PVDF-g-P4VP membrane; Preparation of PVDF/ZIF-8 nanocomposite membrane by contra-diffusion method; UV-Vis spectra of iodine in aqueous solution at room temperature and calibration curve of iodine; SEM image and EDX of PVDF/ZIF-8 nanocomposite membrane after dynamic adsorption of iodine solution at pH=8; XRD patterns of PVDF/ZIF-8 nanocomposite membrane after dynamic adsorption of iodine solution at pH=8. 6. AUTHOR INFORMATION Corresponding Author *Jian-Hua Qiu, E-mail: [email protected] *Juan-Tao Jiang, E-mail: [email protected] ORCID Jian-hua Qiu: 0000-0001-7285-3561 Author Contributions †Xing Long and Ya-Shuo Chen contributed equally to this work.
Notes The authors declare no competing financial interest. 7. ACKNOWLEDGMENTS This work was supported by Nature Science Foundation of China (No. 21868009), Natural Science Foundation of Guangxi Province (No. 2018GXNSFDA281022), Innovation Project of Guangxi Graduate Education (XYCSZ2019057). 8. REFERENCES 29
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35
Highlights
1. PVDF/ZIF-8
nanocomposite
membrane
via
contra-diffusion
method
in
conjunction with P4VP grafted modification. 2. A well-intergrown and continuous ZIF-8 layer can be formed on the surface of PVDF-g-P4VP membrane at Zn/mIm/SF molar ratio of 1:8:1.344. 3. The maximum iodine adsorption capacity of PVDF/ZIF-8 nanocomposite membrane is 73.33 mg/g in batch adsorption. 4. The iodine removal efficiency of PVDF/ZIF-8 nanocomposite membrane is high as 73% until to 180 min in the dynamic adsorption.
Declaration of interests ☐√ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Author Statemen
Xing Long: Investigation, Writing-Original Draft Ya-Shuo Chen: Investigation, Writing-Original Draft, Visualization Qian Zheng: Validation, Formal analysis Xing-Xiao Xie: Investigation, Hao Tang: Investigation, Li-Ping Jiang: Resources Juan-Tao Jiang: Project administration, Writing-Review&Editing Jian-Hua Qiu: Conceptualization, Methodology, Supervision, Funding acquisition
S1383-5866(19)33893-6 https://doi.org/10.1016/j.seppur.2019.116488 SEPPUR 116488
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
29 August 2019 24 December 2019 24 December 2019
Please cite this article as: X. Long, Y-S. Chen, Q. Zheng, X-X. Xie, H. Tang, L-P. Jiang, J-T. Jiang, J-H. Qiu, Removal of iodine from aqueous solution by PVDF/ZIF-8 nanocomposite membranes, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116488
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Removal of iodine from aqueous solution by PVDF/ZIF-8 nanocomposite membranes Xing Long,† Ya-Shuo Chen,† Qian Zheng, Xing-Xiao Xie, Hao Tang, Li-Ping Jiang, Juan-Tao Jiang*, Jian-Hua Qiu*
School of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guangxi Key Laboratory of Low Carbon Energy Materials, Guilin 541004, P. R. China
ABSTRACT: Metal-organic frameworks (MOFs) are capable in capture and storage of iodine ions with high efficiency, and polymer/MOF nanocomposite membrane has the potential to be used for efficient removal of iodine from radioactive wastewater. In this work, we have reported a PVDF/ZIF-8 nanocomposite adsorption membrane prepared by contra-diffusion method. The combination of surface graft of poly(4-vinylpyridien) brushes and optimal addition of sodium formate into ligand contributes to the formation of a well-intergrown and continuous ZIF-8 layer on the metal ions side surface of PVDF-g-P4VP membrane. In the batch adsorption, the PVDF/ZIF-8 nanocomposite membrane exhibits high affinity for iodine following a pseudo-second-order kinetics and fitting the Freundlich isotherm model with a maximum adsorption capacity of 73.33 mg/g. In the dynamic adsorption, the PVDF/ZIF-8 nanocomposite membrane exhibits a high flux of 66.19 L/(m2•h•MPa)
1
and an iodine removal efficiency of 73% until 180 min. Initial iodine concentration has a little influence on the iodine removal efficiency, but solution pH shows a significant effect. The iodine removal efficiency can be maintained around 92% in a weakly alkaline medium of pH=8. The PVDF/ZIF-8 nanocomposite membrane possesses an excellent reusability with an iodine removal efficiency of 73.4% after five cycles.
KEYWORDS: PVDF/ZIF-8 nanocomposite membranes; Iodine removal; 4-vinypyridine; Dynamic adsorption; Contra-diffusion
1. INTRODUCTION Radioactive iodine is one of the most toxic radionuclides presented in radioactive waste due to its high fission yield, volatility, environmental mobility, and significant radiological hazards. Among the iodine isotopes,
129
I stands out due to its extremely
long half-life of 1.57×107 years, which is detrimental to the environment, and 131I has a short half-life of 8.02 days, which can increase the risk of thyroid cancer with its accumulation in thyroid gland of human body [1]. Radioiodine produced in nuclear facilities is often released in off-gas stream [2]. Due to the slight solubility of iodine in water, radioiodine contaminations are often detected in surface water, seawater and groundwater after the Fukushima nuclear power plant accident [3]. Except the nuclear facilities, radioiodine is also employed in the treatment of thyroid cancer, and as result radioactive wastewater is discharged by numerous medical research institutes [4]. Therefore, radioiodine wastewater has to be treated including volume reduction and 2
radioiodine capture to ensure the safe discharge into the environment. There are many methods for radioactive wastewater treatment, such as chemical precipitation, sedimentation, thermal evaporation, ion exchange, and membrane permeation [5]. Membrane separation technology shows advantages in high decontamination factor, large volume reduction and low energy consumption and has been widely applied in removing radionuclides from radioactive wastewater, such as reverse osmosis (RO) and ultrafiltration (UF) [6-9]. RO membrane process performs a favorable rejection against trace nuclides and has been proved to be an effective technology in the treatment of low level radioactive wastewaters (LLRWs) in nuclear power plants, but suffers from relative high energy consumption [10]. Composite membrane comprised of UF membrane and chemical additive, such as adsorbent, coagulant and complexing agent, has been developed as an alternative to the RO membrane treatment of LLRWs [11]. Adsorption membrane characterized with specific functional groups on the surface of membrane, which can bind the nuclide by ion exchange or surface complexation, is a typical composite membrane and can remove target nuclides (e.g., Co2+, Sr2+, Cs+) from LLRWs effectively with higher water permeation than RO membrane [12,13]. Recently, metal-organic frameworks (MOFs) have shown potential as good adsorbents for iodine capture with high capacity, fast uptake, and good reusability [14]. ZIF-8 is a typical class of MOF material that is composed of imidazolate linkers and metal ions in zeolite topologies. It is selected for iodine adsorption due to its large surface area (1810 m2/g) and suitable pore aperture (3.4 Å). Yuan has investigated the 3
iodine adsorption of ZIFs in a multi-component system systematically and proved that ZIF-8 was the efficient and economical adsorbent with high diversity for iodine [15]. Similar conclusion was also confirmed by Bennett on the iodine adsorption and retention properties of ZIF-4, ZIF-8, ZIF-69 and ZIF-mnIm [16]. Nenoff’s group has suggested that iodine adsorption of ZIF-8 was related to the favorable interaction between iodine and 2-methylimidazole linker in the framework and found that 5.4 iodine molecules could be captured by each cage of ZIF-8 [17]. In a follow-up study, the strong chemisorption of iodine with ZIF-8 cage was identified by aqueous solution calorimetry, which is driven by the ideal confinement conditions within ZIF-8 cages, and it was found that ZIF-8 could bind iodine 4 time more strongly than activated carbon [18]. However, the further practical application of ZIFs materials to remove radioiodine in water still faces several hurdles. Firstly, a large mass of radioelement-contaminated solid adsorbents are generated during the desalination process. Secondly, nanomaterials used as adsorbents tend to aggregate easily under some conditions, which will discount the natural and advantageous physicochemical properties of ZIFs. polymer/ZIF-8 nanocomposite membrane, which shows superiorities in improving robustness and enhancing deliverability and also in development as a continuous flow-through unit for the repetitive treatment of iodine wastewater, seems to be a feasible solution and attracts attention of researchers [19]. Recently, some novel methodologies have been reported for preparing polymer/ZIF-8 nanocomposite membrane. Lei et al. have reported a novel layer-by-layer fabrication to prepare a 4
polyamide (PA)/ZIF-8 nanocomposite membrane with a multilayer structure by growing ZIF-8 nanoparticles interlayer on a porous surface of a polymer membrane through in situ growth and then coating it with an ultrathin PA layer through interfacial polymerization [20]. Stark et al. have proposed a membrane synthesis method by restricting ZIF-8 growth in the pores of poly(ether sulfone) (PES) pores by exploiting directed ZnO seed-nanoparticles to prepare PES/ZIF-8 composite membranes [21]. Although the in situ growth is a simple method allowing the simultaneous nucleation and crystal growth of ZIF-8, it is not effective in preparing continuous ZIF-8 layer due to the limited heterogeneous nucleation sites on polymer membrane. In addition, Wang’s group reported a contra-diffusion synthesis method for the intergrowth of ZIF-8 layer on a flexible nylon substrate [22]. This method depends on the surface properties and porous structure of support, and the ZIF-8 film can grow on both sides of support or within porous channels of support [23]. Poly(vinylidene fluoride) (PVDF) is characterized with high mechanical strength, thermal stability, chemical resistance and high hydrophobicity, and is widely used as a membrane material in various membrane separation processes. Khataee’s group has modified the PVDF membranes with different size of ZIF-8 nanocrystals to prepare PVDF/ZIF-8 mixed matrix membranes and found that the modified membranes showed higher water flux compared to the unmodified PVDF owing to their higher porosity
and
selective
pore
size
of
ZIF-8
nanoparticles
[24].
It is commonly recognized that surface modification of PVDF membranes by organic molecular, such as (3-aminopropyl)triethoxysilane, polydopamine, and ethylenediami 5
ne, is an effective method to solve the interfacial binding problem between continuous ZIF-8 film and PVDF membrane and to promote the heterogeneous nucleation of ZIF -8 crystals on its surface [25-27]. Polymer brushes were reported can be used as macromolecular primers to effectively promote the heterogeneous nucleation and crystalline growth of ZIF-8 thin films [28]. In theory, the use of polymer brushes to modify PVDF membrane substrate can also promote the heteromorphic nucleation and growth of ZIF-8 on the PVDF membrane. In our previous work the porous PVDF membrane was first in-situ grafted polymerization of 4-vinylpyridine (4VP) using SI-AGET ATRP, and the pyridine functional groups of poly(4-vinylpyridine) (P4VP) can be a driving factor for maintaining a high concentration of zinc ions selectively near the surface of PVDF support membrane [29]. When combined with the manipulation of the sodium formate to ligand ratios, this approach can lead to continuous ZIF-8 layer on the PVDF support membrane. Therefore, combination of ZIF-8 crystals and P4VP polymer brushes modified PVDF constituting PVDF/ZIF-8 nanocomposite membrane can be predicted to show a positive effect on the iodine ions removal from aqueous solution. In this work, we report a simple and effective method for preparation of well-intergrowth and continues ZIF-8 layer with controllable location on a PVDF substrate based on P4VP polymer brushes modified PVDF support membrane in conjunction with contra-diffusion method. The morphologies of PVDF/ZIF-8 nanocomposite membrane are characterized, and the corresponding iodine removal behavior from iodine aqueous solution including batch adsorption and dynamic 6
adsorption are evaluated. Adsorption kinetics, mechanical properties, regeneration and reusability of the PVDF/ZIF-8 nanocomposite membrane are investigated.
Scheme 1. (A) Preparation of PVDF/ZIF-8 nanocomposite membrane and (B) Schematic illustration
of iodine removal
from
water using
PVDF/ZIF-8
nanocomposite membrane. 2. EXPERIMENTAL SECTION 2.1. Reagents and materials PVDF membranes (average pore size of 0.22 μm) were purchased from Merck Millipore. 4VP (96%) was purchased from Alfa Aesar and distilled under negative pressure over NaOH pellets and then stored at -18 °C under Ar. LiOH•H2O (98%), NaBH4
(98%),
diisobutylaluminium
hydride
(DIBAL-H,
1M
in
hexane),
N,N,N',N',N"-pentamethyldiethylenetriamine (PMDETA, 99%), 2-bromoisobutyryl bromide (98%), ethyl 2-bromobutyrate (99%), CuBr2 (99.95%), ascorbic acid (99.99%), Zn(NO3)2•6H2O (99%), 2-methylimidazole (mIm, 99%) and sodium 7
formate (99%) were purchased from Aladdin Chemical Co., Ltd, China and used without further purification. Iodine (99.8%) and potassium iodine (KI, 99.9%) were purchased from Xi-Long Scientific Co., Ltd, China. Bovine serum albumin (BSA, Mw: 67 kDa) was obtained from Aladdin reagent Co., Ltd. (Shanghai, China). All the other reagents and organic solvents, unless under specific illumination, were purchased from Sinopharm Chemical Reagent Co., Ltd. and used without any further purification. Deionized (DI) water was purified by Milli-Q integral water purification system with a resistivity of 18.2 MΩ/cm. 2.2. Preparation of PVDF-g-P4VP membranes and PVDF/ZIF-8 nanocomposite membranes Due to the nonpolarity of PVDF which cannot provide the heterogeneous nucleation sites for the well-intergrowth of ZIF-8 adsorbent layer, P4VP brushes were grafted firstly onto PVDF membrane via surface-initiated atom transfer radical polymerization with activators generated by electron transfer (SI-AGET ATRP) method. The pyridine groups on P4VP side chains can coordinate to the free zinc ions and provide a large number of nucleation sites. Subsequently, the modified PVDF membrane denoted as PVDF-g-P4VP was employed as the support for the growth of ZIF-8 layer via contra-diffusion method following a previous report, and well-controlled crystal growth in the vicinity of the membrane surface can be obtained with slow diffusion of ligand in contra-diffusion process. Typically, the PVDF-g-P4VP membrane was mounted on a homemade setup, where the zinc acetate solution and mIm solution were separated by the supporting membrane. Zinc acetate 8
solution was prepared by dissolving 0.7348 g Zn(NO3)2•6H2O in 100 mL methanol, and mIm solution was prepared by adding 1.6223 g mIm and 0.9022 g sodium formate in 100 mL methanol. After crystallization of ZIF-8 on the surface of PVDF-g-P4VP membrane for a predetermined time at room temperature, the nanocomposite membranes denoted as PVDF/ZIF-8 were taken out and rinsed with deionized water several times, and finally, were dried in ambient conditions for 12 h. 2.3. Measurement of the adsorption kinetics The batch adsorption experiments were conducted in glass vials containing 100 mL iodine solution with an iodine concentration of 0.25 mmol/L and 8.55 cm2 PVDF/ZIF-8 nanocomposite membranes to evaluate the adsorption capacity for iodine removal from aqueous solution. The glass vials were kept on shaker for 33 h at room temperature. Then the PVDF/ZIF-8 nanocomposite membrane samples were taken out and the concentration of residual iodine in the solution was determined using UV-vis spectrophotometer. For ZIF-8 crystals adsorption experiments, ZIF-8 particles were outgassed under vacuum at 120 °C for 12 h before the iodine ions adsorption and at a certain time point, a small amount of mixture was taken out and filtered through a 0.45 μm porous membrane for UV-vis measurement. The adsorption capacity (qe, mg/g) was calculated as Eq. (1): qe
(c0 ce ) V M
(1)
Where c0 and ce are the initial and equilibrium iodine concentration of aqueous solution respectively, and V is the volume of iodine solution, and M is the weight of dried membrane. 9
2.4. Dynamic adsorption with iodine aqueous solution of PVDF/ZIF-8 nanocomposite membranes The dynamic adsorption experiments were conducted with a dead-end stirred cell (Stirred Cell 8200, Millipore, USA). Iodine solution with an initial concentration of 0.25 mmol/L was used to evaluate the iodine capture capacity of PVDF/ZIF-8 nanocomposite membrane in a dynamic adsorption with a feed pressure of 40 KPa. The iodine solution is 250 mL and the effective area of the PVDF/ZIF-8 nanocomposite membrane is 8.55 cm2. The filtration system was equilibrated with deionized water prior to the experiment. The filtrate was collected and its weight was determined using an electronic balance. The iodine concentration in filtrate was determined by a UV-vis spectrophotometer. The capture capacity for iodine of the membrane was calculated from the change in iodine concentration before and after dynamic adsorption. The water permeation flux (J, L/(m2•h •MPa) of the PVDF/ZIF-8 nanocomposite membrane was calculated as Eq. (2): J
V At P
(2)
Where V is the permeation amount collected under operating pressure ΔP on a time scale t and A is the effective area of the membrane. The iodine removal efficiency (R, %) was calculated as Eq. (3):
R ( 1
cp cf
) 1 0 0 %
(3)
Where cp and cf represent the iodine concentration of filtrate and feed solution, respectively. 2.5. Iodine release and recycling test of PVDF/ZIF-8 nanocomposite membrane 10
The saturated PVDF/ZIF-8 nanocomposite membranes from the iodine adsorption experiments were further used in the iodine release and regeneration studies. I2@PVDF/ZIF-8 nanocomposite membranes were immersed in 10 mL of ethanol (EtOH) at room temperature. To ensure the completion of iodine release, the samples were left in EtOH for 24 h. The iodine adsorption efficiency of regenerated PVDF/ZIF-8 membrane was acquired using dynamic adsorption approach described before. Samples were analyzed for iodine concentration using UV-vis spectroscopy for all experiments. 2.6. Antifouling test A 1.0 g/L protein solution was prepared by dissolving BSA in PBS (phosphate buffer solution, pH=7.2-7.4). In the antifouling test, after DI water was first used as the feed solution of permeation and then displaced by the 1.0 g/L BSA solution, and the Jp was recorded until a steady flux. After protein solution permeation, the membrane was washed with DI water to remove the reversible adsorbed protein. Then, the DI water permeation for the washed membrane was performed again. The flux recover ratios (FRw) of PVDF/ZIF-8 nanocomposite membrane was obtained as Eq. (4): FRw
J w2 100% J w1
(4)
where Jw1 and Jw2 are the steady pure water flux through membrane before and after protein solution permeation, respectively. The permeation flux was resulted from both reversible and irreversible protein fouling, which were defined as Eq. (5) and Eq. (6):
11
Rr Rir
J w2 J p
100%
(5)
J w1 J w 2 100% J w1
(6)
J w1
where Rr and Rir denote the reversible and irreversible iodine removal efficiency, respectively. 2.7. Characterization The surface chemical compositions of the membranes were characterized through ATR-FTIR spectroscopy (Excalibur 3100, Varian), each spectrum was captured by 32 averaged scans at a resolution of 4 cm-1. The membrane morphology was examined by a field-emission scanning electron microscope (SEM, FEI Quanta 200 FEG) with 10 kV accelerating voltage on gold sputter-coated samples. Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku D/Max-IIIc using Cu-Ka radiation. The iodine absorbance was recorded using a UV-vis spectrophotometer at 288 nm (UV-2700, Shimadzu). X-ray photoelectron spectra were obtained on an ESCALAB 250Xi spectrometer with a monochromic X-ray source and the charging shift was corrected by the binding energy of C 1s at 284.6 eV. Mechanical properties of the pristine PVDF, PVDF/ZIF-8 nanocomposite membrane were analyzed by a tensile tense machine (INSTRON 5569, UK). The testing speed was set at a loading velocity of 5 mm/min at ambient temperature. All samples were prepared with a width of 5 mm and a length of 30 mm. Each test was repeated for three times. The quantitative surface roughness analysis of the PVDF/ZIF-8 nanocomposite membrane was measured using Atomic-force microscopy (AFM, Dimension Icon, Bruker) with the 12
spring constant of 0.1 N/m through the contact mode in dry air. All the membrane samples were dried for 12 h in vacuum before the AFM analysis. Contact angle of water droplets on the samples were measured by contact angle analyzer (OCA-15EC, Dataphysics). DI water was deposited on the surface of dried membrane, and images were obtained with water spreading over the membrane. The reported contact angle here was the average value of three locations in one sample. 3. RESULTS AND DISCUSSION 3.1. Fabrication and characterization of PVDF/ZIF-8 nanocomposite membranes The pyridine functional groups on PVDF-g-P4VP membrane were confirmed by ATR-FITR spectra (Fig. S1) and XPS spectra (Fig. S2). ATR-FTIR spectra show the additional peaks at 1596 cm-1, 1556 cm-1, 1494 cm-1, and 1413 cm-1, which are attributed to the vC=C and vC=N stretching at the pyridine ring of grafted 4VP. XPS spectra also show an additional N 1s signal around 398.7 eV, which indicates the successful graft of 4VP onto the PVDF membrane surface. SEM images (Fig. S3) show obvious decrease in the pore size at the top surface of PVDF membrane after graft with P4VP. After the modification of PVDF membrane, ZIF-8 layer can be formed on the PVDF-g-P4VP support membrane by contra-diffusion method. Unlike the conventional contra-diffusion method where the crystals grow on the both side of support, it is interesting to observe that the ZIF-8 crystals grow only on the metal ions surface side of PVDF-g-P4VP membrane in our experiment as verified in the XRD pattern (Fig. 1a). SEM images (Fig. 1b and Fig. 1d) further confirm that a dense and 13
continuous ZIF-8 layer of rhombic dodecahedron morphology with a thickness about 5 μm is formed on the metal ions side of PVDF-g-P4VP membrane. At the same time, no ZIF-8 crystal particles are found on the surface of mIm ligand side of PVDF-g-P4VP membrane (Fig. 1c). In a typical contra-diffusion method, it is difficult to control precisely the position of heterogeneous nucleation and growth of ZIF-8 crystals on the support. Different to the growth of high-quality ZIF-8 layer only on the surface side of ethylenediamine pretreated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) substrate by Shamsaei [30], P4VP polymer brushes were grafted onto the both side of PVDF membrane in our experiment and therefore the pyridine groups on both side of the surface of PVDF membrane are not the main factor controlling the growth position of ZIF-8. Due to the high mIm concentration gradient in mIm side, mIm can slowly diffuse through the PVDF-g-P4VP membrane and then react with zinc ions rapidly, resulting in the enrichment on the other surface side of membrane grafted by P4VP and in-situ growth of ZIF-8 crystals on the zinc nitrate side [31,32]. Therefore, the Zn/mIm molar ratio and solution concentration of reactants on both sides of PVDF-g-P4VP membrane are maybe the key factors determining the growth position of ZIF-8 layer.
14
Fig.1. XRD pattern (a) and SEM images (metal ions side (b), mIm side (c), and Zn2+ side cross-section (d)) of the PVDF/ZIF-8 nanocomposite membrane prepared without sodium formate at room temperature for 24 h. Furthermore, the mechanical properties including tensile strength, Young’s module and breaking elongation of the pristine PVDF and PVDF/ZIF-8 nanocomposite membranes are showed in Table 1. It is obvious that the tensile strength and breaking elongation of the PVDF/ZIF-8 nanocomposite membrane decrease with the growth of ZIF-8 layer on it. The maximum tensile strength and breaking elongation of PVDF/ZIF-8 nanocomposite membranes are 6.40 MPa and 20%, respectively. This confirms that the formation of ZIF-8 layer on the PVDF membrane has reduced the mechanical properties of the PVDF composite membrane resulting from the increase in brittleness and decrease in flexibility of the PVDF membrane by continuous and dense rigid ZIF-8 layer. Table 1. The mechanical properties of the PVDF and PVDF/ZIF-8 nanocomposite 15
membranes prepared by contra-diffusion method. Membrane samples Pristine PVDF membrane PVDF/ZIF-8 nanocomposite membrane
Tensile strength (Mpa)
Young’s module (Mpa)
Breaking elongation (%)
9.37 6.40
10.50 32.41
89 20
In the process of contra-diffusion synthesis of ZIF-8 layer, the crystallization reaction and diffusion occur simultaneously. Theoretically, the crystallization reaction should be faster than the diffusion so that the heterogeneous nucleation and growth can happen before the metal ions are depleted from the support [33]. When the ZIF-8 layer is prepared on the PVDF support membrane by contra-diffusion method, it is an effective method to control the uniformity and continuity of ZIF-8 layer by adjusting the synthesis time. Recently, Khataee’s group reported that a uniform and continuous layer of ZIF-8 nanocrystals were successfully formed on the PVDF support surface by increasing synthesis time to 5 h [34]. However, in this study, we mainly studied the effect of changing the molar ratio of ligand to sodium formate on the ligand solution side on the microstructure of ZIF-8 layer on PVDF membrane. In fact, it was reported that sodium formate (SF) could enhance the crystallization reaction rate leading to the formation of ZIF-8 layer [35-37], and here the effect of the molar ratio of ligand to sodium formate on the ZIF-8 layer microstructure formed on the PVDF-g-P4VP membrane is investigated. Fig. 2 displays the SEM images of the ZIF-8 layer prepared at different molar ratio of ligand to sodium formate. As shown, poorly intergrown layer composed of rhombic decahedron particles covers the Zn side surface but few particles can be observed on the ligand side surface when Zn/mIm/SF 16
molar ratio is 1:8:0.672 (Fig. 2a, 2b and 2c). However, well-intergrown and continuous ZIF-8 layer is formed on the Zn side when Zn/mIm/SF molar ratio increases to 1:8:1.344 (Fig. 2d, 2e and 2f). Further increasing Zn/mIm/SF molar ratio to 1:8:2.688, ZIF-8 layer becomes uneven and the thickness is about 3 μm on the Zn side surface of membrane. In addition, the XRD peak intensities of the ZIF-8 on Zn side are stronger than that on mIm solution side, which confirms the former observation that ZIF-8 layer is mainly formed on the metal ions side surface of PVDF-g-P4VP membrane. These results indicate that the addition of sodium formate to the ligand solution can effectively control the microstructure of the ZIF-8 layer when the Zn/mIm molar ratio is constant. With the increasing addition of sodium formate, ZIF-8 layer becomes more continuous and well-intergrown. The weakly alkaline sodium formate is a proton accepter/scavenger and is expected to play a critical role in deprotonating the bridging mIm ligand and facilitating the heterogeneous nucleation as well as the intergrowth of ZIF-8 crystals on support [38,39]. The addition of sodium formate will increase the solution pH value and decrease the protons concentration in solution, which can shift the equilibrium and thereby drive the deprotonation of mIm ligands. In the meantime, P4VP (pKa=4.7) is a weakly alkaline substance with pH sensitive property. When the PVDF-g-P4VP membrane equilibrate in a weakly alkaline solution, the deprotonated pyridine groups on the P4VP side chain can react with Zn ions, and in turn promote Zn ions to maintain in the vicinity of the metal ions side surface of PVDF-g-P4VP membrane with a relatively high concentration. Because the coordination reaction rate of ZIF-8 17
is faster than the diffusion rate of mIm ligand, it will lead to the fast formation of ZIF-8 nuclei and subsequent crystal growth on the Zn side surface of membrane, and finally the well-intergrown and continuous ZIF-8 layer.
Fig. 2. SEM images of ZIF-8 layer prepared with Zn/mIm/SF molar ratios of 1:8:0.672 (a,b,c), 1:8:1.344 (d,e,f), and 1:8:2.688 (g,h,i) at room temperature for 24 h. (a)
(b )
Zn/mIm/SF=1:8:2.688
Zn/mIm/SF=1:8:1.344
Intensity (a..)
Intensity (a..)
Zn/mIm/SF=1:8:2.688
Zn/mIm/SF=1:8:1.344
Zn/mIm/SF=1:8:0.672
Zn/mIm/SF=1:8:0.672
Simulated ZIF-8
Simulated ZIF-8 5
10
15
20
25
30
5
10
15
20
25
30
2 theta (degree)
2 theta (degree)
Fig. 3. XRD patterns of ZIF-8 layer prepared on PVDF-g-P4VP membranes with
18
varying Zn/mIm/SF molar ratios: The Zn side (a) and the mIm solution side (b). 3.2. Batch adsorption of PVDF/ZIF-8 nanocomposite membranes The potential application of PVDF/ZIF-8 nanocomposite membrane for iodine adsorption from aqueous solution was investigated. The iodine adsorption kinetics by the PVDF/ZIF-8 nanocomposite membrane is shown in Fig. 4. It can be seen that the saturation adsorption capacity of iodine on PVDF/ZIF-8 nanocomposite membrane is 73.33 mg/g. The initial fast adsorption is mainly ascribed to the quick uptake of iodine ions by the ZIF-8 layer located at the surface of PVDF/ZIF-8 nanocomposite membrane. The subsequent slow adsorption is attributed to the slow diffusion of iodine ions through the mesopores of the ZIF-8 particles. The interaction between iodine ions and PVDF/ZIF-8 nanocomposite membrane can be estimated by the adsorption kinetics. Langmuir first-order and pseudo-second-order models are used to investigate the adsorption kinetics and the corresponding kinetic data and kinetic parameters are illustrated in Fig. 5 and Table 2, respectively. It can be seen that the pseudo-second-order kinetic model (R2>0.970) is suitable for the iodine adsorption process with PVDF/ZIF-8 nanocomposite membrane. Compared with the adsorption saturation of ZIF-8 crystals in 300 min, the iodine adsorption of PVDF/ZIF-8 nanocomposite membrane reaches saturation up to 1500 min, indicating that the iodide ions adsorption rate in aqueous solution by PVDF/ZIF-8 nanocomposite membrane is significantly slower than that of ZIF-8 crystals. It is also can been seen that the saturated adsorption capacity of ZIF-8 crystals and PVDF/ZIF-8 nanocomposite membrane is 81.80 mg/g, and 73.33 mg/g under the same original 19
adsorbent mass, respectively. The above experimental results indicate that, for the static iodine adsorption, ZIF-8 crystals formed on the PVDF membrane will significantly decrease the adsorption rate with a slightly decreased iodine adsorption capacity compared with the ZIF-8 crystals. (a)
80
(b) 80
70 60
qe (mg/g)
qe (mg/g)
60
50 40 30 20
40
20
10 0
0 0
500
1000
1500
0
2000
50
100
150
200
250
300
Time (min)
Time (min)
Fig. 4. Effect of adsorption time on the iodine ions adsorption of PVDF/ZIF-8 nanocomposite membrane (a) and ZIF-8 crystals (b) in a batch system.
b 30
a
25 20
1
t/qt
log(qe-qt)
2
15 10
0
5 -1
0 0
800
1600
0
500
1000
1500
2000
Time (min)
Time (min)
Fig. 5. The linear fitted curves of Langmuir first-order model (a) and pseudo-second-order kinetics model (b) of PVDF/ZIF-8 nanocomposite membrane. Table 2. Kinetic parameters of iodine ions adsorption by PVDF/ZIF-8 nanocomposite membrane from aqueous solutions. qe,exp (mg/g) 73.36
Lagergren-first-order
Pseudo-second-order
qe,cal (mg/g)
k1 (min-1)
R2
qe,cal (mg/g)
k2 (min g/mg)
R2
106.89
0.003
0.932
95.60
0.00002
0.970
20
In order to explain the mechanism of iodine adsorption with PVDF/ZIF-8 nanocomposite membrane, the adsorption equilibrium isotherm was studied. In this study, the iodine ions adsorption equilibrium data of the PVDF/ZIF-8 nanocomposite membranes are fitted by Langmuir and Freundlich isotherm models, and the results are presented in Fig. 6 and Table 3. Taking the correlation coefficient as the criterion of fitting degree, the Freundlich isotherm model shows better correlation (R2=0.987) than the Langmuir isotherm model and therefore Freundlich isotherm model can represent the iodine adsorption process with PVDF/ZIF-8 nanocomposite membrane. (b)
(a)
6.4
0.07 6.2
0.06
lnqe
Ce/qe
6.0
0.05
5.8
0.04
5.6
0.03
5.4
5
10
15
20
25
30
35
2.0
40
2.4
2.8
3.2
3.6
lnCe
Ce(mg/L)
Fig. 6. Langmuir isotherm plot (a) and Freundlich isotherm plot (b) for iodine ions adsorption of PVDF/ZIF-8 nanocomposite membrane. Table 3. Langmuir and Freundlich isotherm parameters for PVDF/ZIF-8 nanocomposite membrane. Langmuir isotherm parameters
Freundlich isotherm parameters
qmax (mg/g)
KL (mL/mg)
R2
KF (mg/g)
n
R2
79.365
0.550
0.942
88.352
2.0
0.987
3.3. Dynamic adsorption with iodine aqueous solution of PVDF/ZIF-8 nanocomposite membranes 21
To further investigate the separation capability of PVDF/ZIF-8 nanocomposite membranes for iodine removal from aqueous solution in a more practical continuous operation way, the dynamic adsorption of PVDF/ZIF-8 nanocomposite membranes was conducted at the operating pressure of 0.04 MPa. As shown in Fig. 7a, with the dynamic adsorption processing to 180 min, the water permeation through the PVDF/ZIF-8 nanocomposite membrane decreases from 86.57 to 66.19 L/(m2•h•MPa) and the iodine ions removal efficiency increases from 53% to 73%, which can be ascribed to the decreasing pore size of ZIF-8 crystals and increasing thickness of membrane. In addition, it is interesting to note that the ZIF-8 layer on the surface of nanocomposite membrane seems to be disappeared after the dynamic adsorption, which is confirmed by the XRD characterization of PVDF/ZIF-8 nanocomposite membrane as show in Fig. 7c. The surface morphology of PVDF/ZIF-8 nanocomposite membrane before and after the dynamic adsorption are also shown in Fig. 7b and Fig. 7d, respectively. In order to confirm the chemical state of adsorbed iodine in PVDF/ZIF-8 nanocomposite membrane, the corresponding XPS spectrum was obtained. As seen in Fig. 7f, the iodine spectrum displays two split peaks located at 630.6 and 618.8 eV, which are assigned to the I 3d3/2 and I 3d5/2 orbitals of iodine molecules, respectively. In addition, the I 3d5/2 peak shows two peaks at 618.8 and 619.3 eV, which are attributed to the I3- and I5- anions, respectively [40]. It should be noted that both I3- anions and neutral I2 could form polyiodide. During the ultrafiltration process, the iodine aqueous solution permeates through the ZIF-8 layer and the ZIF-8 crystals are gradually decomposing by water molecules under pressure, 22
which finally leads to the irreversible collapse of ZIF-8 crystalline framework. The possible mechanism for PVDF/ZIF-8 nanocomposite membrane with iodine removal is most likely due to the formation of a charge transfer interaction between iodine molecules and the electron-rich ZIF-8.
Fig. 7. Dynamic adsorption performance of the PVDF/ZIF-8 nanocomposite membrane (a), SEM image of PVDF/ZIF-8 nanocomposite membrane before dynamic 23
adsorption (b), XRD patterns of PVDF/ZIF-8 nanocomposite membrane before and after dynamic adsorption (c), SEM image of PVDF/ZIF-8 nanocomposite membrane after dynamic adsorption (d), XPS survey spectrum of PVDF/ZIF-8 nanocomposite after dynamic adsorption (e), XPS spectrum of I 3d after dynamic adsorption (f). It was reported that ZIF-8 crystal was sensitive to the solution pH. Wu et al. have reported that ZIF-8 crystal is relatively stable in the solution pH range of 5.0-12.0, but will gradually disintegrate when the solution pH is less than 4.0 [41]. Therefore, the effect of the initial iodine concentration as well as solution pH on the iodine ions removal performance of PVDF/ZIF-8 nanocomposite membrane need to be further studied. As shown in Fig. 8a, as the initial iodine ions concentration of aqueous solution increases from 0.061 to 0.2374 mmol/L, the permeation flux remains substantially stable from 36.78 to 35.65 L/(m2•h•MPa), while the iodine removal efficiency decreases from 79.65% to 72.88%. As shown in Fig. 8b, the iodine ions removal efficiency of PVDF/ZIF-8 nanocomposite membrane mainly maintains around 92% at pH=8 but rapidly decreases from 49.67% to 6.38% at pH=3. When ZIF-8 crystals are exposed in acidic aqueous solution, hydrated protons will attack ZIF-8 framework structure and the mIm linkers become protonated, which in turn breaks the Zn-mIm bonds and carves those external crystal surfaces with the high density of Zn-mIm bonds [42]. Therefore, the irreversible destruction of continuous and dense ZIF-8 layer in nanocomposite membrane weakens the iodine removal ability. Besides, the high iodine removal performance of PVDF/ZIF-8 nanocomposite membrane at high pH can be explained by the relationship between pH and zeta 24
potential of ZIF-8. The point of zero charge (pHzpc) of ZIF-8 is approximately occurred at pH=9.3 [43], and this implies that the charge of ZIF-8 crystal is positive at pH range of 5-10 [44]. At pH=8, iodine mainly exists as I2 and I3- with negative charge in solution and the electrostatic attraction between iodine ions and ZIF-8 becomes a critical factor in the removal mechanism, which maintains the removal efficiency at a high level. On the other hand, ZIF-8 crystals are easily decomposed in acidic solution. After filtration of PVDF/ZIF-8 nanocomposite membrane with strong acid iodine solution, the ZIF-8 layer on the surface of PVDF supporting membrane completely disappeared, indicating that ZIF-8 crystals have been completely decomposed by acidic aqueous solution.
70
60
60
50
50
40
40
30
30
20
20
10
10
0
0
0.1217
0.1824
-2
90
Removal efficiency (%)
70
0.061
(b) 100
80
Removal efficiency (%)
P e rm e a n ce R e m o va l e fficie n cy
80
-1
-1
Permeance (Lm h MPa )
(a)
80 pH=8 pH=3
70 60 50 40 30 20 10 0
0.2374
0
C0(mmol/L)
50
100
150
200
250
Time (min)
Fig. 8. Effect of initial iodine concentration (a) and solution pH (b) on the iodine removal efficiency of PVDF/ZIF-8 nanocomposite membranes. 3.4. Regeneration and reusability of PVDF/ZIF-8 nanocomposite membrane The regeneration and reusability are important parameters for a membrane in practical application and are studied here. The encapsulated iodine could be removed from the ZIF-8 framework with the immersion of I2@PVDF/ZIF-8 nanocomposite membrane in organic solvents. In this study, it is found that once the I2@PVDF/ZIF-8 25
nanocomposite membrane was soaked in fresh EtOH at room temperature, the color of EtOH solution would deepen from colorlessness to light yellow quickly with time, which clearly indicates that iodine could be easily desorbed from the nanocomposite membranes. The absorbance of the desorbed iodine solution with time from I2@PVDF/ZIF-8 nanocomposite membrane at room temperature was recorded using UV-vis as shown in Fig. 9a and the corresponding iodine desorption percentage based on the calibration curve of iodine concentration in EtOH was shown in Fig.9b. It is can been seen that the desorbed iodine amount from I2@PVDF/ZIF-8 nanocomposite membrane increases almost linearly with time within 120 min and the highest iodine desorption percentage is 37.72 wt.% at 240 min on the basis of the total captured iodine amount by PVDF/ZIF-8 nanocomposite membrane.
(b) 40 Release Percentage (%)
(a) 1.8
Absorbance
1.6
1.4
1.2
1.0
35
30
25
20 0.8 0
50
100
150
200
250
0
50
100
150
200
250
Time (min)
Time (min)
Fig. 9. Iodine desorption of I2@PVDF/ZIF-8 nanocomposite membranes in EtOH: absorbance of desorbed iodine solution (a) and iodine release percentage (b). The regenerated PVDF/ZIF-8 nanocomposite membranes were further reused for the next round of iodine dynamic adsorption to test its reusability. Fig. 10 a shows the
26
recycled adsorption ability of the PVDF/ZIF-8 nanocomposite membrane. It is can been seen that the iodine removal efficiency still retains 73.4% after five cycles. The dead-end dynamic adsorption was further employed to investigate the antifouling performance of PVDF/ZIF-8 nanocomposite membranes including permeance flux, Rir and Rr. Fig. 10b shows the time-dependent permeation flux of PVDF/ZIF-8 nanocomposite membranes with the feed of water and protein in sequence. The FRw of PVDF/ZIF-8 nanocomposite membrane can be calculated and is equal to 82.51%, which indicates that the PVDF/ZIF-8 nanocomposite membrane can still possess a relative high permeation flux after the protein contamination. In addition, the Rir and Rr of the total fouling is 17.49% and 4.82%, respectively. The multiple factors can be responsible for this phenomenon, including the high hydrophilicity (the water contact is 58.52° as shown in Fig. 10c) and the smooth surface (Rq=0.994 nm as shown in Fig. 10d) of PVDF/ZIF-8 nanocomposite membranes.
27
Fig. 10. Recycled adsorption ability of PVDF/ZIF-8 nanocomposite membrane for dynamic adsorption of iodine solution (a), Permeation flux of PVDF/ZIF-8 nanocomposite membranes with the feed of water and protein in sequence (b), Water contact of the PVDF/ZIF-8 nanocomposite membrane (c), AFM image of the PVDF/ZIF-8 nanocomposite membrane (d). 4. CONCLUSIONS In summary, the work demonstrated a simple and effective method including contra-diffusion method in conjunction with P4VP grafted modification to prepare a well-intergrown and continuous PVDF/ZIF-8 nanocomposite membrane with controllable location. The ZIF-8 crystals grow only on the metal ions side surface of PVDF-g-P4VP membrane and the optimum Zn/mIm/SF molar ratio is 1:8:1.344. The maximum iodine adsorption capacity of PVDF/ZIF-8 nanocomposite membrane is 73.33 mg/g in the batch adsorption and the removal efficiency is high as 73% till to 180 min in the dynamic iodine adsorption. The iodine removal efficiency of PVDF/ZIF-8 nanocomposite membrane slightly decreases with the increase of initial iodine concentration. Weakly alkaline solution has positive effect on the iodine removal of PVDF/ZIF-8 nanocomposite membrane, around 92% at pH=8, while acidic solution shows the opposite. About 37.72 wt.% of adsorbed iodine in PVDF/ZIF-8 nanocomposite membrane can be desorbed until 240 min. The iodine removal efficiency of PVDF/ZIF-8 nanocomposite membrane is still high as 73.4% after five cycles. 5. ASSOCIATED CONTENT 28
Supporting Information ATR-FTIR spectrum of PVDF-g-P4VP membrane; XPS spectrum of PVDF-g-P4VP membrane; SEM images of surface morphology and cross section of PVDF-g-P4VP membrane; Preparation of PVDF/ZIF-8 nanocomposite membrane by contra-diffusion method; UV-Vis spectra of iodine in aqueous solution at room temperature and calibration curve of iodine; SEM image and EDX of PVDF/ZIF-8 nanocomposite membrane after dynamic adsorption of iodine solution at pH=8; XRD patterns of PVDF/ZIF-8 nanocomposite membrane after dynamic adsorption of iodine solution at pH=8. 6. AUTHOR INFORMATION Corresponding Author *Jian-Hua Qiu, E-mail: [email protected] *Juan-Tao Jiang, E-mail: [email protected] ORCID Jian-hua Qiu: 0000-0001-7285-3561 Author Contributions †Xing Long and Ya-Shuo Chen contributed equally to this work.
Notes The authors declare no competing financial interest. 7. ACKNOWLEDGMENTS This work was supported by Nature Science Foundation of China (No. 21868009), Natural Science Foundation of Guangxi Province (No. 2018GXNSFDA281022), Innovation Project of Guangxi Graduate Education (XYCSZ2019057). 8. REFERENCES 29
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Highlights
1. PVDF/ZIF-8
nanocomposite
membrane
via
contra-diffusion
method
in
conjunction with P4VP grafted modification. 2. A well-intergrown and continuous ZIF-8 layer can be formed on the surface of PVDF-g-P4VP membrane at Zn/mIm/SF molar ratio of 1:8:1.344. 3. The maximum iodine adsorption capacity of PVDF/ZIF-8 nanocomposite membrane is 73.33 mg/g in batch adsorption. 4. The iodine removal efficiency of PVDF/ZIF-8 nanocomposite membrane is high as 73% until to 180 min in the dynamic adsorption.
Declaration of interests ☐√ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Author Statemen
Xing Long: Investigation, Writing-Original Draft Ya-Shuo Chen: Investigation, Writing-Original Draft, Visualization Qian Zheng: Validation, Formal analysis Xing-Xiao Xie: Investigation, Hao Tang: Investigation, Li-Ping Jiang: Resources Juan-Tao Jiang: Project administration, Writing-Review&Editing Jian-Hua Qiu: Conceptualization, Methodology, Supervision, Funding acquisition