Surface Decoration of Amino-Functionalized MetalâOrganic

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Surface decoration of amino-functionalized metal-organic framework/graphene oxide composite onto polydopamine coated membrane substrate for highly efficient heavy metal removal Zhuang Rao, Kai Feng, Beibei Tang, and Peiyi Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15873 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on December 31, 2016

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ACS Applied Materials & Interfaces

Surface decoration of amino-functionalized metal-organic framework/graphene oxide composite onto polydopamine coated membrane substrate for highly efficient heavy metal removal Zhuang Rao, Kai Feng, Beibei Tang* and Peiyi Wu*

State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Polymers and Polymer Composite Materials, Department of Macromolecular Science and Laboratory of Advanced Materials, Fudan University, Shanghai 200433, People’s Republic of China.

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ABSTRACT:

A

new

metal-organic

framework/graphene

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oxide

composite

(IRMOF-3/GO) with high adsorption capacity of copper (II) (maximal adsorption amount: 254.14 mg/g at pH 5.0, 25 oC) was prepared. Novel and highly efficient nanofiltration (NF) membrane can be facilely fabricated via surface decoration of IRMOF-3/GO onto polydopamine (PDA) coated polysulfone (PSF) substrate. After decoration of IRMOF-3/GO, membrane surface potential increased from 6.7 to 13.1 mV at pH 5.0, 25 oC. Due to the adsorption effect of IRMOF-3/GO and the enhancement of membrane surface potential, the prepared NF membrane (the loading amount of IRMOF-3/GO is ca. 13.6 g/m2) exhibits a highly efficient rejection of copper (II). The copper (II) rejection reaches up to about 90%, while maintaining a relatively high flux of about 31 L/m2/h at the pressure of 0.7 MPa and pH 5.0. Moreover, the membrane also presents an outstanding stability throughout 2000 min NF testing period. Thus, the newly developed NF membrane shows a promising potential for water cleaning. This work may provide a worthy reference for designing highly efficient NF membranes modified by metal-organic framework (MOF) relevant materials. KEYWORDS: nanofiltration, IRMOF-3/GO, polydopamine, rejection, water cleaning

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INTRODUCTION Heavy metal ions in water have a deleterious impact on the environment and public health. Therefore, it is urgent to find an appropriate technology to remove heavy metal ions in water.1,2 Conventional removal processes of heavy metal ions in water are electrolysis,3 chemical precipitation,4 ion exchange,5 membrane separation,6 adsorption,7 etc. Among them, membrane separation stands out owing to its high efficiency, space saving and environmental friendliness.8,9 Specially, nanofiltration (NF) is a very promising method to remove heavy metal ions in water due to its unique separation character. Satisfying rejection can be achieved by NF at relatively low pressure while maintaining relatively high flux.6,10 A NF membrane commonly consists of porous substrate and thin barrier layer. The construction of barrier layer onto porous substrate is very vital for NF membrane. UV-induced

grafting,11,12

electron

beam

irradiation,13

plasma-initiated

polymerization14,15 and interfacial polymerization16,17 are commonly used to form dense thin layer onto porous substrate. Recently, dopamine has exhibited its efficiency and versatility in construction of thin barrier layer onto porous substrate.18,19 A controllably thin and stable polydopamine (PDA) layer can be formed onto virtually all types of material surfaces via self-polymerization. Besides, some functional materials can be further decorated onto PDA layer. It provides more selections to endow membrane some good performances. Jiang et al. decorated poly(ethylene imine) (PEI) onto the surface of PDA deposited polyethersulfone (PES) 3



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membrane. The achieved NF membrane exhibited the rejections of 73.7, 57.1 and 96.5% to MgCl2, CaCl2 and cationic dyes, respectively.20 This performance was ascribed to the enhancement of surface potential by the introduction of PEI onto membrane surface. Actually, the property of modified material bonded by the PDA deposited layer is a significant factor predominating the resultant membrane performance. Jiang et al. fabricated NF membrane by modifying fluorinated polyamine onto PDA deposited PES membrane surface. The resultant NF membrane presented outstanding antifouling performance. Its flux recovery ratio reached 98.6%,21 mainly due to the promotion of membrane surface hydrophilicity by modification of fluorinated polyamine onto membrane surface. Xu et al. decorated β‑ FeOOH onto PDA-coated membrane substrate for purifying dye solution. The dye degradation efficiency of the membrane reached ~ 100%, which was attributed to the excellent photocatalytic performance of β‑FeOOH.18 Wei et al. decorated PEI onto the surface of PDA-coated nanofibrous membrane to remove Cu2+. The removal capacity of the resultant membrane increased by ~ 53%, compared to that of the pristine membrane, ascribed to the Cu2+ adsorption of PEI.22 Metal-organic frameworks (MOFs), composed of metal units and organic linkers23,24, have expanded many new applications owing to their tunable structure, high porosity, and large surface area.25-27 Particularly, MOFs demonstrate excellent adsorption property due to their ultrahigh surface area.28-31 However, their dispersive forces are not strong for retaining small molecules.32,33 This drawback restricts some of their practical adsorption applications. Besides, most of MOFs are not very stable in water,34 which limits their applications in water treatment. To overcome these drawbacks of MOFs and meet practical adsorption applications, preparing MOF 4


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hybrid materials with enhanced dispersive forces and improved water stabilities has attracted considerable attentions. MOF/graphene oxide (GO) composites are a kind of such hybrid materials that have exhibited potential prospects in water cleaning.35-37 They also displayed potential applications in membrane separation.38,39 Carboxyls, aminos, and thiols are selective adsorption groups for heavy metal ions due to strong affinity. They all displayed great promotion effect on the removal of heavy metal ions.40-42 Among them, amine groups have been testified to be the most effective groups for the removal of heavy metal ions from aqueous solution.43 Therefore, functionalization of MOF/GO composites with amino groups is supposed to be efficient approach to enhance the adsorption capacity of heavy metal ions from wastewater. IRMOF-3 consists of 2-aminoterephthalic acid linkers and ZnO4 clusters. The amino groups in its linkers can bind strongly with heavy metal ions, which is favorable to the removal of heavy metal ions. In the present study, GO was incorporated into the precursors of IRMOF-3 to improve the surface area, dispersive force and water stability of IRMOF-3. An amino-functionalized MOF/GO composite (IRMOF-3/GO) was firstly prepared. The adsorption capacity of IRMOF-3/GO to copper (II) was greatly improved, compared to that of pristine IRMOF-3. Moreover, IRMOF-3/GO showed good selectivity. However, the separation of IRMOF-3/GO from water was not very convenient. In contrast, NF membrane can be separated from water conveniently. Additionally, the water treatment efficiency of NF membrane was relatively high. So decoration of IRMOF-3/GO onto membrane surface can construct high-performance NF membrane, which combined with the advantages of IRMOF-3/GO and NF membrane. IRMOF-3/GO barrier layer onto membrane substrate endowed the resultant NF membrane significantly improved copper (II) 5



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rejection and selectivity. To the best of our knowledge, this is the first work regarding MOF/GO decorated NF membrane. This work provides a facile and effective approach for removing the heavy metal ions from wastewater and also a valuable reference to design NF membrane for water purification.

2 EXPERIMENTAL SECTIONS 2.1 Materials Natural graphite powers, Zn(NO3)2·6H2O, dopamine hydrochloride and tris(hydroxymethyl)aminomethane were provided by Aladin (Shanghai, China). Concentrated sulphuric acid, H2O2 solution (30 wt%) and dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. 2-aminoterephthalic acid was obtained from Sigma-Aldrich. Polysulfone (PSF) ultrafiltration (UF) membrane (molecular weight cut-off: 50 kDa) was purchased from XINLIMO Tech Co., Ltd., Beijing. All the chemicals were used without further purification. 2.2 Preparation Preparation of IRMOF-3: IRMOF-3 was prepared by solvothermal method44. Briefly, 0.4462 g Zn(NO3)2·6H2O and 0.0906 g 2-aminoterephthalic acid were dissolved in 50 mL DMF. Then, the solution was transferred to 100 mL autoclave and placed in an oven with a constant temperature of 100 oC for 24 h. The products were separated by centrifugation after the reaction mixture was cooled to room temperature. Then, the products were washed repeatedly with DMF and dichloroform, respectively. Finally, the products were dried in vacuum at 60 oC for 4 h and stored in a drying apparatus for further characterizations and applications. Preparation of IRMOF-3/GO composite: Hummers method was used to prepare GO. The procedures had been reported in previous works.45,46 Different amount of GO was added to the well dispersive Zn(NO3)2·6H2O/2-aminoterephthalic 6


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acid mixture. Then, GO was dispersed uniformly in solution by ultrasonication. The succedent steps were the same as the preparation of IRMOF-3. The final products were named as IRMOF-3/GO-x, where x denotes the weight percentage of the incorporated GO in the precursors of IRMOF-3 based on Zn(NO3)2·6H2O. The stability of IRMOF-3 in water was effectively improved by GO incorporation (Fig. S1). From Fig. S1, IRMOF-3 and IRMOF-3/GO composite can be regenerated by sodium edetate (EDTA-4Na) aqueous solution after their adsorptions. The GO amounts of the obtained IRMOF-3/GO-0.5, IRMOF-3/GO-1, IRMOF-3/GO-2 and IRMOF-3/GO-5 were about 0.3, 0.6, 1.1 and 2.7 wt% based on the amounts of IRMOF-3 in IRMOF-3/GO composites, respectively. The results were determined based on inductively coupled plasma-optical emission spectroscopy. The mechanism of the interaction between GO and the growing IRMOF-3 is shown in Scheme 1. Specifically, part of Zn2+ ions firstly bound with the oxygen functional groups of GO (e.g. epoxide, hydroxyl and carboxyl groups) owing to electrostatic attraction interaction. Then, the oxygen functional groups functioned as crystallization sites to promote crystallization of IRMOF-3. Finally, 2-aminoterephthalic acid and the remaining Zn2+ ions crystallized along the layers of GO till the whole crystallization process finished. Additionally, TGA results (Fig. S2) are used as suitable experimental evidence for the effect of GO in the composite.

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Scheme 1. Schematic of the interaction between GO and the growing IRMOF-3. Preparation of nanofiltration membrane: Tris buffer solution was prepared by dissolving 1.2114 g tris(hydroxymethyl)aminomethane in 1 L deionized water, then the pH was adjusted to 8.5 by adding 1 M HCl dropwise. Dopamine hydrochloride (2.0 g/L) was dissolved in the fore-mentioned tris buffer solution to obtain aqueous dopamine solution. After wiping off the water on the surfaces, the PSF UF membranes were immersed in the prepared aqueous dopamine solution and kept stirring of 160 r/min at room temperature for 24 h. Then, the membranes were taken out and washed with deionized water thoroughly. Finally, the membranes were stored in deionized water and named PSF@PDA. 25 mg, 50 mg and 75 mg IRMOF-3/GO-1 were added into three 200 mL tris buffer solutions, respectively. Then, the three mixtures were placed in ultrasound bath till they were dispersed uniformly. The 8


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PSF@PDA membranes were dipped into the three IRMOF-3/GO-1 solutions and kept stirring of 160 r/min at room temperature for 24 h, respectively. The subsequent treatments were the same as PSF@PDA. The resultant membranes were denoted as PSF@PDA@IRMOF-3/GO-1(25),

PSF@PDA@IRMOF-3/GO-1(50)

and

PSF@PDA@IRMOF-3/GO-1(75), respectively. In order to determine the loading amounts of IRMOF-3/GO-1 onto membrane surfaces, the nonwovens of membranes were removed, and then the remaining parts were dipped into aqueous HNO3 solutions (40% W/W), respectively. Finally, the mixtures were heated at 70 oC till the remaining parts of membranes were dissolved thoroughly. Inductively coupled plasma-optical emission spectroscopy was applied to analyze the amounts of Zn2+ ions of the resultant solutions. According to the tested amounts of Zn2+ ions, the amounts of IRMOF-3/GO-1 onto the surfaces of PSF@PDA@IRMOF-3/GO-1(25), PSF@PDA@IRMOF-3/GO-1(50) and PSF@PDA@IRMOF-3/GO-1(75) were 5.9 ± 0.2, 13.6 ± 0.6 and 20.3 ± 0.8 g/m2, respectively. The loading amount of IRMOF-3/GO-1 onto every membrane surface was the average value of two experimental results. Additionally, the errors were absolute deviations. 2.3 Characterization X-ray diffraction (XRD) patterns were collected by a Bruker D8 ADVANCE and DAVINCI.DESIGN diffractometer with Cu Kα radiation. A Nicolet Nexus 470 spectrometer was applied to record Fourier transform infrared (FT-IR) spectra. Field emission scanning electronic microscopy (FE-SEM) images were obtained with the use of a Zeiss Ultra 55 microscope. Transmission electron microscopy (TEM) was performed using a JEOL JEM2011 to confirm the existence

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of GO in the crystalline composite. Optical microscope (Leica DM2500P) was employed to observe the crystal morphologies of the samples. Nitrogen isotherms were carried out by applying a Micromeritics TriStar II apparatus at 77 K. The outgassed treatment of the samples was performed at 393 K for 12 h before the test. Thermogravimetric (TG) tests were carried out on a Perkin Elmer Thermal Analyzer. The samples were heated from 50 oC to 800 oC at a heating rate of 20 oC·min-1 under N2 atmosphere. 2.4 Ions adsorption experiment Inductively coupled plasma-optical emission spectroscopy (Atom Scan 2000 Series) was used to measure the concentration of Cu2+ ions in the solution. The adsorption capacities of IRMOF-3/GO-1 at 25 oC were measured using the following procedures. Cu (II) stock solutions were prepared by dissolving certain amount of CuCl2·2H2O into deionized water. Then, the pH was adjusted to 5.0. A series of working solutions with different concentrations (4.6-406.2 mg/L) were obtained by diluting the stock solution with NaOH-HNO3 solution (pH 5.0). For all adsorption experiments, 8 mg IRMOF-3/GO-1 was added into 10 mL operational solution. Then, the mixtures were shaken on a shaker for 8 h. After adsorption, the supernatant was separated by centrifugation, and then the residual Cu (II) concentration of it was analyzed. The amount of copper ions adsorbed at adsorption equilibrium, qe (mg/g), was calculated according to Eq. S1. Langmuir adsorption model (Eq. S2) and Freundlich adsorption model (Eq. S3) were used to analyze the adsorption experimental data. Selectivity coefficient (k) was used to examine the selectivity of the IRMOF-3/GO-1 for Cu (II) over its co-existing metal ions. The definition of the selectivity coefficient is given out in Eq. S4. Specifically, all the adsorption experiments were carried out 10


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twice. Additionally, all the errors of adsorption data were absolute deviations. 2.5 Characterization of the performance of membrane The measurements were carried out using a cross-flow mode. The schematic diagram of the cross-flow system is shown in Scheme 2. The cross-flow cell was mainly composed of two pallets. The dimensions of the upper one and the nether one were 10.9 cm × 7.9 cm × 1.5 cm (L × W × H) and 10.9 cm × 7.9 cm × 2.8 cm (L × W × H), respectively. The tested membrane was pressed firmly in the fillister of the cross-flow cell. Thus, water solution can only pass through the membrane vertically from one membrane surface to the other membrane surface. Water solution cannot pass through the membrane horizontally from the side edges of membrane. The effective membrane area was 26.2 cm2. Before the tests, all the membranes were prepressed at 0.7 MPa pressure for about 1 h, then the flux and rejection were measured at 0.7 MPa at room temperature. For the NF process of PSF@PDA@IRMOF-3/GO-1(50), the cross-flow velocity of pure water was 4079 ± 71 L/m2/h at the pressure of 0.7 MPa. The flow rate of pure water through membrane was 38.7 ± 1.6 L/m2/h at the pressure of 0.7 MPa. The flux can be calculated from the Eq. S6. The rejection was obtained by applying the Eq. S7. The rejections of membrane to Cu2+ and metal salt ions in their binary solutions were determined by the above-mentioned inductively coupled plasma-optical emission spectroscopy. The anions of the used metal ions were all Cl-, unless otherwise stated. Specially, all the membrane experiments were carried out twice. In addition, all the errors of membrane test results were absolute deviations.

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Scheme 2. Schematic diagram of the cross-flow system during NF process.

3 RESULTS AND DISCUSSION 3.1 Characterization of GO, IRMOF-3 and IRMOF-3/GO composite. Fig. 1a shows the XRD patterns of GO, IRMOF-3 and IRMOF-3/GO-x. A single diffraction peak of GO at about 11.2o indicates the interlayer distance of 7.9 Å.47 For IRMOF-3, the diffraction pattern is in agreement with that of the previously reported IRMOF-3.48 After incorporating GO, the diffraction patterns of the composites are rather similar to that of IRMOF-3 and do not show the characteristic diffraction peak of GO. The reason is that the major component of the composites is IRMOF-3. Also, the exfoliation of GO in DMF during synthesis and GO layers’ subsequent dispersion in the resultant composites could be another reason.49 This phenomenon indicates that the composites preserve the crystalline structure of the pristine IRMOF-3. Fig. 1b-f show microscope images of IRMOF-3 and IRMOF-3/GO-x. Noticeably, with increasing GO ratio, the size of IRMOF-3/GO-x first decreases to the minimum of IRMOF-3/GO-1, and then increases. We speculate that GO works as a nucleation agent in the process of crystallization. The incorporated GO (0-1 wt%) provides gradually increasing crystallization nucleation sites with the incremental amount of GO. So the crystallization rate accelerates gradually, finally resulting in a decrease in 12


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crystal sizes from IRMOF-3 to IRMOF-3/GO-1. When the amount of added GO (2-5 wt%) is excessive, part of GO agglomerates result in a decrease in crystallization nucleation sites. Thus, the rate of crystallization slows down when the amount of GO is more than 1 wt%. Moreover, the distance between GO sheets gradually diminishes upon increasing ratio of GO. Hence, the growing crystalline structures on the sheets of GO would combine into entirety when they encounter in the case that the distance between GO sheets reaches a critical value. As a result, IRMOF-3/GO composite with larger particle size would be obtained. Additionally, all the morphologies of the IRMOF-3/GO

composites

exhibit

clear

cubes

without

obvious

structure

transformation, which is in accordance with the results of the XRD test. The result implies that IRMOF-3/GO composites preserve the crystalline characters of the pristine IRMOF-3. In short, the information obtained from Fig. 1 is mainly centered at the preservation condition of the crystal structure after GO incorporation and the change trend of crystal sizes. Besides, according to the observed change of crystal sizes, a speculation that GO works as a nucleation agent in the process of crystallization is put forward. The related information is not expanded on a large scale.

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Fig. 1 (a) XRD patterns of GO, IRMOF-3 and IRMOF-3/GO-x and microscope images of (b) IRMOF-3, (c) IRMOF-3/GO-0.5, (d) IRMOF-3/GO-1, (e) IRMOF-3/GO-2 and (f) IRMOF-3/GO-5. (For IRMOF-3/GO-x, x represents the weight percentage of GO based on Zn(NO3)2·6H2O in the process of preparing the composites) According to the aforementioned speculation about the crystal sizes change of IRMOF-3 and IRMOF-3/GO-x, GO layers are supposed to be embedded in the interiors of the crystals after the completion of crystallization. In order to verify that GO indeed exists in the crystal interiors of the composites, the IRMOF-3/GO-5 was ground, and then ultrasonicated for TEM observation and EDX mapping. Thus, the TEM image of IRMOF-3/GO-5 in Fig. 2 is not consistent with the image in Fig. 1. From Fig. 2 of TEM image, the GO layer labeled by an arrow is located in the bottom of some scattered IRMOF-3 fragments. In addition, EDX mapping (Fig. S3) also shows that scattered IRMOF-3 fragments are located onto GO layer. The TEM image 14


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and EDX mapping powerfully confirm that the layers of GO are embedded in the interiors of the crystals after the crystallization finishes.

Fig. 2 High resolution TEM image of ground IRMOF-3/GO-5. Fig. 3 presents the FT-IR spectra of GO, IRMOF-3 and IRMOF-3/GO-x. For GO, four characteristic peaks clearly appear at 3428, 1734, 1626 and 1406 cm-1, which are assigned to O-H, C=O stretching, O-H bending, and C-O deformation vibrations, respectively.50,51 These peaks confirm that graphite has been oxidized to be GO successfully. The spectrum of IRMOF-3 coincides well with that reported in the reference.52 Two typical peaks at 3472 and 3358 cm-1 are associated with the existence of the amino group.53 The peaks at 1660, 1497, 1427 cm-1 correspond to the aromatic C=C stretching vibration. The band at 1257 cm-1 can be ascribed to C-N vibration.54 The two peaks centered at 1574 and 1380 cm-1 are assigned to asymmetric (νasC-O) and symmetric (νsC-O) vibrations of carboxyl groups, respectively.55 It can be seen that the FT-IR spectra of the composites (IRMOF-3/GO-x) are similar to that of IRMOF-3, which could be explained that MOF is the principal ingredient of the composites.

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Fig. 3 FT-IR spectra of GO, IRMOF-3 and IRMOF-3/GO-x. (x presents the weight percentage of GO based on the Zn(NO3)2·6H2O in the process of preparing the composites) Fig. S4 presents N2 adsorption and desorption isotherms on the IRMOF-3 and IRMOF-3/GO-x. The isotherm profiles belong to type I, which indicates microporous structure characteristic. BET surface areas and total pore volumes of the five samples are shown in Table 1. Except IRMOF-3/GO-5, all the composites obviously present larger surface areas and higher total pore volumes than those of IRMOF-3. Oxygen functional groups on the GO layers function as new crystallization sites in the crystallization process of IRMOF-3. This results in enhanced surface area and porosity. With increasing GO ratio, the BET surface area and total pore volume of IRMOF-3/GO-x first increase to the maximum of IRMOF-3/GO-1, and then decrease, which presents an opposite trend with that of crystal sizes. It may be ascribed to the change in crystallization sites by the incorporation of GO. The amount of GO in IRMOF-3/GO-5 is excessive so that the distorted structures resulting from GO agglomerates exist in the interior of the entirely cubic crystal texture. It results in

16


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lower BET surface area and total pore volume of IRMOF-3/GO-5 than those of IRMOF-3. Table 1 The amount of GO and the structure parameters of IRMOF-3/GO-x. The ratio of GO(%)

SBET (m2/g)

Vp (cm3/g)

0

2588

1.1039

IRMOF-3/GO-0.5

0.5

2884

2.4540

IRMOF-3/GO-1

1

3003

2.6215

IRMOF-3/GO-2

2

2828

2.4002

IRMOF-3/GO-5

5

2069

0.8827

Sample IRMOF-3

3.2 Adsorption capacity of Cu2+ by IRMOF-3/GO-x. Copper ion solutions (53.7 mg/L, 215.6 mg/L) were used for comparison of the adsorption capacities of Cu2+ on the IRMOF-3 and IRMOF-3/GO-x. As shown in Fig. S5, IRMOF-3/GO-1 displays the largest adsorption capacities in both of the two Cu2+ solutions. Also, the adsorption capacities of the samples exhibit a same trend as the surface areas that firstly increase to the highest, and then decrease gradually, with the enhanced amount of GO incorporated. This experimental phenomenon is consistent with the universal discipline that high surface area can undoubtedly improve the adsorption capacity of adsorbent. However, IRMOF-3/GO-5 has a higher capacity than that of IRMOF-3, although its surface area is lower than that of IRMOF-3. It can be attributed to that the incorporation of GO enhances the dispersive forces necessary to the adsorption of Cu2+. In view of the largest capacity of IRMOF-3/GO-1 among the samples, some other adsorption properties of IRMOF-3/GO-1 were investigated below.

3.2.1 Adsorption isotherm and adsorption mechanism. Fig. S6 shows the adsorption isotherm of Cu (II) on IRMOF-3/GO-1. The corresponding constants 17



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simulated by Langmuir and Freundlich models are shown in Table S1. It can be seen that the experimental data are more suitable for Langmuir model than Freundlich model, which suggests that the uptake process is a monolayer adsorption and all sites are equal56. It should be noted that the maximal adsorption capacity of Cu2+ on IRMOF-3/GO-1 is 254.14 mg/g, which is much higher than those of most of adsorbents (including a majority of zeolites) reported in the previous works. (see Table S2) pH is an important factor that greatly influences the metal ions adsorption because the pH of a solution can influence not only the ionic forms but also the surface charge of the adsorbent.57 The effect of pH on the adsorption capacity of Cu2+ has been examined using a series of Cu2+ solutions with different pH values (2-6). As illustrated in Fig. 4a, the adsorption capacity sharply increases in the pH range 2-3, and then increases slowly. This adsorption trend may be influenced by the electrostatic repulsion between protonated amino groups and Cu2+ ions. The protonated amino groups (-NH3+) can transform into the coordination form between -NH2 and Cu2+. Because Cu2+ can bind with -NH2 much easily, compared to H+. (the binding stability constant of Cu (II) complex bound with amino groups is much higher than that of H+ complex bound with amino groups). At low pH, the electrostatic repulsion is stronger so that the resistance of coordination between copper ions and amino groups is larger. Thus, the adsorption capacity at low pH is lower than that at high pH. Cu2+ ions begin to precipitate as Cu(OH)2 at pH>6 in the experiment. On one hand, Cu2+ ions do not precipitate in Cu2+ solution at pH=5. On the other hand, the adsorption capacity of IRMOF-3/GO-1 to Cu2+ ions at pH=5 is relatively high. Thus, a series of other adsorption properties of IRMOF-3/GO-1 are explored at pH=5. It should be noted that carboxyl groups on GO have coordinated with the metal units of 18


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IRMOF-3 and have become a part of the resultant IRMOF-3/GO composite. So the protonation of carboxyl groups on GO is supposed to be neglected. The adsorption mechanism was further characterized by the XPS. As shown in the XPS wide scan spectra (Fig. S7), the Cu2p signal peaks are clearly presented in the spectrum of IRMOF-3/GO-1 after adsorption, which indicates that Cu2+ ions gather on the IRMOF-3/GO-1 during the adsorption. In Fig. 4b, the peaks at 401.00 eV and 402.92 eV are assigned to the neutral amine (–NH2 or >NH) and the protonated amine (–NH3+ or >NH2+), respectively. Evidently, after the adsorption of Cu2+, the characteristic peak of the neutral amine shifts from 401.00 to 401.50 eV (Fig. 4c). This phenomenon can be explained that the nitrogen atom of the neutral amine coordinated with Cu2+ exists in a more oxidized form so that it shows a higher binding energy in the N1s spectrum after adsorption.58 The test of XPS is in accord with the effect of pH on the adsorption, and further verifies the coordination adsorption of Cu2+ on the IRMOF-3/GO-1. Besides, the schematic diagram of coordination adsorption is outlined in Fig. 5. Specially, the possibility that copper ions bind with carboxylic groups in their salt form is supposed to be neglected. Because carboxylic groups have bound with zinc ions to further grow as IRMOF-3 crystals in the process of the solvothermal reaction. Finally, they become a part of the resultant IRMOF-3/GO crystals after the solvothermal reaction finishes. What’s more, the whole adsorption process is conducted in 25 oC. The energy that copper ions substitute the metal parts of the crystals is not sufficient enough.

19



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Fig. 4 Effect of pH on the adsorption capacity of Cu2+ by IRMOF-3/GO-1 (a), XPS spectra of IRMOF-3/GO-1: (b) N1s before adsorption and (c) N1s after adsorption.

Fig. 5 Schematic diagram of the preparation of IRMOF-3/GO and the adsorption of Cu2+ on IRMOF-3/GO. 3.2.2 Adsorption selectivity. In practice, diverse metal ions exist in environmental wastewater. In the case of copper ions removal, the co-existing metal ions may exert competitive interaction with the adsorbent. Na (I), K (I), Ca (II), Mg (II), Pb (II), Ni (II), Co (II) and Fe (III) are commonly co-existing metal ions in wastewater. Consequently, the impacts of these metal ions on the adsorption of Cu2+ ions on the IRMOF-3/GO-1 were investigated for evaluating the adsorption selectivity of the adsorbent. 20


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Binary solutions of interfering metal ions and Cu (II) whose concentrations are both 50.0 mg/L were prepared at pH 5.0 and 25 oC, respectively. As listed in Table 2, the interferences of Na (I), K (I), Ca (II), Mg (II) on the adsorption of Cu (II) can be neglected, while Pb (II), Ni (II), Co (II) and Fe (III) all show minor effects. Hard and Soft Acids and Bases (HSAB) theory can be used to explain this experimental result.59 According to the theory, Na (I), K (I), Ca (II) and Mg (II) ions should belong to hard acids, Pb (II), Ni (II), Co (II) and Fe (III) ions should belong to a kind of limitrophe acids, while IRMOF-3/GO-1 is supposed to be soft base. As a result, Pb (II), Ni (II), Co (II) and Fe (III) ions more easily couple with IRMOF-3/GO-1, compared to the aforesaid hard acids. Accordingly, Pb (II), Ni (II), Co (II) and Fe (III) ions play negative roles on the adsorption of Cu (II) ions. IRMOF-3/GO composite is a porous polymeric material bearing abundant amino groups. It belongs to a kind of polymeric amine material. Commonly, polymeric amine can form more stable complex with Cu (II) ions, compared with those of other heavy metal ions. Because the binding stability constant of the formed Cu (II) complex is higher than those of other heavy metal complexes.60-62 Thus, IRMOF-3/GO-1 displays a reasonably stronger adsorption force of Cu (II) ions than those of Pb (II), Ni (II), Co (II) and Fe (III) ions. These test data (Table 2) effectively demonstrate that the as-prepared IRMOF-3/GO-1 is an excellent adsorbent with a selective adsorption of Cu (II) ions from wastewater.

21



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Table 2 The influences of co-existing metal ions on the adsorption capacity of Cu (II). Co-existing metal ions

Selective coefficients (k)

Na+

∞

K+

∞

Ca2+

∞

Mg2+

∞

Pb2+

6.3 ± 0.2

Ni2+

12.2 ± 0.3

Co2+

9.1 ± 0.2

Fe3+

15.2 ± 0.5

3.3 The performance of the membrane modified with IRMOF-3/GO-1. IRMOF-3/GO-1 was decorated onto the surface of NF membrane (PSF@PDA) to endow the resultant NF membrane a barrier layer with selective Cu2+ rejection. The preparation of PSF@PDA@IRMOF-3/GO-1 is according to the Michael addition reaction and Schiff base reaction63 between the polydopamine surface layer of PSF@PDA and IRMOF-3/GO-1. The preparation process is shown in Fig. 6a. The pure water fluxes of PSF@PDA and PSF@PDA@IRMOF-3/GO-1(50) are 45.3 and 38.7 L/m2/h, respectively (Fig. 7a). It should be noted that the adsorption capacities of the resultant NF membranes are not involved in the NF process. Because adsorption process and membrane filtration process are two entirely different processes. The resultant membranes are under membrane filtration process throughout, and they are not membrane adsorption materials. The whole membrane filtration process is a water flowing process that water solution passes through the membrane from one surface to the other surface. It is completely different with the adsorption process of membrane 22


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adsorption material. Four salt solutions (Na2SO4, MgSO4, CaCl2 and MgCl2) with the concentration of 5 mmol/L were applied to study the salts rejection characteristic of NF

membranes.

As

presented

in

Fig.

7a,

PSF@PDA

and

PSF@PDA@IRMOF-3/GO-1(50) reject salts following the sequence of Na2SO4 < MgSO4 < CaCl2 < MgCl2. This is in accord with the typical feature of positively charged NF membranes.64 The zeta potentials of membrane surfaces in Fig. 7b verify that the NF membranes are positively charged under the testing condition. Donnan exclusion effect could be used to explain the salts rejections. Multivalent cations (Ca2+ and Mg2+) possessing higher co-ion charge are more easily rejected than monovalent cations (Na+) owing to the stronger electrostatic repulsion with membrane surfaces. At the same time, membrane surfaces would exert stronger electrostatic attraction on divalent anions (SO42-) than monovalent anions (Cl-). Additionally, high rejections of Ca2+ and Mg2+ could also be attributed to size exclusion mechanism, since Stokes radii of Ca2+ and Mg2+ are larger than those of Na+, Cl- and SO42-. Moreover, lower rejection of CaCl2 than MgCl2 may also be ascribed to size exclusion mechanism, because the Stokes radius of Ca2+ (0.307 nm) is smaller than that of Mg2+ (0.345 nm).65

23



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Fig.

6

(a)

Schematic

illustration

PSF@PDA@IRMOF-3/GO-1

and

of (b)

the

Page 24 of 39

fabrication

rejection

process

mechanism

of of

PSF@PDA@IRMOF-3/GO-1.

Fig. 7 (a) Salts rejections and pure water fluxes of PSF@PDA and PSF@PDA@IRMOF-3/GO-1(50), PSF@PDA@IRMOF-3/GO-1(25),

and

(b)

zeta

potentials

of

PSF@PDA,


and

PSF@PDA@IRMOF-3/GO-1(75) membrane surfaces. (using 5 mmol/L salt solutions as feeds) 24


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Five feeds of Cu2+, Pb2+, Ni2+, Co2+ and Fe3+ with the concentration of 200.0 mg/L, were prepared at pH 5.0 and 25 oC, respectively. The separation performance to the various heavy metal ions was investigated. As shown in Fig. 8a, the incorporation of IRMOF-3/GO-1 onto PSF@PDA can improve its rejections of Cu2+, Pb2+, Ni2+, Co2+ and Fe3+. The rejections of PSF@PDA@IRMOF-3/GO-1(25) to Cu2+, Pb2+, Ni2+, Co2+ and Fe3+ are 78.2, 67.4, 58.3, 60 and 65.2%, respectively. The rejections of PSF@PDA@IRMOF-3/GO-1(50) to the five heavy metal ions reach 87.5, 74.5, 62.5, 65, and 69%, respectively. In addition, PSF@PDA@IRMOF-3/GO-1(75) possesses Cu2+, Pb2+, Ni2+, Co2+ and Fe3+ rejections of 89.3, 75.3, 63.2, 65.7 and 70.4%, respectively. The improvements of heavy metal ions rejections could be attributed to two factors: the enhancement of membrane surface potential after decoration of IRMOF-3/GO-1 and the high adsorption capacity of IRMOF-3/GO-1. The former makes the electrostatic repulsion between membrane surface and heavy metal ions stronger, which is favorable to the rejection of heavy metal ions. The latter makes some heavy metal ions can be adsorbed onto membrane surface, which is also beneficial to heavy metal ions rejections. It should be noted that all the membranes modified by IRMOF-3/GO-1 present higher rejections of Cu2+ than those of other heavy metal ions. It can be ascribed to that IRMOF-3/GO-1 possesses excellent adsorption selectivity of Cu2+ under the interferences of other heavy metal ions (Table 2). It seems that the rejection difference is less obvious than the adsorption difference between Cu2+ ions and other heavy metal ions. For PSF@PDA@IRMOF-3/GO-1(50), the rejection of Cu2+ is 13, 25, 22.5 and 18.5% higher than those of Pb2+, Ni2+, Co2+ and Fe3+, respectively. Actually, the rejection difference is considerable for membrane filtration process. In adsorption process, the initial Cu2+ concentration (Co), the volume of Cu2+ solution (V) and the weight of adsorbent (W) are fixed values. The 25



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value of adsorption capacity depends on the value of equilibrium Cu2+ concentration (Ce). The relatively big difference of Ce will result in relatively big difference of adsorption capacities. The adsorption selectivity of adsorbent will make the corresponding Ce different obviously, which will further make the difference of adsorption capacities of various heavy metal ions obvious. Whereas, for membrane filtration process, the concentration of the feed (Cf) is a fixed value, and the value of rejection lies on the value of permeation concentration (Cp). The change of Cp will not result in obvious difference of rejections in the numerical value, compared to that of adsorption capacities. The change value of rejections will not exceed 100% all the way. While, the change of adsorption capacities can reach several times value. The treated amount of wastewater in membrane filtration process is much larger than that in adsorption process. So the decreased concentration of the treated wastewater resulted from equal adsorption effect is obviously lower in membrane filtration process, compared to that in adsorption process. It is considerable that the change of Cp is more than ten percentage points (the change of Cp is based on Cf) in membrane filtration process. Therefore, the rejection difference between copper ions and other heavy metal ions is considerable. Additionally, PSF@PDA@IRMOF-3/GO-1(75) just exhibits slightly higher rejections than those of PSF@PDA@IRMOF-3/GO-1(50). It may be attributed to the excessive loading of IRMOF-3/GO-1 onto PSF@PDA@IRMOF-3/GO-1(75) (Fig. S8e). Specifically, PSF@PDA@IRMOF-3/GO-1(50) shows obviously higher Cu2+ rejection than that of PSF@PDA@IRMOF-3/GO-1(25), while presents negligibly lower Cu2+ rejection than that of PSF@PDA@IRMOF-3/GO-1(75). This could be attributed to the good and full loading of IRMOF-3/GO-1 onto membrane surface (Fig. S8d). The adsorption and electrostatic repulsion of IRMOF-3/GO-1 (Fig. 6b) greatly 26


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promote Cu2+ rejection of PSF@PDA@IRMOF-3/GO-1(50). As illustrated in Fig. 8b, the flux of PSF@PDA@IRMOF-3/GO-1(75) decreases greatly than that of PSF@PDA. It may be ascribed to that the excessive loading of IRMOF-3/GO-1 (Fig. S8e)

greatly

enhances

the

permeation

resistance.

Therefore,

PSF@PDA@IRMOF-3/GO-1(50) possesses the best performance. Specifically, NF membranes display higher rejections of heavy metal ions, compared to those of metal salt ions. It could be ascribed to that heavy metal ions are rejected by both of electrostatic repulsion and adsorption effects, while, metal salt ions are rejected only by electrostatic repulsion effect. The situations are same under both the conditions that the loading of IRMOF-3/GO-1 onto membrane surface is full or not.

Fig.

8

Rejections

(a)

and

fluxes

(b)

of

PSF@PDA

and

PSF@PDA@IRMOF-3/GO-1(25, 50 and 75) examined with the feeds containing different heavy metal ions. (The concentrations of heavy metal ions in the feeds are all 200.0 mg/L, pH 5.0, 25 oC) In order to explore the effects of different metal salt ions on the rejection of PSF@PDA@IRMOF-3/GO-1(50) to Cu2+, binary solutions of metal salt ions and Cu (II) ions whose concentrations are both 200.0 mg/L were prepared at pH 5.0 and 25 oC, respectively. As shown in Fig. 9, Na+, Ca2+, Mg2+ have all slightly negative effects on Cu2+ rejection of PSF@PDA@IRMOF-3/GO-1(50). This could be attributed to the 27



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Page 28 of 39

prominent Cu2+ adsorption selectivity of IRMOF-3/GO-1 under the interferences of metal salt ions (Table 2).

Fig. 9 Rejections of PSF@PDA@IRMOF-3/GO-1(50) to Cu2+ and metal salt ions in their binary solutions. (the concentrations of Cu2+ and metal salt ions are both 200.0 mg/L, pH 5.0, 25 oC, the anions of the metal salt ions are all Cl-) Fig. 10 shows the short-term stability of PSF@PDA@IRMOF-3/GO-1(50) towards 200.0 mg/L Cu2+ solution at pH 5.0. Obviously, the membrane presents a relatively

excellent

stability


throughout exhibits

the

testing

sustainable

process. and

stable

In

short, removal

performance for copper (II).

Fig. 10 Short-term stability performance of PSF@PDA@IRMOF-3/GO-1(50). (the concentration of Cu2+ feed is 200.0 mg/L, pH 5.0) 28


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4 CONCLUSIONS In this study, IRMOF-3/GO composites with various ratios of GO have been prepared successfully. The surface areas of the composites firstly increase to the maximum of IRMOF-3/GO-1, then decrease gradually, with increasing GO ratio. Among the samples, IRMOF-3/GO-1 shows the highest adsorption capacity under the same conditions. Thus, a series of adsorption properties of IRMOF-3/GO-1 have been explored. The results demonstrate that as a new adsorbent, IRMOF-3/GO-1 shows an excellent adsorption capacity of Cu2+ with a maximal adsorption amount of 254.14 mg/g at pH 5.0, 25 oC, including good selectivity. Consequently, IRMOF-3/GO-1 is decorated onto PDA deposited membrane substrate to endow the resultant NF membrane a barrier layer with selective copper ions rejection. The rejection of copper (II) reaches up to about 90%, meanwhile maintaining a relatively high flux of about 31 L/m2/h at the pressure of 0.7 MPa and pH 5.0. Moreover, the membrane also presents an excellent stability performance during 2000 min NF test. This finding provides a promising alternative approach for designing novel and effective NF membrane to remove heavy metal ions from wastewater. ASSOCIATED CONTENT Supporting Information. Adsorption capacities of Cu2+ in different cycles by IRMOF-3/GO-1 and IRMOF-3. (pH 5.0, 25 oC, 53.7 mg/L, 8 h, EDTA-4Na aqueous solution with the concentration of 20 mM was used as eluent); TG and DTG curves of GO and IRMOF-3/GO-1; EDX mapping (C and Zn elements) of IRMOF-3/GO-5; Nitrogen adsorption-desorption isotherms at 77 K of IRMOF-3 and 29



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

IRMOF-3/GO-x (For IRMOF-3/GO-x, x represents the weight percentage of GO based on Zn(NO3)2·6H2O in the process of preparing the composites); Adsorption capacities of Cu2+ by IRMOF-3/GO-x at concentration of (a) 53.7 mg/L and (b) 215.6 mg/L. (x presents the weight percentage of GO based on Zn(NO3)2·6H2O in the process of preparing the composites, pH 5.0, 25 oC, 8 h); Adsorption curve of Cu2+ at pH 5.0, 25 oC; Adsorption isotherm model constants for copper ions adsorption; Comparison of Cu (Ⅱ) sorption with other adsorbents; XPS wide scan spectra of (a) IRMOF-3/GO-1 before adsorption and (b) IRMOF-3/GO-1 after adsorption; FE-SEM surface images of (a) PSF, (b) PSF@PDA, (c) PSF@PDA@IRMOF-3/GO-1(25), (d) PSF@PDA@IRMOF-3/GO-1(50) and (e) PSF@PDA@IRMOF-3/GO-1(75) are available as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] and [email protected]. Tel.: +86-21-65643255. Fax: +86-21-65640293. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are very grateful the financial support from the National Natural science Foundation of China (NSFC, No.21276051).

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