Zirconium-based metal organic frameworks loaded on polyurethane foam membrane for simultaneous removal of dyes with different charges

Journal of Colloid and Interface Science 527 (2018) 267–279

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Regular Article

Zirconium-based metal organic frameworks loaded on polyurethane foam membrane for simultaneous removal of dyes with different charges Juan Li a, Ji-Lai Gong a,⇑, Guang-Ming Zeng a,⇑, Peng Zhang a, Biao Song a, Wei-Cheng Cao a, Hong-Yu Liu a, Shuang-Yan Huan b a Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China b State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China

g r a p h i c a l a b s t r a c t The Zr-Based metal-organic frameworks combining polyurethane foam membrane can simultaneously remove RB, MB and CR from RB/MB/CR ternary solution.

a r t i c l e

i n f o

Article history: Received 1 March 2018 Revised 10 May 2018 Accepted 11 May 2018

Keywords: Metal organic framework Polyurethane membrane Wastewater treatment Dye

⇑ Corresponding authors. E-mail address: [email protected] (J.-L. Gong). https://doi.org/10.1016/j.jcis.2018.05.028 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.

a b s t r a c t Treating dye wastewater by membrane filtration technology has received much attention from researchers all over the world, however, current studies mainly focused on the removal of singly charged dyes but actual wastewater usually contains dyes with different charges. In this study, the removal of neutral, cationic and anionic dyes in binary or ternary systems was conducted by using zirconium-based metal organic frameworks loaded on polyurethane foam (Zr-MOFs-PUF) membrane. The Zr-MOFs-PUF membrane was fabricated by an in-situ hydrothermal synthesis approach and a hot-pressing process. Neutrally charged Rhodamine B (RB), positively charged Methylene blue (MB), and negatively charged Congo red (CR) were chosen as model pollutants for investigating filtration performance of the membrane. The results of filtration experiments showed that the Zr-MOFs-PUF membrane could simultaneously remove RB, MB, and CR not only from their binary system including RB/MB, RB/CR, and MB/CR mixtures, but also from RB/MB/CR ternary system. The removal of dyes by Zr-MOFs-PUF membrane was mainly attributed to the electrostatic interactions, hydrogen bond interaction, and Lewis acid-base interactions between the membrane and dye molecules. The maximum removal efficiencies by ZrMOFs-PUF membrane were 98.80% for RB at pH
7, 97.57% for MB at pH
9, and 87.39% for CR at

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pH
3. Additionally, when the NaCl concentration reached 0.5 mol/L in single dye solutions, the removal efficiencies of RB, MB, and CR by Zr-MOFs-PUF membrane were 93.08%, 79.52%, and 97.82%, respectively. All the results suggested that the as-prepared Zr-MOFs-PUF membrane has great potential in practical treatment of dye wastewater. Ó 2018 Elsevier Inc. All rights reserved.

1. Introduction In recent years, dyes are widely used as the colorant in printing, cosmetic, leather, plastic, food, textile, and other industries [1]. The wastewater containing dyes will cause serious threat to human health and the environment once being discharged without proper treatment. Some organic dyes are toxic, carcinogenic, and teratogenic [2–4]. Thus, it is significant to treat dye effluent before releasing them into the environment. To date, a variety of chemical, physical and biological approaches have been developed for separation and removal of dyes from wastewater, such as adsorption [4–7], precipitation and coagulation [8–10], photocatalytic degradation [11,12], ionexchange [13], and membrane filtration [14,15]. Among these methods, membrane filtration technology plays a crucial role in meeting the requirements for wastewater treatment due to its relatively simple operation, low cost, and high efficiency [16,17]. Currently, one of the most attractive aspects in the field of membrane filtration is the innovation of membrane materials. Polyurethane foam (PUF) is a three dimensional porous material which is commercially available. The material has been used as the support substrate in various applications due to its highly mechanical durability and excellent elasticity [18]. For example, Lefebvre et al. used polydopamine-coated polyurethane open cell foam for loading carbon media for dye adsorption [19]. Zhang et al. reported that super-hydrophobic graphene-coated polyurethane sponge showed excellent performance for oil-water separation [20]. Besides, polyurethane foams were also employed in desalination [21] and catalysis [22] as the support materials. Due to its easy accessibility and unique physical and mechanical properties, polyurethane foams have become potential supporting substrate materials for the development of membrane filtration in wastewater treatment. In our previous work, we investigated the removal of organic dyes including positively charged methylene blue (MB), neutrally charged rhodamine B (RB), and negatively charged methyl orange (MO) by PUF membrane stuffed with humic acidchitosan crosslinked gels [23]. We found that the PUF membrane exhibited the ability of selectively separating MB from binary solutions including RB/MB and MO/MB mixed solution when the PUF membrane was only filled with chitosan gels. Noticeably, PUF membrane could simultaneously remove MB and RB from MB/RB binary solution when the membrane was filled with humic acidchitosan crosslinked gels. However, the removal of mixed dye solutions including positively charged dye, neutrally charged dye, and negatively charged dye by PUF-based membrane needs further investigation. In addition, most of reported documents focused on the removal of cationic dyes or anionic dyes alone from wastewater by membrane filtration [24–26]. However, actual wastewater usually contains dyes with different charges. Thus it is necessary and significant to search for more suitable membrane materials to treat the complicated wastewater. Metal organic frameworks (MOFs), also known as porous materials, are composed of inorganic metal ions (or clusters) and organic ligands by coordinate bonds. They have received considerable attention in recent years due to their adjustable pore size, numerous metal active sites, and large surface area [27,28]. These

unique features make them very popular in various applications, such as the separation, adsorption and storage, smart sensors, and heterogeneous catalysis [29,30]. But most of metal organic frameworks still are subjected to insufficient hydrothermal stability, which hinders the applications of MOFs in processes involving water. Nevertheless, a series of zirconium-carboxylate MOFs (ZrMOFs) have been emerged and synthesized with different topologies and various functional groups, mainly including ANH2, ABr, ANO2 [31–33]. The zirconium has a high attraction towards Lewis acid and oxygen ligands characteristic. It was reported that the zirconium-based MOFs can be synthesized from nanocrystal to single-crystal structures by adjusting the category and amount of modulators [34]. The Zr-MOFs exhibit remarkable thermal and chemical abilities via strong coordinate bonds between the zirconium atoms and carboxylate oxygens [31,35,36]. Moreover, zirconium cations can target specific adsorption behavior as open metal sites in the secondary building unit because of its high valence [37]. Benefiting from these outstanding qualities, ZrMOFs family has acquired more and more attention in the area of wastewater treatment, especially in the adsorption of dyes [37,38], capture of toxic heavy metals [39], and removal of persistent organic pollutants [40]. However, because of its powdery or gelatinous nature, it is inconvenient to separate from solutions. And due to the relatively poor mechanical stability of MOFs [41], there is a growing demand for structured supporting materials. In this work, zirconium-carboxylate MOFs combined PUF was designed and fabricated as a novel hybrid membrane (Zr-MOFsPUF membrane) for removing differently charged organic dyes from wastewater. Rhodamine B (RB, neutral dye), Methylene blue (MB, cationic dye), and Congo red (CR, anionic dye) were used as model pollutants. The static adsorption experiments of Zr-MOFsPUF membrane, as well as the effects of flow rate, initial concentration of dyes, pH, coexisting NaCl on single dye removal were investigated. Particularly, the filtration experiments for RB, MB, and CR removal from binary (RB/MB, RB/CR, and MB/CR) and ternary system (RB/MB/CR) by the Zr-MOFs-PUF membrane were also performed. 2. Materials and methods 2.1. Materials Zirconium chloride (ZrCl4, AR, 98%) and 1,2,4,5Benzenetetracarboxylic acid (H4BTEC, AR, 98%) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Flexible PUF of industrial grade (0.035 g/cm3) was obtained from Lianda Technology Industrial Co., Ltd. (Shenzhen, China). Potassium dichromate (K2Cr2O7, AR, 99.8%), MB (AR, 95%), RB (AR, 99%) and CR (AR, 99%) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China) and their chemical structures were shown in Fig. 1. Hydrochloric acid (HCl, GR, 37%), sulfuric acid (H2SO4, GR, 98%), sodium hydroxide (NaOH, AR, 96%), sodium chloride (NaCl, AR, 99.5%), acetone (AR, 99.7%) and ethanol (AR, 99.7%) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All of chemical reagents were used as received without further purification.

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O OH N

N

N+

S

N+

O

N

Rhodamine B (RB)

Methylene blue (MB)

O O

S

ONH2 N

N N

N

NH2 Congo red (CR)

O

S

O-

O Fig. 1. The chemical structure of Rhodamine B (RB), Methylene blue (MB) and Congo red (CR).

2.2. Pretreatment of the PUF The raw PUF materials were tailored to cylinder with a diameter of 30 mm and a higher of 10 mm before respectively immersed them into ethanol and acetone for 3 h at room temperature. Then, the PUF was washed with deionized water for several times, followed by drying them in an oven (60 °C). Then, the dried PUF was soaked in the fresh chromic acid (K2Cr2O7/H2O/H2SO4 = 4.4/7.1/88.5 wt%) for 1 min [42,43]. The treated PUF sample was washed with abundant deionized water in order to remove residual acid completely. And the obtained PUF was dried in an oven at 60 °C for 8 h. 2.3. Synthesis of Zr-MOFs precursors The Zr-MOFs (UiO-66-(COOH)2) nanocrystals were prepared according to previously reported methods with some modifications [44,45]. Briefly, 2.50 mmol of ZrCl4 and 4.25 mmol of H4BTEC were dissolved in 50 mL of deionized water with continuous magnetic stirring for 3 h at ambient temperature. Then, the reaction solution was heated at 100 °C for 24 h. The as-obtained white gel was centrifuged at 10,000 rpm for 10 min and washed with deionized water for at least three times. The synthesized Zr-MOFs nanocrystals were dried and preserved for further characterization

and applications. The UiO-66-(COOH)2 precursors were prepared by the same method as mentioned above without further heating reaction. 2.4. Fabrication of Zr-MOFs-PUF membrane The Zr-MOFs-PUF membranes were fabricated by loading UiO66-(COOH)2 on the pretreated PUF substrate by an in-situ hydrothermal synthesis method [46] and the probable synthesized processes were illustrated in Fig. 2. Concisely, the pretreated PUF and Zr-MOF precursor solutions were placed in a Teflon reactor (100 mL) and heated at 100 °C for 24 h. Then the obtained PUF/ Zr-MOF composite was washed with deionized water cautiously, and dried in an oven at 60 °C for 10 h. Finally, the dried PUF/ZrMOF composite was pressed by hot press equipment at 150 °C. The pretreated PUF membrane without loading Zr-MOFs (noted as PUF membrane) as a control was also prepared under the same experimental conditions. The resulting Zr-MOFs-PUF and PUF membranes were stored at a desiccator for further experiments. 2.5. Characterization The morphologies of Zr-MOFs-PUF membranes were observed by field emission scanning electron microscopy (FE-SEM, S-4800,

Fig. 2. Schematic illustration of fabrication of Zr-MOFs-PUF membrane.

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Hitachi, Japan). X-ray diffraction (XRD) patterns of Zr-MOFs, PUF, and Zr-MOFs-PUF membranes before and after adsorption dyes were obtained using a Bruker-AXS D8Advance X-ray diffractometer in 2h range from 5° to 60°. The Brunauer–Emmett–Teller (BET) surface areas, Nitrogen adsorption-desorption isotherm, and pore size distribution of Zr-MOFs-PUF membrane were conducted using a Quantachrome Instruments. The elemental composition analysis of as-prepared samples was performed by an X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Escalab 250XI). Fourier transforms infrared (FT-IR) spectra were obtained using a Bruker V70 FTIR spectrometer over the wavenumber range of 500–4000 cm1. The zeta-potential measurement was carried out with a Zeta-potential & Particle size Analyzer Mastersizer NaNoZS. 2.6. Static adsorption experiment Adsorption kinetic experiment was conducted by adding 50 mg of Zr-MOFs-PUF membranes to 50 mL of 20 mg/L RB, MB and CR solutions at 150 rpm and 25 °C, respectively. After adsorption, the different single dyes were taking samples at different interval time and the concentrations of dyes were measured by UV–vis spectrophotometer (UV-2550, Shimadzu, Japan) at the wavelength of 554 nm for RB, 664 nm for MB, and 499 nm for CR respectively. 2.7. Evaluation of filtration performance In this study, three differently charged dyes including neutral dye (RB), cationic dye (MB) and anionic dye (CR) were chosen to evaluate the filtration performance of Zr-MOFs-PUF membrane. The filtration experiments of different dyes were executed by a syringe filter (1.13 cm2 effective filter area). The filtration device used in the experiments was depicted in our previous work [23]. A tailored Zr-MOFs-PUF membrane (diameter: 15 mm) was immobilized in the equipment. In order to measure the appropriate flow rate for filtration experiment, 10 mL of RB, MB and CR solutions with a concentra-

tion of 10 mg/L were passed through the filter at different flow rate varying from 0.5 to 3.0 mL/h, respectively. To investigate the effect of initial concentration of dyes, different dye solutions were prepared in deionized water with concentrations ranging from 5 to 30 mg/L. And dyes with different concentrations were filtered by the device with the Zr-MOFs-PUF membrane or the PUF membrane at a flow rate of 1 mL/h at room temperature. The effects of pH and NaCl on the filtration performance of ZrMOFs-PUF membrane for dye removal were also studied. The pH values of dye solution ranging from 3 to 11 were adjusted by NaOH or HCl (0.1 mol/L). And the concentrations of NaCl in dye solutions ranged from 0 to 0.5 mol/L. Moreover, the mixed dye solutions (i.e., binary system and ternary system) filtration experiments by ZrMOFs-PUF membrane were also performed. All filtration experiments by Zr-MOFs-PUF membrane were conducted for at least three times. 3. Results and discussion 3.1. Characterization of Zr-MOFs-PUF membrane The SEM images of pristine PUF membrane (a, b, c), pretreated PUF membrane (d, e, f) and Zr-MOFs-PUF membrane (g, h, i) at different magnification were shown in Fig. 3. For the pristine PUF membrane, it was clearly observed relatively smooth porous network structures in Fig. 3a, and there was some small pores were observed in the wall of hole at high magnification SEM image (see Fig. 3b). However, for the PUF after chromic acid treatment, the SEM image displayed that the pretreated PUF surface was presented to be much more rough than original PUF, which was might be attributed to that the PUF was etched and oxidized by chromic acid [43]. From Fig. 3(g, h, i), it was obvious that Zr-MOFs particles were successfully loaded in the pretreated PUF internal structures. This phenomenon could be especially observed in the cross-section image (see Fig. 3i). It was highlighted the integration of Zr-MOFs with pretreated PUF.

Fig. 3. SEM images of pristine PUF (a, b, c), pretreated PUF (d, e, f) and Zr-MOFs-PUF membrane (g, h, i).

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and 1402 cm1 were weakened compared to the synthesized ZrMOFs. It was attributed to the limited carboxyl groups in the pretreated PUF and a competitive coordination between H4BTEC and the COOH-terminated PUF [49]. The Nitrogen adsorptiondesorption isotherm and the pore size distribution of the ZrMOFs-PUF membrane were displayed in Fig. 4b. The obtained results indicated that the Zr-MOFs-PUF membrane was mesoporous material [7,50]. The calculated BET surface area of ZrMOFs-PUF membrane was 1.451 m2/g and the pore diameter was 5.682 nm (see Fig. 4b). The pore size distribution of Zr-MOFs-PUF

(a) O 1s

Intensity (a.u.)

The FT-IR spectra of pristine PUF, pretreated PUF, Zr-MOFs-PUF membrane and Zr-MOFs (UiO-66-(COOH)2) were shown in Fig. 4a. Comparing the spectra of original PUF and pretreated PUF, the weak peak at 1712 cm1 was linked to the C@O (carboxyl group) stretch vibration. It was indicated that the polyurethane foam might be partially oxidized to carboxylic acids after chromic acid treatment [43]. Hence, the PUF after chromic acid treatment could be used as COOH-terminated substrate which mimicked the carboxylate-based organic linker. Therefore, the pretreated PUF could provide nucleation sites for MOFs growth and enhance the adhesion between the MOFs and substrates [47]. In the spectra of Zr-MOFs, the peaks at 3388 cm1 and 1712 cm1 were assigned to the stretching vibrations of AOH and CAO in carboxyl groups present in UiO-66-(COOH)2, respectively. And the characteristic peaks at 1581 cm1 and 1402 cm1 were linked to the OACAO stretching in the carboxylate groups of the ligands, which corroborated the interaction between the-COOH and Zr (IV) [48]. Moreover, the peak at 655 cm1 was attributed to ZrAO stretching vibration in Zr-MOFs [31]. The presence of the peak at 1506 cm1 represented CAC ring of benzene in Zr-MOFs structure. For the spectrogram of the Zr-MOFs-PUF membrane, these obvious peaks mentioned above were also observed, which corresponds to the spectrum of UiO-66-(COOH)2. These results demonstrated that the Zr-MOFs (UiO-66-(COOH)2) was successful loaded in the ZrMOFs-PUF membrane. However, the peaks around 1581 cm1

C 1s Zr 3p3 Zr 3p1

Zr 3d Zr 4p

750

(a)

600

PUF

300

150

0

Binding Energy (eV)

(b)

pretreated PUF

Transmittance

450

C 1s

C-C 284.84 eV

Intensity (a.u.)

Zr-MOFs-PUF membrane Zr-MOFs (UiO-66-(COOH)2) C=O -OH

4000

3500

O-C=O 288.83 eV 286.53 eV

Zr-O

C-O

O-C-O

3000

2500

2000

1500

1000

500

-1

Wavenumer (cm )

(b)

282

4.0

284

286

288

290

292

180

178

Binding Energy (eV) 0.00035

1.6

(c)

Pore Diameter : 5.682 nm

0.00021

Zr 3d

Zr 3d5/2 182.86 eV

0.00014 0.00007 0.00000 0

15

30

45

60

Zr 3d3/2

Intensity (a.u.)

2.4

0.00028

dV(d) (cc/nm/g)

Volume @ STP (cc/g)

3.2

75

Pore width (nm)

0.8

185.32 eV

Adsorption Desorption

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po) Fig. 4. (a) FT-IR spectra of PUF, pretreated PUF, Zr-MOFs-PUF membrane and Zr-MOFs (UiO-66(COOH)2); (b) Nitrogen adsorption-desorption isotherm of the Zr-MOFs-PUF membrane at 77 K, inset of (b) is the pore size distribution curve of the Zr-MOFs-PUF membrane.

188

186

184

182

Binding Energy (eV) Fig. 5. The XPS spectra of Zr-MOFs (a) full view; (b) C 1s; (c) Zr 3d.

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membrane was estimated by applying the non-local density functional theory (NLDFT) method [7,50]. The XPS spectra of Zr-MOFs were depicted in Fig. 5(a, b, c). The full view of XPS spectra showed (Fig. 5a) six characteristic peaks which corresponding to O 1s (532.08 eV), Zr 3p1 (346.72 eV), Zr 3p3 (333.44 eV), C 1s (285.10 eV), Zr 3d (183.10 eV) and Zr 4p (30.88 eV). It was obviously indicated the presence of oxygen, zirconium and carbon in the Zr-MOFs samples [51]. The C 1s spectra of Zr-MOFs displayed three peaks (see Fig. 5b) at 284.84 eV, 286.54 eV and 288.85 eV, which attributed to CAC, CAO and OAC@O respectively [51]. The results demonstrated the presence of carboxyl (ACOOH) in Zr-MOFs structure, which was consistent with the results of FTIR described above. The Zr 3d spectra of ZrMOFs could be divided into two peaks at 185.32 eV and 182.86 eV (Fig. 5c), which were assigned to Zr 3d3/2 and Zr 3d5/2 [38]. The XRD patterns of Zr-MOFs and PUF were shown in Fig. 6a. For the prepared Zr-MOFs, it was observed three characteristic diffraction peaks at 2h = 7.3°, 8.5° and 25.6° which was in agreement with the previous literature [44,51]. The results indicated that the Zr-MOFs was successfully synthesized and possessed high crystallinity. The pattern of the PUF was observed a broad diffraction peak at 20.6°, which demonstrates the amorphous characteristic of the PUF [52]. In order to measure the stability of Zr-MOFs-PUF membrane, the XRD spectra of Zr-MOFs-PUF membrane before and after adsorption experiments were analyzed, and the results were depicted in Fig. 6b. The samples of dyes adsorbed into the ZrMOFs-PUF membrane were noted as Zr-MOFs-PUF-dye (RB, MB,

(a)

7.3

Intensity (a.u.)

8.5

25.6

Zr-MOFs (UiO-66-(COOH)2)

20.6

Polyurethane Foam (PUF)

10

20

30

40

50

60

2 Theta (degree)

or CR) membranes. It was observed that the XRD pattern of ZrMOFs-PUF membrane showed four diffraction peaks at 2h = 7.9°, 8.7°, 21.1° and 26.0°, which was ascribed to the XRD pattern of Zr-MOFs and PUF respectively (see Fig. 6a). Compared with the Zr-MOFs-PUF membrane, there were no significant changes in peak positions for the Zr-MOFs-PUF-dye (RB, MB, or CR) membranes. It was demonstrated the good stability of Zr-MOFs-PUF membranes after adsorption of RB, MB, or CR. 3.2. Results of adsorption kinetics experiments The adsorption kinetic behavior of Zr-MOFs-PUF membranes towards RB, MB and CR were described in Fig. 7. The pseudofirst-order and pseudo-second-order kinetic adsorption models were used for fitting the experimental data [53].

Pseudo - first - order kinetic model : lnðQ e  Q t Þ ¼ lnQ e  K1 t ð1Þ Pseudo - second - order kinetic model :

t 1 t ¼ þ Q t K2 Q 2e Q e

ð2Þ

where Qe and Qt (mg/g) are the amount of absorbed dyes at equilibrium and time t (min), K1 (1/min) and K2 (g/(mg min)) are the pseudo-first-order and pseudo-second-order rate constants respectively. The results were observed that the adsorption capacities were gradually increased with the increasing of time until the adsorption equilibrium. The experiment data of adsorption kinetics of RB, MB, and CR on Zr-MOFs-PUF membranes were fitted to pseudo-first-order and pseudo-second-order kinetics models respectively, and the results were presented in Table 1. It was indicated that pseudo-second-order kinetic model was better for fitting these data of adsorption kinetics. The results suggested that the adsorption process was mainly attributed to chemisorption and confirmed that surface functional groups interaction between Zr-MOFs-PUF membranes and dyes [23]. For the adsorption of MB and RB on Zr-MOFs-PUF membranes, the main mechanism is electrostatic interaction. Because MB is positively charged, Zr-MOFsPUF membrane surface is negatively charged due to the free carboxyl acid of organic linkers. The cationic dye MB could be adsorbed by the free carboxyl acid of Zr-MOFs-PUF membrane through electrostatic attraction. The neutral dye RB could also be adsorbed by Zr-MOFs-PUF membrane via electrostatic force owing to the RB is a zwitterion dye which containing both cationic and

(b) Zr-MOFs-PUF-CR membrane

20

15

Qt (mg/g)

Intensity (a.u.)

Zr-MOFs-PUF-MB membrane

Zr-MOFs-PUF-RB membrane 7.9 8.7

21.1

26.0

10

RB MB CR pseudo-first-order kinetic model pseudo-second-order kinetic model

5 Zr-MOFs-PUF membrane

0 10

20

30

40

50

60

2 Theta (degree) Fig. 6. XRD patterns of PUF and Zr-MOFs (UiO-66(COOH)2) (a); XRD patterns of ZrMOFs-PUF membrane before and after adsorption experiments (b).

0

500

1000

1500

2000

2500

3000

Time (min) Fig. 7. The adsorption kinetics of Zr-MOFs-PUF membrane towards RB, MB, and CR in static adsorption experiments.

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J. Li et al. / Journal of Colloid and Interface Science 527 (2018) 267–279 Table 1 Adsorption kinetic parameters of RB, MB and CR on Zr-MOFs-PUF membranes in static adsorption experiments (SE represent the Standard Error). Adsorbent

Adsorbate

Pseudo-first-order kinetic model Qe (cal)

Zr-MOFs-PUF membrane

RB MB CR

17.57 12.99 19.57

SE

Pseudo-second-order kinetic model

K1

SE 3

2.19  10 1.16  103 2.43  103

0.37 0.29 0.29

9.64  10 4.75  105 1.19  104

anionic functional groups [54]. As for the adsorption of CR on ZrMOFs-PUF membrane, the phenomenon might be mainly attributed to the follow two aspects. On the one hand, the Zr-MOFs-PUF membrane tends to make the dye solution faintly acid, which was confirmed by the change of solution color from red to blue when the membrane was introduced [55]. This would result in partial protonation of ANH2 in the CR molecules and electrostatic interaction between protonated dye molecules and Zr-MOFs-PUF membrane [56]. On the other hand, zirconium ions, as an open active site, had a high affinity towards sulfonate groups contained in CR. It could form Zr-(SO 3 ) via Lewis acid-base interaction with coordinate bonds [37]. 3.3. Filtration performance of Zr-MOFs-PUF membranes The removal efficiency (R) of single dyes was calculated according to the following equation:

  Cf  100% R¼ 1 C0

(a)

R 5

ð3Þ

Qe (cal)

0.988 0.991 0.994

21.23 17.43 22.02

SE 0.44 0.49 0.25

K2 1.09  10 5.59  105 1.64  104

Zr-MOFs-PUF membrane

Removal efficiency (%)

84 77 RB MB CR

70

6

7.47  10 4.41  106 9.52  106

PUF membrane

100 80 60 40 20

63 0

0.5

1.0

1.5

2.0

2.5

3.0

5

PUF membrane

15

20

Zr-MOFs-PUF membrane

(d)

100

Removal efficiency (%)

Removal efficiency (%)

(b)

10

25

30

MB initial concentration (mg/L)

Flow rate (mL/h) Zr-MOFs-PUF membrane

80 60 40 20

PUF membrane

100 80 60 40 20 0

0 5

10

15

20

25

RB initial concentration (mg/L)

30

0.993 0.993 0.999

3.3.1. Effect of flow rate The effect of flow rate on single dyes removal by Zr-MOFs-PUF membrane was studied and the results were displayed in Fig. 8a. Filtration experiments with Zr-MOFs-PUF membrane were carried out with flow rates of 0.5–3 mL/h whereas the initial concentration of RB, MB, and CR was fixed at 10 mg/L. In Fig. 8a, the removal efficiencies were 97.39% and 96.58% for RB, 96.85% and 97.84% for MB, 91.75% and 92.31% for CR at the low flow rate of 0.5 and 1 mL/h, respectively. However, the removal efficiency was decreased by 8% for RB, 3% for MB and 10% for CR when the flow rate was increased from 1.5 to 3 mL/h, respectively. This might be attributed to the inadequate contact between the dye molecule and the ZrMOFs-PUF membrane, thus less active site of Zr-MOFs-PUF membrane was utilized when the flow rate was increased. Based on this, the following experiments in our work were carried out at the fixed flow rate of 1 mL/h.

98 91

R2

SE 4

where Cf is the concentration of filtrate of dyes (mg/L) and C0 is the initial concentration of dyes (mg/L).

(c) Removal efficiency (%)

2

5

10

15

20

25

CR initial concentration (mg/L)

Fig. 8. Effect of flow rate (a) and initial concentration (b, c, d) on dyes removal by Zr-MOFs-PUF membrane.

30

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3.3.2. Effect of initial concentration of dyes The effect of initial concentration of dyes on the removal efficiency by Zr-MOFs-PUF membrane was investigated and depicted in Fig. 8b, c, and d. Different charged dyes (i.e., RB, MB, and CR) with different initial concentrations were filtered through ZrMOFs-PUF membrane or PUF membrane. As shown in Fig. 8b, the removal efficiency of RB by PUF membrane without Zr-MOFs was decreased from 62.44% to 38.69% with RB concentrations increasing from 5 to 30 mg/L. The removal efficiencies for MB and CR by PUF membrane were less than 35% (Fig. 8c and d). These results were mainly due to the intrinsic adhesion of PUF membrane and electrostatic interactions between dyes and PUF membranes. SEM results demonstrated that the surface of pretreated PUF membrane possessed randomly rough structures after chromic acid treatment (Fig. 3e). And the improvement of adhesion and wettability of the polymer was attributed to the increasing of surface roughness [57]. The rougher surface of pretreated PUF promoted adhesion between dyes and PUF membrane because of an increased contact surface area. Meanwhile, after a limited extent of chromic acid treatment, the partial chemical composition of polyurethane was oxidized to carboxylic acids [43]. The surface charge of PUF membrane could be changed with the introduction of carboxylic acid, inducing the electrostatic interaction between the dyes and the PUF membrane. In addition, it was noted that all Zr-MOFs-PUF membranes prepared in this work exhibited high removal efficiency (>96%) for RB and MB even at a high dye concentration of 30 mg/L. These could be explained by the strong electrostatic interactions between RB

(a) 10 1.42

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(or MB) and Zr-MOFs-PUF membrane. In the case of CR, the removal efficiency of Zr-MOFs-PUF membrane revealed a different trend (see Fig. 8d). The removal efficiency of CR was gradually decreased from 94.71% to 93.06% with an initial concentration of CR increasing from 5 to 15 mg/L. These could be mainly attributed to the electrostatic interactions between low concentration CR and Zr-MOFs-PUF membranes. Besides, the removal of CR by Zr-MOFsPUF membrane may be resulted from hydrogen bond interaction between the carboxylic groups of Zr-MOFs-PUF membrane and amino-groups of CR [58]. And the Lewis acid-base interactions between zirconium ions in Zr-MOFs-PUF membrane and sulfonate groups in CR molecule should also be taken into account [37]. Nevertheless, it was obviously observed that the removal efficiency of CR was dramatically decreased from 93.06% to 69.19% with initial concentration increasing from 15 to 30 mg/L. This might be attributed to the weakening electrostatic interactions between the anionic dye (CR) and the Zr-MOFs-PUF membrane with increasing CR concentrations. And the available active site of Zr-MOFs-PUF membrane was saturated at high concentration of CR. Therefore, compared with PUF membrane, the removal efficiencies of Zr-MOFs-PUF membrane were 97.95 ± 1.36% for RB within 5–30 mg/L concentrations, 97.67 ± 1.05% for MB within 5–30 mg/ L concentrations, and 94.40 ± 1.33% for CR within 5–15 mg/L concentrations. 3.3.3. Effect of pH The pH of the feeding solution was a significant parameter in practical application of the membrane filtration. The solution pH might influence the surface charge density of membrane and ionization of dyes [59], which would finally affect the removal efficiency of Zr-MOFs-PUF membrane for dyes. In this work, the Zeta potential of Zr-MOFs in different pH conditions was depicted in Fig. 9a. Results showed that the Zeta potential of Zr-MOFs was decreased with increasing pH values and the isoelectric point (IEP) value was about 1.42. It was indicated that Zr-MOFs was negatively charged when the pH value over the IEP. Consequently, the surface of Zr-MOFs-PUF membrane was negatively charged in the pH range of 3–11. The effect of pH on the removal of differently charged dyes by Zr-MOFs-PUF membrane was investigated at pH value ranging from 3 to 11 (see Fig. 9b). It was noted that the removal efficiency of Zr-MOFs-PUF membrane for RB was insensitive to pH values. The removal efficiency of RB was slightly increased from 94.62% to 97.86% with the pH increasing from 3 to 5. But the removal effi-

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NaCl concentration (mol/L) Fig. 10. Effect of NaCl concentration on single dyes removal by Zr-MOFs-PUF membrane (initial concentration of MB, RB, and CR: 20 mg/L; flow-rate: 1 mL/h).

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ciency of RB was slightly decreased from 98.80% to 92.24% when the pH varied from 7 to 11. This might be attributed to the specific molecular forms of RB at different pH conditions [60,61]. The RB mainly existed in cationic and monomeric molecular forms at pH < 4, thus the RB might be attracted by negatively charged ZrMOFs-PUF membrane [60]. When the pH value was higher than 4, the zwitterionic form of RB was increased with the pH increasing, resulting in partially electrostatic repulsion between carboxyl group of RB and Zr-MOFs-PUF membrane. In addition, it was observed that the removal efficiency of MB was increased from 46.99% to 97.57% with the pH increasing from 3 to 9, but slightly decreased from 97.57% to 93.85% with pH ranging from 9 to 11 (see Fig. 9b). These might be due to that the MB commonly existed in positive form [62], thus the MB could be attracted by the negatively charged Zr-MOFs-PUF membrane. At lower pH (3–5), abundant of H+ might be competed with MB (cationic dye) for the available active sites [62], causing the weaker electrostatic attraction between MB and Zr-MOFs-PUF membrane. At higher pH (5–9), the stronger electrostatic force was obtained between MB and the more negatively charged Zr-MOFs-PUF membrane (see Fig. 9a). However, the Zeta potential of Zr-MOFs was slightly increased when the pH was higher than 9 (see Fig. 9a), which resulted in a little decrease of negative charge of Zr-MOFsPUF membrane. Thus, the removal efficiency of MB by Zr-MOFsPUF membrane was decreased slightly. In contrast to MB, the removal efficiency of CR displayed a different trend. The removal efficiency of CR was decreased from 87.39% to 70.74% with the pH value varying from 3 to 11. These might be attributed to the change of CR structure under different pH conditions [55,56], resulting in the different degree of electrostatic interaction between CR and Zr-MOFs-PUF membrane. At pH < 5, the amino and/or azo groups in CR molecule might be protonated [56], thus the electrostatic attraction between protonated CR and Zr-MOFs-PUF membrane was favorable for CR removal. At pH > 5, the CR mainly existed in anionic form [55], resulting in the stronger electrostatic repulsion between CR and Zr-MOFs-PUF membrane and causing lower removal efficiency of CR at basic pH. However, the removal efficiency of CR was slightly increased from 67.61% to 75.35% with the pH increasing from 7 to 9. These might be due to the SO 3 in CR molecule was dominant with pH increasing, which was favorable for the Lewis acid-base interactions between zirconium ions sites and sulfonate groups [37]. With the pH increasing from 9 to 11, the excess of OH ions made the negatively charge density of the surface of CR locating more. Thus, resulting in lower removal efficiency of CR because electrostatic repulsion increased with increasing pH [63]. In addition, other mechanisms explaining the removal of dyes by metal-organic framework were proposed, such as hydrophobic interactions, the p-p stacking interactions, influence of framework metal and hydrogen bonds. These interactions could also to further explain the removal of dyes by Zr-MOFs-PUF membrane [64,65]. According to the results of the effect of pH on the removal of differently charged organic dyes by Zr-MOFs-PUF membrane, it can be concluded that the Zr-MOFs-PUF membrane was more suitable for the removal of zwitterionic form of organic pollutants within a wide range of pH. Besides, the cationic organic pollutants were better to be removed by Zr-MOFs-PUF membrane under alkaline conditions, while the anionic organic pollutants were better to be removed in acidic conditions. 3.3.4. Effect of concentration of NaCl In this study, the effect of concentration of NaCl (0.0–0.5 mol/L) in different single dyes solution on the removal efficiency of ZrMOFs-PUF membrane was investigated. As shown in Fig. 10, it was noted that the removal efficiency of CR was increased significantly from 74.94% to 97.20% with the concentration of NaCl rang-

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ing from 0.0 to 0.3 mol/L and remained stable about 97.83% over the range of 0.3–0.5 mol/L. These might be attributed to that the active sites of Zr-MOFs-PUF membrane were increasingly occupied by sodium ions with increasing NaCl concentrations, which might result in the charge changes on the surface of Zr-MOFs-PUF membrane [51,66]. Therefore, it would contribute to electrostatic attraction between CR (anionic dye) and Zr-MOFs-PUF membrane. However, this phenomenon had an adverse effect on the removal of MB (cationic dye) and RB (neutral dye). The removal efficiency of MB was decreased from 98.93% to 79.52% with NaCl concentration increasing to 0.5 mol/L. The removal efficiency of RB was declined from 96.90% to 88.05% with NaCl concentration increasing from 0.0 to 0.1 mol/L. These could be explained by electrostatic repulsions between MB (or RB) and Zr-MOFs-PUF membrane. Besides, the addition of NaCl concentration might decrease Debye length and weaken the electrostatic interaction between the membrane and the dyes. Thus, the screening effect resulted from the increased concentration of NaCl should also be taken into account for the decrease of removal efficiency towards MB (or RB) [67,68]. While the removal efficiency of RB was slightly increased to 93.16% when NaCl concentration at 0.3 mol/L and was almost no changes within the 0.3–0.5 mol/L, maintaining a relatively high removal efficiency of 93.06%. It was mainly related to the presence of diversified functional groups on RB as described above. Therefore, it can be concluded that the Zr-MOFs-PUF membrane still possessed the relatively high removal efficiencies for RB (93.08%) and MB (79.52%) even if the concentration of NaCl reached 0.5 mol/L. And the presence of salt ions enhanced the removal ability of the Zr-MOFs-PUF membrane for CR.

3.3.5. Filtration experiments for mixed dyes in binary and ternary systems In view of differently charged dyes commonly coexisted and the complexity of actual wastewater, it was important to investigate the simultaneous removal efficiencies of differently charged dyes mixtures by Zr-MOFs-PUF membrane. Therefore, we conducted the filtration experiments for the removal of differently charged mixed dyes from binary and ternary systems by Zr-MOFs-PUF membrane. Before filtration experiments, the UV–vis spectra of RB, MB, and CR in single dye solution were measured and the results were depicted in Fig. 11. It was observed that the characteristic peaks at 499 nm, 554 nm, and 664 nm were assigned to CR, RB, and MB, respectively.

Fig. 11. The spectra of RB, MB, and CR in single dye solution (initial concentration = 10 mg/L).

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Fig. 12. The UV–vis spectra of mixed dyes before and after filtration by Zr-MOFs-PUF membrane: (a) RB/MB mixed solution by membrane; (b) RB/CR mixed solution by membrane; (c) MB/CR mixed solution by membrane; (d) RB/MB/CR mixed solution by membrane.

For binary dyes systems, a solution of RB/MB (RB = MB = 10 mg/ L) was passed through the Zr-MOFs-PUF membrane. As shown in Fig. 12a, it was observed that the UV–vis spectrum of RB/MB mixed solution before filtration displayed two strong characteristic peaks at 554 nm and 664 nm which correspond to RB and MB respectively. However, the peaks of RB and MB were disappeared after filtration by the Zr-MOFs-PUF membrane. And the color of RB/MB mixed solution changed from blue-purple to colorless. It was indicated that RB and MB could be simultaneously removed by onetime filtration with Zr-MOFs-PUF membrane via electrostatic interactions. In addition, for simultaneous dyes removal for RB/ CR and MB/CR binary solutions (RB = CR = MB = 10 mg/L) by ZrMOFs-PUF membrane, similar results were obtained. In Fig. 12b and c, it was seen that the color of RB/CR mixed solution changed from bright red to colorless and the color of MB/CR mixed solution changed from dark cyan to colorless. The UV–vis spectra of RB/CR and MB/CR results showed that the characteristic peaks of RB, MB or CR were disappeared after one-time filtration. It was indicated that binary mixed solutions including RB/CR and MB/CR could be simultaneously removed by Zr-MOFs-PUF membrane through one-time filtration. Nevertheless, the main band of CR was redshifted from 499 nm to 517 nm in the UV–vis spectrum of RB/CR (Fig. 12b). It might be attributed to the hydrogen bonds between the functional groups in RB/CR (ACOO, ANH2), resulting in the formation of tight dyes aggregates and causing the changes of

wavelength of dyes [15,69]. Thus, the RB might be served as a bridge to connect CR and Zr-MOFs-PUF membrane, promoting both RB and CR removal by Zr-MOFs-PUF membrane [70]. For the case of MB/CR binary mixed solution, the peak intensity of MB and CR was slightly weakened and the main band of CR was blue-shift from 499 nm to 474 nm in the UV–vis spectrum of MB/CR (Fig. 12c). These phenomena could be explained by the electrostatic interaction between MB and CR with containing opposite charges, resulting in partial flocculation and causing peak intensity of MB and CR decrease [71]. Therefore, the MB-CR complex and CR could be removed via hydrogen bonds between CR (ANH2, SO 3) and Zr-MOFs-PUF membrane (ACOOH) and Lewis acid-base interactions between other free SO 3 groups of CR and Zr-MOFs-PUF membrane [37,69]. And cationic MB could be removed by negatively charged Zr-MOFs-PUF membrane via electrostatic attraction. For ternary dyes system, a mixed solution of RB/MB/CR (RB = MB = CR = 10 mg/L) was filtered by Zr-MOFs-PUF membrane at the same filtration experiment conditions. In Fig. 12d, it was observed that the main band of CR was blue-shift from 499 nm to 483 nm, the main band of MB was slightly red-shifted from 664 nm to 666 nm and the main band of RB was not interference at 554 nm in the UV–vis spectrum of RB/MB/CR mixed solution before filtration. It might be attributed to the different interactions with different charges of dyes, which resulted in dyes aggregates and the changes of wavelength [69,71]. After the first filtration,

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Fig. 13. Illustration of the mainly potential mechanisms of dyes removal by Zr-MOFs-PUF membrane in RB/MB/CR ternary system.

the deep dark red RB/MB/CR mixed solution was clearly faded and the UV–vis spectrum revealed that all the peak intensity of RB, MB, and CR were obviously weakened. After the second filtration by a new Zr-MOFs-PUF membrane, the RB/MB/CR mixed solution was turned to be colorless. And the UV–vis spectrum displayed that the characteristic absorption peak of RB, MB, and CR were almost disappeared. It indicated that RB, MB, and CR could be simultaneously effectively removed through two-times filtration by ZrMOFs-PUF membrane. These might be mainly resulted from electrostatic interactions, hydrogen bonds and Lewis acid-base interactions as discussed in binary dyes systems, and the mainly potential mechanisms were depicted in Fig. 13. Compared to previous literature about cationic (or anionic) dyes removal by membrane, the obtained results in this work by ZrMOFs-PUF membrane showed great advantages in the aspect of mixed dyes removal. For example, Yao et al. reported a new UiO66-Urea-based flexible membrane with loading 70 wt% of UiO66-NH2 that could only selectively adsorb RB from RB/MB mixture by two filtration runs ([RB] = [MB] = 1.00  105 mol/L) [15]. Chen et al. reported a negatively-charged polyethersulfone nanofibrous membrane that could only selectively remove the cationic dye (MB) from MB/MO mixture or MB/amaranth mixture [24]. Zhan et al. reported a positively-charged nanofibrous composite membrane that exhibited high rejection for anionic dye (Direct Blue 14) whereas low rejection for cationic dye (MB) [25]. Bouazizi et al. showed a bentonite supported nano-TiO2 ultrafiltration membrane that could reject cationic dye (MB) and anionic dyes (Direct red 80 and Acid orange 74) respectively. But the high rejection efficiency was dependent on specific filtration conditions (acid medium for MB and very alkaline medium for Direct red 80 and

Acid orange 74) [72]. Hence, the prepared Zr-MOFs-PUF membrane in this study could be used to efficiently remove mixed dyes in actual wastewater.

4. Conclusions In this study, a Zr-MOFs-PUF membrane was successfully fabricated on porous polyurethane foams by employing an in-situ hydrothermal synthesis method and a hot-pressing process. The Zr-MOFs-PUF membrane was used for the removal of neutral dye (RB), cationic dye (MB) and anionic dye (CR) to assess the performance of the membrane. The static adsorption experiments showed that the pseudo-second-order kinetics model was better to describe adsorption kinetics behavior for RB, MB, and CR in single dye system. And it indicated that the adsorption process mainly derived from chemisorption. For filtration performance, Zr-MOFsPUF membrane exhibited high removal efficiency to RB (97.95 ± 1.36%) and MB (97.67 ± 1.05%) within 5–30 mg/L concentrations, at flow rate of 1 mL/h in single dye system. The maximal removal efficiency of CR was 95.73% at the initial concentration of 10 mg/L and flow rate of 1 mL/h. The pH effect on the removal efficiency of different single dyes indicated that the electrostatic interaction played a dominant role for the removal of RB, MB, and CR by ZrMOFs-PUF membrane. In addition, the removal of CR was partially attributed to the Lewis acid-base interaction and hydrogen bond interaction. The coexisting of NaCl had a positive effect on the removal of CR, had a negative influence on the removal of MB, but had no obvious effect on RB removal in single dye system. The simultaneous removal of RB, MB, and CR was realized by Zr-

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MOFs-PUF membrane in RB/MB, RB/CR or MB/CR binary system and RB/MB/CR ternary system. The possible synergetic effect mechanisms in binary or ternary dyes systems could be explained by the electrostatic interactions, hydrogen bond interaction and Lewis acid-base interaction. In comparison, the as-prepared Zr-MOFs-PUF membrane in this work exhibit exceptionally performance in the removal of mixed dyes with different charges [15,23]. The Zr-MOFs-PUF membrane could simultaneously remove different charged mixed dyes, including neutrally charged (RB), positively charged (MB), and negatively charged (CR), while other researches mainly concentrated on the removal of specific singly charged dyes [24,25]. In conclusion, the prepared Zr-MOFs-PUF membrane may be a suitable candidate in wastewater treatment for its high-efficiency removal for dyes, low cost, and environmentally friendly characteristics. The further work could be focused on exploring the wider application of removing different types of pollutants and improving the anti-fouling property of the membrane. In addition, the further investigation needs to be conducted in searching for more special multi-functional materials which can remove specific target pollutants.

Acknowledgements The authors are grateful for the financial supports from National Natural Science Foundation of China (51521006, 51579095, 51378190 and 21675043), the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R17), Hunan Province University Innovation Platform Open Fund Project (14K020), the Interdisciplinary Research Funds for Hunan University, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

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