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Magnetic Zr-MOFs nanocomposites for rapid removal of heavy metal ions and dyes from water
Accepted Manuscript Magnetic Zr-MOFs nanocomposites for rapid removal of heavy metal ions and dyes from water Lijin Huang, Man He, Beibei Chen, Bin Hu PII:
S0045-6535(18)30219-4
DOI:
10.1016/j.chemosphere.2018.02.019
Reference:
CHEM 20777
To appear in:
ECSN
Received Date: 16 February 2017 Revised Date:
30 December 2017
Accepted Date: 4 February 2018
Please cite this article as: Huang, L., He, M., Chen, B., Hu, B., Magnetic Zr-MOFs nanocomposites for rapid removal of heavy metal ions and dyes from water, Chemosphere (2018), doi: 10.1016/ j.chemosphere.2018.02.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Magnetic Zr-MOFs nanocomposites for rapid removal of heavy
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metal ions and dyes from water
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Lijin Huang, Man He, Beibei Chen, Bin Hu*
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Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of
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Education), Department of Chemistry, Wuhan University, Wuhan 430072, Hubei
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Province, P. R. China
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*
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[email protected]
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Corresponding Author. Tel: 86-27-68752701; Fax: 86-27-68754067; E-mail address:
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Abstract
Amino-decorated Zr-based magnetic Metal-Organic Frameworks composites
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(Zr-MFCs) were prepared by a facile and efficient strategy. The nano-sized
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Fe3O4@SiO2 core (about 15 nm) was coated with a shell of Zr-MOFs (about 5 nm) by
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means of in-situ growth. And, Fe3O4@SiO2@UiO-66 and its amino derivatives
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(Fe3O4@SiO2@UiO-66-NH2 and Fe3O4@SiO2@UiO-66-Urea) were successfully
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prepared by using different precursors. The obtained Zr-MFCs were demonstrated to be
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efficient adsorbents for metal ions/organic dyes removal from aqueous solution, with
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high adsorption capacity and fast adsorption kinetics. It was found that the
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amine-decorated MFCs were highly efficient for metal ions/dyes removal compared to
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raw MFC-O. Among them, MFC-N exhibited the highest capacity for Pb2+ (102 mg g-1)
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and methylene blue (128 mg g-1), while MFC-O exhibited the highest capacity for
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methyl orange (219 mg g-1). Moreover, anionic and cationic dyes could be selectively
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separated and removed from the mixed solution just by adjusting the solution pH with
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Zr-MFCs as the adsorbents. And these Zr-MFCs materials can be easily regenerated by
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desorbing metal ions/organic dyes from the sorbents with appropriate eluents, and the
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adsorption capacity can be remained unchanged after 6 recycles. The obtained results
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demonstrated the great application potential of the prepared MFCs as fascinating
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adsorbents for water treatment.
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Keywords: Metal-Organic Frameworks; magnetic nanocomposites; heavy metal ion
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removal; water treatment; dye separation and removal; core-shell structure
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1. Introduction With the rapid industrial development, accidental and purposive dumping of
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industrial waste such as heavy metal ions and dyes has contributed to the serious
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problem of water pollution, which is dangerous for the health of living organisms
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(Wang et al., 2014; Sun et al., 2011). Consequently, a variety of techniques have been
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adopted for the uptake of ions or dyes from aqueous solution, including chemical
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precipitation, adsorption, membrane systems and ion exchange (Hashim et al., 2011;
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Gupta et al., 2009; Kumar and Guliants, 2010; Dabrowski et al., 2004). Among them,
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adsorption featuring with simple and fast separation, efficient and low-cost has been
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extensively used for water treatment (Gupta and Suhas, 2009; Ali and Gupta, 2007).
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The choice of adsorbents largely influence the adsorption capacity and selectivity in the
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adsorption process to a great extent, and the low adsorption capacities and poor
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selective sorption have limited the application of common adsorbents (e.g. activated
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carbon (Mohan et al., 2008), zeolites (Meshko et al., 2001) and natural fibers (Zein et
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al., 2010). Therefore, development of novel adsorbents with high efficiency and
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selectivity for target pollutants has provoked great interest and become the main
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orientation in adsorption field presently. In this regard, some newly appeared materials
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(carbon nanotubes (Liu et al., 2013; Upadhyayuya et al., 2009), grapheme (Han et al.,
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2014; Cong et al., 2012), layered double hydroxides (Ma et al., 2014; Ma et al., 2014),
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and other materials (Taskin et al., 2014; Li et al., 2014) have been investigated as the
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adsorbents for heavy metals trapping, and they have exhibited great potential as the
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adsorbents for water treatment. While tedious high-speed centrifugation or filtration
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separation after adsorption is required, hindering the extensive application of such
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adsorbents. Recently, widespread application of magnetic substrates to address this issue
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facilitates the isolation of trapped species and separation from sample solutions for
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reuse (Kaur et al., 2014). The combination of magnetic nanoparticles (MNPs) and
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desirable building blocks or components could contribute to cooperatively enhanced
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performance during adsorption.
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Metal-Organic Frameworks (MOFs) and/or coordination polymers with permanent
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porosity, which were constructed through connecting inorganic metal nodes and
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organic building blocks via coordination bonds, have received wide attention in
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catalysis, gas storage and drug delivery, etc. Their excellent chemical and thermal
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stability, well-order porosity, high surface area and easy functionalization of their pores
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or outer-surface are highly favourable to adsorption (Hasan et al., 2015; Burtch et al.,
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2014) and separation (Voorde et al., 2014). MOFs have been widely utilized to remove
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hazardous pollutants (Hasan et al., 2015; Ayati et al., 2016; Roushani et al., 2016), such
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as dyes or heavy metal ions in water, demonstrating a great application potential in
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water treatment. Magnetic MOFs composites (MFCs), which integrate the advantages
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of fast separation of magnetic materials and the superior properties of MOFs, have been
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widely used in different technological fields (Ricoo et al., 2013), and offer an attractive
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alternative platform for dyes/heavy metal ions capture. However, a large proportion of
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ACCEPTED MANUSCRIPT the MFCs are functionalized with HKUST-1 currently (Zhao et al., 2015; Xiong et al.,
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2015; Bagheri et al., 2012; Ke et al., 2012; Wang et al., 2013 ), and HKUST-1 is
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unstable under acid solution and would release toxic Cu2+ (Flemming et al., 1989;
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Huang et al., 2015). Comparatively, MOFs containing zirconium metal nodes with
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strong Zr(IV)-O bonds exhibit better stability in water across a broad pH range (Cavka
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et al., 2008; Kandiah et al., 2010; Kalidindi et al., 2015; Piscopo et al., 2015). In
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addition, Zr-MOFs inherit remarkable thermal and mechanical stability (Cavka et al.,
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2008; Wu et al., 2013). Due to the nodes with abundant Zr-bound hydroxides, Zr-MOFs
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exhibited excellent performance for removal of selenium species from drinking water
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(Howarth et al., 2015). However, the preparation/application of magnetic Zr-MOFs in
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water treatment is still scarce (Zhao et al., 2014; Zhao et al., 2014; Zhang et al., 2015).
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Even more distressing is that the present approaches usually require specific surface
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modification for the in-situ growth of Zr-MOFs on the MNPs surface and the prepared
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MFCs are in micro-size (>200 nm). On the other hand, it has been proven that amino
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modification can be an effective strategy to increase the uptake of metal ions or dyes
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from aqueous solution (Ricco et al., 2015; Haque et al., 2010; Huang et al., 2014;
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Roushani et al., 2016). So it is quite attractive to develop more simple and efficient
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methods to prepare amino-functionalized nano-sized Zr-MFCs.
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Herein, a serious of nano-sized Zr-MFCs, including Fe3O4@SiO2@UiO-66 (denoted
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as
MFC-O),
Fe3O4@SiO2@UiO-66-NH2
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Fe3O4@SiO2@UiO-66-Urea (denoted as MFC-U), have been prepared successfully 5
(denoted
as
MFC-N)
and
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showed core-shell structure, and the shell thickness is adjustable through adjusting the
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quantity of predecessor. These nano-sized Zr-MFCs were investigated for the
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extraction of both dyes and metal ions. They exhibited excellent adsorption property
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both for heavy metal ions and cationic/anionic dyes, and relatively high thermal and
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chemical stability. Besides, fast separation and good recovery of MFCs from aqueous
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solution can be realized just by applying external magnetic fields, demonstrating a very
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good application prospect for dyes/heavy metal ions removal from environmental
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water.
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he m eI
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Scheme 1 Schematic illustration of MFCs preparation.
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2. Experimental
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2.1 Chemicals and materials
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All the analytical grade chemicals were commercially available and used without
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further purification. The stock solution (1000 mg L-1) of Hg2+, Cd2+, Cr3+, Co2+, Mn2+,
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Ni2+, Pb2+, Cu2+, Zn2+ was prepared by HgCl2, Cd(NO3)2·4H2O, Pb(NO3)2, 6
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Zn(NO3)2·6H2O, respectively. 1000 mg L-1 of methylene blue (MB) and methyl orange
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(MO) were prepared in deionized water, respectively. The solutions with desired
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concentration were prepared by stepwise dilution of respective stock solution.
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2.2 Synthesis of MFCs
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The MFC-O was synthesized according to our previous work (Huang et al. 2015).
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The MFC-O with different shell thickness were prepared by varying the MOF
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precursors reaction mixture concentration (12.5, 25.0 and 50.0 mM, denoted as
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MFC-O-1, MFC-O and MFC-O-2) while the amount of Fe3O4@SiO2 (MNPs) was kept
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constant. For the preparation of MFC-U, urea (25.0 mM) was added into the reaction
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mixture following a similar procedure with above.
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that the NH2-BDC was used in place of H2BDC.
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2.3 Characterization of materials
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Magnetic separation was conducted through Nd-Fe-B magnet. NEXUS 870 Fourier
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transform infrared spectrometer (FT-IR) (Thermo, Madison, USA) was employed for
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the characterization of the prepared MFCs. Mettler Toledo 320-S pH meter (Mettler
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Toledo Instruments Co. Ltd., Shanghai, China) was applied for solution pH adjustment.
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Thermogravimetric analysis (TGA) was conducted on diamond TG/DTA 6300
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(PerkinElmer, USA). The crystalline structure of MFCs was identified by Bruker D8
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diffractometer (Germany). The surface areas of MFCs were obtained on ASAP 2020
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microscope (SEM, Hitachi, Tokyo, Japan) equipped with an energy dispersive X-ray
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(EDX, SUTW-SAPPHIRE) system were employed to identify the chemical
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composition of the samples.JEM-2010 electron microscope (Tokyo, Japan) was
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adopted to detect the transmission electron micrograph (TEM) image of the samples.
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PPMS-9 vibrating sample magnetometer (VSM) (QUANTOM, USA) was the
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materials’ magnetic properties. The zeta potential of MFCs under different pH was
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measured on a zeta potential analyser (Malvern Nano ZS ZEN 3600, UK). UV-Vis
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spectrophotometer (Shimadzu, UV-1800, Japan) was carried out to determine the
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concentration of MO and MB at 464 and 665 nm, respectively. The concentration of
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target metal ions was determined by intrepid XSP radial inductively coupled plasma
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optical emission spectrometry (ICP-OES) (Thermo, Waltham, MA, USA).
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2.4 Adsorption studies
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10 mg MFCs was spiked into 10 mL of metal ions/dyes solution with appropriate
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pH (pH 6 for metal ions, pH 3 for MO and pH 11 for MB), which was then shaken for
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60 min at room temperature. Subsequently, the concentration of metal ions/dyes in
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remaining solution was analysed using ICP-OES or UV-Vis spectrophotometry,
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respectively.
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The time-dependence of target removal was performed to study the adsorption
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kinetics, and the MFCs were spiked at a level of 1 mg mL-1 at room temperature. The
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concentration of metal ions/dyes at different time (2, 5, 10, 20, 30, 40, 50, 60 min) 8
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experiments, all were performed in triplicate.
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3 Results and discussion
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3.1 Materials characterization
(b)
(a)
MFC-O
MFC-O MFC-N MFC-U Fe3O4@SiO2 simulated
MFC-N MFC-U
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Fe3O4@SiO2
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20
30
2 theta
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Urea
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4000
3000
2000
1500
1000
500
-1
Wavelength (cm )
Fig. 1 PXRD (a) and FT-IR spectra (b) of different kinds of MFCs. Fig. 1(a) presents the XRD patterns of MNPs, MFC-O, MFC-N and MFC-U,
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respectively. The diffraction patterns of MFCs are in accordance to the published data
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(Kandiah et al. 2010). The diffraction peaks at about 31° and 36° are assigned to Fe3O4
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(JCPDS No. 19-0629), which demonstrate the successful introduction of Fe3O4 crystals
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in the hetero-nanostructure. And the characteristic peaks of the prepared MFCs are
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consistent with that of UiO-66, indicating that magnetic modification has no influence
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on the parent MOFs crystal structure.
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Fig. 1(b) showed the FT-IR spectra of the MFCs. For MNPs, the broad peak around
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1090 cm-1 can be designated to O-Si-O stretching vibration. While for UiO-66 and
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MFC-O, the two peaks at 1709 cm-1 and 1645 cm-1 are assigned to C=O bonds, and
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peaks at 1447 cm-1 and 1374 cm-1 are attributed to the C=C stretching vibration on the 9
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the spectra of MFC-N: C-N stretching absorption is found at 1257 and 1340 cm−1, and
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N-H wagging is observed at 764 cm−1. These FT-IR results confirm the presence of
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NH2 group in MFC-N (Kandiah et al. 2010). For the urea containing MFC-U, the peak
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at 1350 cm-1 is designated to the C-N vibration of CO(NH2)2 and the peak around 1600
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cm-1 can be ascribed to the C=O stretching vibration in BDC linker, C-N and N-H in
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urea (Kandiah et al. 2010); the deformation vibration band for urea carbonyl (C=O) is
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observed around 1700 cm-1, revealing that urea was incorporated into MFC-U
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successfully.
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Fig. 2 N2 adsorption-desorption isotherms of MNPs, MFC-O, MFC-N and MFC-U.
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The
porous
structure
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as-prepared
MFCs
was
investigated
by
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Brunauer-Emmett-Teller (BET) gas-sorption experiments, as shown in Fig. 2 and Table
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1. It can be observed that MFC-O exhibited much higher surface area than MNPs,
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which indicated UiO-66 deposited on MNPs successfully. The surface area of MFC-N
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reduced slightly compared with that of MFC-O, since the NH2 groups would like to
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area (~ 33 %) and pore volume (~21 %) are found for MFC-U comparing with that of
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MFC-O. These differences may be caused by the formation of imides between urea and
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the carboxylic acid (Ebrahim et al., 2014), which would bind to zirconium centers
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either via oxygen or nitrogen groups, leading to significant amount of missing-linker
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defects.
Table 1 Surface area and pore volumes for MNPs, MFC-O, MFC-N and MFC-U BET Surface area (m² g-1)
MNPs MFC-O
UiO-66a
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0.34
782
0.47
722
0.42
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Pore volume (mL g-1)
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Note: (a) the data came from ref (Ebrahim et al., 2012).
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TGA was carried out to evaluate the thermal stability of the MFCs (Fig. 3). As can be
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seen, after MOFs loading, a slight weight loss under 200 °C was observed for all MFCs,
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which is ascribed to the loss of water/solvent physically adsorbed on the MFC and the
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dehydroxylation of the zirconium oxo-cluster. For MFC-O, the weight-loss under
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520 °C could be caused by the decomposition of organic ligands to convert into ZrO2
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(Kandiah et al. 2010). In comparison with UiO-66, the thermal decomposition
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temperature of MFC-N is below 350 °C due to the introduced amino group. For
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Fig. 3 TG curves of MNPs, UiO-66-NH2, MFC-O, MFC-N and MFC-U.
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MFC-U, the weight loss of the composite is observed at 420 °C, which may be caused
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by the urea modification. And due to the presence of urea grafted on MOFs, MFC-U
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exhibited more weight loss compared with MFC-O. The differences between the
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weight loss of MFC-O and MFC-U suggest that about 7 % of urea is present in the
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MFC-U. All the results indicate that the prepared MFCs exhibit exceptionally high
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thermal stability.
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Fig. 4 TEM images of MNPs (a), MFC-O (b), MFC-N (c), MFC-U (d) and elemental
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mapping images of MFC-O (e).
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The formation of MFCs can be confirmed via TEM images of magnetic composites
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shown in Fig. 4. The images of TEM show define core-shell structure for all the 12
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about 5 nm (~3.8 nm for MFC-O; ~4.1 nm for MFC-N; ~5.1 nm for MFC-U). SEM was
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employed to further confirm their morphology (Fig. S1). What’s more, the thickness of
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the shell in core-shell structures can be controlled just by adjusting the quantity of
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predecessor. Three MFCs have been prepared, including MFC-O-1, MFC-O and
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MFC-O-2, and the corresponding quantity of UiO-66 predecessor were 12.5, 25.0 and
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50.0 mM. Their structure was characterized by TEM and the results (Fig. S2) showed
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that the shell thickening with the rising of MOF precursor concentration from 12.5 to
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25.0 mM. When the concentration of UiO-66 predecessor was up to 50.0 mM, some of
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the obtained nanoparticles (MFC-O-2) preferred to self-seeding nucleate in the solution
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and grow into much larger crystal rather than forming core-shell structure MFCs.
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EDX spectra was used to determine the relative contents of elements in MFCs(Table
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S1 and Fig. S3). As can be seen, C, N, O, Si, Fe and Zr were involved in the prepared
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MFC-N and MFC-U, while no obvious signal of N was observed for MFC-O. Fig. 4(e),
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Fig. S4 and S5 are the element mapping results for MFC-O, MFC-N and MFC-U,
228
respectively. Fe, Si, O, Zr and C are homogeneously distributed throughout all these
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three materials, and N was found in MFC-N and MFC-U. Taking into consideration the
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results of TEM, it could be speculated that the as-synthesized MFCs are uniform
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core-shell composites rather than a simple mixture of two phases of MNPs and MOF
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crystals.
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The formation mechanism (Scheme 1) of uniform Zr-MOF shell is described by the 13
ACCEPTED MANUSCRIPT following processes: (1) ZrCl4 was firstly adsorbed onto the hydroxylated SiO2 surface,
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reacting with SiO2-OH group and forming a stable complex (Han et al., 2004; Widjaja
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et al., 2003); (2) the H atom of the surface OH group combined with one Cl atom of the
237
ZrCl4 which was adsorbed , forming intermediate complex SiO2-O-ZrCl3 and HCl; (3)
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the by-product HCl would assist the formation of UiO-66 (Katz et al., 2013), which
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grows on the surface of Fe3O4@SiO2-O-Zr3+ as the shell under solvothermal synthesis
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in the presence of H2BDC.
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Fig. 5 VSM magnetization curves of MNPs, MFC-O, MFC-N, and MFC-U (The
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insets digital images show that MFCs can be easily separated from water under an
244
external magnetic field).
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VSM magnetization curves were used to characterize the magnetic properties of the
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MFCs (Fig. 5). As compared to MNPs, the saturation magnetization of the MFCs
247
decreased from 51 to 16, 12 and 6 emu g-1 as the magnetic contents of the MFCs
248
decreased. This can be verified by the ICP-OES results that the Fe contents in the MFCs
249
followed such order: 13.78% (MFC-O)> 12.19% (MFC-N)> 9.72% (MFC-U), which
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coercivity (HC) and retentivity from the expanded hysteresis loop between ±0.02 kOe
252
was found to be 1.7 Oe and 0.47 emu g-1 for MNPs, 7.4 Oe and 0.38 emu g-1 for MFC-O,
253
8.0 Oe and 0.26 emu g-1 for MFC-N, 8.9 Oe and 0.06 emu g-1 for MFC-U, respectively.
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These results clearly demonstrated that all MFCs are superparamagnetic.
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Additionally, due to excellent hydrophilic property of Zr-MOFs and their uniform
256
nano structure, all the MFCs dispersed well in water. Moreover, the inset images of Fig.
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5 showed that the MFCs (using MFC-O as the example) can be easily recovered with a
258
permanent magnet.
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3.2 Effect of pH
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Solution pH plays an important role in adsorption performance of sorbent. A proper
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solution pH not only benefits for adsorption, but also reduces the interference of matrix.
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The effect of solution pH on elimination behaviour of multi heavy metal ions by using
263
MFCs was conducted (Fig. S6). The removal efficiency gradually increased with the
264
increasing pH from 2 to 7. And for MFC-O, Pb2+, Cr3+ and Cu2+ were quantitatively
265
removed at pH 7; for MFC-N, Pb2+, Cr3+, Cu2+ and Hg2+ were quantitatively removed at
266
pH 7; for MFC-U, Pb2+, Cr3+ and Cu2+ were quantitatively removed at pH 4-7, and the
267
removal efficiency of Hg2+ was maintained around 80% in the pH range of 4-7. The
268
elimination mechanism is summarized as follows. The carboxyl and hydroxyl
269
functional groups on the MFC-O are protonated and the protons would combine with
270
-COO- and -O- competing with metal ions under acidic condition, resulting in a lower
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ACCEPTED MANUSCRIPT elimination efficiency (Musico et al., 2013). When the pH is above 4, functional groups
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of -COOH and -OH on MFCs exist as -COO- and -O-, leading electrostatic and
273
chelating interaction between metal cationic and MFCs. The experimental results also
274
showed an improvement in adsorption for Pb2+ by the prepared MFC-N and MFC-U
275
comparing with MFC-O, probably due to the amine modification. The free -NH2
276
groups on MFC-N and MFC-U not only provide more binding sites for adsorbing, but
277
also can interact with Pb2+ by chelating (Aguado et al., 2009), enhancing the adsorption
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of Pb2+ and other species onto MFC-N and MFC-U.
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Simultaneously, the adsorption performance of MFCs for MB (cationic dye) and MO
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(anionic dye) was investigated (Fig. S7). As can be seen, the affecting tendency of pH
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on the removal efficiency of cationic dye is contrary to that of anionic dye. The uptake
282
amounts decrease with the increasing pH of MO solution, which is similar to previously
283
reported results (Haque et al., 2010; Haque et al. 2011), while the adsorbed amounts
284
increase with increasing the pH of MB solution. Based on it, the adsorption mechanism
285
of MO and MB on MFCs may be explained by the electrostatic and π-π stacking
286
interaction between the dyes and MFCs. MO (pKa=3.4) exists in a mixture of neutral
287
molecular and anionic species at pH 3.4, while the surface of MFCs is positively
288
charged at pH 3 as shown in Fig. S8. The negative form of MO can be adsorbed by
289
electrostatic interaction and the molecular form can be adsorbed by π-π stacking and
290
hydrogen bonding between benzene rings of MFCs and MO. MB is an alkaline dye,
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existing in cationic form, and can be adsorbed on the negatively charged MFCs (Fig. S8)
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and hydrogen bonding interaction and π-π stacking may be the main factor.
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Fig. 6 Photographs of the MB-MO mixture before and after adsorption by MFC-O
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under different pH.
In consideration of the different adsorption behaviour for MO and MB under
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different pH condition, MFCs are expected to be effective for the treatment of anionic
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and cationic dye mixture. To verify this application potential, the adsorption
300
performance of the prepared MFCs for dyes mixture (cationic dye MB and anionic dye
301
MO) was studied by mixing 10 mg MFCs with 10 mL solution containing 20 mg L-1 of
302
MB and 20 mg L-1 of MO under different pH (Table 2). As can be seen, with MFC-O as
303
adsorbent, 98% MO and only 8% MB were adsorbed under pH 3, while about 25% MO
304
and 98% MB were adsorbed under pH 11; both MO and MB were adsorbed almost
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17
ACCEPTED MANUSCRIPT completely under pH 7. This means that MFC-O can not only simultaneously adsorb
306
MO and MB under pH 7 but also selectively adsorb MO under pH 3 or MB under pH 11.
307
The photographs of dye mixture before and after treated (MFC-O as an example) under
308
different pH shown in Fig. 6. MFC-N and MFC-U exhibited similar selectivity for MO
309
and MB under pH 3 and 11. These results demonstrate that the prepared MFCs may be
310
an attractive alternative platform for multi dyes separation and removal.
Table 2 Selective removal MB or MO from their mixture under different pH
Removal
efficiency (%)
M AN U
MO
MB
pH 3
pH 7
pH 11
pH 3
pH 7
pH 11
MFC-O
98
100
25
8
98
98
MFC-N
99
84
1
4
95
98
MFC-U
98
89
2
6
93
98
TE D
312
SC
311
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3.3 Adsorption kinetics
Adsorption kinetics of Pb2+ and dyes onto the MFCs were also investigated (Fig.7).
314
The results (Fig. 7(a)) indicated that the adsorption of Pb2+ on MFCs could reach
315
equilibrium within 20 min. Meanwhile, the MFCs also showed similar adsorption
316
kinetics for MB (Fig. 7(b)) and MO (Fig. 7(c)). Pseudo-second-order kinetic rate
317
equation was employed to model the experimental data and the linear regression plot
318
of time (t) and t/Qt shows a good linear pattern with R2=0.999 (Fig. S9). What’s more,
319
the calculated Qe value was in accordance to the experimental data, suggesting the
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18
ACCEPTED MANUSCRIPT 320
adsorption process of Pb2+ and dyes on the as-prepared MFCs follow the
321
pseudo-second-order kinetic model.
(a)
10
0 0
20
40
60
80
Time (min)
322
100
120
-1
Concentration(mg L )
M AN U
(b)
50
SC
MFC-O MFC-N MFC-U
RI PT
-1
Concentration(mg L )
20
40
MFC-O MFC-N MFC-U
30 20
TE D
10 0
0
323
20
40
60
80
100
120
Time (min)
50
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-1
Concentration(mg L )
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(c)
40
20 10 0 0
324 325 326
MFC-O MFC-N MFC-U
30
20
40
60
80
100
120
Time (min)
Fig. 7 Adsorption curve of Pb2+ (a), MB (b) and MO (c) versus incubate time in aqueous solution by using MFCs (initial concentrations of Pb2+ and dye : 50 mg L-1).
19
ACCEPTED MANUSCRIPT 3.4 Adsorption isotherms
(a)
100 80 60 40
MFC-O MFC-N MFC-U
20
0
50
100
150
200
-1
-1
Adsorbed amount (mg g )
328
(b)
M AN U
120
80
250
300
SC
Ce (mg L )
RI PT
-1
Adsorbed amount (mg g )
327
MFC-O MFC-N MFC-U
40
10
20
30
40
-1
TE D
0
Ce (mg L )
-1
Adsorbed amount (mg g )
329
(c)
EP
200 150
330 331 332 333
AC C
100
MFC-O MFC-N MFC-U
50 0 0
10
20
-1
30
Ce (mg L )
Fig. 8 Adsorption curve of Pb2+ (a), MB (b) and MO (c). The effect of concentration of target compounds on adsorption capacity of MFCs was shown in Fig. 8. Fig. 8(a) showed the adsorption isotherm of Pb2+ on MFCs. The 20
ACCEPTED MANUSCRIPT Table 3 Adsorption capacities of metal ions/dyes on different MFCs and Zr-MOFs Capacity (mg g-1)
Adsorbents MO
MB
Cr
Pb
Cu
Hg
MFC-O
219
116
63
68
40
10
MFC-N
130
128
51
102
45
60
MFC-U
183
121
57
UiO-66 (Chen et al., 2015)
39
90
UiO-66-NH2 (Chen et al., 2015)
29
96 48
Composites (Ai et al., 2011)
Fe3O4@ZIF-8 core-shell
50
27
74
M AN U
Fe3O4@MIL-100(Fe) (Shao et al., 2016)
77
SC
Fe3O4-MWCNTs
RI PT
334
20
heterostructure (Zheng et al., 2014)
TE D
Al-based MFCs (Ricco et al., 2015) Pyridine functionalized MFCs
492 190
(Sohrabi et al., 2013)
Covalent triazine-based
291
EP
framework/Fe2O3 composites (Zhang et al., 2011)
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Protonated
194
ethylenediamine-grafted-MIL-101(Cr) (Haque et al., 2010)
MOF-235 (Haque et al., 2011)
477
335
adsorbed amount of Pb2+ on MFCs raised sharply and reached saturation gradually. At
336
equilibrium, MFC-N was able to uptake 102 mg g-1 Pb2+. This adsorption capacity is
337
much greater than that of MFC-O and MFC-U (Table 3). The difference may be caused 21
ACCEPTED MANUSCRIPT by the more NH2 groups anchored on MFC-N which exhibit stronger interaction with
339
Pb2+. This interaction was further verified by the FT-IR spectra (Fig. S10). Comparing
340
with as-prepared MFC-N, the characteristic peaks corresponding to -NH2 shift from
341
3395 cm-1 to 3372 cm-1, which indicates the NH2 may interact with Pb2+ (Hu et al.,
342
2016).
RI PT
338
Due to the precipitation of both dyes (MB and MO) under high concentration, the
344
adsorption isotherm of MB and MO was carried out by fixing the concentration and
345
increasing sample volume (Fig. 8(b) and (c)). As is showed in Table 3, MFC-N showed
346
the highest adsorption capacity for MB, while least for MO. The different adsorption
347
performance for dyes may be attributed to the electrostatic attraction interactions
348
between the adsorbents and dyes. After modification with -NH2, the resulted MFC-N
349
has more negative zeta potential than MFC-O under high pH (Fig. S8), thus favouring
350
the adsorption of more cationic dye MB. For anionic dye MO, MFC-N with lowest zeta
351
potential among the prepared MFCs under acid condition (Fig. S8) presented the lowest
352
adsorption capacity. Comparing with naked MOFs (Chen et al., 2015), MFCs showed
353
larger adsorption capacities attributing to their distinct core-shell structure.
M AN U
TE D
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354
SC
343
Langmuir model was adopted to analyse the adsorption isotherms of MFCs for target
355
compounds (Fig. S11). And the correlation coefficient of linear regression between
356
Ce/qe and Ce was 0.9998, suggesting that the uptake of metal ions (Pb2+) and dyes (MO
357
and MB) on as-prepared MFCs fits Langmuir’s adsorption model well (Table S2).
358
3.5 Stability and recycling 22
100
(a)
80 60 40
MFC-O MFC-N MFC-U
20 0
2
1
6
5
(b)
-1
SC
80
M AN U
Adsorption amounts (mg g )
4
120
MFC-O MFC-N MFC-U
40
0 1
360
2
4
3
Number of cycles
5
6
(c)
TE D
-1
3
Number of cycles
359
Adsorption amounts (mg g )
RI PT
-1
Adsorption amounts (mg g )
ACCEPTED MANUSCRIPT
200 160
361 362
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120
80 40
MFC-O MFC-N MFC-U
0 1
2
3
4
Number of cycles
5
6
Fig. 9 Pb2+ (a), MB (b) and MO (c) removal on recycled MFCs.
363
Solvent desorption was employed for adsorbent regeneration. Methanol, DMF,
364
acetone, ethanol, HNO3 and NaOH were tested as the eluent to regenerate MFCs. And
365
the regeneration was successfully achieved by using 0.5 M HNO3, methanol and 0.01
23
ACCEPTED MANUSCRIPT 366
M NaOH for Pb2+, MB and MO-adsorbed MFCs, respectively. Fig. 9 shows that MFCs
367
can be reused at least 6 times without significant loss of adsorption capacity, indicating
368
that all the MFCs possess a good regeneration ability for water treatment. The XRD patterns of reused MFCs after 5 adsorption/elution cycles of Pb2+ and dyes
370
were studied respectively (Fig. S12). In order to further confirm the stability of MFCs
371
after adsorption, leaching test was carried out by soaking the prepared MFCs in a series
372
of solutions with different pH (2-12) for 2 h, and determining the Zr concentration in
373
the supernatant by ICP-OES. The results (Fig. S13) reveal that < 0.5 mg L-1 Zr was
374
released from the MFCs, even after 15 h soaking. Furthermore, TEM was employed to
375
investigate the microstructure of the adsorbents after five adsorption/elution cycles of
376
metal ions and dyes. As shown in Fig. S14, the structure of the adsorbents after the
377
adsorption/elution cycles kept the original appearance, demonstrating the structural
378
stability of the prepared MFCs during the extraction process. In addition, the BET
379
surface areas and pore volumes of the adsorbents after 5 adsorption/elution cycles
380
(Table S3) decreased slightly. All these results reveal that the structures of all the
381
prepared MFCs are chemically stable under the experimental conditions.
382
4. Conclusions
SC
M AN U
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383
RI PT
369
In this paper, three kinds of core-shell nano Zr-MFCs with controllable encapsulating
384
have been synthesized by facile solvothermal method. Through replacing BDC with
385
NH2-BDC or by adding urea to the components forming UiO-66, two NH2 decorated
386
MFCs were obtained. All the prepared Zr-MFCs are found to be efficient for heavy 24
ACCEPTED MANUSCRIPT metal ions and organic dyes capture. The adsorption results clearly indicate that the
388
introduction of amino groups greatly enhances the ability for metal ions/dyes capture
389
from water. By selecting the appropriate pH, cationic dye (MB) and anionic dye (MO)
390
can be simultaneously removed under pH 7 or selectively removed (MO under pH 3
391
and MB under pH 11). The prepared three kinds of Zr-MFCs can be easily regenerated
392
and reused for more than 6 times. Owing to their flexible structures and intrinsic
393
properties, all of these Zr-MFCs are attractive for rapid and efficient water treatment.
394
Moreover, on account of various studies on Zr-MOFs, the presented magnetic
395
Zr-MOFs may find their application potential in other technological fields.
396
Acknowledgments
M AN U
SC
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387
This work is financially supported by the National Basic Research Program of China
398
(973 Program, 2013CB933900) and the National Natural Science Foundation of China
399
(Grant Nos.: 21175102, 21205090, 21575107).
400
Appendix A. Supplementary data
EP
Supplementary data associated with this article can be found, in the online version,
AC C
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397
402
at http://dx.doi.org/10.1016/j.chemoshpere.************
403
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Xiong, Y., Ye, F., Zhang, C., Shen, S., Su, L., Zhao, S., 2015. Synthesis of magnetic
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porous [gamma]-Fe2O3/C@HKUST-1 composites for efficient removal of dyes and
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heavy metal ions from aqueous solution. RSC Adv. 5, 5164-5172.
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Zein, R., Suhaili, R., Earnestly, F., Indrawati, Munaf, E., 2010. Removal of Pb(II),
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Cd(II) and Co(II) from aqueous solution using Garcinia mangostana L. fruit shell. J.
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Zhang, W., Yan, Z., Gao, J., Tong, P., Liu, W., Zhang, L., 2015. Metal-organic
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framework UiO-66 modified magnetite@silica core-shell magnetic microspheres for
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magnetic solid-phase extraction of domoic acid from shellfish samples. J. Chromatogr.
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A 1400, 10-18.
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Zhao, M., Deng, C., Zhang, X., 2014. The design and synthesis of a hydrophilic core–
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ion affinity platform for phosphoproteome research. Chem. Commun. 50, 6228-6231.
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Zhao, M., Zhang, X., Deng, C., 2015. Facile synthesis of hydrophilic magnetic
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graphene@metal-organic
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Zhao, X., Liu, S., Tang, Z., Niu, H., Cai, Y., Meng, W., Wu, F., Giesy, J.P., 2015.
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Synthesis of magnetic metal-organic framework (MOF) for efficient removal of
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organic dyes from water. Sci. Rep. 5, 11849.
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Zhang, W., Liang, F., Li, C., Qiu, L.G., Yuan, Y.P., Peng, F.M., Jiang, X., Xie, A.J.,
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Shen, Y.H., Zhu, J.F., 2011. Microwave-enhanced synthesis of magnetic porous
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covalent triazine-based framework composites for fast separation of organic dye from
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aqueous solution. J. Hazard. Mater. 186, 984-990.
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Zheng, J., Cheng, C., Fang, W.J., Chen, C., Yan, R.W., Huai, H.X., Wang, C.C., 2014.
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Surfactant-free synthesis of a Fe3O4@ZIF-8 core-shell heterostructure for adsorption of
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methylene blue. CrystEngComm 16, 3960-3964.
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Hu, L.H., Yang, Z.P., Cui, L.M., Li, Y., Ngoc, H.H., Wang, Y.G., Wei, Q., Ma, H.M.,
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Yan, L.G., Du, B., (2016). Fabrication of hyperbranched polyamine functionalized
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ACCEPTED MANUSCRIPT Highlights
Amino-decorated Zr-MFCs were prepared by a facile and efficient strategy.
The Zr-MFCs can effectively removal metal ions/organic dyes from aqueous solution. Anionic and cationic dyes could be selectively separated and removed by
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These Zr-MFCs materials can be easily regenerated for reuse.
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DOI:
10.1016/j.chemosphere.2018.02.019
Reference:
CHEM 20777
To appear in:
ECSN
Received Date: 16 February 2017 Revised Date:
30 December 2017
Accepted Date: 4 February 2018
Please cite this article as: Huang, L., He, M., Chen, B., Hu, B., Magnetic Zr-MOFs nanocomposites for rapid removal of heavy metal ions and dyes from water, Chemosphere (2018), doi: 10.1016/ j.chemosphere.2018.02.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Magnetic Zr-MOFs nanocomposites for rapid removal of heavy
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metal ions and dyes from water
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Lijin Huang, Man He, Beibei Chen, Bin Hu*
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Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of
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Education), Department of Chemistry, Wuhan University, Wuhan 430072, Hubei
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Province, P. R. China
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*
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[email protected]
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Corresponding Author. Tel: 86-27-68752701; Fax: 86-27-68754067; E-mail address:
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Abstract
Amino-decorated Zr-based magnetic Metal-Organic Frameworks composites
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(Zr-MFCs) were prepared by a facile and efficient strategy. The nano-sized
13
Fe3O4@SiO2 core (about 15 nm) was coated with a shell of Zr-MOFs (about 5 nm) by
14
means of in-situ growth. And, Fe3O4@SiO2@UiO-66 and its amino derivatives
15
(Fe3O4@SiO2@UiO-66-NH2 and Fe3O4@SiO2@UiO-66-Urea) were successfully
16
prepared by using different precursors. The obtained Zr-MFCs were demonstrated to be
17
efficient adsorbents for metal ions/organic dyes removal from aqueous solution, with
18
high adsorption capacity and fast adsorption kinetics. It was found that the
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amine-decorated MFCs were highly efficient for metal ions/dyes removal compared to
20
raw MFC-O. Among them, MFC-N exhibited the highest capacity for Pb2+ (102 mg g-1)
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and methylene blue (128 mg g-1), while MFC-O exhibited the highest capacity for
22
methyl orange (219 mg g-1). Moreover, anionic and cationic dyes could be selectively
23
separated and removed from the mixed solution just by adjusting the solution pH with
24
Zr-MFCs as the adsorbents. And these Zr-MFCs materials can be easily regenerated by
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desorbing metal ions/organic dyes from the sorbents with appropriate eluents, and the
26
adsorption capacity can be remained unchanged after 6 recycles. The obtained results
27
demonstrated the great application potential of the prepared MFCs as fascinating
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adsorbents for water treatment.
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Keywords: Metal-Organic Frameworks; magnetic nanocomposites; heavy metal ion
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removal; water treatment; dye separation and removal; core-shell structure
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1. Introduction With the rapid industrial development, accidental and purposive dumping of
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industrial waste such as heavy metal ions and dyes has contributed to the serious
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problem of water pollution, which is dangerous for the health of living organisms
35
(Wang et al., 2014; Sun et al., 2011). Consequently, a variety of techniques have been
36
adopted for the uptake of ions or dyes from aqueous solution, including chemical
37
precipitation, adsorption, membrane systems and ion exchange (Hashim et al., 2011;
38
Gupta et al., 2009; Kumar and Guliants, 2010; Dabrowski et al., 2004). Among them,
39
adsorption featuring with simple and fast separation, efficient and low-cost has been
40
extensively used for water treatment (Gupta and Suhas, 2009; Ali and Gupta, 2007).
41
The choice of adsorbents largely influence the adsorption capacity and selectivity in the
42
adsorption process to a great extent, and the low adsorption capacities and poor
43
selective sorption have limited the application of common adsorbents (e.g. activated
44
carbon (Mohan et al., 2008), zeolites (Meshko et al., 2001) and natural fibers (Zein et
45
al., 2010). Therefore, development of novel adsorbents with high efficiency and
46
selectivity for target pollutants has provoked great interest and become the main
47
orientation in adsorption field presently. In this regard, some newly appeared materials
48
(carbon nanotubes (Liu et al., 2013; Upadhyayuya et al., 2009), grapheme (Han et al.,
49
2014; Cong et al., 2012), layered double hydroxides (Ma et al., 2014; Ma et al., 2014),
50
and other materials (Taskin et al., 2014; Li et al., 2014) have been investigated as the
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adsorbents for heavy metals trapping, and they have exhibited great potential as the
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adsorbents for water treatment. While tedious high-speed centrifugation or filtration
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separation after adsorption is required, hindering the extensive application of such
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adsorbents. Recently, widespread application of magnetic substrates to address this issue
56
facilitates the isolation of trapped species and separation from sample solutions for
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reuse (Kaur et al., 2014). The combination of magnetic nanoparticles (MNPs) and
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desirable building blocks or components could contribute to cooperatively enhanced
59
performance during adsorption.
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Metal-Organic Frameworks (MOFs) and/or coordination polymers with permanent
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porosity, which were constructed through connecting inorganic metal nodes and
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organic building blocks via coordination bonds, have received wide attention in
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catalysis, gas storage and drug delivery, etc. Their excellent chemical and thermal
64
stability, well-order porosity, high surface area and easy functionalization of their pores
65
or outer-surface are highly favourable to adsorption (Hasan et al., 2015; Burtch et al.,
66
2014) and separation (Voorde et al., 2014). MOFs have been widely utilized to remove
67
hazardous pollutants (Hasan et al., 2015; Ayati et al., 2016; Roushani et al., 2016), such
68
as dyes or heavy metal ions in water, demonstrating a great application potential in
69
water treatment. Magnetic MOFs composites (MFCs), which integrate the advantages
70
of fast separation of magnetic materials and the superior properties of MOFs, have been
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widely used in different technological fields (Ricoo et al., 2013), and offer an attractive
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alternative platform for dyes/heavy metal ions capture. However, a large proportion of
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2015; Bagheri et al., 2012; Ke et al., 2012; Wang et al., 2013 ), and HKUST-1 is
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unstable under acid solution and would release toxic Cu2+ (Flemming et al., 1989;
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Huang et al., 2015). Comparatively, MOFs containing zirconium metal nodes with
77
strong Zr(IV)-O bonds exhibit better stability in water across a broad pH range (Cavka
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et al., 2008; Kandiah et al., 2010; Kalidindi et al., 2015; Piscopo et al., 2015). In
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addition, Zr-MOFs inherit remarkable thermal and mechanical stability (Cavka et al.,
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2008; Wu et al., 2013). Due to the nodes with abundant Zr-bound hydroxides, Zr-MOFs
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exhibited excellent performance for removal of selenium species from drinking water
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(Howarth et al., 2015). However, the preparation/application of magnetic Zr-MOFs in
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water treatment is still scarce (Zhao et al., 2014; Zhao et al., 2014; Zhang et al., 2015).
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Even more distressing is that the present approaches usually require specific surface
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modification for the in-situ growth of Zr-MOFs on the MNPs surface and the prepared
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MFCs are in micro-size (>200 nm). On the other hand, it has been proven that amino
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modification can be an effective strategy to increase the uptake of metal ions or dyes
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from aqueous solution (Ricco et al., 2015; Haque et al., 2010; Huang et al., 2014;
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Roushani et al., 2016). So it is quite attractive to develop more simple and efficient
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methods to prepare amino-functionalized nano-sized Zr-MFCs.
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Herein, a serious of nano-sized Zr-MFCs, including Fe3O4@SiO2@UiO-66 (denoted
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as
MFC-O),
Fe3O4@SiO2@UiO-66-NH2
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Fe3O4@SiO2@UiO-66-Urea (denoted as MFC-U), have been prepared successfully 5
(denoted
as
MFC-N)
and
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showed core-shell structure, and the shell thickness is adjustable through adjusting the
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quantity of predecessor. These nano-sized Zr-MFCs were investigated for the
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extraction of both dyes and metal ions. They exhibited excellent adsorption property
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both for heavy metal ions and cationic/anionic dyes, and relatively high thermal and
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chemical stability. Besides, fast separation and good recovery of MFCs from aqueous
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solution can be realized just by applying external magnetic fields, demonstrating a very
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good application prospect for dyes/heavy metal ions removal from environmental
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water.
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he m eI
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Scheme 1 Schematic illustration of MFCs preparation.
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2. Experimental
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2.1 Chemicals and materials
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All the analytical grade chemicals were commercially available and used without
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further purification. The stock solution (1000 mg L-1) of Hg2+, Cd2+, Cr3+, Co2+, Mn2+,
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Ni2+, Pb2+, Cu2+, Zn2+ was prepared by HgCl2, Cd(NO3)2·4H2O, Pb(NO3)2, 6
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Zn(NO3)2·6H2O, respectively. 1000 mg L-1 of methylene blue (MB) and methyl orange
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(MO) were prepared in deionized water, respectively. The solutions with desired
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concentration were prepared by stepwise dilution of respective stock solution.
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2.2 Synthesis of MFCs
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The MFC-O was synthesized according to our previous work (Huang et al. 2015).
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The MFC-O with different shell thickness were prepared by varying the MOF
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precursors reaction mixture concentration (12.5, 25.0 and 50.0 mM, denoted as
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MFC-O-1, MFC-O and MFC-O-2) while the amount of Fe3O4@SiO2 (MNPs) was kept
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constant. For the preparation of MFC-U, urea (25.0 mM) was added into the reaction
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mixture following a similar procedure with above.
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that the NH2-BDC was used in place of H2BDC.
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2.3 Characterization of materials
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Magnetic separation was conducted through Nd-Fe-B magnet. NEXUS 870 Fourier
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transform infrared spectrometer (FT-IR) (Thermo, Madison, USA) was employed for
126
the characterization of the prepared MFCs. Mettler Toledo 320-S pH meter (Mettler
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Toledo Instruments Co. Ltd., Shanghai, China) was applied for solution pH adjustment.
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Thermogravimetric analysis (TGA) was conducted on diamond TG/DTA 6300
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(PerkinElmer, USA). The crystalline structure of MFCs was identified by Bruker D8
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diffractometer (Germany). The surface areas of MFCs were obtained on ASAP 2020
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microscope (SEM, Hitachi, Tokyo, Japan) equipped with an energy dispersive X-ray
133
(EDX, SUTW-SAPPHIRE) system were employed to identify the chemical
134
composition of the samples.JEM-2010 electron microscope (Tokyo, Japan) was
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adopted to detect the transmission electron micrograph (TEM) image of the samples.
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PPMS-9 vibrating sample magnetometer (VSM) (QUANTOM, USA) was the
137
materials’ magnetic properties. The zeta potential of MFCs under different pH was
138
measured on a zeta potential analyser (Malvern Nano ZS ZEN 3600, UK). UV-Vis
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spectrophotometer (Shimadzu, UV-1800, Japan) was carried out to determine the
140
concentration of MO and MB at 464 and 665 nm, respectively. The concentration of
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target metal ions was determined by intrepid XSP radial inductively coupled plasma
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optical emission spectrometry (ICP-OES) (Thermo, Waltham, MA, USA).
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2.4 Adsorption studies
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10 mg MFCs was spiked into 10 mL of metal ions/dyes solution with appropriate
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pH (pH 6 for metal ions, pH 3 for MO and pH 11 for MB), which was then shaken for
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60 min at room temperature. Subsequently, the concentration of metal ions/dyes in
147
remaining solution was analysed using ICP-OES or UV-Vis spectrophotometry,
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respectively.
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The time-dependence of target removal was performed to study the adsorption
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kinetics, and the MFCs were spiked at a level of 1 mg mL-1 at room temperature. The
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concentration of metal ions/dyes at different time (2, 5, 10, 20, 30, 40, 50, 60 min) 8
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153
experiments, all were performed in triplicate.
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3 Results and discussion
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3.1 Materials characterization
(b)
(a)
MFC-O
MFC-O MFC-N MFC-U Fe3O4@SiO2 simulated
MFC-N MFC-U
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Fe3O4@SiO2
10
156 157
20
30
2 theta
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4000
3000
2000
1500
1000
500
-1
Wavelength (cm )
Fig. 1 PXRD (a) and FT-IR spectra (b) of different kinds of MFCs. Fig. 1(a) presents the XRD patterns of MNPs, MFC-O, MFC-N and MFC-U,
159
respectively. The diffraction patterns of MFCs are in accordance to the published data
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(Kandiah et al. 2010). The diffraction peaks at about 31° and 36° are assigned to Fe3O4
161
(JCPDS No. 19-0629), which demonstrate the successful introduction of Fe3O4 crystals
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in the hetero-nanostructure. And the characteristic peaks of the prepared MFCs are
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consistent with that of UiO-66, indicating that magnetic modification has no influence
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on the parent MOFs crystal structure.
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Fig. 1(b) showed the FT-IR spectra of the MFCs. For MNPs, the broad peak around
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1090 cm-1 can be designated to O-Si-O stretching vibration. While for UiO-66 and
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MFC-O, the two peaks at 1709 cm-1 and 1645 cm-1 are assigned to C=O bonds, and
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peaks at 1447 cm-1 and 1374 cm-1 are attributed to the C=C stretching vibration on the 9
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the spectra of MFC-N: C-N stretching absorption is found at 1257 and 1340 cm−1, and
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N-H wagging is observed at 764 cm−1. These FT-IR results confirm the presence of
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NH2 group in MFC-N (Kandiah et al. 2010). For the urea containing MFC-U, the peak
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at 1350 cm-1 is designated to the C-N vibration of CO(NH2)2 and the peak around 1600
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cm-1 can be ascribed to the C=O stretching vibration in BDC linker, C-N and N-H in
175
urea (Kandiah et al. 2010); the deformation vibration band for urea carbonyl (C=O) is
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observed around 1700 cm-1, revealing that urea was incorporated into MFC-U
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successfully.
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Fig. 2 N2 adsorption-desorption isotherms of MNPs, MFC-O, MFC-N and MFC-U.
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The
porous
structure
of
as-prepared
MFCs
was
investigated
by
181
Brunauer-Emmett-Teller (BET) gas-sorption experiments, as shown in Fig. 2 and Table
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1. It can be observed that MFC-O exhibited much higher surface area than MNPs,
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which indicated UiO-66 deposited on MNPs successfully. The surface area of MFC-N
184
reduced slightly compared with that of MFC-O, since the NH2 groups would like to
10
ACCEPTED MANUSCRIPT block the pore or deteriorate crystal formation. As can be seen, the increase of surface
186
area (~ 33 %) and pore volume (~21 %) are found for MFC-U comparing with that of
187
MFC-O. These differences may be caused by the formation of imides between urea and
188
the carboxylic acid (Ebrahim et al., 2014), which would bind to zirconium centers
189
either via oxygen or nitrogen groups, leading to significant amount of missing-linker
190
defects.
Table 1 Surface area and pore volumes for MNPs, MFC-O, MFC-N and MFC-U BET Surface area (m² g-1)
MNPs MFC-O
UiO-66a
51
0.34
782
0.47
722
0.42
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0.57
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Note: (a) the data came from ref (Ebrahim et al., 2012).
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TGA was carried out to evaluate the thermal stability of the MFCs (Fig. 3). As can be
194
seen, after MOFs loading, a slight weight loss under 200 °C was observed for all MFCs,
195
which is ascribed to the loss of water/solvent physically adsorbed on the MFC and the
196
dehydroxylation of the zirconium oxo-cluster. For MFC-O, the weight-loss under
197
520 °C could be caused by the decomposition of organic ligands to convert into ZrO2
198
(Kandiah et al. 2010). In comparison with UiO-66, the thermal decomposition
199
temperature of MFC-N is below 350 °C due to the introduced amino group. For
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Fig. 3 TG curves of MNPs, UiO-66-NH2, MFC-O, MFC-N and MFC-U.
202
MFC-U, the weight loss of the composite is observed at 420 °C, which may be caused
203
by the urea modification. And due to the presence of urea grafted on MOFs, MFC-U
204
exhibited more weight loss compared with MFC-O. The differences between the
205
weight loss of MFC-O and MFC-U suggest that about 7 % of urea is present in the
206
MFC-U. All the results indicate that the prepared MFCs exhibit exceptionally high
207
thermal stability.
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Fig. 4 TEM images of MNPs (a), MFC-O (b), MFC-N (c), MFC-U (d) and elemental
210
mapping images of MFC-O (e).
211
The formation of MFCs can be confirmed via TEM images of magnetic composites
212
shown in Fig. 4. The images of TEM show define core-shell structure for all the 12
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214
about 5 nm (~3.8 nm for MFC-O; ~4.1 nm for MFC-N; ~5.1 nm for MFC-U). SEM was
215
employed to further confirm their morphology (Fig. S1). What’s more, the thickness of
216
the shell in core-shell structures can be controlled just by adjusting the quantity of
217
predecessor. Three MFCs have been prepared, including MFC-O-1, MFC-O and
218
MFC-O-2, and the corresponding quantity of UiO-66 predecessor were 12.5, 25.0 and
219
50.0 mM. Their structure was characterized by TEM and the results (Fig. S2) showed
220
that the shell thickening with the rising of MOF precursor concentration from 12.5 to
221
25.0 mM. When the concentration of UiO-66 predecessor was up to 50.0 mM, some of
222
the obtained nanoparticles (MFC-O-2) preferred to self-seeding nucleate in the solution
223
and grow into much larger crystal rather than forming core-shell structure MFCs.
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EDX spectra was used to determine the relative contents of elements in MFCs(Table
225
S1 and Fig. S3). As can be seen, C, N, O, Si, Fe and Zr were involved in the prepared
226
MFC-N and MFC-U, while no obvious signal of N was observed for MFC-O. Fig. 4(e),
227
Fig. S4 and S5 are the element mapping results for MFC-O, MFC-N and MFC-U,
228
respectively. Fe, Si, O, Zr and C are homogeneously distributed throughout all these
229
three materials, and N was found in MFC-N and MFC-U. Taking into consideration the
230
results of TEM, it could be speculated that the as-synthesized MFCs are uniform
231
core-shell composites rather than a simple mixture of two phases of MNPs and MOF
232
crystals.
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The formation mechanism (Scheme 1) of uniform Zr-MOF shell is described by the 13
ACCEPTED MANUSCRIPT following processes: (1) ZrCl4 was firstly adsorbed onto the hydroxylated SiO2 surface,
235
reacting with SiO2-OH group and forming a stable complex (Han et al., 2004; Widjaja
236
et al., 2003); (2) the H atom of the surface OH group combined with one Cl atom of the
237
ZrCl4 which was adsorbed , forming intermediate complex SiO2-O-ZrCl3 and HCl; (3)
238
the by-product HCl would assist the formation of UiO-66 (Katz et al., 2013), which
239
grows on the surface of Fe3O4@SiO2-O-Zr3+ as the shell under solvothermal synthesis
240
in the presence of H2BDC.
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Fig. 5 VSM magnetization curves of MNPs, MFC-O, MFC-N, and MFC-U (The
243
insets digital images show that MFCs can be easily separated from water under an
244
external magnetic field).
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VSM magnetization curves were used to characterize the magnetic properties of the
246
MFCs (Fig. 5). As compared to MNPs, the saturation magnetization of the MFCs
247
decreased from 51 to 16, 12 and 6 emu g-1 as the magnetic contents of the MFCs
248
decreased. This can be verified by the ICP-OES results that the Fe contents in the MFCs
249
followed such order: 13.78% (MFC-O)> 12.19% (MFC-N)> 9.72% (MFC-U), which
14
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251
coercivity (HC) and retentivity from the expanded hysteresis loop between ±0.02 kOe
252
was found to be 1.7 Oe and 0.47 emu g-1 for MNPs, 7.4 Oe and 0.38 emu g-1 for MFC-O,
253
8.0 Oe and 0.26 emu g-1 for MFC-N, 8.9 Oe and 0.06 emu g-1 for MFC-U, respectively.
254
These results clearly demonstrated that all MFCs are superparamagnetic.
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Additionally, due to excellent hydrophilic property of Zr-MOFs and their uniform
256
nano structure, all the MFCs dispersed well in water. Moreover, the inset images of Fig.
257
5 showed that the MFCs (using MFC-O as the example) can be easily recovered with a
258
permanent magnet.
259
3.2 Effect of pH
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Solution pH plays an important role in adsorption performance of sorbent. A proper
261
solution pH not only benefits for adsorption, but also reduces the interference of matrix.
262
The effect of solution pH on elimination behaviour of multi heavy metal ions by using
263
MFCs was conducted (Fig. S6). The removal efficiency gradually increased with the
264
increasing pH from 2 to 7. And for MFC-O, Pb2+, Cr3+ and Cu2+ were quantitatively
265
removed at pH 7; for MFC-N, Pb2+, Cr3+, Cu2+ and Hg2+ were quantitatively removed at
266
pH 7; for MFC-U, Pb2+, Cr3+ and Cu2+ were quantitatively removed at pH 4-7, and the
267
removal efficiency of Hg2+ was maintained around 80% in the pH range of 4-7. The
268
elimination mechanism is summarized as follows. The carboxyl and hydroxyl
269
functional groups on the MFC-O are protonated and the protons would combine with
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-COO- and -O- competing with metal ions under acidic condition, resulting in a lower
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ACCEPTED MANUSCRIPT elimination efficiency (Musico et al., 2013). When the pH is above 4, functional groups
272
of -COOH and -OH on MFCs exist as -COO- and -O-, leading electrostatic and
273
chelating interaction between metal cationic and MFCs. The experimental results also
274
showed an improvement in adsorption for Pb2+ by the prepared MFC-N and MFC-U
275
comparing with MFC-O, probably due to the amine modification. The free -NH2
276
groups on MFC-N and MFC-U not only provide more binding sites for adsorbing, but
277
also can interact with Pb2+ by chelating (Aguado et al., 2009), enhancing the adsorption
278
of Pb2+ and other species onto MFC-N and MFC-U.
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Simultaneously, the adsorption performance of MFCs for MB (cationic dye) and MO
280
(anionic dye) was investigated (Fig. S7). As can be seen, the affecting tendency of pH
281
on the removal efficiency of cationic dye is contrary to that of anionic dye. The uptake
282
amounts decrease with the increasing pH of MO solution, which is similar to previously
283
reported results (Haque et al., 2010; Haque et al. 2011), while the adsorbed amounts
284
increase with increasing the pH of MB solution. Based on it, the adsorption mechanism
285
of MO and MB on MFCs may be explained by the electrostatic and π-π stacking
286
interaction between the dyes and MFCs. MO (pKa=3.4) exists in a mixture of neutral
287
molecular and anionic species at pH 3.4, while the surface of MFCs is positively
288
charged at pH 3 as shown in Fig. S8. The negative form of MO can be adsorbed by
289
electrostatic interaction and the molecular form can be adsorbed by π-π stacking and
290
hydrogen bonding between benzene rings of MFCs and MO. MB is an alkaline dye,
291
existing in cationic form, and can be adsorbed on the negatively charged MFCs (Fig. S8)
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ACCEPTED MANUSCRIPT by electrostatic interaction; when pH is above pKaMB, MB exists in neutral molecular,
293
and hydrogen bonding interaction and π-π stacking may be the main factor.
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294
296
Fig. 6 Photographs of the MB-MO mixture before and after adsorption by MFC-O
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under different pH.
In consideration of the different adsorption behaviour for MO and MB under
298
different pH condition, MFCs are expected to be effective for the treatment of anionic
299
and cationic dye mixture. To verify this application potential, the adsorption
300
performance of the prepared MFCs for dyes mixture (cationic dye MB and anionic dye
301
MO) was studied by mixing 10 mg MFCs with 10 mL solution containing 20 mg L-1 of
302
MB and 20 mg L-1 of MO under different pH (Table 2). As can be seen, with MFC-O as
303
adsorbent, 98% MO and only 8% MB were adsorbed under pH 3, while about 25% MO
304
and 98% MB were adsorbed under pH 11; both MO and MB were adsorbed almost
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ACCEPTED MANUSCRIPT completely under pH 7. This means that MFC-O can not only simultaneously adsorb
306
MO and MB under pH 7 but also selectively adsorb MO under pH 3 or MB under pH 11.
307
The photographs of dye mixture before and after treated (MFC-O as an example) under
308
different pH shown in Fig. 6. MFC-N and MFC-U exhibited similar selectivity for MO
309
and MB under pH 3 and 11. These results demonstrate that the prepared MFCs may be
310
an attractive alternative platform for multi dyes separation and removal.
Table 2 Selective removal MB or MO from their mixture under different pH
Removal
efficiency (%)
M AN U
MO
MB
pH 3
pH 7
pH 11
pH 3
pH 7
pH 11
MFC-O
98
100
25
8
98
98
MFC-N
99
84
1
4
95
98
MFC-U
98
89
2
6
93
98
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311
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3.3 Adsorption kinetics
Adsorption kinetics of Pb2+ and dyes onto the MFCs were also investigated (Fig.7).
314
The results (Fig. 7(a)) indicated that the adsorption of Pb2+ on MFCs could reach
315
equilibrium within 20 min. Meanwhile, the MFCs also showed similar adsorption
316
kinetics for MB (Fig. 7(b)) and MO (Fig. 7(c)). Pseudo-second-order kinetic rate
317
equation was employed to model the experimental data and the linear regression plot
318
of time (t) and t/Qt shows a good linear pattern with R2=0.999 (Fig. S9). What’s more,
319
the calculated Qe value was in accordance to the experimental data, suggesting the
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ACCEPTED MANUSCRIPT 320
adsorption process of Pb2+ and dyes on the as-prepared MFCs follow the
321
pseudo-second-order kinetic model.
(a)
10
0 0
20
40
60
80
Time (min)
322
100
120
-1
Concentration(mg L )
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(b)
50
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MFC-O MFC-N MFC-U
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Concentration(mg L )
20
40
MFC-O MFC-N MFC-U
30 20
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10 0
0
323
20
40
60
80
100
120
Time (min)
50
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Concentration(mg L )
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(c)
40
20 10 0 0
324 325 326
MFC-O MFC-N MFC-U
30
20
40
60
80
100
120
Time (min)
Fig. 7 Adsorption curve of Pb2+ (a), MB (b) and MO (c) versus incubate time in aqueous solution by using MFCs (initial concentrations of Pb2+ and dye : 50 mg L-1).
19
ACCEPTED MANUSCRIPT 3.4 Adsorption isotherms
(a)
100 80 60 40
MFC-O MFC-N MFC-U
20
0
50
100
150
200
-1
-1
Adsorbed amount (mg g )
328
(b)
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120
80
250
300
SC
Ce (mg L )
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-1
Adsorbed amount (mg g )
327
MFC-O MFC-N MFC-U
40
10
20
30
40
-1
TE D
0
Ce (mg L )
-1
Adsorbed amount (mg g )
329
(c)
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200 150
330 331 332 333
AC C
100
MFC-O MFC-N MFC-U
50 0 0
10
20
-1
30
Ce (mg L )
Fig. 8 Adsorption curve of Pb2+ (a), MB (b) and MO (c). The effect of concentration of target compounds on adsorption capacity of MFCs was shown in Fig. 8. Fig. 8(a) showed the adsorption isotherm of Pb2+ on MFCs. The 20
ACCEPTED MANUSCRIPT Table 3 Adsorption capacities of metal ions/dyes on different MFCs and Zr-MOFs Capacity (mg g-1)
Adsorbents MO
MB
Cr
Pb
Cu
Hg
MFC-O
219
116
63
68
40
10
MFC-N
130
128
51
102
45
60
MFC-U
183
121
57
UiO-66 (Chen et al., 2015)
39
90
UiO-66-NH2 (Chen et al., 2015)
29
96 48
Composites (Ai et al., 2011)
Fe3O4@ZIF-8 core-shell
50
27
74
M AN U
Fe3O4@MIL-100(Fe) (Shao et al., 2016)
77
SC
Fe3O4-MWCNTs
RI PT
334
20
heterostructure (Zheng et al., 2014)
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Al-based MFCs (Ricco et al., 2015) Pyridine functionalized MFCs
492 190
(Sohrabi et al., 2013)
Covalent triazine-based
291
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framework/Fe2O3 composites (Zhang et al., 2011)
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Protonated
194
ethylenediamine-grafted-MIL-101(Cr) (Haque et al., 2010)
MOF-235 (Haque et al., 2011)
477
335
adsorbed amount of Pb2+ on MFCs raised sharply and reached saturation gradually. At
336
equilibrium, MFC-N was able to uptake 102 mg g-1 Pb2+. This adsorption capacity is
337
much greater than that of MFC-O and MFC-U (Table 3). The difference may be caused 21
ACCEPTED MANUSCRIPT by the more NH2 groups anchored on MFC-N which exhibit stronger interaction with
339
Pb2+. This interaction was further verified by the FT-IR spectra (Fig. S10). Comparing
340
with as-prepared MFC-N, the characteristic peaks corresponding to -NH2 shift from
341
3395 cm-1 to 3372 cm-1, which indicates the NH2 may interact with Pb2+ (Hu et al.,
342
2016).
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Due to the precipitation of both dyes (MB and MO) under high concentration, the
344
adsorption isotherm of MB and MO was carried out by fixing the concentration and
345
increasing sample volume (Fig. 8(b) and (c)). As is showed in Table 3, MFC-N showed
346
the highest adsorption capacity for MB, while least for MO. The different adsorption
347
performance for dyes may be attributed to the electrostatic attraction interactions
348
between the adsorbents and dyes. After modification with -NH2, the resulted MFC-N
349
has more negative zeta potential than MFC-O under high pH (Fig. S8), thus favouring
350
the adsorption of more cationic dye MB. For anionic dye MO, MFC-N with lowest zeta
351
potential among the prepared MFCs under acid condition (Fig. S8) presented the lowest
352
adsorption capacity. Comparing with naked MOFs (Chen et al., 2015), MFCs showed
353
larger adsorption capacities attributing to their distinct core-shell structure.
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SC
343
Langmuir model was adopted to analyse the adsorption isotherms of MFCs for target
355
compounds (Fig. S11). And the correlation coefficient of linear regression between
356
Ce/qe and Ce was 0.9998, suggesting that the uptake of metal ions (Pb2+) and dyes (MO
357
and MB) on as-prepared MFCs fits Langmuir’s adsorption model well (Table S2).
358
3.5 Stability and recycling 22
100
(a)
80 60 40
MFC-O MFC-N MFC-U
20 0
2
1
6
5
(b)
-1
SC
80
M AN U
Adsorption amounts (mg g )
4
120
MFC-O MFC-N MFC-U
40
0 1
360
2
4
3
Number of cycles
5
6
(c)
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-1
3
Number of cycles
359
Adsorption amounts (mg g )
RI PT
-1
Adsorption amounts (mg g )
ACCEPTED MANUSCRIPT
200 160
361 362
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120
80 40
MFC-O MFC-N MFC-U
0 1
2
3
4
Number of cycles
5
6
Fig. 9 Pb2+ (a), MB (b) and MO (c) removal on recycled MFCs.
363
Solvent desorption was employed for adsorbent regeneration. Methanol, DMF,
364
acetone, ethanol, HNO3 and NaOH were tested as the eluent to regenerate MFCs. And
365
the regeneration was successfully achieved by using 0.5 M HNO3, methanol and 0.01
23
ACCEPTED MANUSCRIPT 366
M NaOH for Pb2+, MB and MO-adsorbed MFCs, respectively. Fig. 9 shows that MFCs
367
can be reused at least 6 times without significant loss of adsorption capacity, indicating
368
that all the MFCs possess a good regeneration ability for water treatment. The XRD patterns of reused MFCs after 5 adsorption/elution cycles of Pb2+ and dyes
370
were studied respectively (Fig. S12). In order to further confirm the stability of MFCs
371
after adsorption, leaching test was carried out by soaking the prepared MFCs in a series
372
of solutions with different pH (2-12) for 2 h, and determining the Zr concentration in
373
the supernatant by ICP-OES. The results (Fig. S13) reveal that < 0.5 mg L-1 Zr was
374
released from the MFCs, even after 15 h soaking. Furthermore, TEM was employed to
375
investigate the microstructure of the adsorbents after five adsorption/elution cycles of
376
metal ions and dyes. As shown in Fig. S14, the structure of the adsorbents after the
377
adsorption/elution cycles kept the original appearance, demonstrating the structural
378
stability of the prepared MFCs during the extraction process. In addition, the BET
379
surface areas and pore volumes of the adsorbents after 5 adsorption/elution cycles
380
(Table S3) decreased slightly. All these results reveal that the structures of all the
381
prepared MFCs are chemically stable under the experimental conditions.
382
4. Conclusions
SC
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369
In this paper, three kinds of core-shell nano Zr-MFCs with controllable encapsulating
384
have been synthesized by facile solvothermal method. Through replacing BDC with
385
NH2-BDC or by adding urea to the components forming UiO-66, two NH2 decorated
386
MFCs were obtained. All the prepared Zr-MFCs are found to be efficient for heavy 24
ACCEPTED MANUSCRIPT metal ions and organic dyes capture. The adsorption results clearly indicate that the
388
introduction of amino groups greatly enhances the ability for metal ions/dyes capture
389
from water. By selecting the appropriate pH, cationic dye (MB) and anionic dye (MO)
390
can be simultaneously removed under pH 7 or selectively removed (MO under pH 3
391
and MB under pH 11). The prepared three kinds of Zr-MFCs can be easily regenerated
392
and reused for more than 6 times. Owing to their flexible structures and intrinsic
393
properties, all of these Zr-MFCs are attractive for rapid and efficient water treatment.
394
Moreover, on account of various studies on Zr-MOFs, the presented magnetic
395
Zr-MOFs may find their application potential in other technological fields.
396
Acknowledgments
M AN U
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This work is financially supported by the National Basic Research Program of China
398
(973 Program, 2013CB933900) and the National Natural Science Foundation of China
399
(Grant Nos.: 21175102, 21205090, 21575107).
400
Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in the online version,
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402
at http://dx.doi.org/10.1016/j.chemoshpere.************
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ACCEPTED MANUSCRIPT Highlights
Amino-decorated Zr-MFCs were prepared by a facile and efficient strategy.
The Zr-MFCs can effectively removal metal ions/organic dyes from aqueous solution. Anionic and cationic dyes could be selectively separated and removed by
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Zr-MFCs .
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These Zr-MFCs materials can be easily regenerated for reuse.
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