Research Article pubs.acs.org/journal/ascecg
Cr(VI) Reduction and Immobilization by Core-Double-Shell Structured Magnetic Polydopamine@Zeolitic Idazolate Frameworks‑8 Microspheres Kairuo Zhu,†,‡ Changlun Chen,*,†,‡,§ Huan Xu,† Yang Gao,† Xiaoli Tan,† Ahmed Alsaedi,§ and Tasawar Hayat§,∥ †
Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, People’s Republic of China Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, People’s Republic of China § NAAM Research Group, King Abdulaziz University, Jeddah 21589, Saudi Arabia ∥ Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan ‡
S Supporting Information *
ABSTRACT: A well-defined core-double-shell structured magnetic polydopamine@zeolitic idazolate frameworks-8 (MP@ZIF-8) hydrid microsphere consisting of the core of magnetic Fe3O4 nanoparticles, the inner shell of a polydopamine layer, and the outer shell of a porous ZIF-8 nanocrystal was prepared through a facile and green approach to achieve synergistic reduction and adsorptive removal of Cr(VI). The microsphere property was characterized methodically. The batch adsorption experiments showed that the MP@ZIF-8 exhibited high efficiency in the Cr(VI) removal from aqueous solutions, affording Cr(VI) removal capacity of 136.56 mg g−1, surpassing pristine MP (92.27 mg g−1). The pseudo-second-order model fitted the Cr(VI) removal kinetics well. Cr(VI) removal on the MP@ZIF-8 relied highly on pH values. More significantly, with the reduction of nitrogen atom group on ZIF-8 and PDA, Cr(VI) was easily converted into low toxicity Cr(III) and then immobilized on the MP@ZIF-8. Thus, the hybrid microspheres provided excellent adsorptive activity in treating Cr-contaminated wastewater. KEYWORDS: MP@ZIF-8 hydrid microsphere, Cr(VI), Adsorption, Reduction
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INTRODUCTION
Innovation in the structure and morphology of adsorbent is one key of the enhancement of its adsorption areas and adsorption capacity as well as density of reaction sites. As a novel subclass of complex nanostructures, magnetic coredouble-shell structured materials have aroused significant concern.13−15 On the one hand, the magnetic core can provide a facile adsorbent recovery approach after reaction;16 on the other hand, it can improve the stability and dispersity of adsorbent materials.17 What is more, this nanostructure suggests great potential for wider applications such as being a nanoreactor because its permeable shell can prevent the core from aggregation while allowing small active molecules to diffuse in and out of the interior microenvironment.18 Magnetic polydopamine hybrid materials used as templates for convenient fabrication of core−shell structured composites have drawn significant attention for removing heavy metal ions.
Water pollutions from heavy metal ions are of growing worldwide concern for their high detrimental and toxic effects.1−4 Among major heavy metal ions, chromium is considered the second most common inorganic contaminant.5 In general, there are two steady oxidation states of Cr(III) and Cr(VI) for chromium in aquatic environment.6−8 As compared to Cr(III) species, which can readily precipitate in the formation of Cr(OH)3, Cr(VI) is more poisonous to biological systems due to its carcinogenicity and mutagenicity. Thus, converting Cr(VI) into Cr(III) is a crucial motivation for Cr(VI) contaminant elimination. Recently, different purification methods have been implemented to eliminate Cr(VI) contamination, among which the adsorption-reduction method holds promising significance due to its considerable advantages including being fast, efficient, and eco-friendly.9−12 However, investigation on synthesizing advanced adsorbent with relatively simple fabrication procedure and high activity for Cr(VI) adsorption-reduction is still underway. © 2017 American Chemical Society
Received: April 5, 2017 Revised: June 27, 2017 Published: July 5, 2017 6795
DOI: 10.1021/acssuschemeng.7b01036 ACS Sustainable Chem. Eng. 2017, 5, 6795−6802
Research Article
ACS Sustainable Chemistry & Engineering Furthermore, strong magnetic property of the hybrid material brings new prospects for the treatment of environmental pollutants due to its easy separation and fast uptake rate. For example, Li et al.19 found that magnetic polydopamine (PDA)LDH assemblies can simultaneously remove toxic metals and anionic dyes well. Xie et al.20 synthesized Fe3O4@polydopamine (PDA)-Ag core−shell microspheres for MB fast adsorption. Liu et al.21 reported the core−shell Fe3O4/PDA nanoparticles were used as adsorbent, catalyst support, and drug carrier. Magnetic PDA hybrid material possesses the following characteristics in adsorption: (1) PDA shell can protect magnetic particles from corrosion in acid media, and, meanwhile, provide magnetic cores with excellent colloidal stability. (2) Particularly, the redox reactivity of PDA can enable the in situ reduction of adsorbed heavy metal ions. Zeolitic imidazolate frameworks-8 (ZIF-8), a subclass of metal−organic frameworks (MOFs), has been widely investigated in removing inorganic and organic pollutants with excellent capacities due to its porous property and high surface area.22−24 More recently, ZIF-8 has been proved as high efficient material for Cr(VI) adsorptive removal, accompanying the partial reduction process.25 In particular, the highly uniform distribution of pores is ideal for trapping guest molecules and forcing them to participate in chemical coordination in some cases, which endows ZIF-8 with attractive versatility for adsorption related applications. For instance, one-dimensional β-MnO2@ZIF-8 nanostructures possess simultaneous oxidation and adsorptive removal of As(III).26 The Fe3O4@PDA@Zr-MOF can be used as a novel immobilized metal ion affinity platform for phosphoproteome study.27 Motivated by all of these studies, an adsorbent combining the moving core, the redox reactivity of PDA, and the porous of ZIF-8 is expected with high removal capacity and efficiency. Here, a well-defined core-double-shell microsphere composed of a magnetic core, PDA inner shell, and a ZIF-8 outer shell (designated as MP@ZIF-8) was fabricated via a facile and green approach. These synthesized materials can serve as a high-efficient adsorbent to eliminate Cr(VI) from water. In this investigation, the physical and chemical characterization of the synthesized MP@ZIF-8 microspheres was conducted. The adsorption kinetics, isotherms, and reuse, as well as the effect of pH values on their removal capability, were investigated systematically. Moreover, the interaction mechanisms between MP@ZIF-8 and Cr(VI) were proposed by using Fourier transformed infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). This work may provide a potential exploration of MP@ZIF-8 microspheres in adsorbing Cr(VI).
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Scheme 1. Schematic Illustration of the Formation Procedure of the Core-Double-Shell Structured MP@ZIF-8 Microspheres
of MP hybrid material was added to 150 mL of methanol containing 0.225 g of Zn(NO3)2·6H2O with mechanical stirring, and then 0.622 g of 2-methylimidazole (Hmim) was added, and the reaction was continued for 3 h at room temperature for ZIF-8 shell growth. The obtained MP@ZIF-8 microspheres were separated with a magnet and rinsed with Milli-Q water, then freeze-dried at −60 °C under vacuum for 12 h. In addition, ZIF-8 was synthesized through a similar process without MP. Cr(VI) Removal Experiments. The adsorption measurements were carried out in the polycarbonate tubes with 1.2 g L−1 MP@ZIF-8 and 60 mg L−1 Cr(VI) solutions. The initial solution pH was adjusted by using 0.1−0.01 mol L−1 NaOH or HCl solutions. The reaction system achieved uptake equilibrium under oscillation for 24 h at T = 293 K, and then was centrifuged at 8000 rpm for 20 min. The concentration of Cr(VI) was determined with 1,5-diphenyl carbazide method at 540 nm with a detection limit of 0.005 mg L−1 in a UV−vis spectrophotometer (UV-2550, Shimadzu).30 Removal capacities were then calculated as follows:
Qe =
(Co − Ce)·V m
(1) −1
Qe is the equilibrium removal capacity (mg g ), Co and Ce are the initial and equilibrium Cr(VI) concentrations (mg L−1), respectively, V is the solution volume (mL), and m is the adsorbent mass (mg). Characterization, sample preparation after adsorption, and model of adsorption isotherm and adsorption kinetics are described in the Supporting Information.
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RESULTS AND DISCUSSION Characterization of MP and MP@ZIF-8 Microspheres. Figure 1 displays the SEM/TEM images of the as-prepared nanoparticles. Monodisperse pristine Fe3O4 nanoparticles have uniform spherical shape with a rough surface (Figure 1A and B), and their average diameters are approximately 400 nm (Figure S1A). The representative core−shell structure is observed for MP microspheres with a smooth surface, and the thickness of PDA shell is approximately 29 nm (Figures 1C,D and S1B). In Figure 1E and F, the boundary between the ZIF-8 shell and the PDA shell is indistinct, which can be assigned to the slight mass difference of the two components. Of note, ZIF-8 shell is apparent to the PDA shell in morphology in that it is crystalline-like. Moreover, after being coated with ZIF-8, the diameter of the microspheres increases, and the average size of MP@ZIF-8 is about 503 nm (Figure S1C). Time-dependent TEM was used to study how the ZIF-8 shell was formed. As compared to the pristine MP (Figure 1C), tiny ZIF-8 nanocrystals adhered to the surface of the MP microspheres after reacting for 0.5 h (Figure S2B). With the rise in reaction time up to 1.5 h, more nanocrystals are deposited to generate an integrated ZIF-8 shell (Figure S2C). After reacting for 3 h, MP@ZIF-8 with thick ZIF-8 shell thickness (Figure S2D) was successfully synthesized. The EDS spectrum in Figure 2A demonstrates the existence of C, O, N, Fe, and Zn elements in MP@ZIF-8. EDS line distributions are taken across the radius of MP@ZIF-8 microspheres in Figure 2B. Fe element is mainly distributed
EXPERIMENTAL SECTION
Materials. All solvents and chemicals were obtained from Sinoreagent without further purification. All water in this experiment is Milli-Q (Milli-pore, Billerica, MA). K2Cr2O7 was dissolved in MilliQ water to prepared for the 60 mg L−1 of Cr(VI) stock solution. The MP@ZIF-8 microspheres were prepared via three-step routes (Scheme 1). First, the magnetic Fe3O4 microspheres were prepared through a simple solvothermal method.28 Next, 0.3 g of the as-synthesized Fe3O4 microspheres was dispersed in 150 mL of dopamine-tris solution (2 mg mL−1, pH 8.5, 10 mM tris buffer), and the mixture was stirred by mechanical agitator for 24 h at room temperature. Fe3O4 microspheres were embedded into the PDA polymer to prepare MP composites. Finally, the MP nanohybrid was washed and freeze-dried at −60 °C in the vacuum condition for subsequent use. The MP@ZIF-8 microspheres were prepared according to the former reports.29 Briefly, 0.1 g 6796
DOI: 10.1021/acssuschemeng.7b01036 ACS Sustainable Chem. Eng. 2017, 5, 6795−6802
Research Article
ACS Sustainable Chemistry & Engineering
Figure 2. EDS image (A), EDS line scanning of O, Fe, and Zn elements (B) of MP@ZIF-8, XRD patterns (▼, Fe3O4; blue ■, ZIF-8) (C), FTIR spectra (D), TGA curves (F) of Fe3O4, MP, and MP@ZIF8, and (E) FTIR spectra of MP@ZIF-8 before and after Cr(VI) removal.
Figure 1. SEM, TEM images of pristine Fe3O4 (A and B), MP (C and D), and MP@ZIF-8 microspheres (E and F).
in the core part, and O is primarily distributed on the core and partially distributed in the inner shell, while Zn element primarily enriches in the outer shell layer (Figure 2B), which is consistent with the consequence of the elemental mapping image analysis (Figure S3). It is of note that Zn also existed in the core of MP, which can be ascribe to some Zn ions adsorbed into MP during the ZIF-8 growth. By combining the morphology with the component of MP@ZIF-8 through TEM image and XRD pattern analysis, it is sure that ZIF-8 is able to be formed on the surface of MP@ZIF-8. The above results further confirm the successful synthesis of core-doubleshell spheres. Figure 2C shows the XRD patterns of the Fe3O4, MP, and MP@ZIF-8. All of the peaks in the black and red spectra are indexed to the face-centered cubic Fe3O4 (JCPDF card no. 740748). The XRD patterns of the MP and Fe3O4 exhibit similar features, meaning that the amorphous PDA shell does not affect the Fe3O4 crystalline phase. For MP@ZIF-8, the new peaks marked by quadrate come from the cubic phase of ZIF-8, which should be accordingly assigned to the (011), (002), (112), (013), (233), and (134) lattice planes of the ZIF-8 phase.31 Figure 2D displays the FT-IR spectra of Fe3O4, MP, and MP@ZIF-8. In the spectrum of Fe3O4, the absorption peak at 1427 cm−1 is corresponding to the vibration of OCO groups modified on Fe3O4. The broad peak at 1636 cm−1 is corresponding to the vibration of overlapping O−H and O CO groups of Fe3O4.32 The bands at 3440 and 592 cm−1 are associated with the O−H stretching vibration and Fe−O−Fe vibration, respectively.26,33 Plentiful new peaks could be observed in the MP spectrum. The peak of OCO group is covered and the bands at 1615 and 1497 cm−1 appear, which are attributed to the CC stretching vibrations of aromatic
ring.34 A representative peak at 1302 cm−1 might be the primary amine vibration of the PDA shell.35 In the MP@ZIF-8 spectrum, the peak intensities of aromatic ring reduce, notably, the peaks at 1145 and 994 cm−1 are assigned as the plane bending of imidazole ring, and the band at 421 cm−1 is attributed to the Zn−N stretch mode.23 FT-IR was also used to characterize the structure variation of MP@ZIF-8 after Cr(VI) adsorption. Figure 2E shows that the band at 870 cm−1 in the chromium-adsorbed MP@ZIF-8 spectrum corresponds to the stretching vibrations of the CrO4 tetrahedron.36 The characteristic peaks at 1497, 1427, and 1302 cm−1 related to the aromatic ring stretch, N−H vibration, and stretching vibration of phenolic C−O groups of the PDA either reduced or disappeared after Cr(VI) removal, implying that the Cr(VI) species are bonded to both phenolic hydroxyl and amino groups of PDA and certain chemical bonds are formed.37 Figure 2F shows the TG analysis of Fe3O4, MP, and MP@ ZIF-8. The observed curve of Fe3O4 represents two steps of mass loss. The first mass loss stage (
