Fabrication of Stable and Regenerable Amine Functionalized

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Fabrication of stable and regenerable amine functionalized magnetic nanoparticles (MnFe2O4@SiO2-NH2) as a potential material for Pt(IV) recovery from acidic solutions D. Harikishore Kumar Reddy, Wei Wei, Shuo Lin, Myung-Hee Song, and Yeoung-Sang Yun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16813 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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ACS Applied Materials & Interfaces

Fabrication of stable and regenerable amine functionalized magnetic nanoparticles (MnFe2O4@SiO2-NH2) as a potential material for Pt(IV) recovery from acidic solutions D. Harikishore Kumar Reddy†, Wei Wei†, Lin Shuo†, Myung-Hee Song†, and Yeoung-Sang Yun*,†‡ †

Division of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonbuk 561-756, Republic of Korea. ‡

Department of Bioprocess Engineering, Chonbuk National University, Jeonbuk 561-756, Republic of Korea.

Keywords: Magnetic nanoparticle; Adsorption; Pt(IV) adsorption; Grafting; Spinel ferrite

*

Corresponding author:

Prof. Yeoung-Sang Yun E-mail address: [email protected] (Y.-S. Yun) Tel.: +82 63 270 2308; Fax: +82 63 270 2306

ACS Paragon Plus Environment


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ABSTRACT MnFe2O4@SiO2-NH2 magnetic nanocomposite (AFMNC) adsorbent with a particle size of ~50 nm was successfully synthesized using a facile approach. The as-prepared composite particles showed a fast binding of Pt(IV) with easy magnetic solid-liquid separation. The kinetic data were fitted to both pseudo-first and pseudo-second-order rate models, indicating that AFMNC exhibited a much higher rate of Pt(IV) binding (0.125 g mg-1 min-1) compared with commercial ion exchange resin Amberjet 4200 (0.0002 g mg-1 min-1). The equilibrium adsorption data were fitted to the Langmuir isotherm model with a relatively high sorption capacity of 380 mg/g. Scanning transmission electron microscopy (STEM) analysis demonstrated the presence of platinum chloride after sorption on AFMNC, suggesting an adsorbate-adsorbent anion-exchange interaction. In addition, due to its magnetic characteristics, AFMNC can be easily separated from the aqueous medium after sorption process. The novel nanocomposite (AFMNC) may facilitate recovery of Pt(IV) from waste solutions.


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1. Introduction Platinum-group elements (PGEs) are precious metals with a wide range of applications—such as electronic devices, jewelry, automobiles, catalysts, and medical instruments—because of their unique physical and chemical properties.1 These elements are nonrenewable, randomly distributed in the Earth’s crust and have high economic value. Among PGEs, platinum (Pt) is an important and strategic element of particular interest for various industrial applications, due to its good catalytic properties, high thermal stability and chemical resistivity.2 Although the automotive and jewelry industries comprise 60% of the primary demand, Pt also has applications in the healthcare and chemical industries.3 Pt is internationally recognized as a form of currency under ISO 4217 and is expensive (1700 $/ounce).4 Due to its extensive usage, natural Pt resources are becoming depleted. However, demand for Pt is increasing steadily, and the trend that is expected to continue.2,3 Due to its usage in several fields, Pt concentrations in industrial effluents are increasing, leading to its accumulation in the environment5,6. This causes serious pollution in water and soil, and even in the air due to airborne Pt particles5-7. Therefore, from both economic and environmental viewpoints, the recovery of Pt from secondary resources (waste or spent materials) is essential.2,8 Recovery of Pt from industrial wastes would maintain the supply of Pt and convert disposable waste materials into valuable renewable resources. Various conventional and modern techniques—such as solvent extraction,9 ion exchange,10 membrane separation,11 and adsorption12—have been used for the recovery of Pt from solution. Adsorption is a powerful, facile and economic technology13 extensively used for the recovery of Pt from secondary resources. Diverse adsorbents have been developed and used for Pt removal; e.g., biosorbents,12 polymers,1 nanomaterials,14 and ion-exchange resins.15



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Among these adsorbents, nanoparticles have attracted scientific and industrial interest, as they possess a high surface-to-volume ratio, resulting high sorption capacities and rapid kinetics.16 In addition, because of their Brownian motion, a large volume of solvents can be treated in a short period.17 For example, 15 g of nanomagnetite (12 nm) can be used to treat 50 L of arsenic (500 µg/L) in place of 1.4 kg of bulk iron oxide.18,

19

Various nanomaterials—such as hematite,

magnetite, maghemite, spinel ferrites, graphene oxide, carbon nanotubes, and metal oxides— have been used for decontamination of aqueous solutions. Among various nanomaterials, magnetic spinel ferrite nanoparticles are a focus of research and have been extensively used as adsorbents for removal of various pollutants.20 This is due to the fact that spinel ferrite nanoparticles exhibits higher surface area and are stable due to high chemical resistance to oxidation20. Spinel ferrite (MFe2O4, where M denotes a divalent metal ion such as Fe2+, Co2+, Ni2+, Zn2+, Mn2+, etc.) nanoparticles have excellent physicochemical properties, such as high surface area, superparamagnetism, high sorption ability, and easy separation from aqueous solution under an external magnetic field.21, 22 However, certain problems are associated with nanometersize magnetic nanoparticles (MNPs) such as aggregation, stability, leaching, and toxicity.23, 24 In addition, due to their high surface energy, they tend to aggregate in aqueous media, decreasing their adsorption capacity. These weaknesses influence their potential for removal of toxic pollutants. In this context, grafting or coating of magnetic nanoparticles with organic or inorganic layers is performed to overcome the above-mentioned problems.20, 25 Materials such as polymers, surfactants, silica, alumina, and carbon are frequently used for this purpose. Among the above-mentioned materials, non-toxic silica is considered to be one of the best shell materials for coating MNPs due to its surface chemistry, stability and adsorption


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capacity.26, 27 Coating or grafting with silica improves the chemical stability of MNPs even in the medium of lower pH and reduces their toxicity.28, 29 The inner magnetic core with a silica outer shell not only stabilizes the NPs in solution but also provides additional functional terminal groups for surface modification with various chelating ligands.30 In addition, silica coating of the surface prevents the leaching and aggregation of MNPs in solution and reduces their toxicity. Due to these characteristics, magnetic core-shell nanoparticles (MCSNPs) have aroused great attention in various fields due to their biocompatibility, renewability, and stability.31 In the present study, we used MnFe2O4 NPs, which have a relatively high surface area, biocompatibility, excellent chemical stability, and good saturation magnetization for the preparation of amino-functionalized MnFe2O4 NPs. To our knowledge, application of nanoparticles for the recovery of precious metal ions has not been investigated extensively.14, 16 In this study, we report a facile and effective method of synthesizing a MnFe2O4@SiO2-NH2 nanocomposite adsorbent (AFMNC) and evaluated its potential to recover Pt(IV) from aqueous solution. We systematically evaluated the adsorption behavior of Pt(IV) by AFMNC, including the sorption isotherm and kinetics. The kinetic performance of AFMNC was compared with that of the commercial ion-exchange resin Amberjet 4200. The physical structure and chemical characteristics of the nano-adsorbent were characterized in detail by SEM-EDX, STEM, XRD, VSM, and FTIR. Our findings suggest the feasibility of AFMNC as an efficient adsorbent for the recovery of Pt(IV) from chloride media.

2. Experimental 2.1. Materials Tetraethoxysilane (TEOS) (>98.5%, Daejung, Korea), (3-aminopropyl) triethoxysilane



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(APTES) (≥98%, Sigma-Aldrich), ammonium hydroxide (ammonia water) (25.0-28.0%, Daejung, Korea), iron (III) chloride hexahydrate (FeCl3.6H2O), (≥98%, Sigma-Aldrich), manganese

(II)

chloride

tetrahydrate

(MnCl2.4H2O)

(99.0%,

Sigma-Aldrich),

N,N-

dimethylformamide anhydrous (99.8%, Sigma-Aldrich), and toluene anhydrous (99.8%, SigmaAldrich). Amberjet 4200 (Rohm & Haas) was acquired from Samchully Activated Carbon Co. Ltd. (Yeongi-gun, Korea). All other chemicals and solvents used in this study were of analytical reagent grade. 2.2. Fabrication of AFMNC 2.2.1. Synthesis of MnFe2O4 NPs In the present study, MnFe2O4 nanoparticles were synthesized based on the coprecipitation of Mn(II) and Fe(III) under alkaline conditions. Briefly, MnCl2.4H2O (0.01 mol) and FeCl3.6H2O (0.02 mol) were dissolved in deoxygenated distilled water under vigorous stirring until a homogeneous solution was obtained. Then, co-precipitant 2 M NaOH solution was added dropwise with stirring into the homogeneous solution until the pH value reached 11. The resulting solution was maintained at 100°C for 2 h. The obtained MnFe2O4 nanoparticles were isolated by magnetic separation and washed several times with double-distilled water to remove any unreacted content (Na+, OH-, and Cl- ions) and washed once with ethanol. Finally, the NPs were freeze-dried to obtain pure jacobsite nanoparticles. The nanoparticles were synthesized according to the chemical reaction described in equation (1). MnCl2 + 2FeCl3 + 8NaOH → MnFe2O4 (s) +8NaCl + 4 H2O

(1)

2.2.2. Synthesis of MnFe2O4@SiO2 nanocomposite Prior to synthesizing amine-functionalized nanoparticles, silica-coated MnFe2O4 NPs were prepared according to the following procedure: 100 mg of MnFe2O4 nanoparticles were


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suspended in 100 mL ethanol and sonicated for 30 min. To this solution, 2 mL of aqueous ammonia solution (25 wt%) and 20 mL of distilled water were added under sonication at room temperature; sonication was continued for 40 min. Further, this solution was stirred mechanically. Next, 0.6 mL of TEOS was added dropwise into the MnFe2O4 solution slowly for 1 h, and the mechanical stirring was continued for a further 20 h at room temperature. Finally, the resulting silica-coated nanocomposites were isolated by magnetic separation, washed with ethanol several times and dried under vacuum. 2.2.3. Synthesis of MnFe2O4@SiO2-NH2 Amine-functionalized MnFe2O4@SiO2 nanoparticles were obtained by the following synthesis protocol: 50 mg of MnFe2O4@ SiO2 nanoparticles were dispersed in a solution mixture of 20 mL anhydrous DMF and 10 mL anhydrous toluene, this solution was ultrasonicated for 30 min. The solution was kept under mechanical stirring. To the solution, 0.6 mL of APTES was added dropwise slowly for 1 h, and vigorous mechanical stirring was continued for a further 24 h. Finally, the functionalized nanocomposite, MnFe2O4@SiO2-NH2 (AFMNC), was separated from the solution by magnetic separation, washed with ethanol several times, and dried under vacuum. 2.3. Apparatus The crystal structure and phase information of the samples were attained by means of Xray diffraction (XRD) measurements using an XD2711N instrument (Rigaku) in the 2 θ range from 10° to 90°. Fourier transform infrared spectrometry (FTIR, GX Perkin-Elmer, USA) was used to obtain functional group information over the wavelength range 4000-400 cm-1. Pellets were prepared using FTIR-grade potassium bromide (KBr). The magnetic properties were measured using a vibrating sample magnetometer (VSM, Lakeshore 7404, USA) at room



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temperature up to a field of 15 kOe. The morphologies and elemental information of the samples were characterized using a field emission scanning electron microscope (FE-SEM) coupled with an energy dispersive X-ray spectroscope (EDX) (SUPRA 40VP, Carl Zeiss, Germany). Aberration corrected scanning transmission electron microscope (Cs-corrected STEM, JEOL/JEM-ARM200F) was used to analyze the nanocomposite. X-ray photoelectron spectroscopy (XPS) was used to obtain the elemental information of before and after Pt(IV) sorption. The XPS spectra were acquired by means of an AXIS-NOVA spectrometer (Kratos Analytical, Ltd., UK) with monochromated Al Kα radiation (1486.71 eV). A Millipore/directQ3UV (Millipore, USA) was used to produce deionized water.

2.4. Batch sorption of Pt(IV) onto AFMNC Sorption kinetic and isotherm experiments were carried out by the batch method to evaluate the characteristics and mechanism of Pt(IV) sorption onto AFMNC. Pt(IV) stock solution (1000 mg/L) was prepared by dissolving an appropriate amount of H2PtCl6‧ nH2O in 0.1 M HCl solution. For each experiment, 0.03 g of the sorbent was mixed with 30 mL of Pt(IV) solution. Pt(IV) solutions of various concentrations (100, 200, 400, 600, and 1000 mg/L) were shaken for 24 h used to and assess the adsorption isotherm. To evaluate the sorption kinetics, experiments were performed from 0-300 min using 100 mg/L Pt(IV) solutions. The concentration of Pt(IV) after sorption experiments was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, iCAP 7000 series, Thermo Fisher Scientific). The following equation was used to calculate Pt(IV) sorption: Pt(IV) sorption = qe =

(େ౟ ିେ౜ )୚ ୑

(2)


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where Ci and Cf are the initial and final Pt(IV) concentrations (mg/L), m is the mass of MnFe2O4@SiO2-NH2 (g), and V is the volume of solution (L). 2.5. Desorption and regeneration of AFMNC In order to test the recyclability of AFMNC after being used for Pt(IV) recovery from an acidic solution, the metal-loaded sorbent was regenerated by acidified thiourea (0.05 M thiourea + 0.01 M HCl) solution, owing to its good desorption efficiencies of precious metal ions. After sorption, Pt-loaded AFMNC was suspended in 30 mL of acidified thiourea to assess the desorption performance. Three sorption-desorption cycles were carried to determine the reusability performance of AFMNC; during sorption-desorption cycles, AFMNC was washed with double distilled water. 2.6. Stability of AFMNC As an indication of the stability of AFMNC, the released metals (i.e, Mn and Fe) were monitored after contacting with desorption solutions by using ICP-OES. Two different concentrations of acidified thiourea (i) 0.05 M thiourea/0.01 M HCl and (ii) 0.1 M thiourea/0.01 M HCl were used as desorption solutions. 3. Results and discussion 3.1. Synthesis and characterization of AFMNC Various approaches to synthesize magnetic spinel ferrite nanoparticles—such as sol-gel, microemulsion, spray pyrolysis, sonochemical, hydrothermal, etc.—are available.32, 33 The most common technique used for the synthesis of spinel ferrite nanoparticles is chemical coprecipitation of appropriate metal salts. In the present study, we synthesized MnFe2O4 nanoparticles according to the co-precipitation technique using a stoichiometric Fe3+:Mn2+ ratio of 2:1, in an aqueous medium by addition of NaOH solution. To functionalize MnFe2O4



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nanoparticles with amine groups, the MnFe2O4 nanoparticles were first coated with TEOS using the Stober approach.34 TEOS coated MnFe2O4 nanoparticles were further functionalized with APTES to obtain amino groups on the composite surface. The AFMNC nanocomposite fabrication procedure is shown in Figure 1. X-ray diffraction (XRD) was used to identify the crystalline structure and composition of the bare MnFe2O4 and AFMNC (Figure 2). All diffraction peaks in the XRD spectrum can be perfectly indexed with a cubic MnFe2O4 phase, and the observed peaks are consistent with the standard diffraction patterns of MnFe2O4. Similar findings were reported in an earlier study.35 Figure 2 shows the XRD patterns of the AFMNC, most which were similar to that of MnFe2O4. Besides these peaks, an additional broad, weak peak was observed in the 2θ range of 20 to 28°, corresponding to amorphous silica (JCPDS 29-0085).36 These results confirm the presence of an amorphous silica shell on the AFMNC and indicate that the coating process did not influence the phase change of MnFe2O4 nanoparticles. The average crystallite sizes of the nanoparticles were calculated from the (3 1 1) peaks according to Scherrer’s formula D = Kλ/β cos θ. The calculated average crystal size for MnFe2O4 and AFMNC was 21.52 and 20.46 nm, respectively, which indicates that the silica coating had little effect on the crystal size of MnFe2O4. However, a smaller decrease in average crystal size after silica coating was due to the large number of nucleation sites in the SiO2 matrix, which restricts nanoparticle growth.37, 38 FTIR spectra were further used to analyze the formation of MnFe2O4 nanoparticles and the silica-coating process (Figure 3). The vibration bands with peaks observed at 3410 cm-1 and 1627 cm-1 were assigned to the −OH groups on the surface of MnFe2O4. The vibration peak at 578 cm-1 corresponds to the Fe-O stretching vibration.39 In the MnFe2O4@SiO2 spectrum, the peaks at 465, 790, 940, and 1092 cm–1 correspond to the O–Si–O bending mode and the Si–O–Si


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symmetric, Si–O symmetric, and Si–O–Si asymmetric stretching modes of the added silica shell.40-42 The further spectral analysis was performed to confirm APTES coating of the MnFe2O4@SiO2 nanocomposite. Besides the characteristics peaks of silica, few other peaks were observed. The characteristic peaks at 2930 cm-1 and 2876 cm-1 correspond to the C-H2 bondstretching vibrations of aminopropyl from APTES. The peak at 1650 cm-1 can be assigned to the N-H bending vibrations,43 and the broadband at ~3420 cm-1 matches the amino functional groups of APTES that were overlain by O-H stretching vibrations.39 These results confirm that MnFe2O4@SiO2 particles were successfully modified by APTES. The morphologies and structure of the AFMNC were systematically characterized by SEM, STEM, and EDX. Figure S1 shows a representative SEM image of the resultant AFMNC nanoparticles. A relatively smooth surface was observed due to the silica coating, and particles aggregated due to their strong magnetic characteristics. However, the particles dispersed upon addition of AFMNC to the aqueous solution. The SEM image indicates that AFMNC was composed of uniform-size nanoparticles of 40 to 50 nm in diameter. The EDX spectra (Figure S1) revealed the presence of C, N, O, Fe, Si, Mn, and Fe. Although SEM images could not reveal the core-shell structure, silica peaks were observed from EDX spectrum this information shows that coating of SiO2 (shell). STEM characterization was further performed to confirm the AFMNC nanocomposite. TEM image of AFMNC (Supplementary Figure S2), which depicts a single microsphere with a diameter of 30–50 nm. The core-shell structure was not evident, possibly because the difference between the core and thin shells of 1), linear (RL=1), favorable (0

Fabrication of Stable and Regenerable Amine Functionalized

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