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Ind. Eng. Chem. Res. 2005, 44, 816-824
Silanation of Nanostructured Mesoporous Magnetic Particles for Heavy Metal Recovery Pinggui Wu†,‡ and Zhenghe Xu*,†,§ Department of Chemical & Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada, and Department of Chemistry, South China Normal University, Guangzhou, 510631 China
Nanostructured mesoporous magnetic (m-Fe3O4) particles are promising candidate materials for separation, detoxification, and precious metal recovery. The silanation of m-Fe3O4 particles using 3-aminopropyltriethoxysilane is described. The silanized films were characterized using XPS, diffuse-reflectance FTIR spectroscopy, and electrokinetic techniques. The loading capacity of the silanized m-Fe3O4 particles was found to be superior to that of m-Fe3O4 or directly silanized Fe3O4 particles. Extraction and separation of transition metals (II) were investigated using the silanized m-Fe3O4 particles. The effectiveness of metal (II) extraction was found to increase with increasing solution pH, but was less effective at pH below 2. Satisfactory separation with loading efficiency in the order of Cu2+ > Ni2+ > Zn2+ was obtained. The adsorbed metals were successfully desorbed with 1 M aqueous HCl solutions. As a result, the recovered m-Fe3O4 particles can be recycled in industrial applications. 1. Introduction Growing concerns about the depletion of raw material supplies and increasingly stringent environmental requirements for the treatment of municipal and industrial effluents have stimulated great interest in the development of viable methods capable of cleaning up effluents and recovering valuable materials from industrial effluents. The problem of disposing industrial wastes is as old as industry itself.1 Disposing of wastewater or process water containing soluble heavy metals and/or organics is a common problem in the chemical, petroleum, synfuel, mineral, cosmetic, and textile industries. Considerable efforts have been devoted to the study of effective removal or selective recovery of heavy metals from industrial effluents.2-6 Recently, magnetic carrier technology (MCT) has emerged as a new technology with great potential and scientific interest.2,3,7,8 In our previous work,9 mesoporous magnetic nanostructured (hereafter referred to as m-Fe3O4) particles were successfully prepared by molecular templating of self-assembled micelles, followed by a sol-gel process using tetraethoxysilane. The m-Fe3O4 particles consisted of a micrometer-sized magnetic core, surrounded by a mesoporous exterior silica shell of about 100-nm thickness. As described in our previous paper,9 the m-Fe3O4 particles had a high normalized surface area of 520 m2 per gram of coated silica and an average pore size of 2.5 nm, with a strong saturation magnetization. These particles could have a wide range of applications in adsorption and separation * To whom correspondence should be addressed. Address: Department of Chemical & Materials Engineering, University of Alberta, Edmonton, AB Canada T6G 2G6. Tel.: 1-780-492 7667. Fax: 1-780-492 2881. E-mail:
[email protected]. † University of Alberta. ‡ Current address: Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, 1304 W. Green St., Urbana, IL 61801. § South China Normal University.
science and technology. The mesoporous property of the synthesized particles is attractive as it increases their specific surface area and, hence, adsorption capacity. However, to load metal ions effectively and specifically from an industrial effluent, a dense layer of surface functional groups with good chemical stability is essential. The challenge to magnetic carrier technology using m-Fe3O4 particles lies in the fabrication of stable and thin organic films of tailored functional groups on the carrier particles. To achieve a strong affinity for transition metals, functionalization of the mesoporous magnetic particles in the present work was accomplished by silanation. By grafting organosilanes that contain organic functional groups onto the accessible pore surfaces or by incorporating them into the structure during particle synthesis, diverse catalytic activities have been improved.10 For example, Feng et al. reported the formation of organic monolayers within ordered mesoporous silica, MCM-41, using tris(methoxy)mercaptopropylsilane (TMMPS) as the coupling agent. The surfaces functionalized with reactive thiol groups exhibited specific adsorption of heavy metal ions, with a distribution coefficient of up to 340 000 being obtained.11 Antochshuk and Jaroniec12 functionalized mesoporous materials via interfacial reactions in self-assembled silica-surfactant systems. An ideal case for successful functionalization would be chemically anchoring the functional groups onto the carrier surface while maintaining molecular recognition for the target ion(s). Judging from previous research experience,3,4,6,13 3-aminopropyltriethoxysilane (3-APTES) appears to be a logical choice as a silanation agent for functionalizing nanostructured mesoporous magnetic particles that can be used for heavy metal separation and recovery from industrial effluents. Our objective is to functionalize nanostructured mesoporous magnetic particles for metal recovery from mineral processing or hydrometallurgical effluents, detoxification of industrial effluents, and removal of contaminants. The present emphasis is on the recovery
10.1021/ie0495325 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/15/2005
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2. Experimental Section
Figure 1. (A) Conceptual schematics of magnetic carrier technology in metal recovery from an industrial effluent. (B) Pore size distribution of the synthesized m-Fe3O4.
of transition metal ions, the value of which not only provides an economic incentive but also maximizes the use of raw materials. Demonstrated in this article is the potential of amine-functionalized m-Fe3O4 particles for selective removal of heavy metal ions at a desired capacity from industrial effluents and contaminated municipal waters. A unique feature of using magnetic carriers for detoxification and metal recovery from industrial effluents or municipal waters is easy isolation of the loaded magnetic carriers from a complex multiphase system, as schematically illustrated in Figure 1A. In this figure, the functionalized m-Fe3O4 particles (MMC) are added to a hydrometallurgical processing suspension that contains precious metal ions and many other waste solid particulates. The precious metal ions selectively adsorb on the functionalized m-Fe3O4 particles through molecular recognition by surface functional groups. Along with the functionalized m-Fe3O4 particles, the adsorbed metal ions are separated from the suspension by an external magnetic field. The metal ions loaded on the isolated carrier particles can be stripped off by, for example, acid washing, and the particles can then be reused. Precious metals can then be formed from the concentrated ion solution by precipitation, crystallization, and/or electrowinning. In industrial effluent treatment practices, magnetic carrier technology combines the advantages of technical flexibility, environmental acceptability, and great economic value. Thus, the current investigation into the functionalization of nanostructured mesoporous magnetic particles for heavy metal recovery is significant in the search for highly desirable carriers.
2.1. Silanation of Nanostructured Mesoporous Magnetic Particles. Synthesis of the nanostructured mesoporous magnetic particles has been described elsewhere.9 A silane coupling agent, 3-aminopropyltriethoxysilane [3-APTES, NH2-(CH2)3-Si-(CH3CH2O)3, 99%] was purchased from Aldrich and used as received. In-house-distilled toluene with a purity of 99.9% was used. Water used in this study was prepared with a Millipore water purification system. All glassware was cleaned by being soaked in a mixture of 10 wt % sodium hydroxide aqueous solution and ethanol at a 10:1 volume ratio for a sufficiently long period and then rinsed thoroughly with deionized water. To effectively hydrolyze the surfaces, 3 g of mesoporous Fe3O4 (m-Fe3O4) particles of micrometer sizes were steamed for 30 min with boiling deionized water. The hydrolyzed sample was baked at 120 °C for about 1 h to remove free water. The sample was then mixed by vigorous stirring with 90 mL of toluene in a 250-mL three-neck flask equipped with a reflux condensing tube. After the particles were well dispersed, 10 mL of 3-APTES was added slowly to the flask under continuous mechanical stirring. The reaction continued under reflux for 4 h. The resulting suspension was then cooled to room temperature, and the particles were collected with a hand magnet. After being washed twice with toluene, twice with water, and twice with ethanol, the particles were dried in a vacuum oven at room temperature. 2.2. Characterization. (i) Pore Size Distribution Measurement. Pore size distributions were determined from N2 adsorption-desorption isotherms obtained at -196 °C using a Quantachrome Autosorb automated gas sorption system. Before nitrogen adsorption-desorption measurements, the sample was degassed at 250 °C under vacuum for 46 h. The pore size distribution was calculated from the desorption branch of the N2 adsorption-desorption isotherms using the conventional Barrett-Joyner-Halenda (BJH) method. The analysis of the data was completed using the built-in computer programs supplied with the system. (ii) Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The silanized particles were characterized by DRIFTS. Infrared spectra in the range of 4000-600 cm-1 were obtained with an FTS 6000 spectrometer (Bio-Rad Laboratories). A sample of finely crushed KBr was used as the background. All spectra were obtained using 128 scans at a nominal resolution of 4.0 cm-1 and are presented without background correction. (iii) X-ray Photoelectron Spectroscopy (XPS). XPS spectra were obtained on a Physical Electronics PHI 5400 X-ray photoelectron spectrometer (PerkinElmer) with a Mg KR anode (15 kV, 400 W) at a takeoff angle of 45°. The source X-ray was not filtered, and the instrument was calibrated against the C 1s band at 285 eV. The spectra were recorded at a constant pass energy of 35.75 eV. The spectra were taken under a background pressure of ca. 1 × 10-9 Torr. (iv) ζ-Potential. For many applications of functionalized m-Fe3O4 particles, the functional group must be well packed and robust. The ζ-potential of the silanized particles was measured to semiquantitatively assess the degree of functionalization. The measurements were conducted in 1.0 × 10-3 M KCl background electrolyte solutions using a Zetaphoremeter III (SEPHY/CAD,
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Limours, France) instrument. To minimize sedimentation of particles during ζ-potential measurements, magnetite particles were carefully ground before they were synthesized and functionalized. The prepared particles were conditioned in an 80-mL beaker at a given pH. The pH was adjusted using NaOH and HCl stock solutions. The suspension was then used for ζ-potential measurements. The results presented in this article are the averages of three independent measurements with a typical measurement variation within (4 mV. (v) Particle Size. The size of the prepared m-Fe3O4 particles was measured with a Malvern Mastersizer 2000 using the lens suitable for the size range of 0.022000 µm. Particle agglomeration during particle size measurement was minimized by dispersing the samples in deionized water using an ultrasonic bath for 5 min. The Mie scattering theory was used to fit the measured light signals to a particle size distribution. (vi) Magnetization. The magnetic properties of the Fe3O4 particles before and after treatment were measured with a Quantum Design (San Diego, CA) 9TPPMS dc magnetometer/ac susceptometer. The measurements were conducted at a temperature of 2 K with a magnetic field strength from 0 to 9 T (the magnetic induction B) and frequency between 1000 and 5000 Hz. 2.3. Metal Ion Loading. Cupric sulfate, nickel chloride, and zinc chloride were purchased from BDH Chemicals and used without further purification. To examine the loading capacity of the silanized m-Fe3O4 particles, the loadings of the metal ions (copper, nickel, and zinc in this case) on the prepared magnetic carriers were determined. In a typical loading test, 50 mg of carriers was mixed with 25 mL of a metal ion solution of a given initial metal ion concentration. The suspension was stirred and shaken on a horizontal automatic shaker for 1 h. The supernatant was then collected, and the equilibrium concentration of the metal in the supernatant was determined with an atomic absorption spectrophotometer (Perkin-Elmer 310, Perkin-Elmer, Shelton, CT). The amount of metal ions loaded on magnetic carriers was calculated from the difference of the initial and equilibrium metal ion concentrations in the supernatant. The effectiveness of metal loading was evaluated using a metal ion loading distribution coefficient (Kd), defined as
Kd )
Ci - Ce V (mL/g) Ce M
where V is the volume of the aqueous solution (in milliliters); M is the mass of adsorbents (in grams); and Ci and Ce are the metal ion concentrations before (initial) and after (equilibrium) being put into contact with the synthesized m-Fe3O4 particles, respectively. The distribution coefficient defined as such represents the amount of metal ions adsorbed on the particles in reference to the amount of metal ions left in the solution. This definition resembles the conventional partition coefficient of a species in two phases of a solvent extraction system. Its value depends on the nature of the ion, solution pH, initial ion concentration, and loading capacities of the adsorbents. For a separation process to be successful, Kd must have a value greater than 100 mL/g.2 To examine the feasibility of recovering metals and recycling magnetic carriers, stripping tests of metal ions
from the loaded m-Fe3O4 particles were performed. In the stripping tests, the loaded magnetic carriers were mixed by shaking with 25 mL of 1 M HCl solution for about 15 min. The particles (referred to as recycled magnetic carriers) were then collected by a hand magnet and used in the subsequent loading tests. The supernatant was analyzed using the procedures described above to determine the stripping efficiency. The stripping efficiency was calculated as the percentage of the loaded metal ions detached. Reloading of copper ions onto the recycled magnetic carriers was performed using the same procedures as used in the initial loading tests. 3. Results and Discussion 3.1. Identification of Functional Monolayers. (i) Silanation. As presented previously, the synthesized mesoporous magnetic particles exhibit the necessary attributes of strong magnetization and high specific surface area for applications as magnetic carriers.9 The dense liquid silica-coating method followed by a templateassisted sol-gel process resulted in the formation of a uniform mesoporous thin silica coating on the magnetite particles. The pore volume distribution of the synthesized m-Fe3O4 particles as shown in Figure 1B exhibits three distinct distribution peaks in reducing volume at pore sizes of ca. 2.5, 3.9, and 10.5 nm. Although a trimodal pore distribution is observed, the results indicate the absence of micropores and macropores. The resulting m-Fe3O4 particles are found to be well protected from leaching of core iron, while providing silica surfaces amenable for silanation. It is clear that the synthesized m-Fe3O4 particles consist of a micrometersized magnetite core, surrounded by a solid thin silica film, followed by an exterior mesoporous silica shell of about 100 nm thickness. 3-APTES was selected as the silane coupling agent because it has been used previously to form functionalized monolayers and the reactive amino groups exhibit a high affinity for metals.3,11 Furthermore, use of this short-chain alkane amine minimizes blockage of pore channels. The self-assembly of silane coupling agents by silanation is a multistep process. In addition to complex interactions among the substrate, silane, and solvent, many other parameters need to be considered.3 According to Feng et al., the amounts of surface silanol groups and adsorbed water molecules on mesoporous materials are two key parameters in determining the density and quality of the functionalized monolayers.11 As can be seen from the silane-coupling reaction in Figure 2A, surface silanols are essential because they are the active centers for anchoring silane on the particle surfaces through siloxane chemical bonding. Adsorbed surface water is necessary for the hydrolysis of 3-APTES in toluene, which initializes the reaction process. However, the presence of excess free water is deleterious to the formation of a clean monolayer, as 3-APTES is known to polymerize into white solid precipitates in the presence of water. The precipitates can potentially block the pores and hence reduce the effective surface area of the functionalized carriers. For these reasons, a proper amount of water for the hydrolysis of 3-APTES needs to be employed to obtain a monolayer of silanized 3-APTES on the pore surfaces. The absence of H-O vibrational bands in DRIFTS spectrum a of Figure 2B suggests that calcination at 540 °C for 4 h, required to remove surfactant templates and obtain mesoporous particles,9 dehydrated the
Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 819 Table 1. Band Assignment of Infrared Spectra in Figure 2B
Figure 2. (A) Schematic diagram for the synthesis of m-Fe3O4 silanized by 3-APTES. (B) DRIFTS spectra of m-Fe3O4 magnetic particles: after (a) calcination (b), rehydroxylation, and (c) silanation by 3-APTES from toluene.
m-Fe3O4 surface and depleted most of the silanol groups. Such a dehydrated surface would result in a poor surface coverage of functional groups if silanized directly. Evidently, hydrolysis of Si-O-Si siloxane bonds on the calcined m-Fe3O4 surface to form silanols is required. The hydrolysis of 3-APTES is also necessary but must be controlled to avoid intramolecular condensation (precipitation) of silane coupling molecules. The process of steaming followed by controlled drying was investigated in this study to control the presence of surface silanols (Si-OH) and surface water. In other words, to optimize the reaction conditions for depositing alkoxysilane monolayers on mesoporous Fe3O4 surfaces, the particles were rehydrated carefully by steaming, and the amount of surface-adsorbed water was controlled by drying. The strategy of controlling the surface free water rather than the bulk water content of the solvent in this study facilitated the hydrolysis of 3-APTES within the surface region, which, in turn, maximized the surface concentration and minimized bulk condensation of 3-APTES. To be extra cautious, high-quality toluene was used. Toluene was reported in the literature to be excellent for removing excess water and forming organic monolayers.11 As shown in DRIFTS spectrum b of Figure 2B, obtained with the steamed and dried samples, the presence of a sharp H-O vibrational band at 3750 cm-1 confirms the successful hydrolysis of siloxane bonds by steaming. (ii) DRIFTS Characterization. The molecular formula of 3-APTES (Figure 2A) suggests that the DRIFTS spectrum of 3-APTES should feature the characteristic bands of -NH2, -CH3, -CH2, and Si-O-C vibrational modes. A standard DRIFTS spectrum of 3-APTES given by Sigma-Aldrich was used as the reference spectrum (not shown in this article). DRIFTS spectrum c of the silanized m-Fe3O4 in Figure 2B exhibits the featured bands of 3-APTES (see
wavenumber (cm-1)
assignment
3354 3286 2932 2860 1626, 1568 1483, 1405 1200-1040 636
-NH2, asym stretching -NH2, sym stretching -CH2, asym stretching -CH2, sym stretching -NH2, in-plane bending -CH2, bending Si-O-Si, siloxane O-Fe, iron oxide
band assignments in Table 1). As shown, a pair of weak broad bands at 3400-3250 cm-1 is evident. These two bands are assigned to free amino asymmetric and symmetric stretching vibrations. A strong band at 1568 cm-1 is assigned to the deformation bending vibrations of free amine groups on the surface. In addition, two bands at 2932 and 2860 cm-1 assigned to the asymmetric and symmetric stretching vibrations of CH2 in alkyl chains, as well as a band at 1483 cm-1 assigned to CH2 bending vibrations, are evident. These spectral features confirm the silanation of 3-APTES on the particle surfaces. It is interesting to note that the characteristic bands of Si-O-C at 1167, 1105, 1083, and 959 cm-1 almost disappeared after silanization. This finding suggests that most of the ethoxy groups in 3-APTES were hydrolyzed. Two strong bands at 1126 and 1041 cm-1 (characteristic of siloxane stretching vibrations) remained after silanation, indicating that the surface binding of 3-APTES was not by silanols but rather by siloxane bonds. The presence of siloxane binding was further supported by the disappearance of IR bands at 3745 cm-1, assigned to the stretching vibrations of surface silanols in the spectra of silicacoated magnetic particles before calcination9 and of steamed particles (spectrum b in Figure 2B). (iii) XPS Analysis. To determine the bonding characteristics of different elements involved, XPS spectra of the relevant elements on the m-Fe3O4 particles silanized with 3-APTES from toluene solution were acquired and are shown in Figure 3. As a reference, the corresponding XPS spectra of untreated Fe3O4 and m-Fe3O4 particles are also included in this figure. The appearance of a nitrogen band at 399 eV (N 1s) and the presence of a stronger silicon band at 101 eV (Si 2p) after silanation indicate that aminopropyltriethoxysilanes are deposited on the surface. Deconvolution of the broad and asymmetric N 1s band reveals two distinct nitrogen bands centered at ca. 399 and 401 eV. These bands are ascribed to free and protonated amine groups, respectively.3 After silanation, the shape of the O 1s peak becomes complex, showing two distinct shoulders at higher binding energies. Whereas the O 1s band at low binding energy (530.5 eV) is attributed to the oxygen in Fe3O4, the first shoulder located at 532.1 eV is ascribed to the oxygen connected to silicon. The binding environment of oxygen corresponding to the second shoulder centered at 534 eV remains to be identified. (iv) ζ-Potential. To further confirm functionalization of m-Fe3O4 particles by 3-APTES and study the robustness of the functionalized surfaces, the ζ-potential of the m-Fe3O4 particles after silanation was measured. The results are shown in Figure 4. As a reference, the ζ-potentials of the untreated Fe3O4 and m-Fe3O4 particles are also included in this figure. Over the pH range of 1-12, silanation of 3-APTES on m-Fe3O4 particles increases the ζ-potential of the particles significantly
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Figure 3. XPS spectra of elements on the surface of Fe3O4 before and after deposition of 3-APTES: (a) silanation by 3-APTES in toluene, (b) m-Fe3O4, (c) Fe3O4 only.
Figure 4. Changes of the ζ-potentials of magnetic particles after different modifications: (O) untreated Fe3O4, (9) calcined m-Fe3O4, and (2) m-Fe3O4 silanized by 3-APTES.
compared to that of m-Fe3O4. The isoelectric point (iep) of the silanized m-Fe3O4 particles was found to occur at pH 9.6. This value is significantly different from the iep values of pH 2.1 and 4.8 for the m-Fe3O4 particles and untreated Fe3O4 particles, respectively. The obvious changes in ζ-potential further confirm the deposition of 3-APTES on the particle surface. It is important to highlight the electrokinetic similarities of the 3-APTESsilanized m-Fe3O4 particles with air bubbles in dodecylamine (a cationic surfactant) solutions.3,13 In the case of air bubbles in dodecylamine solutions, the cationic amino groups were exposed to the aqueous phase. The similarity suggests not only a nearly full surface coverage of amine groups on mesoporous Fe3O4 surfaces, but also the reactive nature of the amino groups on 3-APTESsilanized mesoporous magnetite surfaces. The above DRIFTS, XPS, and ζ-potential results confirmed successful silanation of 3-APTES from toluene solutions on m-Fe3O4 particles. A cross-linked layer of aminopropyltriethoxysilane was covalently bonded to
and closely packed on the mesoporous surfaces. Amino groups were introduced onto the mesoporous surface of nanostructured magnetic particles as the terminal functional groups of ultrathin organic films. The functionalized mesoporous magnetic carriers are anticipated to efficiently remove/recover copper and other heavy metals (such as zinc and nickel) from contaminated aqueous and organic solutions. 3.2. Properties of the Silanized Magnetic Particles. (i) Stability. As emphasized before, the stability of the functional layers is crucial in real applications of magnetic carrier technology. Figure 2A shows 3-APTES silane coupling through chemical siloxane bonds. Siloxane bonds are known to be chemically stable, which renders the functional layers robust. Leaching tests to detect Fe in leachates are not suitable for studying the stability of 3-APTES films on silica-coated magnetic particles because the magnetic particles would protected by silica coatings9 even if the 3-APTES films were detached. However, ζ-potential measurements can be a useful alternative for studying the stability of silanized films. If the 3-APTES films were completely detached from the surface, the measured ζ-potential variations would return to the case of unfunctionalized nanostructured m-Fe3O4 particles featuring silica-coated surfaces.9 The ζ-potentials of 3-APTES-silanized m-Fe3O4 particles before and after being soaked in acidic or basic solutions are reported in Table 2. In the acidic solution at pH 1.85, the ζ-potential decreased slightly and remained constant with soaking time, indicating a relatively stable 3-APTES film on silica-coated magnetic particles in an acidic environment. However, in the alkaline solution (pH 10.5), the ζ-potential became more negative with soaking time. Increasing the soaking time from 10 to 30 min resulted in a further decrease in the ζ-potential from 48.7 to 45.3 mV. However, even this lowest value is still much higher than the value of
Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 821 Table 2. ζ-Potential Values Measured at pH 2.1 of Silanized m-Fe3O4 Particles with and without Soaking in Leaching Solutionsa 3-APTES film no soaking acidic soaking (pH 1.85) basic soaking (pH 10.5)
soaking time (min)
ζ-potential (mV)
10 30 10 30
57.8 51.2 50.8 48.7 45.3
a ζ-Potential value of unsilanized m-Fe O particles at pH 2.1 3 4 is 0 mV.
Figure 6. Magnetization curves of (a) untreated Fe3O4 and mesoporous Fe3O4 (b) before and (c) after silanization by 3-APTES.
Figure 5. Particle size distribution of (O) untreated Fe3O4, (9) calcined m-Fe3O4, and (2) m-Fe3O4 silanized by 3-APTES.
ca. -40 mV measured at the same pH of 10 for the unsilanized m-Fe3O4 particles, suggesting that a majority of the silane coupling molecules remain anchored on m-Fe3O4 particles. The mechanical stability of the silanized mesoporous silica layers on the m-Fe3O4 particles was determined by measuring similar ζ-potential values for m-Fe3O4 and functionalized m-Fe3O4 particles before and after mechanical agitation, as encountered in various applications. It is therefore safe to conclude that the silanized m-Fe3O4 particles are relatively robust and can sustain severe chemical and mechanical operating environments such as those encountered in mineral processing streams or hydrometallurgical leachates. (ii) Particle Size Distribution. As shown in Figure 5, the measured particle size distribution increased from a mean diameter of ca. 4 µm for the untreated magnetite particles to ca. 5 µm after the template-assisted twostep silica coating9 and ca. 6 µm after 3-APTES functionalization. The results of the particle size measurements suggest that the particles did not aggregate during silica-coating and silanation. Particles in this size range could remain suspended for a reasonable period of time after being well dispersed. This property is suitable for target loading in industrial applications. (iii) Magnetization. The measured magnetization curves for the untreated Fe3O4 particles and the m-Fe3O4 particles with and without silanation are shown in Figure 6. It is clear that all of the samples are ferromagnetic. Only a marginal decrease of 1% in saturation magnetization of the m-Fe3O4 particles occurred upon silanization with 3-APTES. This marginal reduction in saturation magnetization is attributed to the deposition of the organic functional monolayer. The particles remain strongly magnetic at room temperature, allow-
Figure 7. (A) Distribution and (B) adsorption isotherms of copper ions on different magnetic carriers as a function of copper concentration: (a) Fe3O4 coated with silica without templating, (b) m-Fe3O4, (c) Fe3O4 coated with silica without templating but silanized by 3-APTES, and (d) m-Fe3O4 silanized by 3-APTES.
ing for effective magnetic separation. Clearly, the silanized m-Fe3O4 particles exhibit the attributes necessary to be suitable magnetic carriers in heavy metal ion separation, removal, and recovery. 3.3. Application Examples: Separation of Transition Metals. (i) Loading Capacity. Cupric ion adsorption tests were performed on carriers with varying surface treatments. To illustrate the role of molecular templating in synthesis of the silica coating and subsequent capture of target species, the results obtained with the micrometer-sized magnetite coated with silica under the identical conditions but without templating are included for comparison. Plots A and B of Figure 7 show the variation in copper loading from a copper solution at pH 5.2 on these adsorbents. In these two plots, curve a corresponds to copper loading on
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particles coated with silica without templating;9 curve b, on m-Fe3O4 particles without silanation; curve c, on particles coated with silica without templating and then silanized with 3-APTES; and d on m-Fe3O4 particles silanized with 3-APTES. As shown in Figure 7A, the m-Fe3O4 particles (curve b) exhibited a much higher loading capacity than Fe3O4 particles coated with silica without templating (curve a). The increased specific surface area appears to account for the observed increase in copper loading capacity. Because the silica-coated particles are highly negatively charged at the pH of the loading tests, as seen in Figure 4, the electrostatic attraction between the negatively charged particles and the positively charged copper ions is considered to be responsible for copper loading on unsilanized silica surfaces. Silanation with 3-APTES (curves c and d) caused a great increase in copper loading as compared with the corresponding unsilanized counterparts (curves a and b, respectively). For silanized silica-coated particles, the loading capacity (curve c) is even higher than for m-Fe3O4 particles without silanation (curve b). The difference is more pronounced in the case of lower copper ion concentrations. Compared with electrostatic attraction as in cases a and b, strong complexation of copper with the surface amine groups appears to be responsible for the increased copper loading capacity on the silanized silica coating without templating on Fe3O4 particles. The importance of functionalization is further demonstrated by curve d in Figure 7A, as the silanation process further increased the loading capacity of copper on the m-Fe3O4 particles. In this case, a distribution coefficient (Kd) as high as 10 000 is obtained from an effluent of low copper ion concentration. Such a high distribution coefficient implies a near complete scavenging of copper ions from the effluents. To further understand the role of silanation in copper loading, the loading results were plotted in the form of adsorption isotherms, i.e., plotted as the amount of copper adsorbed on the synthesized carrier (Γ) versus equilibrium copper concentration in solution ([Cu]eq), as shown in Figure 7B. The shape of the curve suggests a Langmuir type of adsorption isotherm of copper on the synthesized carriers. It is interesting to note the leveling off of copper adsorption on unsilanized particles, even though the adsorption is considered to be driven by electrostatic attraction between the positively charged copper ions and the negatively charged silica surface. Complete neutralization of the negative surface charge by the positively charged copper ions appears to account for the observed leveling off in the isotherms. The high loading capacity resulting from chemical complexation of amine groups on silanized surfaces with copper is evidenced by the corresponding high levelingoff values of the adsorption isotherms. The isotherms in Figure 7B can be well fitted with a Langmuir adsorption isotherm equation given by
Γ)
Γ∞b[Ceq] 1 + b[Ceq]
where Γ∞ represents amount adsorbed at a monolayer coverage and b is a characteristic constant of system. The fitted values of b are in the order of curve d > curve c > curve b > curve a, suggesting a stronger binding of copper with the silanized surfaces (curves c and d) than with the unsilanized surfaces (curves a and b), clearly
Figure 8. Loadings of transition metal ions on the amineterminated mesoporous magnetic carriers as function of solution pH from (a) single-element solutions and (b) a solution containing copper, nickel, and zinc, each at 0.5 mM concentration.
demonstrating the important role of surface silanation in copper loading by synthesized m-Fe3O4 particles. From the results shown in Figure 7, the following general conclusions can be made: (1) The capability of the particles to capture copper ions increased when mesoporous films were formed on the magnetite particles to produce a higher specific surface area (curves b and d are higher than curves a and c, respectively). (2) Surface functionalization with amine groups by silanation increased the capability of the particles to capture metal ions, arisen from a stronger chemical affinity of the immobilized amine groups for copper ions (curves c and d are higher than curves a and b, respectively). (3) The functionalized mesoporous surfaces showed the highest loading capacity for copper ions (curve d), suitable for detoxification or recovery of copper ions from industrial effluents. (ii) Separation of Transition Metals. Mesoporous magnetic particles functionalized with 3-APTES were tested to determine the loadings of other soluble heavy metals. The extractability of soluble metals was examined by adding 50 mg of particles to 25-mL samples of aqueous solutions, each containing 0.5 mM Cu2+, Zn2+, and/or Ni2+ ions. In this set of tests, the solution pH varied from 2 to 6. The upper pH limit was set at 6 to avoid precipitation of metal hydroxides, which would complicate the interpretation of any results. Figure 8A shows the loading distribution coefficients, Kd, of different metal ions from the corresponding singleelement solutions as a function of the equilibrium solution pH. The extractability obtained at pH < 6 was found to be in the order Cu2+ > Ni2+ > Zn2+. The distribution coefficient for each metal is relatively low for pH < 2. At such a low pH, excess H+ ions compete
Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 823 Table 3. Selectivity Indexes of the 3-APTES-Silanized m-Fe3O4 Particles K1/K2 pH
Cu/Ni
Cu/Zn
Ni/Zn
2.5 5.2
3.48 4.06
563.8 19.2
162.3 4.72
with metal ions for binding with surface amino groups, resulting in a low metal ion loading capacity.13 From the results in this figure, a selective loading of copper ions over nickel and zinc is anticipated. The selective loading was confirmed with loading tests in a solution containing all three of these types of heavy metal ions. The distribution coefficient (Kd) obtained as a function of pH in Figure 8B shows that the extractability of the metal ions from a mixture follows the same order as seen in the single-metal systems, i.e., Cu2+ > Ni2+ > Zn2+. Metal ion loading increases with increasing pH above 2 up to the limiting pH of 5.2 for copper and zinc, but not for nickel. For convenience of discussion on the selectivity of metal ion loading, a selectivity index is defined as the ratio of distribution coefficients. The calculation from the results in Figure 8 showed a higher selectivity index for the mixed system than predicted from single-element solutions. The observed difference is attributed to the competition of metal ions for the limited surface sites in the mixed solutions. The practical implication derived from the selectivity index values listed in Table 3 is that the separation of copper and nickel from zinc is feasible at pH below 3.0. The separation of copper from nickel can be accomplished at pH between 4.5 and 5.2, although it might not be as effective as the separation of copper from zinc. The observed loading capacity in the order of Cu2+ > Ni2+ > Zn2+ is in line with the values of the corresponding binding constant (K). The equilibrium binding constant, K, is a measure of the binding strength of a chelating agent for a given cation. Copper, with a coordination number of 4, is known to complex with many chelating agents.13 In a complexation study, Rivas et al.5 reported a higher binding constant for a poly(Nvinylimidazole) hydrogel with Cu(II) than with Cd(II), Pb(II), and U(VI), resulting in a higher adsorption capacity for copper over the other divalent ions on the hydrogel. EDTA is known to exhibit a higher complex binding constant with copper than with other divalent metal ions.15 Although the binding constants of Cu, Zn, and Ni with an 3-APTES-silanized surface are not readily available in the open literature, the equilibrium binding constant of Cu with ethylenediamine was reported to be 458 M-1, which is higher than the value of 437 M-1 reported for the corresponding nickel complexes.16 The equilibrium constants of dopamine complexing with Ni, Cu, and Zn are found to decrease in the order KCu >KZn >KNi, with KNi and KZn being close to each other.17 Consideration of these literature binding constant values leads us to attribute the observed higher loading capacity of copper over nickel and zinc on amineterminated m-Fe3O4 particles to a higher binding constant of immobilized amine with copper than with nickel and zinc over a pH range from 2 to 6. The observed increase in the maximum loadings of the metal ions with increasing solution pH over the pH range studied is related to the distribution of metal species and the nature of their complexing interactions with amines.18 (iii) Stripping and Recycling. It has been determined from an application point of view that stripping
Table 4. Loading, Stripping, and Reloading of Copper Ions from Amine-Terminated Mesoporous Magnetic Carriersa initial Cu2+ uptake of Cu2+ detached (mg/g) conc (ppm) Cu2+ (mg/g) 22.86 35.14 47.88 63.50
10.80 14.74 15.11 16.47
10.92 14.36 15.38 16.43
detaching reloading efficiency efficiency (%) (%) 101 97 102 100
83 ( 3.2 N/A N/A N/A
a Loading/reloading tests were conducted using 50 mg of magnetic carriers in 25 mL of copper ion solution; stripping tests were conducted in 25 mL of 1 M HCl solution.
of metal ions from loaded magnetic carriers by acid washing is a major step in the subsequent recovery of metals by electrowinning or simple precipitation and regeneration of nanostructured magnetic carriers for recycling. For this purpose, a preliminary study on the detachment of metal ions from the loaded magnetic carriers by acid washing was initiated. In the acid washing, 1 M HCl was used as the stripping liquid. The results of the stripping tests are reported in Table 4. As a reference, the uptake of Cu2+ from initial loading tests is also included in the table. The results in this table show that almost all of the metals that have been loaded onto the carriers are stripped off by 1 M acid, suggesting that the recovery of copper by subsequent electrowinning processes is feasible. The mechanism of copper detachment from silanized m-Fe3O4 particles by acid washing is similar to that involved in the regeneration of exhausted ion-exchange resins often used in wastewater treatment.13 The detachment of copper is most likely accomplished by ion exchange with hydrogen ions because of their high chemical potential in strong acidic solutions, which compete with copper ions for amino groups on the silanized m-Fe3O4 particles. The loading capacity of recycled carriers is another concern in real applications of magnetic carrier technology. In this study, a reloading experiment was conducted with one batch of recycled carriers. The reloading efficiency is calculated as the ratio Kd/Kd0, where Kd0 is the distribution coefficient obtained in the initial loading test and Kd is the distribution coefficient obtained with the recycled carriers in the reloading test using fresh metal ion solutions. The average reloading efficiency of three separate tests is reported in Table 4. The results in Table 4 show a decrease in the loading capacity of the recycled carriers by 17%. The reasons for the observed decrease in the loading capacity of the 3-APTES-silanized m-Fe3O4 particles after acid stripping of the loaded copper remain to be investigated. A similar observation was reported by Liu for the loading of copper using amine-terminated nano-Fe2O3 particles.13 His work led to the conclusion that both the protonation/oxidation of amino groups and some degree of detachment of the silanized 3-APTES films during acid stripping contributed to the reduced copper reloading efficiency. We believe that these effects also account for the observations made in the current study. We are continuing to explore other applications of the functionalized mesoporous magnetic particles and are studying the relevant mechanisms. The above fundamental study demonstrates the feasibility of using 3-APTES-silanized m-Fe3O4 particles in metal recovery and/or removal. The metal ions loaded on the functionalized m-Fe3O4 particles can be detached completely by acid washing. Although the loading
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capacity of copper on the recycled m-Fe3O4 particles is reduced, improving the stability of the silanized film is anticipated to make the recycling of silanized m-Fe3O4 particles feasible in practice. The concept of using m-Fe3O4 particles functionalized with reactive amine terminal groups for the effective recovery or selective removal of metal ions has thus been demonstrated. It should be noted that other functional groups can be attached onto m-Fe3O4 particles by a similar scheme. For example, carboxylic acid-terminated mesoporous magnetic carriers can be readily synthesized following the same procedures as used for silanation of 3-APTES.4 Organic functional groups other than amines are suited to different applications. The surface tailoring method reported here is foreseen to enable diverse design of surface properties of m-Fe3O4 mesoporous materials and could lead to the synthesis of more advanced nanocomposite particles for industrial and environmental applications. 4. Conclusions 1. DRIFTS, XPS, and ζ-potential measurements indicate that the silanation of magnetic particles using 3-APTES was successful when toluene was used as the solvent. In acidic solution, the 3-APTES film silanized from toluene displayed a reasonable stability when compared to that in basic solution. The saturation magnetization of the m-Fe3O4 particles was reduced only marginally after silanization with 3-APTES. The mean diameter of functionalized particles was below 6 µm. These properties of the prepared m-Fe3O4 composite particles justify their application in magnetic carrier technology. 2. Separation of heavy metals (II) was investigated using magnetic carriers and mesoporous magnetic carriers with or without silanation by 3-APTES. Metal ions in aqueous solutions were loaded successfully on the silanized mesoporous magnetic particles under moderate conditions. The loading capacity was found to be related to both the porosity and the functionality of the magnetic particles. For amine-terminated mesoporous Fe3O4 carriers, metal (II) loading was found to increase with increasing solution pH. The results showed that the separation of three metal ions is promising. The extractability is in the order of Cu2+ > Ni2+ > Zn2+. The metals adsorbed on the carriers were desorbed almost completely with 1 M aqueous HCl solution. As a result, the metal ions can be recovered and the magnetic carriers recycled for further use. Acknowledgment This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). XPS was carried out at the Center for Microanalysis of Materials, University of Illinois at Urbana-Champaign,
which is partially supported by the U.S. Department of Energy under Grant DEFG02-91-ER45439. Literature Cited (1) Bolto, B. A.; Pawlowski, L. Wastewater Treatment by Ion Exchange; E. & F. N. Spon Ltd: New York, 1987. (2) Nun˜ez, L.; Kaminski, M. D. Separating Metals Using Coated Magnetic Particles. Chem. Technol. 1998, 9, 41. (3) Xu, Z.; Liu, Q.; Finch, J. A. Silanation and Stability of 3-Aminopropyl Triethoxy Silane on Nanosized Superparamagnetic Particles: I. Direct Silanation. Appl. Surf. Sci. 1997, 120, 269. (4) Shiraishi, Y.; Nishimura, G.; Hirai, T.; Komasawa, I. Separation of Transition Metals Using Inorganic Adsorbents Modified with Chelating Ligands. Ind. Eng. Chem. Res. 2002, 41, 5065. (5) Rivas, B. L.; Maturana, H. A.; Molina, M. J.; Go´mez-Aanto´n, M.; Pie´rola, I. F. Metal Ion Binding Properties of Poly(N-vinylimidazole) Hydrogels. J. Appl. Polym. Sci. 1998, 67, 1109. (6) Lee, B.; Kim, Y.; Lee, H.; Yi, J. Synthesis of functionalized porous silicas via templating method as heavy metal ion adsorbents: the introduction of surface hydrophilicity onto the surface of adsorbents. Microporous Mesoporous Mater. 2001, 50, 77. (7) Williams, R. Colloid and Surface Engineering; ButterworthHeinemann Ltd.: Oxford, U.K., 1992; Chapter 8 Magnetic Carrier Technology. (8) Moffat, G.; Williams, R. A.; Webb, C.; Stirling, R. Selective separations in environmental and industrial processes using magnetic carrier technology. Miner. Eng. 1994, 7, 1039. (9) Wu, P.; Zhu, J.; Xu, Z. Template-assisted synthesis of mesoporous magnetic nanocomposite particles. Adv. Funct. Mater. 2004, 14, 345. (10) Jones, C. M.; Tsuji, K.; Davis, M. Organic-functionalized molecular sieves as shape-selective catalysts. Nature 1998, 393, 52. (11) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J. Functionalized Monolayers on Ordered Mesoporous Supports. Science 1997, 276, 923. (12) Antochshuk, V.; Jaroniec, M. Functionalized Mesoporous Materials Obtained via Interfacial Reactions in Self-Assembled Silica-Surfactant Systems. Chem. Mater. 2000, 12, 2496. (13) Liu, Q. X. An innovative approach in magnetic carrier technology. Ph.D Dissertation, McGill University, Montreal, Canada, 1997. (14) Dwyer, F. P.; Mellor, D. P. Chelating Agents and Metal Chelates; Academic Press: New York, 1964. (15) Welcher, F. J. The Analytical Uses of Ethylenediamine Tetraacetic Acid; D. Van Nostrand Company Inc.: Toronto, Canada, 1958. (16) Otto, S. Catalysis of Diels-Alder reactions in water. Ph.D Dissertation, University of Groningen, Groningen, The Netherlands, 1998; Chapter 3. (17) Kiss, T.; Gergely, A. Complexes of 3,4-Dihydroxyphenyl Derivatives. III. Equilibrium Study of Parent and some Mixed Ligand Complexes of Dopamine, Alanine and Pyrocatechol with Nickel(II), Copper(II) and Zinc(II) Ions. Inorg. Chim. Acta 1979, 36, 31. (18) Stumm, W.; Morgan, J. J. Aquatic Chemistry; John Wiley and Sons: New York, 1995.
Received for review May 29, 2004 Revised manuscript received November 7, 2004 Accepted November 19, 2004 IE0495325
