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Synthesis and characterization of hyaluronic acid-supported magnetic microspheres for copper ions removal
Colloids and Surfaces A: Physicochem. Eng. Aspects 425 (2013) 42–50
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis and characterization of hyaluronic acid-supported magnetic microspheres for copper ions removal Shi Lan, Xiaomin Wu, Linlin Li, Mengmeng Li, Fengying Guo, Shucai Gan ∗ College of Chemistry, Jilin University, Changchun 130026, PR China
h i g h l i g h t s
g r a p h i c a l
• Hyaluronic acid was successfully
Hyaluronic acid-supported magnetic submicron-sized particles were fabricated as a novel adsorbent for the Cu2+ removal and can be separated magnetically through the application of a magnetic field in shorter time after the adsorption performance.
immobilized on the magnetic silica submicron-sized particles. • Hyaluronic acid was utilized as adsorbent for the heavy metal removal. • Magnetism and adsorption were perfectly combined into one single entity.
a r t i c l e
i n f o
Article history: Received 30 October 2012 Received in revised form 22 February 2013 Accepted 28 February 2013 Available online 7 March 2013 Keywords: Magnetic Hyaluronic acid Microspheres Adsorption Separation
a b s t r a c t
a b s t r a c t Magnetic hyaluronic acid (HA) microspheres were fabricated as a novel adsorbent through the immobilization of hyaluronic acid on the magnetic silica microspheres. The as-prepared microspheres were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), X-ray powder diffraction (XRD), and X-ray photoelectron spectra (XPS). The assynthesized microspheres were evaluated for Cu2+ removal by the adsorption, and the effect of pH value, interferential metal ions, initial Cu2+ concentration, and contact time on adsorption capability was investigated, respectively. The adsorption equilibrium study exhibited that the Cu2+ adsorption of hyaluronic acid-supported magnetic microspheres had a better fit to the Freundlich isotherm model than the Langmuir model. The kinetic date of adsorption of Cu2+ on the synthesized adsorbents was best described by the pseudo-second-order equation. The resultant microspheres also revealed super-paramagnetic behavior, which made these adsorbent magnetically separable after the adsorption performance. This work demonstrates that the synthesized hyaluronic acid-supported magnetic adsorbent can be considered as a potential adsorbent for hazardous metal ions from wastewater. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, natural polysaccharides such as chitosan, heparin, chondroitin, keratin, and xanthan have been developed as environmentally friendly materials for removing toxic pollutants from aqueous solution and attracted much attention [1–4]. In particular,
∗ Corresponding author. Tel.: +86 431 88502259. E-mail address: [email protected] (S. Gan). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.02.059
chitosan has been widely suggested as a candidate for an overwhelming scope of adsorption applications, covering almost all the spectrum of biotechnology [5,6]. Hyaluronic acid (HA), one kind of the natural polysaccharide with a similar structure as chitosan, has a variety of functions mainly including roles in joint lubrication, tissue hydration, wound repair, and modulation of inflammation as published in earlier literatures. However, to the best of our knowledge, few have reported the research about the adsorption behavior of HA toward pollutants yet, and it can be expected as a promising candidate for efficient adsorbent of heavy metal ions due to its large
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number of active metal-binding sites including oxygen, nitrogen and carboxyl which can uptake heavy mental ions from wastewater through various mechanisms such as chelation, electrostatic attraction, and ion-exchange, providing possibility for synthesis of HA-based adsorbent materials. However, there is a major drawback to the application of HA as adsorbent originating from the separation due to its high solubility in aqueous solution. Isolation of HA from a large volume of water requires an additional process and further expense, and thus it seems reasonable to immobilize of HA on insoluble component to facilitate the separation process [7–9]. Adsorption performance of the polymeric adsorbent strongly depends on its activated surface area and the quantity of surface activated sites for interaction with metallic species. Nanometer and micrometer sized materials have shown remarkable potential because of their large surface area [10]. Therefore, to enhance adsorption capacity, fabrication of the adsorbent with nanometer or micrometer size to enlarge activate surface area is advisable [11]. However, introduction of nanometer or micrometer sized adsorbent brings them difficulty in separation process, which involves further expense to remove such fine adsorbent nanomaterials from the aqueous suspension [12–14]. Magnetic adsorbents in nanometer and micrometer size have attracted great attention for their potential application in removal of the pollutants from aqueous solution due to their strong adsorption capacity, simple recovery from liquid solution under suitable magnetic field and reusable property [15]. However, pure magnetic particles are prone to form aggregation owing to the magnetic dipolar attraction between magnetite microspheres and magnetic properties affected in liquid systems. To solve this defect, a rational protective layer including inorganic oxide or polymer compounds coated on the surface of magnetic particles is often utilized. As a result, silica-coated magnetic nanoparticles and polymer-coated magnetic nanoparticles are commonly used as the substrate for the synthesis of magnetic-based adsorbent [16]. Herein, novel HA-functionalized magnetic microspheres were fabricated as a magnetic adsorbent through anchoring HA on silicacoated magnetic microspheres. Adsorption behavior of as-prepared microspheres was evaluated by selecting Cu2+ as representative heavy metal, which is considered as an essential nutrient in trace amount, but can cause adverse health effects at high doses [17–20]. Adsorption experiments revealed that the as-synthesized microspheres possess excellent adsorption behavior of Cu2+ ion. In addition, HA-functionalized magnetic microspheres can be enriched completely after the adsorption performance within short time under an external magnetic field. The synergism between magnetism and adsorption would result in promising applications in many fields. 2. Experimental 2.1. Materials HA was commercially obtained from Qufu guanglong Biochem Co., Ltd. Ferric chloride (FeCl3 ·6H2 O), anhydrous sodium acetate (NaAc), polyethylene glycol (PEG molecular weight 10000), ethylene glycol, toluene, anhydrous ethanol, aqueous ammonia solution (28 wt%), and copper nitrate (Cu(NO3 )2 ·3H2 O) were purchased from Beijing Chemical Reagent Research Company. Tetraethoxysilane (TEOS, 98 wt%) was available from Shanghai Chemical Reagents Company. 3-aminopropyltriethoxysilane (APS, 98 wt%) was provided from Aldrich Chemical Company. 2.2. Characterization The particle size and structure of the synthesized magnetic nanoparticles were observed by using a Hitachi 8100 transmission
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electron microscope (TEM, Hitachi, Tokyo, Japan). Scanning electron microscopy (SEM) was performed on a TESCAN 5136MMSEM at an accelerating voltage of 20 Kv. The samples were loaded onto a glass surface previously sputter coated with a homogenous gold layer for charge dissipation during the SEM imaging. The infrared spectra of the nanoparticles were taken in KBr pressed pellets on a NEXUS 670 infrared Fourier transform spectrometer (Nicolet Thermo, Waltham, MA). X-ray diffraction (XRD) measurements were recorded on a Rigaku D/MAXIIA diffractimoter using Cu Ka radiation. X-ray photoelectron spectra (XPS) measurement was carried out on a PHI-5000CESCA system with Mg K radiation (hr = 1253.6 eV). The X-ray anode was run at 250 W, and the high voltage was kept at 14.0 kV with a detection angle at 540. All the binding energies were calibrated by using the containment carbon (C 1s = 284.6 eV). The hysteresis loops were obtained with a vibrating sample magnetometer (VSM 7407, Lake Shore). 2.3. Synthesis of the magnetic silica microspheres Fe3 O4 microspheres were first synthesized via a solvothermal reaction as previously described [21,22]. In the next process, the core-shell magnetic silica microshoeres Fe3 O4 @SiO2 were prepared according to the Stöber process [23,24]. In a typical procedure, the submicron-sized magnetic particles were first treated by HCl (5 mL, 2 M) under ultrasonic vibration for 5 min, and then the Fe3 O4 microspheres were thoroughly rinsed with deionized water for several times. Fe3 O4 microspheres were homogeneously dispersed in a mixture of ethanol (40 mL) and deionized water (10 mL) under ultrasonic vibration for 30 min, then the concentrated ammonia aqueous solution (1 mL, 28 wt%) was added to this solution with the help of ultrasonication to obtain a stable solution, and followed by the addition of tetraethoxysilane every 15 min till the total amount of TEOS reached 0.2 mL under magnetically stirring, and the process was followed 12 h. Finally, the product was collected with the help of magnet and washed with recycle of ethanol and water for several times, and then vacuum dried at 60 ◦ C. 2.4. Synthesis of HA-modified Fe3 O4 @SiO2 microspheres In the functionalization of Fe3 O4 @SiO2 , the APS grafted Fe3 O4 @SiO2 act as core template and hyaluronic acid polymer serve as shell. Typically, the Fe3 O4 @SiO2 magnetic silica microspheres were firstly functionalized with a coupling agent 3-aminopropyltriethoxysilane (APS) through the siloxane linkage. The detailed experimental process was as follows. Fe3 O4 @SiO2 (0.05 g) and coupling agent APS were dispersed in 50 mL toluene and refluxed for 3 h. The obtained products Fe3 O4 @SiO2 -APS were collected and washed with absolute ethanol and deionized water for times [25,26]. In the next step, hyaluronic acid (50 mg) was dissolved in deionized water (180 mL) with the vigorous mechanical stirring to form a clear solution. Fe3 O4 @SiO2 -APS (50 mg) was dispersed in HCI (2 M, 5 mL) for 5 min to activate surface functional group and washed with deionized water for three times, and dispersed subsequently in the freshly prepared solution with the aid of ultrasonication for 10 min, and then the suspension was stirred at 50 ◦ C for another 12 h. The final product Fe3 O4 @SiO2 -HA was collected by magnetic separation and washed with water, and then oven dried at 40 ◦ C. 2.5. Effect of initial Cu2+ concentration on adsorption Batch adsorption experiments were studied by placing 30 mg of obtained Fe3 O4 @SiO2 -HA in 100 mL conical flasks containing 50 mL of various concentration (10–50 mg/L) of Cu2+ solutions in the pH value of 6.8 at 25 ◦ C temperature in a shaker bath for 7 h. The solution was shocked at 120 rpm using a mechanical shaker to reach
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equilibrium. The concentration of Cu2+ before and after treatment was calculated by atomic absorption spectrometer (AAS, Varian 220FS, USA). For accurate adsorption results, the metal adsorption data was analyzed three times and the mean value (error range: ca. ±5%) was represented. The adsorption capacity (qe ) was calculated using the equation [27,28]: qe =
(C0 − Ce ) × V W
where C0 is the initial concentration of metal ions (mg/L), Ce is the equilibrium concentration of metal ions after adsorption (mg/L), V is the volume of metal ions solution (mL), and W is the weight of the synthesized adsorbent (mg). 2.6. Effect of pH value on adsorption The desired pH value of solution was adjusted by adding either 0.1 M HCl or NaOH solution and then 50 mL of the above solution was taken in a conical flask and treated with 30 mg sorbent. The final Cu2+ concentration was determined after equilibrium by atomic absorption spectrometer. 2.7. Effect of interferential metal ions on Cu2+ removal The possibly coexisting cationic metals (i.e., Na+ , Mg2+ , Ca2+ , Ni2+ , Pb2+ , Zn2+ , Cd2+ , etc.) in wastewater were chosen to investigate their effect on Cu2+ removal by the adsorbent. Different concentration of cationic metals and same dosage of sorbent was added in 30 mg/L Cu2+ original concentration solution and tested at room temperature. The different metals solution was prepared by dissolving their nitrates. 3. Results and discussion Hyaluronic acid-functionalized magnetic microspheres Fe3 O4 @SiO2 -HA with magnetic silica microspheres as core and HA as shell were successfully prepared and used as an adsorbent to remove heavy metal Cu2+ form aqueous solution. The fabrication route of the Fe3 O4 @SiO2 -HA microspheres was skillfully designed, and the synthetic procedure was schematically illustrated in Fig. 1A. Magnetic HA-based microspheres can be used as adsorbent for Cu2+ removal from aqueous solution and enriched completely within short time under an external magnetic field as shown in Fig. 1B. The combination of magnetic property and absorption performance into one single entity can make the HA-based materials separable magnetically and significantly facilitate their practical applications. To verify the morphology of the resultant microspheres in detail, the obtained microspheres were characterized by SEM and TEM measurement. Fig. 2 depicted the physical appearance of the iron oxide microspheres, the magnetic silica microspheres, and the HA-supported silica-coated magnetic microspheres. The quasi-monodisperse, spherical, and solid Fe3 O4 microspheres with an average diameter of 105.6 nm were observed in Fig. 2A. After silica coating, the as-synthesized microspheres present relatively smooth particle surface, and the particle size was vividly increased. It can be clearly seen from the insert in Fig. 2B that the Fe3 O4 @SiO2 microspheres presented obvious core-shell structure and the gray outer layer around the magnetic Fe3 O4 nanoparticle were amorphous silica coating with an average shell thickness of 32 nm. This encapsulation was significant because the silica shell not only kept a stable dispersion of magnetic microspheres for a long time in a harsh liquid media but also prevent their corrosion in an acidic environment [29]. As shown in Fig. 2C, the surface of Fe3 O4 @SiO2 HA microspheres was obviously coarser than that of Fe3 O4 @SiO2 , which was further revealed by the insert TEM image in Fig. 2C.
Fig. 1. Synthetic procedure of the Fe3 O4 @SiO2 -HA microspheres (A) and their Cu2+ removal performance by the aid of an external magnetic field (B).
However, no significant change is found in the spherical shape and particle size, suggesting that HA introduction has no effect on the inner core materials. FTIR spectra were recorded to identify the formation of the functional groups on the microspheres at different synthetic steps. FTIR spectra in this case not only confirmed the silica coating on the surface of the magnetic Fe3 O4 microspheres but also verified the formation of Fe3 O4 @SiO2 -HA microspheres. FTIR spectrum of the as-prepared microspheres is shown in Fig. 3. The typical peak of Fe O bond appeared at 580 cm−1 for the spectra in Fig. 3A, B, and D [30]. The absorption peaks of 3423 cm−1 and 1651 cm−1 were observed in all the spectra corresponding to the stretching vibration and bending vibration of O H group for water, respectively [31]. The O H stretching vibration related to SiO H groups appeared in the same range of 3200–3500 cm−1 in the Fig. 3B and D [32]. The SiO2 coated Fe3 O4 microspheres (Fig. 3B) have the characteristic peaks at 1105 cm−1 , 810 cm−1 , and 950 cm−1 , corresponding to the asymmetrical stretching vibration of Si O Si, symmetrical stretching vibration of Si O Si band, and stretching vibration of Si OH, respectively [33–36]. The characteristic peaks of HA polymer displayed in Fig. 3C were evidently observed in the curve of Fe3 O4 @SiO2 -HA microspheres, and the C-H stretching vibration peaks were dramatically enlarged after the HA immobilization in Fig. 3D. The crystallographic structure and composition of the assynthesized microspheres were characterized by X-ray powder diffraction (XRD). Fig. 4A displayed that the position and relative intensity of all characteristic peaks at 2 = 30.1◦ , 35.5◦ , 43.1◦ , 53.4◦ , 57.0◦ , and 62.6◦ could be indexed to the cubic structure of Fe3 O4 powder diffraction data (JCPDS: 65-3107) [37]. This result indicated that the iron oxide microspheres were magnetite and belonged to cubic structure. It was apparent that as shown in Fig. 4B, the diffraction peaks for Fe3 O4 @SiO2 were similar to those of iron oxide, suggesting that the magnetic Fe3 O4 microspheres were successfully encapsulated by the thinner amorphous silica layer without affecting the original crystallinity of Fe3 O4 structure [38].
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Fig. 3. FTIR spectra of Fe3 O4 (A), Fe3 O4 @SiO2 (B), HA (C), and Fe3 O4 @SiO2 -HA (D) microspheres.
surface, and the peak at 402.2 eV was possibly correspond to the protonated amino group (Fig. 5C) [42,43]. The effect of pH value on Cu2+ adsorption with Fe3 O4 @SiO2 -HA microspheres was investigated with 30 mg/L original Cu2+ concentration. The experimental results of Cu2+ removal at varying pH value were presented in Fig. 6. It is obvious that the adsorption capacity was increased from 6.0 to 12.2 mg/g with increasing pH value from 2.0 to 6.8, and slowly decreased to 11.6 mg/g at pH 8.0. The higher concentration H+ ions fact the carboxyl groups of sorbent decreasing the negatively charged surface sites hence result in the low adsorption efficiency. Furthermore, the amino groups and hydroxyl groups on Fe3 O4 @SiO2 -HA microspheres are readily protonated at lower pH value and thus unfavorable for removal of Cu2+ by the positively charged functional groups of adsorbent due to the electrostatic repulsion, resulting in lower adsorption capacity [44–46]. However, the removal efficiency presented slight descending trend with the overly increasing pH value due to the formation of Cu(OH)+ and Cu(OH)2 precipitation at the alkaline condition [47]. In summary, pH value influenced the surface charge of the adsorbent and the concentration of copper ions in solution, so the optimum pH value for further Cu2+ uptake studies was selected as 6.8.
Fig. 2. SEM and TEM images (insert) of Fe3 O4 (A), Fe3 O4 @SiO2 (B), and Fe3 O4 @SiO2 HA (C) microspheres. All scale bars in TEM images are 50 nm.
Immobilization of HA on Fe3 O4 @SiO2 microspheres was further verified by XPS measurement, and the identification of chemical bond was made by deconvolution of the C 1s, O 1s, and N 1s peaks from the total XPS spectrum as shown in Fig. 5. The large component of the C 1s envelope was the C C state at 284.5 eV and 285.1 eV, and the components at 285.7 eV and 286.5 eV were associated to C atoms bonded with N and/or O (Fig. 5A) [39,40]. The binding energy of O 1s at 532.8 eV was corresponding to the C O H, C O C, and C O bond, and the chemical state of O 1s with binding energy of 530.8 eV also appeared owing to the involvement of OH− (Fig. 5B) [41]. Evidently, two N 1s features for the product were observed. The typical peak at 400.4 eV was attributed to the amide group ( CONH ), which acted as unique element marker to provide the evidence that HA presented either at or very near the particle
Fig. 4. XRD patterns of the iron oxide (A) and the magnetic silica (B) microspheres.
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Fig. 6. Effect of pH value on the adsorption capacity of Fe3 O4 @SiO2 -HA microspheres for Cu2+ ions.
earth metal ions (Mg2+ and Ca2+ ) in the range of 10–50 mg/L. The Fe3 O4 @SiO2 -HA microspheres showed Cu2+ adsorption capacity of 12.0 mg/g toward single Cu2+ solution, and 10.8/10.9 mg/g adsorption capacity was found under the condition of 50 mg/L of coexisting ions Mg2+ /Ca2+ . The reduction in Cu2+ adsorption capacity was reached as high as 10% for 50 mg/L Mg2+ and 9.1% for 50 mg/L Ca2+ . Therefore, It was concluded that the divalent ions Ca2+ and Mg2+ had more suppressive effect on Cu2+ adsorption
Fig. 5. XPS spectra of C 1s (A), O 1s (B), and N 1s (C) regions for the Fe3 O4 @SiO2 -HA microspheres.
The heavy metal ion pollutants are often together with alkaline/earth metal ions in wastewater. The impact of coexisting Na+ , Mg2+ and Ca2+ on the uptake of the Cu2+ is displayed in Fig. 7A. The adsorption capacity of Cu2+ with Fe3 O4 @SiO2 -HA microspheres evidently decreased with increasing the concentration of alkaline
Fig. 7. Effect of interferential metal ions concentration on the adsorption capacity of Fe3 O4 @SiO2 -HA microspheres for Cu2+ ions.
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likely due to the stronger interaction ability between the sorbent and Ca2+ /Mg2+ . However, Cu2+ adsorption capacity only decreased to 11.7 mg/g even with higher Na+ concentration of 50 mg/L, showing only 2% reduction [48,49]. The effect of the coexisting heavy metal ions on the removal of Cu2+ by Fe3 O4 @SiO2 -HA microspheres was investigated as well. Compared with alkaline/earth metal ions, heavy metal ions displayed much more competitive performance toward Cu2+ removal. As seen in Fig. 7B, it was observed that the adsorption capacity for Cu2+ ions obviously decreased in the presence of coexisting heavy metal ions even with the low concentration ranged from 0.25 mg/L to 4 mg/L. The adsorption capacity of Cu2+ coexisting with other heavy metal ions (i.e., Ni2+ , Pb2+ , Zn2+ , Cd2+ , etc.) was in the range of 6.8–11.8 mg/g, which was lower than that of the corresponding the single Cu2+ solution (12.0 mg/g). In particular, adsorption capacity for Cu2+ ions reached as low as 6.8 mg/g under the condition of 4 mg/L coexisting Zn2+ , showing 43% reduction. This observation may be explained by considering the decrease in the number of adsorption functional group on the Fe3 O4 @SiO2 -HA microspheres because coexisting ions compete with Cu2+ for adsorption [50,51]. To clarify the Cu2+ adsorption performance of the synthesized HA-based magnetic silica microspheres, adsorption capacity was investigated from the adsorption isotherms and kinetics experimental date. The Cu2+ removal behavior of the synthesized adsorbent was evaluated as a function of the initial Cu2+ concentration in the range from 10 mg/L to 50 mg/L at pH value of 6.8 and 25 ◦ C. Experimental result showed in Fig. 8 that the adsorption capacity of Cu2+ ion increased nearly linear with increasing the initial concentration. Lower the initial Cu2+ concentration of 35 mg/L, the strong ion adsorption capacity can be attributed that adsorbate ions could bind to the abundant adsorption sites on the surface of the synthesized magnetic HA-supported microspheres. Above the initial Cu2+ concentration of 35 mg/L, the rate of increment of adsorption capacity became gradually slow during the initial adsorbate ion concentration increase. The equilibrium adsorption data were applied to fit into two different isotherm models. The Langmuir model can be expressed in equation [52]: qe =
47
where qe is the equilibrium adsorption capacity of adsorbate (mg/g), Ce is the equilibrium concentration of metal ion (mg/L), qm is the maximum amount of metal adsorbed (mg/g), and KL is the constant that refer to the bonding energy of adsorption (L/mg). In general, The Langmuir model assumes that the solid surface active site can be occupied only by one adsorbate and that the active sites are independent. On the contrary, the Freundlich model is based on a heterogeneous adsorption. The Freundlich isotherm is given as [52]: qe = KF ce 1/n The linear form of Freundlich model can be described as equation: Log qe = LogKF +
Logce n
where qe is the equilibrium adsorption capacity of the adsorbent (mg/g), Ce is the equilibrium concentration of Cu2+ (mg/L), KF is the constant related to the adsorption capacity of the adsorbent (mg/L), and n is the constant related to the adsorption intensity. The relationship between initial Cu2+ concentration and the adsorption capacity was analyzed with two different models (Fig. 9). The calculated correlation coefficient (KL , qm , n, and KF ) and linear regression coefficient (R2 ) values for each Langmuir and Freundlich model were shown in Table 1. The adsorption equilibrium study exhibited that the Cu2+ adsorption of HA-supported magnetic silica microspheres had a better fit to the Freundlich isotherm model than the Langmuir model, indicating the multimolecular chemical adsorption.
KL qm ce 1 + KL ce
The linear form of Langmuir isotherm as follows: 1 ce ce = + qe KL qm qm
Fig. 8. Effect of initial Cu2+ concentration on adsorption capacity of Fe3 O4 @SiO2 -HA microspheres. The pH value was adjusted as 6.8 and 30 mg of the Fe3 O4 @SiO2 -HA were contacted with Cu2+ ions for 7 h at 25 ◦ C.
Fig. 9. Adsorption isotherm of Cu2+ onto Fe3 O4 @SiO2 -HA: (A) Langmuir model and (B) Freundlich model.
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Table 1 Adsorption parameters of the Langmuir and Freundlich models for the adsorption of Cu2+ ion onto the Fe3 O4 @SiO2 -HA microspheres. Langmuir model
Freundlich model
qm (mg/g)
kL (L/mg)
R2
Kf
n
R2
29.42
0.036
0.88
1.545
1.429
0.99
Table 2 Kinetic adsorption parameters obtained using pseudo-first-order and pseudosecond-order models. Pseudo-first-order
Pseudo-second-order
qe (mg/g)
K1
R
qe (mg/g)
K (×10−3 )
R2
7.629
0.01
0.97
10.66
1.45
0.99
2
Fig. 11. Magnetic hysteresis loop of the Fe3 O4 (A), Fe3 O4 @SiO2 (B), and Fe3 O4 @SiO2 HA (C) microspheres at 298 K.
In order to investigate adsorption kinetics, the two models (the pseudo-first-order and the pseudo-second-order) were used to test the experiment data. The pseudo-first-order model is given as [52]: In(qe − qt ) = Inqe − K1 t where qt and qe are the amount of Cu2+ adsorbed (mg/g) at time t (min) and at equilibrium, respectively, and K1 is the rate constant of the pseudo-first-order adsorption process (min−1 ). The straight line plots of In(qe − qt ) against t were used to determine the rate constant K1 and correlation coefficient R2 values from these plots. The pseudo-second-order model is given as [52]: 1 1 t = + t qt q2 K2 q22
Fig. 10. (A) pseudo-first-order and (B) pseudo-second-order kinetic adsorption of Cu2+ onto Fe3 O4 @SiO2 -HA microspheres. The test was performed at a pH value of 6.8 and 25 ◦ C. The initial concentration of the Cu2+ ion was 20 mg/L.
where K2 is the constant of pseudo-second-order rate (g mg−1 min−1 ), q2 is the amount adsorbed at equilibrium and qt is the amount adsorbed at any time. The equilibrium adsorption amount (q2 ) and the pseudo-second-order rate parameters (K2 ) can be given from the slope and intercept of plot of t/qt versus t. The corresponding values were presented in Table 2. As can be seen from the results, the correlation coefficients R2 of pseudo-secondorder model (0.99) were higher than that of pseudo-first-order model (R2 ). So the pseudo-second-order model fits better the experimental data than the pseudo-first-order model (Fig. 10). Magnetic characterizations of hybrids containing a magnetite component were carried out by a vibrating sample magnetometer (VSM) at 298 K as shown in Fig. 11. The saturation magnetization moment of Fe3 O4 , Fe3 O4 @SiO2 , and Fe3 O4 @SiO2 -HA microspheres was 68.60 emu/g, 55.21 emu/g, and 44.59 emu/g, respectively. The
Fig. 12. Photographs of the Fe3 O4 @SiO2 -HA microspheres dispersed in aqueous solution without and external magnetic field.
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decrease of the saturation magnetization is most likely ascribed to the existence of outer shell layer on the surface of Fe3 O4 microspheres [53]. The magnetic property of the product is applied to achieve the separation goal in liquid-phase reaction system. As an example, when the Fe3 O4 @SiO2 -HA microspheres were dispersed in water giving a black suspension, upon applying an external magnetic field, the black powder was readily harvested and the background was became transparent, as shown in Fig. 12. This phenomenon provided the facile magnetic separation for their practical application in adsorption field. 4. Conclusion Novel magnetic adsorbent with submicro-size was fabricated through the immobilization of hyaluronic acid on the magnetic silica microspheres. The as-synthesized Fe3 O4 @SiO2 -HA microspheres can be used as an effective adsorbent for the removal of copper ions from aqueous solution. The Cu2+ adsorption activities of the adsorbent were investigated with various experimental conditions. As a result, it was observed that the optimum pH value of Fe3 O4 @SiO2 -HA microspheres for Cu2+ removal was 6.8, and the coexisting metal ions (i.e., Zn2+ , Ni2+ , Pb2+ ) much more competed with Cu2+ than Cd2+ for adsorption. Furthermore, Freundlich isotherm model gave better fittings with adsorption equilibrium data than Langmuir model. Kinetic experiments clearly indicated that adsorption process of copper ions on the synthesized adsorbent was followed pseudo-second-order kinetics models. Fe3 O4 @SiO2 -HA microspheres also displayed super-paramagnetic performance with the saturation magnetization moment of 44.59 emu/g, which made the Cu2+ adsorbed microspheres separable magnetically from wastewater in shorter time by the aid of external magnetic field. Hyaluronic acid modified magnetic adsorbent has opened the possibility to enhance removal of hazardous metal ions from wastewater. Moreover, as a novel adsorbent, its adsorption mechanism toward targeted species needs further illustration, and further study is required on how to obtain these high-performance adsorbents in larger scale. Acknowledgements This present work was financially supported by the key technology and equipment of efficient utilization of oil shale resources, No: OSR-05, and the National Science and Technology major projects, No: 2008ZX05018. References [1] S.M.G. Demneh, B. Nasernejad, H. Modarres, Modeling investigation of membrane biofouling phenomena by considering the adsorption of protein, polysaccharide and humic acid, Colloids Surf. B 88 (2011) 108–114. [2] W. Zhou, J. Wang, B. Shen, W. Hou, Y. Zhang, Biosorption of copper (II) and cadmium (II) by a novel exopolysaccharide secreted from deep-sea mesophilic bacterium, Colloids Surf. B 72 (2009) 295–302. [3] A.H. Chen, S.C. Liu, C.Y. Chen, C.Y. Chen, Comparative adsorption of Cu (II), Zn (II), and Pb (II) ions in aqueous solution on the crosslinked chitosan with epichlorohydrin, J. Hazard. Mater. 154 (2008) 184–191. [4] L.F. Wang, J.C. Duan, W.H. Miao, R.J. Zhang, S.Y. Pan, X.Y. Xu, Adsorption–desorption properties and characterization of crosslinked Konjac glucomannan-graft-polyacrylamide-co-sodium xanthate, J. Hazard. Mater. 186 (2011) 1681–1686. [5] Y. Zhu, J. Hua, J. Wang, Competitive adsorption of Pb (II), Cu (II) and Zn (II) onto xanthate-modfied magnetic chitosan, J. Hazard. Mater. 221–222 (2012) 155–161. [6] M. Monier, D.M. Ayad, D.A. Abdel-Latif, Adsorption of Cu (II), Cd (II) and Ni (II) ions by cross-linked magnetic chitosan-2-aminopyridine glyoxal Schiff’s base, Colloids Surf. B 94 (2012) 250–258. [7] P. Kujawa, P. Moraille, J. Sanchez, A. Badia, F.M. Winnik, Effect of molecular weight on the exponential growth and morphology of hyaluronan/chitosan multilayers: a surface plasmon resonance spectroscopy and atomic force microscopy investigation, J. Am. Chem. Soc. 127 (2005) 9224–9234.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis and characterization of hyaluronic acid-supported magnetic microspheres for copper ions removal Shi Lan, Xiaomin Wu, Linlin Li, Mengmeng Li, Fengying Guo, Shucai Gan ∗ College of Chemistry, Jilin University, Changchun 130026, PR China
h i g h l i g h t s
g r a p h i c a l
• Hyaluronic acid was successfully
Hyaluronic acid-supported magnetic submicron-sized particles were fabricated as a novel adsorbent for the Cu2+ removal and can be separated magnetically through the application of a magnetic field in shorter time after the adsorption performance.
immobilized on the magnetic silica submicron-sized particles. • Hyaluronic acid was utilized as adsorbent for the heavy metal removal. • Magnetism and adsorption were perfectly combined into one single entity.
a r t i c l e
i n f o
Article history: Received 30 October 2012 Received in revised form 22 February 2013 Accepted 28 February 2013 Available online 7 March 2013 Keywords: Magnetic Hyaluronic acid Microspheres Adsorption Separation
a b s t r a c t
a b s t r a c t Magnetic hyaluronic acid (HA) microspheres were fabricated as a novel adsorbent through the immobilization of hyaluronic acid on the magnetic silica microspheres. The as-prepared microspheres were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), X-ray powder diffraction (XRD), and X-ray photoelectron spectra (XPS). The assynthesized microspheres were evaluated for Cu2+ removal by the adsorption, and the effect of pH value, interferential metal ions, initial Cu2+ concentration, and contact time on adsorption capability was investigated, respectively. The adsorption equilibrium study exhibited that the Cu2+ adsorption of hyaluronic acid-supported magnetic microspheres had a better fit to the Freundlich isotherm model than the Langmuir model. The kinetic date of adsorption of Cu2+ on the synthesized adsorbents was best described by the pseudo-second-order equation. The resultant microspheres also revealed super-paramagnetic behavior, which made these adsorbent magnetically separable after the adsorption performance. This work demonstrates that the synthesized hyaluronic acid-supported magnetic adsorbent can be considered as a potential adsorbent for hazardous metal ions from wastewater. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, natural polysaccharides such as chitosan, heparin, chondroitin, keratin, and xanthan have been developed as environmentally friendly materials for removing toxic pollutants from aqueous solution and attracted much attention [1–4]. In particular,
∗ Corresponding author. Tel.: +86 431 88502259. E-mail address: [email protected] (S. Gan). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.02.059
chitosan has been widely suggested as a candidate for an overwhelming scope of adsorption applications, covering almost all the spectrum of biotechnology [5,6]. Hyaluronic acid (HA), one kind of the natural polysaccharide with a similar structure as chitosan, has a variety of functions mainly including roles in joint lubrication, tissue hydration, wound repair, and modulation of inflammation as published in earlier literatures. However, to the best of our knowledge, few have reported the research about the adsorption behavior of HA toward pollutants yet, and it can be expected as a promising candidate for efficient adsorbent of heavy metal ions due to its large
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number of active metal-binding sites including oxygen, nitrogen and carboxyl which can uptake heavy mental ions from wastewater through various mechanisms such as chelation, electrostatic attraction, and ion-exchange, providing possibility for synthesis of HA-based adsorbent materials. However, there is a major drawback to the application of HA as adsorbent originating from the separation due to its high solubility in aqueous solution. Isolation of HA from a large volume of water requires an additional process and further expense, and thus it seems reasonable to immobilize of HA on insoluble component to facilitate the separation process [7–9]. Adsorption performance of the polymeric adsorbent strongly depends on its activated surface area and the quantity of surface activated sites for interaction with metallic species. Nanometer and micrometer sized materials have shown remarkable potential because of their large surface area [10]. Therefore, to enhance adsorption capacity, fabrication of the adsorbent with nanometer or micrometer size to enlarge activate surface area is advisable [11]. However, introduction of nanometer or micrometer sized adsorbent brings them difficulty in separation process, which involves further expense to remove such fine adsorbent nanomaterials from the aqueous suspension [12–14]. Magnetic adsorbents in nanometer and micrometer size have attracted great attention for their potential application in removal of the pollutants from aqueous solution due to their strong adsorption capacity, simple recovery from liquid solution under suitable magnetic field and reusable property [15]. However, pure magnetic particles are prone to form aggregation owing to the magnetic dipolar attraction between magnetite microspheres and magnetic properties affected in liquid systems. To solve this defect, a rational protective layer including inorganic oxide or polymer compounds coated on the surface of magnetic particles is often utilized. As a result, silica-coated magnetic nanoparticles and polymer-coated magnetic nanoparticles are commonly used as the substrate for the synthesis of magnetic-based adsorbent [16]. Herein, novel HA-functionalized magnetic microspheres were fabricated as a magnetic adsorbent through anchoring HA on silicacoated magnetic microspheres. Adsorption behavior of as-prepared microspheres was evaluated by selecting Cu2+ as representative heavy metal, which is considered as an essential nutrient in trace amount, but can cause adverse health effects at high doses [17–20]. Adsorption experiments revealed that the as-synthesized microspheres possess excellent adsorption behavior of Cu2+ ion. In addition, HA-functionalized magnetic microspheres can be enriched completely after the adsorption performance within short time under an external magnetic field. The synergism between magnetism and adsorption would result in promising applications in many fields. 2. Experimental 2.1. Materials HA was commercially obtained from Qufu guanglong Biochem Co., Ltd. Ferric chloride (FeCl3 ·6H2 O), anhydrous sodium acetate (NaAc), polyethylene glycol (PEG molecular weight 10000), ethylene glycol, toluene, anhydrous ethanol, aqueous ammonia solution (28 wt%), and copper nitrate (Cu(NO3 )2 ·3H2 O) were purchased from Beijing Chemical Reagent Research Company. Tetraethoxysilane (TEOS, 98 wt%) was available from Shanghai Chemical Reagents Company. 3-aminopropyltriethoxysilane (APS, 98 wt%) was provided from Aldrich Chemical Company. 2.2. Characterization The particle size and structure of the synthesized magnetic nanoparticles were observed by using a Hitachi 8100 transmission
43
electron microscope (TEM, Hitachi, Tokyo, Japan). Scanning electron microscopy (SEM) was performed on a TESCAN 5136MMSEM at an accelerating voltage of 20 Kv. The samples were loaded onto a glass surface previously sputter coated with a homogenous gold layer for charge dissipation during the SEM imaging. The infrared spectra of the nanoparticles were taken in KBr pressed pellets on a NEXUS 670 infrared Fourier transform spectrometer (Nicolet Thermo, Waltham, MA). X-ray diffraction (XRD) measurements were recorded on a Rigaku D/MAXIIA diffractimoter using Cu Ka radiation. X-ray photoelectron spectra (XPS) measurement was carried out on a PHI-5000CESCA system with Mg K radiation (hr = 1253.6 eV). The X-ray anode was run at 250 W, and the high voltage was kept at 14.0 kV with a detection angle at 540. All the binding energies were calibrated by using the containment carbon (C 1s = 284.6 eV). The hysteresis loops were obtained with a vibrating sample magnetometer (VSM 7407, Lake Shore). 2.3. Synthesis of the magnetic silica microspheres Fe3 O4 microspheres were first synthesized via a solvothermal reaction as previously described [21,22]. In the next process, the core-shell magnetic silica microshoeres Fe3 O4 @SiO2 were prepared according to the Stöber process [23,24]. In a typical procedure, the submicron-sized magnetic particles were first treated by HCl (5 mL, 2 M) under ultrasonic vibration for 5 min, and then the Fe3 O4 microspheres were thoroughly rinsed with deionized water for several times. Fe3 O4 microspheres were homogeneously dispersed in a mixture of ethanol (40 mL) and deionized water (10 mL) under ultrasonic vibration for 30 min, then the concentrated ammonia aqueous solution (1 mL, 28 wt%) was added to this solution with the help of ultrasonication to obtain a stable solution, and followed by the addition of tetraethoxysilane every 15 min till the total amount of TEOS reached 0.2 mL under magnetically stirring, and the process was followed 12 h. Finally, the product was collected with the help of magnet and washed with recycle of ethanol and water for several times, and then vacuum dried at 60 ◦ C. 2.4. Synthesis of HA-modified Fe3 O4 @SiO2 microspheres In the functionalization of Fe3 O4 @SiO2 , the APS grafted Fe3 O4 @SiO2 act as core template and hyaluronic acid polymer serve as shell. Typically, the Fe3 O4 @SiO2 magnetic silica microspheres were firstly functionalized with a coupling agent 3-aminopropyltriethoxysilane (APS) through the siloxane linkage. The detailed experimental process was as follows. Fe3 O4 @SiO2 (0.05 g) and coupling agent APS were dispersed in 50 mL toluene and refluxed for 3 h. The obtained products Fe3 O4 @SiO2 -APS were collected and washed with absolute ethanol and deionized water for times [25,26]. In the next step, hyaluronic acid (50 mg) was dissolved in deionized water (180 mL) with the vigorous mechanical stirring to form a clear solution. Fe3 O4 @SiO2 -APS (50 mg) was dispersed in HCI (2 M, 5 mL) for 5 min to activate surface functional group and washed with deionized water for three times, and dispersed subsequently in the freshly prepared solution with the aid of ultrasonication for 10 min, and then the suspension was stirred at 50 ◦ C for another 12 h. The final product Fe3 O4 @SiO2 -HA was collected by magnetic separation and washed with water, and then oven dried at 40 ◦ C. 2.5. Effect of initial Cu2+ concentration on adsorption Batch adsorption experiments were studied by placing 30 mg of obtained Fe3 O4 @SiO2 -HA in 100 mL conical flasks containing 50 mL of various concentration (10–50 mg/L) of Cu2+ solutions in the pH value of 6.8 at 25 ◦ C temperature in a shaker bath for 7 h. The solution was shocked at 120 rpm using a mechanical shaker to reach
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equilibrium. The concentration of Cu2+ before and after treatment was calculated by atomic absorption spectrometer (AAS, Varian 220FS, USA). For accurate adsorption results, the metal adsorption data was analyzed three times and the mean value (error range: ca. ±5%) was represented. The adsorption capacity (qe ) was calculated using the equation [27,28]: qe =
(C0 − Ce ) × V W
where C0 is the initial concentration of metal ions (mg/L), Ce is the equilibrium concentration of metal ions after adsorption (mg/L), V is the volume of metal ions solution (mL), and W is the weight of the synthesized adsorbent (mg). 2.6. Effect of pH value on adsorption The desired pH value of solution was adjusted by adding either 0.1 M HCl or NaOH solution and then 50 mL of the above solution was taken in a conical flask and treated with 30 mg sorbent. The final Cu2+ concentration was determined after equilibrium by atomic absorption spectrometer. 2.7. Effect of interferential metal ions on Cu2+ removal The possibly coexisting cationic metals (i.e., Na+ , Mg2+ , Ca2+ , Ni2+ , Pb2+ , Zn2+ , Cd2+ , etc.) in wastewater were chosen to investigate their effect on Cu2+ removal by the adsorbent. Different concentration of cationic metals and same dosage of sorbent was added in 30 mg/L Cu2+ original concentration solution and tested at room temperature. The different metals solution was prepared by dissolving their nitrates. 3. Results and discussion Hyaluronic acid-functionalized magnetic microspheres Fe3 O4 @SiO2 -HA with magnetic silica microspheres as core and HA as shell were successfully prepared and used as an adsorbent to remove heavy metal Cu2+ form aqueous solution. The fabrication route of the Fe3 O4 @SiO2 -HA microspheres was skillfully designed, and the synthetic procedure was schematically illustrated in Fig. 1A. Magnetic HA-based microspheres can be used as adsorbent for Cu2+ removal from aqueous solution and enriched completely within short time under an external magnetic field as shown in Fig. 1B. The combination of magnetic property and absorption performance into one single entity can make the HA-based materials separable magnetically and significantly facilitate their practical applications. To verify the morphology of the resultant microspheres in detail, the obtained microspheres were characterized by SEM and TEM measurement. Fig. 2 depicted the physical appearance of the iron oxide microspheres, the magnetic silica microspheres, and the HA-supported silica-coated magnetic microspheres. The quasi-monodisperse, spherical, and solid Fe3 O4 microspheres with an average diameter of 105.6 nm were observed in Fig. 2A. After silica coating, the as-synthesized microspheres present relatively smooth particle surface, and the particle size was vividly increased. It can be clearly seen from the insert in Fig. 2B that the Fe3 O4 @SiO2 microspheres presented obvious core-shell structure and the gray outer layer around the magnetic Fe3 O4 nanoparticle were amorphous silica coating with an average shell thickness of 32 nm. This encapsulation was significant because the silica shell not only kept a stable dispersion of magnetic microspheres for a long time in a harsh liquid media but also prevent their corrosion in an acidic environment [29]. As shown in Fig. 2C, the surface of Fe3 O4 @SiO2 HA microspheres was obviously coarser than that of Fe3 O4 @SiO2 , which was further revealed by the insert TEM image in Fig. 2C.
Fig. 1. Synthetic procedure of the Fe3 O4 @SiO2 -HA microspheres (A) and their Cu2+ removal performance by the aid of an external magnetic field (B).
However, no significant change is found in the spherical shape and particle size, suggesting that HA introduction has no effect on the inner core materials. FTIR spectra were recorded to identify the formation of the functional groups on the microspheres at different synthetic steps. FTIR spectra in this case not only confirmed the silica coating on the surface of the magnetic Fe3 O4 microspheres but also verified the formation of Fe3 O4 @SiO2 -HA microspheres. FTIR spectrum of the as-prepared microspheres is shown in Fig. 3. The typical peak of Fe O bond appeared at 580 cm−1 for the spectra in Fig. 3A, B, and D [30]. The absorption peaks of 3423 cm−1 and 1651 cm−1 were observed in all the spectra corresponding to the stretching vibration and bending vibration of O H group for water, respectively [31]. The O H stretching vibration related to SiO H groups appeared in the same range of 3200–3500 cm−1 in the Fig. 3B and D [32]. The SiO2 coated Fe3 O4 microspheres (Fig. 3B) have the characteristic peaks at 1105 cm−1 , 810 cm−1 , and 950 cm−1 , corresponding to the asymmetrical stretching vibration of Si O Si, symmetrical stretching vibration of Si O Si band, and stretching vibration of Si OH, respectively [33–36]. The characteristic peaks of HA polymer displayed in Fig. 3C were evidently observed in the curve of Fe3 O4 @SiO2 -HA microspheres, and the C-H stretching vibration peaks were dramatically enlarged after the HA immobilization in Fig. 3D. The crystallographic structure and composition of the assynthesized microspheres were characterized by X-ray powder diffraction (XRD). Fig. 4A displayed that the position and relative intensity of all characteristic peaks at 2 = 30.1◦ , 35.5◦ , 43.1◦ , 53.4◦ , 57.0◦ , and 62.6◦ could be indexed to the cubic structure of Fe3 O4 powder diffraction data (JCPDS: 65-3107) [37]. This result indicated that the iron oxide microspheres were magnetite and belonged to cubic structure. It was apparent that as shown in Fig. 4B, the diffraction peaks for Fe3 O4 @SiO2 were similar to those of iron oxide, suggesting that the magnetic Fe3 O4 microspheres were successfully encapsulated by the thinner amorphous silica layer without affecting the original crystallinity of Fe3 O4 structure [38].
S. Lan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 425 (2013) 42–50
45
Fig. 3. FTIR spectra of Fe3 O4 (A), Fe3 O4 @SiO2 (B), HA (C), and Fe3 O4 @SiO2 -HA (D) microspheres.
surface, and the peak at 402.2 eV was possibly correspond to the protonated amino group (Fig. 5C) [42,43]. The effect of pH value on Cu2+ adsorption with Fe3 O4 @SiO2 -HA microspheres was investigated with 30 mg/L original Cu2+ concentration. The experimental results of Cu2+ removal at varying pH value were presented in Fig. 6. It is obvious that the adsorption capacity was increased from 6.0 to 12.2 mg/g with increasing pH value from 2.0 to 6.8, and slowly decreased to 11.6 mg/g at pH 8.0. The higher concentration H+ ions fact the carboxyl groups of sorbent decreasing the negatively charged surface sites hence result in the low adsorption efficiency. Furthermore, the amino groups and hydroxyl groups on Fe3 O4 @SiO2 -HA microspheres are readily protonated at lower pH value and thus unfavorable for removal of Cu2+ by the positively charged functional groups of adsorbent due to the electrostatic repulsion, resulting in lower adsorption capacity [44–46]. However, the removal efficiency presented slight descending trend with the overly increasing pH value due to the formation of Cu(OH)+ and Cu(OH)2 precipitation at the alkaline condition [47]. In summary, pH value influenced the surface charge of the adsorbent and the concentration of copper ions in solution, so the optimum pH value for further Cu2+ uptake studies was selected as 6.8.
Fig. 2. SEM and TEM images (insert) of Fe3 O4 (A), Fe3 O4 @SiO2 (B), and Fe3 O4 @SiO2 HA (C) microspheres. All scale bars in TEM images are 50 nm.
Immobilization of HA on Fe3 O4 @SiO2 microspheres was further verified by XPS measurement, and the identification of chemical bond was made by deconvolution of the C 1s, O 1s, and N 1s peaks from the total XPS spectrum as shown in Fig. 5. The large component of the C 1s envelope was the C C state at 284.5 eV and 285.1 eV, and the components at 285.7 eV and 286.5 eV were associated to C atoms bonded with N and/or O (Fig. 5A) [39,40]. The binding energy of O 1s at 532.8 eV was corresponding to the C O H, C O C, and C O bond, and the chemical state of O 1s with binding energy of 530.8 eV also appeared owing to the involvement of OH− (Fig. 5B) [41]. Evidently, two N 1s features for the product were observed. The typical peak at 400.4 eV was attributed to the amide group ( CONH ), which acted as unique element marker to provide the evidence that HA presented either at or very near the particle
Fig. 4. XRD patterns of the iron oxide (A) and the magnetic silica (B) microspheres.
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Fig. 6. Effect of pH value on the adsorption capacity of Fe3 O4 @SiO2 -HA microspheres for Cu2+ ions.
earth metal ions (Mg2+ and Ca2+ ) in the range of 10–50 mg/L. The Fe3 O4 @SiO2 -HA microspheres showed Cu2+ adsorption capacity of 12.0 mg/g toward single Cu2+ solution, and 10.8/10.9 mg/g adsorption capacity was found under the condition of 50 mg/L of coexisting ions Mg2+ /Ca2+ . The reduction in Cu2+ adsorption capacity was reached as high as 10% for 50 mg/L Mg2+ and 9.1% for 50 mg/L Ca2+ . Therefore, It was concluded that the divalent ions Ca2+ and Mg2+ had more suppressive effect on Cu2+ adsorption
Fig. 5. XPS spectra of C 1s (A), O 1s (B), and N 1s (C) regions for the Fe3 O4 @SiO2 -HA microspheres.
The heavy metal ion pollutants are often together with alkaline/earth metal ions in wastewater. The impact of coexisting Na+ , Mg2+ and Ca2+ on the uptake of the Cu2+ is displayed in Fig. 7A. The adsorption capacity of Cu2+ with Fe3 O4 @SiO2 -HA microspheres evidently decreased with increasing the concentration of alkaline
Fig. 7. Effect of interferential metal ions concentration on the adsorption capacity of Fe3 O4 @SiO2 -HA microspheres for Cu2+ ions.
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likely due to the stronger interaction ability between the sorbent and Ca2+ /Mg2+ . However, Cu2+ adsorption capacity only decreased to 11.7 mg/g even with higher Na+ concentration of 50 mg/L, showing only 2% reduction [48,49]. The effect of the coexisting heavy metal ions on the removal of Cu2+ by Fe3 O4 @SiO2 -HA microspheres was investigated as well. Compared with alkaline/earth metal ions, heavy metal ions displayed much more competitive performance toward Cu2+ removal. As seen in Fig. 7B, it was observed that the adsorption capacity for Cu2+ ions obviously decreased in the presence of coexisting heavy metal ions even with the low concentration ranged from 0.25 mg/L to 4 mg/L. The adsorption capacity of Cu2+ coexisting with other heavy metal ions (i.e., Ni2+ , Pb2+ , Zn2+ , Cd2+ , etc.) was in the range of 6.8–11.8 mg/g, which was lower than that of the corresponding the single Cu2+ solution (12.0 mg/g). In particular, adsorption capacity for Cu2+ ions reached as low as 6.8 mg/g under the condition of 4 mg/L coexisting Zn2+ , showing 43% reduction. This observation may be explained by considering the decrease in the number of adsorption functional group on the Fe3 O4 @SiO2 -HA microspheres because coexisting ions compete with Cu2+ for adsorption [50,51]. To clarify the Cu2+ adsorption performance of the synthesized HA-based magnetic silica microspheres, adsorption capacity was investigated from the adsorption isotherms and kinetics experimental date. The Cu2+ removal behavior of the synthesized adsorbent was evaluated as a function of the initial Cu2+ concentration in the range from 10 mg/L to 50 mg/L at pH value of 6.8 and 25 ◦ C. Experimental result showed in Fig. 8 that the adsorption capacity of Cu2+ ion increased nearly linear with increasing the initial concentration. Lower the initial Cu2+ concentration of 35 mg/L, the strong ion adsorption capacity can be attributed that adsorbate ions could bind to the abundant adsorption sites on the surface of the synthesized magnetic HA-supported microspheres. Above the initial Cu2+ concentration of 35 mg/L, the rate of increment of adsorption capacity became gradually slow during the initial adsorbate ion concentration increase. The equilibrium adsorption data were applied to fit into two different isotherm models. The Langmuir model can be expressed in equation [52]: qe =
47
where qe is the equilibrium adsorption capacity of adsorbate (mg/g), Ce is the equilibrium concentration of metal ion (mg/L), qm is the maximum amount of metal adsorbed (mg/g), and KL is the constant that refer to the bonding energy of adsorption (L/mg). In general, The Langmuir model assumes that the solid surface active site can be occupied only by one adsorbate and that the active sites are independent. On the contrary, the Freundlich model is based on a heterogeneous adsorption. The Freundlich isotherm is given as [52]: qe = KF ce 1/n The linear form of Freundlich model can be described as equation: Log qe = LogKF +
Logce n
where qe is the equilibrium adsorption capacity of the adsorbent (mg/g), Ce is the equilibrium concentration of Cu2+ (mg/L), KF is the constant related to the adsorption capacity of the adsorbent (mg/L), and n is the constant related to the adsorption intensity. The relationship between initial Cu2+ concentration and the adsorption capacity was analyzed with two different models (Fig. 9). The calculated correlation coefficient (KL , qm , n, and KF ) and linear regression coefficient (R2 ) values for each Langmuir and Freundlich model were shown in Table 1. The adsorption equilibrium study exhibited that the Cu2+ adsorption of HA-supported magnetic silica microspheres had a better fit to the Freundlich isotherm model than the Langmuir model, indicating the multimolecular chemical adsorption.
KL qm ce 1 + KL ce
The linear form of Langmuir isotherm as follows: 1 ce ce = + qe KL qm qm
Fig. 8. Effect of initial Cu2+ concentration on adsorption capacity of Fe3 O4 @SiO2 -HA microspheres. The pH value was adjusted as 6.8 and 30 mg of the Fe3 O4 @SiO2 -HA were contacted with Cu2+ ions for 7 h at 25 ◦ C.
Fig. 9. Adsorption isotherm of Cu2+ onto Fe3 O4 @SiO2 -HA: (A) Langmuir model and (B) Freundlich model.
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Table 1 Adsorption parameters of the Langmuir and Freundlich models for the adsorption of Cu2+ ion onto the Fe3 O4 @SiO2 -HA microspheres. Langmuir model
Freundlich model
qm (mg/g)
kL (L/mg)
R2
Kf
n
R2
29.42
0.036
0.88
1.545
1.429
0.99
Table 2 Kinetic adsorption parameters obtained using pseudo-first-order and pseudosecond-order models. Pseudo-first-order
Pseudo-second-order
qe (mg/g)
K1
R
qe (mg/g)
K (×10−3 )
R2
7.629
0.01
0.97
10.66
1.45
0.99
2
Fig. 11. Magnetic hysteresis loop of the Fe3 O4 (A), Fe3 O4 @SiO2 (B), and Fe3 O4 @SiO2 HA (C) microspheres at 298 K.
In order to investigate adsorption kinetics, the two models (the pseudo-first-order and the pseudo-second-order) were used to test the experiment data. The pseudo-first-order model is given as [52]: In(qe − qt ) = Inqe − K1 t where qt and qe are the amount of Cu2+ adsorbed (mg/g) at time t (min) and at equilibrium, respectively, and K1 is the rate constant of the pseudo-first-order adsorption process (min−1 ). The straight line plots of In(qe − qt ) against t were used to determine the rate constant K1 and correlation coefficient R2 values from these plots. The pseudo-second-order model is given as [52]: 1 1 t = + t qt q2 K2 q22
Fig. 10. (A) pseudo-first-order and (B) pseudo-second-order kinetic adsorption of Cu2+ onto Fe3 O4 @SiO2 -HA microspheres. The test was performed at a pH value of 6.8 and 25 ◦ C. The initial concentration of the Cu2+ ion was 20 mg/L.
where K2 is the constant of pseudo-second-order rate (g mg−1 min−1 ), q2 is the amount adsorbed at equilibrium and qt is the amount adsorbed at any time. The equilibrium adsorption amount (q2 ) and the pseudo-second-order rate parameters (K2 ) can be given from the slope and intercept of plot of t/qt versus t. The corresponding values were presented in Table 2. As can be seen from the results, the correlation coefficients R2 of pseudo-secondorder model (0.99) were higher than that of pseudo-first-order model (R2 ). So the pseudo-second-order model fits better the experimental data than the pseudo-first-order model (Fig. 10). Magnetic characterizations of hybrids containing a magnetite component were carried out by a vibrating sample magnetometer (VSM) at 298 K as shown in Fig. 11. The saturation magnetization moment of Fe3 O4 , Fe3 O4 @SiO2 , and Fe3 O4 @SiO2 -HA microspheres was 68.60 emu/g, 55.21 emu/g, and 44.59 emu/g, respectively. The
Fig. 12. Photographs of the Fe3 O4 @SiO2 -HA microspheres dispersed in aqueous solution without and external magnetic field.
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decrease of the saturation magnetization is most likely ascribed to the existence of outer shell layer on the surface of Fe3 O4 microspheres [53]. The magnetic property of the product is applied to achieve the separation goal in liquid-phase reaction system. As an example, when the Fe3 O4 @SiO2 -HA microspheres were dispersed in water giving a black suspension, upon applying an external magnetic field, the black powder was readily harvested and the background was became transparent, as shown in Fig. 12. This phenomenon provided the facile magnetic separation for their practical application in adsorption field. 4. Conclusion Novel magnetic adsorbent with submicro-size was fabricated through the immobilization of hyaluronic acid on the magnetic silica microspheres. The as-synthesized Fe3 O4 @SiO2 -HA microspheres can be used as an effective adsorbent for the removal of copper ions from aqueous solution. The Cu2+ adsorption activities of the adsorbent were investigated with various experimental conditions. As a result, it was observed that the optimum pH value of Fe3 O4 @SiO2 -HA microspheres for Cu2+ removal was 6.8, and the coexisting metal ions (i.e., Zn2+ , Ni2+ , Pb2+ ) much more competed with Cu2+ than Cd2+ for adsorption. Furthermore, Freundlich isotherm model gave better fittings with adsorption equilibrium data than Langmuir model. Kinetic experiments clearly indicated that adsorption process of copper ions on the synthesized adsorbent was followed pseudo-second-order kinetics models. Fe3 O4 @SiO2 -HA microspheres also displayed super-paramagnetic performance with the saturation magnetization moment of 44.59 emu/g, which made the Cu2+ adsorbed microspheres separable magnetically from wastewater in shorter time by the aid of external magnetic field. Hyaluronic acid modified magnetic adsorbent has opened the possibility to enhance removal of hazardous metal ions from wastewater. Moreover, as a novel adsorbent, its adsorption mechanism toward targeted species needs further illustration, and further study is required on how to obtain these high-performance adsorbents in larger scale. Acknowledgements This present work was financially supported by the key technology and equipment of efficient utilization of oil shale resources, No: OSR-05, and the National Science and Technology major projects, No: 2008ZX05018. References [1] S.M.G. Demneh, B. Nasernejad, H. Modarres, Modeling investigation of membrane biofouling phenomena by considering the adsorption of protein, polysaccharide and humic acid, Colloids Surf. B 88 (2011) 108–114. [2] W. Zhou, J. Wang, B. Shen, W. Hou, Y. Zhang, Biosorption of copper (II) and cadmium (II) by a novel exopolysaccharide secreted from deep-sea mesophilic bacterium, Colloids Surf. B 72 (2009) 295–302. [3] A.H. Chen, S.C. Liu, C.Y. Chen, C.Y. Chen, Comparative adsorption of Cu (II), Zn (II), and Pb (II) ions in aqueous solution on the crosslinked chitosan with epichlorohydrin, J. Hazard. Mater. 154 (2008) 184–191. [4] L.F. Wang, J.C. Duan, W.H. Miao, R.J. Zhang, S.Y. Pan, X.Y. Xu, Adsorption–desorption properties and characterization of crosslinked Konjac glucomannan-graft-polyacrylamide-co-sodium xanthate, J. Hazard. Mater. 186 (2011) 1681–1686. [5] Y. Zhu, J. Hua, J. Wang, Competitive adsorption of Pb (II), Cu (II) and Zn (II) onto xanthate-modfied magnetic chitosan, J. Hazard. Mater. 221–222 (2012) 155–161. [6] M. Monier, D.M. Ayad, D.A. Abdel-Latif, Adsorption of Cu (II), Cd (II) and Ni (II) ions by cross-linked magnetic chitosan-2-aminopyridine glyoxal Schiff’s base, Colloids Surf. B 94 (2012) 250–258. [7] P. Kujawa, P. Moraille, J. Sanchez, A. Badia, F.M. Winnik, Effect of molecular weight on the exponential growth and morphology of hyaluronan/chitosan multilayers: a surface plasmon resonance spectroscopy and atomic force microscopy investigation, J. Am. Chem. Soc. 127 (2005) 9224–9234.
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