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Colloids and Surfaces A: Physicochem. Eng. Aspects 425 (2013) 42– 50

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur n al hom ep age: www.elsev ier .com/ locate /co lsur fa

ynthesis and characterization of hyaluronic acid-supported magneticicrospheres for copper ions removal

hi Lan, Xiaomin Wu, Linlin Li, Mengmeng Li, Fengying Guo, Shucai Gan ∗

ollege of Chemistry, Jilin University, Changchun 130026, PR China

i g h l i g h t s

Hyaluronic acid was successfullyimmobilized on the magnetic silicasubmicron-sized particles.Hyaluronic acid was utilized as adsor-bent for the heavy metal removal.Magnetism and adsorption were per-fectly combined into one singleentity.

g r a p h i c a l a b s t r a c t

Hyaluronic acid-supported magnetic submicron-sized particles were fabricated as a novel adsorbent forthe Cu2+ removal and can be separated magnetically through the application of a magnetic field in shortertime after the adsorption performance.

r t i c l e i n f o

rticle history:eceived 30 October 2012eceived in revised form 22 February 2013ccepted 28 February 2013vailable online 7 March 2013

eywords:agnetic

a b s t r a c t

Magnetic hyaluronic acid (HA) microspheres were fabricated as a novel adsorbent through the immo-bilization of hyaluronic acid on the magnetic silica microspheres. The as-prepared microspheres werecharacterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fouriertransform infrared (FTIR), X-ray powder diffraction (XRD), and X-ray photoelectron spectra (XPS). The as-synthesized 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 inves-tigated, respectively. The adsorption equilibrium study exhibited that the Cu2+ adsorption of hyaluronic

yaluronic acidicrospheres

dsorptioneparation

acid-supported magnetic microspheres had a better fit to the Freundlich isotherm model than the Lang-muir model. The kinetic date of adsorption of Cu2+ on the synthesized adsorbents was best described bythe pseudo-second-order equation. The resultant microspheres also revealed super-paramagnetic behav-ior, which made these adsorbent magnetically separable after the adsorption performance. This workdemonstrates that the synthesized hyaluronic acid-supported magnetic adsorbent can be considered as

haza

a potential adsorbent for

. Introduction

Recently, natural polysaccharides such as chitosan, heparin,

hondroitin, keratin, and xanthan have been developed as envi-onmentally friendly materials for removing toxic pollutants fromqueous solution and attracted much attention [1–4]. In particular,

∗ Corresponding author. Tel.: +86 431 88502259.E-mail address: gansc@jlu.edu.cn (S. Gan).

927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfa.2013.02.059

rdous metal ions from wastewater.© 2013 Elsevier B.V. All rights reserved.

chitosan has been widely suggested as a candidate for an over-whelming scope of adsorption applications, covering almost all thespectrum of biotechnology [5,6]. Hyaluronic acid (HA), one kind ofthe natural polysaccharide with a similar structure as chitosan, hasa variety of functions mainly including roles in joint lubrication, tis-sue hydration, wound repair, and modulation of inflammation as

published in earlier literatures. However, to the best of our knowl-edge, few have reported the research about the adsorption behaviorof HA toward pollutants yet, and it can be expected as a promisingcandidate for efficient adsorbent of heavy metal ions due to its large

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umber of active metal-binding sites including oxygen, nitrogennd carboxyl which can uptake heavy mental ions from waste-ater through various mechanisms such as chelation, electrostatic

ttraction, and ion-exchange, providing possibility for synthesis ofA-based adsorbent materials. However, there is a major drawback

o the application of HA as adsorbent originating from the separa-ion due to its high solubility in aqueous solution. Isolation of HArom a large volume of water requires an additional process andurther expense, and thus it seems reasonable to immobilize of HAn insoluble component to facilitate the separation process [7–9].

Adsorption performance of the polymeric adsorbent stronglyepends on its activated surface area and the quantity of surfacectivated sites for interaction with metallic species. Nanometernd micrometer sized materials have shown remarkable poten-ial because of their large surface area [10]. Therefore, to enhancedsorption capacity, fabrication of the adsorbent with nanometerr micrometer size to enlarge activate surface area is advisable [11].owever, introduction of nanometer or micrometer sized adsor-ent brings them difficulty in separation process, which involvesurther expense to remove such fine adsorbent nanomaterials fromhe aqueous suspension [12–14].

Magnetic adsorbents in nanometer and micrometer size havettracted great attention for their potential application in removalf the pollutants from aqueous solution due to their strong adsorp-ion capacity, simple recovery from liquid solution under suitable

agnetic field and reusable property [15]. However, pure mag-etic particles are prone to form aggregation owing to the magneticipolar attraction between magnetite microspheres and magneticroperties affected in liquid systems. To solve this defect, a rationalrotective layer including inorganic oxide or polymer compoundsoated on the surface of magnetic particles is often utilized. As aesult, silica-coated magnetic nanoparticles and polymer-coatedagnetic nanoparticles are commonly used as the substrate for the

ynthesis of magnetic-based adsorbent [16].Herein, novel HA-functionalized magnetic microspheres were

abricated as a magnetic adsorbent through anchoring HA on silica-oated magnetic microspheres. Adsorption behavior of as-preparedicrospheres was evaluated by selecting Cu2+ as representative

eavy metal, which is considered as an essential nutrient inrace amount, but can cause adverse health effects at high doses17–20]. Adsorption experiments revealed that the as-synthesized

icrospheres possess excellent adsorption behavior of Cu2+ ion.n addition, HA-functionalized magnetic microspheres can benriched completely after the adsorption performance within shortime under an external magnetic field. The synergism between

agnetism and adsorption would result in promising applicationsn many fields.

. Experimental

.1. Materials

HA was commercially obtained from Qufu guanglong Biochemo., Ltd. Ferric chloride (FeCl3·6H2O), anhydrous sodium acetateNaAc), polyethylene glycol (PEG molecular weight 10000), ethyl-ne glycol, toluene, anhydrous ethanol, aqueous ammonia solution28 wt%), and copper nitrate (Cu(NO3)2·3H2O) were purchasedrom Beijing Chemical Reagent Research Company. Tetraethoxysi-ane (TEOS, 98 wt%) was available from Shanghai Chemicaleagents Company. 3-aminopropyltriethoxysilane (APS, 98 wt%)as provided from Aldrich Chemical Company.

.2. Characterization

The particle size and structure of the synthesized magneticanoparticles were observed by using a Hitachi 8100 transmission

ochem. Eng. Aspects 425 (2013) 42– 50 43

electron microscope (TEM, Hitachi, Tokyo, Japan). Scanning elec-tron microscopy (SEM) was performed on a TESCAN 5136MMSEMat an accelerating voltage of 20 Kv. The samples were loadedonto a glass surface previously sputter coated with a homoge-nous gold layer for charge dissipation during the SEM imaging. Theinfrared spectra of the nanoparticles were taken in KBr pressedpellets on a NEXUS 670 infrared Fourier transform spectrometer(Nicolet Thermo, Waltham, MA). X-ray diffraction (XRD) measure-ments were recorded on a Rigaku D/MAXIIA diffractimoter usingCu Ka radiation. X-ray photoelectron spectra (XPS) measurementwas 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 highvoltage was kept at 14.0 kV with a detection angle at 540. All thebinding energies were calibrated by using the containment car-bon (C 1s = 284.6 eV). The hysteresis loops were obtained with avibrating sample magnetometer (VSM 7407, Lake Shore).

2.3. Synthesis of the magnetic silica microspheres

Fe3O4 microspheres were first synthesized via a solvothermalreaction as previously described [21,22]. In the next process, thecore-shell magnetic silica microshoeres Fe3O4@SiO2 were preparedaccording 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 Fe3O4microspheres were thoroughly rinsed with deionized water for sev-eral times. Fe3O4 microspheres were homogeneously dispersed ina mixture of ethanol (40 mL) and deionized water (10 mL) underultrasonic vibration for 30 min, then the concentrated ammoniaaqueous solution (1 mL, 28 wt%) was added to this solution with thehelp of ultrasonication to obtain a stable solution, and followed bythe addition of tetraethoxysilane every 15 min till the total amountof TEOS reached 0.2 mL under magnetically stirring, and the pro-cess was followed 12 h. Finally, the product was collected with thehelp of magnet and washed with recycle of ethanol and water forseveral times, and then vacuum dried at 60 ◦C.

2.4. Synthesis of HA-modified Fe3O4@SiO2 microspheres

In the functionalization of Fe3O4@SiO2, the APS graftedFe3O4@SiO2 act as core template and hyaluronic acid poly-mer serve as shell. Typically, the Fe3O4@SiO2 magnetic silicamicrospheres were firstly functionalized with a coupling agent3-aminopropyltriethoxysilane (APS) through the siloxane linkage.The detailed experimental process was as follows. Fe3O4@SiO2(0.05 g) and coupling agent APS were dispersed in 50 mL tolueneand refluxed for 3 h. The obtained products Fe3O4@SiO2-APS werecollected and washed with absolute ethanol and deionized waterfor times [25,26]. In the next step, hyaluronic acid (50 mg) was dis-solved in deionized water (180 mL) with the vigorous mechanicalstirring to form a clear solution. Fe3O4@SiO2-APS (50 mg) was dis-persed in HCI (2 M, 5 mL) for 5 min to activate surface functionalgroup and washed with deionized water for three times, and dis-persed subsequently in the freshly prepared solution with the aidof ultrasonication for 10 min, and then the suspension was stirredat 50 ◦C for another 12 h. The final product Fe3O4@SiO2-HA wascollected by magnetic separation and washed with water, and thenoven dried at 40 ◦C.

2.5. Effect of initial Cu2+ concentration on adsorption

Batch adsorption experiments were studied by placing 30 mg of

obtained Fe3O4@SiO2-HA in 100 mL conical flasks containing 50 mLof various concentration (10–50 mg/L) of Cu2+ solutions in the pHvalue of 6.8 at 25 ◦C temperature in a shaker bath for 7 h. The solu-tion was shocked at 120 rpm using a mechanical shaker to reach

4 Physicochem. Eng. Aspects 425 (2013) 42– 50

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quilibrium. The concentration of Cu2+ before and after treatmentas calculated by atomic absorption spectrometer (AAS, Varian

20FS, USA). For accurate adsorption results, the metal adsorptionata was analyzed three times and the mean value (error range: ca.5%) was represented. The adsorption capacity (qe) was calculatedsing the equation [27,28]:

e = (C0 − Ce) × V

W

here C0 is the initial concentration of metal ions (mg/L), Ce is thequilibrium concentration of metal ions after adsorption (mg/L), Vs the volume of metal ions solution (mL), and W is the weight ofhe synthesized adsorbent (mg).

.6. Effect of pH value on adsorption

The desired pH value of solution was adjusted by adding either.1 M HCl or NaOH solution and then 50 mL of the above solu-ion was taken in a conical flask and treated with 30 mg sorbent.he final Cu2+ concentration was determined after equilibrium bytomic absorption spectrometer.

.7. Effect of interferential metal ions on Cu2+ removal

The possibly coexisting cationic metals (i.e., Na+, Mg2+, Ca2+,i2+, Pb2+, Zn2+, Cd2+, etc.) in wastewater were chosen to inves-

igate their effect on Cu2+ removal by the adsorbent. Differentoncentration of cationic metals and same dosage of sorbent wasdded in 30 mg/L Cu2+ original concentration solution and testedt room temperature. The different metals solution was preparedy dissolving their nitrates.

. Results and discussion

Hyaluronic acid-functionalized magnetic microspherese3O4@SiO2-HA with magnetic silica microspheres as corend HA as shell were successfully prepared and used as andsorbent to remove heavy metal Cu2+ form aqueous solution.he fabrication route of the Fe3O4@SiO2-HA microspheres waskillfully designed, and the synthetic procedure was schematicallyllustrated in Fig. 1A. Magnetic HA-based microspheres can besed as adsorbent for Cu2+ removal from aqueous solution andnriched completely within short time under an external magneticeld as shown in Fig. 1B. The combination of magnetic propertynd absorption performance into one single entity can makehe HA-based materials separable magnetically and significantlyacilitate their practical applications.

To verify the morphology of the resultant microspheres inetail, the obtained microspheres were characterized by SEM andEM measurement. Fig. 2 depicted the physical appearance ofhe iron oxide microspheres, the magnetic silica microspheres,nd the HA-supported silica-coated magnetic microspheres. Theuasi-monodisperse, spherical, and solid Fe3O4 microspheres withn average diameter of 105.6 nm were observed in Fig. 2A. Afterilica coating, the as-synthesized microspheres present relativelymooth particle surface, and the particle size was vividly increased.t can be clearly seen from the insert in Fig. 2B that the Fe3O4@SiO2

icrospheres presented obvious core-shell structure and the grayuter layer around the magnetic Fe3O4 nanoparticle were amor-hous silica coating with an average shell thickness of 32 nm. Thisncapsulation was significant because the silica shell not only kept

stable dispersion of magnetic microspheres for a long time in a

arsh liquid media but also prevent their corrosion in an acidicnvironment [29]. As shown in Fig. 2C, the surface of Fe3O4@SiO2-A microspheres was obviously coarser than that of Fe3O4@SiO2,hich was further revealed by the insert TEM image in Fig. 2C.

Fig. 1. Synthetic procedure of the Fe3O4@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 andparticle size, suggesting that HA introduction has no effect on theinner core materials.

FTIR spectra were recorded to identify the formation of the func-tional groups on the microspheres at different synthetic steps. FTIRspectra in this case not only confirmed the silica coating on thesurface of the magnetic Fe3O4 microspheres but also verified theformation of Fe3O4@SiO2-HA microspheres. FTIR spectrum of theas-prepared microspheres is shown in Fig. 3. The typical peak ofFe O bond appeared at 580 cm−1 for the spectra in Fig. 3A, B, andD [30]. The absorption peaks of 3423 cm−1 and 1651 cm−1 wereobserved in all the spectra corresponding to the stretching vibrationand bending vibration of O H group for water, respectively [31].The O H stretching vibration related to SiO H groups appearedin the same range of 3200–3500 cm−1 in the Fig. 3B and D [32].The SiO2 coated Fe3O4 microspheres (Fig. 3B) have the character-istic peaks at 1105 cm−1, 810 cm−1, and 950 cm−1, correspondingto the asymmetrical stretching vibration of Si O Si, symmetricalstretching vibration of Si O Si band, and stretching vibration ofSi OH, respectively [33–36]. The characteristic peaks of HA poly-mer displayed in Fig. 3C were evidently observed in the curve ofFe3O4@SiO2-HA microspheres, and the C-H stretching vibrationpeaks were dramatically enlarged after the HA immobilization inFig. 3D.

The crystallographic structure and composition of the as-synthesized microspheres were characterized by X-ray powderdiffraction (XRD). Fig. 4A displayed that the position and relativeintensity 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 Fe3O4powder diffraction data (JCPDS: 65-3107) [37]. This result indicatedthat the iron oxide microspheres were magnetite and belonged tocubic structure. It was apparent that as shown in Fig. 4B, the diffrac-tion peaks for Fe O @SiO were similar to those of iron oxide,

3 4 2suggesting that the magnetic Fe3O4 microspheres were success-fully encapsulated by the thinner amorphous silica layer withoutaffecting the original crystallinity of Fe3O4 structure [38].

S. Lan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 425 (2013) 42– 50 45

FH

vbfnaaoC5[T(e

the adsorbent and the concentration of copper ions in solution, sothe optimum pH value for further Cu2+ uptake studies was selectedas 6.8.

ig. 2. SEM and TEM images (insert) of Fe3O4 (A), Fe3O4@SiO2 (B), and Fe3O4@SiO2-A (C) microspheres. All scale bars in TEM images are 50 nm.

Immobilization of HA on Fe3O4@SiO2 microspheres was furthererified by XPS measurement, and the identification of chemicalond was made by deconvolution of the C 1s, O 1s, and N 1s peaksrom the total XPS spectrum as shown in Fig. 5. The large compo-ent of the C 1s envelope was the C C state at 284.5 eV and 285.1 eV,nd the components at 285.7 eV and 286.5 eV were associated to Ctoms bonded with N and/or O (Fig. 5A) [39,40]. The binding energyf O 1s at 532.8 eV was corresponding to the C O H, C O C, and

O bond, and the chemical state of O 1s with binding energy of30.8 eV also appeared owing to the involvement of OH− (Fig. 5B)

41]. Evidently, two N 1s features for the product were observed.he typical peak at 400.4 eV was attributed to the amide group

CONH ), which acted as unique element marker to provide thevidence that HA presented either at or very near the particle

Fig. 3. FTIR spectra of Fe3O4 (A), Fe3O4@SiO2 (B), HA (C), and Fe3O4@SiO2-HA (D)microspheres.

surface, and the peak at 402.2 eV was possibly correspond to theprotonated amino group (Fig. 5C) [42,43].

The effect of pH value on Cu2+ adsorption with Fe3O4@SiO2-HAmicrospheres was investigated with 30 mg/L original Cu2+ concen-tration. The experimental results of Cu2+ removal at varying pHvalue were presented in Fig. 6. It is obvious that the adsorptioncapacity was increased from 6.0 to 12.2 mg/g with increasing pHvalue 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 sor-bent decreasing the negatively charged surface sites hence resultin the low adsorption efficiency. Furthermore, the amino groupsand hydroxyl groups on Fe3O4@SiO2-HA microspheres are readilyprotonated at lower pH value and thus unfavorable for removalof Cu2+ by the positively charged functional groups of adsorbentdue to the electrostatic repulsion, resulting in lower adsorptioncapacity [44–46]. However, the removal efficiency presented slightdescending trend with the overly increasing pH value due to theformation of Cu(OH)+ and Cu(OH)2 precipitation at the alkaline con-dition [47]. In summary, pH value influenced the surface charge of

Fig. 4. XRD patterns of the iron oxide (A) and the magnetic silica (B) microspheres.

46 S. Lan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 425 (2013) 42– 50

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of coexisting ions Mg2+/Ca2+. The reduction in Cu2+ adsorptioncapacity was reached as high as 10% for 50 mg/L Mg2+ and 9.1% for50 mg/L Ca2+. Therefore, It was concluded that the divalent ionsCa2+ and Mg2+ had more suppressive effect on Cu2+ adsorption

ig. 5. XPS spectra of C 1s (A), O 1s (B), and N 1s (C) regions for the Fe3O4@SiO2-HAicrospheres.

The heavy metal ion pollutants are often together with alka-

ine/earth metal ions in wastewater. The impact of coexisting Na+,

g2+ and Ca2+ on the uptake of the Cu2+ is displayed in Fig. 7A. Thedsorption capacity of Cu2+ with Fe3O4@SiO2-HA microspheresvidently decreased with increasing the concentration of alkaline

Fig. 6. Effect of pH value on the adsorption capacity of Fe3O4@SiO2-HA microspheresfor Cu2+ ions.

earth metal ions (Mg2+ and Ca2+) in the range of 10–50 mg/L. TheFe3O4@SiO2-HA microspheres showed Cu2+ adsorption capacityof 12.0 mg/g toward single Cu2+ solution, and 10.8/10.9 mg/gadsorption capacity was found under the condition of 50 mg/L

Fig. 7. Effect of interferential metal ions concentration on the adsorption capacityof Fe3O4@SiO2-HA microspheres for Cu2+ ions.

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exhibited that the Cu adsorption of HA-supported magneticsilica microspheres had a better fit to the Freundlich isothermmodel than the Langmuir model, indicating the multimolecularchemical adsorption.

S. Lan et al. / Colloids and Surfaces A: P

ikely due to the stronger interaction ability between the sorbentnd Ca2+/Mg2+. However, Cu2+ adsorption capacity only decreasedo 11.7 mg/g even with higher Na+ concentration of 50 mg/L,howing only 2% reduction [48,49].

The effect of the coexisting heavy metal ions on the removalf Cu2+ by Fe3O4@SiO2-HA microspheres was investigated as well.ompared with alkaline/earth metal ions, heavy metal ions dis-layed much more competitive performance toward Cu2+ removal.s seen in Fig. 7B, it was observed that the adsorption capacity foru2+ ions obviously decreased in the presence of coexisting heavyetal ions even with the low concentration ranged from 0.25 mg/L

o 4 mg/L. The adsorption capacity of Cu2+ coexisting with othereavy metal ions (i.e., Ni2+, Pb2+, Zn2+, Cd2+, etc.) was in the range of.8–11.8 mg/g, which was lower than that of the corresponding theingle Cu2+ solution (12.0 mg/g). In particular, adsorption capacityor Cu2+ ions reached as low as 6.8 mg/g under the condition of

mg/L coexisting Zn2+, showing 43% reduction. This observationay be explained by considering the decrease in the number of

dsorption functional group on the Fe3O4@SiO2-HA microspheresecause coexisting ions compete with Cu2+ for adsorption [50,51].

To clarify the Cu2+ adsorption performance of the synthe-ized HA-based magnetic silica microspheres, adsorption capacityas investigated from the adsorption isotherms and kinetics

xperimental date. The Cu2+ removal behavior of the synthesizeddsorbent was evaluated as a function of the initial Cu2+ concen-ration in the range from 10 mg/L to 50 mg/L at pH value of 6.8nd 25 ◦C. Experimental result showed in Fig. 8 that the adsorptionapacity of Cu2+ ion increased nearly linear with increasing the ini-ial concentration. Lower the initial Cu2+ concentration of 35 mg/L,he strong ion adsorption capacity can be attributed that adsorbateons could bind to the abundant adsorption sites on the surfacef the synthesized magnetic HA-supported microspheres. Abovehe initial Cu2+ concentration of 35 mg/L, the rate of increment ofdsorption capacity became gradually slow during the initial adsor-ate ion concentration increase.

The equilibrium adsorption data were applied to fit into twoifferent isotherm models. The Langmuir model can be expressed

n equation [52]:

e = KLqmce

1 + KLce

The linear form of Langmuir isotherm as follows:

ce

qe= 1

KLqm+ ce

qm

ig. 8. Effect of initial Cu2+ concentration on adsorption capacity of Fe3O4@SiO2-HAicrospheres. The pH value was adjusted as 6.8 and 30 mg of the Fe3O4@SiO2-HAere contacted with Cu2+ ions for 7 h at 25 ◦C.

ochem. Eng. Aspects 425 (2013) 42– 50 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 theconstant that refer to the bonding energy of adsorption (L/mg). Ingeneral, The Langmuir model assumes that the solid surface activesite can be occupied only by one adsorbate and that the active sitesare independent. On the contrary, the Freundlich model is basedon a heterogeneous adsorption. The Freundlich isotherm is givenas [52]:

qe = KF ce1/n

The linear form of Freundlich model can be described asequation:

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 isthe constant related to the adsorption capacity of the adsorbent(mg/L), and n is the constant related to the adsorption intensity. Therelationship between initial Cu2+ concentration and the adsorptioncapacity was analyzed with two different models (Fig. 9). Thecalculated correlation coefficient (KL, qm, n, and KF) and linearregression coefficient (R2) values for each Langmuir and Freundlichmodel were shown in Table 1. The adsorption equilibrium study

2+

Fig. 9. Adsorption isotherm of Cu2+ onto Fe3O4@SiO2-HA: (A) Langmuir model and(B) Freundlich model.

48 S. Lan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 425 (2013) 42– 50

Table 1Adsorption parameters of the Langmuir and Freundlich models for the adsorptionof Cu2+ ion onto the Fe3O4@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 2Kinetic adsorption parameters obtained using pseudo-first-order and pseudo-second-order models.

Pseudo-first-order Pseudo-second-order

qe (mg/g) K1 R2 qe (mg/g) K (×10−3) R2

7.629 0.01 0.97 10.66 1.45 0.99

Fig. 10. (A) pseudo-first-order and (B) pseudo-second-order kinetic adsorption ofCu2+ onto Fe3O4@SiO2-HA microspheres. The test was performed at a pH value of6.8 and 25 ◦C. The initial concentration of the Cu2+ ion was 20 mg/L.

Fig. 12. Photographs of the Fe3O4@SiO2-HA microspheres dispers

Fig. 11. Magnetic hysteresis loop of the Fe3O4 (A), Fe3O4@SiO2 (B), and Fe3O4@SiO2-HA (C) microspheres at 298 K.

In order to investigate adsorption kinetics, the two models (thepseudo-first-order and the pseudo-second-order) were used to testthe experiment data. The pseudo-first-order model is given as [52]:

In(qe − qt) = Inqe − K1t

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 constantof the pseudo-first-order adsorption process (min−1). The straightline plots of In(qe − qt) against t were used to determine the rateconstant K1 and correlation coefficient R2 values from these plots.The pseudo-second-order model is given as [52]:

t

qt= 1

K2q22

+ 1q2

t

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 adsorptionamount (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 seenfrom the results, the correlation coefficients R2 of pseudo-second-order model (0.99) were higher than that of pseudo-first-ordermodel (R2). So the pseudo-second-order model fits better theexperimental 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 magnetizationmoment of Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2-HA microsphereswas 68.60 emu/g, 55.21 emu/g, and 44.59 emu/g, respectively. The

ed in aqueous solution without and external magnetic field.

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S. Lan et al. / Colloids and Surfaces A: P

ecrease of the saturation magnetization is most likely ascribed tohe existence of outer shell layer on the surface of Fe3O4 micro-pheres [53]. The magnetic property of the product is applied tochieve the separation goal in liquid-phase reaction system. As anxample, when the Fe3O4@SiO2-HA microspheres were dispersedn water giving a black suspension, upon applying an external

agnetic field, the black powder was readily harvested and theackground was became transparent, as shown in Fig. 12. This phe-omenon provided the facile magnetic separation for their practicalpplication in adsorption field.

. Conclusion

Novel magnetic adsorbent with submicro-size was fabricatedhrough the immobilization of hyaluronic acid on the magneticilica microspheres. The as-synthesized Fe3O4@SiO2-HA micro-pheres can be used as an effective adsorbent for the removal ofopper ions from aqueous solution. The Cu2+ adsorption activitiesf the adsorbent were investigated with various experimen-al conditions. As a result, it was observed that the optimumH value of Fe3O4@SiO2-HA microspheres for Cu2+ removalas 6.8, and the coexisting metal ions (i.e., Zn2+, Ni2+, Pb2+)uch more competed with Cu2+ than Cd2+ for adsorption. Fur-

hermore, Freundlich isotherm model gave better fittings withdsorption equilibrium data than Langmuir model. Kinetic exper-ments clearly indicated that adsorption process of copper ionsn the synthesized adsorbent was followed pseudo-second-orderinetics models. Fe3O4@SiO2-HA microspheres also displayeduper-paramagnetic performance with the saturation magneti-ation moment of 44.59 emu/g, which made the Cu2+ adsorbedicrospheres separable magnetically from wastewater in shorter

ime by the aid of external magnetic field. Hyaluronic acid mod-fied magnetic adsorbent has opened the possibility to enhanceemoval of hazardous metal ions from wastewater. Moreover, as aovel adsorbent, its adsorption mechanism toward targeted specieseeds further illustration, and further study is required on how tobtain these high-performance adsorbents in larger scale.

cknowledgements

This present work was financially supported by the key technol-gy and equipment of efficient utilization of oil shale resources, No:SR-05, and the National Science and Technology major projects,o: 2008ZX05018.

eferences

[1] S.M.G. Demneh, B. Nasernejad, H. Modarres, Modeling investigation of mem-brane 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) andcadmium (II) by a novel exopolysaccharide secreted from deep-sea mesophilicbacterium, 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 withepichlorohydrin, 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 crosslinkedKonjac 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 molecularweight on the exponential growth and morphology of hyaluronan/chitosanmultilayers: a surface plasmon resonance spectroscopy and atomic forcemicroscopy investigation, J. Am. Chem. Soc. 127 (2005) 9224–9234.

[

ochem. Eng. Aspects 425 (2013) 42– 50 49

[8] L. Richert, P. Lavalle, E. Payan, X.Z. Shu, G.D. Prestwich, J.F. Stoltz, P. Schaaf,J.C. Voegel, C. Picart, Layer by layer buildup of polysaccharide films: physicalchemistry and cellular adhesion aspects, Langmuir 20 (2004) 448–458.

[9] C. Porcel, P. Lavalle, V. Ball, G. Decher, B. Senger, J.C. Voegel, P. Schaaf, From expo-nential to linear growth in polyelectrolyte multilayers, Langmuir 22 (2006)4376–4383.

10] J.F. Gnichwitz, R. Marczak, F. Werner, N. Lang, N. Jux, D.M. Guldi, W. Peukert,A. Hisch, Efficient synthetic access to cationic dendrons and their applicationfor ZnO nanoparticles surface functionalization: new building blocks for dye-sensitized solar cells, J. Am. Chem. Soc. 132 (2010) 17910–17920.

11] S.T. Nguyen, H.T. Nguyen, A. Rinaldi, N.P.V. Nguyen, Z. Fan, H.M. Duong, Mor-phology control and thermal stability of binderless-graphene aerogels fromgraphite for energy storage applications, Colloids Surf., A 414 (2012) 352–358.

12] Y. Wang, S. Maksimuk, R. Shen, H. Yang, Synthesis of iron oxide nanoparticlesusing a freshly-made or recycled imidazolium-based ionic liquid, Green Chem.9 (2007) 1051–1056.

13] M.I. Shukoor, F. Natalio, M.N. Tahir, V. Ksenofontov, H.A. Therese, P. Theato,H.C. Schröder, W.E.G. Müller, W. Tremel, Superparamagnetic gamma-Fe2O3

nanoparticles with tailored functionality for protein separation, Chem. Com-mun. 44 (2007) 4677–4679.

14] C.H. Yu, C.C.H. Lo, K. Tam, S.C. Tsang, Monodisperse binary nanocomposite insilica with enhanced magnetization for magnetic separation, J. Phys. Chem. C111 (2007) 7879–7882.

15] D.Z. Yin, X. Du, H. Liu, Q.Y. Zhang, L. Ma, Facile one-step fabrication of poly-mer microspheres with high magnetism and armored inorganic particles byPickering emulsion polymerization, Colloids Surf., A 414 (2012) 289–295.

16] P. Lu, J.L. Zhang, Y.L. Liu, D.H. Sun, G.Y. Hong, J.Z. Ni, Synthesis and character-istic of the Fe3O4@SiO2@Eu(DBM)(3)center dot 2H(2)O/SiO2 luminomagneticmicrospheres with core-shell structure, Talanta 82 (2010) 450–457.

17] N. Farnad, K. Farhadi, N.H. Voelcker, Polydopamine nanoparticles as a new andhighly selective biosorbent for the removal of copper (II) ions from aqueoussolutions, Water Air Soil Pollut. 223 (2012) 3535–3544.

18] N. Anuraj, S. Bhattacharjee, J.H. Geiger, G.L. Baker, M.L. Bruening, An all-aqueousroute to polymer brush-modified membranes with remarkable permeabilitesand protein capture rates, J. Membr. Sci. 389 (2012) 117–125.

19] M.F. Sawalha, J.R. Peralta-Videa, J. Romero-González, J.L. Gardea-Torresdey,Biosorption of Cd (II), Cr (III), and Cr (VI) by saltbush (Atriplex canescens)biomass: thermodynamic and isotherm studies, J. Colloid Interface Sci. 300(2006) 100–104.

20] M. Choi, J. Jang, Heavy metal ion adsorption onto polypyrrole-impregnatedporous carbon, J. Colloid Interface Sci. 325 (2008) 287–289.

21] H. Deng, X. Li, Q. Peng, X. Wang, J. Chen, Y. Li, Preparation of graphene-encapsulated magnetic microspheres for protein/peptide enrichment andMALDI-TOF MS analysis, Angew. Chem. Int. Ed. 44 (2005) 2782–2785.

22] Q. Liu, J. Shi, M. Cheng, G. Li, D. Cao, G. Jiang, Monodisperse magnetic single-crystal ferrite microspheres, Chem. Commun. 48 (2012) 1874–1876.

23] X. Xu, C. Deng, M. Gao, W. Yu, P. Yang, X. Zhang, Synthesis of magnetic micro-spheres with immobilized metal ions for enrichment and direct determinationof phosphopeptides by matrix-assisted laser desorption ionization mass spec-trometry, Adv. Mater. 18 (2006) 3289–3293.

24] G. Cheng, J.L. Zhang, Y.L. Liu, D.H. Sun, J.Z. Ni, Synthesis of novelFe3O4@SiO2@CeO2 microspheres with mesoporous shell for phosphopeptidecapturing and labeling, Chem. Commun. 47 (2011) 5732–5734.

25] M. Manzano, V. Aina, C.O. Areán, F. Balas, V. Cauda, M. Colilla, M.R. Delgado,M. Vallet-Regí, Studies on MCM-41 mesoporous silica for drug delivery: effectof particle morphology and amine functionalization, Chem. Eng. J. 137 (2008)30–37.

26] A. Walcarius, M. Etienne, J. Bessière, Rate of access to the binding sites in organ-ically modified silicates I Amorphous silica gels grafted with amine or thiolgroups, Chem. Mater. 14 (2002) 2757–2766.

27] Y. Wang, X.J. Wang, X. Wang, M. Liu, L.Z. Yang, Z. Wu, S.Q. Xia, J.F. Zhao, Adsorp-tion of Pb (II) in aqueous solutions by bamboo charcoal modified with KMnO4

via microwave irradiation, Colloids Surf., A 414 (2012) 1–8.28] J. Zhou, Z. Zhang, B. Cheng, J. Yu, Glycine-assisted hydrothermal synthesis

and adsorption properties of crosslinked porous ˛-Fe2O3 nanomaterials forp-nitrophenol, Chem. Eng. J. 211–212 (2012) 153–160.

29] S. Xuan, F. Wang, X. Gong, S.K. Kong, J.C. Yu, Hierarchical core/shellFe3O4@SiO2@�-AlOOH@Au micro/nanoflowers for protein immobilization,Chem. Commun. 47 (2011) 2514–2516.

30] Z. Xu, Y. Feng, X. Liu, M. Guan, C. Zhao, H. Zhang, Synthesis and characteri-zation of Fe3O4@SiO2@poly-l-alanine, peptide brush–magnetic microspheresthrough NCA chemistry for drug delivery and enrichment of BSA, Colloids Surf.B 81 (2010) 503–507.

31] P. Xu, H. Wang, R. Tong, Q. Du, W. Zhang, Preparation and morphology ofSiO2/PMMA nanohybrids by microemulsion polymerization, Colloid Polym. Sci.284 (2006) 755–762.

32] Z. Xu, B. Feng, S. Bian, T. Liu, M. Wang, Y. Gao, D. Sun, X. Gao, Monodisperseand core-shell structured SiO2@Lu2O3:Ln3 (Ln¼Eu, Tb, Dy, Sm, Er, Ho, and Tm)spherical particles: a facile synthesis and luminescent properties, J. Solid StateChem. 196 (2012) 301–308.

33] L. Wang, Y. Sun, J. Wang, J. Wang, A. Yu, H. Zhang, D. Song, Preparation of surface

plasmon resonance biosensor based on magnetic core/shell Fe3O4/SiO2 andFe3O4/Ag/SiO2 nanoparticles, Colloids Surf. B 84 (2011) 190–484.

34] S.N. Abdollahi, M. Naderi, G. Amoabediny, Synthesis and physicochemical char-acterization of tunable silica–gold nanoshells via seed growth method, ColloidsSurf., A 414 (2012) 345–351.

5 Physic

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

0 S. Lan et al. / Colloids and Surfaces A:

35] D. Shao, K. Xu, X. Song, J. Hu, W. Yang, C. Wang, Effective adsorption and sep-aration of lysozyme with PAA-modified Fe3O4@silica core/shell microspheres,J. Colloid Interface Sci. 336 (2009) 526–532.

36] T.S. Sasikala, S. Raman, P. Mohanan, C. Pavithran, M.T. Sebastian, Effect of silanecoupling agent on the dielectric and thermal properties of DGEBA-forsteritecomposites, J. Polym. Res. 18 (2011) 811–819.

37] X. Zhang, H. Niu, Y. Pan, Y. Shi, Y. Cai, Modifying the surface of Fe3O4/SiO2

magnetic nanoparticles with C-18/NH2 mixed group to get an efficient sorbentfor anionic organic pollutants, J. Colloid Interface Sci. 362 (2011) 107–112.

38] W.K. E-Zawawy, M.M. Ibrahim, Preparation and characterization of novelpolymer hydrogel from industrial waste and copolymerization of poly(vinylalcohol) and polyacrylamide, J. Appl. Polym. Sci. 124 (2012) 4362–4370.

39] E. Vanea, V. Simon, XPS study of protein adsorption onto nanocrystalline alu-minosilicate microparticles, Appl. Surf. Sci. 257 (2011) 2346–2352.

40] O.S. Ishpal, M. Panwar, S. Kumar, Kumar, X-ray photoelectron spectroscopicstudy of nitrogen incorporated amorphous carbon films embedded withnanoparticles, Appl. Surf. Sci. 256 (2010) 7276–7371.

41] T. Herranz, X. Deng, A. Cabot, Z. Liu, M. Salmeron, In situ XPS study of theadsorption and reactions of NO and O2 on gold nanoparticles deposited onTiO2 and SiO2, J. Catal. 283 (2011) 119–123.

42] A. Chatterjee, L. Zhao, L. Zhang, D. Pradhan, X. Zhou, K.T. Leung, Core-level elec-tronic structure of solid-phase glycine, glycyl-glycine, diglycyl-glycine, andpolyglycine: X-ray photoemission analysis and Hartree-Fock calculations oftheir zwitterions, J. Chem. Phys. 129 (2008) 105104.

43] C.H. Yang, L.R. Huang, Y.K. Chih, W.C. Lin, Molecular assembled self-dopedpolyaniline copolymer ultra-thin films, Polymer 48 (2007) 3237–3247.

44] M.K. Sureshkumar, D. Das, M.B. Mallia, P.C. Gupta, Adsorption of uranium fromaqueous solution using chitosan-tripolyphosphate (CTPP) beads, J. Hazard.Mater. 184 (2010) 65–72.

[

ochem. Eng. Aspects 425 (2013) 42– 50

45] H.Z. Mousavi, A. Hosseinifar, V. Jahed, Studies of the adsorption thermodynam-ics and kinetics of Cr(III) and Ni(II) removal by polyacrylamide, J. Serb. Chem.Soc. 77 (2012) 393–405.

46] V.K. Gupta, A. Nayak, Cadmium removal and recovery from aqueous solutionsby novel adsorbents prepared from orange peel and Fe2O3 nanoparticles, Chem.Eng. J. 180 (2012) 81–90.

47] R.A.K. Rao, S. Ikram, Sorption studies of Cu(II) on gooseberry fruit (emblicaofficinalis) and its removal from electroplating wastewater, Desalination 277(2011) 390–398.

48] T.S. Anirudhan, P.S. Suchithra, Humic acid-immobilized polymer/bentonitecomposite as an adsorbent for the removal of copper(II) ions from aqueoussolutions and electroplating industry wastewater, J. Ind. Eng. Chem. 16 (2010)130–139.

49] F. Ge, M. Li, H. Yu, B. Zhao, Effective removal of heavy metal ions Cd2+, Zn2+, Pb2+,Cu2+ from aqueous solution by polymer-modified magnetic nanoparticles, J.Hazard. Mater. 211 (2012) 366–372.

50] T.S. Anirudhan, S. Jalajamony, S.S. Sreekumari, Adsorptive removal of Cu(II) ionsfrom aqueous media onto 4-ethyl thiosemicarbazide intercalated organophiliccalcined hydrotalcite, J. Chem. Eng. Data 58 (2013) 24–31.

51] J. Duan, Q. Lu, R. Chen, Y. Duan, L. Wang, L. Gao, S. Pan, Synthesis ofa novel flocculant on the basis of crosslinked Konjac glucomannan-graft-polyacrylamide-co-sodium xanthate and its application in removal of Cu2+ ion,Carbohydr. Polym. 80 (2010) 436–441.

52] K.Y. Shin, J.Y. Hong, J. Jang, Heavy metal ion adsorption behavior in nitrogen-

doped magnetic carbon nanoparticles: isotherms and kinetic study, J. Hazard.Mater. 190 (2011) 36–44.

53] J.B. Jun, S.Y. Uhm, J.H. Ryu, K.D. Suh, Synthesis and characterization of monodis-perse magnetic composite particles for magnetorheological fluid materials,Colloids Surf., A 260 (2005) 157–164.