adsorption of cu(ii) on β-cyclodextrin modified multiwall carbon nanotube/iron oxides in the...

Research ArticleReceived: 15 August 2011 Revised: 17 October 2011 Accepted: 17 October 2011 Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/jctb.2764

Adsorption of Cu(II) on β-cyclodextrin modifiedmultiwall carbon nanotube/iron oxidesin the absence/presence of fulvic acidJun Hu, Shitong Yang and Xiangke Wang∗

Abstract

BACKGROUND: The adsorption of Cu(II) on β-cyclodextrin (β-CD) modified multiwall carbon nanotubes/iron oxides (denoted asMWCNT/IO/CD) as a function of contact time, pH, adsorbent content, temperature, fulvic acid (FA) and initial Cu(II) concentrationswas investigated using a batch technique under ambient conditions.

RESULTS: The adsorption of Cu(II) was strongly dependent on pH, adsorbent content, temperature and FA. A positive effect ofFA on Cu(II) adsorption was found at pH < 6.5, whereas a negative effect was observed at pH > 6.5. Different effects of FA/Cu(II)concentrations on Cu(II) and FA adsorption were observed, indicating enhanced Cu(II) adsorption on FA bound MWCNT/IO/CD,whereas FA adsorption was decreased in the presence of Cu(II) ions. The adsorption isotherms were well fitted by the linearisotherm model. The adsorption thermodynamic parameters calculated from temperature dependent adsorption isothermssuggested that the adsorption of Cu(II) on MWCNT/IO/CD was an endothermic and spontaneous process.

CONCLUSIONS: MWCNT/IO/CD is a promising magnetic material for the preconcentration and separation of Cu(II) ions fromaqueous solutions in environmental pollution cleanup.c© 2011 Society of Chemical Industry

Keywords: MWCNT/IO/CD; adsorption; copper; fulvic acid

INTRODUCTIONDue to rapid industrialization and increase in the world population,heavy metal pollution in water has attracted considerableattention because of its adverse effect on human health andthe environment. Unlike organic pollutants, metal ions do noteasily convert into harmless end products. Copper ions are ofparticular interest because they are essential to plants, human,and animals. Lack of copper in animal diet may cause anemia,diarrhea, and nervous disturbances. However, if it is ingestedexcessively in the human diet, it may result in itching, dermatitis,vomiting, cramps, convulsion, and even death.1,2 The main sourcesof copper are the industrial waste streams from metal cleaning andplating baths, paper, pulp, paperboard and wood preservative-employing mills, and fertilizer industry, etc.3,4 The greatestcharacteristic of these heavy metal ions is that they are difficultto degrade using microorganisms and thereby cause persistentpollution. Since they cannot be biodegraded they accumulate inliving organisms, causing various diseases and disorders, evenin relatively low concentrations. However, in most cases, heavymetal ions and organic contaminants may present simultaneouslyat many contaminated sites.5 – 7 For example, natural organicmaterials, such as humic acid (HA) and fulvic acid (FA), existubiquitously in natural aquatic environments, and may influencethe physicochemical behavior of heavy metal ions. Therefore, it isimportant to eliminate toxic metals from aqueous solutions in thepresence of organic materials and vice versa.

Adsorption has been widely used as a simple, economical,and cost-effective technology for the removal of heavy metals

from wastewater during the past decade.8 – 10 Adsorbents playa key role in the removal of heavy metal ions since theydetermine the performance of treatment technology, includingadsorption capacity and post-treatment (separation). In recentyears, magnetic adsorbents have aroused researchers’ interest insolving environmental problems due to their high efficiency andeasy separation from aqueous solution using an external magneticfield. The main advantages of this technology are its capabilityof treating large volumes of wastewater within a short time andproducing no secondary pollutants. Banerjee and Chen10 studieda gum arabic modified magnetic nanomaterial for the removal ofcopper ions. Huang et al.11 developed a versatile Al2O3-supportediron oxide for the removal of Pb(II) ions. Ma et al.12 developedmagnetic-chitosan particles for the adsorption of fluoride. Chenet al.13 studied multiwall carbon nanotube/iron oxide (MWCNT/IO)magnetic composites for the removal of Eu(III) ions. Shao et al.14

prepared polyaniline modified MWCNT magnetic composites forthe removal of aniline and phenol. In the adsorption processes,surface functional groups enable high adsorption capacity for theremoval of contaminants. For surface modification application, thelow temperature plasma-induced grafting technique has many

∗ Correspondence to: Xiangke Wang, Key Laboratory of Novel Thin Film SolarCells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126,230031 Hefei, P.R. China. E-mail: [email protected]

Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics,Chinese Academy of Sciences, P.O. Box 1126, 230031 Hefei, P.R. China

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www.soci.org J Hu, S Yang, X Wang

positive characteristics such as the contribution of free radicals,energetic electrons and charged particles. Although these particleshave enough energy to activate, ionize or dissociate the reactantmolecules, they don’t affect the bulk properties, modifying onlythe surface properties.5,15,16

Herein, β-cyclodextrin was used to modify MWCNT/IO using alow temperature plasma-induced grafting technique (denotedMWCNT/IO/CD). The structure of β-cyclodextrin molecules istoroidal, truncated cones containing a polar cavity with primaryhydroxyl groups lying on the outside and secondary hydroxylgroups inside,17 which can form strong complexes with metalions. The objectives of this study were: (1) to study the adsorptionbehavior of Cu(II) and FA on MWCNT/IO/CD as a function ofcontact time, initial pH, temperature, and solid content; (2) toinvestigate the mutual effect of FA/Cu(II), and the effect of FA/Cu(II)concentrations on Cu(II) and FA adsorption; and (3) to determinethe thermodynamic parameters from the temperature dependentadsorption isotherms.

MATERIALS AND METHODSMaterialsβ-cyclodextrin (β-CD) (purity 99.0%) was purchased from TianjinBodi chemical Co., Ltd (China). Cu(ClO4)2 (analytical purity) waspurchased from Sinopharm Chemical Reagent Co. Ltd (China). Thesolutions were prepared with Milli-Q water (resistivity 18.2 M�

cm−1) under ambient conditions.

Synthesis of MWCNT/IO/CDMWCNTs were prepared by a chemical vapor deposition methodusing acetylene in hydrogen flow at 760 ◦C with Ni–Fe nanopar-ticles as catalysts.18 MWCNT/IO was prepared using chemical co-precipitation reaction of ferrous and ferric ions, and carboxylic andhydroxylic groups of MWCNTs in a wet process.13 Plasma-inducedgrafting of β-CD on MWCNT/IO composites was composed of twosuccessive processes: surface activation of MWCNT/IO and graftingof β-CD onto MWCNT/IO. Briefly, 4.0 g MWCNT/IO were activatedusing N2 plasma (power 128 W, voltage 860 V, gas pressure 4.6 Pa)in a custom-built grafting reactor for 40 min under continuousstirring. Then 100 mL of 20 g L−1 β-CD solution was immediatelyinjected into the grafting reactor and the activated MWCNT/IOreacted with β-CD at 80 ◦C for 1 h under continuous stirring. Thesample obtained was repeatedly rinsed with Milli-Q water until noβ-CD was detected (using high performance liquid chromatog-raphy–mass spectrometry, HPLC-MS) in the supernatant. Finally,the MWCNT/IO/CD magnetic composites were dried in an oven at95 ◦C for 12 h.

CharacterizationThe MWCNT/IO/CD was characterized by powder X-ray diffraction(XRD), thermogravimetric analysis (TGA), X-ray photoelectronspectroscopy (XPS), and vibrating sample magnetometry (VSM).The XRD patterns of the oxidized MWCNTs, MWCNT/IO andMWCNT/IO/CD were recorded on a RigaKu D/max 2550 X-rayDiffractometer using Cu Kα radiation (λ = 0.1541 nm) in stepsof 0.05◦ (2θ ) min−1 from 5 to 70◦ (2θ ) at room temperature.The TGA measurements were made using a Shimadzu TGA-50thermogravimetric analyzer from 25 to 700 ◦C at a heating rate of10 ◦C min−1 with an air flow rate of 50 mL min−1. The XPS data wereobtained with a Thermo ESCALAB 250 electron spectrometer fromVG Scientific using 150 W AlKα radiation. The XPS photoelectron

binding energies (BE) of the adventitious carbon species, i.e. theC 1s line at 284.4 eV was used to correct the observed bindingenergies for surface charging. Magnetic curves were obtainedusing a model 155 VSM from 0 to 20.0 kOe.

Adsorption experimentsThe adsorption of Cu(II) was investigated using batch techniquein a 10 mL polyethylene centrifuge tube at three differenttemperatures (T = 293.15, 313.15, and 333.15 K). Sorption ofCu(II) on MWCNT/IO/CD composites as a function of contact timewas carried out at pH = 5.50 and in 0.01 mol L−1 NaClO4 solutions.MWCNT/IO/CD suspension and NaClO4 (0.10 mol L−1) were pre-equilibrated for 1 day. Then FA was added into the suspension andequilibrated with MWCNT/IO/CD for 2 days, and finally Cu(II) wasadded into the suspension. The desired pH values were adjustedwith negligibly small amounts of 0.1 mol L−1 or 0.01 mol L−1 HClO4

or NaOH. After the suspensions were shaken for 2 days to achieveequilibrium, the solid and liquid phases were separated by amagnetic process using a permanent magnet. The concentrationof Cu(II) in supernatant was analyzed by spectrophotometry atwavelength 544 nm using Cu chlorophosphonaze-III complex andthat of FA was analyzed by UV-vis spectrophotometry at 210 nm.

The adsorption (%) and amount of Cu(II)/FA adsorbed on thesolid phase (qe) were calculated from the concentration differencebetween the initial concentration (C0) and the equilibriumconcentration (Ce):

Adsorption(%) = (C0 − Ce)/C0 × 100% (1)

qe = (C0 − Ce) × V

m(2)

where C0 is the initial concentration (mg L−1), Ce is the equilibriumconcentration (mg L−1), qe is amounts of Cu(II)/FA adsorbed onsolid (mg g−1) after equilibrium, V is the solution volume (mL), andm is the mass of solid (g).

RESULTS AND DISCUSSIONCharacterization of MWCNT/IO/CDFigure 1(A) presents the XRD patterns of MWCNT/IO/CD,MWCNT/IO and oxidized MWCNTs. The diffraction peak assignedto MWCNTs at 2θ = 26.2◦ can be clearly seen for three samples, in-dicating that the MWCNT structure is not destroyed in MWCNT/IOand MWCMT/IO/CD. The characteristic peaks (2θ = 30.2◦, 35.5◦,43.2◦, 53.5◦, 57.8◦ and 62.8◦, which are similar to JCPD standards:Fe3O4 (89–3854, 2θ = 30.1◦, 35.4◦, 43.1◦, 53.4◦, 57.0◦ and 62.5◦),or γ -Fe2O3 (89–5892, 2θ = 30.3◦, 35.7◦, 43.3◦, 53.8◦, 57.3◦ and63.0◦) reveal a cubic iron oxide phase. Other peaks (2θ = 21.2◦,33.3◦, 41.2◦ and 59.0◦) is related to the presence of α-FeO(OH).19

The peak at 2θ = 18.3◦ may be related to the presence of β-CDin MWCNT/IO/CD.20 The XPS spectra of Fe 2p3/2 at 710.82 eV andFe 2p1/2 at 724.68 eV in MWCNT/IO and MWCNT/IO/CD (Fig. 2(A))correspond to electron peaks of Fe 2p of Fe2O3 and Fe3O4.21 Theatomic ratio of C : O : Fe is 5.75 : 1.67 : 1 in MWCNT/IO, and that ofC : O : Fe is 7.23 : 1.97 : 1 in MWCNT/IO/CD. Figure 1(C) shows theTGA curves of MWCNT/IO/CD, MWCNT/IO and oxidized MWCNTs.Compared with MWCNTs, MWCNT/IO/CD and MWCNT/IO are lessthermally stable, responding to the presence of the grafted β-CD on MWCNT surfaces. The TGA curves of MWCNT/IO/CD andMWCNT/IO show the characteristic decomposition stages of β-CD.The weight loss at 200–244 ◦C corresponds to the decomposi-tion of β-CD. The final decomposition temperature of CDs is above

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Adsorption of Cu(II) on β-cyclodextrin modified carbon nanotubes www.soci.org

10 20 30 40 50 60 70

(A)

Mh/Mn

CNTs

Mh/Mn

Mh/Mn/CNTs

Mh/Mn

Mh/Mn

Mh/Mn

curve a

2 Theta/degree

curve c

cureve b

FeFeFe

CD

700 705 710 715 720 725 730 735 740

MWCNT/IO/CD

Fe2p1/2 (724.68 eV)Fe2p3/2 (710.82 eV)Inte

nsity

Binding Energy (eV)

(B)

MWCNT/IO

0 100 200 300 400 500 600 7000

10

20

30

40

50

60

70

80

90

100

(C)

MWCNT/IO/CD

MWCNT/IO

Oxidized MWCNTs

Rel

ativ

e w

eigh

t (%

)

Temperature (°C)

-20 -15 -10 -5 0 5 10 15 20-50

-40

-30

-20

-10

0

10

20

30

40

50

(D)

ss = 37.8 emu/g

Mag

netiz

atio

n (e

mu/

g)

Magnetic Field (kOe)

Figure 1. (A) XRD patterns of oxidized MWCNTs (curve A); MWCNT/IO (curve B); and MWCNT/IO/CD (curve C). (Mn: magnetite, Mh: maghemite, Fe:α–FeO(OH)). (B) XPS spectra of Fe 2p high-resolution spectra of MWCNT/IO and MWCNT/IO/CD. (C) TGA curves of oxidized MWCNTs, MWCNT/IO andMWCNT/IO/CD. (D) Magnetization curve of MWCNT/IO/CD.

500◦C, and the residual of CDs is ∼15% in air atmosphere. From theTGA analysis, the amount of β-CD in MWCNT/IO/CD is calculatedto be 161.4 mg g−1,22 and the weight percent of iron oxides inMWCNT/IO is calculated to be 420 mg g−1. The specific saturationmagnetization σs of MWCNT/IO/CD is 37.8 emu g−1 (magneticfield = ±20 kOe) (Fig. 1(D)). Chen et al.13 synthesized MWCNT/IOcomposites and found that the σs value of MWCNT/IO compositeswas 29.2 emu g−1, which was high enough to separate MWCNT/IOfrom aqueous solution by magnetic separation technique. Hereinthe magnetic property indicates that the MWCNT/IO/CD can beseparated from aqueous solution by magnetic separation tech-nique.

Effect of contact timeFigure 2 shows the amount of Cu(II) (mg g−1) adsorbed onMWCNT/IO/CD as a function of contact time at pH 5.50 in theabsence of FA. The amount of adsorbed Cu(II), qe, sharply increaseswith increasing contact time at the first contact time of 20 h andthereafter it proceeds at a slow rate and finally attains equilibrium.

The initial rapid adsorption may be due to an increased numberof available sites at the initial stage. The increase in concentrationgradient tends to increase in Cu(II) adsorption rate at the initialstages. As time proceeds, the concentration gradients becomereduced due to the accumulation of Cu(II) adsorbed on the surfacesites, leading to the decrease in adsorption rate at the laterstages. It can be concluded that the rate of Cu(II) binding onMWCNT/IO/CD is high at initial stages, which gradually decreasesand achieves equilibrium after an optimum period of 20 h. Basedon the kinetic results, 48 h of contact time was applied in thefollowing experiments to sufficiently approach the adsorptionequilibrium.

The kinetic adsorption data is simulated by a pseudo-second-order model, which is expressed by the following equation:23

t

qt= 1

k2q2e

+ 1

qet (3)

where k2 is the rate constant of pseudo-second-order adsorption(g mg h−1)). The value of k2 and qe, calculated from the slope and

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Figure 2. Adsorption kinetic of Cu(II) on MWCNT/IO/CD. m/V = 0.4 g L−1, I = 0.01 mol L−1 NaClO4, pH = 5.50 ± 0.05, T = 293.15 K, and C0 = 10.0 mg L−1.

intercept of the plot of t/qt versus t (inset figure in Fig. 2), are0.137 g mg h−1) and 12.87 mg g−1, respectively. The correlationcoefficient is very close to 1, which suggests that the kineticadsorption can be described by a pseudo-second-order rateequation well.

Effect of pHThe influence of pH on the adsorption of Cu(II) on MWCNT/IO/CDand MWCNT/IO is shown in Fig. 3. The adsorption is pH dependent.The adsorption of Cu(II) on MWCNT/IO/CD increases quickly fromabout 41 to 99% in pH range of 3.3–8.0, and then maintainsa high level with increasing pH. The adsorption of Cu(II) onMWCNT/IO increases slowly with increasing pH from about 0 to90% in pH range of 3.8–9.1. The adsorption of Cu(II) as a functionof pH can be explained by the surface charge of adsorbentand the species of Cu(II) ions. From the hydrolysis constantsof Cu(II) (log K1 = −7.5, LogK2 = −17.3, log K3 = −27.3,log K4 = −39.6), Cu(II) ions present in the forms of Cu2+,Cu(OH)+, Cu2(OH)2

2+, Cu(OH)2 and Cu(OH)3− at pH 3.0–10.0.24

At low pH values, the predominant species of Cu(II) is free Cu2+

ion, and the protonation of the oxygen containing functionalgroups occurs on MWCNT/IO/CD surface (protonation process:Cx OH+H+ ⇔ Cx OH+

2 ), the surface is positively charged. Therefore,the low adsorption can be attributed to the electrostatic repulsionoccurring between Cu2+ and the positively charged surface ofMWCNT/IO/CD. Meanwhile, the competition between H+ andCu2+ will also decrease Cu(II) adsorption.25,26 At high pH values,the surface groups of MWCNT/IO/CD become negatively chargedbecause of the deprotonation process (Cx OH ⇔ Cx O− + H+),which causes a more electrostatic attraction of Cu(II).24 Meanwhile,a decrease in competition between H+ and Cu2+ will alsoincrease Cu(II) adsorption. As more surface functional groupsare deprotonated at high pH values than at low pH values, moreadsorption sites are available for Cu(II), which causes higher Cu(II)adsorption.27,28 The adsorption curve of Cu(II) on MWCNT/IO/CD isshifted left compared with that of Cu(II) on MWCNT/IO, which alsosupports the interpretation mentioned above. This phenomenamay be explained by the increasing hydroxyl for binding Cu(II)

Figure 3. Adsorption of Cu(II) on MWCNT/IO and MWCNT/IO/CD asa function of pH in the presence/absence of FA. m/V = 0.4 g L−1,I = 0.01 mol L−1 NaClO4, T = 293.15 K, contact time = 48 h, andC[Cu(II)]0 = 10.0 mg L−1.

and thereby enhances Cu(II) adsorption. From the precipitationconstant of Cu(OH)2 (log K = −10.36), Cu(II) forms precipitates atpH ∼10.3 if no Cu(II) ions is adsorbed on solid particle at Cu(II)initial concentration of 10 mg L−1. In this study, the precipitationof Cu(II) on Cu(II) adsorption contribution is ignored. However, thehigh adsorption of Cu(II) on solid particles can result in very highconcentrations of Cu(II) on solid particles and may possibly formprecipitation at high pH values on solid surfaces. The possibilityof precipitation of Cu(II) ions as copper hydroxide at alkaline pHvalues may contribute to Cu(II) ion adsorption in the alkaline range.This needs further investigation to study the local atomic structuresof Cu(II) adsorbed on solid particles such as XAFS spectroscopyanalysis.

Adsorption of Cu(II) on MWCNT/IO/CD in the absence/presenceof FA is also shown in Fig. 3. At pH<6.5, Cu(II) adsorption increasesdramatically, while a slight decrease is observed at pH>6.5 inthe presence of 10 mg L−1 or 20 mg L−1 FA. Figure 4 illustrates



Figure 4. Adsorption of FA on MWCNT/IO/CD as a function of pH. m/V =0.4 g L−1, I = 0.01 mol L−1 NaClO4, T = 293.15 K, and contact time = 48 h.

the effects of pH and FA concentration on FA adsorption. Theadsorption of FA gradually decreases with increasing pH andincreasing FA concentration. FA adsorption is strongly dependenton the electrostatic parameters such as the surface charge ofadsorbents, which is dependent on pH. At low pH, FA has a highly‘coiled’ conformation due to the low charge development,29 whichresults in the adsorption of a large number of FA moleculesas each molecule occupies a smaller area. Moreover, at verylow pH values, the negatively charged FA are easily adsorbedon the positively charged surfaces of MWCNT/IO/CD, thereforeenhances FA adsorption. The surface adsorbed FA induces more

functional groups on MWCNT/IO/CD, allowing more Cu(II) ions tobe adsorbed on MWCNT/IO/CD due to the strong complexationof Cu(II) with surface adsorbed FA. As pH increases, the oxygen-containing functional groups of FA and MWCNT/IO/CD becomemore negatively charged, thus, the repulsion between FA andMWCNT/IO/CD would be stronger, hindering the adsorptionof FA on MWCNT/IO/CD. Meanwhile, FA is more ‘expanded’due to electrostatic repulsion at high pH,5 the area occupiedby each molecule will be higher, which will decrease FAadsorption. However, the adsorption of FA on MWCNT/IO/CDis still very high at high pH values. The hydrophobic, hydrogenbonds and strong π –π interactions between FA molecules andMWCNT/IO/CD may also contribute to the high adsorption of FAon MWCNT/IO/CD composites at high pH values.13 At high pHvalues, the electrostatic repulsion between FA and MWCNT/IO/CDbecomes stronger with increasing pH and thereby results in morefree FA molecules in aqueous solutions. Therefore, at high pH,more free FA molecules can form strong complexes with Cu(II)in solution, which diminishes Cu(II) adsorption on MWCNT/IO/CD.According to the mechanisms mentioned above, the proposedmechanisms of Cu(II) adsorbtion on MWCNT/IO/CD are shownin Fig. 5.

Effect of adsorbent contentFigure 6 shows the effect of adsorbent content on Cu(II) adsorptionby MWCNT/IO/CD. As the adsorbent content was increased from0.1 g L−1 to 0.8 g L−1, the Cu(II) adsorption increased from ∼45%to ∼59%. This is attributed to the increased adsorbent surfacearea and more available adsorption sites because of the increasein adsorbent.30 Because a fixed content of MWCNT/IO/CD canonly adsorb a certain amount of Cu(II), the higher the adsorbent

Figure 5. Proposed sorption interactions of Cu(II) on MWCNT/IO/CD in the absence/presence of FA.



Figure 6. Adsorption of Cu(II) on MWCNT/IO/CD as a function of adsorbent contents. I = 0.01 mol L−1 NaClO4, T = 293.15 K, contact time = 48 h, andpH = 5.50 ± 0.05.

content, the larger the amount of Cu(II) that can be adsorbedon solid surfaces. However, the amount of adsorbed Cu(II) onsolid phase, qe, decreased from 45.0 to 7.4 mg g−1 with increasingadsorbent content from 0.1 to 0.8 g L−1. The reason for thisis the split in the flux or the concentration gradient betweensolute concentration in solution and solute concentration in thesurface of the adsorbent. With increasing adsorbent content,the amount of Cu(II) adsorbed onto unit weight of adsorbentreduces, causing a decrease in qe value.28 Evidently, the adsorptionpercentage and equilibrium adsorption capacity are sensitive tothe variation of adsorbent content. An adsorbent content of0.4 g L−1 was selected for all further experiments because of thehigh adsorption efficiency and acceptable adsorption capacity atthis value.

Effect of temperature and thermodynamic dataThe adsorption isotherms reveal the specific relation between theequilibrium concentration of Cu(II) in the bulk and the amount ofCu(II) adsorbed at the solid surfaces. The experimental data of Cu(II)adsorption on MWCNT/IO/CD at three different temperatures arewell reproduced by the linear model (Fig. 7). The slope values ofqe against Ce (i.e. 1.833, 2.014 and 2.199 L g−1 at 293.15, 313.15and 333.15 K, respectively) increases with increasing temperature,which indicates that adsorption is favored at high temperature.High temperature generally increases the rate of Cu(II) diffusionthrough the solution to the external surface of MWCNT/IO/CD, andmay change the equilibrium adsorption capacity of MWCNT/IO/CDfor the particular adsorbate.31 – 34 The linear adsorption isothermssuggest a partitioning process of Cu(II) ions from liquid to solidsurface. The linear adsorption isotherms also indicate that theadsorption of Cu(II) on MWCNT/IO/CD is far from saturation. Thehigh adsorption capacity of MWCNT/IO/CD makes it a very suitablecandidate for the removal of Cu(II) ions from large volumes ofaqueous solutions.

Thermodynamic parameters provide additional in-depth in-formation regarding the inherent energetic changes during theadsorption processes. The values of enthalpy (H0) and entropy(S0) are calculated from the slope and intercept of the plot of

Figure 7. Adsorption isotherms of Cu(II) on MWCNT/IO/CD at differenttemperatures. m/V = 0.4 g L−1, I = 0.01 mol L−1 NaClO4, contact time =48 h, and pH = 5.50 ± 0.05.

ln Kd vs. 1/T (Fig. 8) using the following equations:

Kd = C0 − Ce

Ce· V

m(4)

ln Kd = S◦/R − H ◦

/RT (5)

where Kd is the adsorption coefficient, R (8.314 J mol−1 K−1) is theideal gas constant and T is the temperature (K). The Gibbs freeenergy change (G◦) was determined from the equation:

G◦ = H◦ − TS◦ (6)

The relative data calculated from Equations (5) and (6) are listedin Table 1. The H0 values of Cu(II) adsorption on MWCNT/IO/CDare positive, i.e. endothermic. The Cu(II) ions were well hydrated insolution, they had to lose part of the hydration sheath in order tobe adsorbed. This dehydration process of Cu(II) needed energy andsuperseded the exothermicity of ion adsorption on the surface.10,35



Figure 8. Semi-logarithmic plot of distribution coefficient (Kd) vs. reciprocaltemperature for Cu(II) adsorption onto MWCNT/IO/CD. m/V = 0.4 g L−1,I = 0.01 mol L−1 NaClO4, contact time = 48 h, and pH = 5.50 ± 0.05.

The negative G0 value suggests that the adsorption reactionis a spontaneous process and thermodynamically favorableunder the experimental conditions. The decrease of G0 withincreasing temperature indicates more efficient adsorption athigher temperature, whereas the decrease of G0 with decreasingCu(II) initial concentration indicates greater adsorption of Cu(II)at lower concentration. With decreasing Cu(II) concentration insolution, the competitive Cu(II) adsorption on MWCNT/IO/CDdecreases and thereby increases Cu(II) adsorption. The positiveS0 values indicate the affinity of MWCNT/IO/CD towards Cu(II)ions in aqueous solutions and may suggest some structure changesin the adsorbents.34

Adsorption isothermsFigure 9(A) shows the adsorption isotherms of Cu(II) onMWCNT/IO/CD in the absence/presence of FA. The adsorptionof Cu(II) on MWCNT/IO/CD in the absence/presence of FA is welldescribed by the linear isotherm model. One can see that theadsorption of Cu(II) on MWCNT/IO/CD at high FA concentrationsis much greater than that at low FA concentrations (the slopevalues of qe against Ce for Cu(II) adsorption on MWCNT/IO/CDincrease from 1.833 to 3.441 L g−1 with initial FA concentrationincreased from 0 to 20 mg L−1), in agreement with the resultsshown in Fig. 3. At pH 5.50, the adsorption of FA on MWCNT/IO/CDis very high. From Fig. 4, one can see that ∼97% FA is adsorbed onMWCNT/IO/CD at CFA = 20 mg L−1, and ∼98% FA is adsorbed onMWCNT/IO/CD at CFA = 10 mg L−1. The amount of FA adsorbed

(A)

(B)

Figure 9. Adsorption isotherms of Cu(II) on MWCNT/IO/CD in theabsence/presence of FA (A) and adsorption isotherms of Cu(II) onMWCNT/IO/CD and MWCNT/IO (B). m/V = 0.4 g L−1, I = 0.01 mol L−1

NaClO4, T = 293.15 K, contact time = 48 h, and pH = 5.50 ± 0.05.

on the solid surfaces at CFA = 20 mg L−1 is much higher than thatadsorbed at CFA = 10 mg L−1. At higher FA concentrations, moreFA molecules and more functional groups are available to bindCu(II) ions, thereby resulting in more Cu(II) ions being adsorbedon FA–MWCNT/IO/CD hybrids.

To evaluate the effect of β-CD on the adsorption prop-erty of MWCNT/IO/CD, the adsorption isotherms of Cu(II) onMWCNT/IO/CD and MWCNT/IO are shown in Fig. 9(B). The slopevalues of qe against Ce of Cu(II) adsorption on MWCNT/IO/CD

Table 1. Thermodynamic parameters for Cu(II) adsorption onto MWCNT/IO/CD

−G◦ (kJ mol−1)

Co[Cu(II)] (mg L−1) H◦ (kJ mol−1) S◦ (J mol−1 K−1) T = 293.15 K T = 313.15 K T = 333.15 K

1.7 17.71 75.38 4.39 5.90 7.40

5.0 9.64 42.54 2.83 3.68 4.53

10.0 4.34 22.62 2.29 2.74 3.19

20.0 4.44 21.12 1.76 2.18 2.60

35.0 4.36 20.70 1.71 2.12 2.54



(A)

(B)

Figure 10. Effect of FA concentrations on the adsorption of Cu(II) onMWCNT/IO/CD (A) and effect of Cu(II) concentrations on the adsorptionof FA on MWCNT/IO/CD (B). m/V = 0.4 g L−1, I = 0.01 mol L−1 NaClO4,T = 293.15 K, and contact time = 48 h.

(i.e. 1.833 L g−1) is much higher than that of Cu(II) adsorptionon MWCNT/IO (i.e. 0.084 L g−1), indicating that the adsorptioncapacity of MWCNT/IO/CD is much higher than that of MWCNT/IOunder the experimental conditions. This phenomenon indicatesthat the surface grafted β-CD molecules play an important rolein the enhancement of adsorption capacity. The high adsorptioncapacity of MWCNT/IO/CD may be explained by the large numberof hydroxyl functional groups of β-CD, which can provide effectiveadsorption sites for the binding of Cu(II) ions.

Mutual effects of FA/Cu(II) initial concentrations on Cu(II)/FAadsorptionFigure 10(A) shows the adsorption of Cu(II) at different FA initialconcentrations. It is observed that Cu(II) adsorption increaseswith increasing FA concentrations. As the FA concentrationincreases from 0 to 32 mg L−1, the amount of Cu(II) adsorbedon MWCNT/IO/CD increases from 12.69 to 17.72 mg g−1. Theeffect of FA on Cu(II) adsorption is mainly attributed tothe reaction occurring at the surface of MWCNT/IO/CD. Athigher FA concentrations, more FA molecules are adsorbedon MWCNT/IO/CD, which provides more available sites forsubsequent Cu(II) adsorption.36

The effect of Cu(II) initial concentrations on FA adsorption byMWCNT/IO/CD are also investigated and the results are presentedin Fig. 10(B). The adsorption of FA decreases with increasing Cu(II)initial concentrations. This indicates that the available adsorptionsites for Cu(II) and FA are similar and the competitive adsorptionbetween Cu(II) and FA causes the adsorption of FA to be reduced.

CONCLUSIONSThe kinetic data of Cu(II) adsorption on MWCNT/IO/CD is welldescribed by the pseuso-second-order rate model. Adsorptionof Cu(II) on MWCNT/IO/CD is strongly dependent on pH values.A positive effect of FA on Cu(II) adsorption is found at pH <

6.5, whereas a negative effect is observed at pH > 6.5. Theadsorption isotherms of Cu(II) on MWCNT/IO/CD are fitted wellby the linear model. The adsorption of Cu(II) on MWCNT/IO/CDis an endothermic and spontaneous process. MWCNT/IO/CDmaterial has much higher adsorption capacity for the removalof Cu(II) than MWCNT/IO. The large number of hydroxyl functionalgroups of surface grafted β-CD provide more available adsorptionsites for the removal of Cu(II) ions from aqueous solutions.The presence of FA enhances Cu(II) adsorption whereas thepresence of Cu(II) decreases FA adsorption. The surface adsorbedFA molecules provide more available sites for Cu(II) adsorption,and the competitive adsorption between Cu(II) and FA causesthe FA adsorption to be reduced. The magnetic composites ofMWCNT/IO/CD are very suitable materials for the preconcentrationand immobilization of heavy metals from aqueous solutions.

ACKNOWLEDGEMENTSFinancial support from National Natural Science Foundationof China (21107115; 21071147; 20971126; 21077107), 973projects from the Ministry of Science and Technology of China(2007CB936602; 2011CB933700) and Knowledge Innovation Pro-gram of Chinese Academy of Sciences (085FCQ0121) are acknowl-edged.

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