2.multi-walled carbon nano tubes

Res Chem Intermed(2012)38:2205–2222

DOI10.1007/s11164-012-0537-6

Removal of divalent nickel cations from aqueous

solution by multi-walled carbon nano tubes:

equilibrium and kinetic processes

Iman Mobasherpour?Esmail Salahi?

Mohsen Ebrahimi

Received:28December2011/Accepted:16March2012/Published online:10April2012

óSpringer Science+Business Media B.V.2012

Abstract Release of heavy metals into water as a result of industrial activity may pose a serious threat to the environment.In this study,the potential of multi-walled carbon nano tubes(MWCNT)to remove Ni2?cations from aqueous solutions was investigated in a batch reactor under different experimental conditions.The effects on the removal process of conditions such as initial concentration of Ni2?ions, temperature,and adsorbent mass were investigated.Nickel uptake was quantitatively evaluated by use of the Langmuir,Freundlich,and Dubinin–Kaganer–Radushkevich isotherm models.For20mg/L initial Ni2?cation concentration,adsorption capacity increased from8.12to11.75mg/g when the temperature was increased from25to 65°C,an indication of the endothermic nature of adsorption process.In addition,the adsorption equilibrium was well described by the Langmuir isotherm model;max-imum adsorption capacity was17.86mg/g Ni2?cations on HNO3-treated MWCNT (t-MWCNT).The results obtained in this study show that adsorption of Ni2?on t-MWCNT is a spontaneous and endothermic process.By use of second-order kinetic constants and the Arrhenius equation,the activation energy of adsorption(E a)was determined as5.56kJ mol-1.

Keywords AdsorptionáMWCNTáNi2?áThermodynamicsáKinetics

Introduction

Removal of toxic metals from wastewater is of great interest in water pollution remediation.Numerous metals,for example chromium,mercury,lead,copper, cadmium,manganese,and nickel,are known to be highly toxic[1].A variety of I.Mobasherpour(&)áE.SalahiáM.Ebrahimi

Ceramics Department,Materials and Energy Research Center,P.O.Box31787-316,Karaj,Iran

e-mail:I.Mobasherpour@merc.ac.ir;Iman.Mobasherpour@https://www.360docs.net/doc/5b8053974.html,

2206I.Mobasherpour et al. methods have been used to remove heavy metals from wastewater,including: reduction and precipitation[2],coagulation and?otation[3],adsorption[4,5],ion exchange,membrane technology,and electrolysis[6].Generally,these methods are expensive or ineffective,especially when the metal concentration exceeds100ppm [7].

Nickel salts are commonly used in silver re?neries,in the storage battery industry,for electroplating,zinc base casting,and printing,and in the production of some alloys[8];all these activities discharge signi?cant amounts of nickel in various forms into the environment.At high concentrations,Ni2?causes lung,nose, and bone cancer,headache,dizziness,nausea and vomiting,chest pain,tightness of the chest,dry cough,and shortness of breath,rapid respiration,cyanosis,and extreme weakness[9].Hence,it is essential to remove Ni2?from industrial wastewaters before discharge into natural water systems used as sources of drinking water.

Among all the above methods,adsorption is effective and simple.Different kinds of adsorbent,including activated carbon[10],kaolinite[11],sugar beet pulp[12], activated carbon cloths[13],peanut hulls[14],modi?ed chitosan[15]granular biomass[16],and crab shell[17],have been used for removal of nickel ion. However,because these adsorbents have the disadvantages of low adsorption capacity or low removal ef?ciency,research is being conducted to investigate promising new adsorbents.

The discovery of carbon nano tubes(CNTs)by Iijima[18]has led to much interdisciplinary research.The advantages of CNTs are unique structural,electronic, optoelectronic,semiconductor,mechanical,chemical,and physical properties[19]. CNT have become available in macroscopic quantities and have the potential to substantially affect future nanoscience and nanotechnology[20–23].

CNTs have been proved to have great potential as superior adsorbents for removing many kinds of organic and inorganic pollutants,for example dioxins[24] and volatile organic compounds[25,26]from the atmosphere,and?uoride[27], 1,2-dichlorobenzene[28],trihalomethanes[29],soil organic matter[30],and a variety of divalent metal ions[31–33]from aqueous solutions.

The objective of this study was to investigate the possible use of multi-walled CNTs as an alternative adsorbent material for removal of Ni2?cations from aqueous solutions.The Langmuir,Freundlich,and Dubinin–Kaganer–Radushkevich(DKR) models were used to?t the equilibrium adsorption isotherms.The effects of initial metal ion concentration,adsorbent mass,contact time,and solution temperature on the dynamic behavior of adsorption were investigated.Thermodynamic data were also evaluated from the adsorption measurements.

Experimental

Preparation of the adsorbent

MWCNT prepared by a chemical vapor deposition(CVD)method in the Research Institute of the Iran Petroleum Industry were used as adsorbent without puri?cation.

Removal of divalent nickel cations2207 The purity of the MWCNT was[95%,the outer diameter was10–20nm,length was in the range5–15l m,and the amorphous carbon content was\5wt%.These data were provided by the https://www.360docs.net/doc/5b8053974.html,lipore deionized(DI)water was used for MWCNT sample washing and Ni2?solution preparation.Although the sorption capacity of metal ions by raw CNTs is very low,it increases substantially after oxidation with HNO3solution.This is possibly because the tips of the CNTs are opened and fractures occur at locations where defects such as pentagons and heptagons are present after oxidation.The oxidation process thus improves their dispersivity and results in a large increase in the amount of oxygen-containing functional groups,for example–COOH,–OH,and–C=O,on the surface of the CNTs.These functional groups lead to an increase in the amount of negative charge on the carbon surface and the cation-exchange capacity of the material is increased because the oxygen atoms in the functional groups donate single pairs of electrons to metal ions.

Four grams of raw MWCNT(r-MWCNT)was?rst treated with100mL HNO3(8 molar).This mixture was then sonicated for3h at40°C in an ultrasonic bath to introduce oxygen groups on to the MWCNT surface.After cooling to room temperature,the MWCNT were added dropwise to300mL cold DI water and?ltered through Whatman grade6?lter paper.The?ltered material was washed with DI water until the pH was neutral.The treated MWCNT(t-MWCNT)were dried in a vacuum oven at80°C for24h.

Transmission electron microscopy(TEM)was used to characterize the treated MWCNT.For this purpose,MWCNT were deposited on to Cu grids which support a carbon?lm containing pores.The particles were deposited on to the support grids from a dilute suspension in acetone or ethanol.The shapes and sizes were characterized by diffraction(amplitude)contrast and,by high-resolution(phase-contrast)imaging.The speci?c surface area was determined from the N2adsorption isotherm by the BET method,by use of a Micromeritics model ASAP2010surface area analyzer.The Fourier-transform infrared(FT-IR)spectra of the t-MWCNT were recorded by use of a Perkin–Elmer2000FT-IR spectrometer with a calibrated deuterated triglycine sulfate (DTGS)detector covering the frequency range500–4,000cm-1.The sample cell was purged with nitrogen gas throughout data collection to exclude carbon dioxide and water vapor.Ten milligrams of the dried samples were dispersed in200mg spectroscopic-grade KBr to record the spectra.

Sorption study

All sorption experiments were conducted without imposing any pre-equilibration processes during performance of any https://www.360docs.net/doc/5b8053974.html,ing a batch equilibration technique,the sorption capacity of MWCNT for Ni2?cations,and the effects of the initial concentration of Ni2?cations,adsorbent mass,contact time,and temperature, on sorption,were determined.

t-MWCNT(0.4g)was introduced to a stirred tank reactor containing500mL of the prepared solution.Aqueous solutions containing Ni2?cations of concentration 10,15,20,or30mg/L were prepared from hydrated nickel sulfate(NiSO4.7H2O; Merck no.6725).The temperature of the suspension was maintained constant at

25±1°C and the stirring speed was700rpm.Samples(5mL)were taken,for measurement of the residual metal ion concentration in the solution,after mixing of the adsorbent and the solution of Ni2?cations for0,5,10,20,30,60,or120min. After each speci?ed time,the sorbent was separated from the solution by centrifugation and?ltration through Whatman grade6?lter paper.The exact concentration of metal ions was determined by use of a GBC932Plus atomic absorption spectrophotometer.All experiments were performed in duplicate.

The mass balance of nickel is given by:

mq?V C0àC

eTe1Twhere m,q,V,C0,and C are,respectively,the mass of MWCNT(g),amount of nickel removed by unit of weight of MWCNT(uptake:mg Ni/g MWCNT),volume of nickel solution(L),initial concentration of nickel solution(mg Ni/L),and the concentration of nickel after adsorption for time t(mg Ni/L).After120min,C and q will reach equilibrium values C e and q e.

The percentage removal(%Ni removal)and distribution ratio(K d)were calculated by use of the equations:

%Ni removal?c0àc f

c0

?100e2T

where C0and C f are,respectively,the concentrations of the metal ion in the initial and?nal solutions(after120min),and

K d?amount of metal in adsorbent

?

V

e3T

where V is the volume of the solution(mL)and m is the mass of adsorbent(g).

Results and discussion

Characteristics of the adsorbent

The IR spectra of the r-MWCNT and t-MWCNT are presented in https://www.360docs.net/doc/5b8053974.html,pared with the r-MWCNT,the t-MWCNT had characteristic peaks at wavenumbers1580, 1700,and3425cm-1,which were assigned to hydroxyl groups or carbonyl groups, carboxyl groups,and hydroxyl groups[34],respectively.The increased amounts of these functional groups resulted in increased surface cation exchange and complexation capacity of the r-MWCNT[35].

The TEM micrograph of the MWCNT after modi?cation treatment is shown in Fig.2.The image revealed the t-MWCNT sheet is an entangled network.The MWCNT are highly?exible and not easily broken during oxidation,washing,and drying processes.The TEM image also shows the tube inner diameter is approximately5–10nm and the tube outer diameter is approximately10–20nm. BET analysis of the t-MWCNT sample con?rmed the product had a high surface area(speci?c surface area102m2/g).

2208I.Mobasherpour et al.

Fig.1Fourier-transform infrared spectra of the r-MWCNT (A )and t-MWCNT (B

)

Fig.2TEM micrograph of the t-MWCNT

Removal of divalent nickel cations 2209

Effects of initial Ni 2?concentration and amount of adsorbent

The effect of equilibration time on sorption of nickel by t-MWCNT in water was investigated for time periods from 5to 120min.As shown in Fig.3,removal of Ni 2?by t-MWCNT occurs in two distinct steps—a relatively fast phase (?rst 20min)followed by a slow increase until a state of equilibrium is reached (adsorption rate equal desorption rate and adsorbate concentration does not change with time).The time necessary to reach equilibrium is approximately 120min.Because increasing the time did not result in any noticeable effects,a contact time of 120min was chosen as the equilibration time.

Sorption of Ni 2?cations by t-MWCNT was studied for different initial concentrations of nickel from 10to 30mg/L,at pH 6.5,700rpm,and 120min contact time.The amount sorbed per gram increased as the initial concentration of Ni in solution increased.As shown in Fig.3,when the initial Ni 2?cation concentration was increased from 10to 30mg/L,the uptake capacity of t-MWCNT increased from 4.62to 10mg/g.A higher initial concentration provided an important driving force overcoming all mass-transfer resistance of the pollutant between the aqueous and solid phases,thus increased the uptake [36].

The effect of amount of t-MWCNT is depicted in Fig.4.Evidently,percentage removal increased with increasing mass of sorbent (Fig.4a)and uptake capacity for Ni 2?decreased from 13.75mg/g (27.5%removal)to 4.25mg/g (34%removal)with increasing t-MWCNT concentration from 0.2to 0.8g/L (Fig.4b).This

was Fig.3Effect of initial concentration on removal of Ni 2?by t-MWCNT sorbents (pH 6.5,amount of adsorbent 0.8g/L,stirring speed 700rpm)

2210I.Mobasherpour et al.

Fig.4Effect of amount of adsorbent on percentage removal (a )and uptake capacity (b )of Ni 2?by t-MWCNT (pH 6.5,initial metal concentration =20mg/L,stirring speed 700rpm)

Removal of divalent nickel cations 2211

attributed to the greater amount of sorbent,because the increased surface area provided more adsorption sites,resulting in greater removal of Ni 2?cations.Adsorption isotherms

Analysis of sorption equilibrium data is important for determining how the sorption process occurs;this helps in the design and development of technological devices for water treatment [37].Several isotherm equations,relying on different conceptual models (the Langmuir,Freundlich,and Dubinin–Kaganer–Radushkevich (DKR)models)have been used to describe adsorption equilibrium data.

Equilibrium data for metal ions over the concentration range 10to 30mg /L at 25°C were correlated with the linear version of the Langmuir adsorption isotherm [38]:

C e q e ?1Q 0K tC e Q 0e4T

where C e is the equilibrium concentration of metal in solution (mg/L),q e is the amount absorbed at equilibrium by t-MWCNT (mg/g),Q 0(mg/g),and K (L/mg)are Langmuir constants related to sorption capacity and sorption energy,respectively.Maximum sorption capacity (Q 0)represents monolayer coverage of the sorbent with sorbate and K represents the Gibbs free energy of sorption and should vary with temperature.A linear plot was obtained when C e /q e was plotted against C e over the entire concentration range of metal ions investigated.

The linear Langmuir adsorption isotherms for Ni 2?cations are given in Fig.5a.The Freundlich sorption isotherm,one of the most widely used models for mathematical description,usually ?ts experimental data over a wide range of concentrations.This isotherm gives an expression encompassing the surface hetero-geneity and exponential distribution of active sites and their energies.This adsorption isotherm was also applied to removal of Ni 2?cations from solution by t-MWCNT (Fig.5b).

ln q e ?ln k f t1n ln c e e5T

where q e is the amount of metal ion adsorbed at equilibrium per gram of adsorbent (mg/g),C e is the equilibrium concentration of metal ion in the solution (mg/L),and k f and n are the Freundlich model constants [39,40].k f and n ,were determined by plotting ln q e against ln C e .The numerical value of 1/n \1indicates that adsorption capacity is only slightly suppressed at lower equilibrium concentrations.This iso-therm does not predict any saturation of the sorbent by the sorbate;thus in?nite surface coverage is predicted mathematically,indicating multilayer adsorption on the surface [41].

The DKR has been used to describe the sorption of metal ions on clays.The DKR equation has the form:

Fig.5Linear ?ts of experimental data obtained by using the Langmuir (a ),Freundlich (b ),and DKR (c )sorption isotherms to model adsorption of Ni 2?by t-MWCNT c 2212I.Mobasherpour et al.

Removal of divalent nickel cations2213

ln C ads ?ln X m àbe 2e6T

where C ads is the number of metal ions adsorbed per unit weight of adsorbent (mol/g),X m (mol/g)is the maximum sorption capacity,b (mol 2/J 2)is the activity coef-?cient related to mean sorption energy,and e is the Polanyi potential,which is equal to:

e ?RT ln 1t1=C e eTe7T

where R is the gas constant (8.314kJ/mol.K)and T is the temperature (K).The saturation limit X m may represent the total speci?c micro pore volume of the sor-bent.The sorption potential is independent of the temperature but varies according to the nature of sorbent and sorbate [42].The slope of the plot of ln C ads against e 2gives b (mol 2/J 2)and the intercept yields the sorption capacity,X m (mol/g).The sorption space in the vicinity of a solid surface is characterized by a series of equipotential surfaces having the same sorption potential.The sorption energy can also be determined by use of the relationship:

E ?1=?????????à2b p :e8T

It is known that magnitude of apparent adsorption energy,E ,is useful for estimating the type of adsorption.If this value is below 8kJ/mol the adsorption type can be explained by physical adsorption,between 8and 16kJ/mol the adsorption type can be explained by ion exchange,and over 16kJ/mol the adsorption type can be explained by stronger chemical adsorption than ion exchange [43–45].The plot of ln C ads against e 2for metal ion sorption on t-MWCNT is shown in Fig.5c.The Langmuir,Freundlich,and DKR adsorption constants from the isotherms and their correlation coef?cients are presented in Table 1.

Adsorption isotherms for Ni(II)on t-MWCNT are shown in Fig.5.The experimental data are well ?tted by the three adsorption models.The values calculated from the three models are listed in Table 1.Previous studies have also shown that the Langmuir,Freundlich,and DKR isotherms simulate adsorption of some heavy metal ions well [46].The linear plot of C e /q e against C e with regression coef?cient R 2=0.967is shown in Fig.5a.From the slope of Fig.5a,the maximum Table 1Langmuir,Freundlich,

and DKR constants for

adsorption of Ni 2?by

t-MWCNT Q 0(mg/g)K (L/mg)R 2Langmuir adsorption isotherm constants

17.86

0.060.967k f (mg 1-n L n /g)n R 2

Freundlich adsorption isotherm constants

1.64

1.670.953X m (mg/g)b (mol 2/J 2)R 2

DKR adsorption isotherm constants

95.12-6910-90.961

2214

I.Mobasherpour et al.

adsorption capacity of t-MWCNT for Ni 2?was calculated to be 17.86mg/g under these experimental conditions.The linear plot of ln q e against ln C e with regression coef?cient R 2=0.953is shown in Fig.5b.The magnitude of the constant k f provides quantitative information on the adsorption af?nity for the adsorbed cations,and the magnitude of the constant n is an indicator of linearity of adsorption.Deviation of n from unity indicates that nonlinear adsorption occurring on heterogeneous surfaces.This behavior implies that the adsorption energy barrier increases exponentially as the fraction of ?lled sites on t-MWCNT increases.A linear plot of ln C ads against e 2with regression coef?cient R 2=0.961is shown in Fig.5c.The values of b and X m are evaluated from the slope and intercept,and are found to be -6910-9(mol 2/kJ 2)and 95.12(mg/g).The value of E (9.13kJ/mol)is within the range for ion exchange,therefore it is possible to say that the mechanism of adsorption of Ni(II)on t-MWCNT is ion-exchange.

It is interesting to note that the difference between q max (X m )derived from the Langmuir and DKR models is quite large.This difference may be attributed to the different de?nition of q max in the two models.In the Langmuir model,q max represents the maximum adsorption of metal ions at monolayer coverage whereas in the DKR model it represents the maximum adsorption of metal ions at the total speci?c micropore volume of the sorbent.Thus the value of q max derived from Langmuir model is lower than that derived from DKR model.Such differences have also been reported elsewhere [45,47].

Maximum adsorption capacities reported in the literature for adsorption of Ni 2?cations on different adsorbents are compared in Table 2with the value for the adsorbent in this study.Although direct comparison of t-MWCNT with other adsorbent materials is dif?cult,owing to the different experimental conditions,the maximum adsorption capacity of t-MWCNT was higher than for most the other adsorbents listed in Table 2.

Table 2Adsorption capacities

for sorption of Ni 2?by different

adsorbents Adsorbent Adsorption capacity (mg/g)

Ref.Chabazite 4.5

[48]Clinoptilolite 0.9

[48]Alternanthera philoxeroides biomass 9.73

[49]Waste of tea factory 18.42

[50]PAC 31.08

[51]Fly ash 0.03

[51]Bagasse 0.001

[51]Baker’s yeast 11.40

[52]Sheep manure waste 7.20

[53]Sphagnum moss peat 9.18

[54]Succinated alfalfa biomass 8.5

[55]Calcium-alginate 10.5

[56]Cone biomass of Thuja orientalis 12.42

[57]MWCNT 17.86This work

Removal of divalent nickel cations

2215

It is commonly believed that the chemical interaction between the metal ions and the surface functional groups of CNT is the major sorption mechanism [33].Protons in the carboxylic and phenolic groups of CNT exchange with the metal ions in the aqueous phase.The solution pH dropped after sorption of metal ions on to CNT reached equilibrium,which could be explained by release of H ?from the CNT surface where metal ions are sorbed,consequently reducing solution pH.The drop in pH increased with increasing initial metal ion concentration,which clearly indicates that sorption of more metal ion on to CNT causes release of more H ?ions from the surface sites of the CNTs into the solution.As shown in Fig.6.When the initial Ni 2?cations concentration was increased from 10to 30mg/L,the ?nal pH after 120min decreased from 6.2to 5.5.

Effect of temperature and determination of thermodynamic data

To study the effect of temperature on uptake of Ni 2?cations by t-MWCNT,temperatures of 25,45,and 65°C were selected.Figure 7a illustrates the relationship between amount of Ni 2?cations adsorbed by t-MWCNT after different contact times at different temperatures.For an initial concentration of 20mg/L,adsorption of Ni 2?cations on t-MWCNT increased from 8.12mg/g (32.5%removal)to 10.60mg/g (47%removal)at equilibrium time when the temperature was increased from 25to 65°C.This increase in the amount of metal sorbed with solution temperature indicates that removal of Ni 2?by t-MWCNT is enhanced

at Fig.6Effect of initial concentration on ?nal pH after removal Ni 2?by t-MWCNT sorbent (initial pH 6.5,amount of adsorbent =0.8gr/L,stirring speed 700rpm)

2216I.Mobasherpour et al.

Fig.7Uptake capacity of Ni 2?ions at different temperatures (a )and plot of ln K d against 1/T (b )(pH 6.5,initial metal concentration =20mg/L,amount of adsorbent =0.8gr/L,stirring speed 700rpm)

Removal of divalent nickel cations 2217

higher temperatures.This could be the result of an increase in the mobility of the Ni 2?cations with temperature.An increasing number of molecules could also acquire suf?cient energy to interact with active sites on the surface.Furthermore,increasing the temperature may cause a swelling effect within the internal structure of the t-MWCNT,enabling the large metal ions to penetrate further [58].

The thermodynamic data free energy change (D G °),enthalpy change (D H °),and entropy change (D S °)can be estimated from changes of equilibrium constants as a function of temperature.The free energy change for the sorption reaction is given by the equation:

D G ?àRT ln K d e10T

where D G °is the standard free energy change (J);R is the universal gas constant,

8.314J/mol K,and T is the absolute temperature (K).

D G ?D H àT D S e11T

The distribution ratio (K d )values increased with increasing temperature (Fig.7b),indicating the endothermic nature of adsorption.A plot of Gibbs free energy changes,D G °,against temperature,T (K);was found to be linear.Values of D H °and D S °were determined from the slope and intercept of the plot.Values of the Gibbs free energy change,D G °,are listed in Table 3.The enthalpy,D H °,and entropy,D S °,changes for the sorption process were calculated to be 12,822J/mol and 96.11J/mol K,respectively.The negative values of D G °at different temperatures indicated the spontaneous nature of the adsorption process.The positive value of D S °indicated there is an increase in the randomness of the system solid/solution interface during the adsorption process.In addition,the positive value of D H °indicated that adsorption was endothermic.The positive value of D S °re?ected the af?nity of the t-MWCNT for Ni 2?cations and suggested some structural changes in t-MWCNT

[59].

Sorption kinetics

Sorption kinetic studies were conducted to understand the kinetic behavior of t-MWCNT adsorbent toward Ni 2?.The sorption kinetics include two phases:a rapid metal sorption stage followed by a much slower stage before equilibrium is established.It was found that mass transfer is the key factor in metal sorption [60].The sorption kinetics describes the rate of metal sorption,which in turn de?nes the residence time of Ni in the solution of the batch reactor and,thus,the ef?ciency of sorption process.Of the several kinetic models available for examination of the mechanism controlling the sorption kinetic process and for testing the experimental Table 3Thermodynamic data

for the adsorption of Ni 2?by

t-MWCNT T (K)K d D G °(J/mol)D H °(J/mol)D S °(J/mol K)298

601.85-15856.51282296.11

318

799.18-17670.43381108.49-19701.12218

I.Mobasherpour et al.

data,the Lagrangian equation,or pseudo-?rst-order equation,and the pseudo-second-order equation were used to study the metal sorption kinetics of t-MWCNT.

The linear form of the pseudo-?rst-order equation is:

Log q eàq

eT?Log q e calà

k1

2:303

te12T

where q e is the metal sorbed at equilibrium(mg g-1),q is the amount of the metal adsorbed(mg g-1)at any time t,and k1is the?rst-order rate constant.The?rst-order rate constants k1and q were determined from the slopes and intercepts of plots of log(q e-q)against t for different concentrations of metal.

The linear form of the pseudo-second-order equation for the kinetics of absorption,described by Ho and Chiang[61],is:

t

?

1

k2q2

e cal

t

1

e cal

e13T

The second-order rate constant(k2)and q e cal were determined from the slope and intercept of the line obtained by plotting t/q t against t.

Linear plots of log(q e-q t)against t and t/q t against t for25,45,and65°C are depicted in Fig.8a,b,https://www.360docs.net/doc/5b8053974.html,parison of the results,with the correlation coef?cients,is shown in Table4.The pseudo-second-order kinetic model obtained for Ni2?sorption at different temperatures resulted in better correlation of the results than the pseudo-?rst-order equation model.The correlation coef?cients were high for the second-order kinetic model obtained for a concentration of20ppm at different temperatures.The values of k2at25,45,and65°C varied from0.0175to 0.0135min-1.The higher rate of metal sorption at the beginning(Fig.7a)could be because of the presence of active sites on the t-MWCNT,available for sorption of the metal.when the sorptive sites are exhausted,the rate of uptake may be controlled by rate of intra particle diffusion.The activation energy,E a,was determined by use of the Arrhenius equation[37]:

ln k ad?ln AàE a

RT

e14T

where k ad(k2)is the rate constant for metal adsorption,E a the activation energy in kJ mol-1,T the temperature in Kelvin,and R is the gas constant(8.314kJ mol-1K-1).When ln k ad is plotted against1/T,a straight line of slope-E a/R is obtained.The activation energy for adsorption of Ni2?by t-MWCNT was found as 5.56kJ mol-1from the slope of this plot.As is known when the rate is controlled by an intra-particle diffusion mechanism,the activation energy is low and,hence,it can be concluded that the process is controlled by intra-particle diffusion,which is a physical step in the adsorption process[58].

Conclusions

This investigation showed that t-MWCNT was an effective adsorbent for the removal of Ni2?cations from aqueous solutions.The adsorption process was a Removal of divalent nickel cations2219

Fig.8Linear ?t of the experimental data obtained by use of a pseudo-?rst-order kinetic model (a )and a pseudo-second-order kinetic model (b )(pH 6.5,initial metal concentration =20mg/L,amount of adsorbent =0.8gr/L,stirring speed 700rpm)

2220I.Mobasherpour et al.

function of the amount of adsorbent,initial concentration of Ni 2?cations,and temperature.The ef?ciency of adsorption of Ni 2?cations increased with increasing amount of adsorbent.The adsorption equilibrium was described well by the Langmuir isotherm model with maximum adsorption capacity of 17.86mg/g Ni 2?on t-MWCNT.Thermodynamic calculations showed that the process of nickel sorption on t-MWCNT was endothermic and spontaneous in nature.The kinetics of adsorption of the metal on the t-MWCNT were pseudo-second-order rather than pseudo-?rst-order.The second-order kinetic model was successfully applied to the experimental data,con?rming that adsorption was controlled by intra-particle diffusion.

Acknowledgments This research was completely supported by the Materials and Energy Research Center (MERC),Karaj,Iran,for which we are grateful.

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Table 4Comparison of the

?rst and second-order kinetic

rate constants and calculated

q e cal values obtained at different

temperatures (pH 6.5,initial

metal concentration =20mg/L,

amount of adsorbent =

0.8gr/L,stirring speed

700rpm)Pseudo-?rst-order kinetic model T (K)k 1(min -1)q e cal (mg/g)R 22980.069 6.8390.9213180.0698.3750.9413380.09411.0150.949Pseudo-second-order kinetic model

T (K)

k 2(min -1)q e cal (mg/g)R 2298

0.0178.6950.998318

0.01510.3090.9993380.01312.5000.997Removal of divalent nickel cations

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