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Chemical Engineering Journal 173 (2011) 855– 865

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

emoval of lead from water by amino modified multi-walled carbon nanotubes

oran D. Vukovic a,∗, Aleksandar D. Marinkovic b, Sreco D. Skapinc, Mirjana Ð. Ristic b, Radoslav Aleksic b,leksandra A. Peric-Grujic b, Petar S. Uskokovic b

Nanotechnology and Functional Materials Centre, Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, P.O. Box 3503, 11120 Belgrade, SerbiaFaculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, SerbiaJozef Stefan Institute, Ljubljana, Jamova 39, Sl-1001 Ljubljana, Slovenia

r t i c l e i n f o

rticle history:eceived 16 June 2011eceived in revised form 10 August 2011ccepted 13 August 2011

eywords:eadarbon nanotubesmino-functionalization

a b s t r a c t

Pristine, oxidized, ethylenediamine, diethylenetriamine and triethylenetetramine modified multi-walledcarbon nanotubes (raw-MWCNT, o-MWCNT, e-MWCNT, d-MWCNT and t-MWCNT, respectively) wereemployed as adsorbents in order to study individual and competitive adsorption characteristics of Pb2+

and Cd2+ ions. In batch tests, the influence of functionalization, pH, contact time, initial metal ion con-centration and temperature, on the ion adsorption on MWCNTs was studied. Adsorption of Pb2+ andCd2+ on MWCNTs strongly depends on pH. Time dependent Pb2+ adsorption and adsorption data can bedescribed by pseudo-second-order kinetic model and by Langmuir isotherm, respectively. The maximumadsorption capacities of Pb2+ and Cd2+ on d-MWCNT were 58.26 and 31.45 mg g−1 at 45 ◦C, respectively.

emovaldsorption mechanism

The competitive adsorption studies showed that the metal order affinity with respect to d-MWCNT ande-MWCNT is Pb2+ > Cd2+. Thermodynamic parameters showed that the adsorption of Pb2+ on appropriatenanotubes was spontaneous and endothermic. According to desorption studies, regenerated MWCNTcan be reused over five times with minimal loss of adsorption capacity. Comparison of obtained resultswith capacities and affinities of other adsorbents indicates suitability of amino-functionalized MWCNTapplication for removal of Pb2+ and Cd2+ from aqueous solution.

. Introduction

Carbon nanotubes (CNTs) have attracted enormous scientificttention due to their peculiar properties such as extraordi-ary electrical, mechanical, optical and chemical properties [1,2].ecause of so many outstanding performances, CNTs exhibit greatromise for potential applications in many technological fieldsuch as hydrogen storage [3], catalyst supports [4], chemical sen-ors [5] and nanoelectronic devices [6]. The known ability of CNTso establish �–� electrostatic interactions and their large surfacereas can facilitate the adsorption of many kinds of pollutantsrom water [7,8], such as aniline, phenol and their substitutes9], sodium chloride [10], endrin [11], as well as several divalent

etal ions [8,11–16]. Surface modifications of CNTs have beenpplied recently to enhance the dispersion property and adsorptionapacities of CNTs. Oxidation of CNTs have been widely reported8–17]. During oxidation the surface characteristics are altered

ue to the introduction of new functional groups (e.g., COOH, OH,

O, OSO3H, lactones) [8–17]. Many other functional groups couldlso be appropriate for metal ion adsorption. Amino-containing

∗ Corresponding author. Tel.: +381 11 3303659; fax: +381 11 3370387.E-mail address: goran.vukovic@tmf.bg.ac.rs (G.D. Vukovic).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.08.036

© 2011 Elsevier B.V. All rights reserved.

materials have attracted much attention because of their uniqueproperties derived from the presence of amino groups [18]. Espe-cially, amino functionalities play an important role in constructingcomplex structures in combination with other functional groups[19–22].

Lead, which is classified as prevalent toxic metal and majorenvironmental health problem, could enter the food chain throughdrinking water and crop irrigation. It can accumulate in bones,muscles, liver, kidney and brain. Excessive lead causes mentalretardation, kidney disease, anemia, severe damage to the ner-vous system, reproductive system, liver, brain and causes sickness,sterility, abortion, stillbirths, and neonatal deaths [23]. Accordingto US Environmental Protection Agency, the maximum contami-nant level for lead is 0.015 mg L−1 and the maximum contaminantlevel goal is zero [24].

In order to achieve this goal, since lead does not degrade inenvironment like some organic pollutants, many methods havebeen used to remove it from aqueous solutions. Adsorption is apromising process for the removal of metal ions from pollutedwater and wastewater, since it is a simple and economically fea-

sible method. Many adsorbents have been used for removal of leadions including carbon nanotubes [8,12,25–34] and different kindsof other carbon materials [23,35–43]. These adsorbents were usedin raw state or with modified surface. By reviewing the available

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56 G.D. Vukovic et al. / Chemical En

cientific literature wide dissipation of maximal capacity (from.66 [26] to 549.11 mg g−1 [37]) and affinity (from 0.16 [26] to9,726,392 L mol−1 [21]) of the adsorbents can be noted. In almostll reviewing papers, the discussion of the properties of the adsor-ents is primarily based on adsorption capacity. But if high waterurity is the goal, being the case for drinking water, adsorptionffinity is also an important criterion.

On the other hand, besides above mentioned properties, CNTsave been subject of considerable research because of the extraor-inary fast transport of water [44]. These properties have motivatedheir application for the development of novel CNT immobilizedomposite membranes for removal of divalent metal ions fromater. In such cases, CNTs play role of adsorption sites, which pro-

ided additional pathways for enhanced solute transport [45,46].hemical functionalization at the entrance to CNT cores affectshe selectivity of chemical transport across an aligned membranetructure [47]. Novel membranes based on the unique properties ofNTs may reduce significantly the energy and cost of desalination10,48]. These properties distinguish CNTs as adsorbent from the

ultitude of other adsorbents, although there are adsorbents withigher adsorption capacity and affinity.

The goals of this research were to evaluate the adsorptionehavior of Pb2+ and Cd2+ on amino functionalized MWCNT ando compare their performance with those of other adsorbents inerms of adsorption capacity and affinity. To achieve these goals,he influence of experimental conditions, such as type of function-lization, pH value, contact time, initial Pb2+ concentration andemperature, on the adsorption behavior was investigated. Thedsorption thermodynamics, kinetics and desorption processes onhe MWCNT were also studied. The adsorption of Pb2+ ions byristine, oxidized, ethylenediamine, diethylenetriamine and tri-thylenetetramine modified MWCNT (raw-MWCNT, o-MWCNT,-MWCNT, d-MWCNT and t-MWCNT respectively) were comparednd the adsorption mechanisms were considered. Also, influence ofhe amino functionality on the adsorption capacity and affinity wasiscussed. The o-MWCNT, e-MWCNT, d-MWCNT and t-MWCNTere selected in this study because they have acceptable biocom-atibility in vitro [17], as an important criterion for their practicalse as adsorbents for polluted water and wastewater treatment.

. Materials and methods

.1. Materials and reagents

Commercially available MWCNT (Sigma Aldrich, Serbia), pre-ared by a chemical vapor deposition (CVD) method, were selecteds adsorbents for this study. The length of the MWCNT wasetween 5 �m and 200 �m and the outer and inner diameters were0–30 nm and 5–10 nm, respectively. The purity of MWCNT wasore than 95%. Oxidation of pristine MWCNT (raw-MWCNT) was

resented in the literature [17,18]. The raw-MWCNT were soni-ated for 3 h at 40 ◦C in an ultrasonic bath with a (v/v, 3:1) mixturef concentrated H2SO4 and HNO3 to introduce oxygen containingunctional groups on the raw-MWCNT surface. Functionaliza-ion of oxidized MWCNT (o-MWCNT) by ethylenediamine (EDA),iethylenetriamine (DETA) and triethylenetetramine (TETA) (e-WCNT, d-MWCNT and t-MWCNT, respectively) was performed

hrough carboxylic acid activation followed by direct coupling withmines (the details of this method are described in our previ-us studies [17,18]). Analytical-grade lead nitrate and cadmium

itrate standards (Baker, Serbia) were employed to prepare a stockolution containing 1000 mg L−1 of metal ions, which was furtheriluted with deionized (DI) water to the required metal ion con-entration for the adsorption measurements.

ing Journal 173 (2011) 855– 865

2.2. Characterization of MWCNT

The BET specific surface area, pore specific volume and porediameter were measured by nitrogen adsorption/desorption at77.4 K using a Micromeritics ASAP 2020MP gas adsorption ana-lyzer. Field emission scanning electron microscopy (FE-SEM) wasperformed on a SUPRA 35 VP (Carl Zeiss, Germany) electron micro-scope. The pH values at the point of zero charge (pHPZC) of thesamples, i.e., the pH above which the total surface of the samplesis negatively charged, were measured using the pH drift method[14]. For this purpose, 50 mL of a 0.01 M NaCl solution was placedin a jacketed titration vessel, thermostatted at 25 ◦C, and N2 wasbubbled through the solution to stabilize the pH by preventingthe dissolution of CO2. The pH was then adjusted to successiveinitial values between 2 and 10, by adding either HCl or NaOHand the MWCNT (0.03 g) were added to the solution. The final pH,reached after 48 h, was measured and plotted against the initialpH. The pH at which the curve crosses the line pH (final) = pH (ini-tial) is taken as the pHPZC of the appropriate sample. Transmissionelectron microscopy (TEM) analysis was performed on a TECNAI-FEG F20 electron microscope (FEI Company, USA) at 200 kV. X-raydiffraction (XRD) data were obtained using a BRUKER D8 ADVANCE(Bruker AXS, Germany) with Vario 1 focusing primary monochro-mator (Cu k�1 radiation, � = 1.54059 A). Fourier-transform infrared(FTIR) spectra were recorded in the transmission mode using aBOMEM (Hartmann & Braun) spectrometer. FTIR spectra of thesamples were obtained in the form of KBr disk.

The coordination number (CN) can be obtained from the rela-tionship between the concentration of amine groups (DAKaiser –degree of amination obtained by Kaiser test [17]) and maximumadsorption capacity [49]. Coordination number refers to the num-ber of ligand atoms surrounding the central atom. CN can be veryuseful to understand the interaction between amino functional-ized MWCNT and adsorbed metal ions, as well as the differencesbetween the two kinds of prepared ligands with respect to adsorp-tion capacities. CN was calculated according to Eq. (1):

CN = DAKaiser

qmax/M(M2+)(1)

where DAKaiser is the concentration of amine groups obtained byKaiser test (mmol g−1), qmax is the maximum adsorption capac-ity (mg g−1) obtained by Langmuir model and M(M2+) is the molarmass of the metal ion studied (mg mmol−1).

2.3. Adsorption experiments

All batch adsorption experiments were carried out using 10 mLpolyethylene bottles with addition of 1 mg of MWCNT and 10 mLof Pb2+ and Cd2+ aqueous solution with the desired concentrationand appropriate pH. The bottles were placed in an ultrasonic bath,which was operated at defined temperatures and times. The tem-perature in a ultrasonic bath was maintained using a recirculatingwater system. The aqueous samples were filtered through a 0.2 �mPTFE membrane filter and the concentrations of metal ions in fil-trate were analyzed using the inductively coupled plasma massspectrometry technique (ICP MS).

In order to evaluate the effect of pH on Pb2+ and Cd2+ adsorp-tion, the initial pH values of the solutions were varied between2.0 and 11.0 by adjustment with appropriate concentration ofNaOH and HNO3, at 25 ◦C. The optimum pH was then deter-mined as 6.2 for Pb2+ and used throughout all the adsorptionexperiments. The effect of MWCNT-Pb2+ contact time was exam-

ined in the range of 5–200 min. Adsorption isotherm experimentswere performed with solutions of different initial lead concentra-tions (C0). The range of concentration of lead solution preparedfrom stock solution was varied between 5 and 100 mg L−1. The

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G.D. Vukovic et al. / Chemical En

dsorption thermodynamic experiments were performed at 25, 35nd 45 ◦C. To study the effect of pH on competitive adsorption thenitial concentration of 5 mg L−1 of Pb2+ and Cd2+ solution was usednd pH was varied 2.0 and 11.0. The amount of adsorbed metalons was determined by the difference between the initial and thequilibrium concentration. The results of metal ions adsorptionn polyethylene test tube wall and filters showed that metal iondsorption on this material was negligible.

The data analysis was realized using a normalized standard devi-tion �q (%) calculated using the following equation:

q(%) =

√∑ [(qexp − qcal)/qexp

](N − 1)

2

× 100 (2)

here qexp and qcal are the experimental and calculated amountsf metal ions adsorbed on the MWCNT and N is the number of dataoints. All the experiments were performed in triplicate and onlyhe mean values are reported. The maximum deviation was <3%experimental error). All calculated (estimated) standard errors ofhe isotherm and thermodynamic parameters were determined byommercial software (Microcal Origin 7.0) with a linear regressionrogram. Measurements of Pb2+ and Cd2+ concentrations were real-

zed using an Agilent Technologies 7500ce ICP-MS system (Agilentechnologies, Inc., USA). Standard optimization procedures and cri-eria specified in the manufacturer’s manual were followed. Theetection limit of the method was 4.0 × 10−5 mg L−1 of Pb2+ andd2+ [17].

.4. Desorption and reusability studies

To evaluate the regeneration capacity of MWCNT, adsorp-ion experiments were performed at initial Pb2+ concentrationf 3 mg L−1, as it is described in Section 2.3. After equilibration,WCNT were dried at 60 ◦C for 2 h and then dispersed in DI water

t different pH values (from 1.5 to 6), adjusted using 0.1 and.01 mol L−1 HNO3. The amount of desorbed Pb2+ was measuredfter ultrasound treatment of Pb2+ loaded MWCNT and filtration.fter the adsorbent has been regenerated, it was rinsed withI water and used in subsequent adsorption experiments. Thedsorption–desorption processes have been investigated by fiveime cycles.

. Results and discussion

.1. MWCNT characterization

The detail characterization of raw-MWCNT, o-MWCNT, e-WCNT, d-MWCNT and t-MWCNT using FTIR, TGA and elemental

nalysis techniques has been previously presented [17,18]. Theunctional groups (carboxyls, lactones, phenols, amino) on the sur-aces of raw-MWCNT, o-MWCNT and e-MWCNT (Table S1) wereuantitatively determined by the Boehm method and quantita-ive Kaiser test [17]. Total basic sites and available amino groupsn d-MWCNT and t-MWCNT were defined by Boehm method andAKaiser, respectively (Table S1). Surface modification of o-MWCNTy DETA and TETA was found to increase surface basicity of d-WCNT and t-MWCNT. All of these groups introduced on surface

f the MWCNT can provide numerous adsorption sites and therebyncrease their adsorption capacities.

Morphology of the samples was studied by FE-SEM and rep-esentative images are shown in Fig. S1a–c. It is found that-MWCNT adhere more than e-MWCNT and d-MWCNT, on that

ay inter-space between o-MWCNT is significantly reduced. The

hange of microstructure of MWCNT during modification is ingreement with results obtained by BET method (Table S2). Theurface area, pore volume and average pore diameter increase in

ing Journal 173 (2011) 855– 865 857

order o-MWCNT, e-MWCNT, d-MWCNT and t-MWCNT (Table S2).This could be explained by inter-particle repulsions among aminogroups resulted in smaller-sized “globs” of e-MWCNT, d-MWCNTand t-MWCNT or the additional ultrasound treatment used duringamino-functionalization resulted in smaller aggregates of amino-functionalized than of oxidized MWCNT. Longer chain of amineprovides larger surface area, pore volume and average pore diam-eter of the sample. The presence of the functional groups causeschange in pHPZC of the samples (Fig. S1d). The decrease in thepHPZC of the o-MWCNT, compared to the raw-MWCNT, is a resultof the introduction of acidic oxygen-containing functional groups[14]. The amino groups on MWCNT contribute to increased surfacebasic properties and, thus, the pHPZC of e-MWCNT (5.91), d-MWCNT(5.64) and t-MWCNT (5.52 – the result is not presented in Fig. S1d)are higher than those of the raw-MWCNT and the o-MWCNT. Froman electrostatic interaction point of view, adsorption of divalentmetal ions onto MWCNT is favored at pH values greater than thepHPZC, since the surface of the MWCNT became more negativelycharged. Moreover, it has been shown that functionalized MWCNTare of acceptable biocompatibility in vitro since they are not cyto-toxic even at high concentrations of 50 mg mL−1 [17,18]. This resultindicates possibility of safe use of the functionalized MWCNT asadsorbents in polluted water and wastewater treatment.

The morphology and structure of MWCNT materials were inves-tigated by TEM, and images are shown in Fig. S2. RepresentativeTEM images of raw-MWCNT, o-MWCNT, e-MWCNT and d-MWCNTshow that the surface of MWCNT is smooth and clean, and noobvious change of the surface structure of MWCNT after oxida-tion and amino functionalization was observed. Typical diameterof MWCNT was estimated to be in the range of 20–30 nm. TheMWCNT materials mainly consist of curved shapes or tube bend-ing due to structural defects which are expected to provide activesites for adsorption [15]. Also, presence of functional groups atthe surface of MWCNT significantly influences their dispersibil-ity, providing excellent dispersion stability of modified MWCNT.The pristine MWCNT had a strong tendency to agglomerate dueto their nano size and high surface energy, thus poor dispersionin water was observed (Table S2). However, oxidation introducespolar (hydrophilic) groups on the o-MWCNT surface and therefore,could contribute to electrostatic stability, e.g., different attractiveinteraction with surrounding water molecules (hydrogen bonding,ionic, dipole–dipole interaction etc.) provide a long time stabledispersion in water. The absorbance of the amino-functionalizedMWCNT dispersions in water (Table S2) was different dependingon the structure of the amines presented onto the MWCNT surface.MWCNT modified by DETA and TETA showed lower absorbancein water than e-MWCNT due to higher potential to create hydro-gen bonds between amine functionalities and to form aggregates.Improved dispersibility in water, after modification of pristineMWCNT, indicates suitability for their application as adsorbentsfor polluted water or wastewater treatment.

3.2. Effect of pH

Effect of pH on adsorption of Pb2+ on raw-MWCNT, o-MWCNT,e-MWCNT and d-MWCNT was presented in Fig. 1. It is clear thatpH of solution plays an important role on Pb2+ adsorption char-acteristics on MWCNT. The removal of Pb2+ increases quickly atpH 5–6, decreases slowly in pH range of 6–8, and then decreasessteeply at pH 8–10. It is known that lead species in water solu-tion could be present in the forms of Pb2+, Pb(OH)+, Pb(OH)2 andPb(OH)3

− at different pH values. Equilibrium concentrations of Pb2+

ionic species, at different pH, could be calculated from appropriateconstants (log K) for hydrolysis reactions [12] at 25 ◦C (Table S3).Calculated distribution of Pb2+ species as a function of pH [12],based on the equilibrium constants, is a helpful basis for discus-

858 G.D. Vukovic et al. / Chemical Engineering Journal 173 (2011) 855– 865

Fig. 1. Effect of pH on adsorption of Pb2+ on the raw-MWCNT, o-MWCNT, e-MWCNTand d-MWCNT (in mg g−1, left ordinate). (C[Pb2+]0 = 5 mg L−1, m/V = 100 mg L−1,To

sPcPsr

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equation (Eq. (3)), showing good agreement of the qe values(Table 1) with the results of experimental work (Figs. 1 and 3).

= 25 ◦C). Effect of pH on Pb(OH)2 precipitation (in % of overall available lead, rightrdinate).

ion of the adsorption mechanism. The precipitation constant ofb(OH)2(s) is 1.2 × 10−15, and the precipitation curve of lead at theoncentration of 5 mg L−1 was also shown in Fig. 1. Precipitatedb(OH)2 at pH higher than 8 was determined experimentally andubtracted from the overall available amount of Pb2+ ions, and thuseliable values of adsorbed Pb2+ were obtained.

Significant dependence of Pb2+ adsorption on o-MWCNT, e-WCNT and d-MWCNT and the low adsorption on raw-MWCNT, at

ifferent pH, could be observed (Fig. 1). At pH < 7, the dominant leadpecies is Pb2+, therefore, the low Pb2+ adsorption at low pH can bettributed mostly to the competition between H+ and Pb2+ ions12]. A pH higher than 3 is beneficial for the ionization of the sur-ace acidic groups, such as carboxylic groups (pKa 3–6), which play

significant role in the uptake of Pb2+ ions. The negative chargesenerated at the nanotube surface at pH > pHPZC (2.43) enlarged theation-exchange capacity of o-MWCNT and, also, the electrostaticttraction became more important [13]. A decrease of o-MWCNT,-MWCNT and d-MWCNT adsorption capacity at pH values higherhan 8 is in agreement with the decrease of the Pb2+ concentrationnd an increase of the concentration of ionic species which have aower affinity toward negatively charged adsorbent surface.

The results have shown that less than 25% of Pb2+ is adsorbedn o-MWCNT, e-MWCNT and d-MWCNT at pH < 5 (Fig. 1), stronglyuggesting that o-MWCNT, e-MWCNT, and d-MWCNT are notuitable for Pb2+ removal at low pH. However, 50–75% of Pb2+

s removed from the solution by o-MWCNT, e-MWCNT and d-WCNT at pH 6–7 and C[Pb2+]0 = 5 mg L−1, indicating that theaximum adsorption capacity is at least 3 times higher than that at

H < 5. The optimum pH was determined as 6.2 for Pb2+ and usedhroughout all the adsorption experiments. In the literature, theptimum pH for removal of Pb2+ varied in the range from 4 [32] to.4 [12]. Considering the low o-MWCNT, e-MWCNT and d-MWCNTmounts and the high removal percent at pH 6–7, o-MWCNT, e-WCNT and d-MWCNT could be used as a suitable adsorbent for

eparation of Pb2+ ions from polluted water or wastewater.Additional experiments were performed with Cd2+ to check the

otential of d-MWCNT for removal of divalent metal ions from pol-uted water. Effect of pH on adsorption of Cd2+ on raw-MWCNT,-MWCNT and e-MWCNT was presented in Fig. 2 and it was dis-ussed in detail in our previous study [17]. It can be noticed that-MWCNT shows best sorption capacities in the pH range of 7–9, atH > pHPZC, which indicates that deprotonated amino groups haveain contribution to the sorption of Cd2+. Two amino groups, pri-

ary and secondary in DETA residue and one more secondary group

n TETA could create more favorable coordination and electrostaticnteractions with Pb2+ and Cd2+ cation.

Fig. 2. Effect of pH on adsorption of Cd2+ on the raw-MWCNT, o-MWCNT, e-MWCNTand d-MWCNT (C[Cd2+]0 = 5 mg L−1, m/V = 100 mg L−1, T = 25 ◦C).

In order to study the influence of the amino functionalization onadsorption properties, the effect of pH on Pb2+ and Cd2+ adsorptionby t-MWCNT was examined. Adsorption capacities of t-MWCNTwere similar to d-MWCNT, in pH range of 2–11, (higher capaci-ties around 15–20%) indicating that the introduction of additionalsecondary amino group has considerable influence on t-MWCNTsorption capacity (data not presented).

3.3. Kinetic studies

The removal of Pb2+ ions from aqueous solution by raw-MWCNT, o-MWCNT, e-MWCNT and d-MWCNT at pH 6.2 as afunction of contact time showed that adsorption of Pb2+ on MWC-NTs is fast process and 90 min was sufficient for the adsorptionequilibrium to be achieved (Fig. 3). Kinetic experiment of Cd2+

removal on d-MWCNT was carried out as it was recently described[17].

The pseudo-first and pseudo-second-order rate adsorptionkinetic models were used in this study [50]. Analyzing the regres-sion coefficients (r), �q values and the calculated standard errorsof the parameters for both models, the experimentally obtainedkinetic data could be better fitted by a pseudo-second-order rate

Fig. 3. Effect of time on the adsorption of Pb2+ by raw-MWCNT, o-MWCNT, e-MWCNT and d-MWCNT (C[Pb2+]0 = 5 mg L−1, m/V = 100 mg L−1, pH 6.2, T = 25 ◦C).Lines: pseudo-second-order kinetics model.

G.D. Vukovic et al. / Chemical Engineering Journal 173 (2011) 855– 865 859

Table 1Kinetic parameters of the pseudo-second-order equation for Pb2+ and Cd2+ adsorption on raw-MWCNT, o-MWCNT, e-MWCNT and d-MWCNT.

qe (mg g−1) K′ (g mg−1 min−1) ×102 �q (%) r

Pb2+

Raw-MWCNT 1.61 ± 0.06 2.87 ± 0.69 3.01 0.971o-MWCNT 25.64 ± 0.49 1.18 ± 0.14 2.11 0.989e-MWCNT 29.41 ± 1.02 1.33 ± 0.16 2.19 0.981d-MWCNT 38.76 ± 1.41 1.94 ± 0.18 3.01 0.983Cd2+

a .19 ±

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tdi

q

q

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spatial structure and flexibility of the aminoalkyl chain. Afterwards,incoming metal cation transport is suppressed by electrostatic andsteric repulsion of metal cation loaded aminoalkyl residue.

e-MWCNT 21.23 ± 0.18 3d-MWCNT 27.17 ± 0.41 3

a Ref. [17].

eparation of the variables in the differential form of the pseudo-econd-order equation and integration gives [50]:

t

qt= 1

K ′q2e

+(

1qe

)t (3)

here qe and qt are the amounts of metal ion adsorbed (mg g−1)t equilibrium and at time t, respectively. K′ (g mg−1 min−1) is theseudo-second-order rate constant of adsorption.

Values of qe, K′, and r are calculated from the line plots of t/qt

ersus t and are listed in Table 1. The confirmation of pseudo-econd-order kinetics, which is common for the removal of metalsy carbonaceous materials [15], indicates that the concentrationsf both sorbate (Pb) and adsorbent (raw-MWCNT, o-MWCNT, e-WCNT and d-MWCNT) are involved in the rate determining step

f the adsorption process [51]. Considering the values of K′ constant,t could be concluded that faster equilibrium was achieved in thease of Pb2+ adsorption onto raw MWCNT. The slower adsorptionates on o-MWCNT, e-MWCNT and d-MWCNT indicate that pro-esses with higher energetic barrier [12], such as chemisorptionnd/or surface complexation, are operative.

.4. Adsorption isotherms

Two adsorption models have been used to describe adsorp-ion characteristics of raw-MWCNT, o-MWCNT, e-MWCNT and-MWCNT, namely, the Langmuir (Eq. (4)) and Freundlich (Eq. (5)),

n their linearized forms:

e = bqmaxCe

1 + bCeor

Ce

qe= 1

(bqmax) + Ce/qmax(4)

e = kf Cne or log qe = log kf + nlog Ce (5)

here Ce is the equilibrium concentration of metal ions remainingn the solution (mol L−1); qe is the amount of metal ions adsorbeder weight unit of solid after equilibrium (mol g−1); qmax and bre Langmuir constants related to the adsorption capacity anddsorption affinity, respectively. The maximum adsorption capac-ty qmax is the amount of adsorbate at complete monolayer coveragemol g−1), and b (L mol−1) is a constant relating to the heat ofdsorption. The value of kf (mol1−n Ln g−1) represents the adsorp-ion capacity when the equilibrium metal ion concentration equalso 1 and n represents the degree of dependence of the adsorptionn the equilibrium concentration.

The Langmuir and Freundlich adsorption isotherms are pre-ented in Fig. 4. Isotherm parameters were obtained by fitting thedsorption equilibrium data to the isotherm models, and are listedn Table 2. It can be noticed that the r values for the Langmuir modelre higher, thus indicating that this model better describes adsorp-ion on MWCNT. For all investigated adsorbents, both qmax and b

alues increase with increasing temperature, while the standardrrors of these parameters remain similar. Moreover, these valuesndicate that the best adsorption capacity for Pb2+ was achieved

ith d-MWCNT at increased temperatures, suggesting possible use

0.11 1.98 0.9970.18 2.01 0.987

of functionalized MWCNT for the removal of Pb2+ ions from pol-luted water or wastewater at higher temperatures.

Adsorption isotherms for the removal of Cd2+ by raw-MWCNT,o-MWCNT and e-MWCNT were presented in our previous study[17]. Langmuir isotherm model also describes adsorption of Cd2+

on d-MWCNT and maximum capacity of 31.45 mg g−1 was obtainedat 45 ◦C (data not presented). The maximum sorption capacitiesfor the removal of Pb2+ and Cd2+ by t-MWCNT, calculated fromthe Langmuir isotherm, were 15–20% higher than ones obtainedfor d-MWCNT (data not presented). These results indicate lowercoordination capability of secondary amino group. Due to higheraffinity and availability of primary amino group with respect tometal cation, in the initial adsorption step metal cation/amino bondis created involving mainly primary amino group causing change of

Fig. 4. (a) Adsorption isotherms of Pb2+ on the raw-MWCNT and o-MWCNT at 25,35 and 45 ◦C. (b) Adsorption isotherms of Pb2+ on the e-MWCNT and d-MWCNT at25, 35 and 45 ◦C (m/V = 100 mg L−1, pH 6.2, solid lines: Langmuir model, dot lines:Freundlich model).

860 G.D. Vukovic et al. / Chemical Engineering Journal 173 (2011) 855– 865

Table 2Langmuir and Freundlich isotherm parameters for Pb2+ adsorption on raw-MWCNT, o-MWCNT, e-MWCNT and d-MWCNT.

T (◦C) Langmuir parameters Freundlich parameters

qmax (mg g−1) b (L mol−1) �q (%) r kf (mol1−n Ln g−1) 104 n �q (%) r

Raw-MWCNT25 2.94 ± 0.03 10,925 ± 274 3.27 0.991 7.40 ± 0.21 0.449 ± 0.013 10.11 0.98035 4.16 ± 0.05 11,250 ± 295 4.60 0.993 5.68 ± 0.11 0.446 ± 0.012 13.87 0.98645 5.21 ± 0.09 11,597 ± 303 3.03 0.989 4.50 ± 0.13 0.460 ± 0.014 11.93 0.975o-MWCNT25 37.36 ± 1.39 299,810 ± 8124 3.81 0.994 3.64 ± 0.09 0.085 ± 0.003 9.73 0.88135 39.37 ± 1.58 315,173 ± 10,987 4.10 0.992 3.80 ± 0.11 0.084 ± 0.003 9.78 0.88045 40.79 ± 1.67 317,525 ± 11,117 2.30 0.995 3.68 ± 0.08 0.076 ± 0.001 8.65 0.899e-MWCNT25 40.12 ± 1.51 303,805 ± 8765 2.79 0.991 3.70 ± 0.07 0.079 ± 0.001 9.28 0.88535 42.22 ± 1.29 333,891 ± 9354 1.66 0.997 3.68 ± 0.11 0.072 ± 0.002 8.83 0.88745 44.19 ± 1.63 375,080 ± 11,250 1.26 0.992 3.70 ± 0.10 0.067 ± 0.001 5.37 0.893d-MWCNT

0.980.98

98

3

oV

�

l

wsdovufitna

MvsvaddPttccsp

ipceosietti

changes in the contribution of appropriate adsorption mechanismsto the overall process: the co-existence of physisorption, i.e., ionexchange, electrostatic attraction and chemisorption, i.e., surface

25 54.27 ± 1.67 353,527 ± 10,997 1.99

35 56.35 ± 1.78 364,089 ± 11,157 2.53

45 58.26 ± 1.99 386,565 ± 12,157 1.97 09

.5. Thermodynamic of adsorption processes

The Gibbs free energy (�G0), enthalpy (�H0) and entropy (�S0)f the adsorption processes were calculated using the followingan’t Hoff thermodynamic equations:

G0 = −RT ln(55.5b) (6)

n(55.5b) = �S0

R− �H0

RT(7)

here T is the temperature in K and R is the universal gas con-tant (8.314 J mol−1 K−1). The Langmuir adsorption constant b waserived from the isotherm experiments. �H0 and �S0 can bebtained from the slope and intercept of the linear plots of ln(55.5b)ersus T−1, respectively, assuming the adsorption kinetics to bender steady-state conditions. Well fitting of the data was con-rmed by the high r values and low standard errors of the estimatedhermodynamic parameters (Table 3). The calculated thermody-amic values (Table 3) gives some information concerning thedsorption mechanism for the studied carbon nanotubes.

The negative values of �G0 indicate that Pb2+ adsorption on allWCNT is a spontaneous process. It is noticeable that the �G0

alues decrease with increasing temperature, indicating higherpontaneity at higher temperatures. The lowest and similar �G0

alues were obtained for Pb2+ adsorption on o-, e- and d-MWCNTt 45 ◦C. At higher temperatures Pb2+ ions are readily desolvated, itsiffusion through the medium and within the pores (intra-particleiffusion) are faster processes contributing to higher probability ofb2+ adsorption. It was found that free energy change for physisorp-ion is generally between −20 and 0 kJ mol−1, the physisorptionogether with chemisorption within −20 to −80 kJ mol−1, and purehemisorption in the range of −80 to −400 kJ mol−1 [19]. The cal-ulated �G0 values suggest that the sorption processes of Pb2+ ontudied adsorbents could be considered as contributions of bothhysisorption and chemisorption processes.

The positive values of �H0 show that Pb2+ adsorption on stud-ed MWCNTs is an endothermic process, and thus better adsorbentroperties, at higher temperature, were obtained. Except notifi-ation about endothermicity of adsorption processes, a positiventropy change indicates feasible adsorption. The positive valuesf �S0 indicate a tendency to higher randomness of the studiedystem at equilibrium or such structural changes at the surfacenterface of Pb2+ loaded MWCNT which could contribute to positive

ntropy change. Definite degree of orderliness could be expected athe Pb2+/MWCNT interface, but regardless on that overall adsorp-ion process is entropy driven. In addition, some processes, such ason-exchange, could also contribute to positive entropy change.

9 5.12 ± 0.09 0.081 ± 0.002 10.01 0.8817 5.09 ± 0.13 0.076 ± 0.001 9.54 0.815

5.34 ± 1.15 0.077 ± 0.001 9.78 0.856

3.6. Effect of pH on the competitive adsorption of Pb2+ and Cd2+

Competitive adsorption study was performed in order to deter-mine affinity of appropriate ions with respect to specific adsorbent.Obtained results could be helpful for adsorbent design used forspecific water and wastewater treatment. The effect of pH onthe competitive adsorption of Pb2+ and Cd2+ by e-MWCNT andd-MWCNT is shown in Fig. 5. MWCNT modified by DETA showshigher capacities than e-MWCNT, 24.2 and 17.1 mg g−1 for Pb2+,as well 15.2 and 12.3 mg g−1 for Cd2+, respectively. The competi-tive adsorption study and calculated capacities of d-MWCNT ande-MWCNT showed an affinity order Pb2+ > Cd2+. Similarly, Li et al.[30] showed that the affinity order of three metal ions adsorbed byoxidized MWCNT was Pb2+ > Cu2+ > Cd2+.

3.7. Mechanisms of Pb2+ adsorption by MWCNT

Oxygen-containing functional groups, produced by oxidationof raw-MWCNT, are important sites for the removal of Pb2+ fromsolution with o-MWCNT [12]. The o-MWCNT and amino function-alized MWCNT adsorption capacities increased in the temperaturerange of 25–45 ◦C, indicating a complex adsorption processes and

Fig. 5. Effect of pH on the competitive adsorption of Pb2+, and Cd2+ ions onto d-MWCNT and e-MWCNT (C[Pb2+]0 = 5 mg L−1, C[Cd2+]0 = 5 mg L−1 m/V = 100 mg L−1,T = 25 ◦C).

G.D. Vukovic et al. / Chemical Engineering Journal 173 (2011) 855– 865 861

Table 3Thermodynamic parameters for Pb2+ adsorption onto raw-MWCNT, o-MWCNT, e-MWCNT and d-MWCNT.

T (◦C) Thermodynamic parameters

�G0 (kJ mol−1) �H0 (kJ mol−1) �S0 (J mol−1 K−1) r

Raw-MWCNT25 −33.01 ± 0.29 2.35 ± 0.08 118.59 ± 2.11 0.99535 −34.19 ± 0.2745 −35.38 ± 0.31o-MWCNT25 −41.22 ± 0.33 2.38 ± 0.07 145.94 ± 3.19 0.95735 −42.73 ± 0.3945 −44.13 ± 0.27e-MWCNT25 −41.25 ± 0.31 8.30 ± 0.25 166.14 ± 3.05 0.99435 −42.87 ± 0.3645 −44.57 ± 0.34d-MWCNT

0.33

ct

cbie([

2

dar

wseSrcoeteftias

issC

P

o

P

olryof

25 −41.62 ± 0.33 9.32 ±35 −43.10 ± 0.3745 −44.65 ± 0.41

omplexation. However, amino groups, also, have significant con-ribution to adsorption capacity of amino functionalized MWCNT.

Weak proton-accepting capability of raw-MWCNT has beenonfirmed, i.e., hydrogen bonding interaction of �-electron of theasal plane with water release hydroxide ions [17,52], causing low

ncrease of solution pH. Analogously, Pb2+ could be attracted by �-lectron densities of the graphene structure, according to reaction8), indicating a competition between H+ and Pb2+ ions at lower pH12,52]:

(MWCNT�-H+) + Pb2+ � (MWCNT�)2-Pb2+ + 2H+ (8)

The pH of the solution, at adsorption equilibrium is slightlyecreased, thus indicating that Pb2+/hydrogen exchange hasppropriate contribution to adsorption process with respect toaw-MWCNT.

It was shown in previous studies [8,17] that adsorption capacityas not in direct correlation with MWCNT physical characteristic:

pecific surface area, pore specific volume and mean pore diam-ter, but total surface acidity is a factor of primary significance.ignificantly higher adsorption capacity of o-MWCNT, compared toaw-MWCNT, indicates utmost significance of introduced oxygen-ontaining groups on o-MWCNT adsorption capacity. Ionization ofxygen-containing functional groups (carboxylic, phenol, lactonestc.) increases with the increase of pH, i.e., giving raise to a contribu-ion of negatively charged group (carboxylate and phenolate anion,tc.). These groups provide adsorption sites for Pb2+ uptake at dif-erent extent. It could be supposed that fraction of Pb2+ may entero the inner channel of oxidized MWCNT, at lower velocity, to formrreversible adsorption fractions [12]. This contribution to overalldsorption process seems to be negligible according to desorptiontudy.

Moreover, acidic oxygen-containing groups might behave ason-exchange sites for the retention of Pb2+ creating metal ligandurface complexes [52]. Generally, the adsorption of Pb2+ onto theurface of o-MWCNT having polar functional groups (P) (COOH,

O, OSO3H and OH) could be presented as [51]:

b2+ + 2(MWCNT-P−) � Pb(MWCNT-P)2 (9)

r

b2+ + 2(MWCNT-HP) � Pb(MWCNT-P)2 + 2H+ (10)

r via hydrogen bonding between the surface functional groups andead cations [52,53]. It was shown that carboxyl groups play a key

ole for Pb2+ adsorption on the o-MWCNT [12,53]. Detailed anal-sis of adsorbed forms of Pb2+ on acidified MWCNT was carriedut by XPS techniques [54]. According to the character of sur-ace bonding a description of possible adsorption reactions of Pb2+

171.33 ± 3.98 0.983

with present MWCNT functionality was given [54]. It was provedthat overall adsorption process consisted of two contributions: thespecific surface area and functional groups, and methodology fortheir quantitative determination was presented [54]. Followingsuggested methodology, inorganic deposition of PbO, PbCO3, andPb(OH)2 for all adsorbents, considered as contribution of specificsurface area, showed negligible value <1.5% of overall Pb2+ adsorp-tion at pH 6.2, and it was not included in forthcoming discussion.

Moreover, in relation to the proposed adsorption mechanism(10), a higher decrease of initial pH of o-MWCNT solution duringadsorption (1.2 pH unit), in comparison to raw-MWCNT (0.1 pHunit), was observed. It means that adsorption processes, presentedby reactions (9) and (10), were also operative for the raw-MWCNT,as well as one given by reaction (8) [52]. Presence of oxygen con-taining functional group [17], at raw-MWCNT surface (Table S1), isa consequence of the purification process.

FTIR is a non-destructive technique which could provide infor-mation about chemical interaction of adsorbate and adsorbent’sfunctional groups. The vibration modes of the groups present atadsorbent surface are sensitive to the adsorbed Pb2+ cation. Ingeneral, differences in FTIR spectra of Pb2+-loaded o-MWCNT, e-MWCNT and d-MWCNT and original spectra were observed asdifferences in the peak intensity, peak shifting and peak total or par-tial appearance or disappearance. Change in vibration frequency,caused by adsorbate/adsorbent group interaction is a result of bondstrength change. Band shift to lower or higher frequencies indicatesbond weakening or strengthening, respectively. FTIR spectra of Pb2+

loaded o-MWCNT, e-MWCNT and d-MWCNT and original ones arepresented in Figs. 6–8.

Adsorption capabilities of o-MWCNT surface functional groups,as potential binding sites for Pb2+ ion, depend on the adsorptioncondition, primarily on solution pH. Pb2+ ion might form complexeswith carboxylic and phenol groups [12,54], more favorable interac-tion could be expected with former at pH higher than 6 (pKa 3–6),as ionized form could play significant role in uptake of Pb2+ ion[12]. The both oxygen, in a carboxylate anion, are entities whichpossess a pair of lone electrons (Lewis base), resonantly stabilized,as a center capable for coordination with the electron deficientlead or cation (Lewis acid). Higher electron-donating capabilitiesof oxygen in carboxylate anion have impact on higher adsorptioncapacity of o-MWCNT. The significantly decreased peak intensityat ≈1726 and ≈1260 cm−1 clearly indicates involvement of car-boxyl moiety in a surface complexation (Fig. 6). Concomitantly, it

could be observed an intensity increase of peaks at ≈1635 cm−1 and≈1384 cm−1, assigned to stretching vibration, asymmetric COO−

overlapped with C C and symmetric COO−, respectively, whichindicate that electron density of carboxylate anion is highly affected

862 G.D. Vukovic et al. / Chemical Engineering Journal 173 (2011) 855– 865

Fd(

btpp

gfoap

M

tdgiato[f

Fd(

Fig. 8. FTIR spectra of (a) d-MWCNT and after treatment in aqueous solutions of two

ig. 6. FTIR spectra of (a) o-MWCNT and after treatment in aqueous solutions of twoifferent Pb2+ concentrations, (b) C[Pb2+]0 = 5 mg L−1 and (c) C[Pb2+]0 = 10 mg L−1

m/V = 100 mg L−1, pH 6.2, T = 25 ◦C).

y adsorbed Pb2+ cation. Also, from Figs. 6–8 it could be observedhat peak at ≈1097 cm−1 almost disappeared, what indicate thathenol group, ionized or non-ionized, could significantly partici-ate in Pb2+ binding [12,54].

Free amino and non-reacted oxygen-containing functionalroups are found to be present at e-MWCNT and d-MWCNT sur-aces. Hence, besides the presented adsorption mechanisms for-MWCNT, additional coordination and electrostatic interactionsre possible between Pb2+ ions and unprotonated amino groups atH higher than pHPZC, given by following equations:

WCNT-CONH(CH2)2NH2 + Pb2+ � Pb[MWCNT-CONH(CH2)2NH2]2+ (11)

Significance of the amino group involved in the complexa-ion process was recently unequivocally presented [55]. It wasescribed that Cd2+ is a very soft cation weakly bound by nitro-en adsorbent but strongly bonded to amino groups. A large, softon Pb2+ binds strongly to amino group and also to phenols, alcoholsnd even ether groups [55]. From that point of view, it is important

o define bonding capabilities of the amino groups, primary or sec-ndary, for the cation complexation [55] or chelation interactions19–21]. Similar coordination numbers were obtained (Table S1)or all amino functionalized adsorbents indicating similar type of

ig. 7. FTIR spectra of (a) e-MWCNT and after treatment in aqueous solutions of twoifferent Pb2+ concentrations, (b) C[Pb2+]0 = 5 mg L−1 and (c) C[Pb2+]0 = 10 mg L−1

m/V = 100 mg L−1, pH 6.2, T = 25 ◦C).

different Pb2+ concentrations and (b) C[Pb2+]0 = 5 mg L−1 and (c) C[Pb2+]0 = 10 mg L−1

(m/V = 100 mg L−1, pH 6.2, T = 25 ◦C).

bonding. In general, it could be stated that higher nucleophilic-ity of amino groups, at pH higher than pHPZC, brings to strongerinteraction with lead cation, and preferential bonding with pri-mary amino group is expected. Relevant proof was obtained bysynthesis of ethylamine and 2-ethylamino ethylamine modified o-MWCNT (data not presented), performed analogously to e-MWCNTsynthesis. The modified MWCNT bearing alkyl residues withoutprimary amino group showed significantly lower capacities of8.21 and 15.44 mg g−1 for Pb2+ for ethyl and 2-ethylaminoethylresidue, respectively, clearly indicating utmost significance of pri-mary amino group as sorption site for removal metal cation.Although, initial pH changes for those two sorbents are lower (inthe range 0.2–0.3 unit) than for e-MWCNT and d-MWCNT whichwere 0.6 and 0.8 units, respectively. Structural phenomena shouldbe also included into discussion about alkylamino residue geome-try change caused by amino/Pb2+ bonding. Once Pb2+ is attracted byamino group and coordinated (Eq. (11)) degree of freedom of localsystem is decreased and created positively charged complex sup-press transport incoming Pb2+ ions toward interior, e.g., adsorbentsurface. Secondary amino group could participate in a coordina-tion process, a different type intramolecular and intermolecularbridging creating monodentate or bidentate complex [56].

Some valuable information on bonding type of Pb2+ one-MWCNT and d-MWCNT was obtained from FTIR spectra(Figs. 7a and 8a), considering some recently published results[17,18]. A broad band at ≈1650 and ≈1580 cm−1, assigned to acarbonyl amide stretching vibration (amide I) and N–H in-planevibration, respectively, is gradually shifted to lower frequenciesas Pb2+ binding quantity increases (Figs. 7b and c and 8b and c).In addition, bands at ≈1180 and ≈800 cm−1, correspond to C–Nstretching and out-of-plane NH2 bending mode (twisting), respec-tively, almost completely disappear. This indicates that positivecharge bearing by lead cation has pronounced influences on elec-tronic density at amide and amino group. Also, this result indicatesthat appropriate interaction of Pb2+ cation and amino lone pairrestricts N–H out-of plane movement with small restriction N–Hin-plane vibrations, thus indicating that lead cation coordinate withnitrogen lone pair in a such way to contribute to the steric crowd-ness at amino group. Interestingly, it could be observed in Figs. 6–8an intensity increase of a band at ≈1384 cm−1, assigned to over-lapped stretching vibration of SO2 and symmetric of COO−, whichreflects to the bond strength increase of these groups after Pb2+

adsorption. Broad bend at ≈3458 cm−1 (Fig. 7 a) split up (Fig. 7b andc) indicating N–H vibration mode change, asymmetric and sym-metric, as amount of bonded lead cation increased. Similar results

G.D. Vukovic et al. / Chemical Engineering Journal 173 (2011) 855– 865 863

Table 4Literature data on the adsorption of Pb2+ ions by various adsorbent.

Adsorbent qmax (mg g−1) b (L mol−1) References

Carbon nanotubes Oxidized CNT 2.05 17,800 [12]CNTs (HNO3)/xylene–Fe 14.8 – [25]CNTs (HNO3)/benzene–Fe 11.2 – [25]CNTs(HNO3)/Propylene–Ni 59.8 – [25]CNTs (HNO3)/methane–Ni 82.6 – [25]Oxidized CNT 17.44 0.59 [26]Raw CNT 1.66 0.16 [26]MnO2/CNT 82.6 343,450 [27]Oxidized MWCNT 51.81 302,526 [28]CNTs (HNO3) 30.32 – [29]MWCNTs (HNO3) 97.08 312,887 [30]MWCNTs 16.9 13,468 [31]N2 plasma treated MWCNTs 19.7 13,468 [31]MWCNT-g-PDMA 25.8 161,623 [31]MWCNT-g-PAAM 35.7 116,037 [31]Ethylenediamine-modified MWCNT 54.48 – [32]Oxidized MWCNT/SDBS 66.95 14,460 [33]Oxidized MWCNT 17.5 35,225 [34]d-MWCNT 58.26 386,565 This study

Carbon Functionalized grapheme 406.6 580.19 [23]Sawdust activated carbon 109.82 5260 [35]Activated carbon 5.5 - [36]Activated carbon/zeolite 549.11 24,000 [37]Palm shell activated carbon 95.2 23,071 [38]Activated Carbon Cloths/CS 1501 17.30 – [39]Activated carbon cloths/RS 1301 17.20 – [39]Carbon aerogel 22.57 – [40]Activated carbon (Sorbo-Norit) 54.10 – [41]Bacteria modified activated carbon 26.40 – [41]

ob

ae(alb

fcgtaeroM8atspsfwhs

3

a

Activated carbon (Merck)

Peanut husks carbonDate pits carbon

f Pb2+ bonding to amine groups (–NH– and –NH2) via chelation,ased on XPS, were presented in literature [56].

Analogous analysis stands for FTIR spectra of d-MWCNTt different lead cation loading (Fig. 8). Observable differ-nces are located in the region of N–H stretching vibrations3000–3650 cm−1), originating from contribution of secondarymino group vibration. No observable band splitting is due to over-apping of a number of bands resulting form different Pb2+/amineonding types.

Significance of the introduced oxygen-containing and aminounctionalities could be demonstrated in following: the adsorptionapacity of annealed oxidized MWCNTs (800 ◦C/2 h under nitro-en atmosphere [54] provides a total surface area of 85.42 m2 g−1)o Pb2+ is 5.19 mg g−1, which only accounts for 13.9% of the totaldsorption capacity (Table 2). The adsorption capacities of annealed-MWCNT and d-MWCNT for Pb2+ are 15.7 and 16.9% of the cor-esponding amino functionalized MWCNT. Thus the contributionf the functional groups to the overall adsorption capacity of o-WCNT, e-MWCNT and d-MWCNT with respect to Pb2+ is 86.1,

4.3 and 83.1%, respectively. Concomitantly, the specific surfacerea of annealed e-MWCNT and d-MWCNT are 9.9 and 10.3% higherhan non-treated samples, respectively. Presented result showignificance of introduced groups on MWCNT surface, and alsorimary amino group for metal cation bonding. Applied synthe-is of modified MWCNT offers non-uniform coverage of introducedunctional groups. From that point of view a material synthesisith more uniform distribution of introduced functionality as welligher contribution of primary amino groups will be focus of futuretudy.

.8. Desorption and regeneration studies

Repeated availability of adsorption is an important factor for andvanced adsorbent. Such an adsorbent not only possesses a high

21.50 – [41]113.97 37,297 [42]

30.67 – [43]

adsorption capability, but also exhibits good desorption proper-ties, which significantly reduce the overall cost for the adsorbent.The percentage desorption of Pb2+ ions into solutions of variouspH values is shown in Fig. S3. It is apparent that Pb2+ desorptionincreased with decreasing pH. About 7.6% of Pb2+ was desorbedfrom o-MWCNT at pH 5.5; this increased sharply at pH < 5.5 andreached a value of about 93.9% at pH 1.5. The e-MWCNT, d-MWCNTand raw-MWCNT showed a higher desorption of Pb2+ at pH < 6 andreached 96.7% 97.4% and 98.5% at pH 1.5, respectively. Table S4shows the adsorption capacity and desorption efficiency of theMWCNT over five successive adsorption–desorption cycles. It couldbe seen that little loss of uptake capacity of the MWCNT wasobserved after using it for five times, and the desorption effi-ciency was above 90%. Hence, MWCNT have good recycling valueand wide prospects for practical application. These results showthat the Pb2+ adsorbed by d-MWCNT could be more easily des-orbed than that adsorbed on o-MWCNT and e-MWCNT, suggestinga weaker binding between d-MWCNT and Pb2+, which means thatd-MWCNT can be repeatedly employed in heavy metal wastewatermanagement.

3.9. Comparison of MWCNT adsorbent performance withliterature data

The qmax and b values of the d-MWCNT were compared withthe metal adsorption capacities reported in the literature for otheradsorbents (Table 4), although a direct comparison between theexamined modified MWCNT with those obtained in literature wasdifficult, due to the varying experimental conditions employed inthose studies. However, it may be seen that the qmax and b val-

ues differ widely for different adsorbents (Table 4). Comparison ofqmax values showed that the d-MWCNT sample exhibited a rea-sonable capacity for Pb2+ adsorption from aqueous solutions. It iswell known that not only adsorption capacity, but also adsorption

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64 G.D. Vukovic et al. / Chemical En

ffinity is important. If the main objective of a adsorption tech-ology in practice is to be economic, concerning the amountf adsorbent consumed, and the requirements concerning waterurity are moderate, the adsorption facility will be operated upo near saturation of the adsorbent and hence adsorption capac-ty will be of crucial importance. If extreme water purity is theoal, the facility will be operated “at the left side of the adsorptionsotherm” and the adsorption affinity will be the most importantriterion. Comparison of the b values showed that the d-MWCNTxhibits excellent affinity for Pb2+ adsorption from aqueous solu-ions (Table 4).

The cost of adsorbents is also an important parameter for theirmployment in adsorption processes. The current cost of MWCNT≈50 $/g [57]) is higher than of the other traditional adsorbents, asctivated carbons (≈0.08 $/g) [58], synthetic resins (3–25 $/kg) [59],gricultural waste (100 $/t) [60]. But the encouraging news is thatmproved manufacture and large-scale production have alreadyaused the price of CVD-produced CNTs to fall substantially, fromround 200 $/g in 1999 to 2–50 $/g today [30,57]. CVD is deemedo be a promising route to reduce the cost of CNTs in the future,hich would increase the use of CNTs in environmental protection

pplications. In addition, the practical use of CNTs as adsorbentsn polluted water and wastewater treatment depends on continua-ion of research on the toxicity of CNTs and CNT-related materials.he unique adsorption properties (combination of adsorption affin-ty and capacity) and the extraordinarily fast transport of waterhrough CNTs could be utilized for the production of high-fluxanotube-based filtration membranes, in which aligned nanotubeserve as pores in an impermeable support matrix, in contrast tother materials, such as polymer membranes, with significantlyower fluxes [44].

. Conclusions

The single and competitive adsorption of Pb2+ and Cd2+ byWCNTs was studied, and two kinds of experimental data sets

howed that the adsorption affinity of Pb2+ and Cd2+ to MWC-Ts followed the order Pb2+ > Cd2+. The adsorption properties of

aw-MWCNT were greatly improved by oxidation, as well as bymino-functionalization. It was found that the adsorption capaci-ies change with increasing temperature, whereby the adsorptionapacity for Pb2+ non-linearly increase as the number of aminoroups in alkyl chains increases. Contribution of the functionalroups to the overall adsorption capacity of o-MWCNT, e-MWCNTnd d-MWCNT with respect to Pb2+ is 86.1, 84.3 and 83.1%, respec-ively. The kinetic data of the adsorption on all the investigated

WCNTs were well fitted with the pseudo-second-order kineticodel, suggesting that the rate-limiting step was chemical adsorp-

ion rather than diffusion. The adsorption experimental data ofb2+ on o-MWCNT, e-MWCNT and d-MWCNT follow the Lang-uir adsorption isotherms. The adsorption of Pb2+ on the studiedWCNT is a rather complex and spontaneous process, suggesting

hat mechanism includes both physisorption and chemisorptionechanisms. The adsorption experiments with dendrimer func-

ionalized MWCNT and their influence on adsorption capacities areurrently under investigation in our laboratory.

cknowledgements

The authors acknowledge financial support from Ministry of

cience and Technological Development of Serbia, Project Nos.II45019 and 172007. Goran Vukovic is grateful to the projectP7 REGPOT NANOTECH FTM, GRANT AGREEMENT 245916 for thenancial support.

[

ing Journal 173 (2011) 855– 865

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cej.2011.08.036.

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