Chemical Engineering Journal 279 (2015) 344–352

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Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /ce j

Removal of noxious Cr (VI) ions using single-walled carbon nanotubesand multi-walled carbon nanotubes

http://dx.doi.org/10.1016/j.cej.2015.04.1511385-8947/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding authors at: Tehran University of Medical Sciences, School of Public Health, Department of Environmental Health Engineering, Tehran, Iran (B.Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India. Tel.: +91 1332285801; fax: +91 1332273560 (V.K. Gupta).

E-mail addresses: vinodfcy@iitr.ac.in, vinodfcy@gmail.com (V.K. Gupta).

Mohammad Hadi Dehghani a,b, Mahdieh Mohammad Taher a, Anil Kumar Bajpai c, Behzad Heibati a,d,⇑,Inderjeet Tyagi e, Mohammad Asif f, Shilpi Agarwal e, Vinod Kumar Gupta e,g,h,⇑a Tehran University of Medical Sciences, School of Public Health, Department of Environmental Health Engineering, Tehran, Iranb Institute for Environmental Research, Center for Solid Waste Research, Tehran, Iranc Bose Memorial Research Laboratory, Department of Chemistry, Government Autonomous Science College, Jabalpur 482001, Indiad Health Science Research Center, Faculty of Health, Mazandaran University of Medical Sciences, Sari, Irane Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, Indiaf Department of Chemical Engineering, King Saud University, Riyadh, Saudi Arabiag Center for Environment and Water, The Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabiah Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africa

h i g h l i g h t s

�MWCNTs and SWCNTs used asadsorbent for the removal of Cr (VI)ion.� The maximum efficiency of adsorbent

was noticed at pH 2.5.� Specific surface area (BET) of SWCNTs

and MWCNTs were 700 and 270 m2/grespectively.� qm (Maximum adsorption capacity) of

MWCNTs and SWCNTs was 1.26 and2.35 mg/g respectively.� Cr (VI) ion removal percentage

decreased in the presence SO42� ion.

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

SEM and TEM micrographs of SWCNTs and MWCNTs.

a r t i c l e i n f o

Article history:Received 25 February 2015Received in revised form 25 April 2015Accepted 30 April 2015Available online 18 May 2015

Keywords:ChromiumAdsorption isothermMWCNTsSWCNTs

a b s t r a c t

The adsorption capacity of two efficient adsorbents namely MWCNTs and SWCNTs for the rapid removalof noxious Cr (VI) ion from the polluted aqueous source was well studied and investigated. The impact ofseveral influential parameters such as contact time, initial pH, initial Cr (VI) ion concentrations and theadsorption capacity of the adsorbent in the presence of competing anion was well elucidated and opti-mized. It was observed that the removal of efficiency of Cr (VI) ion depends on the pH of solution andthe maximum efficiency was noticed at pH 2.5. Furthermore, the uptake of Cr (VI) ion was hindered inthe presence of the competing anion, SO4

2�. Experimental equilibrium data were fitted to four differentisotherm models by linear regression method, however, the adsorption equilibrium data were well inter-preted by the Langmuir model. The maximum adsorption capacity of Cr (VI) ion by MWCNTs andSWCNTs was 1.26 and 2.35 mg/g respectively, which is calculated by the Langmuir isotherm model.Kinetic studies are well suited and found in good agreement with pseudo-second order. The results evi-dently indicated that MWCNTs and SWCNTs would be suitable adsorbents for Cr (VI) ion in wastewaterunder specific conditions.

� 2015 Elsevier B.V. All rights reserved.

Heibati).

M.H. Dehghani et al. / Chemical Engineering Journal 279 (2015) 344–352 345

1. Introduction 2. Materials and methods

Chromium is one of the heavy metals present in toxic effluentsejected out by the aerospace, electroplating, leather, mining, dye-ing, fertilizer and photography industries [1]. Cr (VI) ion generallyexists in the form of extremely soluble and highly toxic chromateions (HCrO4

� or Cr2O72�), which can transfer freely to the biotic

organisms prevailing in the aquatic ecosystem and food chain[2]. Persistent exposure to Cr (VI) ion lead to the severe carcino-genic effect on human health and it causes cancer in the digestivesystem and lungs, and it may lead to severe other health hazardssuch as skin dermatitis, bronchitis, perforation of the nasal sep-tum, severe diarrhea and other hemorrhaging problems [1,3]. Onthe other hand, Cr (III) ion is comparatively stable, have low sol-ubility and mobility in soils and aquifers, and are usually consid-ered as a much less hazardous pollutant [4]. Plentifulconsumption of chromium compounds in industries and thenejection of the chromium contained toxic products in the aquaticecosystem would possess serious un-natural effect on the biomeas well as biotic organisms [5]. Under the usual oxidation condi-tions, chromium can be found in the nature in both trivalent (Cr(III)) and hexavalent (Cr (VI) ion) forms. The Cr (VI) ion concentra-tion above 0.05 milligrams per liter (mg/L) can be toxic to most ofthe microorganisms. On the other hand, it was proved to be car-cinogen for most of the biological creatures. Additionally, it cancause irritation and corrosion of the skin in humans, but Cr (III)ion is less toxic and can be readily precipitated or adsorbed ona variety of inorganic and organic substances either at alkaline(<7) or neutral pH (7). Cr (VI) ion is highly soluble in aqueoussolution and result in the formation of bivalent anions such aschromate (CrO4

2–), dichromate (Cr2O72–), and hydrogen chromate

in different pH [6–8]. Several techniques like electrochemical oxi-dation and sensors [9–17], sorption, chemical coagulation, solventextraction, bioremediation, photo catalytic degradation andadsorption were reported for the removal of noxious impuritiesfrom polluted aquatic source, but among all the adsorption wasproved to be a most economical and efficient method for theremoval of Cr (VI) ion from aqueous solution, it has been exten-sively applied because it is a simple and cost effective techniqueand low cost adsorbents such as activated carbon [18], clay min-erals [19], organic polymers [20], metallic oxidants [21], naturaladsorbents such wheat bran [5] and maize bran [22] are knownfor their efficiency in the fast adsorption of chromium, and severalother previously developed adsorbents such as carbon nanotubes[23–30], MWCNTs [31,32], nanoparticles and nanocomposites[33–37], rubber tire [38,39], and other low cost adsorbents [40–45] etc., are extensively used for the rapid removal of noxiousimpurities from the aqueous solution.

Nanotechnology is the engineering of functionalizing thesystems at molecular level, which lead to formation of newproducts and process alternatives for the treatment of pollutedwater and make it available for drinking purpose, the advantageof these nanomaterials was that they possess a large surface tovolume ratio [46]. In recent years, synthesized nanosized adsor-bents, such as single-walled carbon nanotubes (SWCNTs) [47]and multi-walled carbon nanotubes and (MWCNTs) [48] hadbeen extensively studied and demonstrate promising resultsfor Cr (VI) ion removal from aqueous environment.

The objectives of the present study were to obtain an under-standing of the equilibrium adsorption of Cr (VI) ion on SWCNTsand MWCNTs by investigating the influence of different experi-mental influential parameters on the adsorption process andadsorption capacity of the developed adsorbent. Moreover, theequilibrium and kinetics of the adsorption system were alsostudied.

2.1. Chemicals and materials

All chemicals were of reagent grade and used without any fur-ther purification. Potassium dichromate, K2Cr2O7 (Fisher scien-tific > 99.5%) was used to prepare Cr (VI) ion stock solution.Hydrochloric acid and sodium hydroxide were used to adjust thepH of the working solutions. MgSO4 salt was used to create thecompeting ions during the experimentation. Adsorbents surfacetextural and morphological features was determined using scan-ning electron microscopy (SEM), transmission electron microscopy(TEM), X-ray diffraction (XRD) and Brunauer, Emmett and Teller(BET). The FT-IR spectra of the samples were recorded on aPerkinElmer Spectrum 100 spectrophotometer to characterize thechange in the functional groups of the material surface (using aKBr disk technique in the range of 500–4000 cm�1).

2.2. Experimental

Batch adsorption studies were performed in 200 mL Erlenmeyerflasks inside an incubator container. The contents of all Erlenmeyerflasks were mixed thoroughly using magnetic stirrers with a fixedsetting to achieve a constant speed. Required concentrations of Cr(VI) ion ion standards were prepared by appropriate dilution of theabove Cr (VI) ion stock solution. For every examination, at first200 mL of a stock solution was added into an Erlenmeyer flask;HCl or NaOH (0.01 N) was used to adjust the pH of the workingsolution, the adsorbent was put in the Erlenmeyer flask and under-goes continuous shaking in a regulated speed shaker, differentadsorbent doses and adsorption times were considered. After theadsorption, the samples were filtered and analyzed for Cr (VI) ionconcentration. The adsorption process was studied as a functionof pH (3–9), initial Cr (VI) ion concentration (0.2–1.0 mg/L), adsor-bent dose (5–40 mg/200 cc), and contact time (5–240 min).Theamount of Cr (VI) ion adsorbed onto the sorbent, qt (mg/g) was cal-culated as follows:

qt ¼ðC0 � CtÞ � V

mð1Þ

where C0 (mg/L) and Ct (mg/L) are the initial concentration of Cr (VI)ion and the Cr (VI) ion concentration after an absorption time t,respectively. While qt (mg/g) is the amount of Cr (VI) ion adsorbed,m (g) the mass of the adsorbent and V (L) is the volume of the liquidphase. After equilibrium was reached, the sample solutions were fil-tered through a 0.45 lm membrane filter.

3. Results and discussion

3.1. SWCNTs and MWCNTs specifications

According to the SEM analysis of SWCNTs and MWCNTs(Fig. 1a and b), it was specified that the developed adsorbents haveporous morphological structure. The specific surface area (BET) ofSWCNTs and MWCNTs were 700 and 270 m2/g, respectively.Based on TEM images (Fig. 1c and d), it was observed that thedeveloped CNTs are in spherical form when used as a purifier inthe solution. These spherical particles have tendency to adheretogether and form aliphatic chains.

The X-ray diffraction spectrum of CNTs was shown in Fig. 1(e).As can be revealed from Fig. 1e, the peaks at 25 and 43 degree arerelated to graphene structure of carbon nanotubes.

The FT-IR spectra of the developed CNTs was shown in Fig. 1(f).The band observed near 1580 cm�1 in all samples shows the pres-ence of the cylinder like carbon structure (rolled graphene sheet).

346 M.H. Dehghani et al. / Chemical Engineering Journal 279 (2015) 344–352

Several infrared active modes may have a wave number near1580 cm�1, while the wave number depends on the geometry ofthe CNT. Besides, infrared wave numbers are dependent on thediameter of nanotube. The broadness of the band observed at1580 cm�1 for MWCNTs samples can be explained on the basis ofthe polydispersity index in the geometry of nanotube.

XPS is a surface-sensitive quantitative spectroscopic techniquethat allows the elemental composition of the surface to be ana-lyzed. In XPS the sample is irradiated with a beam of X-rays andthe quantity of emerging electrons and their kinetic energies aremeasured. XPS has generally been used to study the structuralmodification of the nanotube surface by chemical reactions carriedout to functionalize these systems. Fluorination of multi-walledcarbon nanotubes has been evaluated by XPS. It was found that thisreaction starts on the external layers of the nanotubes and thenprogresses to the internal layers with graphene deterioration[49]. Two different approaches for the amino-functionalization ofSWCNTs (amide and amine groups) have been characterized indetail using this technique [50]. Doping of CNTs with nitrogen

Fig. 1. SEM micrographs of (a) SWCNTs, (b) MWCNTs and TEM micrographs of (c) SWC

[51] and non-covalent functionalization by the immobilization ofdiverse biological molecules on the CNT sidewall [52] has also beenstudied by XPS.

Raman spectroscopy is one of the most powerful characteriza-tion techniques for CNTs. It is commonly employed for the charac-terization and purity analysis of CNTs due to the simple samplepreparation and the non-destructive and non-invasive nature ofthis technique [53]. Raman spectra of CNTs present two main firstorder bands, the G band (graphitic band) is observed at around1500–1600 cm�1 and is associated with the vibrations in theplanes of the graphene sheets. The D band (defect or disorder band)is observed at around 1300–1350 cm�1, depending on the excita-tion source, and is related to the defects in the sidewalls of theCNTs. Therefore, the measurement of the ratio between the areasof the two bands (D/G ratio) provides a good estimation of the levelof defects in a given CNT sample: a small D/G ratio indicates a lowlevel of defects in the CNT walls [54]. However, if the sample con-tains a significant level of impurities, high D/G ratios principallycould indicate the presence of carbonaceous residues in the

NTs, (d) MWCNTs, (e)XRD pattern of CNTs and (f) FTIR of SWCNTs and MWCNTs.

0

20

40

60

80

100

0 10 20 30 40 50 60

Rem

oval

of

effe

cien

cy (

%)

Time (min)

(A)

0.2mg/l 1mg/l 0.5mg/l

0

20

40

60

80

100

0 10 20 30 40 50 60

Rem

oval

of

effe

cien

cy

Time (min)

(B)

0.2 mg/l 0.5 mg/l 1 mg/l

Fig. 2. Contact times of Cr (VI) ions adsorption onto (A) SWCNTs, (B) MWCNTs at Cr(VI) concentrations 0.2,0.5, 0.5 mg/l; dose 0.2 g/L.

0

20

40

60

80

100

0 1 2 3 4 5 6 7 8 9 10

Rem

oval

of

effe

cien

cy

pH

SWCNT

Fig. 3. Effect of pH on adsorption Cr (VI) ion onto SWCNTs and MWCNTs.

M.H. Dehghani et al. / Chemical Engineering Journal 279 (2015) 344–352 347

sample. López-Lorente et al. [55] described how the aggregationstate of the sample influences the intensities of the G and D bands,and consequently the D/G ratio. In order to avoid the effect pro-duced by CNT aggregation these authors propose a sample treat-ment consisting of dispersion of the sample using surfactants(e.g., Triton X-100). Appropriate sample preparation and homoge-nization are essential stages to achieve reliable results in charac-terization by means of Raman spectroscopy [56]. Ramanspectroscopy has also been used for structural and size determina-tion of CNTs. Different CNT structures are obtained depending onhow the graphene sheets roll up to make the CNT. If n and m areintegers that define the diameter and chiral angle, different valuesof (n,m) give rise to diverse structures such as zig-zag, chiral andarmchair. Jorio et al. [57] showed the possibility of determiningthe chirality of CNTs by measuring the radial breathing mode fre-quency by a Raman scattering technique.

3.2. Effect of contact time and initial concentration

The contact time experiment carried out at various concentra-tions of Cr (VI) ion at constant pH 2.5 are plotted in Fig. 2. As canbe revealed from Fig. 2, the removal efficiency increased with timeuntil it reaches an equilibrium constant nearly after 60 min forboth adsorbents, corresponding to the saturation of the adsorbent.The removal of Cr (VI) ion was found to be rapid initially and thenit slows down with the increase in the contact time. This was prob-ably due to the abundant number of active sites are present on theadsorbents, whereas, with the gradual increased occupancy ofthese sites, the adsorption process becomes less efficient [58].

The percentage removal efficiency for Cr (VI) ion decreased withincreasing of Cr (VI) ion concentration in the aqueous solutions.These results indicate that energetically less favorable sitesbecame involved in increasing Cr (VI) ion concentrations in theaqueous solution. Therefore, the adsorbent would have excellentadsorption properties for Cr (VI) ion, even though the solutionhad a low concentration of Cr (VI) [59].

3.3. Effect of solution pH

The pH of the solution is an important factor for controlling thesurface charge of the adsorbent and the degree of ionization of thematerials in solution. Cr (VI) ion ions are often present at a concen-tration ranging from ppb up to several hundred ppm, and are oftenacidic in nature [1]. At low pH, the oxo-anionic form of Cr (VI) ion ispredominant [1]. The process was found to be pH dependent(Fig. 3), which indicates that ion exchange and electrostatic inter-actions might be involved in the adsorption mechanism of Cr (VI)ion on SWCNTs and MWCNTs. The removal efficiency of Cr (VI)ion on SWCNTs and MWCNTs decreased when the pH of the work-ing solution increased from 2.5 to 9.

With respect to Cr (VI) ion, it exists mainly in the form of HCrO4

� in the pH range from 2 to 7, associated with small concentrationof H2CrO4 and CrO4

2�, as well as a portion of Cr2O72� formed by con-

densation of two HCrO4� ions at weak acidic solution. While CrO4

2�

is the predominant species of Cr (VI) ion when pH > 7, at that timeit is associated with small and tiny concentration of HCrO4

� andH2CrO4, respectively [60]. In acidic medium, the surface will besurrounded by high quantities of hydronium ions (H+). The hydro-xyl groups are protonated and as such are positively charged,which will electrostatically attract the negatively charged Cr (VI)ions. These findings are supported by the measurements of pointof zero charge (pHzpc) of SWCNTs and MWCNTs, which is near2.5. Hence, the electrostatic interaction may occur between theanions of Cr (VI) and functional groups present on the surface ofSWCNTs and MWCNTs, and therefore, Cr (VI) anions were reservedon the absorbent. After pH > 6.0, the relatively low Cr (VI) ion

removal on SWCNTs and MWCNTs at high pH can be due to theexistence of a negative charge on the SWCNTs and MWCNTs sur-face. Thus the removal efficiency decreases due to the repulsionbetween negatively charged chromium ions and OH� ions [1].

3.4. Effect of adsorbent dose

Under experimental conditions, the initial concentration of Cr(VI) ion of 0.2 mg/L and at pH 2.5, the effect of adsorbent dosageon adsorption of chromium onto SWCNTs and MWCNTs was car-ried out and the results are presented in Fig. 4. As can be seen fromthe figure, the Cr (VI) ion removal efficiency of SWCNTs andMWCNTs increased with increased adsorbent amount, which canbe due to the large number of vacant adsorption sites and thegreater surface area hence favoring more Cr (VI) ion adsorption.

0

20

40

60

80

100

Rem

oval

eff

ecie

ncy

(%)

co-existing anion

Blank

Sulfat (MWCNTs)

Sulfate(SWCNTs)

Fig. 5. Effects of co-existing anion on Cr (VI) ion removal.

348 M.H. Dehghani et al. / Chemical Engineering Journal 279 (2015) 344–352

3.5. Effect of background ions

Batch equilibrium experiments were carried out to find theindividual effect of coexisting anion SO4

2� on adsorption of Cr (VI)ion. The percentage removal of Cr (VI) ion was compared to sam-ples with no background ions. The results are shown in Fig. 5. Itwas observed that the Cr (VI) ion removal percentage decreaseddramatically in the presence of SO4

2� anion. The presence of SO42�

enhanced the percentage removal. The researcher group [61] indi-cated that a relatively high level of SO4

2�will enhance the reductionof Cr (VI) ion to Cr (III) ion, the reason for this is because it is likelythat SO4

2� does not serve as a competitive electron acceptor in Cr(VI) ion reduction by biomass

3.6. Adsorption isotherms

Equilibrium data, commonly known as adsorption isotherms, itdescribes how the adsorbate interacts with adsorbents and gives acomprehensive understanding of the nature of interaction. It isimportant to optimize the design of an adsorption system.Several isotherm equations have been developed and employedfor such analysis, and the three important isotherms, namelyLangmuir, Freundlich, Halsey and Dubinin Radushkevich isothermsare applied in this study.

3.6.1. Langmuir isotherm modelThe equation of Langmuir can be expressed as:

qe ¼qmbCe

1þ bCeð2Þ

It can conveniently be written in a linearized form:

Ce

qe¼ 1

qmbþ 1

qmCe ð3Þ

where: qe is the adsorbed amount at equilibrium (mg g�1), Ce is theadsorbate concentration at equilibrium (mg L�1), qm is the maxi-mum adsorption capacity and b is the Langmuir constant relatedto the energy of adsorption [62]; qm and b can be deduced fromthe slope and intercept, by plotting Ce/qe versus Ce. The influenceof the adsorption isotherm shape can be discussed to examinewhether adsorption is favorable in terms of RL, a dimensionless con-stant referred to as separation factor or equilibrium parameter. RL isdefined by the following relationship:

RL ¼1

1þ bCoð4Þ

RL values between 0 and 1 indicates favorable adsorption, whileRL > 1, RL = 1, and RL = 0 indicate unfavorable, linear, and irre-versible adsorption isotherms, respectively [63].

0

20

40

60

80

100

0 5 10 15 20 25 30 35 40

Rem

oval

eff

ecie

ncy

(%)

Dose (mg)

MWCNTs SWCNTs

Fig. 4. Effect of adsorbents dose of Cr (VI) ion removal.

3.6.2. Freundlich isotherm modelThe Freundlich isotherm can be expressed as:

qe ¼ Kf C1=ne ð5Þ

The equation is conveniently used in its linear form by takingthe logarithm of both sides as:

log qe ¼ log Kf þ1n

log ce ð6Þ

where: Kf and n are the Freundlich constants. For favorable adsorp-tion, the value of n should be in the range from 1 to 10 [64].

3.6.3. Dubinin Radushkevich isotherm modelIn order to understand the adsorption type, equilibrium data

were tested with Dubinin Radushkevich isotherm (D.R.) [65]. Thelinearized D.R. equation can be written as:

In qe ¼ In qm � Ke2 ð7Þ

where: e is Polanyi potential, and is equal to RT ln(1 + 1/Ce), qe is theamount of Cr (VI) ions adsorbed per unit mass of adsorbents, qm isthe theoretical adsorption capacity, Ce is the equilibrium concentra-tion of Cr (VI) ions, K is the constant related to mean free energy, Ris universal gas constant and T is the absolute temperature (K). Themean free energy of adsorption (E) was calculated from the con-stant ‘‘K’’ using the relation [66]:

E ¼ ð2KÞ�0:5 ð8Þ

It is defined as the free energy change when 1 mol of ion istransferred to the surface of the solid from infinity to solution.

3.6.4. Halsey isotherm modelThe Halsey [67] adsorption isotherm can be given as:

lnqe ¼1n

ln K� �

� 1n

lnCe ð9Þ

This equation is suitable for multilayer adsorption. In particular,the fitting of this equation can be best used for heterotopous solids.1/qe versus log Ce Harkins–Jura isotherm and lnqe versus lnCe

Halsey adsorption isotherm are given in Fig. 6.Isotherm studies of Cr (VI) ion ion removal are shown in Fig 6.

The experimental data values were fitted to four different isothermmodels and the results are presented in Table 1. As can be seenfrom the correlation coefficients (R2), for both SWCNTs andMWCNTs, the Langmuir model was well fitted and found to be ingood agreement with the experimental values better than that ofother models. Conformation of the data into the Langmuir iso-therm model demonstrated the formation of monolayer coverageof Cr (VI) ion at the outer surface of adsorbents and showed thehomogeneous nature of Cr (VI) ion adsorption onto adsorbentswhich suggests that adsorption sites are identical and energetically

Table 1Isotherm constants for the adsorption of Cr (VI) ion on SWCNTs and MWCNTs.

Model Parameter Parameter Parameter Parameter

Freundlich Kf (mg g�1) N R2

MWCNTs 0.623 5.91 – 0.775SWCNTs 0.279 4.23 – 0.824

Langmuir qm (mg g�1) b (L mg�1) RL (L mg�1) R2

MWCNTs 1.26 0.789 0.86 0.986SWCNTs 2.35 0.424 0.92 0.997

D. R qm (mg g�1) E (kJ mol�1) R2

MWCNTs 1.378 80.34 0.82SWCNTs 2.72 77.3 0.867

Halsey N K R2

MWCNTs 5.91 17.38 0.775SWCNTs 4.23 248.43 0.824

M.H. Dehghani et al. / Chemical Engineering Journal 279 (2015) 344–352 349

equivalent [46]. The values of KF and 1/n were calculated from theslope and intercept of the plot in Fig. 6B and the value obtained arereported in Table 1. Kf is a constant indicative of the adsorptioncapacity of the adsorbent, while n is an empirical constant relatedto the magnitude of the adsorption driving force [46]. The resultspresented in Table 1 and Fig. 6B reveals that the theoretical valueof the adsorption capacity is higher for MWCNTs than that ofSWCNTs, and this is probably due to the highly porous nature ofMWCNTs. However, the Langmuir constant b, connected to theadsorption free energy and specifying the adsorbents affinity forCr (VI) ion ions binding, is higher for MWCNTs than that ofSWCNTs, indicating a more favorable capability of Cr (VI) ion mole-cules to form a stable complex with MWCNTs, rather thanSWCNTs. The 1/n value from the Freundlich equation indicates thatthe relative distribution of energy sites depends on the nature andstrength of the adsorption process. Perhaps, the value of 1/n ofadsorption Cr (VI) ion ions onto SWCNTs surface is 0.236, indeedthis value refers to 24% of the active sites that have equivalentenergy where the adsorption takes place. Furthermore, the values

0

0.05

0.1

0.15

0.2

0.00 0.05 0.10 0.15 0.20

Ce/

qe

Ce(mg/l)

(A)MWCNTs SWCNTs

-0.10

0.00

0.10

0.20

0.30

0.40

-2.0-1.5-1.0-0.50.0

Log

qe

Log Ce

(B) MWCNTs SWCNTs

-0.2

0

0.2

0.4

0.6

0.8

1

-5.00 -4.00 -3.00 -2.00 -1.00 0.00

lnqe

lnCe

(C) MWCNTs SWCNTs

-0.2

0

0.2

0.4

0.6

0.8

1

0 20000000 40000000 60000000 80000000100000000

lnqe

2

(D) MWCNTs SWCNTs

Fig. 6. (A) Langmuir; (B) Freundlich; (C) Halsey; (D) Dubinin Radushkevichadsorption isotherm of Cr (VI) for SWCNTs and MWCNTs.

of n closer to 1 specifies homogeneous surface [58]. The values ofconstant RL for the MWCNTs and SWCNTs are 0.86 and0.92 L mg�1 respectively, which reveals the favorable adsorptionon both the adsorbents. In addition, the n values of theFreundlich model for MWCNTs and SWCNTs were 5.91 and 4.23,respectively (Table 1), indicating strong interactions between theadsorbents and Cr (VI) ion. Fig. 6D shows the plot of ln qe againste2, which was almost linear with a correlation coefficient (R2) of0.82 and 0.867 for MWCNTs and SWCNTs, respectively. The valueof qm was 1.26 and 2.35 mg g�1 for MWCNTs and SWCNTs respec-tively. Moreover, the mean free energy evaluated using the D–Rmodel are 80.34 and 77.3 kJ mol�1 for MWCNTs and SWCNTsrespectively. Table 3 indicating that the adsorption of Cr (VI) ionon MWCNTs and SWCNTs was a chemisorptions process whichoccurred by ion exchange reactions [66].

3.7. Adsorption kinetics studies

The kinetic of adsorption describes the rate of Cr (VI) ion ionadsorb on the adsorbents, which controls the equilibrium timeand influences the adsorption mechanism. For an adsorption pro-cess, there are typically three major steps that are taking place atthe solid–liquid interface: external diffusion, intra-particle diffu-sion and surface reaction. The ‘‘external diffusion’’ is not often deci-sive, especially when the experimental system is well agitated[68]. In order to adequately correlate the experimental data, thepseudo-first order equation, pseudo-second order equation andintraparticle diffusion were used to investigate the kinetics ofadsorption. The kinetic models can be linearized as shown inTable 2.

The most accurate model was considered based on the regres-sion coefficient (R2) and the comparison of the qe values to theexperimental ones. Kinetics of Cr (VI) ion ion removal is shownin Fig 7. The corresponding parameters are collected in Table 3.By contrast, the experimental data of kinetic model werewell-fitted by the pseudo-second-order model for both adsorbents.This was generally in agreement with other research results, whichshowed that the pseudo-second-order model was able to properlydescribe the kinetic process of other adsorbents for Cr (VI) ion ion

Table 2Kinetic models and their linearized expressions.

Kinetic models Equations Plot Ref.

Pseudo-first-order lnðqe � qÞ ¼ ln qe � K1t ln(qe � qt) versust

[59]

Pseudo-second order tqt¼ 1

K2 q2eþ 1

qet t/qt versus t [59,60]

Intraparticlediffusion

qt = kPp

t + C qt versus t1/2 [61,62]

Table 3Parameters of kinetic equations.

Adsorbent Concentration Pseudo-first-order model Pseudo-second-order model Intraparticle diffusion model

qe exp(mg g�1)

qe cal(mg g�1)

K1 R2 qe cal. (mg g�1) K2 R2 kip

(g mg�1 min�1/2)R2

MWCNTs 0.2 0.190 0.198 0.007 0.98 0.188 0.289 0.994 0.011 0.878SWCNTs 0.2 0.175 0.203 0.003 0.81 0.193 1.555 0.999 0.008 0.612

0

200

400

600

800

1000

1200

1400

1600

0 50 100 150 200 250 300

t/qt

Time (min)

(A)MWCNTsSWCNTs

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

00 50 100 150 200 250 300

Log

(1-

qt/q

e)

(B)

Time (min)

MWCNTs SWCNTs

00.020.040.060.08

0.10.120.140.160.18

0.2

0 5 10 15 20

qt

√t

(C)

MWCNTs SWCNTs

Fig. 7. Adsorption kinetics: (A) Pseudo-second order; (B) Pseudo-first order;(C)Intra- particle diffusion kinetics for adsorption isotherm of Cr (VI) for SWCNTs andMWCNTs.

Table 4Comparison between various adsorbents for the removal of Cr (VI) ion.

Adsorbent Metalion

Adsorptioncapacity (mg g�1)

References

Pine Cr (VI) 0.47 [73]Oak Tree Cr (VI) 1.74 [74]Maize corn cob Cr (VI) 0.82 [75]Walnut Cr(VI) 2.28 [73]Coconut Cr(VI) 0.68 [73]Lignin (Origin not known) Cr(VI) 0.20 [76]Rice straw Cr(VI) 3.15 [77]DBSA doped polyaniline/multi-

walled carbon nanotubesCr(VI) 55.55 [78]

poly(2-amino thiophenol)/MWCNTs nanocomposite

Cd(II) 178.7 [79]Pb(II) 186.4 [79]

SWCNTs Cr (VI) 2.35 ProposedmethodMWCNTs 1.26

350 M.H. Dehghani et al. / Chemical Engineering Journal 279 (2015) 344–352

adsorption [69]. The sorption kinetics may be described by apseudo second-order model. In despite of the first order model,the plot of t/qt versus t for the pseudo-second-order kinetic modelgives a straight line with a high correlation coefficient that k2 andequilibrium adsorption capacity (qe) were calculated from theintercept and slope of this line, respectively. The values of R2 andcloseness of experimental and theoretical adsorption capacity(qe) value show the applicability of the second order model toexplain and interpret the experimental data (Table 3).The R2 valuefor pseudo-second-order kinetic model was found to be higher(0.994 for SWCNTs and 0.999 for MWCNTs) and the calculated qe

value is mainly close to the experimental adsorption capacity valueunder different physico-chemical conditions.

Several adsorption experiments were carried out to determinethe effect of initial Cr (VI) ion ion concentration on its kineticbehavior. Though, initial concentration, Co, provides an importantdriving and attraction force to overcome all the mass transfer resis-tance of Cr (VI) ion ion between the aqueous solution and theadsorbent surfaces. Consequently, high Cr (VI) concentration willimprove the adsorption processes [69–72]. Fig. 7 reveals that theCr (VI) ion concentration barely affected by the time required toreach adsorption equilibrium, since 15 and 5 min of contact timewas sufficient to attain complete removal for MWCNTs andSWCNTs, respectively. Fig. 7 showed that adsorption of Cr (VI)ion was very fast in the first few minutes (0–5 min) and (0–15 min) for SWCNTs and MWCNTs, respectively. Sharp slopecurves of Cr (VI) ion ion adsorption onto SWCNTs and MWCNTsrevealed an immediate adsorption which could be attributed tothe effect of surface functional groups. Consequently, the adsorp-tion behavior and mechanism of Cr (VI) ion for both adsorbentswas believed to happen via surface adsorption till the surface func-tional active sites were entirely occupied. Subsequently, Cr (VI) ionmolecules diffused into the pores of the SWCNTs and MWCNTs forfurther adsorption [69]. This behavior might be quite different forSWCNTs in which the kinetic adsorption in the first few minuteswas faster that of MWCNTs. This means that the surface adsorptionfor SWCNTs was predominant than pore adsorption. As the poros-ity of SWCNTs was higher than that of MWCNTs, intra-particle dif-fusion was expected in the adsorption processes for SWCNTs.Consequently, the slope of a linear portion was associated to theintra-particle diffusion parameter, kp, and is specific to the limitingstep-adsorption rate. The kp is related to the Cr (VI) ion ionadsorbed bed amount per gram of SWCNTs and MWCNTs at timet. Though, the intra-particle diffusion kinetic model applied theexperimental results and is shown in Table 3. Nevertheless, itappears that the intra-particle diffusion could not be active inthe Cr (VI) ion control of adsorption kinetics as the surface func-tional groups of the adsorbents are perhaps good enough to adsorband remediate the total amount of Cr (VI) ion, especially forSWCNTs.

Table 5Comparison between various adsorbents for the removal of Cr (VI) ion.

Adsorbent Metalion

Contacttime(min)

References

Fe0 nanorods modified with chitosananodic alumina

Cr (VI) 400 [80]

Titanium oxide with Ag compositeadsorbents

Cr (VI) 720 [81]

Graphene sand composite Cr (VI) 100 [82]Magnetic graphene oxide via

ethylenediamineCr (VI) 360 [83]

SWCNTs Cr (VI) 60 ProposedmethodMWCNTs 60

M.H. Dehghani et al. / Chemical Engineering Journal 279 (2015) 344–352 351

3.8. Comparable studies with other adsorbents

The SWCNTs and MWCNTs prepared in this work had a rela-tively large adsorption capacity of Cr (VI) compared to some otheradsorbents reported in the literature. Tables 4 and 5 lists the com-parison of maximum monolayer adsorption capacity of Cr (VI) onvarious adsorbents and time to reach the equilibrium by theadsorbate-adsorbent system. The adsorption capacity for proposedmethod in comparison with all of the adsorbents are preferableand superior to the literature which shows satisfactory removalperformance for Cr (VI) as compared to other reported adsorbents[73–81].

4. Conclusion

The present study confirmed that the SWCNTs and MWCNTshave effective adsorbents for removal of Cr (VI) ion from aqueoussolution. It was also a function of initial concentration, contacttime, pH, sorbents dose and co-existing anion of the solution.Also adsorption equilibrium data follows Langmuir isothermmodel. The kinetic study showed that the pseudo second ordermodel is suitable for describing the experimental data and film dif-fusion might be implicated in the sorption process. The presence ofSO4

2� enhanced the removal percentage, whereas, Cr (VI) ionremoval percentage decreased in the presence of this anion, whichcan be due to the competition of SO4

2� with Cr (VI) ion for the bind-ing sites.

Acknowledgments

The authors would like to thank Tehran University of MedicalSciences for financial and other supports. Also, VKG, IT and SAare thankful to the Department of Science and Technology, NewDelhi, India, for the financial support under project grantDST/INT/South Africa/P-03/2014. Support of the Deanship ofScientific Research grant for the Research Group RGP VPP-292 atKing Saud University is appreciated by M. Asif.

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