Nitrogen-functionalised carbon nanotubes as a novel adsorbent for the removal of Cu(ii) from aqueous solution

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Nitrogen-functionalised carbon nanotubes as

a novel adsorbent for the removal of Cu(II) from

Cite this: RSC Adv., 2016, 6, 2731

aqueous solution†

Oluwaseun A. Oyetade, Vincent O. Nyamori,* Bice S. Martincigh

and Sreekantha B. Jonnalagadda

0

00

This study investigated the introduction of 4 -(4-hydroxyphenyl)-2,2 :6 ,2 -terpyridine (HO-Phttpy) onto

the surface of multiwalled carbon nanotubes (MWCNTs) to obtain nitrogen-functionalized MWCNTs

(MWCNT-ttpy). This novel material was characterised and tested for its possible use in the remediation of

wastewater contaminated with heavy metal ions. Its eﬃcacy was compared with that of acid-

2+

functionalized MWCNTs (MWCNT-COOH) for the removal of the heavy metal ion Cu

through

adsorption. HO-Phttpy was ﬁrst synthesized, followed by the functionalization of MWCNT-COOH to

aﬀord MWCNT-ttpy. MWCNT-ttpy showed signiﬁcant textural enhancement due to an increase in the

extent of functionalization. This was demonstrated by an increase in the surface area and pore volume of

2

ꢀ1

3

ꢀ1

MWCNT-ttpy, from 126.8 to 189.2 m

g

and 0.692 to 1.252 cm

g

, respectively, relative to MWCNT-

), i.e. 19.44 to 31.65

2+

COOH. Its application for Cu removal showed a marked increase in uptake (q

e

ꢀ1

mg g , compared with MWCNT-COOH. This is attributed to the introduction of more active/chelating

sites for adsorption. Adsorption experiments were conducted at pH 5 at which equilibrium was reached

after 360 min. The results showed that the adsorption process was best described by the pseudo-

second order model. Among the isotherms tested, the Langmuir isotherm provided the best ﬁt for the

equilibrium data. Thermodynamic studies revealed that the adsorption process was spontaneous and

Received 12th November 2015

Accepted 21st December 2015

2+

endothermic. Desorption studies demonstrated a better removal eﬃciency of Cu from MWCNT-ttpy,

2+

DOI: 10.1039/c5ra23900a

indicating its possible regeneration and the recovery of the Cu adsorbate for reuse. Thus, MWCNT-ttpy

www.rsc.org/advances

shows superior properties for wastewater remediation compared to MWCNT-COOH.

application for diﬀerent purposes. Covalent functionalization is

oen carried out through oxidation of CNTs with acids such as

1

. Introduction

Carbon nanotubes (CNTs) are tubular shaped carbon nano- HNO

and H SO , to introduce oxygen-containing groups such

2 4

3

4

materials, possessing excellent electrical, mechanical and as –OH and –COOH onto the walls of the tubes. This approach

1,2

optical properties and thermal stability. They are made up of improves the wettability of CNTs, thereby increasing their dis-

a hexagonal lattice of carbon atoms, consisting of hollow persibility in aqueous/organic solutions. New functional

graphite, rolled at specic angles into cylinders. CNTs are groups, such as –NR, –F, –Cl and –SR, can also be incorporated

5

classied into single-walled carbon nanotubes (SWCNTs), onto the walls/sides of the tubes through solution chemistry.

double-walled carbon nanotubes (DWCNTs) and multiwalled

In recent times, the application of CNTs in various elds

carbon nanotubes (MWCNTs), depending on the number of such as medicine, engineering, environmental science, energy,

3

concentric sheets contained in the tubes. They form large catalysis and catalyst supports, amongst many others, has been

4,6,7

aggregates with low dispersibility in aqueous and organic reported by various authors.

This is largely attributed to their

solutions due to strong van der Waals interactions between inherent properties such as large surface area, pore volume and

them. This requires the development of both covalent and non- the ease in introducing new functional groups onto their walls.

covalent strategies for CNT functionalization, in order to In spite of these studies, functionalization of CNTs to contain

introduce functional groups which will enhance their multiple functional groups which will further enhance chemical

1

reactions with other molecules is still under-researched.

0

The tridentate chelating ligand, 4 -(4-hydroxyphenyl)-

,2 :6 ,2 -terpyridine (HO-Phttpy), containing three nitrogen

School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus,

Private Bag X54001, Durban 4000, South Africa. E-mail: nyamori@ukzn.ac.za; Fax:

0 0 00

2

atoms is a good contender for the functionalization of

MWCNTs, in order to introduce nitrogen-containing donor

atoms to the structure of CNTs. HO-Phttpy has profound

+

†

1

27 31 260 3091; Tel: +27 31 260 8256

Electronic supplementary information (ESI) available. See DOI:

0.1039/c5ra23900a

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

exibility to form bridged metal centres by using two or more desorption studies were undertaken to investigate the potential

moieties, construction of supramolecular structures and reuse of the adsorbent and recovery of the adsorbate.

formation of macrocyclic ligands. Due to the strong aﬃnity of

this ligand for transition metal ions, the chelates produced have

been used in luminescence and chemical sensing devices. In

8

2. Experimental

spite of the vast amount of work on CNTs, to the best of our 2.1. Materials

knowledge, no study has investigated the eﬀect of functionali-

Copper metal powder was obtained from Johnson Matthey

Chemicals (Pty) Ltd (Gauteng, South Africa) while sodium

hydroxide (NaOH, 98%) and sodium bicarbonate (NaHCO₃,

zation of oxidized MWCNTs (MWCNT-COOH) with HO-Phttpy

for their increased activity as adsorbents.

One possible area of application of CNTs functionalized with

nitrogen-donor ligands is the remediation of wastewater

contaminated with heavy metal ions. In this work, the synthe-

99%) were purchased from Merck Chemicals (Pty) Ltd (Gauteng,

South Africa). Sodium carbonate (Na CO , 99%) was purchased

2

3

from Associated Chemical Enterprises (Johannesburg, South

Africa). Chemicals such as 4-hydroxybenzaldehyde (99%),

2

+

sized material was applied for the removal of Cu as an

example of a typical heavy metal ion discharged into the envi-

ronment. The choice of copper was based on the fact that

copper and its compounds are regularly used in agriculture for

fertilizer production, electronics, textiles, medicine and metal

2

-acetylpyridine (99%), indium bromide (InBr

solvents such as absolute ethanol, N,N-dimethylformamide

DMF, 99%), dimethyl sulfoxide-d (DMSO-d , 99%) and trie-

3

, 99%) and

(

6

thylsilane (Et SiH, 97%) were purchased from Sigma-Aldrich (St

3

9

cleaning purposes. The discharge of copper-contaminated

Louis, USA). Tetrahydrofuran (THF, 99%), chloroform (99%)

eﬄuents into aquatic environments introduces humans to

necrotic changes in the liver and kidney, gastrointestinal irri-

tations such as diarrhoea and melena, and including a number

of capillary diseases; resulting in severe deleterious health

2

and thionyl chloride (SOCl , 99%) were purchased from Merck

Chemicals (Pty) Ltd (Gauteng, South Africa) while aqueous

ammonia (25%) was purchased from Associated Chemical

Enterprises (Johannesburg, South Africa). Nitric (55%), sulfuric

10

eﬀects. Although copper is a known essential metal at low

(98%) and hydrochloric acids (32%) were obtained from C C

concentrations, an excessive amount is considered harmful to

Imelmann Ltd (Robertsham, South Africa). All materials and

chemicals were of analytical grade and used as received from

suppliers without further purication. MWCNTs (purity > 95%)

11

both aquatic and human life. As a result of the numerous

health consequences, a variety of techniques have been used for

2

+

the removal of Cu from municipal wastewaters and industrial

(

P-MWCNTs), synthesized by chemical vapour deposition

CVD), were obtained from Cheap Tubes Incorporation (Brat-

1

2,13

eﬄuents.

Of these methods, adsorption is considered most

13

reliable for its removal from wastewater. This approach is

simple, eﬃcient, and cost-eﬀective, with a high potential for

recycling the metal ion and the adsorbent. Thus, the removal of

Cu from aqueous solution by adsorption has been reported

through the use of tree fern, bagasse, peanut hull, rice The ligand, 4 -(4-hydroxyphenyl)-2,2 :6 ,2 -terpyridine, was

tleboro, USA).

2

+

2.2. Characterization

14

15

16

0

00

17

18

19

20

husk, magnetite, peat and activated carbon. Reports have characterized by using Fourier transform infrared (FTIR) spec-

shown that these adsorbents possess good potential for Cu

2

+

troscopy, nuclear magnetic resonance (NMR) and mass spec-

removal; however, drawbacks such as slow sorption and trometry, and melting point measurements. The FTIR spectra

19,21

regeneration of adsorbents limits their usage.

Pristine and were recorded on a Perkin Elmer RX 1 spectrophotometer and

functionalized CNTs have also been applied as adsorbents for the melting point was determined by using a Bibby Stuart

2

+

10,22,23

1

13

Cu removal.

However, increased removal eﬃciency can Scientic model SMP3 apparatus. H and C NMR spectra were

be achieved by using CNT-based nanocomposites as adsor- obtained from a 400 MHz Bruker Avance III spectrometer and

2

4

25

bents. Composites such as CNT/bagasse, CNT/magnetite,

mass spectra were obtained on a Waters Synapt G2 mass spec-

26

and CNT/chitosan have demonstrated good sorption ability trometer in electrospray positive mode.

for Cu removal. To further improve the eﬃciency of CNT-

2

+

Structural characterization of the adsorbents was carried out

based nanomaterials, nitrogen-containing ligands can be used with a transmission electron microscope (TEM) (JEOL, TEM

as modiers for CNTs, hence, increasing the number of 1010) and a scanning electron microscope (SEM) (JEOL, TSM

13

chelating sites available for adsorption on the adsorbent.

6100) to visualize the morphology, structure, shape and size

In this communication, we report the synthesis of nitrogen- distribution of the nanomaterials. Images were captured by

functionalized MWCNTs (MWCNT-ttpy) through the function- means of a Megaview 3 camera and analysed on iTEM soware.

alization of acid-functionalized MWCNTs (MWCNT-COOH) by The surface area of the adsorbents was determined with

0

00

incorporating 4 -(4-hydroxyphenyl)-2,2 :6 ,2 -terpyridine (HO- nitrogen as the ow gas by means of a Micromeritics Tristar II

Phttpy) onto their structure. The eﬀectiveness of this material 3020 surface area and porosity analyser. Data were captured and

2

+

in the adsorption of Cu from aqueous solution was examined analysed by using Tristar II 3020 version 2 soware. Fourier

through a series of batch adsorption processes and compared transform infrared (FTIR) spectra of the synthesized materials

with that of MWCNT-COOH. The eﬀects of pH, contact time, were obtained with the samples were embedded into KBr pellets

adsorbent dose, temperature and initial Cu concentration and were recorded on a Perkin Elmer Spectrum RX 1 spectro-

were investigated to determine the optimum conditions for the photometer, in order to identify the surface functional groups

2

+

2

+

eﬀective remediation of Cu -polluted wastewater. In addition, present on the materials. Raman spectroscopy (DeltaNu

2732 | RSC Adv., 2016, 6, 2731–2745

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ꢂ

Advantage 532™) measurements were also performed to reuxed at 70 C for 24 h. The resulting mixture was ltered and

3

33

provide information on the purity and crystallinity of the the solid washed with anhydrous THF (5 ꢁ 5 cm ). The solid

ꢂ

adsorbents. Thermogravimetric (TGA) analyses (Q Series™ was dried in a vacuum oven at 80 C, and stored under an inert

Thermal Analyzer DSC/TGA Q600) were performed to determine atmosphere of argon for further analysis.

the thermal stability and fraction of volatile components in the

samples while the Boehm titration was applied to quantitatively

estimate the amount of acidic and basic groups present on the

2

.6. Preparation of MWCNT-COOttpy

MWCNT-COCl (100 mg) were added to HO-Phttpy (100 mg) in

a round-bottomed ask containing 20 cm of dry THF and 2–3

drops of glacial acetic acid. The mixture was reuxed at 64 C for

27,28

adsorbents.

Elemental analysis (ThermoScientic Flash

3

2000) of adsorbents was also carried out to investigate the

ꢂ

percentage composition of carbon, hydrogen, oxygen and

nitrogen present in the samples. The point of zero charge

24 h under an inert atmosphere of argon. Aer cooling to room

temperature, the resulting suspension was ltered and the

product obtained was washed with THF and dried in a vacuum

oven at 80 C overnight.

4

(

pH_P_Z_C) was also performed as described by Oyetade et al. and

29

Khan et al. to estimate the electrical charge density of the

adsorbents. Details of the Boehm titration and point of zero

charge determination are provided in the ESI.†

ꢂ

2

.7. Preparation of MWCNT-ttpy

0

2

.3. Procedure for the synthesis of 4 -(4-hydroxyphenyl)-

,2 :6 ,2 -terpyridine (HO-Phttpy)

The reduction of MWCNT-COOttpy to obtain an ether was

carried out as reported by Sakai et al. MWCNT-COOttpy

0

00

34

3

0,31

(100 mg), InBr

3 3

(10.6 mg, 0.03 mmol) and Et SiH (380 mL,

The ligand was synthesized as reported by Patel et al.,

with

3

2.4 mmol) were added to a 30 cm volume of freshly distilled

some modications. 2-Acetylpyridine (2.423 g, 20.0 mmol) was

added to 15 cm of a 2 : 1 (v/v) mixture of ethanol and water

containing 4-hydroxybenzaldehyde (1.221 g, 10.0 mmol). To the

solution, NaOH pellets (1.458 g, 26.0 mmol) and 30 cm

aqueous NH were added and stirred continuously at room

3

temperature for 8 h to yield a cream-coloured precipitate. The

resulting mixture was ltered, the solid obtained was washed

ꢂ

3

chloroform, and reuxed at 60 C for 1 h under an inert atmo-

sphere of argon. The suspension was ltered and the solid ob-

tained was washed with chloroform, followed by water until

a neutral pH was obtained. The resulting solid was dried in

a vacuum oven at 80 C overnight and stored in a desiccator for

future analysis.

3

ꢂ

3

with deionised water (5 ꢁ 10 cm ), followed by absolute ethanol

3

2.8. Adsorbate preparation

(

3 ꢁ 5 cm ) to obtain the crude white product (508.8 mg, 42%).

ꢂ

ꢀ1

1

2+

m.p. 199–201 C; IR (ATR, cm ) 3375, 1614, 1588, 1565; H

A standard stock solution of Cu was prepared by dissolving

3

NMR (400 MHz, DMSO-d ) d: 6.92 (d, 2H, J ¼ 8.6 Hz), 7.49–7.52

6

approximately 1.0 g of pure copper metal into 50 cm of 5

ꢀ

3

(

m, 2H), 7.75 (d, 2H J ¼ 8.68 Hz), 7.99–8.04 (m, 2H), 8.67–8.74

mol dm nitric acid. The solution was made up to 1000 dm

13

m, 6H); C NMR (400 MHz, DMSO-d

6

) d: 160.2, 155.4, 155.1,

with deionized water. From this solution, working solutions

were prepared by diluting the stock solution in accurate

proportions to obtain the required concentrations.

1

49.4, 149.2, 137.3, 128.0, 126.8, 124.3, 120.8, 116.8, 116.4; HR-

MS [C21H N O] ES: [M + H ] m/z calcd 326.1215, found

15 3

+

326.1293. Additional spectral information is shown in ESI

Fig. S1–S3.†

2.9. Sorption experiments

2

+

The adsorption of Cu on prepared adsorbents was investi-

gated by using batch adsorption experiments. All adsorption

experiments were conducted in duplicate by using 50 cm

2

.4. Preparation of MWCNT-COOH

Oxidation of MWCNTs was carried out as reported by Oyetade

et al. and Santangelo et al. In brief, pristine MWCNTs (1.5 g)

were placed in a round-bottomed ask containing 100 cm of

concentrated hydrochloric acid, and stirred for 4 h to remove

residual metal impurities from the tubes. The resulting mixture

was ltered, and the solid was washed with deionised water

until a neutral pH was obtained. The sample obtained was dried

in a vacuum oven at 80 C overnight and stored in a desiccator

for future analysis. The puried MWCNTs were then oxidized by

using a mixture of sulfuric and nitric acids in a volume ratio of

1

diluted with deionised water, ltered, and the residue obtained

was washed continuously with deionised water until a neutral

pH was obtained.

3

4

32

polypropylene plastic vials. Freshly prepared working solutions

3

ꢀ3

2+

of 100 mg dm Cu were prepared from the stock solution

daily. Adsorption experiments were performed by agitating

3

2+

ꢀ3

2

0 cm of a known Cu concentration (100 mg dm ) in

ꢂ

a thermostated water bath at a xed temperature (20 C) for 24 h

with an adsorbent dose of 50 mg. The pH of the solution was

ꢂ

ꢀ3

adjusted by adding appropriate amounts of 0.1 mol dm NaOH

3

or HNO to obtain the desired pH. Aer the required time

interval, the mixtures were ltered and the nal concentration

ꢂ

: 3, and reuxed at 80 C for 12 h. The resulting mixture was

2+

of Cu in the ltrate was determined by inductively coupled

plasma-optical emission spectroscopy (ICP-OES) (Perkin Elmer

Optima 5300 DV). The eﬀect of pH, amount of adsorbent, initial

2+

Cu concentration, contact time and temperature was studied

in order to determine the optimum conditions necessary for

Cu removal from aqueous solution. The adsorption eﬃciency

2.5. Preparation of MWCNT-COCl

2+

e

MWCNT-COOH (150 mg) were placed in a solution containing (% adsorbed) and adsorption capacity (q ) were calculated by

3

2

0 cm of a 20 : 1 (v/v) mixture of SOCl and dry DMF, and then using eqn (1) and (2), respectively.

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ꢀ

ꢁ

2+

ꢂ

C

i

ꢀ Ceq

a thermostated water bath at 20 C. The concentration of Cu

desorbed was then determined by using ICP-OES.

%

adsorbed ¼

ꢁ 100

(1)

(2)

C

i

ꢀ

ꢁ

C

i

ꢀ Ceq

2.10. Analysis of real samples

q

eq

¼

ꢁ V

m

Three water samples were collected from three sites on the

Umgeni river: (i) the tributary on the conuence of the Umgeni

and Msunduzi rivers, (ii) the outow from the EThekwini

wastewater treatment plant and (iii) the Blue Lagoon (mouth of

Umgeni rivers). The initial concentrations of Pb , Zn and

Cu were determined by using ICP-OES. An aliquot of 25 cm of

the water samples conditioned to pH 5, was measured into

0 cm polypropylene bottles with an addition of 50 mg of

MWCNT-ttpy. The suspensions were agitated in a thermostated

water bath at 20 C for 1 h. Aer agitation, the suspensions were

2

+

ꢀ3

i

C is the initial Cu concentration (mg dm ), Ceq is the equi-

librium concentration of Cu (mg dm ), q_e_qis the adsorption

capacity (mg g ), m is the mass of the adsorbent (mg) and V is

the volume (dm ) of the adsorbate solution used.

.9.1. Kinetics, isotherm and thermodynamic studies.

Kinetics studies were performed by agitating 20 cm aliquots of

00 mg dm Cu solution with an adsorbent dose of 50 mg.

The mixtures were placed in a thermostated water bath at 20 C

2

+

ꢀ3

ꢀ

1

2

+

2+

3

2

+

3

2

3

ꢀ

3

2+

5

1

ꢂ

and agitated for diﬀerent time intervals in the range of 5 to

440 min. The pH of the solutions was adjusted to 5 by using



ltered and their supernatants analysed for the nal concen-

1

0

ꢀ

3

trations of metal ions in solution. The removal eﬃciency and

adsorption capacity for each metal ion were evaluated according

to eqn (1) and (2), respectively.

.1 mol dm

NaOH/HNO . Aer the pre-determined time

3

intervals, the samples were ltered by gravity and the equilib-

2

+

rium concentration of Cu determined by ICP-OES. The

experimental data obtained were applied to the pseudo-rst

14,35

14,19,35,36

37

2.11. Data analysis

order,

pseudo-second order,

intraparticle diﬀusion

38

and Elovich kinetics models. The equations for the models are

given in ESI Table S1.†

The data obtained were tted to the isotherm and kinetics

models by means of the nls nonlinear regression routine in the

R statistical computing environment. The R statistical so-

ware takes into account the minimization of the sum of squared

residuals (SSR) and the residual square errors (RSE). A

comparison of all SSR and RSE values was done in order to

assess the adequacy of the models. The model chosen was that

with the lowest SSR.

Adsorption isotherms were investigated by using varying

47

2

+

ꢀ3

Cu

concentrations, ranging from 10–100 mg dm , at

3

a constant pH of 5. Aliquots of 20 cm were mixed with 50 mg of

the adsorbents and agitated on a thermostated shaking water

bath under varying temperatures of 293, 303, 313 and 318 K for

2

4 h. The solutions were ltered by gravity and the concentra-

2

+

tions of Cu in the ltrates determined by ICP-OES. The

experimental adsorption equilibrium data were analysed by

eight adsorption isotherm models consisting of both two-

3

. Results and discussion

39

40

41

(Langmuir, Freundlich, Temkin and Dubinin–Radushke- The ligand and nanomaterials synthesised were characterized

4

2

4

3

4

vich models) and three-parameter (Sips, Toth, Redlich– by a series of techniques to ascertain the authenticity and

Peterson and Khan ) models. The equations of the models are properties of the adsorbents for Cu removal. Subsequently,

45

46

2+

2

+

given in ESI Table S2.† Thermodynamic parameters such as the eﬃciency of the novel material to adsorb Cu was assessed

ꢂ

change in Gibbs energy (DG ), change in enthalpy (DH ), and and compared with that of acid-functionalized MWCNTs by

ꢂ

change in entropy (DS ) were also calculated over the studied means of batch adsorption processes.

temperature range.

0

00

2.9.2. Selectivity of MWCNT-ttpy for diﬀerent metal ions.

3

.1. Synthesis of 4 -(4-hydroxyphenyl)-2,2 :6 ,2 -terpyridine

To investigate the selectivity of MWCNT-ttpy for diﬀerent metal

(

HO-Phttpy)

3

ions, 25 cm of a solution containing equal concentrations of

0

00

The synthesis of 4 -(4-hydroxyphenyl)-2,2 :6 ,2 -terpyridine was

carried out by using a “greener” synthetic approach as illus-

trated in Fig. 1. The use of green solvents such as ethanol and

water at room temperature aﬀords an opportunity of obtaining

products in moderate yields. Upon reacting 4-hydroxy-

benzaldehyde and 2-acetylpyridine in an ethanolic solution,

a cream-coloured precipitate was formed. The completion of the

reaction was monitored by thin layer chromatography (TLC)

and FTIR and NMR spectroscopy. The formation of the product

was conrmed by the disappearance of the carbonyl absorption

2

+

2+

ꢀ3

Pb , Zn , Cd and Cu between 10 and 50 mg dm was

conditioned to pH 5.5 with the addition of appropriate amounts

ꢀ

3

of 0.1 mol dm NaOH or HNO . About 200 mg of the adsorbent

was added into the solution and thereaer agitated on a ther-

ꢂ

mostated water bath at 20 C for 24 h. The suspensions were



ltered and the nal concentration of metal ions in the ltrates

was determined by means of ICP-OES.

.9.3. Desorption experiments. Desorption studies were

2

3

2+

carried out by rst contacting aliquots of 20 cm of Cu solu-

tion with a concentration of 100 mg dm and an adsorbent

ꢀ3

ꢀ1

band at z1700 cm and the appearance of sharp peaks at

1

6

dose of 50 mg for 24 h. The solution was ltered and the loaded-

ꢀ1

634 cm (C]N str), 1250–1335 cm (C–N str), 712 cm and

ꢂ

adsorbent obtained was dried in a vacuum oven at 80 C. The

ꢀ1

34 cm (indicating g and d Py-ring in-plane and out-of-plane

2

+

remaining Cu concentration in the ltrate was determined. A

deformation vibrations respectively) in the FTIR spectra

3

5

0 mg mass of the loaded-adsorbent was added to a 10 cm

48–50

1

(Fig. 4c).

H NMR spectrometry further conrmed the

ꢀ

3

aliquot of 0.1 mol dm

HCl and agitated for 30 min in

formation of the product by the disappearance of the methyl

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0

00

Fig. 1 Synthesis of 4 -(4-hydroxyphenyl)-2,2 :6 ,2 -terpyridine (HO-Phttpy).

ꢀ

1

ꢀ1

proton resonance peak (z2.10 ppm) and the appearance of 2970 cm (CH and CH groups), 3500 cm (O–H str), 1250–

2

ꢀ

1

ꢀ1

aromatic proton resonance peaks (z7.5–8.0 ppm) (see ESI 1335 cm (C–N str), 712 cm and 634 cm (indicating g and

1

3

Fig. S1†). Further verication of the product was done by

NMR and mass spectroscopy (see ESI Fig. S2 and S3†).

C

d Py-ring in-plane and out-of-plane deformation vibrations

respectively) were obtained.

48–50

Of importance to note is that

MWCNT-ttpy (Fig. 4d) showed a similar prole to that of the

ligand (Fig. 4c). This therefore conrms that the successful

functionalization of MWCNT-COOH with HO-Phttpy was ach-

ieved, since functional groups characteristic to the ligand were

present for MWCNT-ttpy.

3

.2. Synthesis of MWCNT-ttpy

The synthesis of the novel nanomaterial was carried out rst by

the acylation of MWCNT-COOH with a mixture of SOCl and

DMF in a volume ratio of 20 : 1 as illustrated in Fig. 2.

Acylated nanotubes (MWCNT-COCl) were further reacted in

a mass ratio of 1 : 1 with HO-Phttpy to aﬀord esteried

MWCNTs (MWCNT-COOttpy). Reduction of MWCNT-COOttpy

was carried out in distilled CHCl

a reducing agent and InBr as a catalyst, to aﬀord ether-

functionalized CNTs (MWCNT-ttpy). The route to the

synthesis of MWCNT-ttpy is shown in Fig. 3.

2

To conrm the shape and structure, and determine the

morphology of the synthesized adsorbents, images were

collected from transmission electron microscopy (TEM) and

scanning electron microscopy (SEM). The micrographs shown

in Fig. 5 represent the SEM images of (a) P-MWCNT, (b)

MWCNT-COOH and (c) MWCNT-ttpy while Fig. 6 shows the

TEM images of (a) P-MWCNT, (b) MWCNT-COOH and (c)

MWCNT-ttpy. As shown in Fig. 5a and 6a, a large entanglement

of tubes was observed for P-MWCNT demonstrating a high

presence of metal catalyst (shown with arrows) and amorphous

carbon on the surface of the tubes. More alignment and less

agglomeration was observed for MWCNT-COOH (i.e. tubes

puried and functionalized with acids) (Fig. 5b). This is in

3 3

, by using Et SiH as

3

3.3. Characterization of adsorbents

The synthesized products were characterized by FTIR spec-

troscopy. Fig. 4 shows the FTIR spectra of (a) P-MWCNT, (b)

MWCNT-COOH, (c) HO-Phttpy and (d) MWCNT-ttpy. For pris-

tine MWCNTs (P-MWCNTs), the peaks observed at z1498 cm

are assigned to C]C stretching vibrations (Fig. 4a) which are

representative of graphitic structures characteristic of CNTs.

The peaks observed around z3500 and z2700 cm

attributed to the presence of water molecules and carbon

dioxide, respectively. Acidic functional groups such as

carboxylic groups were introduced onto the walls of tubes aer

51

ꢀ

1

agreement with the study of Rosca et al. Oxidation of CNTs

accounts for the shortening of tubes, thereby introducing new

functional groups, such as carboxylate groups, onto open ends

of CNTs. Purication of CNTs removes the metal catalyst and

1

ꢀ1

are

1

amorphous carbon present on the surface of the tubes. Also,

Fig. 6c further demonstrates that the tubular structure charac-

teristic of MWCNTs was preserved aer functionalization with

the nitrogen ligand was carried out. A higher interspaced region

was also noticed between tubes when compared with MWCNT-

COOH (Fig. 5b vs. c). The images shown in Fig. 6a and

b demonstrate the straight morphology for P-MWCNTs and

MWCNT-COOH, while a curved/bent morphology was obtained

ꢀ

1

oxidation, hence a new peak at z1750 cm , assigned to C]O

stretching, was noticeable (Fig. 4b), indicating the presence of

new groups aer oxidation was carried out. For HO-Phttpy

23

ꢀ

1

(Fig. 4c), peaks at 3050 cm

(aromatic C–H str), 1475–

ꢀ

1

ꢀ1

1585 cm (aromatic C]C str), 1634 cm (C]N str), 2890–

Fig. 2 Acylation of pristine MWCNTs.

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Fig. 3 Route to the synthesis of MWCNT-ttpy.

for MWCNT-ttpy (Fig. 6c) due to the introduction of the ligand.

Evidence of functionalization was also noted as long strands of

CNTs were cut to obtain short and open-ended tubes.

In order to establish the thermal stability, purity and amount

of volatile components in the synthesized adsorbents, ther-

mogravimetric analysis (TGA) was carried out. Fig. S4† repre-

sents the (a) thermograms (TGA) and (b) derivative

thermograms (DTG) of P-MWCNT, MWCNT-COOH and

MWCNT-ttpy. As observed from Fig. S4a,† both P-MWCNTs and

MWCNT-COOH were more thermally stable than MWCNT-ttpy.

A lower decomposition temperature was obtained for MWCNT-

ttpy than for P-MWCNTs and MWCNT-COOH (Fig. S4a†). This

could be attributed to the increase in defects induced by the

functionalization of MWCNT-COOH. This result is consistent

with data obtained from Raman spectroscopy (Table 1), which

demonstrates that MWCNT-ttpy had a higher I_D/I_Gratio than

either P-MWCNTs or MWCNT-COOH. A higher ratio was also

obtained for MWCNT-COOH when compared with P-MWCNTs.

Hence, decreasing crystallinity in MWCNT-COOH and MWCNT-

ttpy was associated with increasing functionalization, which

thereby induces an increase in defects on P-MWCNTs, resulting

Fig. 4 FTIR spectra of (a) P-MWCNT, (b) MWCNT-COOH, (c) HO-

Phttpy and (d) MWCNT-ttpy.

in a larger I

D G

/I ratio. An increase in defective sites also

accounts for a decrease in the decomposition temperature of

Fig. 5 SEM images of (a) P-MWCNT, (b) MWCNT-COOH and (c) MWCNT-ttpy.

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Fig. 6 TEM images of (a) P-MWCNT, (b) MWCNT-COOH and (c) MWCNT-ttpy.

2

+

MWCNT-COOH and MWCNT-ttpy obtained from TGA. The of the adsorbents for the removal of Cu from aqueous

thermal stability of MWCNTs therefore increased in the order solutions.

MWCNT-ttpy < MWCNT-COOH < P-MWCNTs. These results are

Elemental analysis of the adsorbents conrmed the presence

of nitrogen atoms attached to the ligand on the MWCNT-ttpy.

1

consistent with recently published studies by Zhao et al.,

7

52

Ombaka et al., and Chizari et al., indicating that increasing However, as expected, nitrogen atoms were absent in

functionalization results in a reduced decomposition tempera- P-MWCNTs and MWCNT-COOH (Table S3†). Thus, the presence

D G

ture and larger I /I ratio. Purication of P-MWCNTs also of nitrogen atoms in MWCNT-ttpy conrms the successful

resulted in the decrease of metal catalyst present in MWCNT- functionalization of MWCNT-COOH with HO-Phttpy. The

COOH (Fig. S4a†) as indicated by the residual mass remaining elemental analysis also showed an increase in the amount of

ꢂ

aer 620 C. Three thermal decomposition stages were obtained oxygen in the order P-MWCNTs < MWCNT-COOH < MWCNT-

for MWCNT-ttpy. The rst stage (in circle) showed a mass loss ttpy. Acidic functionalization of P-MWCNTs accounts for the

ꢂ

(

Dm ¼ 9.88%) in the interval of 282–412 C, suggesting the increase in the oxygen content of MWCNT-COOH due to the

release of CH

(

4

O (9.85%, calcd). The second decomposition step incorporation of oxygen-containing functional groups onto the

4

shown with arrow) demonstrates a mass loss (Dm ¼ 35.84%) in walls of tubes. An increase in the oxygen content of MWCNT-

ꢂ

the temperature range of 434–555 C, suggesting the release of ttpy is accounted for by the functionalization of MWCNT-

(35.08%, calcd). The third decomposition step was COOH, which introduces nitrogen and more oxygen atoms

9 6

C H

observed for all three types of CNTs (i.e. P-MWCNTs, MWCNT- onto the tubes. This result is consistent with data obtained from

COOH and MWCNT-ttpy), representing the total decomposi- the Boehm titration (Table 2) which demonstrates an increase

tion of the graphitic structure of CNTs.

The surface area of the adsorbents was noticed to increase in functionalization increases.

the order P-MWCNTs < MWCNT-COOH < MWCNT-ttpy (Table The nature of the acidic and basic functional groups on the

). The results obtained indicated an increase in surface area adsorbents was determined by the Boehm titration method.

in the surface acidic groups of the MWCNTs as the extent of

28,53

1

and pore volume of MWCNT-COOH when compared with Functionalization of P-MWCNT with acids markedly increased

P-MWCNT (Table 1, Entry 2 vs. 1). This is attributed to the the concentration of carboxyl, phenolic and lactonic groups on

treatment of P-MWCNT, which results in cutting of large MWCNT-COOH (Table 2). This was consistent with the FTIR

entanglements of tubes (Fig. 6a), thereby causing an increase in spectra (Fig. 4b) obtained, which revealed the appearance of

7

ꢀ1

surface area and pore volume as observed in MWCNT-COOH.

a new peak at z1750 cm corresponding to the formation of

Furthermore, an increase in surface area and pore volume of carboxylic groups. This result conforms with data reported by

27

MWCNT-ttpy (Table 1, Entry 3 vs. 2) was obtained, due to Biniak et al. wherein an increase in acidic properties was

increasing functionalization of MWCNT-COOH. This result attributed to oxidation of adsorbents with acids. However,

conrms that surface area/pore volume of nanomaterials can be further functionalization with the nitrogen ligand decreased the

increased based on the extent of functionalization. All adsor- concentration of the carboxyl groups but increased the

bents were mesoporous in nature since the pore diameters were concentrations of phenolic and lactonic groups. It is also worthy

less than 50 nm. Hence, MWCNT-COOH and MWCNT-ttpy of note that an increase in the total number of basic groups of

possess moderately large surface areas and increased pore the adsorbents was observed as the extent of functionalization

volume, which might enhance faster and better sorption ability increased. MWCNT-ttpy contained the largest amount of basic

Table 1 Textural characterization of synthesized nanomaterials

2

ꢀ1

3

ꢀ1

Entry

Adsorbents

Surface area/m g

Pore volume/cm g

Pore diameter/nm

D G

I /I

1

2

3

P-MWCNT

MWCNT-COOH

MWCNT-ttpy

108.8

126.8

189.2

0.494

0.692

1.252

18.44

22.95

27.26

1.17

1.19

1.31

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Table 2 Surface chemistry of P-MWCNTs, MWCNT-COOH and MWCNT-ttpy determined by the Boehm titration method

Carboxyl/

mmol g

Lactonic/

mmol g

Phenolic/

mmol g

Total acidic groups/

mmol g

Total basic groups/

mmol g

ꢀ

1

ꢀ1

Adsorbents

pHPZC

P-MWCNTs

MWCNT-COOH

MWCNT-ttpy

0.136

0.719

0.613

0.014

0.104

0.165

0.114

0.401

0.544

0.264

1.224

1.322

0.145

0.226

0.752

5.04

4.02

4.48

+

groups justifying the presence of nitrogen-containing groups on H ions for active sites on the adsorbent. Increasing the pH of

+

the adsorbent.

the solution led to a decrease in H , which enabled electrostatic/

2

+

coordination attraction of Cu onto the active sites of the

adsorbents. Cationic adsorption is favourable at pH values

55

2

+

3.4. Batch adsorption processes

higher than pHPZC

; hence, sorption of Cu was propitious at

pH conditions greater than 4.5. Higher removal eﬃciencies of

Sorption processes were carried out to examine the eﬀectiveness

and eﬃciency of MWCNT-COOH and MWCNT-ttpy for the

2

+

Cu were noticeable from pH 3 to 7 when MWCNT-ttpy was

used as adsorbent. This is attributed to an increase in the

number of chelating sites on the adsorbent, which permits

strong binding between the nitrogen-donor atoms in the

2

+

removal of Cu from a simulated wastewater. The role of

various parameters that inuence adsorption such as pH,

contact time, adsorbent dose, temperature and initial adsorbate

concentration were investigated to ascertain the ideal condi-

2

+

adsorbent and Cu . Modication of MWCNT-COOH with HO-

Phttpy signicantly improved the textural properties of

MWCNT-COOH, resulting in an increase in surface area, pore

volume (Table 1) and active sites on MWCNT-ttpy (Table 2),

2

+

tions suited for Cu removal. Kinetics, isotherm and thermo-

dynamic studies were also carried out by using the data

obtained.

2

+

2

+

hence, enabling better sorption ability for Cu removal.

Although the data suggest that pH 7 is optimal for the removal

3

.4.1. Eﬀect of pH. The extent of adsorption of Cu onto

the studied adsorbents was investigated at diﬀerent pH values

2

+

2

+

of Cu , further adsorption studies were carried out at a pH

ranging from 1.0 to 10.0. Removal of Cu from aqueous solu-

tion is usually inuenced by the initial pH of the solution, since

it inuences the surface charges present on the adsorbent and

2

+

value of 5, in order to limit the eﬀect of precipitation on Cu

removal.

54

3

.4.2. Eﬀect of contact time. The eﬀect of contact time for

the speciation of the metal ion in solution. Fig. 7b shows the

2

+

2

+

the adsorption of Cu was examined at varying time intervals,

between 5 and 1440 min, while keeping the initial Cu

concentration (100 mg dm ), pH (5.0) and adsorbent dose

50 mg) constant. The percentage removal of Cu onto the

speciation of Cu as a function of pH in aqueous solution. Free

2

+

2

+

2+

Cu exist only at pH values # 5. Precipitation of Cu as hydroxy

ꢀ

3

species occurs when the pH of the solution is further increased.

2

+

2

+

(

A signicant increase is noticed for the removal of Cu from

aqueous solution as the pH of the solution is increased (Fig. 7a).

The surface charge of the adsorbents is positive at pH values

less than their pHPZC (Table 2), resulting in a low removal of the

studied adsorbents was noticeably increased with time, attain-

ing equilibrium for MWCNT-COOH and MWCNT-ttpy at

180 min and 360 min, respectively (Fig. 8). Aer this stage, the

2

+

percent adsorption was observed to remain steady with little or

no further increase. Initially, more active sites are available for

adsorption, hence, enabling faster removal. As the binding sites

adsorbate due to strong competitive activity between Cu and

2+

Fig. 7 (a) Eﬀect of pH on sorption of Cu onto MWCNT-COOH and

3

ꢀ3

2+

MWCNT-ttpy, [conditions: 20 cm of 100 mg dm

Cu , 24 h

equilibration time, 50 mg adsorbent dose, agitation speed 150 rpm,

ꢂ

2+

temperature 20 C] and (b) speciation of Cu as a function of pH in

2

+

aqueous solution. Numerical values of log b for the metal hydroxides Fig. 8 Eﬀect of contact time for the adsorption of Cu

onto

3

used in the calculation of the speciation curves were obtained from MWCNT-COOH and MWCNT-ttpy [conditions: 20 cm of 100

56

ꢀ3

2+

critical stability constants compiled by Smith and Martell, and plots mg dm

obtained with the aid of HySS software.

Cu , 50 mg adsorbent dose, pH 5.0, agitation speed

57

ꢂ

150 rpm, temperature 20 C].

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on the surface of the adsorbents were occupied, further removal adsorbents. Similar results have been reported by Ho and

2

+

61

26

10

62

of Cu was observed to be minimal. Though adsorption was McKay, Popuri et al. Mobasherpour et al. and Yu et al. for

2

+

rapid in both adsorbents, MWCNT-ttpy exhibited higher removal of Cu from aqueous solutions. The Elovich model is

removal eﬃciency at each time than MWCNT-COOH. This also based on the concept of chemisorption (i.e. the interac-

3

6

could be attributed to the greater number of chelating sites on tion through sharing or exchange of electrons). This further

2

+

MWCNT-ttpy, induced through the functionalization of elucidates that the adsorption of Cu onto both adsorbents

MWCNT-COOH with a nitrogen-containing compound. In this proceeds via chemical interactions between the active groups

2

+

study, the maximum time for agitation was xed at 24 h to on the adsorbents and cationic Cu .

2

+

ascertain the complete removal of Cu from aqueous solutions

under all conditions.

Adsorption proceeds via one/more of these four stages: (i) the

transfer of solute from the solution to the surface of the

3.4.3. Kinetics studies. Four kinetics models, namely, the adsorbent, (ii) solute transfer from the bulk solution to the

pseudo-rst order, pseudo-second order, Elovich and intra- boundary lm which surrounds the adsorbent surface (lm

particle diﬀusion models were applied to the experimental data diﬀusion), (iii) solute transfer through the internal pores of the

in order to determine the dynamics and rate-determining step adsorbent (intraparticle diﬀusion), and (iv) interaction between

of the adsorption process. Table S1† gives the equations of the adsorbate molecules with the active sites on the external surface

models used in this study. The experimental data obtained were of the adsorbent. One of these processes usually determines the



tted into the kinetics models by non-linear least squares rate at which the adsorbate is removed from aqueous solutions.

(NLLS) analysis. Previous studies have shown that to eliminate To further elucidate the diﬀusion mechanism involved in

error distributions associated with models in linearized forms, adsorption, experimental data were modelled with the intra-

pﬃﬃ

5

8–60

NLLS analysis

is a preferred choice. The curves showing particle diﬀusion model. A plot of q against was t plotted to

t

a comparison of all the kinetics models tted for the adsorption obtain a straight line which did not pass through the origin.

of Cu onto MWCNT-COOH and MWCNT-ttpy are shown in ESI This indicates that adsorption proceeded through the intra-

2

+

2

+

Fig. S5.† Data illustrating the comparison of all models with particle diﬀusion of Cu to the pores of the adsorbent;

63–65

their associated sum of squared residuals (SSR) and residual however, it was not the only rate-controlling step.

Adsorp-

square errors (RSE) is given in Table 3. It was observed that the tion therefore occurred via a multi-step process involving an

data for MWCNT-ttpy best t the pseudo-second order model, initial rapid stage, followed by the intraparticle diﬀusion of

while that for MWCNT-COOH t the Elovich model better as adsorbates to the pores of adsorbents and then to a slower

reected by the lowest SSR values (Table 3). The pseudo-second phase which proceeds towards saturation due to low adsorbate

2

+

58

order model assumes that the rate-limiting step for Cu

concentration. Higher kid and l values were obtained for

adsorption may be due to interaction through sharing or MWCNT-ttpy than MWCNT-COOH, showing that adsorption

2

+

exchange of electrons between hydroxyl/nitrogen groups and was boundary-controlled and indicates better Cu removal for

2+

14,26,36,61

Cu .

Hence, this indicates that the rate of removal of MWCNT-ttpy.

2

+

Cu from aqueous solution was determined by a bimolecular 3.4.4. Eﬀect of adsorbent dose. The eﬀect of increasing the

2

+

interaction between the adsorbate and active sites on the amount of adsorbent on Cu ion removal was investigated at

varying adsorbent dosage ranging from 30–400 mg. An increase

2

+

in the percentage removal of Cu was achieved as the mass of

adsorbent increased, until a limiting value was reached, where

removal remained steady (Fig. 9). This is attributed to the fact

that as the dose of adsorbent increases, there is an increase in

Table 3 Kinetics parameters for the adsorption of divalent copper ion

from aqueous solution

MWCNT-

COOH

MWCNT- the surface area and number of active sites available for

Model

Parameter

ttpy

2+

adsorption. A high eﬃciency of Cu removal was obtained for

ꢀ

1

MWCNT-ttpy with percentages in the range of 80 to 100% being

obtained in contrast to 60% for MWCNT-COOH. This illustrates

that complete removal of Cu can be achieved with MWCNT-

Pseudo-rst order

k

q

1

/min

0.259

10.06

0.741

8.242

0.044

10.44

0.447

2.994

0.491

7.764

5.639

508.8

5.237

1.864

0.217

0.705

0.073

33.30

3.350

168.3

0.003

35.25

1.858

51.77

1.573

14.66

15.64

3916

ꢀ

1

eq^/^ma ^g^g

2+

RSE_b

SSR

ttpy; hence, it has promising applications for remediating

wastewater generated from industries. The increase in the

number of metal binding sites on MWCNT-ttpy is responsible

ꢀ1

Pseudo-second order

Intraparticle diﬀusion

Elovich

k

2

q

RSE

SSR

k

l/mg g

RSE

SSR

/g mg ₁min^ꢀ¹

ꢀ

eq/mg g

2

+

for the high removal of Cu observed. An adsorbent dose of 100

mg of MWCNT-ttpy achieved 99% removal eﬃciency, aer

which a state of equilibrium was reached with increase in

ꢀ

1

ꢀ0.5

id/mg g min

ꢀ1

2+

adsorbent dose due to the limited amount of Cu in solution.

ꢀ

1

ꢀ1

This state of equilibrium was achieved with a dose of 300 mg for

MWCNT-COOH. However, it is worthy of note that as the dose of

adsorbent is increased, the adsorption capacity (qeq) decreased

for both adsorbents. This is due to the fact that increasing the

adsorbent dose at the same concentration and volume of

adsorbate solution leads to unsaturation of adsorption sites.

a/mg g min

192.7

0.287

2.082

65.00

ꢀ

1

b/g mg

RSE

SSR

a

b

RSE – residual square error. SSR – sum of squared residuals.

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temperature variation for MWCNT-COOH, whereas very little

eﬀect was observed for MWCNT-ttpy with change in tempera-

ture (Fig. 10b). This illustrates that MWCNT-ttpy will be eﬀec-

2

+

tive for Cu removal, and could prove advantageous regardless

of change in temperature. Also, the application of MWCNT-ttpy

2

+

will be suﬃcient for the removal of Cu at point source, as

eﬄuents are normally discharged at above ambient tempera-

tures. It is also worthy of note that the amount adsorbed (qeq) by

MWCNT-ttpy was also greater than for MWCNT-COOH at all

temperatures.

3.4.7. Adsorption isotherms. Adsorption isotherms are

usually used to describe how adsorbate molecules/ions interact

2

+

Fig. 9 Eﬀect of adsorbent dose for the adsorption of Cu onto

3

with the active sites on the surface of the adsorbent and to

MWCNT-COOH and MWCNT-ttpy [conditions: 20 cm of 100

ꢀ3

2+

4

mg dm

Cu , 24 h equilibration time, pH 5.0, agitation speed optimize adsorbent use for large-scale removal. Eight

ꢂ

1

50 rpm, temperature 20 C].

isotherms, including two- and three-parameter models, namely,

Langmuir, Freundlich, Temkin, Dubinin–Radushevick (D–R),

Sips, Toth, Redlich–Peterson (R–P) and Khan, were applied to

3

.4.5. Eﬀect of initial adsorbate concentration. The inu- the equilibrium data obtained in this study. The equations for

ence of increasing the adsorbate concentration was examined the isotherms are given in Table S2.† NLLS analysis was used to

2

+

for diﬀerent Cu concentrations, ranging from 10–100 mg t the data and the lowest sum of squared residuals (SSR) gave

ꢀ3

dm , on MWCNT-COOH and MWCNT-ttpy. A decrease in the an indication of the best t for the equilibrium data obtained.

2

+

2+

percentage removal of Cu from 98.2% to 47.6% and 100.0% to The parameters obtained for the adsorption of Cu onto

8.9% was obtained for MWCNT-COOH and MWCNT-ttpy, MWCNT-COOH and MWCNT-ttpy are presented in Table 4.

respectively. This could be attributed to the greater number of

Table 4 demonstrates that the Freundlich isotherm best

7

2

+

active sites available at lower Cu concentrations, resulting in describes the experimental data obtained for MWCNT-COOH,

higher removal eﬃciencies. However, saturation of active sites while the Langmuir isotherm best described the data ob-

occurred at higher concentrations, resulting in lower removal tained for MWCNT-ttpy. The curves for the best-t isotherms for

eﬃciencies. Also, an increase in the amount of Cu removed MWCNT-COOH and MWCNT-ttpy are shown in ESI Fig. S6 and

per unit mass from 3.87 to 19.01 mg g and 3.94 to 31.89 mg S7,† respectively. The Langmuir maximum adsorption capacity

2

+

ꢀ1

g

was obtained for MWCNT-COOH and MWCNT-ttpy, (q

respectively. Higher initial concentrations of Cu enhanced

m

ꢀ1

) varied from 15.50 to 19.44 mg g and 31.65 to 34.13 mg

for MWCNT-COOH and MWCNT-ttpy, respectively over the

2

+

g

the adsorption process, due to an increase in the driving force studied temperature range. Table 4 further indicates a decrease

by overcoming mass transfer resistance between the solid– in q as the temperature of the adsorbate solution was

solution interface. Similar trends for the removal of Cu from increased for MWCNT-COOH, whereas an increase in q was

aqueous solution as concentration is varied have been reported obtained with increasing temperature for MWCNT-ttpy. The

m

54

2+

m

54

66

62

by Tong et al. Ekmekyapar et al. and Yu et al.

.4.6. Eﬀect of temperature. Increasing the adsorbate adsorptive binding power obtained for MWCNT-ttpy, is indica-

Langmuir adsorption constant, b (Table 4), indicating the high

3

temperature results in an increase in the rate of diﬀusion across tive of good adsorptive forces resulting in the enhancement of

the external boundary layer, decreases the viscosity of the the binding strength as the temperature is increased.

67

2+

solution and increases the internal pores of the adsorbents.

The amount of area on the adsorbent covered by Cu can be

The extent of adsorption was investigated over the temperature established by dividing the theoretical specic surface area (S)

range of 293–318 K. An increase in the adsorption capacity (q_e_q) into the BET surface area. The theoretical surface area was

6

8

with an increase in temperature was obtained for MWCNT-ttpy calculated by using eqn (3) as described by Ho et al.

Fig. 10b), whereas a decrease in qeq was obtained with

increasing temperature for MWCNT-COOH (Fig. 10a). An

(

m A

q N A

S ¼

(3)

M

2

+

increase in temperature favours faster mobility of Cu , as was

2

+

S is the area covered by the adsorbed Cu , N

A

+

is Avogadro's

the case for MWCNT-ttpy, hence resulting in higher qeq values.

2

ꢀ20

2

+

constant, A is the cross-sectional area of Cu (1.58 ꢁ 10

Also, higher Cu uptake by MWCNT-ttpy at higher tempera-

tures may be attributed to an increase in the porosity and pore

volumes of the adsorbent, hence, enabling the pores for better

68

2 2+

m ) and M is the molar mass of Cu . The values obtained for S

ꢂ

at 20 C for MWCNT-COOH and MWCNT-ttpy were calculated

as 2.91 and 4.74 m g , respectively. As given by Table 1, the

BET surface area of MWCNT-COOH and MWCNT-ttpy was 126.8

g , respectively. Hence, the percentage of

adsorbent surface covered by Cu was calculated as 2.30% and

values at 20 C were compared

with the total number of acidic and basic groups in MWCNT-

COOH and MWCNT-ttpy determined by the Boehm titration

2

ꢀ1

4

uptake of adsorbate. However, increasing the adsorbate

temperature also results in a decrease of the physical adsorptive

forces responsible for adsorption. This phenomenon was

observed for MWCNT-COOH (Fig. 10a), and resulted in

a decrease in qeq values as the temperature of the adsorbate

increased. Fig. 10a also showed a signicant change in qeq with

2

ꢀ1

and 189.2 m

2

+

ꢂ

2

.51%, respectively. Equally, q

m

2740 | RSC Adv., 2016, 6, 2731–2745

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Paper

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2+

Fig. 10 Eﬀect of varying temperature on the adsorption of Cu by (a) MWCNT-COOH and (b) MWCNT-ttpy.

(Table 2). The values obtained indicated that 25.0% of acidic ions on this adsorbent do not interact with each other,

groups on MWCNT-COOH and 66.2% of basic groups on achieving monolayer coverage of the adsorbate. In the case of

2

+

MWCNT-ttpy were primarily involved in adsorption of Cu onto MWCNT-COOH, the values of the Freundlich constant n indi-

active sites of adsorbents. These results indicate that chelation cate a favourable adsorption process since values were #10 (see

2

+

54

of Cu with nitrogen-donor atoms in MWCNT-ttpy and elec- Table 4).

trostatic interactions with oxygen atoms in MWCNT-COOH were Table 5 shows the results of some previously published

primarily responsible for interaction in this study. Hence, studies investigating the sorption process of Cu from aqueous

2

+

2

+

adsorption of Cu could be said to be enhanced signicantly solutions onto carbon-structured materials. These results were

due to the presence of N donor atoms on MWCNT-ttpy. compared with those obtained in this study. A signicant

69

70

2+

Also, as suggested by Hamza et al. and Soon-An et al., the increase in sorption uptake of Cu was obtained by using

nature of the adsorption process can be estimated depending on MWCNT-ttpy in comparison with 8-hydroxyquinoline-

the values of the separation factor (R ) as expressed in eqn (4). functionalized MWCNTs (8-HQ-MWCNT), also functionalized

L

Adsorption is assumed to be favourable if 0 < R

L

< 1, unfav- with a N-donor ligand but of lower denticity. Table 5 further

69–71

2+

ourable if R > 1, irreversible if R

L

¼ 0 and linear if R

L

¼ 1.

demonstrates better uptake of Cu

comparison with composite adsorbents such as MWCNT/

bagasse and MWCNT/Fe . Thus, the results reported in this

on MWCNT-ttpy in

1

R

L

¼ ₁

(4)

2 4

O

þ bC

i

study compare very favourably with data obtained from other

studies.

2

+

ꢀ3

3

i

C is the initial Cu concentration (mg dm ) and b is the

ꢀ1

Langmuir constant obtained from Table 4 (dm mg ). All R

values obtained in this study were found to fall between 0 < R

, hence adsorption of Cu onto MWCNT-COOH and MWCNT-

L

3

.4.8. Thermodynamic parameters of adsorption. Ther-

L

<

modynamic parameters such as change in enthalpy (DH ),

ꢂ

2

+

1

ꢂ

change in entropy (DS ) and change in Gibbs energy (DG ), were

studied in order to examine the spontaneity of the adsorption

process at varying temperatures. An adsorption process is

ttpy is favourable.

The Langmuir isotherm assumes monolayer coverage of

adsorbates onto the surface of adsorbent, while the Freundlich

isotherm assumes that the surface of adsorbents is heteroge-

ꢂ

considered feasible and spontaneous if negative values for DG

are obtained. The process is endothermic if the adsorption

4

neous in nature. Since MWCNT-ttpy t the Langmuir isotherm,

capacity (q ) increased with an increase in temperature and the

eq

it can be inferred that the active sites on this adsorbent are

equivalent with a uniform surface, hence, the adsorbed Cu

reverse yields an exothermic process.

2

+

2+

Table 4 Isotherm parameters for the adsorption of Cu onto MWCNT-COOH and MWCNT-ttpy

MWCNT-COOH

MWCNT-ttpy

303 K

Isotherm

Langmuir

Parameter

293 K

303 K

313 K

318 K

313 K

318 K

ꢀ

1

q

m

/mg g

19.44

0.169

1.887

28.48

5.244

3.056

0.604

2.922

16.87

0.184

1.386

15.37

4.756

3.188

0.575

2.644

16.35

0.123

0.510

2.077

3.876

2.964

0.701

3.926

15.59

0.068

0.478

1.828

2.533

2.507

0.585

2.740

33.04

5.494

3.302

87.25

23.88

6.641

4.295

147.6

34.13

9.065

3.176

80.71

26.05

7.006

4.589

168.5

33.97

17.25

5.670

257.2

28.54

8.991

5.951

283.3

3

ꢀ1

b/dm mg

a

RSE_b

SSR

Freundlich

K

n

F

RSE

SSR

a

b

RSE – residual square error. SSR – sum of squared residuals.

RSC Adv., 2016, 6, 2731–2745 | 2741

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2+

m

Table 5 Comparison of Langmuir maximum capacity (q ) for Cu adsorbed onto various carbon-structured materials

ꢀ1

Adsorbents

Conditions

m

q /mg g

References

ꢀ

3

P-MWCNTs

MWCNT-COOH

pH 7.0, C

i

1.0 mg dm , 250 mg dose, 120 min, 298 K

0.080

0.398

72

22

10

72

24

25

3

i

0.5 mg dm , 125 mg dose, 120 min, 298 K

ꢀ3

pH 7.0, C

pH 5.5, 100 mg dm , 50 mg dose, 360 min, 301 K

i

20 mg dm , 30 mg dose, 120 min, 293 K

12.34

0.080

15.60

8.920

19.44

31.65

ꢀ

3

8

-HQ-MWCNT

MWCNT/bagasse

MWCNT/Fe

i

1.0 mg dm , 250 mg dose, 120 min, 298 K

ꢀ

3

2

O

4

pH 5.50, 10 mg dm , 40 h, 353 K

ꢀ

3

MWCNT-COOH

MWCNT-ttpy

pH 5.0, C

i

100 mg dm , 50 mg dose, 24 h, 293 K

This study

3

i

100 mg dm , 50 mg dose, 24 h, 293 K

ꢂ

2+

The change in Gibbs energy of adsorption (DG ) is calculated process for the removal of Cu from aqueous solutions by using

14

thus (eqn (5)):

MWCNT-COOH and MWCNT-ttpy was a physico-chemical

process, indicating that adsorption was facilitated by both

processes.

75

ꢂ

DG ¼ ꢀRT ln K

(5)

Thermodynamic parameters therefore suggest that the

application of MWCNT-ttpy will be eﬀective for remediating

metal-polluted eﬄuents discharged directly from industries.

ꢂ

ꢀ1

where DG is the standard Gibbs energy change (J mol ), R is

ꢀ

1

ꢀ1

the universal gas constant (8.314 J K mol ), and T is the

absolute temperature in Kelvin. The value of K was obtained

3.4.9. Selectivity of MWCNT-ttpy towards the removal of

from the product of q

m

and b obtained from the Langmuir plot

diﬀerent metal ions. The selectivity of the novel adsorbent was

investigated to understand its sorption aﬃnity for diﬀerent

metal ions in complex situations. A typical wastewater contains

a mixture of metal ions, hence, the selectivity of an adsorbent

towards metal ion removal is important. Adsorbate solutions

containing equal initial metal ion concentrations between 10

69,73

(

Table 4).

by multiplying by 1000.

A plot of ln K against 1/T was found to be linear, and the

The value of K was corrected to be dimensionless

74

ꢂ

values of DH and DS were obtained from the slope and

intercept of the plot, respectively, according to the van't Hoﬀ

equation (eqn (6)):

ꢀ3

2+

and 50 mg dm of Pb , Cd , Cu and Zn were conditioned

to pH 5.5, and agitated in a thermostated water bath for 24 h,

aer the addition of 200 mg of MWCNT-ttpy. Fig. 11 shows

a gradual decrease in the extent of removal as the initial

concentration of metal ions was increased from 10 to 50 mg

ꢂ

DH

DS

ln K ¼ ꢀ

þ

(6)

RT

R

Table 6 presents the thermodynamic parameters for the

adsorption of Cu onto MWCNT-COOH and MWCNT-ttpy.

Negative values were obtained for DG , indicating that the

process was spontaneous and feasible for both adsorbents. An

ꢀ3

2

+

dm . This was attributed to the competition for active sites

amongst adsorbates as the initial metal ion concentration

increased. At each metal ion concentration, the removal of Pb

from solution was highest, and Zn exhibited the lowest eﬃ-

ciency (Fig. 11). It is worthy of note that the removal of Cu

from mixed metal ion solutions was considerably close to Pb

ꢂ

2

+

2

+

ꢂ

increase in negative values for DG was obtained as the

2

+

temperature of the adsorbate solution was increased for

MWCNT-ttpy. This conrms that increasing the temperature of

the adsorbate solution resulted in better sorption for the

2

+

removal, obtaining eﬃciencies greater than 70% over the

concentration range studied. This result signies that MWCNT-

ꢂ

adsorbent. In contrast, a decrease in the negative values of DG

2

+

ttpy was selective for Cu , regardless of the presence of other

metal ions in solution.

was obtained as temperature is increased when MWCNT-COOH

was used as adsorbent. An exothermic process of adsorption

was obtained with MWCNT-COOH, indicated by the negative

76

As proposed by Chen et al., the removal of divalent metal

ꢂ

ions is inversely proportional to the hydrated radii of the metal

DH value obtained. On the other hand, the application of

MWCNT-ttpy as adsorbent yielded an endothermic process,

ꢂ

which is conrmed by the positive DH value obtained.

2+

Table 6 Thermodynamic parameters for the adsorption of Cu onto

MWCNT-COOH and MWCNT-ttpy

Table 6 further shows a positive DS value for MWCNT-ttpy,

which indicates an increase in the disorderliness at the solid/

solution interface resulting in

process. However, a negative DS was obtained for MWCNT-

a

favourable adsorption

ꢂ

ꢀ1

ꢂ

ꢀ1

ꢂ

ꢀ1

Adsorbent

T/K DG /kJ mol

DH /kJ mol

DS /J K mol

ꢂ

COOH indicating a decrease in disorderliness as the adsor- MWCNT-COOH 293 ꢀ19.72

3

03 ꢀ20.26

13 ꢀ19.79

18 ꢀ18.42

bent interacts with the adsorbate. Hence, the adsorption

process for MWCNT-ttpy is entropy-driven, while the process is

enthalpy-driven for MWCNT-COOH. The values of DH ob-

ꢀ32.36

ꢀ99.19

ꢂ

MWCNT-ttpy

293 ꢀ28.11

303 ꢀ30.50

313 ꢀ32.90

318 ꢀ35.11

tained show that the heat evolved/absorbed was greater than for

ꢀ

1

physisorption (2.1–20.9 kJ mol ), but lower than for chemi-

50.97

211.9

ꢀ

1

4,75

sorption processes (80–200 kJ mol ). This indicates that the

2742 | RSC Adv., 2016, 6, 2731–2745

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Table 7 Textural properties of spent-adsorbents after adsorption

Surface₂

Pore volume/

cm g

Pore

diameter/nm

ꢀ1

3

ꢀ1

Adsorbents

area/m g

MWCNT-COOH

MWCNT-ttpy

104.22

149.1

0.314

0.527

15.18

16.85

eﬃcient when applied to real samples. Hence, the application of

MWCNT-ttpy should be further explored for the remediation of

wastewater or eﬄuents discharged from industries.

Fig. 11 The eﬀect of equal initial metal ion concentration in a multi-

2

+

2+

component system for the adsorption of Pb , Zn , Cd and Cu

[

3.6. Desorption studies

3

conditions: 25 cm of adsorbate solution, pH 5.5, 24 h equilibration

time, adsorbent dose 200 mg, agitation speed 150 rpm, temperature The process of desorption regenerates an adsorbent for reuse,

ꢂ

20 C].

thereby reducing the cost and availability of sorbents for

adsorption. This process also limits the introduction of spent

adsorbents into the environment, thereby reducing the disposal

2

+

2+

species in solution. The ionic radius of Pb , Cu , Cd and of secondary pollutants. Similarly, the isolation of copper is

2

+

˚

Zn are 1.33 A, 0.71 A, 0.95 A and 0.83 A respectively. The possible, and may encourage its reutilization for other valuable

smaller the ionic radii of the metal ion species, the greater its purposes.

2

4,76,77

2+

tendency to hydrolyse, resulting in decreased sorption.

Prior to the desorption of Cu from loaded adsorbents, they

Hence, the hydrated radii of the studied metal ions are in the were characterized by means of TEM and BET surface area

2

+

2+

˚

order of Pb (4.01 A) < Cu (4.19 A) < Cd (4.29 A) < Zn

measurements. These were performed in order to conrm the

2

4,76,77

(

4.30A^˚ ).

Metal ions with smaller hydrated radii therefore presence of adsorbate on the adsorbents. The data obtained

have easy access to the adsorbent sites, hence accounting for exhibited lower surface areas and pore volumes for both

increased sorption in the sequence Pb > Cu > Cd > Zn (Fig. 11). adsorbents aer adsorption as compared with their corre-

Therefore, this principle could be said to be obeyed, since their sponding values before adsorption (Tables 7 vs. 1). This trend

removal was in the same order.

can be explained by the increase in agglomeration of nanotubes

The selectivity of metal ions obtained in this study was observed aer adsorption as shown in Fig. S8.† The increase in

2

+

mostly in agreement with previously reported studies. Srivas- CNT agglomeration aer the deposition of Cu onto their

78

79

tava et al. and Li et al. reported the same sequence order of surface (Fig. S8†), resulted in a decrease in the surface areas

Pb > Cu > Cd from a multicomponent system onto MWCNT- exhibited by both adsorbents (Table 7). Further, the hydrated

80

76

2+

24,77

COOH. Sitko et al. and Chen et al. also reported similar radius of Cu (0.419 nm)

is low compared with the pore

trends in their studies by using graphene oxide and nano- volumes of the synthesized adsorbents (Table 1). Hence, copper

hydroxyapatite, respectively. However, a sequence order of Pb species might ll the pore volumes of the adsorbent, thereby

8

1

82

>

Cu > Zn > Cd was reported by Srivastava et al. and Sdiri et al.

from a multicomponent system onto kaolinite and clay, aer adsorption (Table 7).

respectively. This order was slightly diﬀerent from that obtained

Desorption experiments were carried out by agitating a Cu -

resulting in a reduction of the pore volumes of the adsorbents

2

+

3

ꢀ3

in this study, hence, the removal of metal ion species is prin- loaded adsorbent with 10 cm of 0.1 mol dm HCl in a ther-

cipally dependent on the textural characteristics of the sorbent mostated water bath at 20 C for 30 min. The mixture was

ꢂ

2

+

used.

ltered and the nal concentration of Cu determined. Results

obtained showed good desorption eﬃciencies of 62% and 73%

for MWCNT-COOH and MWCNT-ttpy, respectively. These

values signify that the adsorbents can be regenerated for reuse

3

.5. Analysis of real samples

In order to aﬃrm the selectivity of MWCNT-ttpy and test its and the adsorbate solution can be isolated as chloride salt and

eﬃcacy for metal ion removal in a real life scenario, water reused for other industrial applications such as electroplating.

samples were collected from three diﬀerent rivers in Durban, Again, in this respect MWCNT-ttpy performed better, showing

2

+

South Africa. The initial metal ion concentrations found in the better desorption of Cu than MWCNT-COOH.

water samples is given in Table S4.† Interestingly, a removal

eﬃciency greater than 95% was obtained for all three metal ions

4

. Conclusions

in the three water samples (Table S4†). Although the initial

2

+

2+

0

concentrations of Pb , Cu and Zn in samples were in A novel nanomaterial, synthesized by the use of 4 -(4-hydroxy-

ꢀ

3

0

00

measurable amounts ($10 mg dm ), MWCNT-ttpy success- phenyl)-2,2 :6 ,2 -terpyridine as

a

modier for acid-

fully sequestered these toxic heavy metal ions from the envi- functionalized multiwalled carbon nanotubes (MWCNT-

ronmental samples. This further authenticates that MWCNT- COOH), to produce nitrogen-functionalized MWCNTs

ttpy and the method of adsorption presented are eﬀective and (MWCNT-ttpy), was successfully synthesized. The adsorbent

RSC Adv., 2016, 6, 2731–2745 | 2743

View Article Online

RSC Advances

Paper

2

+

proved eﬀective and eﬃcient for the removal of Cu from 18 S. Sai Bhargav and I. Prabha, Int. J. Chem. Biol. Eng., 2013, 3,

aqueous solutions, obtaining a higher Langmuir monolayer

107–112.

ꢀ

1

ꢀ1

adsorption capacity (q ) of 31.65 mg g than the 19.44 mg g

19 Y. S. Ho and G. McKay, Water, Air, Soil Pollut., 2004, 158, 77–

m

obtained for MWCNT-COOH. Also, a higher sorption uptake of

97.

2

+

Cu was exhibited by MWCNT-ttpy when compared with other 20 H. Liu, S. Feng, N. Zhang, S. Du and Y. Liu, Front. Environ.

MWCNT-containing sorbents from other studies. Sci. Eng., 2014, 8, 329–336.

This study revealed that MWCNT-ttpy can be used as an 21 Y. Jiang, H. Pang and B. Liao, J. Hazard. Mater., 2009, 164, 1–

alternative adsorbent for the removal of heavy metal ions from 9.

municipal wastewater and industrial eﬄuents, due to the high 22 M. Abdel Salam, Int. J. Environ. Sci. Technol., 2013, 10, 677–

2

+

eﬃciency exhibited towards Cu removal from aqueous solu-

688.

tion. The regeneration of this adsorbent was successful, sug- 23 M. S. Tehrani, P. A. Azar, P. Ehsani Namin and

2

+

gesting that the spent adsorbent and Cu can be recovered and

S. M. Dehaghi, J. Appl. Environ. Biol. Sci., 2014, 4, 316–326.

made available for reuse, and thereby reducing secondary 24 I. A. A. Hamza, PhD Thesis, University of KwaZulu-Natal,

pollutants.

Durban, South Africa, 2013.

25 J. Hu, D. Zhao and X. Wang, Water Sci. Technol., 2011, 63,

9

17–923.

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Durban, for research facilities and the National Research

Foundation (NRF) for provision of funds for the completion of

this work.

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2

29 T. A. Khan, M. Nazir and E. A. Khan, Toxicol. Environ. Chem.,

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RSC Adv., 2016, 6, 2731–2745 | 2745

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Nitrogen-functionalised carbon nanotubes as a novel adsorbent for the removal of Cu(ii) from aqueous solution

DOI: 10.1039/c5ra23900a

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