Synthesis and characterization of dithiocarbamate carbon nanotubes for the removal of heavy metal ions from aqueous solutions

Accepted Manuscript Title: Synthesis and characterization of dithiocarbamate carbon nanotubes for the removal of heavy metal ions from aqueous solutions Author: Qiong LiJingang Yu Fang Zhou Xinyu Jiang PII: DOI: Reference:

S0927-7757(15)30052-2 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.06.034 COLSUA 19991

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

5-4-2015 15-6-2015 18-6-2015

Please cite this article as: Qiong LiJingang Yu, Fang Zhou, Xinyu Jiang, Synthesis and characterization of dithiocarbamate carbon nanotubes for the removal of heavy metal ions from aqueous solutions, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2015.06.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and characterization of dithiocarbamate carbon nanotubes for the removal of heavy metal ions from aqueous solutions Qiong Li,Jingang Yu, Fang Zhou, Xinyu Jiang*

School of Chemistry and Chemical Engineering, Central South University,

Changsha 410083, China

Highlights

1. Dithiocarbamate-groups modified MWCNTs were prepared and characterized.

2. The adsorption capacities of DTC-MWCNT for Cd (Π), Cu (Π) and Zn (Π) were evaluated.

3. The possible adsorption mechanism of DTC-MWCNT for metal ions was proposed.

Graphical abstract

Abstract

A new carbon nanotube (CNT) composite, dithiocarbamate groups functionalized *

Corresponding author: Tel.: +86-0731-88830833; E-mail address: [email protected]

multi-walled CNT (DTC-MWCNT), was prepared by reaction of oxidized MWCNT with ethylenediamine and carbon disulfide. The physical structure and chemical properties of DTC-MWCNT were characterized using Fourier transform infrared (FT-IR) spectroscopy, thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM). The adsorption performance of Cd (II), Cu (II) and Zn (II) onto DTC-MWCNT was evaluated. The adsorption conditions such as pH value, adsorption time and initial concentration were systematically investigated. The adsorption isotherms, adsorption kinetics, adsorption thermodynamics and adsorption mechanism were discussed in detail. The results indicated that the adsorption process matched well with the pseudo-second-order kinetic model and the Langmuir model. The values of ∆Gθ and ∆Hθ calculated from the experiment data indicated that the adsorption process was spontaneous and endothermic in nature. DTC-MWCNT possesses higher maximum adsorption capacities for Cd (II), Cu (II) and Zn (II) of 167.2, 98.1 and 11.2 mg/g, respectively.

Keywords:Adsorption; Dithiocarbamate; Functionalization; Heavy metal ions; Multi-walled carbon nanotubes.

1 Introduction

Environmental pollution by toxic metals occurs globally. Heavy metal pollutants mainly come from industries, such as mining, metal processing, rubber, leather, plastic and medicine[1]. The most common poisonous heavy metal ions, including lead, mercury, zinc, nickel and copper, are different from other pollutants due to their

accumulation in living organisms[2, 3]. Both animals and plants would be endangered by the heavy metals polluted water and soil. Heavy metalscan enter human body and causeirreversible damage to bone, liver and brain, and the toxicity is not easy to eliminate[4]. Therefore, the removal of heavy metal ions from water environment benefits both public health and environment. Various separation methods including chemical deposition, ion exchange, membrane filtration, flotation, electrochemical and adsorption [1, 5] have been developed to remove heavy metal ions from aqueous solutions. Among the existing separation technologies, the adsorption was recognized as one of the most effective method for removing heavy metal ions due to the significant advantages such as high removal efficiency, simplicity and low cost.

Nowadays, the selective adsorption of heavy metal ions by carbon nanotubes (CNTs) has been of great concern because of the simpler preparation process, higher adsorption ability and shorter adsorption time. CNTs could be used to remove various pollutants such as dyes, phenols, aniline and divalent metal ions from aqueous solutions [6,7] due to the II-II interactions, electrostatic interactions and large specific surface areas [8]. However, the dispersion of CNTs into solvent was poor due to the strong intermolecular van der Waals interactions between tubes, which has largely decreased their adsorption performance [9]. To enhance the dispersion and the adsorption capacity of CNTs,the surface modification to generate functional groups is one of the most common and most effective approaches. Numerous functionalized CNTs have been used to remove heavy metal ions from aqueous solution or wastewaters. For example, functionalized CNTs containing iodo functional groups

have been used to remove heavy metal ions from compact fluorescent bulbs and water streams[10]. Amino modified CNTs were used for the adsorption of lead and cadmium ions [11]. CNTs containing various oxygen-containing functional groups (COOH, C=O, OH) were developed to remove various heavy metal ions [12,13]. The sulphur-containing complex agent anchored adsorbents were also used for removal of heavy metal ions[8, 14, 15].

Based on the theory of hard and soft acids and bases (HSAB), dithiocarbamate, a soft base, could be used as selective adsorption material for removal of heavy metal ions [16]. Dithiocarbamate modified chelating resins showed high adsorption capacity for various heavy metal ions such as Pb(II), Hg(II), Cd(II), Ni(II) and Cu(II) [17-20]. To the best of our knowledge, dithiocarbamate-modified MWCNT (DTC-MWCNT) as adsorbent have never been reported in previous literature for the removal of heavy metal ions.

In this paper, a novel adsorbent, dithiocarbamate groups functionalized multi-walled CNT (DTC-MWCNT) was developed and used to remove Cd (II), Cu (II) and Zn (II) from aqueous solution. The morphology of DTC-MWCNT was characterized by Fourier transform infrared (FT-IR) spectroscopy, thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM). The adsorption performance of Cd (II),Cu (II) and Zn (II) onto DTC-MWCNT was investigated. The adsorption conditions pH value, adsorption time and initial concentration were investigated in detail. The adsorption isotherms, adsorption kinetics, adsorption thermodynamics were discussed. The adsorption mechanism was also proposed.

2 Experimental

2.1. Chemicals

CNTs (purity>95%) were purchased from Shenzhen Nanotech Port Co., Ltd. The range of diameters is 20-40 nm and the length exceeds 5 µm. Ethylenediamine (EDA) and N´N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium ethoxide, sulfoxide chloride and carbon disulfide were obtained from Tianjin Kermel Chemical Reagent Co., Ltd. Metal salts including Cd(NO3)2·4H2O, Cu(NO3)2·3H2O and ZnCl2 were used as sources for Cd (II), Cu (II) and Zn (II), respectively. All the chemicals used were of analytical grade and used as received without any further.

2.2. Pretreatment of multi-walled carbon nanotubes

In a 100 mL round bottomed flask was added 500 mg of raw MWCNTs and 50.0 ml of nitric acid, which was dispersed well by sonication for 10 min. Then this mixture was stirred continuously at 120 °C for 24 h to introduce oxygen groups onto the MWCNTs surface. After cooling to room temperature, the mixture was added to 500 mL of deionizer water and filtered through a 0.45 µm PTFE membrane. The oxidized MWCNTs (o-MWCNTs) was washed with deionized water until the pH was neutral. Then, it was dried under vacuum at 70 °C for 24 h. Next step, 300 mg o-MWCNTs were converted to chloride-functionalized MWCNTs by reacting with 50.0 ml of sulfoxide chloride and 1.5 mL of coupling agent (DMF) at 70 °C for 24 h. The residual sulfoxide chloride was removed by distillation. Subsequently, 45.0 mL of

EDA were added under magnetic stirring and reacted at 120 °C for 24 h. The obtained product was diluted with 300 mL of deionized water and filtered using a 0.45µm PTFE membrane, then washed extensively with excess methanol. Thus obtained amine group functionalized MWCNTs (MWCNTs-CONHCH2CH2NH2, abbreviated as e-MWCNTs) were dried in a vacuum oven at 70 °C for 24 h.

2.3. Synthesis of dithiocarbamate group functionalized MWCNTs

400 mg of e-MWCNTs in 50.0 mL sodium ethoxide solution was reacted with 5.0 ml CS2 at 50 °C for 72 h under magnetic stirring. Then, the mixture was separated through a 0.45 µm PTFE membrane. The solid residuals were washed with deionized water, diluted HCl solution, diluted NaOH solution and acetone in sequence. Finally, the obtained DTC-MWCNT was kept in a vacuum oven at 70 °C for 24 h. The procedure for the functionalization of MWCNTs was depicted in Fig. 1.

2.4. Sorption experiment

The effect of pH on the adsorption of the investigated metal ions was studied at 25 °C. For these experiments, a series of 50 mL flasks were used. Each flask was added with 5.0 mg of adsorbent and 20.0 mL of metal ions with concentration of 10 mg/L. The desired pH was adjusted by using aqueous solution of 0.1 mol/L HCl and 0.1 mol/L NaOH. A pH range of 2.0 to 6.0 was used to avoid the precipitation of metal hydroxides. The flask was shaken in the thermostat shaker for a certain time. After filtration, the residual concentration of each metal ion was determined by atomic absorption to calculate the adsorption capacities of metal ions according to Eq.

1: qe =

co − ce ×V m

(1)

where co and ce (mg/L) are the initial and equilibrium concentrations of metal ions in the liquid phase, respectively, qe is the adsorption capacity of metal ions (mg/g), V is the volume of the solution (L), and m is the mass of the absorbent used in adsorption experiments (mg).

The equilibrium sorption experiments were performed by agitating 5.0 mg of adsorbent with 20.0 mL of solution containing a known amount of each metal ion (varying from 5.0 to 100.0 mg/L) at the optimum pH value for 2 h. The temperature was maintained at 25, 35 and 45 °C, respectively. After equilibration, the adsorbent was filtered and the residual concentration of each metal ion was determined.

For the kinetic adsorption, 5.0 mg of adsorbent was weighted into the flasks with 20.0 ml of metal ions solution at a concentration of 10 mg/L. The pH was kept at 6 throughout kinetic studies. The flasks were agitated for 2 h at 25 °C in a shaking thermostatic bath. The concentration of each metal ion was measured at different time intervals up to 150 min. Each data point was obtained from an individual flask.

2.5. Adsorption Isotherm

Numerous adsorption isotherms are employed to describe the interaction between adsorbents and adsorbates. In this study, Langmuir and Freundlich equations were used to describe the equilibrium characteristics of adsorption.

Langmuir isotherm was originally used to describe the gas molecules adsorption onto the solid surface. The adsorption can be expressed as:

qe =

abce (2) 1 + bce

Eq. (2) can be transformed into a linear equation as follows: ce ce 1 = + (3) qe qm qm kd Where ce is the equilibrium concentration of metal ions in solution (mg/g), qe is the amount of metal ions adsorbed at equilibrium(mg/g), a and b are the isotherm constant, qm is the maximum adsorption capacity (mg/g), and kd is the Langmuir constant which is related to affinity of binding sites.

The Freundlich isotherm is an empirical equation which describes heterogeneous systems (multilayer adsorption). This isotherm can be represented as follows:

qe = k f c1/n (4) e Eq. (4) can be transformed into a linear equation as follows: 1 log qe = log k f + log ce (5) n Where qe is the adsorption capacity at equilibrium (mg/g), ce is the equilibrium concentration of metal ions (mg/L), kf is physical constants of Freundlich isotherm which is an indicator of adsorption capacity, and 1/n is a measurement of adsorption effectiveness. The values of kf and 1/n are obtained from the intercept and slope of the linear Freundlich equation.

2.6. Adsorption Kinetics

In order to interpret the kinetic experimental date, and predict the rate-limiting step, the pseudo-first-order, pseudo-second-order and intraparticle diffusion equation were used in this study.

The pseudo-first-order rate equation given by Lagergren and Svenskais expressed as: log(q e − q t ) = logq e −

k1 t 2.303

(6)

Where qt is the adsorption capacity at any time(mg/g), and qe is the equilibrium adsorption

capacity(mg/g).

k1

is

the

rate

constant

of

pseudo-first-order

adsorption(1/min).

The pseudo-second-order rate is based on the sorption capacity of the solid phase and is represented as:

t 1 1 = + t (7) 2 qt k2 qe qe Where k2 is the pseudo-second-order rate constant (g/mg·min). The intraparticle diffusion rate is expressed according to equation given by Weber and Morris: qt = k3t 0.5 (8)

Where k3 is the intraparticle diffusion rate constant(mg/g·min0.5) and the value can

be obtained from the plot slope of qt versus t0.5. 2.7. Adsorption thermodynamics

Equilibrium distribution coefficient kd for the adsorption process was calculated by the follow equation[21]:

kd =

co − ce V × (9) ce m

Where co and ce are the initial and equilibrium concentration of metal ions in solution (mg/L), V is the volume of the solution(L), and m is the dosage of the adsorbent(g). The standard Gibbs free energy changes (∆Gθ), standard enthalpy changes(∆Hθ), and standard entropy changes(∆Sθ) of the adsorption process were carried out by the following Van´t Hoff thermodynamic equations[22]:

ln kd =

∆Gθ ∆H θ − (10) R RT

∆Gθ = ∆H θ − T ∆S θ (11)

Where R is the universal gas constant(8.314J/mol·K) and T is the absolute temperature(K). The values of ∆Sθ and ∆Hθ were calculated from the intercept and slope of the plots of lnkd versus 1/T. 3 Result and discussion

3.1. Characterization of MWCNTs

FT-IR spectra of the synthesized materials were compared in Fig. 2a in the range of 500-4000 cm-1. FT-IR spectra of o-MWCNTs exhibited a broad peak at 3441 cm-1 and 1701 cm-1, which are the stretching vibrations of υ(OH) and υ(C=O) of the carboxylic group(COOH), respectively(Fig. 2a)[7]. The peaks at 2921 cm-1 and 2843 cm-1 are due to the asymmetric and symmetric stretching vibrations of υ(CH2). The broad peaks at 1166 cm-1 is caused by the υ(C-O) stretching vibration[23]. The overlapped vibration of carbonyl group(C=O) is seen at 1628 cm-1. The appearance of O-H, C-O and C=O bonds and the increased intensity of the OH peak suggested that the OH, COOH and C=O groups were successfully introduced on the surface of the raw MWCNT after oxidation. For the e-MWCNTs(Fig. 2a), the peaks at 3427 cm-1 is attributed to the NH2 stretch of the amine group. Further, the peak at 1701 cm-1 assigned to υ(C=O) vibration of the carboxylic group(COOH) shift to 1627 cm-1, which is assigned to stretching of amide carbonyl (C=O). In additional, the appearance of new bonds at 1174 cm-1 and 1560 cm-1 assigned to υ(C-N) bond stretching and υ(N-H), respectively[11, 24]. These data indicated that amines were covalently attached to the MWCNTs. The FT-IR spectra of DTC-MWCNT showed the appearance of new peaks at 1209 cm-1 and 1016 cm-1, which are representatives of the υ(C=S) vibration and υ(N-C=S) vibration of dithiocarbamate group[15, 25]. The peak at 3446 cm-1 is due to the NH2 stretching vibration of the amine group. These above dates verified that the dithiocarbamate groups were successfully introduced to MWCNTs. All these functionalized MWCNTs can provide numerous adsorption sites and thereby increase the sorption capacity.

The thermal gravimetric analysis (TGA) was performed to evaluate thermal stability of the functionalized MWCNTs. As shown in Fig. 2b, the TGA curves of o-MWCNTs, e-MWCNTs and DTC-MWCNT were multistage processes because of the different functional groups introduced onto the surface of the MWCNTs. The first weight loss below 200 °C of all the samples was due confidently to the loss of water which is present in external surface and internal pores or cavities[26]. The weight loss above 200 °C of the o-MWCNTs, e-MWCNTs and DTC-MWCNT can be attributed to the thermal decomposition of the organic functional groups. Therefore, according to the thermo gravimetric analysis, the amount of dithiocarbamate covalently bonded to the MWCNTs was estimated, based on the total weight of e-MWCNTs, to be about 0.6 wt

in relation to the e-MWCNTs.

Fig. 3 presented the typical SEM images of functionalized MWCNTs, which shows that these materials were usually curves and had cylindrical shapes. The raw-MWCNTs (Fig. 3a) had diameter in the range of 20-40 nm and length over 5 µm. From the SEM images, the functionalized MWCNTs are regarded as entangled MWCNTs networks and there was no detectable change in the surface morphology, indicating that these materials were not easily broken during the functionalization, washing and drying procedures.

3.2. Influence of pH

The pH of the aqueous solution is an important parameter in the heavy metal ions adsorption process. Thus, the influence of the initial pH on the adsorption of metal

ions was investigated at 25 °C in the pH range of 2.0-6.0 and the results were presented in Fig. 4. It can be seen that the capacity of DTC-MWCNT for each metal ion was pH depended. The decrease of metal ions adsorption amount when pH decreased may be attributed to two reasons. Firstly, the number of negatively charged active sites decreased and the number of positively charged sites increased at low pH value, which would decrease the electrostatic adsorption between metal ions and DTC-MWCNT. Secondly, at low pH, the competive interaction between metal ions and protons with active sites increased, resulting in a low adsorption capacity of metal ions.

As indicated in Fig. 4, the adsorption of Cu(II) on the DTC-MWCNT was clearly more favorable at pH value of 5.0, compared with Cd(II) and Zn(II) at 6.0. Therefore, all the following experiments were carried out at these optimum pH values. Above the optimum pH, the metal ions formed the hydroxyl complexes.

3.3. Adsorption Isotherms

Adsorption isotherm is fundamental to understand how the adsorbates interact with adsorbents, which is the most important parmeter for determining the adsorption behavior of an adsorption process. The isotherm curves demonstrate the adsorption as a function of the equilibrium concentration of the adsorbates in solution. Fig. 5 shows the adsorption isotherm at optimum pH value. As seen from Fig. 5, the isotherm results indicated a good adsorption capacity of DTC-MWCNT for Cd(II), Cu(II) and Zn(II) ions. In additional, the capacity of DTC-MWCNT for each metal ions followed

the sequence of Cd(II) > Cu(II) > Zn(II). The Langmuir, Freundlich and Langmuir-Freundlish isotherms were used to normalize the adsorption. The parameters of the Langmuir and Freundlich isotherms were shown in Table 1. The correlation coefficients values(R2) indicated that the adsorption of Cd(II), Cu(II) and Zn(II) ions onto DTC-MWCNT was fitted better by the Langmuir isotherm equation(R2=0.963-0.999) than the Freundlich isotherm equation(R2=0.698-0.917). Therefore, the adsorption of DTC-MWCNT of Cd (II), Cu (II) and Zn (II) ions can be mainly considered as monolayer adsorption. A comparison of the present DTC-MWCNT with those different types of carbon nanotubes in recent references is shown in Table 2.

3.4. Adsorption kinetics

The effects of contact time on the adsorption of DTC-MWCNT for Cd (II), Cu (II) and Zn (II) ions were shown in Fig. 6a. The adsorption of DTC-MWCNT for three metal ions increased sharply during the first 40 min and then tended to be equilibrium. It can be seen from Fig. 6a that the adsorption capacity of Cd(II), Cu(II) and Zn(II) were 29.20, 37.24 and 10.80 mg/g, respectively. So the adsorption time was set at 120 min in subsequent experiments.

The parameters of thethree kinetic equations were shown in Table 3. Obviously, the correlation coefficients of the pseudo-second-order equation (R2=0.961-0.999) were higher

than

the

results

obtained

from

the

pseudo-first-order

equation

(R2=0.710-0.951). Therefore, the adsorption behavior of Cd(II), Cu(II) and Zn(II)

onto DTC-MWCNT fitted well with the pseudo-second-order equation. And the fitting lines of the adsorption of Cd(II), Cu(II) and Zn(II) by pseudo-second-order equation were shown in Fig. 6b.

As

seen from Fig. 6c, the adsorption process was divided into three stages: (1)

rapid transportation of metal ions onto the surface of DTC-MWCNT; (2) adsorption stage where intraparticle diffusion is the rate-limiting step; (3) final equilibrium step due to the lower concentration of metal ions in aqueous solution.

3.5. Adsorption thermodynamics The thermodynamic parameters of ∆Hθ and ∆Sθ can be obtained respectively from the slope and intercept of lnkd versus 1/T plots in Fig. 7a. It can be seen from Fig. 7a that the distribution coefficient(kd) increased with temperature increasing. The values of thermodynamic parameters including ∆Hθ, ∆Sθ and ∆Gθ(Table 4) gave useful information about the adsorption mechanism of the DTC-MWCNT. As seen from Table 4, the positive values of ∆Hθ indicated that the adsorption of DTC-MWCNT for Cd(II), Cu(II) and Zn(II) were endothermic processes. Furthermore, the positive values of ∆Sθ demonstrated a tendency to higher disorder at the solid/solution interface during the adsorption. And the increase of the randomness of the system may be related to the liberation of water of hydration and the ion exchange. Finally, the spontaneity of metal ions adsorption onto the DTC-MWCNT was indicated by the negative values of ∆Gθ. The values of ∆Gθ between -20 and 0 KJ/mol were generally assumed as the

comparable values for the physical adsorption processes, the change of ∆Gθ for physisorption together with chemisorption was frequently between -20 and -80 KJ/mol and for chemisorption between -80 and -400 KJ/mol. From Table 4, all the values of ∆Gθ were between -23.429 to -29.851 KJ/mol for the metal ions adsorption of DTC-MWCNT, which revealed that both chemisorption and physisorption coexisted during the adsorption processes.

3.6. Adsorption mechanism

To interpret the mechanism of the adsorption capacity order of Cd (II)> Cu (II) >Zn (II) onto DTC-MWCNT, the characteristics of the functional groups and the properties of the metal ions should be taken into account. According to the theory of hard and soft acids and bases(HSAB), the functional groups containing sulfur are considered to be soft base, and the heavy metal ions like Cd(II), Cu(II) and Zn(II) are considered to be soft acids, so the dithiocarbamate functionalized carbon nanotubes can be used to remove Cd(II), Cu(II) and Zn(II) from aqueous solution.

The properties of metal ions, such as atomic number, ions radius, ions potential, should be considered as important impacting factors to the adsorption capacity onto the same adsorbents. However, it is difficult to array the metal ions just by single factor. Nieboer and Richardson [27] proposed a concept of covalent index, which was a complex parameter calculated by the expression of Xm2r, where Xm is the electro-negativity and r is ionic radius[28]. Covalent index reflected the importance of DTC-MWCNT interactions with heavy metal ions relative to ionic interactions. The

parameters of Cd(II), Cu(II) and Zn(II) were obtained from literatures[29, 30] and listed in Table 5. The covalent index has been found to be an important factor to the adsorption capacity. The relationship between adsorption capacities and the covalent indexes corresponding to Cd(II), Cu(II) and Zn(II), was demonstrated in Fig. 7b. As can be seen from the Fig. 7b, the greater the covalent index value of metal ion is, the greater are the potential to form covalent bonds with DTC-MWCNT and the adsorption capacities of heavy metal ions.

4. Conclusion

In summary, DTC-MWCNT was prepared successfully and used as adsorbents for removing Cd(II), Cu(II) and Zn(II) from aqueous solutions. The optimum adsorption pH of heavy metal ions was in the range of 5-6 depending on the metal ion used. Adsorption kinetic and isotherm processes for three metal ions were found to correlate with pseudo-second-order model and Langmuir isotherm equation, respectively. The maximum adsorption capacities were in the order of Cd(II) >Cu(II) >Zn(II). Furthermore, energy changes were -23.429, -27.957 and -27.197 KJ/mol for Cd(II), Cu(II) and Zn(II) at 25 °C, which revealed that the adsorption processes including chemisorption and physisorption were spontaneous. In conclusion, this novel material shows efficiency and selectivity for the removal of heavy metal ions from aqueous solutions.

Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 21176262).

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Table 1

Fitting results with Langmuir and Freundlich isotherm for Cd (Π), Cu (Π) and Zn (Π).

Metal ion

Langmuir isotherm qm(mg/g)

Freundlich isotherm

kd

R2

n

kf

R2

Cd(Π)

202.429

0.1093

0.963

0.5008

30.154

0.806

Cu(Π)

101.523

1.0478

0.999

0.2949

43.903

0.698

Zn(Π)

16.625

0.0655

0.967

0.5912

1.5743

0.917

Table 2

A comparison for the maximum adsorption capacities of functionalized MWCNTs with those of some other MWCNTs reported in literatures for the adsorption of Cd (Π), Cu (Π) and Zn (Π).

Adsorbent

qm (mg/g)

T(℃ ℃)

pH

Ref.

Cd(Π)

25.70

45

8.0

[8]

Cd(Π)

31.45

45

6.2

[11]

Cd(Π)

75.84

−

−

[27]

Cu(Π)

50.37

Zn(Π)

58.00

CNTs/CA

Cu(Π)

72.99

20

5.0

[28]

MWCNTs(3N+1S)

Zn(Π)

10.21

25

7.0

[29]

DTC-MWCNTs

Cd(Π)

167.20

25

6.0

present

Ethylenediamine

Metal ions

MWCNTs Triethylenediamine MWCNTs Oxidized carbon nanotubes sheets

98.10

5.0

Cu(Π)

11.20

6.0

Zn(Π)

Table 3

Kinetic parameters for the adsorption of Cd (Π), Cu (Π) and Zn (Π).

Equations

Parameters

Cd(Π)

Cu(Π)

Zn(Π)

First-order-kinetic

qm (mg/g)

28.267

36.458

10.362

k1 (min-1)

0.046

0.077

0.046

R2

0.710

0.915

0.736

q (mg/g)

29.709

37.636

11.149

k2 (min)

0.011

0.018

0.018

R2

0998

0.998

0.996

K3 (mg/(gmin1/2))

2.100

2.423

0.816

R2

0.571

0.435

0.650

Second-order-kinetic

Intraparticle diffusion

Table 4

Thermodynamic parameters for the adsorption of Cd(Π), Cu(Π) and Zn(Π) onto DTC-MWCNTs.

Metal ions

∆Hθ(J/moL)

∆Sθ(J/KmoL)

T(K)

∆Gθ(KJ/moL)

R2

Cd(Π)

193.38

79.27

298

-23.43

0.921

308

-24.22

318

-25.02

298

-27.96

308

-28.90

318

-29.85

298

-27.20

308

-28.12

318

-29.04

Cu(Π)

Zn(Π)

263.99

94.70

305.71

92.29

0.943

0.992

Table 5

Parameters of Cd (Π), Cu (Π) and Zn (Π).

Metal ions

r(Å)

Xm

Xm2r(Å-1)

Cd(Π)

0.95

1.69

2.71

Cu(Π)

0.73

1.90

2.64

Zn(Π)

0.75

1.65

2.52

Fig. 1 Schematic diagram of the functionalization of the MWCNTs.

Fig. 2 (a)The FT-IR spectra of r-MWCNTs, o-MWCNTs, e-MWCNTs and DTC-MWCNTs; (b)The TGA curves of r-MWCNTs, o-MWCNTs, e-MWCNTs and DTC-MWCNTs.

Fig. 3 SEM images of (a) r-MWCNTs, (b) o-MWCNTs, (c) e-MWCNTs and (d) DTC-MWCNTs.

Fig. 4 Effect of pH on the adsorption of Cd(Π), Cu(Π) and Zn(Π) by DTC-MWCNTs at 25℃.

Fig. 5 (a) Adsorption isotherms of Cd(Π) and Cu(Π) at optimum pH and at 25℃; (b) Adsorption isotherm of Zn(Π) at optimum pH and at 25℃；(c) Langmuir isotherms for the adsorption of Cd(Π), Cu(Π) and Zn(Π) by DTC-MWCNTs at optimum pH and at 25℃.

Fig. 6 (a) Effect of time on the adsorption of Cd(Π), Cu(Π) and Zn(Π) by DTC-MWCNTs at optimum pH and at 25℃; (b) Pseudo-second-order kinetic for the adsorption of Cd(Π), Cu(Π) and Zn(Π) onto DTC-MWCNTs at optimum pH and at 25℃; (c) Plot of intraparticle diffusion model for the adsorption of Cd (Π), Cu (Π) and Zn (Π) onto DTC-MWCNTs.

Fig. 7 (a) Plots of lnkd versus 1/T for the adsorption of Cd (Π), Cu (Π) and Zn (Π); (b) Plots of the adsorption capacities versus the covalent indexes.