Accepted Manuscript

Composite nano floral clusters of Carbon Nano tubes and Activated alumina:

An efficient sorbent for heavy metal removal

Nalini Sankararamakrishnan, Meha Jaiswal, Nishith Verma

PII: S1385-8947(13)01123-6

DOI: http://dx.doi.org/10.1016/j.cej.2013.08.070

Reference: CEJ 11175

To appear in: Chemical Engineering Journal

Received Date: 25 June 2013

Revised Date: 14 August 2013

Accepted Date: 16 August 2013

Please cite this article as: N. Sankararamakrishnan, M. Jaiswal, N. Verma, Composite nano floral clusters of Carbon

Nano tubes and Activated alumina: An efficient sorbent for heavy metal removal, Chemical Engineering Journal

(2013), doi: http://dx.doi.org/10.1016/j.cej.2013.08.070

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Composite nano floral clusters of Carbon Nano tubes and Activated alumina: An efficient 1

sorbent for heavy metal removal 2

3

Nalini Sankararamakrishnan$*, Meha Jaiswal$,# and Nishith Verma$,@ 4

$Centre for Environmental Science and Engineering, Indian Institute of Technology Kanpur, 5

Kanpur, U.P. 208016, India 6

@Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, U.P. 7

208016, India 8

#Mahatma Gandhi Chitrakoot Gramoday Vishwavidyalay, Satna, Chitrakoot, M.P. 485331, India 9

10

*author for correspondence, Tel: 915122596360, Email: nalini@iitk.ac.in 11

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Abstract 25

Carbon nano tubes (CNTs) were grown over Fe- and Ni-doped activated alumina (AA) by 26

chemical vapor deposition (CVD) and washed with acid to produce nano floral clusters (NCs). 27

The produced NCs were characterized by Scanning Electron Microscopy (SEM), Energy 28

Dispersive X-ray (EDAX), Brunauer, Emmett, and Teller (BET) surface area analyzer, Raman 29

and Fourier Transform Infra Red (FTIR) Spectroscopy. Batch mode experiments were used to 30

study the effect of various operating conditions such as pH and contact time on the adsorbent 31

capacity for Cr(VI) and Cd(II) . It was found that maximum adsorption of Cr(VI) took place at 32

pH 2 and for Cd(II) in the pH range of 7 to 9. The measurements of the sorption capacity of NC 33

revealed that it had the saturation capacity of 264.5 and 229.9 mg g-1 for both Cr(VI) and Cd(II) 34

respectively. Sorption data were fitted by both Langmuir and Freundlich models. 35

Thermodynamic parameters such as ΔS, ΔH and ΔG indicated the suitability of NC for the 36

removal of Cr(VI) and Cd(II). The results indicate that NC is a promising candidate for the 37

treatment of heavy metals from waste water industrial effluents. 38

39

Key Words: CNTs, Alumina, Cr(VI), Cd(II), CVD, Nano Clusters, Adsorption 40

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1. Introduction 46

In the past decades, carbon nanotubes (CNTs) have attracted considerable attention owing to the 47

unique structural and excellent mechanical, conductive and thermal properties [1-3]. High 48

sorption capability of chemical pollutants by CNTs is attributed to its hollow and layered 49

nanostructure with a characteristically large surface area [4-6]. CNTs have been found useful for 50

the removal of various contaminants such as Pb(II) [7], Ni [8], Cu [9] and Cd [10]. Owing to the 51

high sorption potential towards heavy metals, both multiwalled carbon nanotubes (MWCNTs) 52

[11] and functionalized MWCNTs [12,13] have been reported for the solid phase extraction of 53

many heavy metal ions from various environmental matrices. Chemical vapor deposition (CVD) 54

technique is cost effective technique to synthesize CNTs in large amounts and variety of 55

substrates like activated carbon fibres [14], activated carbon [15-17] and porous substrates such 56

as MgO [18 – 20] , Al2O3 [21-22] and SiO2 [23] are used. Compared to a flat substrate coated by 57

metal catalyst, a three-dimensional porous substrate coated catalyst has the advantage of higher 58

available metal content per substrate mass and a higher catalyst active surface [24, 25] . It is also 59

well known that activated alumina is one of the widely used adsorbents for metal sorption [26]. 60

Thus use of activated alumina will serve the dual purpose as the porous substrate to hold the 61

catalyst and an effective adsorbent as well. With these advantages in mind, this study is focused 62

on the synthesis of CNTs over activated alumina (AA) by chemical vapor deposition (CVD) 63

using benzene as the carbon source. The as-prepared composite floral clusters were characterized 64

by various techniques such as SEM, EDAX, BET surface area, X-Ray Diffraction spectroscopy, 65

FTIR and Raman and applied to the removal of model heavy metal ions, namely, Cr(VI) and 66

Cd(II) from aqueous solution. 67

2. Materials and Methods 68

2.1 Substrate preparation 69

Around 6 g of AA (particle size < 0.4 mm) obtained from Bhargava Industries, Surat, 70

India) was placed in 20 ml of ethanol and sonicated for 3 to 4 h and heated until the ethanol was 71

vaporized completely. Next, 0.5 g of Fe(III)-nitrate and 0.3 g Nickel sulphate in 20 ml of 72

ethanol was added to the treated AA. This mixture was stirred and heated for 1h at 80 ˚C. 73

2.2 Synthesis of NCs 74

An indigenously assembled vertical CVD apparatus was used for the preparation of NCs. 75

Fig. 1 describes the schematic representation of CVD reactor used in the study. The details of the 76

assembly are described elsewhere [14]. Around 3 g of the catalyst prepared by the procedure 77

described in the above section was placed in the furnace and calcined at nitrogen atmosphere for 78

2 h at 450˚C. Further, the reduction was carried out at 550˚C for 2 h using hydrogen gas at 100 79

cc per min and 2 bar , followed by the pyrolysis of benzene vapor. During pyrolysis the 80

temperature was raised to 750˚C. Pyrolysis was carried out for 2 h to ensure complete growth of 81

CNTs. The product was washed with 0.01 N HCl in a sonicator. Ni(II) nitrate and Fe(III) nitrate 82

act as catalysts for the growth of CNTs. During calcinations both Fe and Ni salts are converted 83

to their respective oxides, and then to metallic state when reduced under H2 atmosphere [14]. Fe 84

and Ni metals catalyze the formation of NCs on AA. Washing with HCl ensures the removal of 85

the metals from the tip of the produced NCs. 86

2.3 Adsorption Batch Experiments 87

Batch experiments were carried out with synthetic solutions of Cd(II)/Cr(VI) in 100 mL 88

flasks with stopper. The flasks were placed in an incubator shaker (orbital stirring rate = 100 89

rpm) at room temperature (~ 30 oC). The contact time of the solutions with the adsorbents was 90

maintained at 4 h. After equilibration, the solutions were filtered with Whatman No. 42 filter 91

paper, diluted and analyzed for Cd(II) and Cr(VI). Unless otherwise stated the parameters used 92

for the tests were: sample volume = 20 mL, sorbent dose = 2.5 g L-1, initial metal ion 93

concentration = 100 mg L-1, equilibration time = 4 h. The initial solution pH was maintained at 2 94

and 7.5 for Cr(VI) and Cd(II), respectively. For pH studies the pH of the solution was varied 95

from 1 to 10 keeping the other conditions constant. For equilibrium studies the initial metal ion 96

concentration was varied from 100 to 2000 mg/l. Thermodynamic studies were conducted by 97

equilibrating Cr(VI)/ Cd(II) solutions using the above mentioned conditions at three different 98

temperatures namely 25, 35 and 45˚C. Kinetic studies were conducted by equilibrating 20 ml of 99

500 mg L-1 of Cr(VI)/Cd(II) at a dose rate of 2.5 g L-1 and the amount of Cr(VI)/Cd(II) adsorbed 100

were monitored at regular intervals of time. 101

2.4 Metal concentration analysis 102

Dissolved Cadmium in water was determined by Aanalyst 400 Perkin Elmer Atomic 103

Absorption Spectrophotometer using an air-acetylene burner. The measurements were done at 104

wavelength of 228.8 nm using a slit width of 0.7 nm. Hexavalent Cr in water was determined by 105

spectrophotometry using diphenylcarbazide method [27] after suitable dilutions. The test 106

samples were filtered using Whatman 0.45 mm filter paper, and the filtrates after suitable 107

dilutions were analyzed. Control experiments showed that no sorption occurred on either 108

glassware or filtration systems. All assays were carried out in triplicate and only mean values are 109

presented. 110

3. Results and Discussion 111

3.1 Characterization of the nano clusters (NC) 112

The field emission scanning electron microscopy (FE-SEM, Supra 40 VP, Zeiss, 113

Germany) was used to observe the surface morphology of NC. The samples were gold coated to 114

improve their conductivity to obtain good images. Additionally, EDX elemental spectra of a few 115

spots on the samples were taken for determining the elemental compositions. We present here 116

the representative images and spectra of the samples. 117

Fig. 2 describes the SEM images of the prepared adsorbents at different magnifications 118

(figs 2(a), 2(b) and 2(c). NCs may be observed on the surface of the adsorbents. The presence of 119

Fe, Ni and Al in the material was confirmed from the EDAX graph (Fig.2(d)). The elemental 120

mapping of the acid washed composite (Figs. 2(e), 2(f), 2(g)) revealed the uniform distribution 121

of Fe, Al and Ni particles. From EDAX graph the weight percentages of Al, Fe and Ni were 122

found to be 20.8, 0.8 and 8.6% respectively. The TEM image of NC is shown in Fig.3 where 123

CNTs with Fe-Ni particles were observed. 124

The FT-IR spectra of plain CNT, activated alumina and NC (Fig.4) were acquired by 125

Tensor 27 (Bruker, Germany) in the attenuated total reflectance (ATR) mode using Ge crystal. 126

The sample chamber was continuously purged with nitrogen during the measurement. The broad 127

absorption band around 3428 cm−1 for NC (Fig.4c) could be attributed to the O-H stretching of 128

the adsorbed water molecules and intra-molecular hydrogen bonding. The bands between 2850 – 129

2950 cm-1 observed in Fig. 4 C are due to the symmetrical and assymetrical stretching of –CH 130

vibrations of –CH3 and –CH2 groups. Carbonyl stretching of plain CNT (Fig. 4A) is observed at 131

1628 cm-1, whereas the vibration of activated alumina (Fig. 4B) at 1634 cm-1 is due to the 132

stretching Al-OH framework. In the case of NC (a composite of AA and CNT) (Fig.4C), a 133

strong band is observed at 1626 cm-1 which arises due to the merging of carbonyl and Al-OH 134

stretching vibrations. Stretching and bending vibrations of Al-O-Al are also observed in the 135

spectrum of NC (Fig.4C) at 820 and 592 cm-1 with a slight shift compared to the virgin activated 136

alumina [28]. Form the above discussions, it is evident that NC consists both alumina and nano 137

carbon. 138

The adsorbent (NC) prepared in this study was analyzed for the specific surface area, 139

pore volume, and PSD by N2-physisorption using Autosorb-1C instrument (Quantachrome, 140

USA). Chemisorption analysis was also carried out to measure the active metal surface area. 141

The multipoint Brunauer, Emmett, and Teller (BET) surface area was measured from the 142

nitrogen adsorption/desorption isotherm. The BET surface area of NC was found to be 203 143

m2g1. The total pore volume was found to be 0.345 ccg-1 and average pore size was 6.79 nm. 144

Some examples of specific surface areas reported for CNTs are 80.9 m2 g-1 [29] , 90.2 m2 g-1 145

[30], 74 m2 g-1 [31]. High specific surface area in NCs indicates higher adsorption capacity of 146

the solutes which is discussed later in this article. The Raman spectra of NC (Fig.5) showed two 147

characteristic peaks at 1333 cm-1 (D band) and 1585 cm-1 (G band). The G band originates from 148

the in-plane tangential stretching of the carbon-carbon bonds in graphene sheets, whereas the D 149

band originates from either defect sites in the hexagonal framework of multi walls carbon 150

nanotubes (MWCNTs) or the presence of amorphous carbon [32]. The ratio between the areas of 151

these two bands (AG/AD) has been used to quantify the degree of purification [33-35] of multi-152

wall carbon nanotubes. The higher intensity of D band compared to G band indicates the 153

presence of amorphous carbon in the structure. For the present material, the ratio is calculated as 154

0.993, indicating the presence of disordered graphite components in NCs. The X-ray diffraction 155

measurements were carried out to examine the structure of the floral clusters (Fig.6). The peak 156

observed at 26.0° is the characteristic (002) reflection of graphitic peak arising due to the 157

presence of MWCNTs in the sample. The peaks observed at 32°, 38° and 44° correspond to the 158

standard body-centered cubic (bcc) phase of iron (JCPDS no. 06-0696) [36]. Three distinct 159

reflections, located at 2θ = 38° (311 reflection), 45.9° (400) and 67.1° (440), which were 160

assigned to γ-alumina [37] were also observed. Additional peaks observed at 36.5°, 44.0°, 161

51.5°and 61.5° are attributed to the presence of nano crystalline Ni-O [38]. 162

3.2. Effect of initial pH 163

Initially, zero point charge of the sorbent (pHzpc) was determined through the plots of 164

pHfinal versus change of initial and final pH (ΔpH) in 0.01M KNO3 solution. The pHzpc of NC 165

was found to be 7.2 (Fig.7). The effect of initial pH on the adsorption behavior Cd(II) and Cr(VI) 166

was examined using NC as the sorbent (Fig. 8). The pH was adjusted between 2 to 10 using 167

either hydrochloric acid or sodium hydroxide (0.1M). No efforts were made to maintain the pH 168

throughout the adsorption tests and equilibration was carried out for 2h. 169

Speciation diagram of Cr(VI) at various pH values [39] are depicted in Fig.9. It evident 170

that at acidic pH values, the predominant species of hexavalent chromium are salts of chromic 171

acid (H2CrO4), hydrogen chromate ion (HCrO4−) and chromate ion (CrO4

2−), depending on the 172

pH. H2CrO4 predominates at pHs less than about 1.0, HCrO4− at pHs between 1.0 and 6.0, and 173

CrO42− at pHs above about 6.0. The dichromate ion (Cr2O7

2−), a dimer of HCrO4−, is formed at 174

concentrations higher than 1 g/L. The equilibrium reactions between various forms chromium 175

ions could be depicted in the following equations [40, 41]. 176

177

H2CrO4 H+ + HCrO4

- (1) 178

HCrO4- H+ + CrO4

2- (2) 179

2HCrO4- Cr2O7

2- + H2O (3) 180

With increase in pH, the degree of protonation of the surface reduces gradually and hence 181

adsorption is decreased. Fig. 8 shows the decrease in Cr(VI) adsorption at high pH values. The 182

pHzpc of the sorbent was found to be 7.2. Thus below pH 7.2.5, the carboxylic groups and 183

hydroxyl groups of carbon are fully protonated. Thus an electrostatic interaction takes place 184

between the positively charged sorbent molecules and chromate anion. Further the hydroxyl 185

groups on the surface of NCs could act as electron donors and reduce Cr(VI) to Cr(III) [42]. 186

Thus, in further experiments the initial pH for Cr(VI) adsorption experiments were carried out at 187

pH 2. It was found that after equilibration the final pH was around 1.9. 188

Exactly a reverse effect is observed with cadmium ions. (Fig.8). Maximum adsorption is 189

observed in the pH range of 7.5 – 9.0. Cadmium ions exists as Cd2+, Cd(OH)+, Cd(OH)2 in the 190

pH range of 1 – 11 (fig.7b). Thus further experiments, were carried out at pH 7.5 to prevent 191

precipitation of Cd(OH)2. Since the pHzpc of NC is 7.2, at pH values >7.2 the surface of the 192

NCs are negatively charged arising due to the presence of carboxyl groups and there exists an 193

electrostatic interaction followed by the complexation of Cd(II) ions. It was also observed that 194

after equilibration the final pH changed from 7.5 to 6.19. This could be attributed to the release 195

of proton during the complexation and the inherent acidity of NCs. Experiments were also 196

conducted to evaluate the amount of catalyst metals ions namely Fe(III) and Ni(II) leached 197

during sorption. It was found that at pH ~ 2 (optimum pH for Cr(VI) adsorption) around 0.8 and 198

0.9 mg L-1 of Fe(III) and Ni(II) leached into the solution respectively and at pH 7.5 (optimum 199

pH for Cd(II)) the amount of Fe(III) and Ni(II) leached were found to be < 0.1 mg L-1 . 200

3.3 Sorption Equilibrium 201

Equilibrium studies were conducted at pH 2 for Cr(VI) and pH 7.0 for Cd(II). The 202

resulting data were analyzed with the Langmuir and Freundlich isotherms. The data were found 203

to be best fitted the Langmuir isotherm, which assumes a monomolecular layer on the surface of 204

the adsorbent. The linearized form of the Langmuir isotherm equation is: 205

1/qe = 1/QbCe + 1/Q (4) 206

Where, qe is the amount of solute adsorbed (mg g-1) at equilibrium and Ce is the equilibrium 207

concentration (mg L-1). The empirical constants Q and b denote the monolayer capacity and 208

energy of adsorption, respectively, and were calculated from the slope and intercept of plot 209

between 1/Ce and 1/qe . 210

Fig. 10 describes the linearized Langmuir adsorption plots for the nanoclusters. The 211

maximum adsorption capacity for Cd(II) and Cr(VI) using NC as adsorbent was found to be 212

229.9 and 264.5 mg g-1 of NC respectively. The values obtained for various Langmuir constants 213

are shown in Table 1. Some of the earlier work using adsorbents like activated alumina, activated 214

carbon, MWCNT and functionalized MWCNT are listed in the Table 2. For example using 215

activated alumina and activated carbon Mor et al. [45] reported a capacity of 7.44 and 12.87 mg 216

g-1 for Cr(VI) respectively. Guanghhui et al [46] reported a nano composite consisting of 217

nanometer AlO(OH) loaded on the fiberglass with activated carbon fibers yielding a capacity of 218

128.5 mg g-1 towards Cd(II) removal. Comparing the carbon based or activated alumina based 219

adsorbents reported in the literature, to the best of our knowledge, the capacities obtained for 220

these two metals are the highest (Table 2). Enhanced capacities could be attributed to the 221

presence of amorphous carbon (as evident from Raman studies), CNT and alumina (evident from 222

XRD and FTIR studies) in the composite. In addition to their excellent adsorption capacity, the 223

presence of both CNTs and alumina will result in good mechanical and thermal stability [47,48]. 224

It should be worthwhile mentioning that these nano clusters were synthesized indigenously in 225

our laboratory and with these high capacities and stabilities have enormous potential as 226

adsorbents for the application in waste water treatment. 227

A further analysis of Langmuir model could be arrived based on a dimensionless equilibrium 228

parameter (RL) [49]. 229

01

1

bCRL +

= (6) 230

where Co is the initial concentration of Cr(VI) or Cd(II) and ‘b’ is the Langmuir adsorption 231

equilibrium constant (ml/mg). The value of RL indicates the isotherm shapes to be favorable 232

(0<RL<1), linear (RL =1), irreversible process (RL =0) or unfavorable process (RL <1). For an 233

initial concentration of 100 mg/l RL values for Cd(II) and Cr(VI) were found to be 0.599 and 234

0.592 respectively. These values suggest the adsorption of both the metal ions with NC is a 235

favorable process. 236

The freundlich equilibrium isotherm model describes adsorption by heterogeneous 237

energetic distribution of adsorption sites accompanied by interaction between solute molecules. 238

It is commonly expressed as 239

fee KCn

q loglog1

log += (7) 240

Where, qe is the amount of solute adsorbed (mg/g) at equilibrium and Ce is the equilibrium 241

concentration (mg L-1). The n and Kf are the Freundlich parameters, for values range 1 < n < 10, 242

which indicated adsorption is considered to be favorable. From the results (Table 1) it is evident 243

the values of n ranged from 1.38 to 1.84 with a regression coefficient of 0.88 to 0.97. These 244

values signify good affinity between the Cd(II)/Cr(VI) ions with the nano clusters. 245

3.4 Kinetic and Thermodynamic parameters 246

Kinetics studies were conducted to ascertain the time required to attain the equilibrium between 247

the adsorbent and Cd(II) and Cr(VI) solutions. In both Cd(II) and Cr(VI) amount adsorbed 248

increased rapidly and reached equilibrium at 1.5 and 2 h for Cr(VI) and Cd(II) respectively 249

(Fig.11). The kinetic adsorption parameters for both the systems were modeled by the pseudo 250

second order model [50]. 251

qe

t

qekqt

t +=2'

1 (8) 252

Where k’ the pseudo second-order rate constant of adsorption (g/mg/min) qe and qt are the 253

amounts of metal ion sorbed (mg g-1) at equilibrium and at time t, respectively. Linear plots of 254

t/qt Vs t for Cd(II) and Cr(VI) systems yielded straight line with correlation coefficients of 0. 255

992 and 0.999, respectively. The calculated values of various constants are tabulated in Table 3. 256

Further to ascertain, whether the adsorption through diffusion is the sole rate determining step, 257

the kinetic data obtained were modeled through Weber- Morris intraparticle diffusion model [51] 258

given by the following equation 259

Ctkqt += 5.0int )( (9) 260

Where qt is the amount of Cr(VI)/Cd(II) adsorbed at a given time t and kint is the intraparticle 261

diffusion constant. The plot obtained for Cd(II) and Cr(VI) with NCs are shown in Fig. 12. The 262

values obtained from this model are given in Table 3. It is evident from the plots that the slope of 263

the plot of qt Vs √t is not a straight line passing through the origin which confirms that in 264

addition to intraparticle diffusion, the adsorption process is controlled by chemisorptions and 265

external diffusion processes also. 266

The common thermodynamic parameters such as ΔS°, ΔH° and ΔG° were calculated from the 267

adsorption data [52]. First, Kc, the equilibrium constant was determined by eq. (3) 268

e

AC C

CK = (3) 269

where, CA( mg L-1) is the concentration of solute in the aqueous phase and Ce is the equilibrium 270

concentration (g/l). ΔG° was calculated using the following equation: 271

KcRTG ln−=Δ � (4) 272

where R is the gas constant, T is the temperature in Kelvin. Using Van’t Hoff equation (5) the 273

value of ΔS and ΔH was determined: 274

RT

HSKc

303.2303.2log

�� Δ−Δ= (5) 275

Based on the above-calculated data, a linear plot of log Kc vs. 1/T was drawn for both Cd(II) and 276

Cr(VI). Using these plots, ΔS° and ΔH° were determined from the intercept and slope, 277

respectively. The data obtained are presented in Table 4. Chemisorption is indicated by the 278

positive values of ΔS° and ΔH° for both metal ions. The positive value of the entropy indicates 279

the irreversibility and stability of adsorption [53]. Also, the negative value of ΔG° indicates the 280

feasibility and spontaneity of the process. 281

4 Conclusions 282

Novel floral nano clusters were synthesized by CVD using alumina as the substrate. The 283

prepared clusters were characterized by various spectral techniques and applied to the removal of 284

Cr(VI) and Cd(II). The adsorption equilibrium data were best fitted by Langmuir model. The 285

various model parameters were evaluated from the adsorption data. The monolayer adsorption 286

capacity calculated from the Langmuir model for NC (264.5 and 229.9 mg g-1 for Cr(VI) and 287

Cd(II) respectively)) was found to be considerably higher than that those reported in the 288

literature for other adsorbents. High capacity is attributed to the presence of activated alumina, 289

CNT, amorphous carbon and various surface functional groups such as carboxyl, carbonyl and 290

hydroxyl present in the clusters. Thus, indigenously prepared nanoclusters with very high surface 291

area, mechanical strength and adsorption potential paves way to the synthesis of unique brand of 292

adsorbents for cleaning up the environment. 293

Acknowledgement Funding from Organization for prevention of chemical weapons (OPCW) to 294

carry out this work is gratefully acknowledged. The authors would like to thank Department of 295

Science and Technology unit on Nanoscience at IIT Kanpur for providing the SEM facility. 296

297

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458

459

460

461

List of Figures 462

Fig.1 Schematic representation of vertical furnace CVD reactor containing activated alumina 463

Fig.2 SEM and EDAX and Elemental mapping of Nano Clusters 464

Fig.3 TEM image of Nano Clusters 465

Fig 4. FTIR spectra of Activated alumina, Plain CNT and Nano Clusters 466

Fig . 5 Raman Spectra of Nano clusters 467

Fig. 6 X-Ray Diffraction spectra of Nano Clusters 468

Fig.7 Determination of Zero point charge of NC in 0.01 M KNO3 469

Fig. 8 Effect of initial pH on the adsorption behavior of Cr(VI) and Cd(II) by NC 470

Fig. 9. Speciation diagram of Cr(VI). Source: Dionex [39] 471

Fig.10. Adsorption Isotherms and Linearized Langmuir adsorption Iostherms of Cr(VI) and 472

Cd(II) on nano clusters 473

Fig.11. Adsorption kinetics of Cd(II) and Cr(VI) on nano clusters 474

Fig.12 Weber-Morris Intraparticle diffusion model plot 475

List of Tables 476

Table 1. Langmuir and Freundlich Model Constants 477

Table 2. Comparison of adsorption capacities of CNT’s or Modified CNT’s as adsorbents for the 478

removal of Cadmium and Chromium ions 479

Table 3. Kinetic data for the adsorption of dichromate and Cd(II) by NC 480

Table 4. Thermodynamic parameters on the adsorption of Cr(VI) and Cd(II) on Nano Clusters 481

482

483

12

3

6

7

4

58

484

485

Fig.1 Schematic representation of vertical furnace CVD reactor containing activated 486

alumina 487

488

489

490

491

492

493

494

495

496

497

498

1 Gas inlet 2 Gas outlet 3 Thermal sensor 4 Perforated disc 5 Sample holder 6 Moving furnace 7 Quartz tube 8 Activated alumina

499

500

501

Fig.2 SEM, EDAX and elemental images of Nano Clusters 502

(b)

(c) (d)

(f)

(a)

(g)(e)

(h)

503

504

505

506

507

Fig.3 TEM image of Nano Cluster 508

509

510

511

512

513

Fe/Ni catalyst particles

3500 3000 2500 2000 1500 1000 50020

30

40

50

60

70

80

90

Wavelength

% o

f Tra

nsm

itten

ce

3428

2923

1626

820

96

97

98

99

100

101

102

% o

f Tra

nsm

itten

ce

1628

A

B

C

A - Plain CNT

B - AA

C - NC

1634

575

3443

514 515

Fig 4. FTIR spectra of Activated alumina, Plain CNT and Nano Clusters (NC) 516

517

518

519

520

521

522

523

524

525

526

527

528

529

530

Fig . 5 Raman Spectra of Nano clusters 531

532

533

534

535

536

537

D-Band G-Band

20 30 40 50 60 70 80120

140

160

180

200

220

240

260

280

Ni-O

Ni-O

Ni-Oγ-

Alu

min

a (3

11)

γ-Alumina (400)

Inte

nsi

ty

Angle

γFe(

111)

αFe(

110)CN

T(0

02)

γ-Alumina (440)

Ni-O

538 539

540

Fig. 6 X-Ray Diffraction spectra of Nano Clusters 541

542

543

544

545

546

547

548

549

550

551

Fig.7 Determination of Zero point charge of NC in 0.01 M KNO3 552

553

554

555

556

557

558

559

560

561

562

563

564

565

566

567

568

569

570

571

572

573

574

Fig.8 Effect of initial pH on the adsorption behavior of Cr(VI) and Cd(II) by NC 575

576

577

578

579

580

581

582

583

584

585

586

Fig. 9.a. Speciation diagram of Cr(VI). Source: Dionex [39], b. Eh-pH diagram of Cd(II) 587

species [44] 588

589

590

(a)

(b)

591

592

593

594

595

0 200 400 600 800 1000 12000

50

100

150

200

250

300

350

0.00 0.02 0.04 0.06 0.080.00

0.01

0.02

0.03

0.04

0.05

0.06

1/q

e

1/Ce

R2 0.97Intercept 0.00435

Slope 0.64216

0.000

0.008

0.016

0.024

0.032R2 0.92737Intercept 0.00378Slope 0.54715

Cd(II)Cr(VI)

Am

ou

nt

of

Cr(

VI)

/Cd

(II)

ad

sorb

ed (

mg

/g)

Equilibrium Concentration (Ce) (mg/l)

596

597

Fig.10 Adsorption Isotherms and Linearized Langmuir adsorption Iostherms of Cr(VI) 598

and Cd(II) on nano clusters 599

600

601

602

603

604

605

606

607

608

609

610

Fig.11. Adsorption kinetics of Cr(VI) and Cd(II) on nano clusters 611

612

613

614

615

616

617

618

619

620

621

622

623

624

625

626

0.4 0.6 0.8 1.0 1.2 1.460

80

100

120

140

160 Cd(II) Cr(VI)

Am

ou

nt

of

Cr(

VI)

/Cd

(II)

ad

sorb

ed (

mg

/g)

(t)0.5

627

Fig.12 Weber-Morris Intraparticle diffusion model plot 628

629

630

631

632

Table 1. Langmuir and Freundlich Model Constants 633

634

Langmuir Constants Freundlich constants Species

Qmax

(mg/g)

b R2

RL

(Co = 100 mg/l) n kf R2

Cd(II) 229.9 0.0067 0.97 0.599 1.845 7.727 0.88

Cr(VI) 264.5 0.0069 0.93 0.592 1.382 3.249 0.97

635

636

637

638

Table 2. Comparison of adsorption capacities of CNTs or Modified CNTs as adsorbents for the removal of Cadmium and Chromium ions

Adsorbent pH Metal Ion Capacity (mg/g)

Reference

CNT sheets 7.0

Cd(II) 92.59

[43]

MWCNT 5.0 Cd(II) 10.86

[10]

Oxidized MWCNT 2.0 Cr(VI) 2.69

[54]

Non modified Carbon nanotubes

7.0 Cr(III) 0.37

[55]

CNT (Amino-functionlization)

8.0 Cd(II) 25.7

[56]

SWCNTs-COOH 5.0 Cd(II) 77.00

[57] AlO(OH) loaded activated carbon fiber

5.8

Cd(II)

142.85

[46]

Oxidized CNT 6.5 Cd(II) 128.5

[58]

Activated Carbon supported CNT 3.0 Cr(VI)

9.0

[59]

Activated alumina-CNT nano clusters 2.0 Cr(VI) 264.5 Present Work

Activated alumina-CNT nano clusters 7.5 Cd(II) 229.9 Present Work

Table 3. Kinetic data for the adsorption of dichromate and Cd(II) by NC

Pseudo Second Order model constants Weber-Morris Model Analyte

Qe (mg/g) K’

(gmg-1h-1)

R2 Kint

(mgg-1h-0.5)

R2

Cr(VI) 142.8 0.025 0.992 63.044 0.80

Cd(II) 200.0 00.049 0.999 44.692 0.88

Table 4. Thermodynamic parameters on the adsorption of Cr(VI) and Cd(II) on Nano Clusters

Ion t (°C) T(K) 1/T CA(g/l) Ce(g/l) Kc lnKc Log Kc ∆G(kJ/mol) ∆S (J/mol) ∆H [kJ/(mol/K)]

25 298 0.00335 0.09219 0.0078 11.804 2.468 1.072 -6.1188

35 308 0.00324 0.08044 0.0196 4.112 1.414 0.614 -3.6226 -27.08 -73.18

Cd(II)

45 318 0.00314 0.0649 0.0351 1.849 0.615 0.267 -1.6258

25 298 0.00335 0.08723 0.0128 6.831 1.921 0.834 -47.6292

35 308 0.00324 0.07892 0.0211 3.744 1.320 0.573 -33.8207 -14.465 -405.5

Cr(VI)

45 318 0.00314 0.07098 0.0290 2.446 0.894 0.388 -23.6581

Research Highlights

Synthesis of Nanoclusters of Fe doped CNT and AA for the first time Structural characterization High surface area, hence a very high adsorption capacity Removal of both cationic and anionic contaminant (Cd(II) and Cr(VI)) Potential candidate for cleaning variety of water contaminants.

1

2

3

6

7

4

58

0 200 400 600 8000

50

100

150

200

250

300

350Cd(II)Cr(VI)

Am

ou

nt

of

Cr(

VI)

/Cd

(II)

ad

sorb

ed (

mg

/g)

Equilibrium Concentration (Ce) (mg/l)

Benzene,

AA with

Fe(II) and

Ni(II)

Cr(VI), Cd(II)

Fe loaded CNT-AA Nano clusters Vertical CVD unit