Potential of carbon nanomaterials for removal of heavy metalsfrom water

J.P. Rupareliaa, S.P. Duttaguptab, A.K. Chatterjeec, S. Mukherjia*aCentre for Environmental Science and Engineering, bDepartment of Electrical Engineering,

Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India

cUnited Nanotechnologies Pvt. Ltd., Mumbai, India

Abstract

Heavy metals, such as, cadmium, lead, nickel and zinc can be removed from water using sorbents. The rate andextent of removal may be enhanced by choice of appropriate sorbents. In this study heavy metal sorption wasstudied on indigenously synthesized carbon nanomaterials (CNMs). Two CNMs differing in surface morphologywere synthesized using turpentine oil in a chemical vapour deposition (CVD) setup by varying the process parameters.Activation and catalyst removal were achieved by post-treatment with HNO3 and KOH. Characterization of theCNMs produced revealed that both comprised of graphitic amorphous carbon, however, while the nanocarbon(NC) produced using cobalt catalyst in N2 atmosphere comprised of varying grain sizes indicative of soot, thenanoporous carbon (NPC) produced using silica catalyst in H2 atmosphere had a distinctive uniformly poroussurface morphology. Comparative sorption studies with cadmium, lead, nickel and zinc also revealed greater sorptionon NPC compared to NC. Batch isotherms for the various heavy metals using NPC and a commercial activatedcarbon (AC) widely used for metal sorption revealed that NPC is characterized by significantly higher metal sorptioncapacity and more favourable sorption energetics. The superior performance of NPC as a sorbent may be due to itsunique nanoporous structure.

Keywords: Heavy metal; Adsorption; Sorption; Carbon nanotubes; Nanoporous carbon

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1. Introduction

Extensive industrialization and improper dis-posal are attributed to be a prime factor respon-sible for the release of heavy metals into the eco-systems. Once released, the heavy metals tend tobioaccumulate in higher trophic levels of the foodchain. Almost all heavy metals are toxic to livingbeings and excessive levels are known to causeboth acute and chronic toxicity [1]. Heavy metalscannot be degraded or destroyed; moreover, thenatural process of metal mineralization is veryslow. Hence, removal of heavy metals from wa-ter and wastewater is best accomplished by im-mobilization and concentration on suitable sor-bents. The sorbed metals can be removed and re-used as raw materials [2].

Cadmium, lead, nickel and zinc were chosenfor this study due to their widespread use in in-dustries and potential water pollution impact.Cadmium exposure may cause nausea, salivation,muscular cramps and anaemia. Extended expo-sure to cadmium may also cause cancer [3]. Leadpoisoning is associated with gastrointestinal dis-orders, constipation, abdominal pain, and centralnervous system effects [4,5]. Exposure to nickelmay cause cancer of lungs, nose and bones. More-over, it may cause extreme weakness, dermatitis,headache, dizziness and respiratory distress [6].Zinc toxicity from excessive ingestion is less com-mon, however, it may cause damage to varioussystems in the human body [7,8].

Sorbents which have been studied for adsorp-tion of metal ions include activated carbon, flyash, crab shell, coconut shell, zeolite, manganeseoxides, and rice husk. However, these adsorbentshave poor removal efficiencies for low concen-trations of metal ions. Moreover, the slow removalrates often fail to meet pollution control require-ments [9–12]. To prevent deteriorating surfacewater quality, legislation governing the levels ofheavy metals, such as cadmium, lead, nickel andzinc discharged from industries is becoming pro-gressively more stringent. Thus, there is a need

for exploring alternative adsorbents, with bettermetal removal efficiency for low aqueous con-centrations.

With the emergence of nano science and tech-nology in the last decade, research has been initi-ated to exploit the unusual and unique propertiesof carbon nanomaterials (CNMs). CNMs mayexist in several forms, such as, single-walled car-bon nanotubes (SWCNTs), multi-walled carbonnanotubes (MWCNTs), carbon beads, carbon fi-bres and nanoporous carbon. CNMs have beenstudied widely for potential applications in cata-lyst supports, optical devices, quantum computer,and biochips. However, their sorption potentialhas not been studied extensively. CNMs are en-gineered materials targeted to exhibit unique sur-face morphologies, hence, they may prove to begood sorbents [13].

Carbon nanotubes (CNTs) have shown excep-tional adsorption capability and high adsorptionefficiency for various organic pollutants such asbenzene [14], 1,2-dichlorobenzene [15], trihalo-methanes [16] and polycyclic aromatic hydrocar-bons (PAHs) [17]. CNTs were found to be supe-rior sorbents for inorganic pollutants such as fluo-ride [18], and several divalent metal ions [7,8,19–25]. Li et al. [24] reported that CNTs with moredefects and poor quality possess higher surfacearea and exhibit better lead sorption capacity com-pared to aligned CNTs. Activation of CNTs playsan important role in enhancing the maximum sorp-tion capacity. Activation causes modification inthe surface morphology and surface functionalgroups and causes removal of amorphous carbon.Activation of CNTs under oxidizing conditionswith chemicals such as HNO3, KMnO4, H2O2,NaOCl, H2SO4, KOH, and NaOH have beenwidely reported [8,21,26,27]. During activation,the metallic impurities and catalyst support mate-rials are dissolved and the surface characteristicsare altered due to the introduction of new func-tional groups [24,26].

Both Langmuir and Freundlich adsorptionmodels have been used for representing sorption

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of heavy metals on CNTs. Maximum sorptioncapacity for heavy metals determined based onLangmuir model varies based on the type of metal.The maximum sorption capacity for cadmium,lead, nickel and zinc is reported to be in the rangeof 10–11 mg/g [21], 49–97 mg/g [20,22,24], 9–47 mg/g [19,25] and 11–43 mg/g [7,8], respec-tively. Hence, with CNTs as sorbent, metal ionuptake followed the order Pb2+ > Ni2+ > Zn2+ >Cd2+.

Characteristics of CNTs synthesised by chemi-cal vapor deposition (CVD) process depend uponthe type of catalyst and carrier gas used [28–32].Ryoo et al. [33] proposed a novel silica template-mediated approach for synthesis of nanoporouscarbon using CVD. The resulting high surface areamaterials with uniform pores were suitable for awide variety of applications: as adsorbents, cata-lyst supports, and materials for advanced electron-ics applications. Han et al. [34] used the NPC syn-thesized using silica template for dye adsorption.The adsorption capacity of nanoporous carbonwas found to be 10 times higher than that of com-mercial activated carbon for removal of directblue-78 dye. Apart from the template-mediatedmethod, another nanoporous carbon synthesisapproach involves functionalization and introduc-tion of nanopores in activated carbon by chemi-cal oxidation and heat treatment. Such function-alized activated nanoporous carbon has been usedfor adsorption of Ca2+, Cd2+, Pb2+, and Hg2+, how-ever, surface morphological analysis has not beenreported. The maximum amount adsorbed de-creased in the order Hg2+ > Pb2+ > Cd2+ > Ca2+

[35].The current study discusses the synthesis, treat-

ment and characterization of CNMs, i.e.,nanocarbon (NC) and nanoporous carbon (NPC)produced by variation of process parameters inCVD. Investigations were further carried out todetermine the sorption potential of NC and NPC.Sorption isotherms were generated for removalof cadmium, lead, nickel and zinc from aqueoussolution. The objective was to compare heavy

metal sorption capacities of novel carbon materi-als synthesized in the laboratory with tradition-ally used activated carbon.

2. Experimental procedure

2.1. Preparation of purified CNMs

CNMs were produced in the laboratory usinga CVD unit consisting of two furnaces joined sideby side with a quartz tube traversing both of them(Fig. 1). Detailed catalyst preparation, synthesisand treatment methods have been described else-where [36,37]. Pyrolysis of organic precursors athigh temperature facilitated by catalyst leads toCNM formation. In the CVD process, 7 g of crudeturpentine oil was filled in a quartz boat and keptin the vaporizing furnace. Two quartz boat eachcontaining 0.5 g of catalyst were kept in thepyrolysing furnace. After inserting the quartzboats within the quartz tube, one end was con-nected to the carrier gas inlet and the other endwas connected to the gas bubbler through whichthe gas was vented. Initially, carrier gas flow wasinitiated (500 mL/min) to ensure an oxygen-freeenvironment inside the quartz tube. Thepyrolysing furnace set at a predetermined tem-perature was switched on after 15 min. Thevaporising furnace also set at the desired tempera-ture was switched on after the temperature of thepyrolising furnace was stabilized. As the turpen-tine oil started vaporizing, dark brown fumes wereobserved to emerge through the gas bubbler. Af-ter 2–3 min, the dark brown vapour was no longerobserved, thus indicating complete vaporizationof turpentine oil. Both furnaces were then switchedoff and allowed to cool. Throughout the process,the carrier gas flow rate was maintained constant.After cooling of the furnace, the catalyst boatsand precursor boat were taken out. The carbondeposited on each boat was weighed and storedfor further purification.

By varying the process parameters, i.e., cata-lyst (cobalt/silica), type of gas (N2/H2) and the

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Vaporizing furnace Pyrolysing furnace

Gas bubbler

Quartz boats with catalyst Quartz boat with precursor Rotameter

Carrier gas

Quartz tubular reactor

pyrolysis temperature (600°C/700°C), CNMswith different characteristics were deposited andthese were designated as nanocarbon (NC) andnanoporous carbon (NPC), respectively. The va-porization temperature was kept constant at500°C. Nanocarbon and the cobalt catalyst wereseparated by treating with strong HNO3 for 3 h.Purification involved a 3 h reflux in 65% nitricacid at 80°C where approximately 1 L of acid wasrequired per 50 g of raw material. Subsequently,the black solution was centrifuged (10000 rpm,10 min) and the nanocarbon pellet was recoveredafter decanting off the acid. The nanocarbon pel-let still contained substantial trapped acid whichwas removed by repeatedly re-suspending thepellet in DI (deionized) water (by shaking vigor-ously), centrifuging, and decanting the superna-tant liquid. This process was repeated until thesolution attained neutral pH. Metal catalyst par-ticles were digested with strong HNO3 and washedaway. The NC was further treated with 30% KOHsolution (80°C in a reflux set-up) and repeatedlywashed with DI water until neutral pH was at-tained. Purified NC was oven dried and stored ina dessicator. The same procedure was applied to

Fig. 1. Schematic of the CVD setup.

NPC deposited on silica catalyst. Post-treatmenttime for removal of catalyst was optimized and 3h of 65% HNO3 treatment followed by 1 h of 30%KOH treatment was found to be sufficient. Withcrude turpentine oil as precursor, the yield for NCand NPC was about 40–45%.

2.2. Characterization

Crystallinity and purity of CNMs were deter-mined by X-ray diffraction (XRD, PhilipsPW3040/60 X’pert PRO, Netherlands) employ-ing Cu/Kα radiation. Surface morpholgy wasanalysed using scanning electron microscope(SEM, Hitachi S3400N, Japan). Surface hydro-phobicity of CNMs were analysed by powderwettability instrument (GBX InstrumentationCAP26, France). The aqueous phase heavy metalconcentration was determined using an atomicabsorption spectrophotometer (AAS, GBC-902Scientific Equipments Pvt. Ltd., Australia).

2.3. Metal sorption experiments

A kinetic study was performed for cadmium,lead, nickel and zinc using the nitrate salt (S.D.

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Fine-Chem Ltd., Mumbai, India) of each metal.The sorbents used were NC, NPC and commer-cially available AC (Loba Chemie Ltd., Mumbai,India). A kinetic study was conducted to decidethe contact time required for the adsorption pro-cess to reach equilibrium. Polypropylene batchreactors were filled with 100 mL of aqueous so-lution containing the metal ion (M2+) at a concen-tration of 1 meq/L and 0.5 g/L sorbent. The aque-ous solution in all experiments contained 0.01 MNaNO3 for maintaining constant ionic strength.The initial pH was adjusted to 6.5 using 0.01 NNaOH. The reactors were capped and kept in atumbler set at 15 rpm and maintained at room tem-perature. Over a period of 30 min–48 h, a pair ofsamples were removed and analyzed at predeter-mined time intervals. The sorbent was separatedout by centrifugation (12000 rpm, 15 min), pH ofaqueous phase was measured and aqueoussamples were stored at 4ºC until analysis was con-ducted. A study was carried out at a fixed initialconcentration (Co for M2+ = 1 meq/L) to checkthe sorption potential of each sorbent. Sorptionof all metals was compared on AC, NC and NPC.Subsequently, the equilibrium studies were con-ducted over the initial metal concentration range0.5–5.0 meq/L, while maintaining a constant sor-

0 20 40 60 80 100 1202θ

Inte

nsity

(a.u

.)

(004

)

(002

)

AC

NC

NPC

(004

)

(002

)

Fig. 2. XRD pattern of AC, NC and NPC.

bent dose of 0.5 g/L. For the equilibrium isothermstudies all samples were extracted and analyzedsimilarly after allowing sufficient time for equili-bration. For all studies, controls devoid of sor-bent were maintained and analyzed. All experi-ments were carried out in triplicate.

3. Results and discussion

3.1. Characteristics of carbon materials

The AC, NC and NPC samples were charac-terized using XRD. The XRD patterns (Fig. 2) ofAC, NC and NPC samples were compared andinterpreted with standard data of the InternationalCentre of Diffraction Data (ICDD). The peaks at12.05° and 24.01° can be attributed to (002) and(004) graphitic planes. The wide peaks indicatethe presence of amorphous nanocarbon [38]. TheXRD pattern further reveals that the cobalt andsilica catalysts were efficiently removed from NCand NPC, respectively, by the post-treatment pro-cess.

The SEM images of the carbon samples at vari-ous magnification are shown in Fig. 3. The mor-phology of the AC sample is fairly heterogeneousand comprised of grains with particle sizes greater

150

Fig. 3. SEM images of AC (a–d), NC (e–h) and NPC (i–l) at various magnifications.

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than 5 μm. Porosity was not observed even at ahigher magnification (Figs. 3a–d). The NC pow-der exhibits a porous structure as compared to ACpowder (Figs. 3e–h). Multi-folded graphene sheetsand cylinders are observed in NC. However, thestructural uniformity is lacking and grains of vary-ing particle sizes, indicative of the presence ofsoot and the related substances are observed. Incontrast, the NPC sample demonstrates a highlyporous morphology (Figs. 3i–l). This unique struc-ture of NPC is distinctly different from AC andNC. At a high magnification, a unique honey-comb-like structure with a uniform porosity andinterconnected pores is observed throughout theNPC sample. The average pore dimension is aboutone micron. The difference in the morphology ofNC and NPC discussed above can be attributedto the difference in the process parameters usedfor synthesis of CNMs. The use of hydrogen as acarrier gas in NPC synthesis may lead to less sootformation. Various researchers have reported thatthe hydrogen concentration inside the CVD reac-tor has an effect on the nature and quality of thedeposited materials [32,39].

The water contact angle of any powder reflectsits surface hydrophobicity. The greater the watercontact angle, the greater the surface hydropho-bicity of the powder. The water conact angle mea-sured for AC was 50°. The water contact anglesof NC and NPC as produced were 68° and 74°,respectively, representing hydrophobic surfaces.Post-treatment with HNO3 and KOH reduced thecontact angle values of NC and NPC to 55° and57°, respectively. This transition to hydrophilicsurfaces is possibly due to the introduction ofacidic oxygen containing functional groups. Simi-lar observations were reported by various re-searchers [7,19,21,25] for oxidation of CNTs withHNO3. These surface modifications are also re-ported to enhance the sorption of metal (M2+) ions.

3.2. Metal sorption studies

Kinetic study conducted with lead, cadmium,nickel and zinc with all carbon types AC, NC and

NPC indicated that the samples essentially reachequilibrium after 1 h as shown in Fig. 4 for leadsorption. Similar time scale, i.e., 1 h equilibrationtime, for sorption of metal ions on CNTs has beenreported by various researchers [7,19] althoughlonger time periods have also been reported[21,22] for lead, copper and cadmium sorptionon CNTs.

The equilibrium sorption studies were set-upfor 12 h in all cases to allow sufficient time forachieving equilibrium. The metal loading on thesorbents (qe, meq/g) was determined by mass bal-ance. The results of a single point sorption study(at a fixed initial concentration of 1 meq/L) shownin Fig. 5 indicated that the removal efficiency ofNPC is much higher compared to AC and NC.

NPC could sorb all the heavy metals studiedsuch that a high degree of removal was observed.The objective of this preliminary study conductedat a fixed initial concentration was to identify thebetter sorbent amongst the two CNMs. Subse-quently, isotherm studies were conducted withNPC, and sorption on NPC was compared to thatof AC.

3.3. Isotherm studies

The sorption isotherm data (Ce vs. qe) were fit-ted to the Langmuir and Freundlich isotherm

0

0.4

0.8

1.2

1.6

2

0 5 10 15 20 25Time (h)

qe (m

eq/g

)

AC NC NPC

Fig. 4. Kinetic study for lead sorption on AC, NC andNPC.

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Fig. 5. Removal of heavy metals by sorption on AC, NC and NPC.

models. The Langmuir model [Eq. (1)] assumesmonolayer coverage on sorbent whereas theFreundlich model [Eq. (2)] is an empirical modelallowing for multilayer adsorption on sorbent [40].

1

oa e

ee

Q bCqbC

=+ (1)

ne F eq K C= (2)

where qe = loading of adsorbate on adsorbent atequilibrium (μeq/g); Qa

o = maximum capacity ofadsorbent for adsorbate (μeq/g), b = sorption en-ergy (L/μeq), KF = indicator of sorption capacity(μeq1–nLng–1), n = adsorption energetics and Ce=aqueous concentration of adsorbate at equilibrium(μeq/L).

Isotherms generated for selected heavy met-als using AC and NPC as sorbents are shown inFig. 6 along with the Langmuir and Freundlichmodel fit to the data. The Langmuir and Freund-lich model parameters are listed in Table 1. Theparameter values for the Langmuir and Freundlichisotherm models were obtained by linear regres-sion based on the experimental data.

The R2 value of the Langmuir and Freundlichmodels reveals that the none of the models fitswell to the experimental data. However, fromFig. 6 it is observed that the model fits are rela-tively good and may be used to make comparisionacross the two sorbents and the various heavymetals. A comparison of the Langmuir model pa-rameter Qa

o representing the sorption capacity re-veals that sorption on NPC is significantly morethan on AC for sorption of all metals. Moreover,the sorption enegetics represented by the Lang-muir parameter b is also more favourable as indi-cated by the significantly higher b values for NPC(except for zinc sorption studies). The larger bvalues in NPC indicate that utilization of the sorp-tion capacity of NPC is likely to be higher in anytreatment scenario since 50% monolayer cover-age would be achieved at lower Ce (=1/b) values.The unique structure and uniform surface mor-phology of NPC may have facilitated heavy metalsorption.

NPC demonstrates high sorption capacity forall the metals studied. Sorption of heavy metalson NPC may occur due to ion exchange with theacidic oxygen containing functional groups intro-

0

20

40

60

80

100

Cadmium Lead Nickel Zinc

Perc

ent r

emov

al

AC NC NPC

153

Fig. 6. Freundlich ( - - - ) and Langmuir (——) model fits to experimental sorption data on NPC and AC forcadmium (a), lead (b), nickel (c) and zinc (d).

0

500

1000

1500

0 1000 2000 3000 4000Ce (μeq/L)

qe ( μ

eq/g

)

AC

NPCa

Table 1Isotherm model parameters for heavy metal sorption on AC and NPC

0

500

1000

1500

2000

2500

0 1000 2000 3000 4000Ce (μeq/L)

qe ( μ

eq/g

)

AC

NPCb

0

500

1000

1500

2000

0 1000 2000 3000 4000Ce (μeq/L)

qe ( μ

eq/g

)

AC

NPCc

0

500

1000

1500

2000

2500

0 1000 2000 3000 4000Ce (μeq/L)

qe ( μ

eq/g

)

AC

NPC

d

Langmuir isotherm Freundlich isotherm Metal Sorbent Qa

o (μeq/g)

b (L/ μeq)

R2 KF (μeq/g)*(L/μeq)n

n R2

Cd AC 217.4 0.0031 0.4103 16.84 0.3179 0.5906 NPC 1428.6 0.0092 0.4397 475.5 0.1389 0.5897 Pb AC 909.1 0.0025 0.7707 120.3 0.233 0.6027 NPC 2000 0.01 0.6216 1044 0.0844 0.3617 Ni AC 909.1 0.0004 0.5787 2.586 0.6652 0.6616 NPC 1428.6 0.0075 0.5422 567.02 0.1124 0.4689 Zn AC 625 0.0043 0.7362 69.5 0.2792 0.8087 NPC 2000 0.0035 0.7088 220.85 0.2707 0.8277

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duced by activation during the post-treatment pro-cess. The sorption capacity (expressed in μeq/g)is comparable for lead and zinc. The sorption ca-pacities for cadmium and nickel are comparablebut lower than for lead/zinc. The lower capacityfor cadmium and nickel sorption may be due tothe lower penetration and their inability to reachall the sorption sites that are accessed by lead andzinc. For lead, the ionic radius is the largest(1.19 Å), hence, due to its lowest hydrated ionicradius it is likely to penetrate effectively into theporous matrix. However, this consideration alonecannot explain the lower penetration of cadmium(ionic radius 0.97 Å) compared to zinc (ionic ra-dius 0.74 Å). Moreover, the electronegativity ofzinc is significantly lower compared to the othermetals. Thus, comparable sorption capacity forlead and zinc is an unexpected result. It may benoted that the R2 values for the Langmuir fit(Table 1) are significantly lower for cadmium andnickel compared to those for lead and zinc.

In this study, the sorption capacity is reportedover the equilibrium concentration range 10–4000 μeq/g and pH 6.5 for all the metals. Althougha direct comparison with the literature reportedstudies for CNTs cannot be made due to differ-ences in pH, Ce range and other parameters, thesorption capacity on NPC needs to be reviewedin comparison to that reported for CNTs. Li et al.[21] treated CNTs with different oxidizing agentsand generated isotherms for cadmium sorption atpH 5.5 over the equilibrium concentration range10–130 μeq/L. For the various CNTs studied, thesorption capacity of cadmium was significantlylower than for NPC (1428 μeq/g) and was reportedto be in the range of 35–200 μeq/g. A lead sorp-tion study with CNTs was carried out by Li et al.[20] at pH 7 and equilibrium concentration rangeof 10–80 μeq/L where the sorption capacity wasreported as 482 μeq/g. In another study, Li et al.[24] reported that the lead sorption capacity ofCNTs at pH 5 was 797 μeq/g. The lead sorptioncapacity on NPC is significantly higher (2000 μeq/g)than on these CNTs. Lu and Liu [19] and Lu and

Chiu [7] reported nickel and zinc sorption capac-ity on CNTs as 1630 μeq/g and 1335 μeq/g, re-spectively, at pH 7 over the equilibrium concen-tration of 200–2200 μeq/L which is comparableto that observed in this study for NPC (1428 and2000 μeq/g, respectively). It is thus establishedthat, by virtue of its unique morphology, NPC hasa significant sorption potential for heavy metalswhich is superior or comparable to the literaturereported values for CNTs.

4. Conclusions

Surface morphology plays an important rolein sorption. Two types of CNMs differing in sur-face morphology were synthesized by CVD byvarying the process parameters, i.e., catalyst, car-rier gas and temperature of the furnace. XRDanalysis demonstrated that both CNMs were gra-phitic in nature. SEM images revealed that nano-porous carbon (NPC) prepared using silica cata-lyst in a hydrogen atmosphere had a distinctivesurface morphology comprised of uniform poros-ity with interconnected pores of average pore di-ameter 1 μm. In contrast, the nanocarbon (NC)prepared using cobalt catalyst in a nitrogen atmo-sphere comprised grains with varying particlesizes which indicated the presence of soot andrelated particles. The porous NPC with distinc-tive surface morphology exhibited superior sorp-tion of heavy metals compared to NC and AC, acommercially-available activated carbon. Thesorption capacity and energetics for heavy metalsorption on NPC were more favourable comparedto AC. This implies that at a low aqueous concen-tration of heavy metals, the sorption capacity ofNPC will be efficiently utilized for the removalof heavy metals. NPC exhibited a high sorptioncapacity for all the heavy metals — lead, cadmium,nickel and zinc — used in this study. These val-ues were significantly higher or comparable tothose reported for carbon nanotubes by variousresearchers. Thus, nanoporous carbon has a goodpotential for use in water treatment applications.

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Acknowledgements

The authors gratefully acknowledge the Na-tional Doctoral Fellowship (NDF) awarded by AllIndia Council for Technical Education (AICTE),New Delhi, India, which provided funds for stu-dent support. The authors would like to acknowl-edge the Department of Metallurgical Engineer-ing and Material Science, IIT Bombay, for theXRD and SEM analyses, and the Department ofChemical Engineering, IIT Bombay, for the con-tact angle measurements.

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