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Adsorption of lead ions from aqueous solution using porous carbon derivedfrom rubber tires: Experimental and computational study

Tawk A. Saleh a, Vinod K. Gupta a,b,, Abdulaziz A. Al-Saadi aaDepartment of Chemistry, King Fahd University of Petroleum & Minerals, Dhahran, Saudi ArabiabChemistry Department, Indian Institute of Technology Roorkee, Roorkee 247 667, India

a r t i c l e i n f o

Article history:Received 5 December 2012Accepted 17 January 2013Available online 31 January 2013

Keywords:Porous carbonLead ion removalFT calculations

a b s t r a c t

Effective activated porous carbon (AC) was prepared by thermal treatment of waste rubber tires and wasfurther activated using oxidizing agents like nitric acid and hydrogen peroxide. The tire-derived porouscarbon was characterized by means of FTIR and X-ray diffraction. Careful analysis of the IR spectra of thesynthesized AC reveals a number of bands centered at about 3400, 2350, 1710, 1650, and 13001000 cm1, proving the existence of hydroxyl and carboxylic groups on the surface of AC in addition toC@C double bonds. The developed AC was tested and evaluated as a potential adsorbent for the removalof lead (II) ions. Experimental parameters, such as contact time, initial concentration, adsorbent dosage,and pH were optimized. AC was effective in a pH range between 4 and 7 with a highest uptake of leadions at pH 5 and 6. For further understanding of the chemistry behind the process, density functional the-ory (DFT) calculations were performed at the B3LYP/6-31G(d) level adopting a functionalized pyrenemolecule as a model. The binding energy of Pb(II) ion toward carboxylic acid, carbonyl, and hydroxylgroups was calculated. A binding energy in the range of 310340 kcal/mol, which is considered to be highand to be indicative of a chemisorptions process, was predicted. The adsorption of the lead ion toward theC@O groups in relatively all cases shows more stable binding compared to the sorption toward the alco-hol groups.

2013 Elsevier Inc. All rights reserved.

1. Introduction

Contamination of the environment from different activities hasbecome an increasingly serious problem in recent years. Heavymetals are one class of toxic pollutants released into the surfaceand ground water as a result of various activities such as indus-tries, mining, and agriculture [1]. The rapid development of indus-try has led to severe problems of water pollution. Industries andmunicipal authorities have been forced to treat wastewater beforedischarging. Various methods are available for water treatment.However, adsorption technology is considered as an efcient anduniversal method of water treatment as per the guidelines ofWHO and EPA. This is because of its cost effectiveness and environ-mental friendliness. The cost effectiveness of this technology is dueto the use of effective adsorbents by converting solid waste to va-lue-added products.

Tire rubber is a mixture of different elastomers such as naturalrubber, butadiene rubber, and styrene butadiene rubber plus otheradditives like carbon black, sulfur, and zinc oxide. Approximately

32% by weight of the waste tire is mainly constituted of carbonblack in which the carbon content is as high as 7075 wt.% [24].

The increase in waste rubber tires and other rubber materialsimposes serious threats on the public health and safety as wellas environment. Different methods are used for rubber disposalincluding grinding which converts it into granulates used as llerfor thermoplastics [57]. One potential application for the wasterubber tire char is to produce activated carbon (AC) which is usedas adsorbent. AC is of high porosity and high surface area materialmanufactured by carbonization and activation of carbonaceousmaterials by either physical or chemical activation methods [8,9].

Water pollution by heavy metals is considered a serious menaceto the environment and especially to human health [10]. Accordingto the World Health Organization, the most common toxic metalsfound in wastewater are lead, copper, cadmium, chromium, zinc,and nickel. Lead is widely used in processing industries such aselectroplating, paint and dyes, explosive manufacturing, and leadbatteries [11]. The presence of lead ion in water even at a verylow level, 5 ng mL1, would be harmful to aquatic life and endan-ger human health because of its toxicological, potential carcino-genic, and neurological effects [12].

The maximum permissible limit (MPL) of lead in drinking wateris 0.1 mg/L according to the World Health Organization since leadis cumulative poison [13]. Hence, the appropriate treatment of

0021-9797/$ - see front matter 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jcis.2013.01.037

Corresponding author at: Chemistry Department, Indian Institute of TechnologyRoorkee, Roorkee 247 667, India. Fax: +91 1332286202.

E-mail address: [email protected] (V.K. Gupta).

Journal of Colloid and Interface Science 396 (2013) 264269

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industrial wastewater which releases lead into the aquatic and ter-restrial systems is very important. Various adsorbents have beenreported for the removal of metal ions and organic compoundsfrom waters [1444]. Most of these adsorbents might not be ef-cient in removing heavy metal at very low concentrations andcould be relatively expensive. Therefore, it is essential to furtheractivate the adsorbent surface to enhance adsorption capacity.

The objective of the present study was to prepare activate por-ous carbon from waste rubber tire with a combination of physicaland chemical treatment in an attempt to develop surface porosityas well as to incorporate desired functional groups on the surfaceof the resultant activated carbon. The adsorption capacity of thedeveloped carbon was further evaluated for lead ions removal fromwaste water. Factors that are likely to affect the adsorption capac-ity of the developed carbon were studied. In addition, a computa-tional study was conducted.

2. Experimental section

2.1. Reagents

Nitric acid and lead nitrate were obtained from SigmaAldrich.Stock solutions of the lead ions were prepared in deionized water(the resistance of the deionized water is 0.5 lS). Lead solutions ofdifferent initial concentrations were prepared by diluting the stocksolution in appropriate proportions. In order to prevent metal con-tamination from laboratory glassware, glassware was kept over-night in a 10% (v/v) HNO3 solution. Safety precaution wasconsidered in handling all chemicals and conducting the experi-mental work [45,46]. All reagents used in the study were of analyt-ical grade.

2.2. Development of the adsorbent

Waste rubber tire is cleaned and ground. The tire granules wereheated up to 300 C to separate the produced oil; black tire crudeoil and yellow distilled diesel oil. Then, thermal pyrolysis was per-formed at a temperature of around 500 C for 3 h in the presence ofhelium to break down the cross-linkage between carbon atoms.The activation process was performed using steam as activatinggas at 900 C for 2 h, for further development of the porosity of tirecarbon [47].

Further modication was accomplished via HNO3 and H2O2treatment in order to develop oxygen surface groups on the rubbertire carbon. For this purpose, the product was treated with HNO3(4 M concentration) with a ratio of 1 g of carbon to 20 mL ofHNO3 [48]. The mixture (carbon and acid solution) was stirredand heated at 60 C for 24 h. Then, it was washed thoroughly anddried at 100 C. After that, the product was treated with H2O2(6% concentration) with a ratio of 1 g/20 mL carbon/H2O2. The mix-ture (carbon and acid solution) was stirred and heated at 60 C for24 h. Then, it was washed thoroughly and dried at 100 C whichwas then be characterized. This is referred to as RAC throughoutthe study.

2.3. Characterization of activated carbon

The activated carbon produced from waste rubber tires (RAC)was characterized by means of scanning electron microscope (FES-EM, FEI Nova-Nano SEM-600, Netherlands) for scanning the adsor-bent surface and energy-dispersive X-ray spectroscopy (EDX) forthe quantitative analysis of the components of RAC. Fourier trans-form infrared spectroscopy (FT-IR) was used for identifying typesof chemical bonds and functional groups and oxygen containinggroups. The IR spectra of the RAC were recorded on a PerkinElmer

FTIR 180 spectrophotometer using KBr pellets over the range4000400 cm1. For the FTIR study, 10 mg of nely sized particleof the adsorbent was encapsulated in 300 mg of KBr keeping theratio 1:30, in order to prepare the translucent sample disks.

2.4. Adsorption and kinetic studies

The kinetic and isothermal studies were performed using a ser-ies of 250-mL Erlenmeyer asks which were lled with 100 mLlead solution of varying concentrations (120 ppm), maintainedat desired temperature and pH. The pH of the solution was keptconstant by adding 0.2 M NaOH or 0.2 M HNO3. An equal amountof adsorbent was added separately into each individual ask. Theasks were agitated in a thermostatted shaker at a speed of150 rpm. Liquid samples were taken out at a given time intervaland centrifuged at 1500 rpm for 3 min. The concentration ofremaining lead ions in the adsorption medium was determinedby the ICP-MS. The adsorption capacity for lead ions uptake, qe(mg/g), was determined using the following equation:

qe C0 CV=W 1and the percentage of removal was calculated using the followingequation:

%removal C0 C=C 100 2where C0 and C are the initial and nal lead concentrations (ppm),respectively, V is the volume of solution (L) and W is the weightof adsorbent (g). All the experiments were repeated three times,and average values were reported. The standard deviation wasfound to be 0.2%; values of the correlation coefcient averaged 0.99.

The parameters considered for study were initial concentrationof adsorbate, contact time, adsorbent dose, pH, and temperature.

2.5. Computational part

The representative calculations for Pb(II) ion adsorption on sev-eral functionalized pyrene systems have been carried out with theG09 program package14 utilizing the DFT-B3LYP method with the6-31+G basis set for H, C and O atoms and the SDD pseudo-poten-tial for Pb(II) ion. Full geometry optimizations were performed onthe models shown in Fig. 8 without any constraint. The stationarypoints have been conrmed by frequency calculation, and the ini-tial binding energy was predicted from the following equation:

BEPbPyr EPyr EPbII EPbPyr 3

3. Results and discussion

3.1. Characterization of RAC

The morphology of the activated carbon produced from wasterubber tires was characterized by SEM, Fig. 1. The SEM imageshows the porous structure of the activated carbon. The imageexhibits two distinct morphologies. One of the morphological re-gions is granular in nature (the granules are 0.5 lm in diameter),and a large number of the granules can be observed in the image.High resolution of the image of the developed activated carbonindicates that there are different pore structures in the adsorbent.The basic parameters for an effective adsorbent are high surfacearea and pore structure. When the porosity increases, the surfacearea also increases [4951].

EDX measurements were conducted for the compositional anal-ysis of the RAC. The spectrum in Fig. 2 conrms the presence of car-bon and oxygen in the tire-derived carbon. Table 1 presented theEDX quantitative microanalysis of the tire-derived carbon.

T.A. Saleh et al. / Journal of Colloid and Interface Science 396 (2013) 264269 265


The IR spectra of the tire-derived activated carbon were mea-sured by recorded on a PerkinElmer FTIR 180 spectrophotometerusing KBr pellets over the range 4000400 cm1. The spectrum,not shown, displays a number of bands. The assignment of the car-bonyl band to an ester is conrmed by observation of a strong bandin the CAO stretching region at about 1180 cm1 (broad band11001300 cm1) [52]. The band at 1580 cm1 is attributed toC@C double bonds [53]. The carboxylic acid group is usually pre-sumed to take part in the adsorption of metal ions from aqueoussolution [54]. The bands at 1710 and 1650 cm1 are ascribable inturn to stretching (m) (C@O) vibrations of carboxyl and carbonylin acidic oxygen surface groups. The band at 3400 cm1 is attrib-uted to stretching (OAH) vibration in hydroxyl groups. The bandat 2350 cm1 is attributed to CC stretching vibration of the al-kyne group [55].

3.2. Experimental parameters

3.2.1. Effect of contact timeIn order to establish the rate of metal removal and the equilib-

rium time for maximum uptake and to know the kinetics of thesorption process, the adsorption of lead ions by developed acti-vated carbon was carried out under contact times from 0 to140 min, Fig. 3. The adsorption efciency of lead ions increasesgradually with increasing contact times and reaches equilibriumat around 80 min at which point the majority of Pb(II) is removed.At this point, the amount of lead being adsorbed onto the RAC wasin a state of dynamic equilibrium with the amount of lead des-orbed from the adsorbent. According to the results, the equilibriumtime is xed at this time for the rest of the batch experiments tomake sure that equilibrium is reached.

3.2.2. Effect of lead initial concentrationsThe effect of the initial concentration of lead ions on the adsorp-

tion process was also investigated. The removal extent of lead isdependent on the initial concentration. This is reasonable sincethe more the lead ions available in the solution, the faster theadsorption due to the enhancement in the dynamic contact be-tween the adsorbent and the adsorbate. As depicted in Fig. 4, theuptake slightly increases by decreasing the initial lead concentra-tion which indicates perhaps that at higher concentration rates,equilibrium is not attained in 80 min.

3.2.3. Effect of agitation speedThe agitation speed is considered an important parameter in the

adsorption process. The agitation speed was varied between 0 and

Fig. 1. SEM image of the surface structure of the tire-derived activated carbon.

Fig. 2. EDX spectrum of activated carbon derived from the tire.

Table 1Energy dispersive X-ray analysis (EDX) quantitative microanalysisof the tire-derived activated carbon.

N# Element Weight% Atomic%

1 C K 93.95 95.392 O K 6.05 4.61

Total 100.00

Fig. 3. The effect of contact time on the amount of lead ions adsorbed on the tire-derived activated carbon. (Conditions: initial lead concentration 20 ppm; dosage ofadsorbent = 0.5 mg; pH 5; agitation speed = 150 rpm; contact time = variable.)

Fig. 4. Effect of lead initial concentrations on the adsorption on the tire-derivedactivated carbon. (Conditions: initial lead concentration = variable; dosage ofadsorbent = 0.5 mg; pH 5; agitation speed = 150 rpm; contact time = 80 min.)

266 T.A. Saleh et al. / Journal of Colloid and Interface Science 396 (2013) 264269


150 rpm keeping all other experimental variables (initial lead con-centration 20 ppm; dosage of adsorbent = 0.5 mg; pH 5; contacttime = 80 min) constant. It was observed that the percentages oflead adsorbed increased as the agitation speed was increased,Fig. 5. The removal ratio of lead increased from about 25% toaround 85% by increasing the speed between 20 and 150 rpm.The adsorption capacity was greatest when agitation speed of120 and 150 rpm was used. Fig. 5 shows that the extent of adsorp-tion is not independent of agitation rate until the latter exceeds120 rpm. Thus, at slower agitation rates, equilibrium is not at-tained in 80 min.

3.2.4. Effect of adsorbent dosageThe dose of adsorbent (m) was varied from 0.0 to 1.0 g/L, keep-

ing all other experimental variables constant. As depicted in Fig. 6,the percentage of lead removal increases up to adsorbent dose of1 g/L. This may be attributed to an increased sorbent surface areaand availability of more adsorption sites resulting from the in-creased dose of the adsorbent. The percentage of removal of Pb(II)increases from 15% to 85% with an increase in the amount of adsor-bent from 0.1 to 0.5 g, respectively.

3.2.5. Effect of the pHThe effect of the pH value of the solution is an important con-

trolling parameter in the adsorption process. It inuences both

the adsorbent surface metal binding sites and the metal chemistryin water [56].

As a function of solution pH, Pb2+ is the dominant species belowpHvalue of about 6. Between pH 6 and 8, Pb undergoes hydrolysis toPb(OH)+. Above pH 9, solid lead hydroxide Pb(OH)2 is thermody-namically the most stable phase, while PbOH3 is predominant atpH above 11 [57,58]. In order to nd the optimal pH value for thesorption process, the pH of feed solution was examined using solu-tions of different pH levels, covering a range of 2.09.0. AtpH > 7.0, the Pb(II) gets precipitated due to hydroxide anionsforming a lead hydroxide precipitate. As shown in Fig. 7, the rangebetween 2 and 7 was chosen to avoid metal solid hydroxide precip-itation. The results indicate that the removal of lead ions onto theactivated carbon is pH dependent. At low pH range between 2.0and 3.0, hydrogen ions compete with lead ions for the surface ofthe adsorbent which would hinder Pb(II) ions from reaching thebinding sites of the sorbent caused by the repulsive forces. Theincrease in metal removal as pH increases to 4 can be explained onthe basis of a decrease in competition between protons (H+) andpositively charged metal ions at the surface sites. The maximumuptake of lead ions is obtained at pH 5 and 6 with highest efciencyof 85%. This can be explained by the point of zero charge of thedeveloped activated carbon which was measured and found to beat 4.5 (pHpzc = 4.5). According to the Pb(II) speciation diagram andin this range of pH, the dominant species of sorption are Pb2+ [59].At pH of 5 or 6, the adsorbent surface is negative and the lead pre-sents as Pb2+ which reect the electrostatic adsorbate/adsorbentinteractions.

3.3. Computational study

For further understanding of the chemistry behind the process,density functional theory (DFT) calculations were performed at the

Fig. 5. The effect of agitation speed on the amount of lead adsorbed on the tire-derived activated carbon. (Conditions: initial lead concentration 20 ppm; dosage ofadsorbent = 0.5 mg; pH 5; agitation speed = variable; contact time = 80 min.)

Fig. 6. The effect of dosage on the amount of lead ions adsorbed on the tire-derivedactivated carbon. (Conditions: initial lead concentration 20 ppm; dosage of adsor-bent = variable; contact time = 80 min; pH 5; agitation speed = 150 rpm.)

Fig. 7. The effect of pH on the amount of lead adsorbed on the tire-derivedactivated carbon. (Conditions: initial lead concentration 20 ppm; contact time =80 min; dosage of adsorbent = 0.5 mg; pH = variable; agitation speed = 150 rpm.)

Table 2Binding energies (kcal/mol) and bond distances (angstroms) of the adsorption ofmono-binding lead ion on functionalized pyrenes as calculated at the B3LYP/6-31+Glevel of theory.

Site type BE (kcal/mol) Bond distance ()

E Ecorrected

Pyr-COOH 337 337 2.446Pyr-C@O 333 334 2.185Pyr-OH 330 329 2.498Pyr-COOH 319 319 2.519



B3LYP/6-31+G level. The initial binding energy of Pb(II) ion towardcarboxylic acid (two possible positions), carbonyl, and hydroxylgroups was calculated Fig. 8, and results are listed in Table 2. Abinding energy in the range of 310340 kcal/mol was predicted.It is considered to be relatively high and indicative of a possiblechemisorption process. The adsorption of the lead ion toward theC@O groups in relatively all cases shows more stable binding com-pared to the sorption toward the alcohol groups. In the case of apurely carbonyl group, the lead ion tends to approach much closerand more conveniently toward the carbonyl oxygen atom com-pared to the other cases. The initial binding energy, yet, seems tobe in this case more sensitive toward the nature of the functionalgroup rather than the proximity of the lead ion to the binding site.

4. Conclusion

Waste rubber tires were converted into a carbonaceous adsor-bent and used for the removal of aqueous lead from wastewater.The activation process facilitated in the incorporation of carbonyland hydroxyl functional groups onto the adsorbent surface; theseultimately helped in the adsorption process. Further, this has beensupported by the density functional theory (DFT) calculations,which indicate the high binding energy (310340 kcal/mol) ofPb(II) ion toward carboxylic acid, carbonyl, and hydroxyl groups,respectively.

On the basis of the results obtained, it can be safely concludedthat the AC developed from waste rubber tires act as potentialadsorbents for the removal of lead ions from aqueous media.Waste rubber tires are a cheap and easily available material thatthus can act as a better replacement source for commercial AC.Being a waste product, the use of waste rubber tires as adsorbentwould also solve their disposal problem.

Acknowledgments

The authors would like to acknowledge the support provided byKing Abdulaziz City for Science and Technology (KACST) through

the Science & Technology Unit at King Fahd University of Petro-leum & Minerals (KFUPM) for funding this work through ProjectNo. 10-WAT1400-04 as part of the National Science, Technologyand Innovation Plan.

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