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Journal of Environmental Management 128 (2013) 787e797

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Lead sorption onto acrylamide modified titanium nanocompositefrom aqueous media

Ajit Kumar Sharma, Byeong-Kyu Lee*

Department of Civil and Environmental Engineering, University of Ulsan, Nam-gu, Daehak ro 93, Ulsan 680-749, Republic of Korea

a r t i c l e i n f o

Article history:Received 31 December 2012Received in revised form13 May 2013Accepted 22 June 2013Available online 14 July 2013

Keywords:AdsorptionPb(II)Acrylamide-TiO2

Nanocomposite

* Corresponding author. Tel.: þ82 52 259 2864; faxE-mail address: bklee@ulsan.ac.kr (B.-K. Lee).

0301-4797/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.jenvman.2013.06.030

a b s t r a c t

The purpose of this study was to investigate the adsorption of lead (Pb(II)) onto acrylamide (AM) dopedTiO2 nanocomposites (TieAM) using batch techniques for evaluation of isothermal and kinetic proper-ties. Chemical, structural and textural characteristics of the material were determined by FTIR, XPS, XRD,SEM and EDAX analysis. XPS results showed that a change in oxidation state occurred due to leadadsorption. The adsorption conditions for the adsorbent were optimized by varying several experimentalparameters, i.e., contact time, initial lead concentration, adsorbent dose, pH, and electrolyte amount ofthe solution. The adsorption data were modeled using both the Langmuir and Freundlich isotherms. Themaximum adsorption capacity using Langmuir isotherm (qmax) for the nanocomposite was found to be476.19 mg g�1. Adsorption showed pseudo-second-order kinetics with a rate constant of 8.7 � 10�4 and1.2 � 10�4 g mg�1min�1 at 100 and 500 mg L�1 initial Pb(II) concentrations, respectively. Acrylamideconcentration in nanocomposite synthesis up to 1 g had greater influence on the sorption of lead. Themost favorable pH for the adsorption was pH 5.5. With increasing concentrations of three electrolytes(NaCl, Na2SO4 and Na2CO3) from 0.01 to 1.0 M Pb(II), the lead removal decreases from 76.3 to 32.8 mg g�1,from 97.4 to 68.7 mg g�1, and from 98.8 to 72.5 mg g�1, respectively. Further, the TieAM nanocompositeis amenable to efficient regeneration by a 0.05 N HCl solution for repeated (up to six cycles) use withoutany significant capacity loss, making this approach very economical.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Adsorption behavior of lead ions from aqueous media hasbecome of great importance in the last decade. Among the heavymetal ions, Pb(II) is of particular interest since it is toxic and notbiodegradable, tends to accumulate in vital organs, and causesvarious diseases and disorders in humans and animals as a result oflong-term exposure (Benhimaa et al., 2008; Jiang et al., 2012). Leadis frequently used in industrial processes such as electroplating,mining, extracting, printing, painting, textile and batterymanufacturing, tanning, metallurgy activities, metal processingand others (Pimentel et al., 2008; Googerdchian et al., 2012). Thehigh toxicity of lead ions leads to serious ecological damage, thus,there is an increasing necessity to develop processes to eliminate orat least to greatly reduce the concentration in wastewater prior torelease into the environment.

As per the World Health Organization (WHO), the maximumallowed concentration value for lead is 0.05mg/l (Singh et al., 2007)

: þ82 52 259 2629.

All rights reserved.

in drinking water and other types of waters. Many physicochemicaltreatment processes have been developed to remove heavy metalions from industrial effluents. These include reverse osmosis, ionexchange, electrodialysis, evaporation, chemical precipitation,flocculation, membrane separation and chelation. Most often, thesemethods have high reagent requirements, generate secondarypollutants or are simply inefficient and particularly require highcost in removing toxic metal ions in trace quantities from solutions(Fu and Wang, 2011; Ghorai et al., 2012). Above all the variousmethods, adsorption has been recognized as one of the mosteconomical (as compared to ion exchange, extraction or electro-dialysis methods) and effective methods for the removal of the leadfrom aqueous media due to flexibility in design and operation(technically easy, as compared chemical precipitation or reverseosmosis methods) offered by the adsorption process (Okoye et al.,2010; Hao et al., 2012).

Adsorption using different polymeric materials and metal ox-ide sorbents is the method of choice in many wastewater treat-ment processes for removing metals from chemical industries insome developed countries (Akkaya and Ulusoy, 2008; Pan et al.,2009; Ulusoy and Akkaya, 2009; Ngah and Fatinathan, 2010).Many researchers have reported on the use of hydrous metal

A.K. Sharma, B.-K. Lee / Journal of Environmental Management 128 (2013) 787e797788

oxides as adsorbents for the removal of lead ion from aqueousmedia (Hua et al., 2012). Titanium dioxide has been intensivelyinvestigated as a potential sorbent due to its high chemical sta-bility in the pH range 2e14 and because the process is simple witha fast rate of adsorption and desorption (Samarghandi et al., 2007;Li et al., 2010a,b; Tehrani et al., 2011). In addition, nano-size tita-nium dioxide possesses many unique features, such as high sur-face area, more surface atoms, high surface reactivity, uniquecatalytic activity and high suspension stability when compared tolarger size particles (Jia et al., 2009). Titanium dioxide nano-particles (Hu and Shipley, 2012) and amorphous titanium phos-phate (Jia et al., 2008) have high potential for lead ion adsorption.Therefore, research has been conducted recently to include sur-face modification of TiO2 to enhance the adsorption capacity ofTiO2. Modification of the TiO2 surface with polymer, due to thecombined characteristics of titanium and polymer, is potentiallyan even more attractive candidate to be used as an adsorbent, astheir combined properties represent an improvement upon thethermal, mechanical, and porosity properties as compared to thehomologous characteristics of the individual organic and inor-ganic components (Sanchez et al., 2001). Polymers such as, cel-lulose microspheres, polystyrene, polyethylene oxide (Nagaokaet al., 2002; Zan et al., 2004; Sung et al., 2006) and chitosan(Chena et al., 2012) used as supports would have some advantagesincluding low cost and easy separation from reaction solution.Acrylamide (AM) is a well known water soluble polymer con-taining large numbers of amide groups; the hydrophobicallyassociating cationic polyacrylamide, one kind of water-solublehydrophobically associating copolymers, polymerized by cationicmonomer, hydrophobic monomer and acrylamide (Yang et al.,2010). Also, AM has been widely grafted to various adsorbentmaterials; polymerization occurs only in supersaturated waterswith respect to silica (Demadis and Stathoulopoulou, 2006).Acrylamide based materials are efficient adsorbents for theremoval of toxic metal ions (Rivas et al., 2001; Li et al., 2005) andorganic pollutants (Meng et al., 2005). The adsorption efficiency ofmaterials can be greatly enhanced after modification with AM. Theproperties of the functional groups (i.e., active site types, andamide group, etc.) are very important for the adsorption capacityof adsorbents. In order to evaluate and to compare the effects offunctional groups and active site type on the adsorption capacityof materials, the same content of functional groups on the surfacesof the adsorbent are required.

The major aim of the present study is to explore the adsorptionbehavior of Pb(II) cation from aqueous media onto acrylamide-TiO2using IR, XRD, XPS and SEM/EDX analysis. In particular, this study isaimed to examine the effects of operational parameters, such ascontact time, initial pH of the ionic solution and initial concentra-tion of ions to optimize the adsorption capacity. Also, theequilibrium adsorption data have been evaluated by adsorptionisotherms (Langmuir and Freundlich), and kinetics parameters aredetermined.

2. Materials and methods

2.1. Materials

All the chemicals used in this study were of analytical reagentgrade and were utilized as received without further purification.Titanium (IV) isopropoxide in the liquid form and potassium per-sulfate as radical initiator for the synthetic reaction of acrylamide(AM) were obtained from Daejung Chemical and Metals Co. LTD.,South Korea. Milli-Q deionized water was used throughout thestudy. Aqueous solutions of lead nitrate were prepared by dis-solving Pb(NO3)2 (Merck) in deionized water.

2.2. Synthesis of Acrylamide-TiO2 (TieAM) composites

Nano titaniumwas synthesized by solegel method (Kallel et al.,2009). In-situ doped acrylamide titanium nanocomposite (TieAM)was synthesized by a solegel process, where acrylamide monomerwas doped into nano titanium during the solegel process. After-ward, the reaction was completed with initiator by polymerizationand the products were characterized as described earlier. Acryl-amidemonomer (0.1, 0.2, 0.5, 1, 2 g) was dissolved in distilled water(120 mL) at ambient temperature. The typical synthesis proceduresfor nano TiO2 and acrylamide doped nano-TiO2 are as follows.Titanium(IV) isopropoxide (5.58 mL) was hydrolyzed using glacialacetic acid (10.74 mL) at 35 �C. To this solution 120 mL acrylamide/water was added drop-wise under vigorous stirring for 30 min.Acrylamide-TiO2 solutionwas stirred and heated to 70 �C in awaterbath for 15 min and initiator potassium peroxydisulfate (0.1 g) wasadded. The mixture was kept stirring for 1 h longer until a clearsolution of acrylamide-TiO2 nanocrystals was formed. The preparedsolution was kept in the dark for the nucleation process for 14 h.After this period, the solution was placed in an oven at a temper-ature of 85 �C for a period of 14 h for the gelation process. The gelwas then dried at 120 �C and subsequently the nanocomposite wascrushed into fine powder.

2.3. Characterization of the composites

The experimental data were corrected for the background ab-sorption of the solvent. Infrared spectra (PerkineElmer infraredspectrophotometer) were carried out on Nicolet Nexus 470 FTIRspectrometer in the region 3800e500 cm�1. For this, the sampleswere prepared in the form of pellets with KBr at ambient conditionsand were measured in transmission mode. Moreover, nano-TiO2and acrylamide-TiO2 surfaces were characterized by X-ray photo-electron spectroscopy (XPS) using a Thermo Scientific K-Alpha XPSspectrometer to determinate the superficial change on adsorbentsbefore and after adsorption at pH 5.5 when the lead adsorptionamount was relatively higher than nano titanium. The X-raydiffraction (XRD) patterns were recorded on a Bruker AXN in the 2qrange of 10e80� using Cu-Ka radiation (l ¼ 1.5418 �A). The diffrac-tion data at a 0.02� step were collected at room temperature(T ¼ 296 K). The accelerating voltage and the applied current were40 kV and 30 mA, respectively. Semi-quantitative analyses wereperformed on an energy-dispersive X-ray (EDX) spectrometerconnected to a Hitachi S-4700 scanning electronmicroscope (SEM).

2.4. Lead quantification

Dissolved lead was determined using a Varian AA240 AtomicAbsorption Spectrophotometer (AAS) using an air-acetylene flameat a wavelength of 283.3 nm using a slit width of 0.7 nm. Experi-mental samples were filtered using whatman 0.45 mm filter paperand the filtrates after suitable dilutions were analyzed. This is watersample so no need to acid digestion because during adsorption pHis 5.5 (Sing et al., 2007). However AAS analysis is used to calculatethe initial and final concentration of the lead ions from the aqueousmedium. Control experiments showed that no sorption occurred oneither glassware or filtration systems. All assays were carried out intriplicate and only mean values are presented.

2.5. Adsorption in batch system

Adsorption experiments were carried out under optimizedconditions using the composite as an adsorbent in a temperaturecontrolled incubator shaker set at 120 rpm maintained at 30 �C for1.5 h.

A.K. Sharma, B.-K. Lee / Journal of Environmental Management 128 (2013) 787e797 789

2.5.1. Optimization of adsorption conditionsA known amount of adsorbent was thoroughly mixed with

50 mL of respective Pb(II) solutions of known concentration andpH. The pH of the solution was measured with a Digital pH meter(pH Thermo Scientific) equipped with a glass working electrodeand a reference Ag/AgCl electrode. The pH of the reaction mixturewas initially adjusted using either hydrochloric acid or sodiumhydroxide (1M). After the flaskwas shaken for the desired time, thesuspension was filtered through whatman 0.45 mm filter paper,and the filtrate after suitable dilution was analyzed to determinethe Pb(II) concentration. Control experiments showed that nosorption occurred on either glassware or filtration systems.

For optimizing the adsorption, one parameter was varied at atime keeping the others fixed. Effect of acylamide concentration inthe composite, In-situ doped acrylamide titanium nanocomposite(TieAM) was synthesized by a solegel process, where acrylamidemonomer was doped into nano titanium during the solegel pro-cess. Afterward, the reaction was completed with initiator bypolymerization and the products were characterized as describedearlier. Acrylamide monomer (0.1, 0.2, 0.5, 1, 2 g) was dissolved indistilled water (120 mL) at ambient temperature.

To evaluate the initial pH effect of the solution, 50 ml of100 mg L�1 Pb(II) solutions were adjusted to various pH rangingfrom 2 to 10 and the optimum pH was determined using a 50 mgadsorbent dose. Various doses of the adsorbent ranging from 0.01 gto 1 g were used for studying the effect of adsorbent dose on theadsorption of the Pb(II) at 500 mg L�1 concentrations. The effect ofelectrolyte was studied using different concentrations (1.00e0.01 M) of Na2SO4, NaCl and Na2CO3.

2.5.2. KineticIn order to evaluate kinetic data, the agitation time was varied

from 15 to 120 min, and the kinetic studies were performed at twodifferent Pb(II) concentrations (at 100 and 500 mg L�1) using a50 mg adsorbent dose. Separate flasks were prepared for each timeinterval. The mixtures were filtered after 120 min and the amountof the adsorbed metal ions was determined by AAS.

2.5.3. IsothermComposite samples of different acrylamide concentration (0.2e

1 g) were used to study the effect of acrylamide concentration onPb(II) removal under optimum adsorption conditions. To study theeffect of initial lead concentration, the range for different initiallead concentrations used was 50e1000 mg L�1 at a 50 mg adsor-bent dose. The amount of metal ions adsorbed (Singh et al., 2007)

Fig. 1. FTIR of TiO2, acrylamide (AM), nanocomposite of TieAM

per gram of the composite was calculated by the difference be-tween the initial and the final readings using Equation (1):

qe�mg g�1

�¼ Co � Ce

�mg L�1

�� VðLÞ=WðgÞ (1)

where qe is the amount of the lead adsorbed (mg g�1) on theadsorbent, Co is the initial concentration of Pb(II) (mg L�1), Ce is theequilibrium concentration of Pb(II) in solution (mgL�1), V is thevolume of the solution used (L), and W is the weight of the adsor-bent used. The pH, initial concentration of Pb(II), adsorbent dose,contact time and the electrolyte amount (ionic strength) werevaried one at a time keeping the other parameters fixed. Unlessotherwise stated, the experiments were carried out at 100 mgL�1 Pb(II) concentration, 50 mg adsorbent dose, 50 mL reactionvolume, 120 rpm and 1.5 h shaking time. Under optimum condi-tions, the results were verified by measuring the Pb(II) concentra-tion using an atomic absorption spectrophotometer. Sorptionisotherms were modeled using linearized Langmuir and Freundlichmodels (Singh et al., 2007) as describe in result and discussionsection.

2.6. Desorption

To examine the desorption behavior of lead ions from acryl-amide-TiO2 nanocomposite, adsorption experiments were firstconducted by placing a 50 mg amount of acrylamide-TiO2 in 50 mLof a lead ion solution (with an initial concentration of 500 mg/L anda solution pH value of 5.5) for 1.5 h, and then the final lead ionconcentration in the solution was analyzed. The acrylamide-TiO2adsorbed with lead ions was separated from the solution byfiltration and then added into 20 mL of a 0.05 M EDTA, 0.05 N HCl,and 0.1 N NaOH solutions in a flask for lead ion desorption. Thecontents of the flask were stirred at 150 rpm at room temperaturefor a time period of up to 1 h, and the lead ion concentrations in thesolution were analyzed. Adsorbents were finally collected from thesolution by filtration, washed with DI water, and then reused in thenext cycle of adsorption experiments. The adsorptionedesorptionexperiments were conducted for six cycles.

3. Results and discussion

3.1. Characterization of the composites

An optimized nanocomposite (TieAM) sample (in terms ofPb(II) binding) was used for the characterization and sorption

and Pb(II) loaded TieAM nanocomposite (TieAMePb).

A.K. Sharma, B.-K. Lee / Journal of Environmental Management 128 (2013) 787e797790

study. The representative nanocomposite samplewas characterizedusing infrared spectroscopy (Fig. 1), X-ray photoelectron spectros-copy (Fig. 2), X-ray diffraction (Fig. 3), and SEM/EDX analysis(Fig. 4).

3.1.1. FT-IR spectroscopyAs seen from the IR spectrum (Fig. 1), the presence of external

water within the titanium composite particles in addition to thestrongly hydrogen-bonded OH or extremely strongly coordinatedH2O was confirmed by the peaks at region 3440e3580 cm�1 and1648e1660 cm�1 (Jia et al., 2008; Yang et al., 2010). The IR spec-trum of nano-TiO2 (Fig. 1a) shows a broad intense band below1200 cm�1 is due to TieOeTi vibration (Jia et al., 2008). In the IR

Fig. 2. XPS spectra of TiO2, nanocomposite of TieAM and Pb(II) loaded TieAM

spectrum of acrylamide (Fig. 1b), the absorption peak at 1171 cm�1

belongs to the eNH2 bending vibration, while the absorption peakat 1490 cm�1 belongs to the CeN stretching vibration. The NeHstretching of the secondary amide I occurs at 2880 cm�1, whileamide II (C]O) stretching at 1875 cm- also shown in the acrylamideIR spectrum. In the IR spectrum of the TieAM composite (Fig. 1c),the characteristic peaks are at 1658 cm�1 (nCeO), 2944 cm�1 (vCH2

),1458 and 1409 cm�1 (dCH2

), and 3550e3300 cm�1 (nNH), whichprovided substantial evidence of acrylamide grafting onto titanium.The absorption peaks at 1950 cm�1 and 930 cm�1 are induced bythe CeOeH deformation vibration and TieOeC vibration, whichindicates titanium, has reacted with hydroxyl groups at the acryl-amide surface.

nanocomposite (TieAMePb) with peaks of C1s, O1s, N1s, Ti2p and Pb4f.

Fig. 3. XRD pattern of TiO2, acrylamide (AM), nanocomposite of TieAM and Pb(II) loaded TieAM nanocomposite (TieAMePb).

A.K. Sharma, B.-K. Lee / Journal of Environmental Management 128 (2013) 787e797 791

Significant change in the spectrum of the TieAM composite(Fig. 1d) is observed in the region of 2000 cm�1 to 600 cm�1 afterPb(II) binding. Intensity of symmetric TieOeC stretching vibra-tions at 1850 cm�1 to 1500 cm�1 shows peak shifting after Pb(II)binding. Further, new sharp peaks at 1204 and 1653 cm�1 wereobserved due to adsorption of lead species onto the nano-composite. Moreover, the lead loaded composite (TieAMePb)showed significantly different absorption patterns in the region of1200e2000 cm�1. Shifts of both OeH stretching and NeHstretching peaks confirm that there is more than one mechanisminvolved in binding. The corresponding peaks in this region arematched with the literature value (Vahur et al., 2009; Gundogduet al., 2009; Li et al., 2010a,b). The same was also inferred fromthe successful modeling of the adsorption data to both Langmuirand Freundlich isotherms.

3.1.2. X-ray photoelectron spectroscopy (XPS)XPS spectra demonstrated the sensitivity for identifying ele-

ments on the surface (Fig. 2). Surface chemical composition data ofnano titanium, TieAM and lead loaded nanocomposites (TieAMe

Pb) are summarized in Table 1, where the content of Ti (16.83%) ishigher than acrylamide modification due to pure nano titanium.The presence of C in the nanoparticles of TiO2 originated from Ti-tanium (IV) isopropoxide which was used as prime source materialof TiO2 in all the prepared samples. The increase in carbon contentin TieAM and TieAMePb as compared TiO2 indicates doping of

acrylamide onto nano titanium. The presence of lead content(0.63%) shows the binding of Pb(II) onto the nanocomposite. In theanalysis of the chemical composition demonstrated by XPS spectrain the Ti2p region (Fig. 2), the symmetric shapes of the Ti 2p3/2 XPSlines indicate that the titanium is present only as Tiþ4. The Ti2pregion is composed of 2p3/2 and 2p1/2 peaks with binding energies(BE) at 458.8 and 464.5 eV, respectively. The binding energy of theTi2p region decreases slightly (from 459.7 to 458.8 eV), while thebinding energy of O1s increases slightly (from 529.1 to 530.2 eV). Anew peak appears in the O1s region after modification. Thesechanges clearly show that acrylamide binds to TiO2. The O1s regionof pure TiO2 was decomposed into two contributions: the TieObond in TiO2 and the surface hydroxyl. After modification, the O1sregion has two organic contributions as well as the inorganic ox-ygen species in pure TiO2 (Hu and Shipley, 2012). It has been sug-gested that the formation of C]O double bonds results from anirreversible hole trap to an electron-hole recombination center(Pascal andMéar, 2009). In the case of the N1s core level before leadadsorption, there are two peaks at 399.53 and 401.34 eV for sampleTieAM are characteristic of nitrogen in acrylamide (Wang et al.,2012) and some quaternary amide nitrogen (formed in smallamounts during the nanocomposite synthesis), respectively. In theXPS spectra in the Pb 4f region, two symmetric XPS peaks due to Pb4f7/2 and Pb 4f5/2 were observed. The XPS results confirm thepresence of two types of lead in the nanocomposite: (i) Pb2þ ionswith binding energies of Pb 4f7/2 (142.1 eV) and Pb 4f5/2 (138.02 eV),

Fig. 4. SEMphotographs of (a) TiO2, (b) TieAM nanocomposite (c) TieAM loaded with lead (TieAMePb) and EDX graphs of (d) TiO2, (e) TieAM and (f) TieAMePb.

A.K. Sharma, B.-K. Lee / Journal of Environmental Management 128 (2013) 787e797792

respectively, corresponding to those of PbOeTi, and (ii) Pb0metalliclead with binding energies Pb 4f7/2 (142.1 eV) and Pb 4f5/2(137.1 eV). This result confirms that lead ions are coordinated to thenanocomposite through N and O atoms, most likely from the amidemoieties. This also indicates that some N atoms exist in a moreoxidized state on the nanocomposite surfaces due to lead adsorp-tion. This phenomenon can be attributed to the formation ofR � NH2Pb

2þ complexes, in which a lone pair of electrons in thenitrogen atom is donated to the shared bond between the N andPb2þ, and, as a consequence, the electron cloud density of the

Table 1Surface analysis results of titanium, acrylamide modified titanium composite andlead loaded nanocomposite(TieAMePb).

Sample Atomic concentration (%)

C N O Pb Ti

TiO2 38.09 e 45.08 e 16.83TieAM 63.22 11.74 22.61 e 2.43TieAMePb 68.36 10.37 17.89 0.63 2.75

nitrogen atom is reduced, resulting in a higher peak binding energy.Lead (0) and (II) binding energies corresponding to Pb 4f7/2 and Pb4f5/2 have been found at similar values showing that lead (II) re-mains present in the same environment. Nevertheless, the relativearea of the peaks corresponding to lead metal increases, while therelative area of the Pb 4f peaks corresponding to lead in a PbOeTienvironment decreases. Since both FTIR and XPS spectra provideclear evidence that the chemical bond associated with the N and Oatoms on nanocomposite are significantly changed after leadadsorption, it may be speculated that the contribution ofR � NH2Pb

2þ and PbeO interaction to lead adsorption onto thenanocomposite was mainly through a chemical interaction. Also, anonspecific interaction (physical adsorption, electrostatic attrac-tion, etc.) with the oxygen atoms attributed to the high flexibility ofpolymer chains of nanocomposite, which could facilitate closecontact between the surface functional groups and ions, may beinvolved.

Based on above observation, Scheme 1 has been proposed forthe removal of Pb(II) by the prepared adsorbent due to contributionof R � NH2Pb

2þ and PbeO onto the nanocomposite, mainlythrough a chemical interaction.

Scheme 1. Proposed structure of reaction pathways for the adsorption of Pb(II) ontonanocomposite.

0.0 0.5 1.0 1.5 2.0 2.5

0

100

200

300

400

500

Pb

2+

ad

so

rb

ed

(m

g/g

)

Arcylamide (g) in composite synthesis

Fig. 5. Adsorption at various different concentration of acrylamide at 500 mg/L initialPb(II) concentration, adsorbent dose 50 mg, pH 5.5, contact time 1.5 h at30 �C.

A.K. Sharma, B.-K. Lee / Journal of Environmental Management 128 (2013) 787e797 793

3.1.3. X-ray diffractionFig. 3 shows the X-ray diffraction patterns of the nanocomposite

before and after lead adsorption. XRD of TiO2 (Fig. 3a) shows thedistinctive peaks at 2q ¼ 37.9�, 54.1� and 62.6�, 69.5�, corre-sponding to the rutile phase, and peaks at 2q ¼ 25.29�, 47.6� cor-responding to the anatase phase (Jia et al., 2008; Hu and Shipley,2012). Changes in the XRD pattern in the 2q ¼ 45o-65� regionrepresent lead adsorbed by titanium due to rutile phase crystal-linity. On the other hand, the XRD spectra of the TieAM (Fig. 3c),which shows stronger diffraction intensity at 2q ¼ 43.1� for TieAM,indicates a larger amount of acrylamide bound at the titaniumsurface that is same of (Senvaitiene et al., 2007). In the lead loadednanocomposite (Fig. 3d), crystalline peaks (2q ¼ 43.1�) are lost,indicating the loss of crystallinity on the binding (Yahya et al.,2006), which is strong evidence for adsorption of lead onto theTieAM nanocomposite.

3.1.4. Scanning electron microscopy and energy-dispersive X-ray(EDX)

Comparisonof the texture andmorphologyof thenanocompositesample before and after adsorption of Pb (II) ions was performed bySEM and EDX analysis, and their low and high magnification imagesare shown in Fig. 4. The SEM image of nano TiO2 showed particlesaround or below one micron (seen in both magnification images)having a distinct crystalline form (Fig. 4a). The SEM image of the TieAM (Fig. 4b) showed the acrylamide-incorporated nanocompositehad a flattened flaky thin sheet-like appearance. Adsorption of leadonto the nanocomposite leads to distinct changes in the structure ofthe composite morphology, depicted in Fig. 4c. The micrographsclearly exhibit significant lead loading and show significantlydifferent morphologies, i.e., nanocomposite strand like small de-positions of lead, on the TieAMsurface. Thepresenceof leaddepositson the composite was further confirmed by means of EDX spectra,shown in Fig. 4 (d, e and f). The obvious characteristic peaks for C, O,Ti, and Pb elements, originating from the nanocomposite, are foundin Fig. 4, indicating that TiO2 is indeed incorporated in the TieAMnanocomposite. AnEDX spectrumof Fig. 4 represents thepresence ofTi and O as major elements in the nanocomposite. An EDX spectrum(Fig. 4d) of the nano-synthesized TiO2 shows 54.14% oxygen and37.21% titanium. In Fig. 4f, 9.75% lead element observed afteradsorption on to the TieAM nanocomposite confirmed Pb(II) sorp-tion by the nanocomposite (Fig. 4f).

3.2. Adsorption in batch system

3.2.1. Effect of acylamide concentration in lead adsorptionWith an increase concentration of acrylamide (from 0.1 to 2 g) in

the titanium solution and this mixture of solution used for nano-composite (adsorbent) synthesis. However adsorption increasedfrom 112.02 to 390.12 mg g�1 at a fixed adsorbent dose (50 mg), pH(5.5), initial Pb(II) concentration (500 mg/L), rpm (120), and contacttime (1.5 h) at 30 �C. The sample synthesized under optimumconditions was fully water insoluble and was used for the leadadsorption studies and the conditions for the adsorption wereoptimized and under optimum conditions, samples with differentacrylamide concentration were evaluated for the Pb2þ removalfrom the aqueous solutions. Acrylamide concentration in synthesisplays a significant role in increasing the functional group concen-tration on the composite adsorbent, as shown in Fig. 5. With anincrease in acrylamide concentration, there was an increase in theadsorption for lead. The initial increase in acrylamide concentrationup to 1 g had greater influence on the sorption, thereafter 2 g ofacrylamide concentration was nominal.

3.2.2. Effect of the pHA pH effect test was performed to determine the pH of

adsorption at which maximum uptake of metal occurs. Also, pH isan important parameter that controls the adsorption of metalcations at the metal oxideewater interface (Chauhan andSankararamakrishnan, 2008). The pH was initially adjusted from2 to 10 with either hydrochloric acid or sodium hydroxide (1M). Noefforts were made to maintain the pH throughout the adsorptionprocedure. Fig. 6 shows the adsorption of Pb(II) by acrylamide-TiO2nanocomposite for various initial pH conditions. The hydrogen ionconcentration of the heavy metal solution is a crucial factor in leaduptake by acrylamide-TiO2 nanocomposites, as shown in Fig. 6.Uptake capacity beyond pH 8.0 was avoided because insoluble leadhydroxide precipitates were observed in the solution. Themaximum uptake capacity decreased with decreasing initial pHvalues, as shown in Fig. 6 and there are two possible explanationsfor this. First, hydrogen ions compete with lead ions to the samebinding sites on the adsorbent. Second, the amide group is knownto be unstable in a strong acid solution and can dissociate from theacrylamide-TiO2 nanocomposite backbone. There are hydroxylgroups on the surfaces of metal oxides that interface with theaqueous solution. Surfaces ionized by those hydroxyl groups tendto adsorb counter ions. Therefore, the hydroxyl groups are

0 2 4 6 8 10 12

0

20

40

60

80

100

120

Ti-AM

TiO2

Pb

2+

a

ds

orb

ed

(m

g/g

)

pH

Fig. 6. Effect of pHonequilibriumconcentrationof Pb(II) (initial concentration¼ 100mg/L, amount of adsorbents ¼ 50 mg, reaction volume ¼ 50 mL, rpm ¼ 120,T ¼ 30 �C).

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0

100

200

300

400

500

600

(mg/g)

(%)

Adsorbent dose (g)

Pb

2+

a

ds

orb

ed

(m

g/g

)

Adsorption

20

40

60

80

100

120

Pb

2+

ad

so

rb

ed

(%

)

Fig. 7. Adsorption at various adsorbent dose at 500 mg/L initial Pb(II) concentration,pH 5.5, contact time 1.5 h, at 30 �C.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

20

40

60

80

100

120

Pb

2+

ad

so

rb

ed

(m

g/g

)

Concentration of electrolytes (M)

NaCl

Na2SO

4

Na2CO

3

Fig. 8. Adsorption with changing concentration of electrolyte at fixed adsorbent doseTieAM ¼ 50 mg, pH ¼ 5.5, 100 mg/L Pb(II) concentration, temperature 30 �C reactionvolume 50 mL, and contact time 1.5 h.

A.K. Sharma, B.-K. Lee / Journal of Environmental Management 128 (2013) 787e797794

important in controlling the adsorption and desorption of metalsfrom the nano-TiO2 surface (Kim et al., 2003). Since lead ions arebound to the amide group of acrylamide-TiO2, any loss of amidecould lead to a reduction in the uptake capacity. The amountadsorbed increased from 46.78 to 98.89 mg g�1 when the pH wasincreased from 2 to 5.5, and thereafter a slight decline in amountadsorbed was observed with increase in pH to 10. The optimum pHfor the removal of Pb(II) by acrylamide-TiO2 was found to be 5.5.

3.2.3. Effect of the adsorbent doseFig. 7 shows the experimental results demonstrating the effect

of adsorbent dose on the adsorption of Pb(II) by acrylamide-TiO2nanocomposite. The percentage adsorption of Pb(II) increasesfrom 19.2 to 99.7% by increasing the adsorbent dose from 0.01 g to1 g in 50 mL of 500 mg L�1 Pb(II) solution at 30 �C, rpm 120 and acontact time of 1.5 h. A further increase in the dose had a negli-gible effect on the sorption. This increased because, at a higherdose of composite, more binding sites are available. This indicatesthat extra adsorption sites are available with increased surfacearea for the adsorption. Also, with increasing adsorbent dose, thequantity of lead adsorbed on to the unit weight of the adsorbentgets reduced, thus causing a decrease in qe value with increasingadsorbent dose.

3.2.4. Effect of electrolytesFig. 8 shows experimental results demonstrating the effect of

electrolyte (NaCl, Na2SO4 and Na2CO3) on the adsorption of Pb(II)by acrylamide-TiO2 nanocomposite. The sorption performance ofadsorbent toward a given ion in solution could therefore vary. Someanions can have an affinity toward metal, so that they form aninsoluble or soluble complex displaced with difficulty in the pres-ence of the sorbent material (Singh et al., 2007). The role of anions(Cl�, SO2�

4 and CO3) possibly present in wastewater in leadadsorption by acrylamide-TiO2 was investigated. Lead sorption canbe inhibited only by anions forming sufficiently stable complexes.Stability constants (log k) of Cl�, SO4

2� and CO3 are 1.98, 2.4 and4.02, respectively (Singh et al., 2007). As expected from theincreasing order stability constant values of chloride, sulfate andcarbonate, they affect the adsorption behavior of lead, as shown inFig. 8. With increasing concentrations of NaCl, Na2SO4 and Na2CO3from 0.01 to 1.0 M Pb(II), removal decreases from 76.3 to32.8 mg g�1, from 97.4 to 68.7 mg g�1and from 98.8 to 72.5 mg g�1,respectively, for 50 mL of 100 mg L�1 lead solution at pH 5.5, 30 �C,120 rpm and a contact time of 1.5 h.

3.2.5. KineticKinetics of metal sorption governs the rate, which determines

the residence time, one of the important characteristics definingthe efficiency of an adsorbent. Fig. 9 shows kinetic results of leadion adsorption onto acrylamide-TiO2 nanocomposite. It can beobserved that lead uptake on the TieAM nanocomposite is a fastprocess. The amount of adsorption increased rapidly in the first0.5 h, contributing to about 65% of the ultimate adsorption amount,and then increased slowly thereafter. Adsorption equilibrium wasachievedwithin about 1.5 h in this case and no change in the uptakecapacity was observed up to 24 h. The pseudo-second-order kineticmodel has often been used to fit the experimental kinetic adsorp-tion data and determine whether an adsorption process is domi-nated by chemical adsorption phenomenon. When adsorptioninvolves a surface reaction process, the initial adsorption is rapid,and then a slower adsorption would follow as the availableadsorption sites gradually decrease (Chauhan andSankararamakrishnan, 2008). Kinetics of heavy metals adsorptionwas modeled by (Fig. 9A and B) the pseudo-second-order equationand the second-order rate equations shown below as Equations (2)and (3), respectively (Chauhan and Sankararamakrishnan, 2008).

t=qt ¼ 1=k1 q2e þ t=qe (2)

0 20 40 60 80 100 120 140

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

100 PPM

500 PPM

t/q

t

(m

in

.g

/m

g)

Time (min)

A

0 20 40 60 80 100 120 140

0.0

0.1

0.2

0.3

0.4

0.5

100 PPM

500 PPM

1/q

0-q

t

Time (min)

B

Fig. 9. Kinetic models for the equilibrium adsorption data: (A) pseudo second order (B)second order model plot of Pb(II) adsorption at two different lead concentrations (at100 and 500 mg L�1) using 50 mg adsorbent dose by TieAM, at pH ¼ 5.5, reactionvolume (50 mL),temperature 30 �C.

Table 2Sorption kinetics of the Pb(II) adsorption by TieAM.

(Pb2þ)(mg/L)

Pseudo second order Second-order model

qe (mg/g) k1 (g/mg/min) R2 qe (mg/g) k2 (g/mg/min) R2

100 111.11 8.7 � 10�4 0.998 15.87 4.0 � 10�4 0.933500 454.54 1.2 � 10�4 0.997 416.66 6.0 � 10�5 0.939

A.K. Sharma, B.-K. Lee / Journal of Environmental Management 128 (2013) 787e797 795

1=ðqe � qtÞ ¼ 1=qe þ k2 t (3)

Here, “k1 is the pseudo-second-order rate constant of adsorption (g/mg/min) and k2 is the second-order rate constant (g/mg/min), qeand qt are the amounts of metal ion sorbed (mg g�1) at equilibriumand at time, t, respectively” (Sing et al., 2007). The kinetic data ofPb(II) adsorption was best fit by the pseudo-second-order kineticmodel. Pseudo second-order adsorption kinetic data of the Pb(II)adsorption on TieAM was ascertained since the plot of t/qt versus twas found to be a straight line in accordance with the pseudo-second order Equation (2). The observed data gave a straight lineplot between t/qt vs. t, indicating pseudo-second-order kinetics forthe adsorption (Fig. 9A), and coefficient of determination and rateconstant have been summarized in Table 2, indicating that rate ofadsorption was higher at 500 mgL�1 Pb(II) concentration. Thisfurther strengthens the conclusion based on earlier results that theadsorption of lead is primarily a chemisorptions phenomenon.

3.2.6. IsothermSorption isotherms were modeled using linearized Langmuir

and Freundlich models (Singh et al., 2007). The Langmuir isothermis valid for monolayer sorption due to a surface of a finite number ofidentical sites and is expressed in the linear form below (Equation(4))

qe ¼ qmaxbCe=ð1 þ bCeÞ (4)

where qe is the equilibrium adsorbate loaded on the adsorbent(mg g�1), Ce is the equilibrium concentration of the adsorbate

(mg L�1), qmax is the ultimate capacity (mg g�1) representing themonolayer adsorption capacity, and b is the relative energy (in-tensity) of adsorption, also known as binding constant (L/mg). Theessential feature of the Langmuir adsorption can be expressed bymeans of RL, a dimensionless constant referred to as the separationfactor or equilibrium parameter for predicting whether anadsorption system is favorable or unfavorable. RL is calculated usingthe following (Equation (5))

RL ¼ 1=1þ bC0 (5)

where C0 is the initial Pb(II) concentration (mg/L). If RL values liesbetween 0 and 1, the adsorption is favorable.

The Freundlich isotherm describes heterogenous surface en-ergies by multilayer adsorption, and is expressed in linear form as(Equation (6))

In qe ¼ In Kf þ 1=n In Ce (6)

where Kf indicates adsorption capacity (mg g�1) and n is anempirical parameter related to the intensity of adsorption, whichvaries with the heterogeneity of the adsorbent. Higher values of the1/n represent better favorability of adsorption. Equilibriumadsorption data of nano TiO2 and synthesized nanocomposite TieAM could be modeled satisfactorily to both the Langmuir andFreundlich isotherms (Fig. 10A and B). The equilibrium data fitted toLangmuir (R2 ¼ 0.983 and 0.986) model better than Freundlichmodel (R2 ¼ 0.919 and 0.921) for titanium and TieAM nano-composite respectively, indicating surface homogeneity of theadsorbent and monolayer adsorption. Acrylamide-TiO2 exhibitedapproximately three times the uptake capacity of the nano titanium.From Langmuir, an adsorption capacity (qmax) of 476.19mg Pb/gwasobserved for acrylamide-TiO2 nanocomposite compared to140.84 mg/g for nano TiO2 (Fig. 10A) indicating that the nano-composite had a high capacity to remove Pb(II) ions. b were calcu-lated to be 0.05 and 0.30 ml/mg for TiO2 and TieAM respectively,thus adsorption is favorable. The Freundlich constant Kf and 1/n forTiO2 and TieAMwere calculated to be 24.38, 88.61 and 0.321, 0.285,respectively from Freundlich isotherm (Fig. 10B). Coefficient ofdetermination and isotherm parameters has been summarized inTable 3. The value obtained is considerably higher than those ob-tained on coffee residue activated carbon (Boudrahem et al., 2009).The sorption capacity of the adsorbent is compared with other re-ported adsorbents in Table 4. The qmax in our current studies issignificantly higher than other reported adsorbents. According toHSAB (’hard’ or ’soft’, and ’acid’ or ’base’ to chemical species) theory,soft bases tend to form stable complexes with metals such as Cd2þ,Pb2þ and Cu2þ (Winter,1994). HSAB is the acronym for hard and softacids and bases it assigns the terms ’hard’ or ’soft’, and ’acid’ or ’base’to chemical species. ’Hard’ applies to species which are small, havehigh charge states (the charge criterion applies mainly to acids, to alesser extent to bases), and are weakly polarizable. ’Soft’ applies tospecies which are big, have low charge states and are stronglypolarizable (Pearson, 1963). The gist of this theory is that soft acidsreact faster and form stronger bonds with soft bases, whereas hardacids react faster and form stronger bondswith hard bases, all other

0.0 0.5 1.0 1.5 2.0 2.5

0.000

0.005

0.010

0.015

0.020

0.025

0.030

Ti-AM (y = 0.007x + 0.0021, R2

= 0.986)

TiO2

(y = 0.129x + 0.0071, R2

= 0.983)

1/q

e

1/Ce

A

1 10 100 1000

10

100

1000

Ti-AM (y = 88.61x0.285

, R2

=0.921)

TiO2 (y = 24.38x0.321

, R2

=0.919)

Pb

2+

lo

ad

in

g q

e (m

g/g

)

Final concentration of Pb2+

(mg/l)

B

Fig. 10. Adsorption equilibrium (A) Langmuir and (B)Freundlich isotherm onTiO2, andnanocomposite adsorbent(TieAM) at T ¼ 30 �C, pH ¼ 5.5.

1st cycle-- 2 nd cycle-- 3 rd cycle-- 4th cycle-- 5 th cycle-- 6 th cycle--

0

20

40

60

80

100

Effic

ie

nc

y (%

)

Adsorption-Desorption

Adsorb

Desorb

Fig. 11. Adsorption (adsorb) and desorption (dsorb) cycles: Adsorptiondsamplevolume ¼ 50 mL, Pb(II) concentration ¼ 500 mg/L, adsorbent dose (TieAM) ¼ 50 mg,initial pH ¼ 5.5, equilibration time ¼ 1.5 h; Desorptiondsample volume ¼ 20 mL of0.05NHCl, equilibration time ¼ 1 h.

A.K. Sharma, B.-K. Lee / Journal of Environmental Management 128 (2013) 787e797796

factors being equal (Pearson, 2005). The classification in the originalworkwasmostly based on equilibrium constants for reaction of twoLewis bases competing for a Lewis acid. Hard acids and hard basestend to have: small size, high oxidation state, lowpolarizability, high

Table 3Isotherm parameters and coefficient of determination for adsorption of lead fromaqueous solution by TieAM composite.

Adsorbents Langmuir isotherm Freundlich isotherm

qmax (mg/g) b (ml/mg) R2 Kf (mg/g) 1/n R2

TiO2 140.84 0.05 0.983 24.38 0.321 0.919TieAM 476.19 0.30 0.986 88.61 0.285 0.921

Table 4Comparison of lead uptake using acrylamide-TiO2 with different adsorbents.

Adsorbents Pb(II) adsorptioncapacity (mg/g)

Ref.

TiO2 nanomaterial 81.3 (Hua et al., 2012)Nanometer calcium titanate 124 (Hua et al., 2012)Titanium dioxide

nanoparticles42.93 (Hu and Shipley, 2012)

Nanocrystalline TiO2 80.64 (Googerdchian et al., 2012)Impregnating titanium

phosphate351.42 (Jia et al., 2009)

Polyacrylamide-graftediron(III) oxide

211.42 (Pan et al., 2009)

Polyacrylamideeapatitecomposite

167.67 (Ulusoy and Akkaya, 2009)

Chitosan entrappedin polyacrylamide

173.88 (Akkaya and Ulusoy, 2008)

Acrylamide-TiO2

nanocomposite476.19 This study

electronegativity, energy low-lying (bases) or energy high-lying(acids) (Koch, 2005). This was an expected result based upon ourprevious discussion of IR, XPS, XRD and surface morphology, wherewe have already mentioned about the changes observed in the FTIRpeaks, XRD peaks and elements on the surface through XPS of TieAM after Pb(II) adsorption.

3.2.7. Desorption studiesTo make the sorbent economically competitive, the prepared

composite material should be reused for ‘n’ number of cycles.Among the various stripping solutions, only 30% of the loaded Pb(II)was stripped in 0.5 N NaOH, 52% was stripped with 0.01 M EDTAand 73% was stripped with 0.01 N HCl in one cycle. Addition ofNaOH might lead to the precipitation of Pb(OH)2 on the surface ofthe adsorbent, which eventually blocks adsorption sites, preventingfurther adsorption. Hence, further experiments were carried outonly with 0.01 N HCl solutions. Adsorbed Pb2þ could be stripped offin 1 h by the introduction of protons that competed withmetal ionsfor binding sites. In an acidic medium, the protons in solutionreplace the metal ions on the TieAM nanocomposite, while poorrecovery was observed in a basic media such as NaOH. This may bedue to the coordinating ligands being deprotonated, hence bound-metal ions find it difficult to detach from the nanocomposite. In thesecond cycle, 69% Pb(II) was removed, and 65% desorption wasachieved. In the fourth cycle, 52.6% adsorption and 44.6% desorp-tion was possible (Fig. 11). The removal decreased nominally percycle up to the sixth cycle, suggesting very high efficiency of theadsorbent. Even in the sixth cycle, the adsorbent could remove37.5% Pb(II), which could be desorbed to 30.2%. Hence, it appearedthat during desorption only the metal ions adsorbed by physicalforces can be desorbed. It was likely that both the electrostatic andcomplexation reactions occurred between the sorbent and themetal ion. Therefore, complete desorption was not possible, whichis a similar to earlier reports where the percent adsorbed wasreduced in subsequent cycles (Singh et al., 2007).

4. Conclusions

Acrylamide-Ti nanocomposite (TieAM) proved to be an effectiveadsorbent for Pb2þ, from aqueous media through an ion-exchangemechanism. Acrylamide doped titanium nanocomposite with an

A.K. Sharma, B.-K. Lee / Journal of Environmental Management 128 (2013) 787e797 797

amide group increased the adsorption capacity three timescompared to the nano-TiO2. The nanocomposite TieAM showedmaximum Pb(II) binding at pH 5.5. The adsorption data were fitbetter to a Langmuir adsorption isotherm, indicating surface ho-mogeneity, and unilayer adsorption also showed pseudo-second-order kinetics. Equilibrium sorption capacity of 476.19 mg g�1

was determined from the Langmuir isotherm and is very highcompared to the other reported adsorbents. TieAM nanocompositeis amenable to an efficient regeneration by 0.05 N HCl solutions forrepeated (upto six cycles) use without any significant capacity loss,making this approach very economical. The adsorbent was found tobe easily disposable.

Acknowledgment

This work was supported by the National Research Foundationof Korea (NRF) grant funded by the Ministry of Education(2013R1A2A2A03013138).

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