Three-Dimensional Porous Spinel Ferrite as an Adsorbent for Pb(II)

8/10/2019 Three-Dimensional Porous Spinel Ferrite as an Adsorbent for Pb(II)

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Three-Dimensional Porous Spinel Ferrite as an Adsorbent for Pb(II)Removal from Aqueous SolutionsD. Harikishore Kumar Reddy and Seung-Mok Lee*

Department of Environmental Engineering, Kwandong University, 522 Naegok-dong, Gangneung-si, Gangwon-do 210-701, Republicof Korea

*S Supporting Information

ABSTRACT: Application of porous magnetic materials for detoxication of pollutants from aqueous solutions has attractedgreat attention in recent years because of their ease of separation and enhanced efficiency. In this light, a three-dimensional (3D)porous NiFe2O4 adsorbent (PNA) having signicant magnetic properties was synthesized and employed in detoxication ofPb(II) contaminated aqueous solution. The PNA was obtained by a solgel process using chitosan as precursor and was fullycharacterized by using FTIR, XRD, SEM, and EDAX analytical techniques. The surface morphology obtained from the SEMimages of the PNA clearly exhibited the three-dimensional porous structure; pores were well interconnected. The kinetic

mechanism was best described by pseudo-second-order and double-exponential models. The equilibrium state sorption data wasbest tted to the Langmuir and Sips isotherm models. The studies clearly demonstrated that the PNA could be an efficientmaterial for treatment of Pb(II) contaminated wastewaters.

INTRODUCTION

Water pollution is one of the major environmental threatsobserved around the globe, consequent upon rapid industrial-ization and urbanization. The ever-increasing contamination ofwater bodies with a variety of organic, inorganic, and biologicalpollutants is one of the key environmental problems. However,among all of these pollutants, heavy metals are notorious waterpollutants with high toxicity and reported carcinogenicity.1,2

Lead is one of the heavy metals of particular concern in theaqueous environment, and its toxicity with its persistency hasattracted extensive attention.3,4 Sources of Pb(II), its effects onthe environment and human health, and also permissible limitsfor Pb(II) by various international and national regulatorybodies are well articulated elsewhere in several reviews.57

Therefore, lead removal from aqueous solutions to protectbiodiversity, hydrosphere ecosystems, and human beings is ofutmost importance. Conventional technologies reported for theremoval of Pb(II) from water environment include chemicalprecipitation, membrane separation, ion exchange, biologicalremoval, etc.8 However, rapid, efficient, and economicalremoval of toxic Pb(II) from the aquatic environment was animportant technological challenge. Adsorption is one of the

most effective processes of advanced wastewater treatmenttechnologies due to its ease of operation and the availability of awide range of adsorbents (activated carbons, biomaterials,nanomaterials, industrial byproducts, clays, etc.) assessedpreviously in the removal of Pb(II) from aqueous solutions.912

However, some difficulties were encountered in separation andregeneration of used adsorbnet for subsequent application.Therefore, the design and exploration of novel adsorbents is acontinuous effort for many researchers. In recent yearssignicant effort was devoted to the development of novelmagnetic adsorbent materials for effective, efficient, andeconomical removal of Pb(II).

Magnetic materials were the most attractive class of materialsfor removal of toxic contaminants due to quick and effectivemagnetic separation from treated water.13 Magnetic propertiesof adsorbent materials are useful for overcoming some of theissues present with ltration, centrifugation, or gravitationalseparation, and for saving energy.1416 Magnetic adsorbentswith porous structures possessed signicantly high specicsurface areas, effectively facilitated mass transfer, andsubstantially lowered the diffusion resistance. Due to their

large surface areas and good adsorption capacities, inorganicporous materials were widely used as adsorbents.17 Amongthese materials spinel ferrites were known with uniqueadvantages of magnetic and stable chemical properties. So far,there have been several reports about the applications of spinelferrites andtheir composites as adsorbents or catalysts in watertreatment.1821 Spinel ferrites are represented by the generalformula MFe2O4, where M is a metal or a group of metallicelements with two different valences.22 In particular, theMFe2O4 (M = Mn, Fe, Co, Ni) ferrites with spinel structureexhibit interesting magnetic, magnetoresistive, and magneto-optical properties that were potentially useful for a broad rangeof applications. Among spinel ferrites, nickel ferrite (NiFe2O4)was known as a soft magnetic material and was the most

important spinel ferrite. NiFe2O4with ferromagnetic characterwas originated from the magnetic moment of antiparallel spinsbetween Fe3+ ions at tetrahedral sites and Ni2+ ions atoctahedral sites.23 However, metal ferrites conned withporous structure were an especially interesting class ofmaterials. This structure was combined with key advantagesas magnetic ferrite possessed unique magnetic response while

Received: December 6, 2012Revised: June 20, 2013Accepted: October 18, 2013Published: October 18, 2013

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porous matrixes exhibited highly developed internal specicsurface areas, good chemical stability, and high adsorptioncapacity.2426 Furthermore, porous materials with connectedpores having irregular and wide pore size distribution, perhaps,provided effective sites for the adsorption process and werelikely to be good candidates as adsorbents.27

However, to prepare a porous spinel ferrite requires atemplate material as precursor. Searching for a simple andreliable precursor for the preparation of porous spinel ferrite,chitosan would be an available ligand and template tosynthesize porous material. Chitosan could be used as atemplate to form porous oxides, because the chitosanmacromolecule was featured with homogenously well-dispersedand naturally spaced amino groups (5.8 mmol), providingactive surface functional groups to coordinate with metallicspecies.28,29 Chitosan (obtained from chitin) next to cellulose isthe second most abundant organic resource, imparted ratherless expensively.30 For instance, Kadib et al.31,32 preparedporous metal oxide microsphere using chitosan as precursor.

The present contribution reports the synthesis of porousmaterial containing spinel nickel ferrite through a solgelmethod and calcination using chitosan as precursor. This

method has the advantages of simple preparation, costeffectivenes, and a gentle chemical route resulting in neporous magnetic material. The objective was to reveal theeffectiveness of porous NiFe2O4 adsorbent (PNA) for theremoval of Pb(II) from aqueous solution and to study theinuence of various parameters on the adsorption process.

EXPERIMENTAL SECTION

Materials. All reagents used in the present investigationwere of analytical or equivalent grade and were used withoutfurther purication. Chitosan (low molecular weight) wasobtained from Aldrich (CAS Registry No. 9012-76-4).Deionized double distilled water was used throughout theexperimental studies. The distilled and degassed water wasstored in a polyethylene airtight bottle to minimize the mixingof carbon dioxide from atmosphere. Pb(NO3)2was used for thepreparation of 1000 mg/L stock solution.

Preparation of PNA. Three-dimensional (3D) porousadsorbent was synthesized by a solgel method as reportedearlier.33 In the rst step chitosan solgel solution (2 wt %)was prepared by dissolving chitosan in 2% (v/v) acetic acidsolution and stirring for 2 h. Then, 5.9346 g of Ni(NO3)26H2O and 16.4060 g of Fe(NO3)39H2O were mixed with 5mL of water and added slowly into chitosan solution withvigorous stirring. The metal solution occupied the voids ofchitosan gel template. The solution was concentrated byheating to 80 C; the active organic groups of chitosan couldreact with metal ions (Ni2+ and Fe3+) to form complex

precursors. The assembly of Ni2+Fe3+/chitosan was calcined ina muffle furnace at 400 C for 5 h, and the organic part wasremoved. The obtained product was quenched using ice andwater to form the 3D porous spinel ferrite. The as-preparedmaterial was washed several times with water, and the nalproduct obtained was denoted as PNA.

Equipment. X-ray diffraction (XRD) measurements werecarried out using a Rigaku D/MAX 2500 diffractometerequipped with a copper anode (Cu K = 1.5406 ) over ascanning interval (2) from 20 to 70. The infrared spectrum ofthe samples was obtained by using a Fourier transform infraredspectrometer (FT/IR-300E, Jasco, Japan). The sample wasprepared as a KBr pellet by investigating the peaks within the

range 4000400 cm1. The morphology of the solid wasobserved by scanning electron micrographs obtained by using aMini-SEM (Mini-SEM, SNE-1500M, Korea). Further, theelemental composition was obtained by using SU-70, Hitachi,Japan.

Batch Adsorption Assay. Experimental solutions for theequilibrium tests were prepared from the 1000 mg/L stocksolution to the desired concentrations through successivedilutions. To determine the effect pH on adsorption, theexperiments were carried out in the initial pH range from 2 to 8at 298 K with an adsorbent dose of 100 mg and 25 mg L1

initial metal ion concentration. The adsorption kinetic studieswere obtained using three different metal concentrations (10,20, and 40 mg/L) from the time intervals 0 to 150 min at 298K by using the optimum pH 5 and an adsorbent dose of 25 mg.The effect of the initial metal ion concentration (10600 mg/L) on the removal of Pb(II) was studied from 298 to 308 K.The sample solutions were stirred on a thermostatic mechanicalshaker operating at constant speed. After adsorption, thesamples were treated with a magnet to separate the PNAadsorbent from the solution and the equilibrium concentrationof each solution was analyzed for residual metal concentrationusing an atomic absorption spectrophotometer. The amount ofmetal ion retained in the solid phase (adsorbent) was calculatedas the difference between the initial concentration of metalsolution and the equilibrium concentration of metal ions whichwas represented in the following equation:

=

qV C C

m

( )e

0 e

(1)

Potentiometric Titrations. Potentiometric titrations wereperformed in order to determine deprotonation constants andabsolute concentrations of specic functional groups present onthe sorbent surface. The data may enable the deduction of thepHPZC(point of zero charge) of the solid employed. Titrations

were performed in a nitrogen atmosphere with 100 mMNaNO3electrolyte at room temperature. A quantity of 0.5 g ofPNA was suspended in 100 mL of aqueous solution having theknown ionic strength 100 mM NaNO3, separately in twodifferent beakers. The solutions were titrated separately with100 mM NaOH or 100 mM HCl. The suspension pH wasrecorded after each addition of titrant using a digital pH meter,employed prior to its calibration with standard buffers.

Nonlinear Regression Analysis.Estimation of the best tof the kinetic and equilibrium model to the experimental kineticand equilibrium data is necessary. The typical assessment of thequality models t to the experimental data is based on themagnitude of the correlation coefficient for the regression.Nonlinear regression analysis was performed in order to

determine the model which best t the experimental resultsusing Origin 8.0 software. The correlation coefficient (R2) andthe chi-square value (2) were used to evaluate the goodnessof t of curves to the experimental data. The value R2

represents the percentage of variability in the dependentvariable that has been explained by the regression line and mayvary from 0 to 1. On the other hand, 2 is basically the sum ofthe squares of the difference between the experimental data anddata obtained from the models, with each square differencedivided by the corresponding data obtained by calculation fromthe models. If the values calculated from the model are similarto the experimental data,2 should be a small number and viceversa. This can be represented mathematically as

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=(experimental value model value)

model value2

2

(2)

RESULTS AND DISCUSSIONCharacterization of the PNA. XRD Analysis. The phase

composition of synthesized PNA was investigated by X-raydiffraction analysis and is presented in Figure1. As can be seen

from the XRD pattern, high intensity and sharp, well-denedpeaks indicated the high purity and the high crystalline natureof spinel NiFe2O4. No impurity phase was observed in thispattern. The diffraction peaks observed at 2= 30.42, 35.81,37.41, 43.46, 54.03, 57.55, and 63.08 indicated the planes of(220), (311), (222), (400), (422), (511), and (440),respectively. By referring to the XRD patterns of the standarddata (JCPDS 10-0325), all the detected peaks of PNA wereindexed with NiFe2O4 with an inverse spinel structure. Thediffraction patterns provide clear evidence of the formation ofthe pure inverse spinel structure of nickel ferrite.

FTIR Analysis.The formation of the porous spinel NiFe2O4structure was further supported by comparing the Fouriertransform infrared (FTIR) spectra of chitosan (CH) and PNA(Figure 2). The FTIR spectrum of CH possessed majorcharacteristic absorption bands at 3469 cm1 (OH and NHstretching), 1590 and 1654 cm1 (NH bending vibrations ofthe secondary amide), and 1379 cm1 (CO stretching ofamide group). However, a signicant difference in the spectralbands was observed after the formation of PNA from chitosan.The strong absorption peaks in the spectral data of PNA at

3399 and 1628 cm

1 were assigned to the stretching andbending vibrations of surface hydroxyl groups (OH). The bandat 590 cm1 corresponds to intrinsic stretching vibrations ofthe metal at the tetrahedral site (FeO, typical of spinelferrite).34 Overall, the FTIR spectrum provided supportiveevidence for the formation of NiFe2O4 adsorbent.

Morphology, Pore Structure, and Elemental Composition.The morphology of the PNA was investigated by scanningelectron microscopy (SEM). Figure3provides a comparison ofchitosan (CH) and PNA morphology, and it is clearly observedthat a marked difference between the surface morphology ofchitosan akes and that of PNA occurred. PNA exhibited 3Dporous structure contained highly porous scaffolds with

interconnected porosity. The samples synthesized at 400 Cwere pure phase of NiFe2O4 with porous structures, and theaverage pore size was estimated to be about 5.0 m. Further,energy dispersive X-ray analysis (EDAX) was also used todetermine the chemical composition of the as-prepared PNAproduct. The EDAX spectra of the representative samplesobtained at an applied potential of 13 kV and the compositionof the elements present in the samples are given in Figure S1 inthe Supporting Information. The EDAX results of the as-prepared product PNA showed the existence of Ni, Fe, and Oelements with the absence of any impurities. The analysisrevealed that Fe and Ni were evenly distributed within thepores and surface of the solid. On the basis of the SEMobservations, it was conrmed that the interconnectedcharacter of the porosity and the pore size of the scaffolds issuitable for metal ion adsorption.

Surface characteristics from Potentiometric Titration.Surface charge characteristics impact the adsorption capacitiesand adsorption mechanism. Potentiometric titrations wereconducted in order to characterize the surface characteristics ofPNA adsorbent. Moreover, the potentiometric titrationprovides the important characteristics of the surface chargeproperties, i.e., the point of zero charge (PZC) along with thedeprotonation constants of the solids (see eqs 3and4). ThepHPZCwas determined and found to be 7.21 for PNA. Similarly,the deprotonation constants pKa1 and pKa2 were found to be3.73 and 10.69, respectively. Further, the total concentration ofadsorption sites available on the PNA surface was estimatedand was found to be 2.93 104 mol/g, giving the theoretical

maximum sorption capacity of Pb(II) of 60.70 mg g

1.Effect of pH and Lead Speciation. The pH values of an

aqueous solution play a critical role in the adsorption of metalions onto an adsorbent. To optimize the pH for maximumremoval efficiency, the sorption experiments were conducted inthe initial pH range from 2 to 8 in batch adsorption studiescontaining 25 mL of solution with a concentration of 25 mgL1 metal solution and 100 mg of PNA (Figure 4a). It wasobserved that the adsorption of Pb(II) onto PNA was stronglypH dependent. The removal efficiency was increased sharply asthe pH was increased from 2.0 to 5.0 and remained nearly>90% during the pH region 5.08.0. In order to describe thebinding of Pb(II) onto the functional groups present on the

Figure 1.XRD spectrum of PNA.

Figure 2.FTIR spectra of chitosan, PNA, and Pb(II) loaded PNA.




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surface of the adsorbent, speciation of Pb(II) was also studied.It was a known fact that the pH inuenced the ionization andspeciation of metals in aqueous solution. The lead speciationdiagram (Figure 4b,c) showed the concentration of dissolvedspecies and the pH at which hydrolysis could occur. The molefractions were calculated using the Visual MEDUSA speciationprogram. It was observed that lead predominantly exists as Pb 2+

and PbOH+ cations within the pH range from 2.0 to 6.0. Theconcentration of cationic species, i.e., Pb2+, started to decrease

at pH >6.0 due to its precipitation. A number of other leadspecies such as Pb2OH

3+, Pb(OH)2, Pb(OH)42, and Pb-

(OH)3 were present in solution between pH 5.0 and 12.0.

However, the dominant species were only Pb(OH)2 andPb(OH)4

2.The surfaces of metal oxides in aqueous medium could

acquire a charge by protonation or deprotonation of the neutralsites (OH groups) in accordance with the following reactions

Figure 3.SEM images of chitosan and PNA at different magnications.




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referring tothe two different proton dissociation constants, viz.,Ka1and Ka2:

35

++ +S OH S OH H2 (3)

+ +S OH SO H (4)

At lower pH values due to the protonation reaction3, theconcentration of protonated sites are present in higher amount(SOH2

+). Otherwise, there would be a competition betweenprotons and metal ions toward the surface hydroxyl groups.Therefore, the adsorption of Pb2+ and PbOH+ was suppressedand hence less uptake of Pb(II) occurred at very low pH values,

Figure 4.(a) Effect of pH on removal of Pb(II) using PNA. Conditions: adsorbent dose, 100 mg; initial concentration, 25 mg L 1; temperature, 298K. (b and c) Speciation of Pb(II).




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i.e., 2.0. Lead removal was increased with gradually increasingpH until 5.0; such an increase in adsorption was due to thegradual dissociation of surface functional groups since the pKa1

was found to be 3.74. Hence, there was a gradual increase inneutral (SO) species followed by completely deprotonatedsites (SO) species (reaction4) since the pKa2obtained was

Figure 5. Pseudo-rst-order, pseudo-second-order, and double-exponential nonlinear kinetic models and comparison of experimental and modeladsorption capacities. Conditions: pH, 5.0; adsorbent dose, 25 mg; temperature, 298 K.




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10.70. This resulted in the surface complexation between Pb2+

and PbOH+, the surface functional group available on the PNAsurface. This, perhaps, facilitated a sharp increase of Pb(II)adsorption. At very higher pH values, i.e., beyond pH 6.0, theprecipitation of Pb(II) may contribute to the high uptake ofPb(II). In order to visualize the surface sorption process ofPb(II), the entire adsorption experiments were conducted atpH 5.

Time Prole of Metal Adsorption. The contact timerequired for the system to reach equilibrium is an importantparameter for designing an adsorption process.36,37 Experi-ments were performed at three different initial sorptiveconcentrations, and data was collected within the time period0150 min. Results obtained are depicted in Figure 5, wherethe solid dots show experimental qt values. The maximumadsorption capacity of Pb(II) was increased with increasinginitial concentration as shown in the Supporting Information(Table S1). It was noted that the fast adsorption process ofPb(II) occurred during the rst few minutes and an apparentequilibrium was reached within 60 min. The fast adsorptioncould be ascribed to the 3D pore structure exhibiting moreadvantages in mass diffusion and transportation.33 Thisphenomenon could be explained as follows: the adsorptionkinetics is initially fast because the adsorption process takesplace predominantly on the outer surface, followed by a slowadsorption step on the inner surface or even the diffusionwithin the pores of the adsorbent.38 For the purpose ofresearching the kinetic mechanismofthe adsorption process, apseudo-rst-order (PFO) model,39 a pseudo-second-order(PSO) model,40 a double-exponential model (DEM),41 andan intraparticle diffusion42 model were applied and the resultswere compared in specic terms.

Modeling of the Kinetic Data (PFO, PSO, and DEM). Thekinetics of the adsorption describing the adsorbate uptake rateis one of the important characteristics which control theresidence time of adsorbate uptake at the solidliquid

interface.43 In general, three mechanisms were possible duringthe adsorption process by porous-adsorbent particles: (i)external mass transfer, (ii) intraparticle transport, and (iii)chemisorption.44 A pseudo-rst-order (PFO) model, a pseudo-second-order (PSO) model, and a double-exponential model(DEM) were used to identify the kinetic behaviors of theadsorption process and the prevailing mechanisms. Theapplicability of a model was checked by constructing nonlinearplots oftvs qt, and these plots are shown in Figure5. In Figure5, the experimental points are shown together with themodeled curves from PFO, PSO, and DEM equations.Whatever the kinetic equation used (Supporting Information,Table S1), the description of the adsorption kinetics wassatisfactory. The calculated constants of the three kinetic

equations along with R2 values at different initial Pb(II)concentrations are presented in the Supporting Information(Table S1).

Based on the kinetic data (Supporting Information, TableS1) obtained, it was seen that for all the studied concentrationsthe correlation coefficients for the pseudo-rst-order kineticmodel were low, which revealed that PFO equation under-estimates the Pb(II) uptake. Contrary to the PFO equation, thetting of the kinetic data in the PSO equation showed the bestt with a high correlation coefficient (R2 > 0.97) over theconcentration range of 1040 mg/L. This suggested that theadsorption of Pb(II) was not a simple rst-order reaction butwas a pseudo-second-order reaction. Results were showed that

the data could be described by two-step models as the pseudo-rst-order model had correlation coefficients that were not asgood as those of the other two models. From the resultsobtained, it was also observed that the best ts to theexperimental data were obtained with the double-exponentialand pseudo-second-order models. However, the adsorption ofPb(II) seemed to be better described by the double-exponential

model.Adsorbents with a porous structure normally possessed twoseparate adsorption sites: one at the outer surface which waseasily accessible and another inside the pores. The rate ofadsorption on the outer surface was more rapid, and that on theinner surface was relatively slower. A double-exponential modelequation was applied, and the parameters for the slow and rapidsteps and model parameters obtained from experimental dataare given in theSupporting Information(Table S1). The modelt very well for all three concentrations (R2 > 0.99), and datashown in Figure5conrmed that all of the experimental pointscoincide with the curve developed by the model. Thus uptakeof Pb(II) onto PNA material followed the DEM model;according to this model, during the rst step, rapid metal

uptake took place involving external and internal diffusion.Subsequently, a slow step prevailed; the intraparticle diffusioncontrolled the adsorption rate and nally the metal uptakereached its equilibrium value. Thus KD1and KD2were diffusion

parameters controlling the overall kinetics. Since there wereequal numbers of pore cavities and windows in these materialsas shown from SEM studies, diffusion through both structuralfeatures supposed to contribute equally. The kinetics wasdescribed by two processes with single relaxation times: (a)slow diffusion through windows with high activation energy and(b) fast diffusion along pore cavities with low activation energy.

In fact, to say a kinetic model is effectively tted to theexperimental kinetic data, it is required that the calculatedequilibrium adsorption capacity values,q

etheoretical, should be

in accordance with the experimental qevalues.45 The values of

qe theoretical and qe experimental for all the applied kineticmodels were plotted and are shown in Figure 5. The values ofqe experimental differed from those of qe theoretical (model)from the PFO model, and the values were perfectly inagreement for DEM. Table S1 in the Supporting Informationillustrates the calculated values of the kinetic parameters, therelated correlation coefficient (R2), and 2 obtained from thethree kinetic models. These results suggested that theadsorption of Pb(II) on PNA followed the PSO and DEMmodels.

Intraparticle Diffusion Modeling. Adsorption kinetics areusually controlled by different mechanisms, of which the most

limiting ones are the diff

usion mechanisms. The evaluation ofthe diffusion mechanism is not possible from the PFO and PSOmodels. For a solidliquid adsorption process, the solutetransfer process is usually characterized by either external masstransfer (boundary layer diffusion) or intraparticle diffusion orboth. Generally, a sorption process is diffusion controlled if itsrate is dependent upon the rate at which adsorbate andadsorbent diffuse toward one another. A number of diffusionmodels have been applied for the dening kinetics ofadsorption.46 In the present investigation the most widelyapplied Weber and Morris model was used to predict theintraparticle diffusion based mechanism. The WeberMorrismodel assumes the following stages: external mass transfer and




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stages of intraparticle diffusion in larger and smaller pores untilthe saturation of the solid surfaces is attained.47

For the plot of qtversus t1/2 (Figure S2 in theSupporting

Information), the modeling exhibited multilinearity, meaningthe existence of several steps in the adsorption process. Themultilinear plots were divided into three portions. The rstsharper portion corresponds to the instantaneous adsorptionstage or external surface adsorption. The second portion is thegradual adsorption stage, where intraparticle diffusion is rate-limiting. The third portion is the nal equilibrium stage, whereintraparticle diffusion starts to slow down due to the extremelylow adsorbate concentrations left in the solution. The linearplots at various concentrations do not pass through the origin,which shows that the intraparticle diffusion was not the onlyrate-controlling step. Other kinetic models may also control therate of adsorption with all of them operating simultaneously.This was indicative of some degree of boundary layer control.The values of the intercept C were used for indirectcharacterization of the boundary layer thickness, which meansthat the larger the intercept the greater the boundary layereffect.48 The boundary layer effect on lead complex sorptionwas more pronounced at high sorptive concentrations(Supporting Information, Table S2), and it increased withincrease in initial concentration. The values of the intercept inthe Supporting Information (Table S2) provide informationabout the thickness of the boundary layer and the resistance tothe external mass transfer. The larger the intercept that wasobtained showed higher external resistance. On the other hand,by increasing the initial metal ion concentration, the actualamount of metal adsorbed per unit mass of PNA was increased.The higher initial concentration of the PNA provided animportant driving force to overcome the mass transferresistance for Pb(II) transfer between the solution and thesurface of the PNA. In the process, the Pb(II) primarilyencountered the boundary layer effect and then diffused fromthe boundary layer lm onto the adsorbent surface and nally

diffused into the porous structure of the adsorbent, which couldtake a relatively longer contact time. TheR2values (SupportingInformation, Table S2) for this model were lower compared tothose obtained from the PFO, PSO, and DEM models. Fromthese results, it was concluded that intraparticle diffusion wasnot the dominating mechanism for the adsorption of Pb(II)from aqueous solution by PNA. Although the correlationcoefficients were high (R2 values ranged from 0.8727 to0.9858), the straight line obtained when tting the experimentaldata did not pass through the origin, which also indicated thatpore diffusion was not only the controlling step.

Modeling of Sorption Equilibria. Analysis of equilibriumsorption data is important for optimizing the design of sorptionsystems, and theadsorption data is modeled using adsorption-

type isotherms.49 The mathematical description of an isothermrelates the concentration of adsorbate in solution Ce (mg/L)and its accumulation onto adsorbent qe (mg/g) at a specictemperature.50 The distribution of Pb(II) between a solidsorbent and the solution in equilibrium over a range ofconcentrations at three different temperatures (298308 K)was studied. During this study the equilibrium data obtainedwas tted using four mathematical expressions (Langmuir (L),Freundlich (F), RedlichPeterson (RP), and Sips (S)) and isgiven in theSupporting Information(Table S3). All isothermswere positive and concave to the concentration axis, and theexperimental equilibrium values of the amount of Pb(II) sorbedon the PNA was increased with the increase in Pb(II)

concentration in solution. Isotherm data obtained at 308 K isshown in Figure6. The detailed isotherm parameters for three

different temperatures are compiled in the SupportingInformation(Table S3), along with the results of the nonlinear2 test analysis.

Initially,the two parameter isotherm models Langmuir51 andFreundich52 were applied. Among two-parameter models, highcorrelation coefficients and low2 values were observed in thecase of the Langmuir model. These results suggest that Pb(II)ions were strongly adsorbed as a monolayer covering the solidadsorbent surface. From the results it was also observed that qmparameter from the Langmuir isotherm was increased withincreasing temperature. Increasing the temperature is known toincrease the rate at which diffusion processes occur. Theessential characteristics of the Langmuir isotherm wereexpressed in terms of a dimensionless separation factor, RL(RL = 1/(1 + KLC0)), which was then determined at differenttemperatures over a broad concentration range. The RLvalueswere found to vary within a range, 0.9250.172, 0.9090.142,and 0.8840.113 for the temperatures 298, 303, and 308 K,respectively. All the observed RL values were found to bebetween 0 and 1, indicating favorable Pb(II) adsorption ontothe PNA. Further, from these results the Freundlich isothermmodel was studied, and it indicated that the sorption capacity(KF) was increased with increase in temperature (SupportingInformation, Table S3). From the Freundlich model themagnitude of 1/ngives a measure of favorability of adsorptionas included in theSupporting Information(Table S3), that the

values of 1/nsmaller than 1 reect a favorable adsorption.Further, the three parameter model LangmuirFreundlich

isotherm, also known as the Sips isotherm,53 which describesthe relationship of the equilibrium concentration of the sorbatebetween the solid and liquid phases in heterogeneous systems,was employed. At low sorbate concentrations the Sips modeleffectively reduces the Freundlich isotherm, while at highsorbate concentrations, it predicts a monolayer sorptioncapacity characteristic of the Langmuir isotherm.54 Thisisotherm is capable of modeling both homogeneous andheterogeneous binding surfaces. The value of the exponentnLFin the Sips equation was 1, which means that Pb(II) sorptiononto the PNA is more of a Langmuir type than of a Freundlich,

Figure 6.Adsorption isotherms for Pb(II) onto the PNA. Conditions:

pH 5.0; adsorbent dose, 100 mg; temperature, 308 K.




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since for nLF = 1 the Sips equation reduces to the Langmuirequation in which the variable bLF corresponds directly tobinding affinity (KL). The predicted value of qmax was lowerthan the corresponding value (qm) of the Langmuir model(Supporting Information, Table S3). The RedlichPetersonisotherm55 also incorporates features of both Langmuir andFreundlich, and the equation was used in further investigation.This model was related with a three-parameter empiricalequation and was a hybrid isotherm featuring both Langmuirand Freundlich isotherms.56 It was used to understand theadsorption mechanism of homogeneous or heterogeneoussystems due to its versatility. In this study, the value of theexponent b was approximately equal to 1, suggesting, like theSips equation, that the equilibrium data was preferablytted bythe Langmuir model rather the Freundlich. This indicated thatthe Langmuir condition was the ideal one for the adsorptionmechanism.57 The results of the RedlichPeterson isothermalso supported the suitability of the Langmuir model.

In order to verify the model for the adsorption system, thedata was analyzed using 2 and R2 analysis. The smaller theerror function value and the closer the R2 value was to unity,the better the curve tting. The values of errors from thenonlinear method are listed in the Supporting Information(Table S3). The experimental data yielded excellent ttingfollowing the isotherm order Langmuir > Sips > RedlichPeterson > Freundlich, based on R2 and 2 values. Among thetested three-parameter equations, the better and perfectrepresentation of the experimental result of the adsorptionisotherms was obtained using the Sips model. The maximumadsorption capacities were identical to those obtained using theLangmuir isotherm.

Effect of Ionic Strength. Ionic strength is one of thesignicant parameters that inuence the metal ion removalfrom aqueous solution. Further, it provides signicantinformation about the specic and nonspecic adsorption ofmetal ions onto the surfaces of solids.58 In this study, the

inuence of the ionic strength (sodium nitrate as backgroundelectrolyte) on Pb(II) sorption was studied by varying theconcentration of sodium nitrate solution from 0.01 to 1.0 mol/L (i.e., 100 times increase) . As shown in Figure S3 in theSupporting Information, 100 times increase in ionic strengthinuenced insignicantly the uptake of Pb(II) by this solid,suggesting that inner sphere complexation reactions account forthe adsorption. In the inner sphere surface complexes, theadsorbed molecules or ions and the surface functional groupsapparently were sorbed by strong chemical bonds. These resultswere in accordance with the thermodynamic data showing thatan endothermic uptake of Pb(II) by PNA occurred since anincrease in temperature favored the uptake of Pb(II), asreported earlier for other divalent metal ions.59,60

Comparison of Adsorption Capacities. The adsorptioncapacity is an important factor that determines how muchadsorbent is required for quantitative enrichment of the targetanalyte from a given solution.61 The maximum adsorptioncapacity of PNA for the removal of lead was compared withthose of other potential adsorbents reported in the literatureand was presented in theSupporting Information(Table S4).The maximum adsorption capacity values were based on theLangmuir adsorption capacity.6268 The maximum adsorptionefficiency of PNA for Pb(II) was found to be 48.98 mg/g; thisadsorption capacity was comparable to and moderately higherthan that of many other corresponding sorbents reported in theliterature. However, higher adsorption capacities of Pb(II) were

reported by some other researchers with different adsorbents.Moreover, the total surface active sites estimated with the helpof potentiometric titrations was found to be 2.93 104 mol/g,and an estimated apparent capacity for Pb(II) was found to be60.70 mg/g. This value was very comparable to the capacityestimated by this method. Nonetheless, this comparison is notworth emphasizing, since the experimental conditions main-tained were different and also combinations of materialsemployed was different in those studies. Further, the success ofany adsorption process in wastewater treatment depends largelyon the cost of the adsorbent used. In the present investigationthe adsorbent PNA was prepared from the easily availablenatural biopolymer chitosan in a simple synthetic route. Hencethe cost of the adsorbent would likely to be economical whencompared with various other adsorbents reported in theliterature.

Adsorption Mechanism.The two major challenges in theadsorption eld were underlined as (i) to prepare the mostpromising adsorbent from readilyavailable materials and (ii) toidentify the sorption mechanism.68 The present investigationwas addressed to fully meet these problems with a wide rangeof physicochemical parametric studies. The most abundant

surface functional group participating in the reactions on PNAsurfaces was the hydroxyl group. It was predicted that thesurface hydroxyl group (SOH) were predominaantly andpreferably involved in surface adsorption of Pb(II) onto thesolid surface since the pHPZCwas found to be 7.22. Further, theinvolvement of hydroxyl groups was affirmed by the FTIRspectral data (Figure2). It was found that a peak at 3399 cm1

in PNA was observed which corresponds to the hydroxylgroups. However, after adsorption of Pb(II) these groups wereshifted to a wavenumber of 3408 cm1; this conrms theinvolvement of hydroxyl groups in Pb(II) removal. Along withthis from the SEM results it was hypothesized that the presenceof pores in the PNA adsorbent further enhances the adsorptionof Pb(II).

CONCLUSIONSIn summary, as-prepared NiFe2O4 adsorbent with 3D porousstructure possesses attractive adsorption properties towardPb(II) removal from aqueous solutions. The adsorption rate isso fast that it reaches equilibrium in a short time (almost 60min). This fast adsorption rate comes from the interconnectedpore structure of PNA, which is benecial for the diffusion ofmetal ions to the adsorption sites on PNA. The adsorptionkinetics followed the mechanism of the DEM, evidencing atwo-stage adsorption process. From FTIR spectral data it wasfound that surface hydroxyl groups play a role in the adsorptionof lead ions from aqueous solution onto the surface of PNA.Also, the magnetic separation of the PNA from treated Pb(II)

solution by a magnet was achieved within 2 min; this is anadvantageous property of the as-prepared materials (Figure S4in the Supporting Information). Moreover, the magneticadsorbent was easily dispersed in aqueous solution and canbe rapidly separated by applying a magnetic eld. In conclusion,as-prepared adsorbent PNA showed a promising prospect fortreatment of Pb(II) containing wastewater.

ASSOCIATED CONTENT*S Supporting InformationEDX spectrum of PNA adsorbent, WeberMorris plots andeffect of ionic strength on Pb(II) adsorption, picture showingseparation of PNA after adsorption by using a magnet, and




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tables representing parameters of kinetics models, isothermmodels, and comparison of maximum adsorption capacities ofvarious adsorbents. This material is available free of charge viathe Internet athttp://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +82 33 649 7535. Fax: +82 33 642 7635. E-mail:[email protected].

Notes

The authors declare no competing nancial interest.

ACKNOWLEDGMENTS

This work was supported by a National Research Foundation ofKorea (NRF) grant funded by the Korean government(MEST) (No. 2012R1A2A4A01001539). This subject is alsosupported by Korea Ministry of Environment as "Convergingtechnology project".

NOMENCLATURE

aRP = RedlichPeterson isotherm constant, dm3

mg1

bLF= LangmuirFreundlich isotherm constant, dm3 mg1

bRP = RedlichPeterson isotherm exponentC= intercept from WeberMorris equation, mg g1

C0= initial concentration of adsorbate in aqueous solution,mg/LCe = concentration of adsorbate in aqueous solution atequilibrium, mg/L

D1 = sorption constant rate parameter of the rapid step ofdouble-exponential model, mmol L1

D2 = sorption constant rate parameter of the slow step ofdouble-exponential model, mmol L1

k1 = rate constant of the rst-order kinetic model, min1

k2= rate constant of the second-order kinetic model, g mg1

min1

KD1 = parameter of the rapid step of double-exponentialmodel, min1

KD2 = parameter of the slow step of double-exponentialmodel, min1

KF= Freundlich isotherm constant, (mg g1)(L mg1)1/n

kWM= WeberMorris kinetic constant, mg g1 min0.5

PZC = point of zero chargeq= amount of adsorbate adsorbed, mg g1

qe = mass of adsorbate adsorbed at equilibrium, mg/gqt= amount of sorbate sorbed at time t, mg g

1

qexp= experimental mass of adsorbate adsorbed, mg/gqmax= maximum biosorption capacity, mg g

1

R2 = correlation coefficientT= temperature, Kt= time, minV= volume of the solution, Ml2 = chi-square function

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Three-Dimensional Porous Spinel Ferrite as an Adsorbent for Pb(II)

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