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Chemical Engineering Journal 166 (2011) 814821

Contents lists available at ScienceDirect

Chemical Engineering Journal

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c e j

Thermodynamical and analytical evidence of lead ions chemisorption ontoPimenta dioica

J. Cruz-Olivares a, C. Prez-Alonso a, C. Barrera-Daz b,, R. Natividad b, M.C. Chaparro-Mercado c

a Universidad Autnoma del Estado de Mxico, Facultad de Qumica, Paseo Coln interseccin Paseo Tollocan S/N, C.P. 50120, Toluca, Estado de Mxico, Mexicob Centro Conjunto de Investigacin en Qumica Sustentable UAEM UNAM, Carretera Toluca-Atlacomulco, km 14.5, Unidad El Rosedal, C.P. 50200, Toluca, Estado de Mxico, Mexicoc Departamento de Ingenieras, Universidad Iberoamericana, Prol. Paseo de la Reforma 880, Lomas de Santa Fe, lvaro Obregn, D.F. 01219, Mexico

a r t i c l e i n f o

Article history:Received 22 September 2010Received in revised form 9 November 2010Accepted 10 November 2010

Keywords:BiosorptionLeadPimenta dioicaIsothermsXPS

a b s t r a c t

Residue of allspice (Pimenta dioica L. Merrill) obtained as a by-product from the hydro-distillation oilprocess, has been studied as a low cost biosorbent for removing lead (II) ion from water solution atdifferent temperatures. Batchexperiments were performed withaqueous leadsolutions of concentration25mgL1 , at pH 5 and adsorbent dosage 1.0 g biosorbent per liter of solution. According to pseudo-second order kinetic model, the maximum adsorption capacity was 22.37mg g1 of Pb (II) on residueof allspice (RA). This value was reached at 90 min and temperature of 308 K. Langmuir, Freundlich andDubininRadushkevich(DR) adsorptionisotherm models wereapplied as an attempt to mathematicallyrepresent adsorption data. These three equations were found to be applicable to this adsorption system,in terms of relatively high regression values. Thermodynamic parameters showed that the adsorption oflead(II) onto RA was feasible, spontaneous, andendothermic under the studied conditions.The elementalanalysis from scanning electron microscopy (SEM) before and after the contact showed that lead wasadsorbedbyRA.Diffusionresults,thevalueofthefreeenergy E(kJmol1),XPSandFTIRanalysisconfirmedthat the lead (II) adsorption process onto RA was controlled by chemisorption.

2010 Elsevier B.V. All rights reserved.

1. Introduction

The presence of heavy metals in industrial effluents is an envi-ronmental problem mainly because of their undesirable effects onhumans (i.e. damage to kidneys, nervous and reproductive system,liver and brain) [1]. Among heavy metals, lead has been recog-nized as one of the most toxic metals, mainly in ionic state. Severeexposure to lead has been associated with sterility, abortion, still-births and neonatal death [2]. For lead ions removal from aqueoussolutions, adsorption has led to important results and therefore isa worthwhile alternative to explore [3,4]. The economical conve-nience and efficiency of adsorption highly depend on adsorbenttype. Within this context, low-cost adsorbents (i.e. agriculturalwaste) have exhibited a promising performance in the removalof metallic ions [5]. Some authors have reported that functionalgroups present in agricultural waste, like hydroxyl, carboxylic andpolyphenolic of the cellulose, hemicellulose and lignin, could formbinding with lead ions [6,7]. The mechanism of adsorption of leadon thecellulosic materialsis notwell established.In some cases theadsorption process is governedby physical phenomena or chemical

Corresponding author. Tel.: +52 722 296 5514; fax: +52 722 296 5541.E-mail address: [email protected] (C. Barrera-Daz).

phenomena. By means of diffusion, kinetics and thermodynamicsstudies it is possible to establish whether a process is governed bya physical or chemical phenomenon.

In this study the residue of allspice (RA) was used as biosor-bent to study the process of lead (II) ions adsorption as functionof temperature. The residue of the allspice oil extraction processis at least 95.5% in weight of the dried fruit. Annually, this reaches1500tonnesof biosorbent in Mexicoand currentlythe finaldisposalof such a waste is a problem. This work aims to evaluate a lowcost adsorbent for the lead ions removal from aqueous solutionsat different conditions. RA mainly contains cellulose, hemicellu-lose and lignin. If lead (II) ions are able to bind with some of thefunctional groups of the cellulosic biosorbents, then it would beexpected that RA works efficiently. The effects of physical andchemical parameters on the performance of the adsorption pro-cess were studied in order to elucidate whether physisorption orchemisorption was occurring during the process. The biosorbentadsorption capacity was determined by applying the pseudo-firstorder, pseudo-second order kinetic models and Elovich equationat different temperatures. The thermodynamic parameters of theadsorption process were also obtained. The diffusion phenomenonand the sorption kinetics were studied by means of the externalmass transfer diffusion and intraparticle mass transfer diffusionmodels.

1385-8947/$ see front matter 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.cej.2010.11.041


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J. Cruz-Olivares et al. / Chemical Engineering Journal 166 (2011) 814821 815

2. Materials and methods

2.1. Adsorbent preparation

The crushed de-oiled residue of allspice was obtained as a by-product from a hydro-distillation process. This waste was firstly

washed with diluted nitric acid (0.1M) solution and then withethanol (99.9 purity) in order to eliminate colouring and remain-ing substances. It was then dried at 60 C for 24h in a stove. Oncethe adsorbent cooled off, it was sieved through a 20 mesh to obtainparticles of size smaller than 0.836 mm, and stored in desiccators.

2.2. Adsorbent physical and chemical analysis

The residue of allspice was analyzed to establish the presenceof cellulose, hemi-cellulose fibre content, particle size, density andhumidity,in accordance with food legislation [8] and Soest method[9].

2.3. Adsorption experiments

The initial concentration of lead (II) solutions was 25 mg L1.Thesesolutions were prepared by dissolving Pb(NO3)2 in deionisedwater. The pH of the working solutions was adjusted to desiredvalues with 0.1 M HNO3. All the chemicals employed were analyt-ical grade. The adsorption experiments were carried out within anorbital stirringshaker (Lab-Line Incubator-Shaker, USA)at 200rpm.The pH was measured with a Conductronic pH 130. The adsorptionexperiments were conducted at various time intervals (5, 10, 15,30, 45, 60, 75 and 90min) and temperatures (16, 25 and 35C). Thetest solutions were centrifuged to separate the adsorbent mate-rial and the supernatant. The adsorbent material was dried andcharacterized using scanning electron microscopy (SEM) (JEOL-

JSM-6510LV), while the supernatant was analyzed for aqueousmetalconcentration usingthe standardmethodof leaddetection byatomic absorption spectrophotometry (Perkin-Elmer AA300) [10].All experiments were conducted by duplicate.

2.4. Sorption kinetics

To examine the plausible rate-controlling step (i.e. chemicalreaction, diffusion control or mass transfer) of the adsorption pro-cess, the fitting of some kinetic models was evaluated [11,12].

2.4.1. Pseudo-first order equationThe pseudo-first order equation is generally expressed as fol-

lows:

dqt

dt =k1(qe

qt) (1)

where qe and qt (mgg1) are the amount of sorbate at equilibriumandat time t(min), respectively, and k1 (min1) is the rate constantof the pseudo-first order equation.

2.4.2. Pseudo-second order equationThe pseudo-second order equation is expressed as:

dqtdt

= k1(qe qt)2 (2)

where k1 is the rate constant of the pseudo-second order equation(gmg1 min1).

2.4.3. The Elovich equation

The Elovich equation is of general application to chemisorp-tion kinetics. The equation has been satisfactorily applied to somechemisorption processes and has been found to cover a wide range

of slow adsorption rates. The same equation is often valid forsystems in which the adsorbing surface is heterogeneous, and isformulated as:

dqtdt

= eqt (3)

where (mgg

1 min

1) is theinitialadsorptionrate and (gmg

1)isrelatedtotheextentofsurfacecoverageandtheactivationenergyinvolved in chemisorption.

2.5. Equilibrium isotherm models

An adsorption isotherm describes the relationship betweenthe amount of metal adsorbed and the metal ion concentrationremaining in solution [13]. The equilibrium adsorption isothermsare one of the most important data to understand the mecha-nism on the sorption process [14]. There are many equations foranalyzing experimental adsorption equilibrium data. The equa-tion parameters and underlying thermodynamic assumptions ofthese equilibrium models often provide some insight into boththe adsorption mechanism and the surface properties and affin-ity of the sorbent. In this work three important models wereselected for evaluation purposes (i.e. Langmuir, Freundlich andDubininRadushkevich).

2.5.1. The Langmuir isothermTheLangmuir model is described by thefollowing equation[13]:

qe =qmaxKLCe1+ KLCe

(4)

where Ce (mgL1) and qe (mgg1) are the equilibrium concentra-tions in the liquid and solid phase, respectively; qmax is a Langmuirconstant that expresses the maximum metal uptake (mgg1) andKL is also a Langmuir sorption constant related to the sorption pro-cess energy and the affinity of the binding sites (L mg1).

Whenthe initial metal concentrationrises, adsorption increaseswhile the binding sites are not saturated. The linearized Langmuirisotherm allows the calculation of adsorption capacities and Lang-muir constants. This isotherm is given by the followingexpression:

Ceqe

= 1qmaxb

+ Ceqmax

(5)

Linear plots of Ce/qe vs. Ce show that adsorption may be wellpredicted by the Langmuir adsorption model.

2.5.2. Freundlich isothermThe Freundlich isotherm is an empirical model employed to

describe heterogeneous systems. The Freundlich equation is,

qe = KFC1/n

e (6)where Ce is the equilibrium concentration (mg L1), qe is theadsorbed amount (mg g1), KF and n are constants incorporatingall parameters affectingthe adsorption process, such as adsorptioncapacity and intensity,respectively [15]. The linearized form of Fre-undlich adsorption isotherm is used to evaluate the sorption dataand is represented as:

ln qe = ln KF+1

nln Ce (7)

KF and n are calculated from the intercept and slope of the Fre-undlich plots.

2.5.3. The DubininRadushkevich isotherm

TheDubininRadushkevich(DR)isothermis moregeneralthanthe Langmuir isotherm since it does not assume a homogeneoussurface or constant adsorption potential. This model is applied to


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816 J. Cruz-Olivares et al. / Chemical Engineering Journal 166 (2011) 814821

distinguish between thephysicaland chemical adsorption [16].TheDR isotherm equation [17] is:

qe = qme2

(8)

where isaconstantrelatedtothemeanfreeenergyofsorptionpermol of sorbent (mol2J2), qm is the theoretical saturation capacity

and is the Polanyi potential, which is equal to RTln(1+(1/ce)),where R (Jmol1 K1) is the gas constant and T(K) is the absolutetemperature.

The mean free energy E(kJmol1) of sorption per molecule ofadsorbate when it is transferred to the surface of the solid frominfinity in the solution can be calculated using the following rela-tionship [18]:

E= 12

(9)

This parameter gives information about the sorption mecha-nism, either chemical ion-exchange or physical sorption. If themagnitude of E is between 8 and 16kJmol1, the sorption pro-cess proceeds via chemical ion-exchange [19], while for values of

E


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Table 1

Kinetic parameters of the sorption of lead (II) onto residue of allspice at various temperatures.

T(C) Pseudo-first order Pseudo-second order Elovich equation

k1 (min1) q1 (mgg1) r2 k2 (gmg1 min1) q2 (mgg1) r2 (mgg1 min) (gmg1) r2

16 1.970 17.391 0.928 0.020 17.953 0.993 733.057 0.596 0.98525 1.737 19.608 0.917 0.018 20.325 0.998 1858.299 0.573 0.994

35 1.242 21.978 0.949 0.026 22.371 0.992 83331.374 0.695 0.983

JSM-5900 LV microscope to obtain information on the compo-sition and general features of the structures. Scanning electronmicroscopyprovidessecondary electron images of the surface withresolution in the micrometer range, while energy dispersive X-rayspectroscopy offers in situ chemical analysis of the bulk. Imageswere observed at 20kV. The chemical composition was determinedby a DX-4 analyzer coupled to the SEM, before and after contactwith the lead aqueous solution.

2.10. X-ray photoelectron spectroscopy

XPSanalysis of the biomass before andafter the lead adsorptionwas carried out on a JEOL spectrometer (JSP9200) with an Al X-raysource to determine the C, O, N and Pb atoms onto the surface.

3. Results and discussion

3.1. Sorption kinetics

The lead (II) adsorption kinetics was studied in the tempera-ture range of 1635 C. Equilibrium time for 16, 25 and 35 C wasfound to be less than 90 min (Fig. 1) indicating that the equilib-rium time is independent of temperature.It was observed that leadremoval by allspice residue is a typical sorption of metals, involv-ing metabolically biomass, where metal removal from solution is

purely due to the chemical or physical sorption, which reachesequilibrium relatively fast during the initial 030min followed bydiffusion into the adsorbent material which is particularly slower[28].

Fig. 2 depicts only the kinetic profile of lead (II) adsorption at25 C because similar performance was obtained for all tempera-tures. The relatively short contact time necessary to achieve theequilibrium condition is considered as an initial indication that theadsorption lead (II) could be a chemical reaction controlled ratherthan a diffusion controlled process [29].

Three kinetic models were used to fit experimental data. As itcan be seen in Table 1, pseudo-second order and Elovich equa-

Fig. 1. Effect of time and temperature on the adsorption capacity of Pb(II) ontoallspice residue (RA).

12

13

14

15

16

17

18

19

20

21

1009080706050403020100

q(mgg-1)

Time (min)

Experimental

Pseudo-first order

Pseudo-second order

Elovich

Fig. 2. Sorption kinetics of lead (II) onto residue of allspice (RA) at 25 C.

tion provide the best fitting to the experimental data and this isevidenced by the statistical parameters shown in Table 1.

The equilibrium adsorption capacities obtained with the

pseudo-second order model are much more reasonable than thoseof the pseudo-first order when comparing predicted results withexperimental data. According to pseudo-second order equationthere is a slightlydependence of the adsorption capacity with tem-perature and the maximum adsorption capacity was 22.37 mg g1

of Pb (II) on residue of allspice at 35 C. This adsorption capac-ity is not too different to many others as is show in Table 2. Thedependence of temperature is more evident in the initial adsorp-tionrate results withElovich equation.The Elovich equation,whichis based on a general second-orderreactionmechanism foradsorp-tion process, assumes that the active sites of the adsorbent areheterogeneous [30] and therefore exhibit different activation ener-gies for chemisorption. The constant in the Elovich equation isrelated to the rate of chemisorption. In this case, its value signifi-

cantly increases when increasing temperature. The other constant,, is related to the surface coverage, and remains practically con-stant.

Table 2

Reported adsorption capacities of different types of waste biomass for Pb(II).

Sorbent Adsorptioncapacity (mg g1)

Reference

Tea waste 2 Ahluwalia and Goyal [28]Saw dust 3 Shukla et al. [31]Bagasse fly ash 4 Gupta and Ali [32]Rice husk 11 Chuah et al. [33]Tree leaves 21 Baig [34]

Tree barks 21 Martin-Dupoint et al. [35]Residue of allspice 22 This workCoca shells 33 Meunier et al. [36]


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Table 3

Sorption isotherms constants for the sorption of lead (II) onto residue of allspice atdifferent temperatures.

16 C 25 C 35 C

Langmuirqmax (mgg1) 7.876 12.090 15.869KL (Lmg1) 0.232 0.468 1.102RL 0.147 0.079 0.035r2 0.982 0.995 0.998n 1.46 2.42 4.28FreundlichKF (mgg1) (Lmg1)1/n 0.016 0.252 1.648r2 0.989 0.986 0.984DubininRadushkevichqmax (mgg1) 0.669 2.107 5.809 (mg2 kJ2) 232.69 139.96 70.84r2 0.990 0.984 0.980E(kJmol1) 9.60 12.38 17.41

3.2. Equilibrium isotherm models

Table 3 shows the parameter sets obtained by fitting the exper-imental data to different models. The Langmuir isotherm providesaccurate fitting at high temperature while DubininRadushkevichisotherm is better at low temperature. Despite experimentaldata being reasonably fitted by Freundlich isotherm (r2 =0.98),the adsorption capacity value, qmax, obtained with Langmuirand DubininRadushkevich models, represents in a better waythe experimentally obtained adsorption capacity. In Langmuirisotherm, the equilibrium parameter RL, which is defined asRL =1/(1+ KLC0) in the range of 0< RL < 1, reflects a favourableadsorption process [37] where KL (Lmg1) is the Langmuirs con-stant and C0 (mgL1) is the initial adsorbate concentration. In thiswork, the equilibrium parameter (Table 3) was found to be in therange of 0< RL < 1 therefore indicating that the adsorption processwas favourable.

The Freundlich isotherm constants, KF and n, are constantsincorporating all factors affecting the adsorption process such asadsorption capacity and intensity of adsorption. The values of nbetween 1 and 10 (i.e. 1/n less than 1) represent a favourableadsorption [38]. The values of n, which reflects the intensity ofadsorption, also reflected the same trend. The n values obtainedfor the adsorption process represented a beneficial adsorption.

The values of the mean free energy E(kJmol1) calculated withDubininRadushkevich isotherm confirm that the lead (II) adsorp-tion process onto allspice residue is controlled by chemisorption.According to some authors [20] if E


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Table 6

External masstransfercoefficient for thesorptionof lead(II) ontoresidueof allspiceat different temperatures.

16 C 25 C 35 C

(m s1) 2.04E03 2.38E03 2.84E03

Table 7

Physical and chemical properties of allspice residue.

Parameters

Hemicellulose (%) 25.77Lignin (%) 28.64Cellulose (%) 30.22Ash (%) 3.44Moisture (%) 11.94Particle size (mm)


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Fig. 8. SEM and EDS of (a) residue of allspice before the contact with lead solution and (b) residue of allspice after the contact with lead solution.

Table 8

Elemental analysis of RA before the contact with Pb.

Element Weight% Atomic%

C K 56.01 63.29O K 42.74 36.26Al K 0.16 0.08K K 0.52 0.18Ca K 0.57 0.19

Totals 100

that theoxygen is interacting with the lead and that theoxygen hastwo plausible sources: hydroxyl group and ether group.

3.8. Scanning electron microscopy (SEM) analysis

The capacity of adsorption of allspice residue was corroboratedby thepresence of lead ions onto itssurface. Fig.8a and b shows the

SEMimage and theelemental analysis of adsorbentbefore andaftercontact with lead solutions, respectively. It canbe seen that rawall-spice residue contains aluminium, calcium and potassium and onlylead ions are present in allspice residue after the sorption process.This indicates that aluminium, calcium and potassium are trans-ferred to the aqueous solution. As shown in Table 8 the percentageof Al, Ca and K is quite low compared with C and O.

Regardingon the waylead ion is bound to the functional groupsof the adsorbent, it has been suggested [40], accordingto XPS stud-ies that lead ions can be found forming two complexes with theoxygen present in the cellulosic groups, being the CHOH the pre-dominant one.

4. Conclusions

The adsorption capacity of lead (II) on allspice is evident and itsvalue increases when the temperature is raised. According to the

kinetic model,the maximum adsorption capacity was 22.37mg g1

of Pb(II). Thermodynamic parameters showed that the adsorptionof lead (II) onto RA was feasible, spontaneous, and endothermicunder the studied conditions.

Diffusion results, the free energy value E (kJmol1) and XPSanalysis, confirm that the lead (II) adsorption process is controlledby chemisorption. The elemental analysis and FTIR results indicatethat lead is adsorbed and likely interacting with some functionalgroups of the cellulosic component.

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