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Chemical Engineering Journal 166 (2011) 814–821

Contents lists available at ScienceDirect

Chemical Engineering Journal

journa l homepage: www.e lsev ier .com/ locate /ce j

Thermodynamical and analytical evidence of lead ions chemisorption ontoPimenta dioica

J. Cruz-Olivaresa, C. Pérez-Alonsoa, C. Barrera-Díazb,∗, R. Natividadb, M.C. Chaparro-Mercadoc

a Universidad Autónoma del Estado de México, Facultad de Química, Paseo Colón intersección Paseo Tollocan S/N, C.P. 50120, Toluca, Estado de México, Mexicob Centro Conjunto de Investigación en Química Sustentable UAEM – UNAM, Carretera Toluca-Atlacomulco, km 14.5, Unidad El Rosedal, C.P. 50200, Toluca, Estado de México, Mexicoc Departamento de Ingenierías, Universidad Iberoamericana, Prol. Paseo de la Reforma 880, Lomas de Santa Fe, Álvaro Obregón, 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. Batch experiments were performed with aqueous lead solutions of concentration25 mg L−1, 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 22.37 mg g−1 of Pb (II) on residueof allspice (RA). This value was reached at 90 min and temperature of 308 K. Langmuir, Freundlich andDubinin–Radushkevich (D–R) adsorption isotherm models were applied 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, and endothermic under the studied conditions. The elementalanalysis from scanning electron microscopy (SEM) before and after the contact showed that lead wasadsorbed by RA. Diffusion results, the value of the free energy E (kJ mol−1), XPS and FTIR analysis confirmedthat 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 the cellulosic materials is not well established. In some cases theadsorption process is governed by physical phenomena or chemical

∗ Corresponding author. Tel.: +52 722 296 5514; fax: +52 722 296 5541.E-mail address: cbd0044@yahoo.com (C. Barrera-Díaz).

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 reaches1500 tonnes of biosorbent in Mexico and currently the final disposalof 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.

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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 firstlywashed with diluted nitric acid (0.1 M) solution and then withethanol (99.9 purity) in order to eliminate colouring and remain-ing substances. It was then dried at 60 ◦C for 24 h 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 L−1.These solutions 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 stirring shaker (Lab-Line Incubator-Shaker, USA) at 200 rpm.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 90 min) and temperatures (16, 25 and 35 ◦C). 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 aqueousmetal concentration using the standard method of lead detection 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:

dqtdt

= k1(qe − qt) (1)

where qe and qt (mg g−1) are the amount of sorbate at equilibriumand at time t (min), respectively, and k1 (min−1) 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(g mg−1 min−1).

2.4.3. The Elovich equationThe 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

= ˛e−ˇqt (3)

where˛ (mg g−1 min−1) is the initial adsorption rate andˇ (g mg−1)is related to the extent of surface coverage and the activation energyinvolved 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 andDubinin–Radushkevich).

2.5.1. The Langmuir isothermThe Langmuir model is described by the following equation [13]:

qe = qmaxKLCe1 + KLCe

(4)

where Ce (mg L−1) and qe (mg g−1) are the equilibrium concentra-tions in the liquid and solid phase, respectively; qmax is a Langmuirconstant that expresses the maximum metal uptake (mg g−1) andKL is also a Langmuir sorption constant related to the sorption pro-cess energy and the affinity of the binding sites (L mg−1).

When the initial metal concentration rises, 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 following expression:

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/ne (6)

where Ce is the equilibrium concentration (mg L−1), qe is theadsorbed amount (mg g−1), KF and n are constants incorporatingall parameters affecting the 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 + 1n

ln Ce (7)

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

2.5.3. The Dubinin–Radushkevich isothermThe Dubinin–Radushkevich (D–R) isotherm is more general than

the Langmuir isotherm since it does not assume a homogeneoussurface or constant adsorption potential. This model is applied to

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distinguish between the physical and chemical adsorption [16]. TheD–R isotherm equation [17] is:

qe = qme−ıε2(8)

where ı is a constant related to the mean free energy of sorption permol of sorbent (mol2 J−2), qm is the theoretical saturation capacityand ε is the Polanyi potential, which is equal to RT ln(1 + (1/ce)),where R (J mol−1 K−1) is the gas constant and T (K) is the absolutetemperature.

The mean free energy E (kJ mol−1) 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 = 1√2ı

(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 16 kJ mol−1, the sorption pro-cess proceeds via chemical ion-exchange [19], while for values ofE < 8 kJ mol−1, the sorption process is mainly of physical nature [20].

2.6. Thermodynamic parameters

Thermodynamic parameters such as free energy (�G0),enthalpy (�H0), and entropy (�S0) change of adsorption can beevaluated from the following equations [21,22]:

Kd = qeCe

(10)

where Kd is the sorption distribution coefficient. The Kd values areused to determine the�G0,�H0, and�S0:

�G0 = −RT ln Kd (11)

�G0 (J mol−1) is the free energy of adsorption, T (K) is the absolutetemperature, and R is the universal gas constant.

The Kd constant may be expressed in terms of the�H0 (J mol−1)and�S0 (J mol−1 K−1) as a function of temperature:

ln Kd = �H0

RT+ �S0

R(12)

the values of �H0 and �S0 can be calculated from the slope andintercept of the ln Kd vs. 1/T plot.

2.7. Diffusion models

The uptake of adsorbate by the adsorbent is carried out by diffu-sion or mass transfer process. In this process three steps take placeand apply to lead removal [23,24]:

Step 1. Lead transfer from the boundary liquid film to the surfaceof the solid.

Step 2. Lead transfer from the surface to the intra-particle activesites.

Step 3. Metal ion uptake by active sites. This process may occurvia complexation, sorption or intra-particle precipitationphenomena.

Step 1 describes film mass transfer resistance. Step 2 is relatedto the intraparticle diffusion model. Step 3 is fast and in some casesnon-limiting stage.

Various models of diffusion have been examined, includingsingle steps of external or intraparticle diffusion or combined phe-nomena [25].

2.7.1. External mass transfer diffusion modelThis model, as an application of the Fick’s law, describes the

evolution of the solute concentration in the solution C (mg L−1), asa function of the difference in the concentrations of the metal ionin the solution, C and at the particle surface, Cs (mg L−1), accordingto the following equation [26]:

dC

dt= − S(C − Cs) (13)

where is the mass transfer coefficient (m s−1) and S is the surfacearea of the adsorbent per solution volume (m−1). The coefficientsare determined after making some assumptions such as surfaceconcentration Cs being negligible at t = 0, the concentration in solu-tion tending to the initial concentration C0 and also intraparticlediffusion being negligible. So the previous equation can be simpli-fied to,[dC/C0

dt

]t→0

= − S (14)

The initial rate of sorption, − S (s−1), is obtained by polynomiallinearization of C/C0, and subsequent derivation at t = 0.

In this model, the surface area is approximated to the externalsurface area. Moreover, the particles are assumed spherical and Sis calculated as the external surface compared to the solid/liquidratio in solution:

S = 6mdp�app

(15)

where m is the sorbent mass concentration in the solution (g m−3),dp the particle size diameter (m) and �app the apparent volumemass of the sorbent (g m−3).

2.7.2. Intraparticle mass transfer diffusion modelAccording to the intraparticle diffusion model proposed by

Weber and Morris, which has been used in several studies [21,26],the initial rate of intraparticular diffusion is calculated by lineariza-tion of the curve represented by:

q = Kit0.5 (16)

where q is the amount of adsorbed metal ion onto the adsorbent atany time t (mg g−1), t is time (s) and Ki is the diffusion coefficientin the solid (mg g−1 s−0.5).

Another kind of intraparticle diffusion model was proposed byUrano and Tachikawa [27]. In this model, the adsorption rate is con-sidered as independent of the stirring speed and external diffusionis assumed negligible relative to the low overall adsorption rate.The model to obtain the diffusion coefficient is:

− ln

[1 −

(q

qe

)2]

= 4�2Dit

d2p

(17)

where Di is the diffusion coefficient in the solid (m2 s−1).

2.8. Fourier transform infrared spectroscopy

Infrared spectra of the biomass before and after the contactexperiments were obtained using a Fourier transform infraredspectrometer (FTIR Nicollet AVATAR 360). For the FTIR study, 30 mgof finely ground biomass was encapsulated in 30 mg of KBr (Sigma)in order to prepare translucent sample disks. This analysis allowsto observe the changes in the chemical bonds and to elucidate thechemical groups involved in the metal sorption process.

2.9. Characterization (SEM and EDX)

The SEM characterization was carried out on biomass sam-ples before and after the contact with the lead ions using a JEOL

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Table 1Kinetic 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 (min−1) q1 (mg g−1) r2 k2 (g mg−1 min−1) q2 (mg g−1) r2 ˛ (mg g−1 min) ˇ (g mg−1) 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 0.99435 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 electronmicroscopy provides secondary 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 20 kV. 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

XPS analysis of the biomass before and after 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 16–35 ◦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 ispurely due to the chemical or physical sorption, which reachesequilibrium relatively fast during the initial 0–30 min 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 (m

g g-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 thepseudo-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 slightly dependence of the adsorption capacity with tem-perature and the maximum adsorption capacity was 22.37 mg g−1

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-tion rate results with Elovich equation. The Elovich equation, whichis based on a general second-order reaction mechanism for adsorp-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,ˇ, is related to the surface coverage, and remains practically con-stant.

Table 2Reported adsorption capacities of different types of waste biomass for Pb(II).

Sorbent Adsorptioncapacity (mg g−1)

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 3Sorption isotherms constants for the sorption of lead (II) onto residue of allspice atdifferent temperatures.

16 ◦C 25 ◦C 35 ◦C

Langmuirqmax (mg g−1) 7.876 12.090 15.869KL (L mg−1) 0.232 0.468 1.102RL 0.147 0.079 0.035r2 0.982 0.995 0.998n 1.46 2.42 4.28FreundlichKF (mg g−1) (L mg−1)1/n 0.016 0.252 1.648r2 0.989 0.986 0.984Dubinin–Radushkevichqmax (mg g−1) 0.669 2.107 5.809ˇ (mg2 kJ−2) 232.69 139.96 70.84r2 0.990 0.984 0.980E (kJ mol−1) 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 Dubinin–Radushkevichisotherm is better at low temperature. Despite experimentaldata being reasonably fitted by Freundlich isotherm (r2 = 0.98),the adsorption capacity value, qmax, obtained with Langmuirand Dubinin–Radushkevich 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 (L mg−1) is the Langmuir’s con-stant and C0 (mg L−1) 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 (kJ mol−1) calculated withDubinin–Radushkevich isotherm confirm that the lead (II) adsorp-tion process onto allspice residue is controlled by chemisorption.According to some authors [20] if E < 8 kJ mol−1, the adsorptionprocess is of physical nature, but if 8 < E < 16 kJ mol−1 then it is achemical phenomenon.

3.3. Thermodynamic parameters

The values of thermodynamic parameters such as free energy ofadsorption (�G0), heat of adsorption (�H0), and standard entropychanges (�S0) provide information about the nature of removallead process. The results are given in Table 4. The Gibbs free energyindicates the degree of spontaneity of the sorption process. �H0

and �S0 were obtained from the slope and intercept of a plot ofln Kd against 1/T (Fig. 3).

Table 4Thermodynamic parameters for the sorption of lead (II) onto residue of allspice.

�G0 (J mol−1) �H0 (J mol−1) �S0 (J mol−1 K−1)

16 ◦C 25 ◦C 35 ◦C

−2341.933 −3958.712 −5541.064 46321.451 168.417

0.50

1.00

1.50

2.00

2.50

3.503.453.403.353.303.253.20

lnKd

1000/T(K-1)

Fig. 3. A plot of ln Kd vs. 1/T for the adsorption process of Pb(II) on RA.

r² = 0.9216

r² = 0.945

r² = 0.9062

10.0

12.0

14.0

16.0

18.0

20.0

22.0

24.0

10.08.06.04.02.0

q(m

g g-1

)

t^0.5

16°C

25°C

35°C

Fig. 4. Intraparticle diffusion plot for the adsorption process of Pb(II) on RA at dif-ferent temperatures.

When temperature was increased from 16 to 35 ◦C, the mag-nitude of free energy change shifted to a high negative valuesuggesting that the adsorption was energetically favourable, rapidand spontaneous [39]. The positive value of adsorption heat(46.32 kJ mol−1) confirmed the endothermic nature of adsorp-tion process and the positive value of standard entropy changessuggested randomness increase at the adsorbent–solute interfaceduring the adsorption process [14].

3.4. Diffusion models

As shown in Fig. 4 and Table 5, the intraparticle models did notprovide a good fitting to the experimental data. This fact couldconfirm that the occurring adsorption process is controlled by apossible chemical reaction (complexation or ion exchange) ratherthan by a physical phenomenon.

Table 5Intraparticle diffusion parameters for the sorption of lead (II) onto residue of allspiceat different temperatures.

T (◦C) Ki (mg g−1 s−1/2) r2 Di (m2 s−1)

16 0.632 0.922 5.61E−0925 0.663 0.945 6.91E−0935 0.538 0.906 8.14E−09

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Table 6External mass transfer coefficient for the sorption of lead (II) onto residue of allspiceat different temperatures.

16 ◦C 25 ◦C 35 ◦C

(m s−1) 2.04E−03 2.38E−03 2.84E−03

Table 7Physical and chemical properties of allspice residue.

Parameters

Hemicellulose (%) 25.77Lignin (%) 28.64Cellulose (%) 30.22Ash (%) 3.44Moisture (%) 11.94Particle size (mm) <0.836Bulk density (g cm−3) 0.42

Moreover, the low values of Ki and Di coefficients suggest thatthe intraparticle diffusion is almost negligible in comparison withthe external mass transfer phenomena as it can be seen in Table 6.

3.5. Physical and chemical analysis of the adsorbent

The physical and chemical properties of the adsorbent are sum-marized in Table 7. The high content of cellulosic molecules couldbe responsible of adsorption process. These components containfunctional groups such as carbonyl, methyl, and hydroxyl groupsthat interact with the metal ions present in aqueous solutions. TheXPS analysis suggests that the lead is forming 2 complexes withthe oxygen present in the cellulosic groups [40]. The first complexcorresponds to a lead oxygen interaction of the group CH2–OH andwith oxygen of the ether group. The second indicates the lead oxy-gen interaction of the CH–OH; these groups are abundant in thesorbent.

3.6. FTIR analysis

In order to determine which functional groups were respon-sible for the metal uptake, a FTIR analysis of the biosorbent wasperformed before and after the contact with lead solutions.

As shown in Fig. 5, the spectra display a number of adsorptionpeaks. The identified adsorption bands in the spectra are similar toother cellulosic biomaterials [41,42]. The 3286.7 cm−1 wave num-ber indicates the presence of –OH groups. The intensity of this banddecreased only in the FTIR spectrum of the lead (II)-loaded sorbent.The change in the intensity in the spectrum indicates that this groupis involved in the lead (II) sorption.

70

75

80

85

90

95

100

105

60010001400180022002600300034003800

Tra

nsm

itanc

e

Wavenumbers (cm-1)

BA

Fig. 5. FTIR spectra of the biomaterial (A) after and (B) before the contact with25 mg L−1 lead solution, at pH 5.

50

150

250

350

450

550

650

750

850

540535530525520

cps

BE(eV)

Fig. 6. XPS signal with oxygen 1s before the contact with the lead ions. Total signal(=), oxygen corresponding to C–OH (– –) and oxygen of the ether group (C–O–C) (—).

The strong absorption peaks at 2846.9 cm−1 and 2916.4 cm−1

are indicative of symmetric and asymmetric stretching vibrationsof the –CH3 and –CH2 groups. The band at 1604.7 cm−1 corre-sponds to C O bond and has been slightly displaced to 1600.9 cm−1

after the sorption process. Another shift of band is observed at1296.2 cm−1 and is due to O–C–O interactions. This signal peakrepresents lead interaction with oxygen from cellulosic compo-nents. The 1085 cm−1 band is due to C–O stretching of carbonylgroups and the bending vibration of hydroxyl groups for unloadedsorbent. However, the disappearance of this band after the lead con-tact suggests that these functional groups are likely to participatein metal binding. The wave number observed at 1018.4 cm−1 canbe ascribed to the presence of C–O bond in carboxylic and alcoholicgroups.

3.7. XPS results

Fig. 6 shows the XPS oxygen signal. It is possible to note thatthere are two deconvolutions corresponding to the covalent unionof C–OH and the ether C–O–C. As expected the hydroxyl group sig-nal is more intense. After the contact of the biosorbent with thelead ions, the XPS oxygen signal is modified as depicted in Fig. 7.The signal now is deconvoluted in four peaks. These peaks indicatethat the lead is forming a complex with the oxygen, since the dis-placements do not correspond to a covalent union. This evidences

Fig. 7. XPS signal with oxygen 1s after the contact with the lead ions. Total signal(=), oxygen corresponding to C–OH (– –), oxygen of the ether group (C–O–C) (+),lead–oxygen interaction with the C–OH (– � –) and lead-oxygen interaction withC–O–C (—).

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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 8Elemental 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 the oxygen is interacting with the lead and that the oxygen hastwo plausible sources: hydroxyl group and ether group.

3.8. Scanning electron microscopy (SEM) analysis

The capacity of adsorption of allspice residue was corroboratedby the presence of lead ions onto its surface. Fig. 8a and b shows theSEM image and the elemental analysis of adsorbent before and aftercontact with lead solutions, respectively. It can be seen that raw all-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.

Regarding on the way lead ion is bound to the functional groupsof the adsorbent, it has been suggested [40], according to XPS stud-ies that lead ions can be found forming two complexes with theoxygen present in the cellulosic groups, being the CH–OH 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 22.37 mg g−1

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 (kJ mol−1) 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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