Journal of Colloid and Interface Science 386 (2012) 325–332

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On the use of a natural peat for the removal of Cr(VI) from aqueous solutions

Ana León-Torres, Eduardo M. Cuerda-Correa ⇑, Carmen Fernández-González, María F. Alexandre Franco,Vicente Gómez-SerranoDepartment of Organic and Inorganic Chemistry, Faculty of Sciences, University of Extremadura, Avda. de Elvas, s/n, E-06006 Badajoz, Spain

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 May 2012Accepted 12 July 2012Available online 2 August 2012

Keywords:Natural peatChromium (VI) removal

0021-9797/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcis.2012.07.038

⇑ Corresponding author.E-mail address: emcc@unex.es (E.M. Cuerda-Corre

A natural peat has been used as an adsorbent for the removal of hexavalent chromium from aqueoussolution. The peat was firstly characterized in terms of particle size and chemical composition (ash con-tent, pH of the point of zero charge, FT-IR and thermal analysis). Next, the kinetic and equilibrium aspectsof the adsorption of Cr(VI) by this adsorbent were studied. The kinetic data were satisfactorily fitted to akinetic law of partial order in C equal to one. The specific adsorption rates are around 10�4 s�1, increasingas temperature does. A noticeable influence of diffusion on the global adsorption process has been dem-onstrated. Finally, the equilibrium isotherms were satisfactorily fitted to a previously proposed model.The adsorption capacity of Cr(VI) was similar to some other previously reported and the affinity of Cr(VI)towards the active sites of the adsorbent increases as temperature rises.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

A great variety of versatile adsorbents are used at present forthe removal of pollutants from wastewater. Silica gel, alumina,activated carbons and synthetic exchange resins are among themost widely used adsorbents [1]. In addition to adsorption, someother processes such as chemical precipitation [2], membrane fil-tration [3], ion exchange [4] and biosorption [5] are to be men-tioned. A bibliographic review (see [6,7] and references therein)reveals that the development of low-cost adsorbents has receiveda great deal of attention. Among these latter, peat is one of themost versatile materials to be used for the removal of pollutantsfrom wastewater. Peat is the generic name of a recently formedmineral produced by the incomplete combustion of vegetal organicmatter. Presence of clean water and wet air as well as absence ofoxygen is required for the formation of peat. Based on the natureof parent materials, peat is classified into four groups, namely mosspeat, herbaceous peat, woody peat and sedimentary peat [8].

On the other hand, although small amounts of chromium –mainly as Cr(III) – are essential for human life, many Cr(VI) speciesare well-known carcinogens and extremely toxic. Consequently,the removal of Cr(VI) compounds from wastewater is a topic ofcurrent concern. A wide variety of materials have been tested withsuch an aim. However, many of these materials are either syntheticadsorbents (e.g., Amberlite ion-exchange resins [9], functionalizedsilica materials [10], supported biofilms [11], modified natural fi-bers [12] or chemically-modified biomass [13]). In both cases the

ll rights reserved.

a).

cost of the material used for the removal of Cr(VI) is a serious dis-advantage from a practical standpoint.

In this work, a natural peat has been tested as a potential adsor-bent of hexavalent chromium with promising results.

2. Materials and experimental procedure

2.1. Characterization of the adsorbent

The peat used as the adsorbent in the present study was kindlyprovided by Biomatrix Gold (Mérida, Spain). This material is a com-mercial product that is currently being used in a successful mannerfor the removal of organic pollutants in wastewaters. In this work,the ability of the peat for removing inorganic anions from aqueoussolutions has been analyzed. In order to determine the physicaland chemical properties of the adsorbent the peat has been charac-terized in terms of granulometry, ash content, thermal behavior,pH of the point of zero charge and surface chemistry.

The granulometric study of the sample was carried out with theaid of a sieves tower CISA RP.09, by weighing the quantity of sam-ple retained in each sieve. Stainless steel sieves, having mesh sizescomprised between 3.350 and 0.053 mm were used. The tower wasplaced on a vibrating table. The analysis time was equal to 30 min.The resulting values allow one to obtain the experimental particlesize distribution (PSD) curves. These represent the percentages byweight vs. particle size.

The ash content was determined in triplicate after heating thesample in a ceramic crucible inside an oven maintained at 650 �Cfor 12 h. The solid residue in the crucible once elapsed such timeis considered as the ash content. The pH of the point of zero charge

Table 1Granulometric analysis of the natural peat.

Mesh size, lm Fraction, g Fraction, % % Cumulative weight, under

53 1.62 3.39 3.3975 5.68 11.85 15.24

150 3.44 7.18 22.42212 3.84 8.02 30.43300 6.01 12.53 42.96425 5.23 10.91 53.88600 5.77 12.04 65.92850 8.48 17.68 83.59

1700 6.73 14.04 97.633350 1.14 2.37 100.00

326 A. León-Torres et al. / Journal of Colloid and Interface Science 386 (2012) 325–332

(pHPZC) is considered as the pH at which the electric charge densityof a given surface equals zero. In other words, it is the pH value atwhich a solid submerged in an electrolyte exhibits a surface netelectric charge equal to zero. For the determination of this param-eter an operative method previously described in the literaturewas used [14]. An infrared spectrum of the sample was performedfollowing a procedure previously described elsewhere [15]. A Per-kin–Elmer 1720 spectrophotometer was used to register the FT-IRspectrum in the wavelength range comprised between 4000 and500 cm�1. TG and DSC analyses were performed with the aid of aTA Instruments SDT Q600 equipment. A fixed amount of sample(approximately 20 mg) was placed in the crucible and next heatedfrom room temperature up to 1000 �C with a heating rate of10 �C min�1 under an air flow equal to 100 mL min�1.

2.2. Study of the adsorption kinetics of Cr(VI)

Fixed amounts of peat (0.1 ± 0.0001 g) were kept in contactwith 20 mL of aqueous solution of potassium dichromate of initialconcentration equal to 4 � 10�3 M under 50 rpm stirring. Kineticexperiments were performed at four different temperatures,namely 15, 25, 35 and 45 �C in a Selecta Unitronic-OR thermostaticbath. The concentration of Cr(VI) in the solution was determinedafter a preset time interval with the aid of a Thermo Electron S Ser-ies atomic absorption spectrophotometer (AAS).

2.3. Definition of the adsorption equilibrium isotherms of Cr(VI)

20 mL of Cr(VI) solution of initial concentration equal to4 � 10�3 M were kept in contact with different amounts of peat(comprised between 0.01 and 0.5 g) for a period of time longerthan that strictly necessary for reaching the equilibrium. Sampleswere maintained in the Unitronic-OR bath under the same condi-tions of temperature and stirring described above. The Cr(VI) con-centration in the supernatant was also determined by AAS.

3. Results and discussion

3.1. Characterization of the adsorbent

The application of mathematical models to the experimentaldata of PSD curves makes it possible to obtain the correspondingdistribution and density functions. A bibliographic revision revealsthat the most commonly used models are those proposed by Ro-sin–Rammler, RR [16], and Gates–Gaudin–Schuhmann, GGS [17].The RR model is particularly suited to represent powders obtainedby grinding, milling and crushing operations. In many grindingcases, mainly during the last stages of the process, the cumulativeparticle size distribution is found to follow the RR equation, theparameters of which can be used as essential criteria to the opera-tion progress [18]. This model has been applied to a number of par-ticulate systems such as, for instance, stream sediments and minetailings [19], Bovine Serum Albumin (BSA)-based microspheres[20], circulating fluid bed (CFB) boiler ashes [21], and pharmaceu-tical aerosols [22]. The distribution function provided by the RRmodel is:

Fð/Þ ¼ 1� exp � /l

� �m� �ð1Þ

Or, in linearized form

Lnð�Lnð1� Fð/ÞÞÞ ¼ mLnð/Þ �mLnðlÞ ð2Þ

where F(/) is the distribution function, / is the particle diameter, lis the average particle size and m is a characteristic parameter of thegranulometric distribution. The experimental data are fitted to

Eq. (2) in order to test the applicability of the RR distributionfunction to the PSD as well as to calculate its fitting parameters.The correlation coefficient may be used as the parameter forgoodness of fit.

The density function in the RR model will be:

f ð/Þ ¼ mlm /m�1 exp � /

l

� �m� �ð3Þ

The use of the RR model may provide valuable help to carry outthe modeling during the design phase of milling circuits. For in-stance, it facilitates making correct use of the particle sizes to ob-tain more homogeneous cork agglomerate samples in the corkindustry [23].

Table 1 summarizes the values of the weights of the differentparticle sizes obtained in the sieving operations of the peat sample,as well as the cumulative percentages by weight. The correspond-ing PSD curve is depicted in Fig. 1, which shows that the sievedmass is retained in a progressive manner in the different sieves,although most of the sample is retained in the sieves of mesh sizecomprised between 300 and 850 lm.

On the other hand, the number of particles (expressed as parti-cle mass or volume) whose diameters are comprised between twosizes (i.e. /1 and /2) is given by the area under the curve in thatinterval:

Fð/2Þ � Fð/1Þ ¼Z /2

/1

f ð/Þ � dð/Þ ð4Þ

The presence of a PSD curve analogous to that shown in Fig. 1indicates that the material under study is well-suited to be ana-lyzed by using this type of model.

The RR model was applied to the experimental data given inTable 1 and depicted in Fig. 1 by fitting such data to the linearizedform of the RR model given by Eq. (2). The value of the correlationcoefficient (R2 = 0.9895) suggests that the RR model provides a verygood fitting for the PSD of the sample here studied. In fact, the RRmodel usually fits reasonably well the PSD curves of different kindsof materials. Furthermore, the RR model is the most commonlyused function for approximation of the PSD in Europe and severalstudies in the field prove that this formula can give the best ap-proach to the PSD of the mass of ground industrial products.

The resulting distribution function obtained by application ofRR model is given by the following expression:

Fð/Þ ¼ 1� exp � /516

� �1:03" #

ð5Þ

Thus, the average particle size of the peat resulted to be516 lm. Eq. (5) has been used to plot the solid lines depicted inFig. 1. It may be observed that the predicted values (solid line) sat-isfactorily fit the experimental data.

Once the average particle size (i.e., 516 lm) was determined,the mass fraction retained between the 425 and 600 lm mesh

( m)0 500 1000 1500 2000 2500 3000 3500

F(

)

0.0

0.2

0.4

0.6

0.8

1.0

Experimental data

Calculated F( )

Fig. 1. Particle size distribution of the natural peat sample.

A. León-Torres et al. / Journal of Colloid and Interface Science 386 (2012) 325–332 327

sizes was used as the adsorbent in order to discard the minimumamount of sample.

On the other hand, the density function obtained by applyingthe RR model to the sample under study is given by the followingequation:

f ð/Þ ¼ 1:65� 10�3/0:03 exp � /516

� �1:03" #

ð6Þ

The application of the distribution and density functions to the PSDdata allows one to extrapolate (with estimative purpose only) thepercentage of material smaller than a certain particle diameter (/), at points that do not correspond to the sieving classification sys-tem used, thus obtaining information at the extremes of the particlesize diagrams.

The ash content of the sample is only 3.39 ± 0.22%. This fact sug-gests that the inorganic matter content of the peat is very low,which is consistent with a low mineralization rank of the sample.The pH of the point of zero charge of the sample is approximatelyequal to 4.0. Thus, at operating pH below 4 the surface of the adsor-bent will be positively charged and the sample will tend to adsorbanions. On the contrary, at pH above 4 the adsorption of cationswill be favored. This is graphically illustrated in Fig. 2.

Fig. 2. Schematic representation of the surface charge of a carbonaceou

Fig. 3 (left) shows the FT-IR spectrum of the natural peat sam-ple. The FT-IR analysis reveals the existence of a well-developedaromatic structure as well as of a wide variety of oxygen groups(e.g., carbonyl, carboxyl and ether) in the surface of the peat. Thisis corroborated by the temptative assignation of the bands thathas been summarized in Table 2.

Furthermore, the application of thermogravimetric and differ-ential thermal analysis techniques shows the existence of threemass loss effects (see Fig. 3, right). Firstly, at a temperature com-prised between 50 and 150 �C an endothermic effect suggests theloss of moisture. The second effect, which is strongly exothermicin nature, ranges from 225 up to 300 �C and is compatible withthe pyrolysis of the material. Finally, a third slightly exothermic ef-fect is observed between 300 and 550 �C and is attributable to thegasification of the partially carbonized material. At temperaturesabove 550 �C no significant mass variation is observed.

3.2. Study of the adsorption kinetics of Cr(VI)

The C vs. t plots corresponding to the removal of Cr(VI) by thepeat here used are shown in Fig. 4 (left).

Such plots suggest that the process occurs in a reversible man-ner until reaching equilibrium between the solid and liquid phases.In all cases the equilibrium time is observed to be of approximately100 h. The adsorption kinetics has proved to respond to a kineticlaw of partial order in C equal to one. As a consequence, the exper-imental results have been fitted to Eq. (7) which results from theintegration of such kinetic law as previously described elsewhere[24]. This equation is

C ¼ C0 þ ka � Ce � t1þ ka � t

ð7Þ

where C0 is the initial concentration of Cr(VI) in solution; ka is thespecific adsorption rate constant; Ce is the equilibrium concentra-tion and t is time. The values of ka calculated from the experimentaldata are summarized in Table 3.

The values of ka are, in general, around 10�4 s�1 and increaseprogressively as the operation temperature grows. This trendmay also be deduced from the shape of the C vs. t plots since theinitial portion of such plots becomes steadier with increasing tem-perature. From the values of the specific adsorption rate the activa-tion thermodynamic functions of the adsorption process have beencalculated according to the transition state theory:

s adsorbent in connection with the pH of the point of zero charge.

4000 3500 3000 2500 2000 1500 1000 500

469529591

900

1058

1161

12581398

1519

1622

17142922

T (ºC)0 200 400 600 800 1000

m (m

g)

0

2

4

6

8

10

12

14

16

18

DTA

(%/ºC

)

-0.5

0.0

0.5

1.0

1.5

2.0

TG plotDTA plot

Tran

smitt

ance

, a.u

.

-1Wavenumber, cm

Fig. 3. FT-IR spectrum (left) and TG-DTA plots (right) of the natural peat sample.

Table 2Temptative assignation of bands in the FT-IR spectrum of natural peat.

Band position/cm�1 Assignation Functional group

�3400 m(OAH) Hydroxyl (R–OH)2922 mas(CH2) Alkanes (R–(CH2) n-R0)1714 m(C@O) Carboxylic acids, ketones1622 m(C@C) Olefins, aromatic nuclei1519 m(C@C) Skeletal vibration (aromatic and benzene rings)1398 dsy(CH3) �C (CH3)3

1258 mas(CAOAC) Esters, ethers, epoxides1161(shoulder)–1058 m(CAO) Hydroxyl and ether groups900 d(CAH) Aromatic groups591–469 c(OAH) Hydroxyl groups

Key: m, stretching vibration; d, bending vibration (in-plane); c, bending vibration (out-of-plane); sy, symmetric; as; asymmetric.

Fig. 4. Adsorption kinetics (left) and isotherms (right) of Cr(VI) in aqueous solution at different temperatures.

Table 3Specific rate constant and diffusion parameters of the adsorption process.

T (�C) ka � 10+4 (s�1) DH⁄ (kJ mol�1) DS⁄ (kJ mol�1 K�1) D � 10+7 (cm2 s�1) E�a ðkJ mol�1Þ D0 � 10+7 (cm2 s�1)

Ban. C–H Ban. C–H Ban. C–H

15 3.33 0.94 1.8925 6.39 1.66 2.4335 9.01 34.9 �0.19 2.21 3.70 31.9 30.5 5.8 5.845 15.3 3.45 6.28

328 A. León-Torres et al. / Journal of Colloid and Interface Science 386 (2012) 325–332

t (s) ·10-3

0 50 100 150 200 250

D (c

m2 /s

)·10+7

0

1

2

3

4

5

6

7

T = 15ºCT = 25ºCT = 35ºCT = 45ºC

A. León-Torres et al. / Journal of Colloid and Interface Science 386 (2012) 325–332 329

ka ¼R � TN � h � e

DS�Rð Þ � e �DH�

R�Tð Þ ð8Þ

where R is the molar constant of gases, h is Planck’s constant and Nis Avogadro’s number. Thus, a plot of Ln (ka/T) vs. 1/T (not shown forthe sake of brevity) leads to a straight line. From the slope and inter-cept of such line the activation enthalpy (DH⁄) and entropy (DS⁄)may be calculated, respectively. Such values have also been in-cluded in Table 3.

It may be observed that the formation of the activated speciesthrough which the adsorption process takes place is endothermicand exoentropic in nature, that is, there is a loss of degrees of free-dom when the activated complex is formed.

3.2.1. Influence of diffusion on the adsorption processIn order to analyze the influence of diffusion on the global

adsorption process, the diffusion coefficients have been calculatedat each temperature by applying two methods previously reportedin the literature, namely those proposed by Banerjee, Ban. [25], andCarman and Haul, C–H [26]. Both of them are based in the integra-tion of the Fick’s second law and assume certain approximationsregarding the shape and size of the particles as well as the shapeof the pores of the adsorbent. Thus, the values of diffusion coeffi-cients must be considered as only orientative.

Fick’s second law is given by the equation:

dCdt¼ D � d2C

dx2

!ð9Þ

where x = distance covered by the diffused species at a time t.D = diffusion coefficient. C = concentration of the chemical adsorbedinto the pore, at a distance (x) from its entrance, at a time t.

The value of the diffusion coefficient (D) depends on both, tem-perature and the so named ‘‘frequency factor’’ (D0) which is the va-lue that would take D at T =1K. The relationship between D and D0

is given by an Arrhenius-type exponential law,

D ¼ D0 � e�EaR�T ð10Þ

where Ea is the activation energy of the diffusion process.The integration of Eq. (9) leads to different equations depending

on the type of solid and the adsorbable molecule and the previ-ously fixed conditions to carry out the integration. For this reasona number of tentative methods are used which give rise to severalequations providing very similar information.

Among the most widely used integrated equations to determinethe value of D from the kinetic data, it is worth noting the Banerjeeapproximation [25],

h ¼ nst

nse¼ 1� 6

p2 � exp �p2 � Dr2 � t

� �ð11Þ

where nst = moles of solute adsorbed per mass unit of sorbent at a

given time, t; nse = moles of solute adsorbed per mass unit of sorbent

at the equilibrium, t = te; D = diffusion coefficient; r = radius of theparticle – supposed spherical – through which diffusion takes place(calculated from the Rosin–Rammler equation).

On the other hand, Carman and Haul [26] proposed anotherequation that makes it possible to calculate the diffusion coeffi-cient as a function of the coverage fraction, h,

h ¼ nst

nse

¼ ð1þ aÞ � 1� m1

m1 þ m2� e:erfc

3 � m1

a� D � t

a2

� �1=2" #( )

� ð1

þ aÞ � m2

m1 þ m2� e:erfc

�3 � m2

a� D � t

a2

� �1=2" #( )

ð12Þ

where

m1 ¼12� 1þ 4 � a

4

� �1=2

þ 1

" #ð13Þ

m2 ¼ m1 � 1 ð14Þe:erfcðZÞ ¼ expðZÞ2 � erfcðZÞ ð15Þ

a ¼ 3 � V4 � p � r3 ð16Þ

By applying the Carman–Haul Eq. (12) to the experimental ki-netic data, the values of D depicted in Fig. 5 have been calculated.

Firstly, it is worth noting that the D vs. t plots depicted in thisfigure reveal an increase in the diffusion coefficient as the opera-tion temperature rises. From the shape of these plots it may beconcluded that the diffusion of the solute in the liquid–solid inter-face does not appear to be important. This is probably due to thecontinuous stirring of the system along the experiments. As a con-sequence, the first ascending tract of the D vs. t plot is attributableto the beginning of the diffusion of the solute inside the pores ofthe adsorbent which results in a displacement of solvent moleculesthat were previously occupying such pores. Once this process be-gins effectively in the larger pores, diffusion reaches its maximumintensity and the highest value of D is reached. From that moment,as the at least partially solvated Cr(VI) species access the pores thediffusion is progressively hindered and the value of D decreases.

For comparative purpose with the method proposed by Banerjee,the value of the maximum in the D vs. t plot has been considered asthe diffusion coefficient from the Carman–Haul model. Values of Dobtained by both methods have been included in Table 3 and reachapproximately 10�7 cm2 s�1, increasing as temperature does.

By applying an Arrhenius-type equation, the frequency factor(D0) and the activation energy (Ea) of the diffusion process alsoshown in Table 3 have been determined. It is worth noting that thisvalue is very close to that of the activation enthalpy (DH⁄) obtainedby applying Eq. (8) to the experimental data. This is indicative of anoticeable influence of the diffusion on the adsorption process ofCr(VI) by the peat. In other words, the energy barrier to be over-come in order to form the activated complex through whichadsorption takes place (DH⁄) is mainly determined by the diffusiv-ity of Cr(VI) inside the pores of the adsorbent.

3.3. Study of the adsorption equilibrium of Cr(VI)

The adsorption isotherms of Cr(VI) by the peat used as the adsor-bent are shown in Fig. 4 (right). It is worth noting that all the iso-therms are well defined within the whole relative concentration

Fig. 5. Plot of the diffusion coefficient, D, as calculated from C–H Eq. (12) vs. time.

Table 4Equilibrium parameters corresponding to the adsorption of Cr(VI).

T (�C) ns0 � 10þ4 ðmol g�1Þ KCi Km d R2 DH� (kJ mol�1) DS� (kJ mol�1 K�1) DG� (kJ mol�1)

15 0.86 1867 2.98 � 10+16 7.75 0.9446 �18.125 1.01 3413 2.02 � 10+5 3.57 0.9733 �20.035 1.15 5842 33.4 2.04 0.9864 36.5 0.19 �21.945 1.31 7669 28.0 1.96 0.9665 �23.8

330 A. León-Torres et al. / Journal of Colloid and Interface Science 386 (2012) 325–332

range comprised between 0 and 1. From the shape of these iso-therms it may be concluded that all of them correspond to the Stype, S-3 subtype of the classification proposed by Giles et al.[27]. Furthermore, from the shape of the isotherms it may be con-cluded that the adsorption process takes place in a reversible man-ner. This assertion is based on the slope of the initial tract of the ns

vs C/C0 plots (Fig. 4, right). In fact, if this tract exhibits a steep slope(i.e., ideally being coincident with the Y-axis) it may be concludedthat the value of KCi in Langmuir equation (first term of the sumin Eq. (17)) is very high and, hence, the affinity of the solute towardsthe active sites of the adsorbent’s surface is also very high. Thus, theprocess may be considered as irreversible or – at least – with a highdegree of irreversibility. On the contrary, if the slope of the initialtract is low, the values of KCi as well as the adsorbent-solute affinityare moderate. This results in a more reversible process, as it is thecase in the present study.

When the global adsorption process is constituted by an onlysingle process the experimental isotherm consists of only one tractor segment. This is the case in the 1 and 2 subtypes of the referredclassification. On the contrary, if the global process consists of twoor more single individual processes the adsorption isotherm showsmore than one tract, each of them corresponding to a single indi-vidual process. This kind of behavior has been previously describedin the literature [28,29]. The adsorption isotherms here studied ap-pear to be constituted by two different tracts, each one corre-sponding to a single process. The first of those processes isattributable to the adsorption of chromate ions directly on the ac-tive sites of the solid surface, tending to complete a monomolecu-lar layer. The second tract is compatible with the adsorption ofanother Cr(VI) species and/or with the contribution of another kindof active sites of the adsorbent that begin to be available for theadsorption only when a certain relative concentration value isreached. Such value appears to be smaller as temperature in-creases. Taking into consideration all the exposed, the followingequation has been proposed for the fitting of the experimentaldata.

ns ¼ ns0 � KCi � Ce

1þ KCi � Ceþ Km � Cd

e ð17Þ

where ns0 is the monolayer adsorption capacity; KCi is the equilib-

rium adsorption constant; Km is the potential equilibrium constantand d is an exponent that provides information regarding kinetic as-pects of the second stage of the adsorption process [30]. The firstterm of Eq. (17) corresponds to the formation of the monolayer thatmay be fitted to the well-known Langmuir’s equation. The secondterm represents the formation of the multilayer.

The experimental data have been fitted to Eq. (17). The fittingparameters are summarized in Table 4.

When the values of ns0 summarized in Table 4 are compared

with those previously reported in the literature it may be con-cluded that peat is able to adsorb similar amounts of Cr(VI) permass unit than some other low-cost adsorbents (again, see theexcellent reviews by Gupta and Ali [6] and Mohan and Pitman[7], and references therein). Particularly, when compared withother adsorbents of analogous nature [31] the adsorption capacity

is very similar. Anyhow, in sight of the FT-IR and pHPZC analyses, itmay be thought that this adsorbent may behave as a poly-electro-lyte, thus being more suitable for the removal of metal cations insolution.

On the other hand, from the values of KCi summarized in Table 4it may be concluded that the affinity of Cr(VI) towards the activesites of the peat is relatively low, mainly at low treatment temper-atures. As temperature increases, the affinity does as well. Fromthe values of KCi, plotted in the form Ln KCi vs. 1/T, it is possibleto calculate the values of the average standard enthalpy (DH�), en-tropy (DS�) and free energy (DG�) of the adsorption process. Suchvalues are also summarized in Table 4. From such values it maybe deduced that the adsorption process takes place in an endother-mic manner and with a slight increase of entropy. This latter fact isattributable to a noticeable desolvatation of both the solute mole-cules and the active sites of the adsorbent. Furthermore, the valuesof the Gibb’s free energy are negative at all the operation temper-atures. This fact indicates that the adsorption process is spontane-ous within the whole interval of temperatures. Furthermore, suchprocess is more favorable as temperature increases.

3.4. Differential enthalpy and entropy of the adsorption process

As the adsorption process takes place, differential enthalpy(DH) and entropy (DS) vary as h increases and the adsorbatereaches less active sites on the sorbent surface. In order to obtaininformation concerning the variation of DH and DS vs. h, modifiedClausius–Clapeyron equations have been used as described in aprevious paper [30]

DH ¼ R � T1 � T2

T2 � T1� ln

C2CS2C1CS1

!h

ð18Þ

and

DS ¼ RT2 � T1

� T1 � lnC1

CS1� T2 � ln

C2

CS2

� �h

ð19Þ

where C1 and C2 represent the Cr(VI) concentrations in solution attemperature T1 and T2, and CS1 and CS2 the adsorbate solubility inwater at those temperatures. Differential enthalpy and entropy val-ues are calculated for fixed values of h, then being plotted vs. h, inorder to obtain further information about the energy distributionof the active sites on the solid surface.

From Eqs. (18) and (19) and using the experimental data corre-sponding to the adsorption isotherms ns vs. Ce/C0 at T1 = 15 �C andT2 = 45 �C values of DH and DS have been calculated and plotted vs.h, as shown in Fig. 6.

If the energy distribution of the active sites at the solid surfaceis supposed to be ideally homogeneous, then a plot of DH vs. hshould fit the Maxwell–Boltzmann law. Hence, DH should be lessthan zero in the whole interval (0 6 h 6 1) increasing from DHequal to �1, for h = 0, up to DH ’ 0, for h = 1. Fig. 6, though, showsa noticeable variation of both, differential enthalpy and entropy asthe value of the coverage fraction, h increases. This fact suggests aremarkable degree of heterogeneity in terms of the energy of the

Fig. 6. Differential adsorption enthalpy (left) and entropy (right) of Cr(VI).

A. León-Torres et al. / Journal of Colloid and Interface Science 386 (2012) 325–332 331

active sites towards the Cr(VI) adsorption from solution. In thisconnection, it is worth noting that the adsorption of Cr(VI) is ther-modynamically favored in the first steps of the adsorption process.Hence, at low coverage fraction values (approximately between 0and 0.1) a decrease in the DH vs. h plot (Fig. 6, left) may be ob-served. This suggests an intense interaction between the Cr(VI)species adsorbed and the surface of the peat. However, at valuesof h above 0.1 the curve rises progressively tending to reach anasymptotic value of DH very close to zero for values of h close tothe unity. This behavior may be attributed to the occurrence of sideinteractions between the adsorbed Cr(VI) species.

On the other hand, Fig. 6 (right) shows that the DS vs. h plot ispositive along the whole interval of coverage fraction comprisedbetween 0 and 1. This suggests that the adsorption of Cr(VI) takesplace with an at least partial desolvation of both, the solute and thesurface active sites of the adsorbent. Furthermore, as usual theshape and variation of the DS vs. h plot is qualitatively inverse tothat of DH vs. h.

4. Conclusions

From the results obtained in this study the following conclu-sions may be drawn.

� A natural peat has been successfully used as the adsorbent forthe removal of a highly toxic chemical species such as Cr(VI).The adsorbent has been characterized in terms of particle sizeanalysis, ash content, pH of the point of zero charge, FT-IR spec-troscopy and thermal analysis.� The adsorption of Cr(VI) has been analyzed from the kinetic and

equilibrium standpoint. A kinetic model derived from a kineticlaw that assumes a reversible adsorption process of partialorder in C equal to one has been satisfactorily used to fit the Cvs. t experimental data. An analysis of the diffusion phenome-non clearly indicates that the kinetics of the adsorption processis remarkably influenced by the diffusion of the retained chem-ical species.� The adsorption equilibrium isotherms have been defined and

fitted to a model that assumes the occurrence of a global pro-cess constituted by two single unitary processes. The first ofsuch processes is related to the formation of a monomolecularlayer of Cr(VI) adsorbed on the active sites of the peat. The sec-ond process, which corresponds to a steep final tract of the iso-therm, is assigned to the formation of a second (and successive)layer(s) of adsorbed Cr(VI) species, probably due to the avail-ability of new active sites that are only accessible at large valuesof coverage fraction.

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