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Removal of Diethyl Phthalate from Aqueous Media byAdsorption on Different Activated Carbons: Kinetic andIsotherm StudiesTatianne Ferreira de Oliveira a , Benoît Cagnon a , Henri Fauduet a , Marina Licheron b &Olivier Chedeville aa Institut de Chimie Organique et Analytique, (ICOA, CNRS-UMR 6005), Université d'Orléans,Institut Universitaire de Technologie, Orléans-Cedex, Franceb Conditions Extrêmes et Matériaux: Haute Température et Irradiation, (CEMHTI, CNRS-UPR3079), Orléans-Cedex, France

Available online: 24 Feb 2012

To cite this article: Tatianne Ferreira de Oliveira, Benoît Cagnon, Henri Fauduet, Marina Licheron & Olivier Chedeville (2012):Removal of Diethyl Phthalate from Aqueous Media by Adsorption on Different Activated Carbons: Kinetic and Isotherm Studies,Separation Science and Technology, 47:8, 1139-1148

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Removal of Diethyl Phthalate from Aqueous Media byAdsorption on Different Activated Carbons: Kineticand Isotherm Studies

Tatianne Ferreira de Oliveira,1 Benoıt Cagnon,1 Henri Fauduet,1 Marina Licheron,2

and Olivier Chedeville11Institut de Chimie Organique et Analytique, (ICOA, CNRS-UMR 6005), Universite d’Orleans,Institut Universitaire de Technologie, Orleans-Cedex, France2Conditions Extremes et Materiaux: Haute Temperature et Irradiation, (CEMHTI, CNRS-UPR3079), Orleans-Cedex, France

The adsorption of diethyl phthalate (DEP) was studied on fouractivated carbons (ACs) with different chemical and microporousproperties. The kinetic results showed that the non-linear form ofthe pseudo-second order kinetic model provided the best parameters.Results further showed that the DEP adsorption kinetics is favored byACs which have a ratio of a specific microporous surface to a specificexternal surface that is relatively low. Besides, an increase in tem-perature induced an increase in the rate constant k2, but the adsorp-tion capacity is temperature independent. Two models, (Langmuir(linear and non-linear forms) and Dubinin-Radushkevich-Kaganer(DRK)), were tested from experimental data. While the Langmuirmodel provided the best correlation on all the ACs studied. The sur-face occupied, calculated with the Langmuir parameter obtained bythe non-linear form, evidenced the importance of the external surfaceand the mean pore size. The results show that the non-electrostaticinteractions are predominant in the DEP adsorption and are essen-tially due to dispersion and hydrophobic interactions (for L27 andX17). In the case of F22 and S21 the DEP adsorption is only dueto dispersion interaction. A comparison of the thermogravimetricanalysis of the ACs washed and saturated shows that the DEP istotally desorbed between 300 and 500�C with a characteristic peak.

Keywords activated carbons; adsorption; diethyl phthalate;isotherms; kinetics

1. INTRODUCTION

In recent years, the presence ofmicropollutants in the aqua-tic environmenthas increased.Phthalate esters aremicropollu-tants frequently used as plasticizers to increase the flexibility ofpolymers. As phthalates are not bonded to the material, theycan migrate into the water environment. Phthalates are often

mentioned as suspected endocrine disrupters and carcinogeniccompounds (1,2). Given this potential health hazard, a toler-ance limit of phthalates in industrial wastewater has thereforebeen fixed in Europe at 1.3mg L�1 (diethylhexyl phthalate(DEHP) is the standard), and a 30% reduction in phthlate con-centration in industrialwastewaters has to be achievedby 2021(WFD, 2000=60=EC). Diethyl phthalate (DEP), which has alarge number of industrial applications (cosmetics industries,printing inks, food packaging), was chosen for this studybecause it represents pollutants commonly encountered inwater and wastewater treatment and in industrial residues inthe Region Centre (France). Adsorption on activated carbon(AC) could be an interesting method to remove these mole-cules (3,4). To optimize this adsorption treatment process, itis very important to understand pollutant=AC interactions.The aim of this work was to study the influence of both thechemical and textural properties of ACs on DEP adsorption.Four commercial ACs manufactured by PICA S.A. (F22,L27, S21, and X17) were studied to performDEP adsorption.The textural and chemical properties of these ACs were firstcharacterized and the adsorption kinetics and isotherms ofDEP on these ACs were then determined. The influence ofthe temperature was studied on the adsorption kinetics andthe isotherms of the most efficient activated carbon at 5, 10,20, and 40�C. Several kinetic models and adsorption modelswere tested from experimental data and compared so as tounderstand the DEP adsorption mechanism and to chooseefficient ACs. Thermogravimetric experiments (TG-DSC)were carried out to determine the decomposition rate and tem-perature rangeofDEPdesorptionbya thermal treatment afterthe adsorption process with each activated carbon.

2. MATERIAL AND METHOD

2.1. Characterization of the Activated Carbons

The four commercial activated carbons studied(L27, F22, S21, and X17) were supplied by Pica (Vierzon,

Received 15 July 2011; accepted 25 November 2011.Address correspondence to Dr. Benoıt Cagnon, Institut de

Chimie Organique et Analytique, (ICOA, CNRS-UMR 6005),Universite d’Orleans, BP 16729, Institut Universitaire de Techno-logie, 16 rue d’Issoudun, 45067 Orleans-Cedex 2, France, IUTd’Orleans, Orleans 45067, France. Tel.: þ033 2 38 49 44 31;Fax: þ033 2 38 49 44 25. E-mail: benoit.cagnon@univ-orleans.fr

Separation Science and Technology, 47: 1139–1148, 2012

Copyright # Taylor & Francis Group, LLC

ISSN: 0149-6395 print=1520-5754 online

DOI: 10.1080/01496395.2011.645184

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France). Their porosity was characterized by conventionalnitrogen adsorption isotherm at 77K using a MicromeriticsASAP 2020. The samples were previously degassed at370�C for 24 h under a residual vacuum of less than10�4 Pa. The adsorption isotherms were carried out withrelative pressures ranging from 7.9� 10�4 to 0.99. Thenitrogen adsorption isotherms were analyzed according tothe Dubinin theory (5). The specific microporous volumeW0 (cm3 g�1) and the mean pore size L0 (nm) were esti-mated from the linear part of the Dubinin-Radushkevichplot (5,6). Sing aS plots were used to determine the externalspecific surface Sext (m2 g�1) (7) and assuming that forslit-shaped micropores the specific micropore surface Smicro

(m2 g�1) could be estimated using the specific microporousvolume and the mean pore size (5). The total surface (Stot inm2 g�1) is the sum of the external specific surface and thespecific surface of micropores.

The chemical properties of the materials were also deter-mined. The four ACs were washed before each analysis andexperiment to eliminate any residual acidity or basicity dueto the activation treatment. They were dried overnight at80�C. The surface oxygen groups were determined accordingto the Boehm method (8). A weight of 0.2 g of activated car-bon was introduced into 25mL of the following 0.1mol L�1

solutions: NaOH, Na2CO3, NaHCO3 and HCl. The AC=solution mix was placed in a thermostated multi-agitationapparatus at 25�C under mechanical stirring at 150 rpm for48h. The suspension was filtered through a 0.45mm mem-brane filter and the excess of base or acid was titrated with0.1molL�1 solutions of HCl or NaOH, respectively.Following the Boehm method, the carboxylic, phenolic,and lactonic groups were quantified. The number of surfacebasic groups was calculated (but not identified) from theamount of HCl which reacts with the carbon. The methodof Rivera-Utrilla and Sanchez-Polo (9) was used to deter-mine the pH of point of zero charge (pHPZC). The FTIRspectra of the materials were obtained on an FT-IR NicoletiS10 spectrophotometer. The samples were dried overnightat 80�C before the spectra were recorded. Themean diameterof particles was determined using a laser granulometer(Coulter LS 230). These ACs are similar in terms of granulo-metry (approximately 1300mm) except for L27 which has amean particle size of 718mm.

2.2. Kinetic and Adsorption Experiments

The DEP used was supplied by Aldrich and the purityfor this compound was �99.5%. The adsorption kineticsexperiments were carried out with an initial concentrationof 9� 10�4mol L�1. The equilibrium time was determinedby the following procedure: a weight of 0.1 g of washedactivated carbon was introduced under mechanical stirringat 150 rpm into 200mL of the DEP solution. The pH valuewas fixed at 5. The reactors were placed in a thermostatedmulti-agitation apparatus (Vankel Vanderkamp 600) at

20�C. The solutions were withdrawn from the reactormedium at suitable time intervals, filtered through a0.45mm membrane filter, and analyzed. The equilibriumconcentrations (Ce in mol L�1) of the solution were deter-mined by using a UV=Visible spectrophotometer (JascoV-530) at a wavelength of 228 nm. For the isotherms, a timeof 500 minutes, based on kinetic results, was chosen foreach experiment.

The adsorption isotherms were carried out in the sameexperimental conditions with DEP initial concentrationsranging from 1.7� 10�4 to 9� 10�4mol L�1. The amountsof matter adsorbed at time t, qt (mol g�1) and at equilib-rium qe (mol g�1) were calculated according to the follow-ing equations:

qt ¼C0V0 � CtVt

Wand qe ¼

C0V0 � CeVe

Wð1Þ

where C0, Ct, and Ce are the DEP concentrations respect-ively in the initial solution, at time t and at equilibrium(mol L�1), V0, Vt, and Ve are the volumes of the solution(L), respectively, in the initial solution, at time t and at equi-librium, and W is the weight of activated carbons used (g).

2.3. Isotherms and Kinetic Models

Adsorption kinetic data were modelled by using theLagergren pseudo-first order model (10,11) and Ho’spseudo-second order model (linear and non-linear method)(12–14) to determine the rate constant and the amounts ofadsorption at equilibrium. The pseudo-first order model isbased on the sorption capacity of the solid phase. Com-monly, it is applicable over the first 30min of the sorptionprocess. The pseudo-first order kinetic equation is expressedas follows:

dq

dt¼ k1 � ðqe � qtÞ ð2Þ

where k1 is the rate constant of pseudo-first order adsorp-tion (min�1), qe the amount of DEP adsorbed at equilib-rium (mol g�1), and t the time (min). The integrated formof this expression is:

lnðqe � qtÞ ¼ ln qe � k1 � t ð3Þ

The kinetic parameters k1 and qe in Eq. (3) can be obtainedfrom the linear plot of ln(qe� qt) versus t. The pseudo-second order model is also based on the sorption capacityof the solid phase.

The pseudo-second order kinetic equation is expressedas follows:

dq

dt¼ k2 � ðqe � qtÞ2 ð4Þ

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where k2 is the rate constant of pseudo-second orderadsorption (gmol�1min�1). The integrated form of thisequation is:

t

qt¼ 1

k2 � q2eþ t

qeð5Þ

The linear form of the pseudo-second order model enablesthe values of k2 and qe to be determined from the slopeand the intercept of t=qt versus t. The non-linear methodwas also used to determine the kinetic parameters (14).

The intraparticle diffusion model (15) was used to deter-mine the rate limiting step of the adsorption kinetics. It isexpressed as:

qt ¼ Ki � t0;5 þ C ð6Þ

where Ki is the intraparticle diffusion rate constant (mol g�1

min�0.5) and C a constant (mol g�1). Plots may present amulti-linearity corresponding to successive steps in theadsorption phenomenon. The first step, very short, corre-sponds to a limitation of adsorption by external diffusion.The second step corresponds to a gradual adsorption stageduring which intraparticle diffusion is the rate limiting fac-tor. The third step is the final equilibrium in which adsorb-ate concentration in aqueous solution becomes very weak,thus slowing down the intraparticle diffusion phenomenon.The value of the intercept C indicates whether externaldiffusion limits the adsorption phenomenon: if the straightline passes through the origin, intraparticle diffusion is therate limiting factor; otherwise, external diffusion operatessimultaneously to control the rate of adsorption (16,17).

In order to determine the isotherm parameters and toestimate the important parameters in terms of porousproperties, two equilibrium adsorption isotherm models(Langmuir and Dubinin-Radushkevich-Kaganer (DRK)equations) were tested from experimental isotherms andcompared. It is well-known that the Langmuir equation isused for a monolayer adsorption (18,19). Using the linearLangmuir equation, the maximum amount of adsorptioncorresponding to complete monolayer coverage (qm in molg�1) can be determined:

Ce

qe¼ Ce

qmþ 1

bqmð7Þ

whereCe is the equilibrium concentration (molL�1), qe is theadsorbed amount at equilibrium (mol g�1), and b is theLangmuir constant (Lmol�1). The values of qm and b canbe determined from the linear plot of Ce=qe versus Ce fromthe slope and the intercept, respectively. These parameterscan also be determined by the non-linear approach (20,21).

In the case of liquid-phase adsorption, several studieshave shown that the adsorption energy can be estimated

according to the Dubinin-Radushkevich-Kaganer (22)equation (DRK):

qe ¼ qDRK exp �RT

ESLn

Csat

Ce

� �� �n� �ð8Þ

where qDRK is the limiting amount filling the micropores atequilibrium (mol g�1), Csat and Ce are respectively the con-centration at saturation and the equilibrium concentration(mol L�1), ES is the most probable energy of adsorption (Jmol�1), R is the gas constant (8.314 J mol�1K�1) and T thetemperature (K). As shown elsewhere (6), the value n¼ 4leads to a better overall fit for the experimental data andwas chosen for this study.

2.4. DTA-TG Experiments

In order to analyse the decomposition of DEP after theadsorption process for each activated carbon DTA andTG experiments were combinated. The rate and the tem-perature of the decomposition of DEP have been determi-nated using a differential scanning calorimeter (DSCMulti HTC, Setaram) and a thermobalance (Setsys 2400,Setaram). The washed raw activated carbons were driedovernight at 80�C to eliminate residual water. Samples wereanalysed systematically in platinum crucibles under anargon flow at 10L min�1. A weight of 0.20 g of samplewas heated from Room Temperature to 800�C using a rateof 10�Cmin�1. The samples were then allowed to cool downto room temperature at the same rate (10�C min�1). Theequipments were calibrated using three standards at thesame conditions used for the samples measurements. Theseexperiments were carried out on the washed raw activatedcarbons, on the activated carbons saturated with DEPand on the activated carbons after the thermal treatment(to verify the total desorption of the DEP).

3. RESULTS AND DISCUSSION

3.1. Characteristics of the Activated Carbons

The results, presented in Table 1, show that these ACshave very different chemical properties. The pHpzc variesfrom 3.0 to 8.2, total acid functions vary from 0.20 to1.57meq g�1 and basic functions vary from 0.18 to0.85meq g�1. There is one acid AC (L 27), two neutral (F22 and S 21), and one basic (X 17). L27 and X17 are the acti-vated carbons which present the greatest number of surfacegroups. The results obtained by the Boehm method are inagreement with FT-IR spectra (Fig. 1) and with pHpzc

values. AC L27 exhibits the most intense bands, in particularthe bands for the 1040 cm�1–1080 cm�1 domain. This maybe related to vibrations of phenolic C–O groups, as the1100 cm�1–1250 cm�1 domain corresponds to OH bendingvibrations in the carboxyl group (1125 cm�1) and phenol(1175 cm�1) and at 1590 cm�1 (C=O stretching vibration

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in the carboxyl group). For all the ACs studied (exceptedfor S21 which exhibits very low intensity bands) there arespecific bands at 1710 cm�1–1730 cm�1 (C=O stretchingvibration in carboxyl functions) and at 1736 cm�1–1800 cm�1 indicating the presence of lactones, quinine esteror carboxylic anhydrides (23,24). The ACs also have differ-ent porous properties. The N2 adsorption-desorption iso-therms for the four ACs used (Fig. 2) show that the L27,F22, and X17 ACs were microporous but also containedsomemesopores, as indicated by the shape and the hysteresisloop of the N2 isotherms. Furthermore, Fig. 2 shows thatS21 is microporous only, which is confirmed by the valuesof its Smicro and Sext. The mean pore size (L0) varies from9.7 A to 18.5 A, the specific external surface (Sext) variesfrom 18 to 444m2 g�1 and the specific microporous volume(W0) varies from 0.29 to 0.57 cm3 g�1.

3.2. Equilibrium Time Determination

The results presented in Fig. 3 show that the DEPadsorption equilibrium was reached after different contacttimes for the four ACs, that is, 80, 120, 250, and 500minrespectively for the F22, L27, X17, and S21 ACs. Kineticmodels can be applied to determine the rate constant. Thelinear form of the pseudo-second order kinetic model fits

correctly the experimental data and describes DEP adsorp-tion on these ACs (R2� 0.998), as shown on Table 2 andFig. 3. Nevertheless, as described by Ho (2006) (14), thenon-linear method permits to obtain more precision on thekinetic parameters. The results also show that the experi-mental and calculated values of the amount adsorbed atequilibrium are very close whatever the model form used.The results, obtained by the non-linear method, show thatthe rate constant (k2) for DEP adsorption varies from12.71 to 65.46 g mol�1 min�1 and may be classified as fol-lows: k2 (L27)> k2 (F22)> k2 (X17)> k2 (S21). With regardto the porous properties of the ACs, these results demon-strate that the adsorption kinetics of DEP is favored byACs which have a ratio of a specific microporous surfaceto a specific external surface that is relatively low, since thepresence of mesopores, shown by the hysteresis of the N2

isotherms in Fig. 2, seems to favor access to the micropores.As seen previously, F22, L27, and X17 contain some

mesopores while S21 is microporous only, having the lowestvalue of the mean pore size (9.7 A). This value is very close tothe size of the DEPmolecule calculated with theChemsketchsoftware (8.23 A� 8.23 A). It is therefore more difficult andtakes longer to fill the S21 micropores (intraparticlediffusion phenomenon) than those of the other ACs. The

FIG. 2. N2 adsorption-desorption isotherms at 77K of the four ACs

used (L27 (&), F22 (�), S21 (.) and X17 (D)).

TABLE 1Chemical and porous properties of the ACs used

Stot Sext Smicro

Smicro=Sext

W0 Lo Acidity Basicity pHpzc

AC (m2 g�1) (m2 g�1) (m2 g�1) (cm3 g�1) (A) (meq g�1) (meq g�1)

L27 1060 444 616 1.39 0.57 18.5 1.57 0.18 3.0F22 870 256 614 2.40 0.39 12.7 0.32 0.26 7.5S21 987 18 969 53.83 0.47 9.7 0.35 0.33 7.4X17 514 130 384 2.95 0.29 15.1 0.20 0.85 8.2

FIG. 1. FTIR spectra of the four ACs used (L27 (&), X17 (D), F22 (�)and S21 (.)).

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presence of acid functions could enhance the water adsorp-tion and could disfavor the DEP adsorption and so, decreasethe adsorption rate. The amounts adsorbed at equilibriumvary from 8.46� 10�4 to 1.01� 10�3mol g�1 and are verysimilar.

The intraparticle diffusion model was applied to dataobtained at 20�C and at pH¼ 5.0 on the four ACs used inthe process (Table 3). For each AC, modelling exhibitedmulti-linearity, meaning the existence of several steps inthe adsorption process (Fig. 4). The low values of C (inthe range of 10�5) indicate that the external diffusion stepwas not rate controlling. Thus, the first stage correspondsto an intraparticle diffusion rate control (effect of themacropores diffusion) (25). The results, obtained by theintraparticle diffusion model, show that the intraparticlediffusion rate constant (Ki) for DEP adsorption varies from5.0 10�5 to 1.0 10�4mol g-1 min�0.5 and may be classified asfollows: Ki (F22)>Ki (L27)>Ki (X17)>Ki (S21). It isimportant to note that the Ki rate constant decreased whenthe AC has a small microporous surface and a lesswell-developed porosity (X17) or when the AC is micropor-ous only (S21). The second step, marked by the lack of evol-ution of qt, corresponds to the equilibrium step due to thedecrease in DEP concentration in the aqueous solution.

In the light of these results, the influence of the tempera-ture was studied on the adsorption kinetics of F22 whichseems to be the AC the most efficient AC in terms of adsorp-tion capacity. The time necessary to reach equilibrium wastemperature-dependent: the equilibrium state was reachedmore quickly when the temperature increased. Thus, forF22, equilibrium was reached in approximately 80min at40�C and in 300min at 5�C, indicating that the adsorptionkinetics of DEP was enhanced when the temperatureincreased (Fig. 5).

The pseudo-second order kinetic model (linear andnon-linear methods) was also applied to determine the rateconstant and the adsorption capacity at equilibrium(Table 4). An increase in temperature induced an increasein the rate constant k2, which rose from 12.67 to 70.00 gmol�1min�1 when the temperature increased from 5 to40�C. Moreover, the results show that the adsorptioncapacity is constant when the temperature increased: qeis equal to 1.0� 10�3mol g�1 (�0.4) when temperatureincreased from 5�C to 40�C. Adsorption is generally knownto be an exothermic process but DEP adsorption on F22 istemperature independent.

FIG. 3. Evolution of DEP adsorption (experimental (symbol) and

pseudo second-order model (line)) as a function of the contact time at

20�C and at pH¼ 5.0 for (F22 (�), L27 (&), S21 (.) and X17 (D)); (a)linear form and (b) non-linear form.

TABLE 2Pseudo-second-order kinetic parameters obtained by using the linear and non-linear methods for DEP

adsorption on the four ACs used in the process at 20�C and at pH¼ 5.0

Linear method Non-linear method

ACqe exp.

(mol g�1)qe calc.

(mol g�1)k2

(g mol�1 min�1) r2qe exp.

(mol g�1)qe calc.

(mol g�1)k2

(g mol�1 min�1) r2

F22 9.1 10�4 9.2 10�4 110.48 0.999 9.1 10�4 1.0 10�3 41.30 0.951L27 8.8 10�4 8.9 10�4 65.46 0.999 8.8 10�4 8.9 10�4 65.46 1S21 9.0 10�4 9.5 10�4 14.09 0.998 9.0 10�4 9.8 10�4 12.71 0.993X17 7.9 10�4 7.9 10�4 39.55 0.999 7.9 10�4 8.5 10�4 25.45 0.963

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3.3. Adsorption Isotherms

Adsorption isotherms were determined with 0.1 g of ACand in the same experimental conditions as for the kineticsstudy. According to Giles’s classification (26), Fig. 6 showsthat the DEP isotherms of these activated carbons are of typeL (Langmuir class). The isotherm shape seems to show thatDEP adsorption follows a process with the coating of bothmicropore walls and specific external surface (22). It can beseen that F22 and L27 have a higher adsorption capacitythan X17 and S21 (the worst adsorption capacity). TheLangmuir results are shown in Table 5. The Langmuir modelwas successfully applied for the four ACs used (R2> 0.982).The maximum amount of adsorption calculated byLangmuir (qmL) corresponding to complete monolayer

coverage is different for the four ACs and varies from 1.1610�3 to 1.71 10�3mol g�1. These results show that DEPadsorption occurs mainly in a molecular monolayer andare in agreement with the shapes of the isotherms given inFig. 6. It is possible to calculate the surface occupied bythe DEP with the following equation (22):

Socc: m2 g�1

� �¼ qmLSDEPNA

FIG. 4. Intraparticle diffusion model applied on the four ACs used (L27 (&), F22 (�), S21 (.) and X17 (D)).

TABLE 3Intraparticle diffusion model parametersfor DEP adsorption on the four ACs usedin the process at 20�C and at pH¼ 5.0

AC Ki (mol g�1min�0.5) r2

L27 9.0 10�5 0.974F22 1.0 10�4 0.984X17 7.0 10�5 0.961S21 5.0 10�5 0.980

FIG. 5. Evolution of DEP adsorption as a function of the contact time

for F22 at pH¼ 5.0 and at 5�C (&), 10�C (�), 20�C (D) and 40�C (.).

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with NA the Avogadro number (mol�1) and SDEP the DEPmolecular area (m2). The DEP surface was calculated withthe Chemsketch software (SDEP¼ 6.77 10�19m2) consideringthat DEP is adsorbed in flat position. The results show thatthe surface occupied (with regard to the total surface) variesfrom 100% (X17) to 48% (S21). In sum, X17 has a goodaffinity for DEP (showing that basic functions enhance inter-action with DEP) but this AC has a small microporous

surface and is saturated; S21 is microporous only and itsmicroporosity does not seem to be accessible with a meanpore size of 9.7 A (value which is very close to the DEP size);L27 has a relatively low ratio of specific microporous surfaceto specific external surface but its surface is acid and itspHPZC (Table 1) is below the solution pH¼ 5 which can cre-ate repulsive interactions with the DEP molecule. F22 is themost efficient AC. These results are in agreement with kineticresults and highlight the importance of the AC mean poresize, of the specific microporous volume, and of the surfacegroups.

The DRKmodel is not retained in this study the determi-nation coefficients (R2> 0.929) are lower than those of theLangmuir model. Nevertheless, Table 6 shows that the qDRK

are close to qexp.According to Moreno-Castilla (27), adsorption of

organic molecules from dilute aqueous solutions on ACis a complex interplay between electrostatic andnon-electrostatic interactions. In this study, the DEP is inits molecular form, and there is no electrostatic interaction.The non-electrostatic interactions are predominant and areessentially due to dispersion and hydrophobic interactions.According to the pH value, the surface charge of the L27 ispredominantly negative and the presence on carbon surfaceof oxygen groups (shown by the Boehm method) couldfavor H-bonds with water and thus reduce the adsorptioncapacity as shown by the determination of the occupied

TABLE 4Pseudo-second-order kinetic parameters obtained by using the linear and non-linear methods for DEP

adsorption on the F22 AC at pH¼ 5.0 and at different temperatures

Linear method Non-linear method

T(�C)

qe exp.

(mol g�1)qe calc.

(mol g�1)k2

(g mol�1 min�1) r2qe exp.

(mol g�1)qe calc.

(mol g�1)k2

(g mol�1 min�1) r2

5 9.4 10�4 9.9 10�4 18.54 0.994 9.4 10�4 1.1 10�3 12.67 0.99910 9.2 10�4 9.3 10�4 42.51 0.998 9.2 10�4 9.6 10�4 33.45 0.98020 9.1 10�4 9.1 10�4 110.48 0.997 9.1 10�4 1.0 10�3 41.43 0.95140 9.4 10�4 9.5 10�4 147.64 0.999 9.4 10�4 1.0 10�3 70.00 0.986

TABLE 5Langmuir isotherm parameters obtained by the non-linear method for DEP adsorption on

the four ACs used in the process at 20�C and at pH¼ 5.0

qmax exp. qmL. b Socc. % Socc. r2

AC (mol g�1) (mol g�1) (L mol�1) (m2 g�1) (%)

F22 1.52 10�3 1.67 10�3 60623 652 75 0.951L27 1.53 10�3 1.68 10�3 43855 697 66 0.984S21 1.14 10�3 1.09 10�3 76132 473 48 0.966X17 1.24 10�3 1.16 10�3 153970 514 100 0.937

FIG. 6. DEP adsorption isotherms (experimental (symbol) and

Langmuir model (line)) at pH¼ 5.0 and at 20�C on the different ACs used

(F22 (�), L27 (&), X17 (D) and S21 (.)).

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surface. This is confirmed by the results obtained with theX17, which has the lowest porous properties of all theACs used but adsorbs preferentially the DEP (which isshown by the value of its occupied surface). The presenceof basic groups on this carbon surface could decrease thewater adsorption and could favour the dispersion interac-tion between the p-electrons of the aromatic ring and thoseof the graphene layers. In the case of both F22 and S21which are neutral ACs, the dispersion interaction betweenthe p-electrons of the aromatic ring and those of the gra-phene layers are predominant.

3.4. Thermogravimetric Analysis

Figure 7 shows the rate of desorption in terms of differ-ential weight loss (DTG profile) against temperature for aheating rate of 10�C min�1 (right ordinate: dm=dT) andrelative weight loss (left ordinate: mass loss) as a functionof temperature for the four ACs (washed (dot curve) andsaturated with DEP (line curve)). As shown in Fig. 7, thethermal desorption of the DEP occurs at different ratesand within different distinct temperature ranges.

For each AC, the cumulated weight loss observedbetween 93�C and 168�C, corresponds to the departure ofwater which is initially physisorbed in the various raw mate-rials. For the washed AC, this peak is rather high, represent-ing approximately 5%, 12%, 17% and 30% of the initialmass, respectively, for F22, X17, S21, and L27, despite thefact that all the ACs were dried before the analysis. TheDTG profile of both L27 and X17 presents a peak from500�C to 800�C which may be representative of thedecomposition of the surface oxygen groups (28). This is inagreement with the Boehm results: L27 and X17 ACs arethe activated carbons which present the largest number ofsurface groups contrary to F22 and S21. For the ACs satu-rated with DEP, all the profiles are similar from a qualitative

TABLE 6DRK isotherm parameters for DEP adsorption on the four

ACs used in the process at 20�C and at pH¼ 5.0

qmax exp. qDRK Er2AC (mol g�1) (mol g�1) (kJ mol�1)

F22 1.52 10�3 1.70 10�3 15.1 0.929L27 1.53 10�3 1.65 10�3 14.5 0.931S21 1.14 10�3 1.12 10�3 14.9 0.933X17 1.24 10�3 1.20 10�3 17.2 0.944

FIG. 7. TG-DTG experimental curves at 10�C min�1 under N2 for washed and DEP saturated ACs: F22 (a), L27 (b), S21 (c) and X17 (d).

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point of view: all the ACs except for the X17 presentshoulders from 300�C to 500�C. This temperature range cor-responds to DEP desorption and this result is in agreementwith the literature: Tatavarti et al. (28) have shown thatthe vaporization temperature of DEP at atmospheric press-ure is approximately 300�C. For the saturated ACs, thismass loss represents approximately 10.5%, 13.5%, 15%,and 21% of the initial mass respectively for X17, S21, F22,and L27, which is in agreement with the adsorption isothermresults. A second thermogravimetry analysis was carried outon each AC after desorption and no mass loss was observed,showing that one thermal treatment is sufficient to desorbDEP completely (these curves are not shown here). Thesedifferences in the desorption rate can be partly explainedby the difference in the adsorption process of the ACs, whichhave different porous properties. So, there is also a differencein the desorption mechanism. The DSC analyses were car-ried out to evaluate the heat flow and to calculate the DEPdesorption enthalpy on each AC. Figure 8 shows that thedesorption heat flow profiles of F22 and L27 are similar withshoulders corresponding to the DTG profiles. This can prob-ably be attributed to two kinds of adsorption sites withdifferent energies. The desorption heat flow profile of S21is different but corresponds to its DTG profiles. The DEPdesorption enthalpy values calculated are very close for

L27 and F22 (respectively 191 J g�1 and 170 J g�1) and lowerfor S21 (95.1 J g�1). It is difficult to calculate the DEPdesorption enthalpy for X17 because the curve of the ther-mal effects extends over a wider temperature domain. Theseresults are in agreement with the adsorption isotherms.

4. CONCLUSION

Four commercial ACs were studied to remove DEP inwater. The pseudo-second order kinetic model (non-linearform) is the most suitable one to describe DEP adsorptionon these ACs. First, the adsorption rate was very high forthe AC L27 which has a relatively low ratio of specificmicroporous surface to specific external surface. The studyof the influence of temperature on the adsorption kinetics ofF22 shows that when the temperature increases, the rateconstant increases but there is no influence on the adsorp-tion capacity. Then, it was shown that the Langmuir iso-therm best represents the DEP adsorption isotherms onall the ACs studied. The surface occupied values calculatedwith the Langmuir parameter show that the presence of acidfunctions seems to decrease the efficiency of the AC. Theefficiency of the DEP adsorption process appears to beparticularly influenced by the specific microporous volume,the chemical surface (basic functions), the Smicro=Sext ratio,

FIG. 8. DTG (dot)-DSC (line) experimental curves at 10�C min�1 under N2 for DEP saturated ACs: F22 (a), L27 (b), S21 (c) and X17 (d).

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and the mean pore size (The F22 AC is the AC the moreefficient in this study). The results show that the non-electrostatic interactions are predominant in the DEPadsorption and are essentially due to dispersion and hydro-phobic interactions (for ACs with surface groups). In thecase of ACs with a low number of functional groups, theDEP adsorption is only due to dispersion interactionBesides, the thermogravimetry analysis, with the TG-DTG-DSC results, it shows that L27, F22, and S21 have twokinds of adsorption sites with different energies and theDEP desorbs from 300�C to 500�C.

ACKNOWLEDGEMENTS

The authors wish to thank Xavier Bourrain and theAgence de l’Eau Loire Bretagne for their technical andfinancial support and Pica S.A. for gratuitously supplyingthe ACs.

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