Lead ion adsorption from aqueous solutions in

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Nov 16, 2011 - Oubagaranadin JUK, Murthy ZVP. Adsorption of divalent lead on a montmorillonite-illite type of clay. Ind Eng Chem Res. 2009;. 48:10627–36.
Lead ion adsorption from aqueous solutions in modified Algerian montmorillonites A. Zehhaf, A. Benyoucef, R. Berenguer, C. Quijada, S. Taleb & E. Morallon

Journal of Thermal Analysis and Calorimetry An International Forum for Thermal Studies ISSN 1388-6150 Volume 110 Number 3 J Therm Anal Calorim (2012) 110:1069-1077 DOI 10.1007/s10973-011-2021-8

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Author's personal copy J Therm Anal Calorim (2012) 110:1069–1077 DOI 10.1007/s10973-011-2021-8

Lead ion adsorption from aqueous solutions in modified Algerian montmorillonites A. Zehhaf • A. Benyoucef • R. Berenguer C. Quijada • S. Taleb • E. Morallon



Received: 23 August 2011 / Accepted: 19 October 2011 / Published online: 16 November 2011 Ó Akade´miai Kiado´, Budapest, Hungary 2011

Abstract The adsorption of lead (II) ions on three Algerian montmorillonites (sodium, non-sodium, and acidic-activated) was studied. Transmission electron microscopy coupled with energy dispersive X-ray analysis, X-ray fluorescence and physical adsorption of gases were used to characterize the clays. This characterization has shown than the activation with acid increases the surface area as a consequence of the rupture of the laminar structure. The effect of the pH in the lead adsorption capacity was analyzed. The results show that adsorption is strongly depended on the pH. At low pH values, the mechanism that governs the adsorption behavior of clays is the competition of the metal ions with protons. Between pH 2 and 6, the main mechanism is an ion exchange process. The kinetics of the adsorption is tested with respect to pseudo-first-order and second-order models. The adsorption process, gives a better fit with the Langmuir isotherm, being the monolayer capacity ranging between 18.2 and 24.4 mg g-1. The A. Zehhaf  A. Benyoucef (&) Laboratoire de Chimie Organique, Macromole´culaire et des Mate´riaux, Universite´ de Mascara, Bp 763, 29000 Mascara, Algeria e-mail: [email protected] R. Berenguer  E. Morallon Departamento de Quı´mica Fı´sica e Instituto Universitario de Materiales, Universidad de Alicante, Apartado 99, 03080 Alicante, Spain C. Quijada Departamento de Ingenierı´a Textil Y Papelera, Universidad Polite´cnica de Valencia, Pza Ferrandiz i Carbonel, 03801 Alcoy (Alicante), Spain S. Taleb Departement de Chimie, Universite´ de Sidi Belabas, 22000 Sidi Belabas, Algeria

adsorption of lead decreased in the order AcidicM2 [ M2 [ M1. Thermodynamic parameters such as DH, DS, and DG were calculated. The adsorption process was found to be endothermic and spontaneous. The enthalpy change for Pb(II) by M1 adsorption has been estimated as 60 kJ mol-1, indicating that the adsorption of Pb(II) by all montmorillonites used corresponds to a physical reaction. The adsorption capacity of washed Acidic-M2 was very high compared to M2 and M1. Keywords Adsorption  Clay  Ion exchange  Lead  Montmorillonite

Introduction As a result of their numerous uses, lead can pollute water and soils, producing a serious environmental problem. Several methods have been developed for the removal of these metal ions present in industrial wastewaters and soils, such as chemical precipitations, conventional coagulation, reverse osmosis, ion exchange, and adsorption on activated carbon [1]. Among these methods, adsorption appears to be the most widely used for the removal of heavy metals [2–4]. Clay minerals are non-pollutant and have a significant adsorption capacity so they can have a privileged role in the treatment of water [5, 6]. Pillaring and acid activation processes are used in order to improve clay properties [7–12]. On one hand, pillaring confers to the mineral a high thermal stability, a developed microporous surface and a great acidity [13]. On the other hand, acid activation [14] increases the number of active sites, the specific surface area, acidity, and porosity. These changes depend on several factors such as the pillaring agent, the nature of the acid, the nature of clay, duration of the treatment and the temperature.

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The use of various clay adsorbents such as kaolinites [15–18], illites [19], bentonites [20, 21], montmorillonites [15, 18, 22], zeolites [23, 24], and sepiolites [25, 26] have been reported for the removal of heavy metal ions from aqueous solutions. The chemical composition and pore structure of the clays usually determine their adsorption ability [27]. The chemical nature of the metal clay interaction changes with increasing pH: at low pH, cation exchange is the dominating process [28, 29], whereas at high pH values, the uptake of heavy metal ions is accompanied by a release of hydrogen ions, and seems to be more specific than the uptake at low pH values. Thus, the classical ion exchange model does not cover the whole range of adsorption phenomena and part of heavy metal adsorption occurs at sites created by displacement of protons from surface hydroxyls (i.e., surface complexation) [30]. The aim of this study is to investigate the adsorption capacity of two Algerian natural montmorillonites and an acid modified montmorillonite for the removal of Pb(II) cations from aqueous solution. The effects of different variables have been analyzed. The Langmuir and Freundlich isotherm models have been used to describe equilibrium data and a kinetic study has also been performed.

prepared from analytical grade Pb(NO3)2 (Merk) dissolved in double-distilled water in a 0.01 M NaCl solution. The pH was adjusted to the required value by adding 0.1 M NaOH or 0.1 M HNO3 solution, respectively, to each solution. Clay adsorbents

Experimental section

Two clays were obtained from Algeria ‘‘a non-sodium montmorillonite from Mostaganem (M1) and a sodium montmorillonite from Telemecem (M2)’’. The clay samples were washed with distilled water to remove impurities; the raw-montmorillonites (20 g) were crushed for 20 min using a Prolabo ceramic balls grinder. They were then dried at 423 K for 2 h and stored in tightly stoppered glass bottles for later use (samples M1 and M2). The M2 sample was activated in acid (acidic-M2) by the procedure of Belbachir et al. [31]. In brief, this procedure consists on refluxing 20 g of montmorillonite (M2) in 200 mL of 0.25 M H2SO4 for 3 h. The resulting acidic-activated clay was centrifuged and washed with water several times until it was free of SO42- and the pH of the washing was 6.8. Finally the sample was dried at 378 K in air until constant mass. This acid activation also removes sodium from its composition. The chemical composition of the three different clays is included in Table 1.

Reagents

Characterization of the clay adsorbent

Lead(II)-containing solutions with concentration of the heavy metal ion ranging from 1 to 2,500 mg L-1 were

The porous texture of all samples was determined by physical adsorption of gases (N2 at 77 K and CO2 at

Table 1 Composition mass% of M1, M2, and acidic-M2 montmorillonites before and after adsorption of lead Composition/mass%

M2

M1

Acidic-M2

Before

After

Before

After

Before

After

SiO2

78.88

67.58

63.97

69.26

74.62

53.54

Al2O3

12.75

14.32

16.72

17.91

17.88

12.97

MgO

1.91

2.38

3.37

2.12

2.87

1.91

Fe2O3

2.81

3.20

2.45

2.23

1.78

3.07

K2O

2.31

1.94

2.28

2.63

2.60

0.00

TiO2

0.14

0.37

0.17

0.21

0.12

0.00

Rb2O

0.02

0.00

0.03

0.00

0.05

0.02

ZrO2

0.01

0.05

0.01

0.04

0.01

0.08

SO3

0.11

0.00

0.00

0.00

0.08

0.00

Cl CaO

0.73 0.00

0.00 0.00

0.53 0.01

0.00 0.01

0.00 0.00

0.00 0.01

Na2O

0.00

0.00

10.40

0.00

0.00

0.00

P2O5

0.33

0.01

0.00

0.01

0.00

0.01

MnO

0.00

0.00

0.06

0.00

0.00

0.00

PbO

0.00

10.15

0.00

5.58

0.00

28.37

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273 K) using an automatic adsorption system (Autosorb-6, Quantrachrome Corporation) after sample out-gassing at 383 K under vacuum for 4 h. Nitrogen adsorption at 77 K was used for determining the total volume of micropores (VDR(N2)) (pore size smaller than 2 nm) by applying the Dubinin–Radushkevich (DR) equation (range of relative pressures used for the DR analysis was: 0.005 \ P/ P0 \ 0.17) and for determining the specific surface area by the BET equation (SBET), whereas the adsorption of CO2 at 273 K was used to assess the narrowest micropores (VDR(CO2)) (pore size smaller than around 0.7 nm) also by application of the Dubinin–Radushkevich equation at relative pressures below 0.025 [32–34]. For TEM observations, the samples were dispersed in water and supported on TEM grids. The images were collected using a JEOL (JEM-2010) microscope, working at an operation voltage of 200 kV. The TEM is coupled with EDX for the elucidation of chemical composition of the samples. X-ray fluorescence spectroscopy of the powder samples was made using a Philips PW1480 equipment with a UNIQUANT II software to determine element concentration in a semi quantitative way. Lead adsorption studies The clay samples were dried at 80 °C under vacuum for 24 h before adsorption. Then, 0.5 g of adsorbent was put in contact with 50 mL of an aqueous solution of Pb(NO3)2 with metal concentration ranging from 1 to 2,500 mg L-1 at 25 °C and for 24 h. Each adsorption experiment was done two times. The pH of the Pb(II) initial solution ranged from 2 to 6.0, avoiding the pH for precipitation of Pb(II) in aqueous solutions (pH [ 6.5). The concentration of Pb(II) in the solution was determined by atomic absorption spectroscopy coupled with plasma inductive (ICP) (PerkinElmer 7300-DV)). The Pb(II) concentration was also determined at different times in order to follow the kinetics of the process. The amount of adsorbed Pb(II) was calculated by difference between the initial concentration and that after a time t, according to the following equation: qt ¼

ðc0  ct ÞV 1; 000M

ð1Þ

where qt (mg g-1) is the amount of metal ion adsorbed on the adsorbent at time t, c0 is the initial metal ion concentration (mg L-1), ct is the concentration of metal ion in solution at time t (mg L-1), V is the volume of metal ion solution used (mL), and M is the mass of the adsorbent used (g). Langmuir and Freundlich isotherms were used to analyze the experimental results. The Langmuir model assumes that uptake of metal cations occurs on a homogeneous surface by monolayer adsorption without

any interaction between adsorbed cations. The model results in the following equation: ce 1 ce ¼ þ qe K l c m qm

ð2Þ

where qe is the amount adsorbed (mg g-1), ce is the equilibrium concentration of the adsorbate (mg L-1), and qm (mg g-1) and Kl (L mg-1) are Langmuir constants being qm the maximum adsorption capacity of adsorbent in a monolayer and Kl is related to the free energy of adsorption [35]. The Freundlich isotherm is an empirical equation based on adsorption on a heterogeneous surface. The equation is commonly represented by: 1 ln qe ¼ ln Kf þ ln ce n

ð3Þ

where qe is the amount adsorbed (mg g-1), ce is the equilibrium concentration of the adsorbate (mg L-1) and Kf (mg1-1/n L1/n g-1) and n are the Freundlich constants characteristics of the system, corresponding to the adsorption capacity and the strength of adsorption, respectively. In order to determine the adsorption kinetics of Pb(II) ions, first-order and second-order kinetics models were checked. The first-order rate expression [36] is expressed as follows: dqt ¼ k1 ðqe  qt Þ dt

ð4Þ

where qe and qt are the amounts of metal ions adsorbed onto the montmorillonite (mg g-1), respectively, at equilibrium and at time t and k1 is the first-order rate constant (min-1). After integration from t = 0 to t and from qt = 0 to qe, it becomes the Lagergren’s rate equation: logðqe  qt Þ ¼ log qe 

k1 t 2:303

ð5Þ

In most cases, the first-order equation of Lagergren does not apply well throughout the whole range of contact times and is generally applicable over the initial 20–30 min of the adsorption process. A pseudo-second-order rate law expression was also used; the kinetic rate equation is expressed as [37]: dqt ¼ k2 ðqe  qt Þ2 dt

ð6Þ

where k2 is the second-order rate constant (g mg-1 min-1). At boundary conditions from t = 0 to t and from qt = 0 to qe, the rate law becomes t 1 1 ¼ þ t qt k2 q2e qe

ð7Þ

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Adsorption thermodynamics The thermodynamic parameters of the adsorption, i.e. the standard enthalpy DH, Gibbs free energy DG, and entropy DS were calculated using the following equations: DG ¼ RT ln KL ln KL ¼

ð8Þ

DS  DH  T R

ð9Þ

DG ¼ DH  T  DS

ð10Þ

where R is the ideal gas constant (kJ mol-1 K-1), KL = kads/kd is the Langmuir constant and T is the temperature (K). DH and DS values can be obtained from the slope and intercept, respectively, of Van’t Hoff plots of lnKL (from the Langmuir isotherm) versus 1/T.

Results and discussion Characterization of the adsorbents Figure 1 shows the nitrogen adsorption/desorption isotherms at 77 K for natural and acidic activated samples. According to IUPAC classification, the shape of these isotherms is similar to Type II. The three isotherms are reversible at low relative equilibrium pressures, but at higher relative pressures they exhibit a hysteresis loop of the H3 type [38]. The shape of the isotherms indicates that the samples are mainly mesoporous solids but also contain some micropores. Such hysteresis loop appears in porous materials with slit-shaped pores or pores with narrow necks and wide bodies and, then, they can be associated with capillary condensation of liquid nitrogen in mesopores. The hysteresis phenomenon becomes more prominent after acid activation because the amount of mesopores increases by the structural deformation and reflects a change from the

160 140

cm3/g

120 100 80 60 40 20 0 0

0.2

0.4

0.6

0.8

1

p/p 0 Fig. 1 N2 adsorption isotherms at 77 K of the non-sodium montmorillonite (M1: striaght line), the sodium montmorillonite (M2: short dashed line) and the acid activated montmorillonite (acidic-M2: long dashed line)

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laminar structure of the raw clay to a delaminated structure [39]. Figure 2 contains the TEM images of the M1 (Fig. 2a) and M2 (Fig. 2b) samples, which show a laminar structure. In the case of M1 clay it is possible to determine the sep˚ . However, it can aration between layers being around 3.6 A be clearly observed differences in the morphology of both samples with respect to that observed for acidic-M2 clay (Fig. 2c) that shows a mainly exfoliated morphology in agreement with the nitrogen isotherms. The specific surface area of the samples was obtained from the BET method by using the adsorption data. Table 2 includes the porous texture characterization data of the three samples. It can be observed that the specific surface area increases from 25 m2 g-1 for the M2 sample to 140 m2 g-1 for the acid activated sample. The micropore and mesopore volumes also increase in the acidic activated sample, possibly as a consequence of the dissolution of the exchangeable cations-like sodium and the partial dissolution of the structural cations like Fe3? and Mg2? (Table 1) [40]. Figure 3 shows the thermogravimetric analysis (TG) of the clays. The TG experiments contain the typical features for montmorillonites, that is, two main processes at around 85 and 550 °C. The first one corresponds to the evolution of weakly bonded water molecules with a loss between 20 and 30% by mass, due to the evolution of interlayer water. The second one can be associated to dehydroxylation of the octahedral sheet [41]. Above 650 °C the clays are completely calcined. Lead cations adsorption Figure 4 shows the adsorption isotherms obtained for the three samples at 25 °C. It can be observed that the shapes of the isotherms are similar for the three montmorillonites. The maximum adsorption capacity decreases in the order acidicM2 [ M1 [ M2. The acidic-montmorillonite (acidic-M2) shows the maximum adsorption capacity in agreement with the increase in the BET surface area values. The adsorption isotherms were adjusted mathematically to the Langmuir and Freundlich models and the characteristic parameters of these isotherms and the values of R2, which represents goodness-of-fit of experimental data to the models, are listed in Table 3. The value of n in the Freundlich model which is in the range 1–10 indicates favorable adsorption [42]. The best fit to the experimental data is provided by the Langmuir isotherm. The maximum adsorption capacities in the monolayer obtained from the Langmuir isotherm increase from 18.2 (M2) to 24.4 mg g-1 (acidic-M2). The Freundlich isotherm provides a fair fit to the data. The value of the constant, Kf, has been also calculated being between 3.40 and 5.44 (mg1-1/n L1/n g-1); however, in this case the coefficient of regression is lower than in the case of Langmuir isotherm.

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Fig. 2 TEM images of a nonsodium montmorillonite (M1), b sodium montmorillonite (M2), and c acid-activated montmorillonite (acidic-M2)

Table 2 Textural characterization of non-sodium montmorillonite (M1), sodium montmorillonite (M2), and acid-activated montmorillonite (acidic-M2) SBET/m2 g-1

Sample M1

32

M2 Acidic-M2

VDR(N2)/cm3 g-1

VDR(CO2)/cm3 g-1

Vmeso/cm3 g-1

0.09

0.01

0.16

25

0.09

0.01

0.16

140

0.19

0.01

0.68

After lead adsorption, the three montmorillonite clays were dried at 105 °C overnight and their composition was measured by X-ray fluorescence (see data in Table 1). It can be observed that the amount of lead adsorbed follows the same tendency to that obtained in the adsorption studies: acidic-M2 [ M1 [ M2. However, in the acidic-M2 sample, which does not contain initially sodium because it has been replaced by protons, a substitution of potassium is also produced. Then, it can be concluded that lead ions are replacing the proton sites. Effect of pH

three adsorbents at different pH values, obtained from an adsorption experiment with an initial concentration of Pb(II) of 2500 mg L-1. The lead cation adsorption increases as the pH value increases from 2 to 6 reaching a maximum. At lower pH values, the active sites of the montmorillonite are positively charged leading to an increase in the competition between protons and lead cations for the exchangeable sites in the adsorbent. As pH increases, this competition decreases and the sites of the montmorillonite become more negatively charged, what favors the adsorption of Pb(II) cations through electrostatic attraction [44]. At pH higher than six, precipitation of

The pH of the aqueous solution is an important controlling parameter in the adsorption process [43]. Thus, the effect of pH of the solution ranging from 2 to 8 was examined. Figure 5 contains the amount of Pb(II) adsorbed in the

M1 M3 M2

Mass/%

90

20

q/mg g–1

100

25

15

10

80

5 60

0

40

0 0

100

200

300

400

500

600

700

800

T/°C Fig. 3 Thermogravimetric analysis (TG) of non-sodium montmorillonite (M1), sodium montmorillonite (M2), and acid-activated montmorillonite (acidic-M2)

500

1000

1500

2000

Ceq/mg l–1 Fig. 4 Lead (II) adsorption isotherms on non-sodium montmorillonite (M1: asterisk), sodium montmorillonite (M2: inverted filled triangle), and acid-activated montmorillonite (acidic-M2: filled square) at pH 6, T = 298 K; m = 0.25 g of montmorillonite). Equilibrium time 24 h

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Table 3 Freundlich and Langmuir coefficients obtained from the adsorption isotherms of Pb(II) on the montmorillonite samples at 298 K Adsorbents

Freundlich coefficients 1-1/n

Kf/mg

L

1/n

g

Langmuir coefficients

-1

n

R

2

qm/mg g-1

Kl/L mg-1

R2

5.44

0.83

0.89

24.3

0.004

0.98

3.40

0.83

0.90

19.4

0.005

0.98

Acidic-M2

4.73

0.83

0.85

26.5

0.004

0.98

25

25

20

20

15

15

q/mg g–1

Absorption capacity/mg g–1

M1 M2

10

10 Acidic-M2 M2 M1

5

5

0

0 0

2

4

6

8

10

0

1

pH Fig. 5 Effect of pH on Pb(II) adsorption on non-sodium montmorillonite (M1: aterisk), sodium montmorillonite (M2: filled circle), and acid-activated montmorillonite (acidic-M2: open circle). Initial concentration of Pb(II) = 2500 mg L-1, adsorption time = 24 h

insoluble metal hydroxides takes place restricting the true adsorption process. Then, Pb(II) adsorption is maximum at pH 6, a value which agrees with the results obtained by other authors [45–47]. This change in montmorillonite adsorption behavior with pH suggests that its immobilization effect could be less effective at low pH. Hence, the addition of a higher amount of montmorillonite or a combination with lime would be required for lead(II) remediation at low pHs.

2

3

4

5

Time/H Fig. 6 Amount of Pb(II) adsorbed with time. Initial concentration = 2500 mg L-1 pH 5

experimental data. It can be observed that the second-order model fits better the experimental data in the whole range of Pb(II) concentrations used (from 1 to 2500 mg L-1) obtaining a correlation coefficient higher than 0.997 in all the cases. It can also be observed a good agreement between the experimental and the calculated qe values. This second-order model is based on the assumption that the rate limiting step may be a chemical adsorption involving valence forces through sharing or exchange of electrons between adsorbent and adsorbate [25]. Thermodynamic studies

Adsorption kinetics The kinetics of the lead adsorption on the three samples used in this study was examined. This study can be helpful to understand the mechanism and to design a suitable adsorbent for metal removal in water. In this study, batch adsorption kinetics were studied in terms of pseudo-firstorder and pseudo-second-order kinetics. Figure 6 shows the amount of Pb(II) adsorbed (qe) with time for an initial concentration of Pb(II) of 2500 mg L-1. Table 4 includes the relevant kinetic parameters, together with the correlation factor R2 for different lead concentrations, obtained after applying the two models commented in the experimental section (first-order and second-order kinetics) to the

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The results of thermodynamic calculations are shown in Table 5. The negative value for the Gibbs free energy for Pb(II) adsorption shows that the adsorption process is spontaneous and that the degree of spontaneity of the reaction increases with increasing temperature. The overall adsorption process seems to be endothermic (DHM1 = 47.75, DHM2 = 56.25, and DHAcidic-M2 = 58.40 kJ mol-1). Although not very high, these values of DH can be interpreted on the basis of considerably strong interaction between lead ions and montmorillonites surface. This result also supports the suggestion that the adsorption capacity of all montmorillonites used for Pb(II) increases with increasing temperature. One possible explanation of

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Table 4 Comparison of the first- and second-order adsorption rate constants, for calculated (qe,cal) and experimental (qe,exp) values at different lead concentrations Adsorbent

Concentration/ppm

qe.Exp/mg g-1

First-order kinetic model k1/min

M1

M2

Acidic-M2

-1

qe.Cal/mg g

R

k2.ads/g mg-1 min-1

qe.Cal/mg g-1

R2

21.7

0.021

20.5

0.984

0.0023

22.1

0.999

1,500

21.7

0.018

21.3

0.938

0.0016

22.2

0.999

1,000 2,500

17.9 17.8

0.012 0.025

14.2 17.4

0.935 0.990

0.0020 0.0028

18.4 18.2

0.999 0.999

1,500

17.8

0.019

17.4

0.999

0.0021

18.3

0.998

1,000

15.1

0.015

13.9

0.996

0.0017

15.6

0.997

2,500

23.8

0.022

22.8

0.983

0.0021

24.3

0.999

1,500

23.8

0.033

39.6

0.852

0.0015

24.5

0.998

1,000

20.0

0.016

18.9

0.993

0.0018

20.5

0.999

Table 5 Thermodynamic constants for the adsorption of Pb(II) on M1, M2, and acidic-M2 montmorillonites at various temperatures Adsorbent

Temperature/K

DG/ kJ mol-1

M1

298

-43.68

308

-46.38

318

-49.08

298

-78.21

308

-81.51

318

-84.81

298

-31.31

308

-33.71

318

-36.11

Acidic-M2

Second-order kinetic model 2

2,500

positive DH is that the Pb ions are well-solvated. In order for the Pb ions to be adsorbed, they have to lose part of their hydration sheath. This dehydration process of the ions requires energy. This energy of dehydration supersedes the exothermicity of the ions getting attached to the surface. We can say that the removal of water from the ions is essentially an endothermic process and it appears that endothermicity of desolvation process exceeds that of the heat of adsorption to a considerable extent. The values of free energy change DG are negative as expected for a spontaneous adsorption process [48]. Table 5 also shows that the DS value was positive. This occurs as a result of redistribution of energy between the adsorbate and the adsorbent. Before adsorption occurs, the heavy metal ions near the surface of the adsorbent will be more ordered than in the subsequent adsorbed state and the ratio of free heavy metal ions to ions interacting with the adsorbent will be higher than in the adsorbed state. As a result, the distribution of rotational and translational energy among a small number of molecules will increase with increasing adsorption by producing a positive value of DS and randomness will increase at the solid-solution

M2

-1

DH/ kJ mol-1

DS/ kJ mol-1 K-1

36.78

0.27

20.13

0.33

40.21

0.24

interface during the process of adsorption. Adsorption is thus likely to occur spontaneously at normal and high temperatures because DH [ 0 and DS [ 0.

Conclusions This study demonstrates that the two Algerian montmorillonites studied (sodium, M2, and no-sodium, M1) are effective adsorbents for Pb(II) removal from aqueous solutions. The results obtained show that the adsorption of lead on acidic-activated montmorillonite (acidic-M2) is higher than on the other two montmorillonites. Moreover, both adsorption capacity and adsorption rate are strongly dependent on the pH of solution. Adsorption equilibrium can be modeled by Langmuir isotherms. The adsorption capacities were 21.6, 18.2, and 24.4 mg g-1 for M1, M2, and acidic-M2, respectively, according to the Langmuir model. The kinetics of adsorption can be described by a model of a pseudosecond-order because of the strong correlation of the experimental results obtained. It can be concluded that the use of montmorillonite as an adsorbent may be an alternative to more costly materials such as activated carbon for the treatment of liquid wastes containing metal ions. The thermodynamic parameters such as DH, DS, and DG were computed from the experimental data. These values are more consistent of a physical sorption process. The low values of enthalpy validate well the assumption of a physical adsorption. In the case of montmorillonites used, the positive value of DS indicates the increasing randomness of the system. Acknowledgements This study has been financed by the AECID (projects AECID-PCI A/019533/08 and A/023858/09) and Ministerio de Ciencia e Innovacio´n (project MAT2010-15273). The National Agency for the Development of University Research (CRSTRA), the

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Author's personal copy 1076 Directorate General of Scientific Research and Technological Development (DGRSDT) of Algeria.

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