Preparation, characterisation and application of thermally treated Algerian halloysite

Microporous and Mesoporous Materials 158 (2012) 47–54

Contents lists available at SciVerse ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Preparation, characterisation and application of thermally treated Algerian halloysite Samir Kadi a, Salima Lellou a, Kheira Marouf-Khelifa b, Jacques Schott c, Isabelle Gener-Batonneau d, Amine Khelifa b,⇑ a

Laboratoire de Chimie et Environnement, Université Ibn Khaldoun Tiaret, BP 78 Zaaroura, Tiaret 14000, Algeria Laboratoire de Structure, Elaboration et Applications des Matériaux Moléculaires (S.E.A.2M.), Département de Chimie, Université de Mostaganem, B.P. 981, R.P., Mostaganem 27000, Algeria c Laboratoire Géosciences Environnement Toulouse (GET)-CNRS-IRD-OMP-Université de Toulouse, 14, Avenue Edouard Belin, 31400 Toulouse, France d Institut de Chimie des Milieux et Matériaux de Poitiers IC2MP – UMR 7285, 4 rue Michel Brunet, 86022 Poitiers, France b

a r t i c l e

i n f o

Article history: Received 4 January 2012 Received in revised form 4 March 2012 Accepted 6 March 2012 Available online 14 March 2012 Keywords: Halloysite Thermal treatment Characterisation Adsorption Pb(II)

a b s t r a c t Natural halloysite was treated at different temperatures in the range 200–1000 °C. The resulting materials were characterised by TGA, DTA, TEM, XRD, FTIR and nitrogen adsorption. The characterised halloysitic solids were employed as Pb(II) adsorbents from aqueous solutions. The crystal structure of halloysite is stable up to 400 °C. The thermal treatment at 600 °C results in the formation of dehydroxylated halloysite, due to –OH release from the structure. A poorly organised structure and a progressive amorphisation of the structure were obtained not only for H600 but also H800 (halloysite treated at 800 °C). For H1000, a recrystallisation process arose from amorphous substance, materialised by the formation of c-Al2O3 and amorphous SiO2. Nanotubular particles were obtained for H600 proving that the thermal treatment at 600 °C conserves the original morphology of halloysite. The size distribution of the tubes was found to be quite broad. H1000 presents both tubular and plate morphologies. The nanotubes were however damaged. The values of specific surface area remained more or less constant up to 800 °C, beyond which it drastically decreased from 63 m2/g for H200 to 17.8 m2/g for H1000. Whatever the sample, total volume was mainly represented by mesopores, while micropores volume was negligible. The adsorption isotherms of Pb(II) were of L-type. The affinity follows the sequence: H200 > H400 > H800 > H600 > H1000. This evolution was explained in terms of the surface characteristics of the adsorbents. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Halloysite is a dioctahedral 1:1 clay mineral in which an alumina octahedral sheet is related to a silica tetrahedral sheet. These sheets are bound to each other by covalent bonds, forming layers. In the formation of the halloysite lattice, hydroxyl groups of one layer are bound to O2 ions of another layer by hydrogen bonds. Halloysite differs from kaolinite, another 1:1 clay mineral, by the presence of interlayer water [1] and by the morphology of the crystals, which are curved or rolled. A diagrammatic sketch of the structure of hydrated halloysite is given in Fig. 1 [2]. Halloysite was employed in many fields. The incorporation of a small amount of halloysite nanotubes (HNTs) increased the strength, stiffness and toughness of polymers [3,4]. Halloysite was also used as adsorbent for 5-amino salicylic acid [5], Cu(II) [6], Cr(VI) [7], methylene blue [8] and malachite green [9]. A ⇑ Corresponding author. E-mail address: [email protected] (A. Khelifa). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.03.014

heterogeneous catalyst in the methylic and ethylic esterification of lauric acid was prepared from raw halloysite [10]. The industrial uses of halloysite are closely related to its reactivity and surface properties, which are remarkably improved after modification. The latter can be achieved inter alia by thermal activation. If many studies were dedicated to the thermal transformation of kaolinite [11], a systematic investigation of the thermally modified halloysites properties is sparsely available in the literature. The objective of this study was to investigate the modifications undergone by Algerian halloysite owing to thermal treatment at different temperatures, in the range 200–1000 °C and at interval of 200 °C. The obtained materials were characterised by TGA, DTA, TEM, XRD, FTIR and nitrogen adsorption. The modified halloysitic solids were applied as Pb2+ adsorbents from aqueous solutions and the results compared to lead(II) adsorption properties of a number of different materials [12–16]. The use of clays and other low-cost materials as alternative adsorbents received a special attention for removing pollutants from wastewater [17–26].

48

S. Kadi et al. / Microporous and Mesoporous Materials 158 (2012) 47–54

Fig. 1. Diagrammatic sketch of the structure of hydrated halloysite.

2. Materials and methods 2.1. Materials Halloysite from Djebel Debbagh, Guelma (eastern region of Algeria) was ground, ultrasonically dispersed in deionised water and dried, after separation by filtration. Its characteristics were reported in a previous work [27]. Samples of the starting material were heated in a muffle furnace at 200, 400, 600, 800 and 1000 °C, at a rate of 10 °C/min. Each sample was processed at the relevant temperature for 2 h. It is well-known that 2 h is a time sufficient so that heat penetrates until the interior of the particle [28,29]. The samples were named H, H200, H400, H600, H800 and H1000. 2.2. Characterisation The modified halloysitic materials were characterised by thermal analysis (TG-DTA), transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR) and sorptometric study (nitrogen adsorption). Thermal analysis (TG-DTA) was performed on a Netzsch Sta 409 C instruments (Germany). Approximately 190 mg of halloysite was heated in an alumina crucible with a heating rate of 17 °C/min, under an atmosphere of high purity N2. TEM images were determined with a Jeol 2100 electron microscope (Japan). An EDAX detector for X-ray energy dispersive analysis is attached to this microscope. The

clay sample was previously ultrasonically dispersed in ethanol for 5 min. X-ray powder diffraction patterns were obtained using a Philips PW 1830 diffractometer (Netherlands) with CoKa radiation (k = 0.1789 nm) operating at 40 kV and 25 mA. The XRD data were collected over a 2h range of 5–90° with a step width of 0.03°. Fourier transformed infrared (FTIR) spectra were acquired on a Nexus Nicolet spectrometer (USA), using an Attenuated Total Reflection (ATR) accessory with a zinc selenide crystal. Each spectrum was recorded in absorption mode by accumulating at least 256 scans with a resolution of 2 cm1 in the region from 4000 to 400 cm1. The data were processed with the EZ OMNIC software. The assessment of the samples porosity and the crystallites surfaces were realised by nitrogen adsorption–desorption. These measurements were performed at 77 K using a Tristar instrument (Micromeritics, Norcross, GA, USA). Before measurement, the samples were outgassed under secondary vacuum at 623 K for 12 h. Specific surface areas were calculated by BET method. External surface areas and micropore volumes were determined by the t-plot method. Mesopore volumes and pore size distributions were calculated from the desorption branch of the corresponding nitrogen isotherm, using the Kelvin equation and the BJH method with the parameters for the thickness of the adsorbed layers from the Harkins–Jura equation.

2.3. Adsorption procedure The adsorption experiments of lead(II) (Lead(II)-chlorid, Merck, Germany) were performed via the batch method at 25 ± 1 °C and a pH of 6. 0.04 g of the halloysitic clay were mixed with 20 mL of aqueous lead nitrate solution in a concentration range of 10– 200 mg Pb2+/L. pH of the dispersions was adjusted to 6 by adding negligible volumes of 0.1 M HNO3 or 0.1 M NaOH. After equilibration, the solution was separated by filtration. The amount of adsorbed lead ions was determined from the difference between the initial and final concentrations. The concentration of Pb(II) in the sample was determined by flame atomic absorption spectrophotometry (FAAS), using a Perkin-Elmer 400 series spectrophotometer (USA). The instrumental working conditions were: air/ acetylene flame, lamp current: 10 mA, wavelength: 283.3 nm, detection limit: 0.02 mg/L. Before the analysis, the sample was diluted to have the concentration in the range of 0.2–20 mg/L.

Fig. 2. DTA and TG curves for halloysite.

S. Kadi et al. / Microporous and Mesoporous Materials 158 (2012) 47–54

3. Results and discussion 3.1. Thermal analysis TG and TDA curves are depicted in Fig. 2. These experiments show that the decomposition process of the Algerian halloysite takes place in three main steps: (i) A first endothermic peak in the range 50–240 °C, corresponds to the release of water adsorbed on the surface of the particles, (ii) structural decomposition of halloysite occurs in the second endothermic peak between 480 and 640 °C. The peak centred at about 580 °C is assigned to the dehydroxylation of structural aluminol groups present in halloysite [30] and (iii) an exothermic peak at 993 °C could be explained by the formation of amorphous SiO2 and c alumina [31]. The thermogravimetric curve, TG, highlights a continual mass loss between 25 and 1100 °C. The total loss is 12.5%. In the dehydroxylation range, a steep slope was obtained. It corresponds to a major weight loss of 6.7%, between 400 and 700 °C. The dehydroxylation of halloysite consists in the following reaction:

d-spacing of 4.4 Å, with relative intensity of 100%, which is in accordance with the results obtained by TEM. Also observed were quartz (q) and calcite (c) reflections. The low intensities indicated minor quantities of quartz and calcite. The thermal treatment at 200 and 400 °C did not cause significant changes of structure except that the intensity of the peaks characteristic of halloysite increased for H200 and H400. This increase may be ascribed to that of relative content of halloysite in the sample after the removal of physisorbed water. This shows that the crystalline structure of halloysite is stable up to 400 °C. For solids treated at 600 and 800 °C, no clear diffraction peak was observed. This indicates a poorly organised structure and a progressive amorphisation of the structure, which is due to the release of OH radicals. The latter, belonging to the halloysite structure, require higher temperatures to break –OH bond. For H1000, a recrystallisation process arose from amorphous substance, materialised by the appearance of several reflections. The peaks

dehydroxylation

Al2 O3  2SiO2  2H2 O

!

Al2 O3  2SiO2 þ 2H2 O

ð1Þ

By calculating the theoretical mass loss from reaction 1, one found 14%. The difference between this value with that experimental, i.e. 6.7%, would be explained by a partial dehydroxylation, which could be due to a structure resistance for releasing the totality of hydroxyl groups. 3.2. Transmission electron microscopy TEM images of halloysite, H600 and H1000 are presented in Fig. 3. The starting material, H, evidences particles having a cylindrical shape and contains a transparent central area that runs longitudinally along the cylinder, indicating that the nanotubular particles are hollow and open-ended. The particles are of rather different sizes both in diameter and in length. Their external diameters vary from 50 to more than 100 nm while internal diameter is about 10 nm. These rolled tubes consist in a number of aluminosilicate sheets, curved, and closely packed. One finds 2 layer distances, one of 7.2 and an other of 4.4 Å corresponding to the reflections (0 0 1) and (1 1 1), respectively. Nanotubular particles were also obtained for H600 proving that the thermal treatment at 600 °C conserves the morphology of halloysite. Their external and internal diameters vary from 30 to 180 nm and from 10 to 30 nm, respectively. A phase rich in Al, O and Mn was evidenced by EDAX in the microscope. This phase consists of agglomerated small plates of diameter about 10 nm. Interlayer spacing could not be highlighted. This is probably due to the fact that the layers are immediately destroyed under the beam of electrons. H1000 presents both tubular and plate morphologies. The nanotubes are damaged. Their external diameters are about 70 nm. The dehydroxylation phenomenon followed by the formation of amorphous SiO2 and c-Al2O3, at 1000 °C, alter the morphology of halloysitic clays. 3.3. XRD analysis XRD patterns of unheated and activated halloysites at 200, 400, 600, 800 and 1000 °C are shown in Fig. 4. The XRD diagram of the starting material showed a basal reflection at 7.6 Å (2h = 13.5°) with a shoulder at 10 Å (hh) (2h = 10.3°) indicating a partially hydrated halloysite. A feature of the X-ray pattern of halloysite having common tubular morphology is the very intense reflection at 4.4 Å. The X-ray diffractogram of the Algerian halloysite showed a prominent reflection at 2h value of 23.5°, corresponding to

49

Fig. 3. Transmission electron microscopy images of halloysitic solids.

50

S. Kadi et al. / Microporous and Mesoporous Materials 158 (2012) 47–54

Fig. 4. XRD patterns of halloysitic solids; hh: hydrated halloysite; h: halloysite; c: c-alumina; c: calcite; q: quartz.

in question were identified as being due to c-Al2O3. In parallel to these reflections, the broad band observed in the 2h range of 5– 18° is probably due to amorphous SiO2. In other words, the structural rearrangement of dehydroxylated halloysite results in the formation of c-Al2O3 and amorphous SiO2. All these observations are consistent with the results obtained from the thermal analysis.

3.4. FTIR analysis The FTIR spectra of unheated and thermally treated halloysites between 200 and 1000 °C, are shown in Fig. 5. The halloysite spectrum showed three bands in the region 3700–3600 cm1, at 3699, 3674 and 3616 cm1. This region is specifically assigned to

51

S. Kadi et al. / Microporous and Mesoporous Materials 158 (2012) 47–54

Fig. 5. IR spectra of halloysitic solids.

Table 1 Textural parameters of the modified halloysites. Samples

H200 H400 H600 H800 H1000

Specific surface area SBET (m2/g)

External surface area Sext (m2/g)

Internal surface area Sint (m2/g)

Total volume of pores (cm3/g)

Micropores volume (cm3/g)

Mesopores volume (cm3/g)

Mesopores v olume Total v olume

63 62.7 60.5 54 17.8

50.1 54.8 48.9 49.5 9.6

12.9 7.9 11.6 4.5 8.2

0.288 0.276 0.306 0.265 0.089

0.006 0.003 0.005 0.002 0.004

0.282 0.273 0.301 0.263 0.085

97.9 98.9 98.4 99.2 95.5

stretching vibrations of hydroxyl groups. The bands at 3699 and 3674 cm1 are attributed to the inner surface hydroxyl groups. The band at 3616 cm1 is due to the inner hydroxyl-stretching vibration [32]. Interlayer water is indicated by the stretching vibration band at 3531 cm1. The band centred at 1593 cm1 is attributed to bending vibrations of adsorbed water [33]. The 1124 cm1 band is assigned to stretching mode of apical Si–O. The bands between 1000 and 400 cm1 are attributed to Si–O–Si, Al–O–H and OH vibrations. The bands observed at 2899 and 2337 cm1 are due to calcite and quartz, respectively. No substantial changes were observed up to 400 °C. The featureless vibrational spectra of H600 and H800 combined to the broadening of the 1092 cm1 band (1119 cm1 for H800) indicate an amorphisation of the structure. In particular, the disappearance of the three bands of the region 3700–3600 cm1 confirms the structure dehydroxylation. At 1000 °C, the appearance of new absorption bands implies the formation of a new substance from dehydroxylated halloysite. On the basis of the discussion above,

 100

(%)

it is probably about c-Al2O3. Sohlberg et al. [34] showed that c-alumina has a spinel structure, which exists over a range of hydrogen content associated to the empirical formula H3mAl2mO3. For H1000, the three bands observed in the region 3900–3600 cm1 correspond to the hydrogen bond between the various hydroxyl groups available in c-alumina. 3.5. Textural analysis The nitrogen adsorption–desorption isotherms depicted in Fig. S1 in the supplementary data belong to the type IV, according to the IUPAC classification. This isotherm type is typical of mesoporous structures. The textural parameters are summarised in Table 1. SBET remained more or less constant up to 800 °C, beyond which it drastically decreased from 63 m2/g for H200 to 17.8 m2/g for H1000. Whatever the sample, the values of external surface are higher than those of internal surface, while the volumes of micropores are negligible. Total volume is mainly represented by

52

S. Kadi et al. / Microporous and Mesoporous Materials 158 (2012) 47–54

0.018 0.016

H200

0.014 δ vp/δ r p

0.012 0.01 0.008 0.006 0.004 0.002 0 0

5

10

15

20

25

30

35

r p (nm ) 0.07

0.06 0.05

0.06

H400 δvp/δr p

0.04 δ vp/δ r p

H600

0.05

0.03 0.02

0.04 0.03 0.02

0.01

0.01

0 0

5

10

15

20

25

30

0

35

0

5

10

r p (nm )

20

25

30

35

r p (nm )

0.009

0.05 0.045

0.008

H800

0.04

H1000

0.007

0.035

0.006

0.03

δvp/δr p

δ vp/δ r p

15

0.025 0.02

0.005 0.004

0.015

0.003

0.01

0.002

0.005

0.001

0

0

0

5

10

15

20

25

30

35

0

5

10

r p (nm )

15

20

25

30

35

r p (nm )

Fig. 6. Pore size distribution diagrams of halloysitic solids.

mesopores. They represent up to 99% of total volume for H800. The lowest value of total volume of pores was found for H1000. The solid obtained from halloysite calcined at 1000 °C consists of Al2O3 grains and amorphous silica. By calcining kaolin minerals (same family than halloysite) at around 950 °C, Okada et al. [35] obtained a microtexture consisting of very fine spinel-phase grains of c-alumina precipitating within an amorphous silica matrix. Such a microtexture would somewhat explain the structure thickening of our halloysite treated at 1000 °C. The pore-size distribution diagrams are shown in Fig. 6. For H200, the diagram shows a broad peak at a radius of 6.9 nm. This again confirms the mesoporous character of the Algerian halloysite.

The distribution is multimodal. Each mode corresponds to a maximum of the curve. The intensity of these maxima is unequally distributed. The H200 curve presents three very distinct modes centred on pores radii of 6.9, 14.3 and 19.8 nm. The curves of H400, H600 and H800 are almost identical. The pore size was less segmented; the obtained peaks were less distinct. This proves that the thermal treatment from 400 to 800 °C leads to the formation of an irregular porosity, heterogeneous, inside the halloysitic matrix. The pores of H1000 showed the sharpest distribution consisting of three modes. The pore sizes were 10.2, 12.3 and 16.1 nm. H1000 presents a narrower distribution comparatively to H200, inside 10.2–16.1 nm against 6.9–19.8 nm, respectively. This might be

S. Kadi et al. / Microporous and Mesoporous Materials 158 (2012) 47–54

to increasing SO-groups, can thereby more easily take place. High removal efficiency at pH > IEP can be attributed to electrostatic interactions between the positively charged lead(II) ions and negatively charged binding sites on halloysitic materials surface. As seen from Fig. 7, the affinity of the heated halloysites towards lead follows the sequence: H200 > H400 > H800 > H600 > H1000. The highest and lowest affinities manifested by H200 and H1000, respectively, are in line with that of specific surface area. The uptake capacity was 1.9 times larger in comparison with that of the sample calcined at 1000 °C. The reason why Pb2+ adsorption onto halloysitic materials evolves as mentioned above might be explained by a specific interaction between this adsorbate and hydroxyl groups of surface (samples H200 and H400), or siloxane groups (samples H600 and H800). The same considerations have been developed during the adsorption of benzene by halloysites treated between 200 and 1000 °C [43].

30

H200 H400 H600

25

H800 H1000 20

Q e(m g/g)

53

15

10

5

4. Conclusion 0 0

20

40

60

80

100

120

140

160

180

200

Ce(mg/g) Fig. 7. Adsorption isotherms of Pb(II) onto halloysitic solids at 25 °C.

explained by the uniform microtexture formed at 1000 °C, which consisted of fine c-alumina grains interspaced with residual amorphous silica [36]. 3.6. Lead(II) adsorption The equilibrium adsorption of lead(II) was studied via a batch process at 25 ± 1 °C and pH of 6. The adsorption isotherms were carried out in triplicate. The results were determined with a variation coefficient lower than 5%. No changes in lead concentrations of the solution were observed after 2 h. Thus, a shaking time of 2 h was sufficient length to achieve equilibrium. Sari et al. [37] showed that 98% of Pb(II) was adsorbed by Turkish kaolinite during first 30 min. Unuabonah et al. [38] showed that 80 min was sufficient to adsorb most Pb2+ ions onto phosphate-modified kaolinite. H200 adsorbed 27.4 mg g1 more than that adsorbed by montmorillonite (21.7 mg g1) [39] but less than the amount adsorbed by black gram husk (BGH) (49.97 mg g1). However for BGH, the authors [40] considered lead(II) concentrations of 800 mg/L against 200 mg Pb2+/L, in our case. Giles et al. [41] proposed that for adsorption from dilute solutions there were four basic types of adsorption isotherm. The mechanistic interpretation of these isotherms was based upon their initial slope and the shapes that the curves assumed at higher solute concentrations. Using this classification, the experimental isotherms obtained in the present study were of L-type (Fig. 7). This isotherm shape indicates that there is no strong competition between the solvent and the adsorbate for surface sites. The initial curvature of the L curve shows that the contaminant has a high affinity for the surface. As more surface sites are occupied, it becomes increasingly difficult for the adsorbate to find a vacant site. As mentioned in a previous work [27], Algerian halloysite has an IEP approximately equal to 2.5. The surface charge of this clay is positive at pH < 2.5 and negative for pH > 2.5. Knowing that the sorption experiments were performed at pH 6, the surface of halloysite becomes charged negatively as follows [42]:

BSOH þ OH ¼ BSO þ H2 O

ð2Þ

At pH values higher than IEP, the association of lead cations with more negatively charged halloysitic materials surface, owing

The effect of thermal treatment on the physico-chemical and adsorptive properties of Algerian halloysite was investigated. Thermal treatment involves a series of solid-state chemical transformation on the temperatures interval explored. Thermal analysis showed that the decomposition takes place in three main steps: release of physically bounded water in the range 50240 °C, structural decomposition through a dehydroxylation of structural aluminol groups between 480 and 640 °C and a recrystallisation process at about 1000 °C. The halloysitic solids conserved the original nanotubular morphology up to 600 °C. At 1000 °C, the nanotubes were damaged. The crystal structure of halloysite is stable up to 400 °C. A poorly organised structure and a progressive amorphisation of the structure were obtained between 600 and 800 °C. The featureless FTIR spectra of H600 and H800 confirm this structure amorphisation. For H1000 (halloysite treated at 1000 °C), a recrystallisation process arose from amorphous substance, materialised by the formation of c-Al2O3 and amorphous SiO2. Specific surface area drastically decreased from 63 m2/g for H200 to 17.8 m2/g for H1000. Whatever the sample, total volume was mainly represented by mesopores, while the pore-size distribution was multimodal. The affinity of the halloysitic materials towards Pb(II) follows the sequence: H200 > H400 > H800 > H600 > H1000. The uptake capacity of H200 was 1.9 times larger in comparison with that of the sample calcined at 1000 °C. The sequence affinity might be explained by a specific interaction between Pb2+ ions and hydroxyl groups of surface (samples H200 and H400), or siloxane groups (samples H600 and H800). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso. 2012.03.014. References [1] G.J. Churchman, R.M. Carr, Clays Clay Miner. 23 (1975) 382–388. [2] H.H. Murray, Applied Clay Mineralogy, Occurrences, Processing and Application of Kaolins, Bentonites, PalygorskiteSepiolite, and Common Clays, Elsevier, Amsterdam, 2007. [3] Y. Lin, K.M. Ng, C.M. Chan, G. Sun, J. Wu, J. Colloid Interface Sci. 358 (2011) 423–429. [4] U.A. Handge, K. Hedicke-Höchstötter, V. Altstädt, Polymer 51 (2010) 2690– 2697. [5] M.T. Viseras, C. Aguzzi, P. Cerezo, C. Viseras, C. Valenzuela, Micropor. Mesopor. Mater. 108 (2008) 112–116. [6] S. Mellouk, A. Belhakem, K. Marouf-Khelifa, J. Schott, A. Khelifa, J. Colloid Interface Sci. 360 (2011) 716–724. [7] J.H. Wang, X. Zhang, B. Zhang, Y.F. Zhao, R. Zhai, J.D. Liu, R.F. Chen, Desalination 259 (2010) 22–28.

54

S. Kadi et al. / Microporous and Mesoporous Materials 158 (2012) 47–54

[8] M.F. Zhao, P. Liu, Micropor. Mesopor. Mater. 112 (2008) 419–424. [9] G. Kiani, M. Dostali, A. Rostami, A.R. Khataee, Appl. Clay Sci. 54 (2011) 34–39. [10] L. Zatta, J.E.F. da Costa Gardolinski, F. Wypych, Appl. Clay Sci. 51 (2011) 165– 169. [11] K. Okada, A. Shimai, T. Takei, S. Hayashi, A. Yasumori, K.J.D. MacKenzie, Micropor. Mesopor. Mater. 21 (1998) 289–296. [12] V.K. Gupta, A. Rastogi, J. Hazard. Mater. 152 (2008) 407–414. [13] V.K. Gupta, I. Ali, J. Colloid Interface Sci. 271 (2004) 321–328. [14] V.K. Gupta, M. Gupta, S. Sharma, Water Res. 35 (2001) 1125–1134. [15] V.K. Gupta, D. Mohan, S. Sharma, Sep. Sci. Technol. 33 (1998) 1331–1343. [16] S.K. Srivastava, V.K. Gupta, D. Mohan, J. Environ. Eng. 123 (1997) 461–468. [17] V.K. Gupta, P.J.M. Carrott, M.M.L. Ribeiro Carrott, Crit. Rev. Environ. Sci. Technol. 39 (2009) 783–842. [18] V.K. Gupta, A. Rastogi, J. Hazard. Mater. 153 (2008) 759–766. [19] V.K. Gupta, I. Ali, Environ. Sci. Technol. 42 (2008) 766–770. [20] I. Ali, V.K. Gupta, Nat. Protoc. 1 (2007) 2661–2667. [21] V.K. Gupta, I. Ali, V.K. Saini, J. Colloid Interface Sci. 315 (2007) 87–93. [22] V.K. Gupta, R. Jain, A. Mittal, M. Mathur, S. Sikarwar, J. Colloid Interface Sci. 309 (2007) 464–469. [23] V.K. Gupta, R. Jain, S. Varshney, J. Hazard. Mater. 142 (2007) 443–448. [24] V.K. Gupta, A. Mittal, L. Kurup, J. Mittal, J. Colloid Interface Sci. 304 (2006) 52– 57. [25] V.K. Gupta, A. Mittal, V. Gajbe, J. Mittal, Ind. Eng. Chem. Res. 45 (2006) 1446– 1453. [26] V.K. Gupta, A. Rastogi, M.K. Dwivedi, D. Mohan, Sep. Sci. Technol. 32 (1997) 2883–2912. [27] S. Mellouk, S. Cherifi, M. Sassi, K. Marouf-Khelifa, A. Bengueddach, J. Schott, A. Khelifa, Appl. Clay Sci. 44 (2009) 230–236.

[28] R. Marouf, N. Khelifa, K. Marouf-Khelifa, J. Schott, A. Khelifa, J. Colloid Interface Sci. 297 (2006) 45–53. [29] R. Marouf, K. Marouf-Khelifa, J. Schott, A. Khelifa, Micropor. Mesopor. Mater. 122 (2009) 99–104. [30] P. Yuan, P.D. Southon, Z. Liu, M.E.R. Green, J.M. Hook, S.J. Antill, C.J. Kepert, Phys. Chem. C 112 (2008) 15742–15751. [31] G. Qiu, T. Jiang, G. Li, X. Fan, Z. Huang, Scand. J. Metall. 33 (2004) 121–128. [32] R.L. Frost, J. Kristof, E. Horvath, J.T. Kloprogge, J. Colloid Interface Sci. 226 (2000) 318–327. [33] R.L. Frost, J. Kristof, G.N. Paroz, J.T. Kloprogge, J. Colloid Interface Sci. 208 (1998) 216–225. [34] K. Sohlberg, S.J. Pennycook, S.T. Pantelides, J. Am. Ceram. Soc. 121 (1999) 7493–7500. [35] K. Okada, H. Kawashima, Y. Saito, S. Hayashi, A. Yasumori, J. Mater. Chem. 5 (1995) 1241–1244. [36] Y. Saito, T. Motohashi, S. Hayashi, A. Yasumori, K. Okada, J. Mater. Chem. 7 (1997) 1615–1621. [37] A. Sari, M. Tuzen, D. Citak, M. Soylak, J. Hazard. Mater. 149 (2007) 283–291. [38] E.I. Unuabonah, K.O. Adebowale, B.I. Olu-Owolabi, J. Hazard. Mater. 144 (2007) 386–395. [39] S. Sen Gupta, K.G. Bhattacharyya, Appl. Clay Sci. 30 (2005) 199–208. [40] A. Saeed, M. Iqbal, M.W. Akhtar, J. Hazard. Mater. 117 (2005) 65–73. [41] C.H. Giles, T.H. Mac Ewan, S.N. Nakhwa, D. Smith, Chem. Soc. 93 (1960) 3973– 3993. [42] M. Alkan, B. Kalay, M. Dogan, O. Demirbas, J. Hazard. Mater. 153 (2008) 867– 876. [43] F. Carrasco-Marin, M. Domingo-Garcia, I. Fernandez-Morales, F.J. LopezGarzon, J. Colloid Interface Sci. 126 (1988) 552–560.