Alteration of non-swelling clay minerals and magadiite by acid activation

Applied Clay Science 44 (2009) 95–104

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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y

Alteration of non-swelling clay minerals and magadiite by acid activation A. Steudel a,c,⁎, L.F. Batenburg b, H.R. Fischer b, P.G. Weidler a, K. Emmerich a,c a b c

Institute for Functional Interfaces (IFG former ITC-WGT), Forschungszentrum Karlsruhe GmbH, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany TNO Science and Industry, De Rondom 1, 5612AP Eindhoven, The Netherlands Competence Center for Material Moisture (CMM), University of Karlsruhe, Kaiserstr. 12, D-76131 Karlsruhe, Germany

a r t i c l e

i n f o

Article history: Received 14 March 2008 Received in revised form 2 February 2009 Accepted 4 February 2009 Available online 21 February 2009 Keywords: Acid activation Kaolinite Sepiolite Illite Magadiite H2SO4 Surface area

a b s t r a c t The bulk material of three kaolins, a sepiolite, an illite and one magadiite were treated with 1, 5 and 10 M H2SO4 at 80 °C for several hours. The alteration of the non-swelling clay mineral structures was controlled by the individual character of each mineral (chemical composition and initial particle size). The reaction resulted in a successive dissolution of the octahedral sheets by edge attack. The number of substitutions by Mg or Fe in the octahedral sheet promoted the dissolution of these layers and the formation of a silica phase. High amounts of Al in the tetrahedral sheet of the clay minerals caused partial dissolution of these sheets. The dissolution of the octahedral cations occurred in the following order: Mg N Fe N Al. Thus, dioctahedral clay minerals were more stable against acid attack than trioctahedral clay minerals. The release of the octahedral cations caused the lightening of the material together with a development of micropores and an increasing specific surface. Dissolution and splitting of the particles required longer reaction times compared to swellable smectites and vermiculites. Acid activation of non-swelling clay minerals can be used to produce layered materials with simple chemical composition and high specific surface area. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Non-swellable clay minerals like illite, sepiolite and kaolinite are widely used in many industrial applications like ceramic, food, chemical and paper industries. They are applied in their natural form or after specific modifications (acid or alkali activation). The behavior of sepiolite against acid was predominately investigated. HCl was mainly used in these (Abdul-Latif and Weaver, 1969; Vicente-Rodríguez et al., 1995; Yebra-Rodríquez et al., 2003). The changes of the acid treated sepiolite were investigated by FTIR spectroscopy. The influence of acid on several clay minerals and nonclay minerals was subject of studies by Jozefaciuk and Bowanko (2002) and Carroll and Starkey (1971). They also used HCl, the clay minerals were montmorillonite, biotite, illite, kaolinite, halloysite, and vermiculite. Snäll and Liljefors (2000) activated mica- and chloriterich bulk clays and other minerals like amphibole, titanite, epidote and plagioclase with HCl. Only few workers have looked onto the behavior of kaolinite against acids (Aglietti et al., 1988; Hradil et al., 2002; Makó et al., 2006; Volzone and Ortiga, 2006). Aglietti et al. (1988) investigated the structural alteration of kaolinite reacted with 1 M H2SO4 at 170 °C by

⁎ Corresponding author. Institute for Functional Interfaces (IFG former ITC-WGT), Forschungszentrum Karlsruhe GmbH, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany. Tel.: +49 7247826805; fax: +49 07247823478. E-mail address: [email protected] (A. Steudel). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.02.001

FTIR spectroscopy. Hradil et al. (2002) used HCl and H2SO4, but their studies were carried out at 25 °C. Makó et al. (2006) used mechanochemically activated kaolinite. Volzone and Ortiga (2006) treated kaolinites calcined at 600 °C with H2SO4. The focus of this study was to prepare materials by acid reaction at conditions to maintain the layered or fibrous morphology and to develop a high specific surface area for a possible application as precursors for clay–polymer-nanocomposites. The present study proceeds previous work on alteration of swellable clay minerals by acid attack (Steudel et al., 2009). Differences in the dissolution mechanisms of swellable and non-swellable clay minerals (interlayer and edge attack versus merely edge attack) depending on reaction conditions and the properties of the resulting material were especially of interest. Although there are many studies on acid attack of non-swelling clay minerals, the characterization of the resulting materials was mainly restricted to the use of a single method. Acid attack causes a corrosion of the octahedral sheet, while the SiO4 and SiO3OH groups of the tetrahedral sheet stay largely intact. It is still under discussion if this process is complete or not. Thus, development of new applications focuses unanswered questions like resulting morphology and charge density for incorporation of cationic monomers or polymers. We used pristine material because for most industrial applications purification and separation of clay fractions (b2 μm) is too time consuming and too expensive. Several non-swelling dioctahedral and trioctahedral clay minerals (illite, kaolinite and sepiolite) with different chemical compositions were reacted with H2SO4. Special

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Table 1 Origin and the suppliers of samples. Sample

Product name

Main phase

Provenance

Supplier

9_Illite 10_PangelS9 13_Pol 14_Kaolex 15_Rogers 16_Mag

Arginotec NX Sepiolite Pangel S9 Kaolin Polwhite Kaolin Kaolex Kaolin Rogers Magadiite

Illite Sepiolite Kaolinite Kaolinite Kaolinite Magadiite (synthetic)

France Spain GB USA USA –

Commercial product of B + M Tolsa; Spain IMERYS; GB Kentucky-Tennessee Clay Company; Langley, South Carolina, USA Kentucky-Tennessee Clay Company; Sandersville, Georgia, USA TNO Science and Industry; Eindhoven, The Netherlands

attention was drawn on the dissolution kinetics of the octahedral sheet with respect to Al, Mg and Fe. 2. Materials Five pristine bulk materials containing non-swelling clay minerals of several clay groups as well as one synthesized alkali silicate (magadiite) were used (Table 1). The selected non-swelling clay minerals differ in their morphology and layer charge. Magadiite was included in the study due to its layered structure to prove the stability of layered silicates under acid attack. 3. Methods 3.1. Acid activation Amounts of 4 g of each sample were dispersed in 100 ml deionised water under continuous stirring. 300 ml of diluted acid were added to the clay dispersions to achieve the final concentration of 1–10 M H2SO4 (prepared from H2SO4 98%, p.a., Merck). Subsequently, the dispersions were heated to 80 °C. We investigated the influence of the reaction period with 5 M H2SO4 (Table 2) at 80 °C. Preliminary tests were performed at 20 h and the reaction time was adjusted according to degree of corrosion of the starting material. The reactions were terminated by cooling for 5 to 10 min in an ice bath. The samples were centrifuged, the supernatant was removed and the sediment was washed with deionised water until the conductivity of the supernatant solution was below 5 μS/cm corresponding to the conductivity of the deionised water in our laboratory. Extensive washing was mandatory for a consistent mineralogical characterization of the remaining solid material to avoid obscuring influence of excess chemicals. The extension of washing or the required purity of the produced materials for e.g. an application in nanocomposites has to be tested by performance tests while incorporating in polymers. Finally the dispersions were freeze-dried.

(Kleeberg and Bergmann, 2002). An internal standard (ZnO, 10%) was used to quantify X-ray amorphous SiO2 in the raw materials and in the reaction products. The cation exchange capacity (CEC) of the raw clays was measured with 0.01 M Cu-triethylentetramine (Meier and Kahr 1999; Amman et al., 2005) to estimate the maximum smectite content of the clays. The chemical composition of the natural and acid treated materials was determined with X-ray fluorescence analysis (XRF), using a MagiXPRO spectrometer (Phillips). The Fourier Transform Infrared Spectroscopy (FTIR, DRIFT) were obtained with a Bruker IFS66/S spectrometer combined with a diffuse reflectance accessory from Spectra-Tech Inc (DRIFT). Raw DRIFT data were transformed by a Kubelka–Munk function. The PeakFit program 4.0 (Jandel Scientific) was used for peak deconvolution. The peak areas of vibration bands connected to octahedral cations were calculated. Those data were compared with the adjusted amount of the unreacted octahedral cations like Al, Mg and Fe, which were measured by XRF. ICP-OES was used to determine the amount of Si, Al, Fe, Mg, Na, Ca and K in the supernatant reaction solutions. Changes in particle morphology were observed with an Environmental Scanning Electron Microscope (ESEM; Philips ESEM XL 30 FEG). Nitrogen adsorption using a Quantachrome Autosorb-1MP and BET evaluation (p/p0 = 0.05–0.32) was used to measure specific surface area (As) (Brunauer et al., 1932; Gregg and Sing, 1991). The micropore area (AMP b 2 nm) was determined by the t-plot according to De Boer et al. (1966). The samples were outgassed 24 h under vacuum at 95 °C. The solubility of aluminum salts in H2SO4 at concentrations of 1, 5 and 10 M was checked with aluminum sulphate (Al2(SO4)3 ⁎ 18H2O, M = 666.3 g⁎mol- 1).

4. Results 4.1. Mineralogy and appearance of the raw material

3.2. Characterization methods A detailed description of the used methods is given in Steudel et al. (2009). The mineralogical composition of raw and acid treated materials was determined on powdered samples by X-ray diffraction analysis (XRD), using a Siemens D5000 diffractometer (CuKa radiation), equipped with a graphite secondary monochromator. The patterns were recorded between 5° and 80° 2θ (0.02° 2θ, 5 s). Quantitative analysis was performed using the Rietveld program “Autoquan” (Agfa NDT Pantak Seifert GmbH_CO.KG, Version 2.7.0) Table 2 Reaction times of clay dispersions with 5 M H2SO4 at 80 °C. Sample

Reaction time (h) 1.5

9_Illite 10_PangelS9 13_Pol 14_Kaolex 15_Rogers 16_Mag

X X

5 X X

20 X X X X X

72

96 X

X

X

X X X

The phase content of all raw materials except of magadiite (pure phase) is shown in Table 3. All samples except magadiite contain Table 3 Phase content of the raw samples. Sample phases

9_Illite [%] 10_PangelS9 [%] 13_Pol [%] 14_Kaolex [%] 15_Rogers [%]

Kaolinitea Illitea Sepiolitea Phlogopitea Muscovite/ illitea Smectitea Quartz Calcite K-feldspars (orthoclase)a Plagioclase (anorthite)a Anhydrite Apatite Anatase

5.4 ± 0.7 76.4 ± 2.0 – 7.8 ± 1.2 –

– – 90.7 ± 2.2 – 8.2 ± 2.1

71.4 ± 1.4 – – – 9.2 ± 0.7

85.3 ± 1.3 – – – 6.6 ± 1.1

86.1 ± 2.6 – – – 3.3 ± 1.0

– 0.4 ± 0.3 2.4 ± 0.4 4.4 ± 0.6

– – 1.1 ± 0.8 –

5.1 ± 1.0 1.5 ± 0.3 – –

9.1 ± 2.9 – – –

1.1 ± 0.9

–

6.9 ± 1.1 1.7 ± 0.4 – 10.8 ± 0.6 –

–

–

1.4 ± 0.3 0.7 ± 0.4 –

– – –

– – –

– – 1.5 ± 0.3

– – 1.4 ± 0.3

a

Al2O3 containing phases.

A. Steudel et al. / Applied Clay Science 44 (2009) 95–104 Table 4 CEC of the kaolin. Sample

13_Pol

14_Kaolex

15_Rogers

CEC [meq/100 g]

6

4

10

associated mineral phases like other layer silicates and feldspars that contribute to the total Al2O3 content. The amount of smectite determined in the kaolin samples by Rietveld analysis was verified by the measured cation exchange capacity (Table 4). For this purpose a mean CEC for smectites of 82 meq/100 g and low CEC for illite + mica of b10 meq/100 g were assumed. Based on this calculation the CEC of the kaolinite in all three kaolin samples was 0–2 meq/100 g. The raw illite (9_Illite) consists of small agglomerates with slatshaped particles. Illite is characterized by a layer charge between 0.6 and 0.9 eq/FU. Illite has lower K+ contents in the interlayer space than the muscovite because K+ can be substituted by H3O+. Its Sepiolite contains a high content of Mg2+ in the octahedral sheet. In comparison to the other clay minerals like smectite, vermiculite and mica, sepiolite has only few substitutions in the octahedral and tetrahedral sheets. Therefore layer charge and CEC (measured: 15 meq/100 g) are low. In contrast to the other non-swelling minerals kaolinite (13_Pol, 14_Kaolex, 15_Rogers) belongs to the 1:1 clay minerals with a layer charge of about zero. Kaolinite consists of platy particles with a pseudohexagonale habitus. The three kaolin samples vary in chemical composition (Table 5) and particle sizes. Sample 13_Pol contains 40% (m/m) of the fraction b2 μm, the other two kaolin samples 80% of the fraction b2 μm. We used a synthetic magadiite synthesized by TNO Science and Industry (NL). Magadiite is not a clay mineral but a layered sodium silicate with the approximated formula Na2H2Si14O30 xH2O (Lagaly and Beneke, 1975; Scholzen et al., 1991; Schwieger et al., 1991). There are also many papers on the silicic acid formed by reaction of magadiite with acids. The magadiite consists of thick agglomerates with an irregular shape and partly sharp grain boundaries.

97

Table 6 XRF-analysis (not adjusted) of the acid treated illite (20 h), sepiolite (1.5 h), kaolinites (96 h) and magadiite (72 h) (5 M H2SO4, 80 °C), with respect to the dry mass. Oxides

9_Illite 10_PangelS9 13_Pol 14_Kaolex 15_Rogers 16_Mag

Time of treatment [h] 20

1.5

96

96

96

72

SiO2 [%] Al2O3 [%] MgO [%] Fe2O3 [%] TiO2 [%] MnO [%] Na2O [%] CaO [%] K2O [%] P2O5 [%]

95.97 2.66 0 0.19 0.11 b 0.01 0 b 0.01 0.97 0

85.55 9.44 0.07 0.36 0.04 b0.01 0 0 4.54 0

95.70 2.38 0 0.22 1.18 0 0 0 0.48 0.03

96.30 2.91 0.07 0.22 0.03 0 0 0.06 0.37 0.03

93.29 0.37 0 0.12 0.02 0 0 0 0 0

92.30 4.86 0 0.21 0.52 b 0.01 0 0 2.08 b 0.01

content compared to 14_Kaolex and 15_Rogers. Both had a similar Al2O3 content after 96 h (Table 6). The Al2O3 contents were not corrected by considering the mass loss by acid activation. After 1.5 h acid reaction the total mass of solid of the sepiolite sample was significantly reduced. Approximately 50% of the material was dissolved. In contrast, the mass loss of magadiite was only 20% after 72 h. The total mass of kaolin was decreased by 20–35% after 20 h, of illite by 60%. After 96 h the mass of kaolin was reduced by 45– 55%. Color changes during acid attack are described here because many applications e.g. in nanocomposites require light colored or transparent particles. The raw materials are colored. 10_PangelS9 and 13_Pol were of creamy color. 14-Kaolex was yellowish and 15_Rogers reddish. 9_Illite was dark brown. 16_Mag was white. After acid attack all materials showed different grades of white color.

4.2.2. Long range order Due to the acid attack the intensity of the basal reflection (d001) of the illite (1.01 nm) and d060 of 0.1498 nm decreased with increasing period of reaction (Fig. 2). After 20 h the (001) reflection was small but detectable. The characteristic reflections of the accessory minerals like

4.2. Reaction with H2SO4 4.2.1. Composition and color The chemical composition of the untreated material is reported in Table 5. The magadiite mainly consisted of Na+ and Si4+. The MgO and Fe2O3 content decreased below 0.5% after 20 h for 9_Illite and after already 1.5 h for 10_PangelS9 (Table 6). After 72 h the Na+ in the magadiite structure (16_Mag) was no longer detectable (Table 6). The total Al2O3 content of the kaolin samples decreased with the period of acid attack (Fig. 1). After 96 h 13_Pol had a higher Al2O3

Table 5 XRF-analysis of the bulk material including LOI. Oxides

9_Illite

10_PangelS9

13_Pol

14_Kaolex

15_Rogers

16_Mag

SiO2 [%] Al2O3 [%] MgO [%] Fe2O3 [%] TiO2 [%] MnO [%] Na2O [%] CaO [%] K2O [%] P2O5 [%] LOI [%]

47.56 22.02 3.27 7.93 0.08 0.06 0 1.46 6.82 0.41 11.1

53.59 2.51 22.93 0.81 0.12 0.03 0.09 0.34 0.63 0.05 18.9

49.72 33.85 0.30 0.96 0.04 0.02 0 0.03 3.02 0.16 11.9

44.72 36.34 0.08 1.58 1.58 0 0 0 0.47 0.10 14.2

45.69 35.98 0.33 0.97 1.39 0 0.00 0.16 0.27 0.07 15.1

77.91 0.36 0 0.20 0.03 0 6.09 0 0.02 0 15.4

Fig. 1. Al2O3 content of the residual solid including the unreacted oxides in % determined by XRF (error ± 2%) The material was reacted with 5 M H2SO4 for 0, 1.5, 5, 20, 72, 96 h at 80 °C.

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A. Steudel et al. / Applied Clay Science 44 (2009) 95–104

Fig. 2. XRD-pattern of the raw and acid activated illite (9_Illite) (5 M H2SO4, 80 °C); It: illite reflection with increasing 2θ (001); (002); (130); (060) Qz: quartz; Kao: kaolinite; Ph: phlogopite (060).

Fig. 4. XRD-pattern of the raw and acid activated kaolinite Polwhite (13_Pol) (5 M H2SO4, 80 °C); Kao: kaolinite reflection with increasing 2θ (001); (002) Qz: quartz; M: muscovite.

phlogopite and calcite also disappeared. The (110) reflection of the sepiolite disappeared after 1.5 h (Fig. 3). In contrast, the basal spacing d001 of muscovite was present after 1.5 h reaction with 5 M H2SO4 at 80 °C. The intensity of the basal reflections of the kaolinite in the three kaolins decreased with increasing acid attack (Fig. 4). After 96 h all kaolins showed small basal reflection. Fig. 5 displays the XRD patterns of the natural and acid treated magadiite under air dry conditions. The d001 peak is the strongest line in the XRD pattern with a spacing of 1.56 nm. After acid attack this basal spacing reduced to 1.32 nm.

4.2.3. Short range order The FTIR spectra of all samples showed distinctive changes, especially between 400 and 1200 cm- 1. Assignments are listed in Table 7 (Farmer, 1974; Wilson, 1994; Vicente-Rodríguez et al., 1995; Komadel et al., 1996; Frost et al., 2001; Liu, 2001; Makó et al., 2006). The intensity of the OH deformation bands at 912 and 877 cm- 1 and the weak absorption bands near 825 and 750 cm- 1 as well as the intensity of the tetrahedral Si–O–Al vibration at 538 cm- 1 decreased during prolonged acid attack of illite (Fig. 6). After 96 h the bands nearly disappeared and only a small and broad band at 538 cm- 1 was still observable.

The OH bands at 694 and 655 cm- 1 in the FTIR spectra of sepiolite (Fig. 7) as well as the vibration at 445 cm- 1 disappeared after 1.5 h of treatment. The Si–O vibrations at 1103 and 1024 cm- 1 disappeared in the asymmetric Si–O band at 1076 cm- 1, which had a broad shoulder at higher wavenumbers (1195 cm- 1). Also the Si–O band at 505 cm- 1 disappeared in the broad band at 464 cm- 1. Fig. 8 illustrates the FTIR spectra of raw kaolin (13_Pol) and the acid treated samples after 5, 20 and 96 h. The bands at 939, 917, 792, 754, and 539 cm- 1 are related to Al in the octahedral and tetrahedral position. The intensity of the OH bands at 939 and 917 cm- 1 as well as the area of the vibrations decreased during prolonged acid attack (Fig. 9) and after 96 h the bands nearly disappeared. Fig. 9 displays the content of unreacted Al compared to the area of the OH deformation bands at 939 and 917 cm- 1. The intensity of the bridging Si–O–Al vibrations at 792 and 754 cm- 1, as well as the intensity of the tetrahedral Si–O–Al vibration at 539 cm- 1 decreased during prolonged acid attack. The vibration at 792 cm- 1 disappeared in the new absorption band at 798 cm- 1, while the band at 754 cm- 1 disappeared completely. Only a small and broad band at 539 cm- 1 still existed after 96 h. Kaolins 14_Kaolex and 15_Rogers showed a similar behavior. The FTIR spectra of the untreated and treated magadiite are displayed in Fig. 10. In contrast to the other minerals the acid attack of magadiite caused minor modifications in the FTIR spectra. The po-

Fig. 3. XRD-pattern of the raw and acid activated sepiolite (10_PangelS9) (5 M H2SO4, 80 °C); M: muscovite; Qz: quartz; Fsp: feldspars.

Fig. 5. XRD-pattern of the raw and acid reacted magadiite (16_Mag) (5 M H2SO4, 80 °C).

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Table 7 Positions and assignment of the vibrational bands of the non-swelling clay minerals in cm- 1. 9_Illite 10_PangelS9 13_Pol 14_Kaolex 15_Rogers Assignment 1101

1195

1110

1112

1110

1030

1103 1076

1030

1033

1035

939

935

939

917

916

917

792 754

792 754

792 754

644 539

644 541

646 539

472

470

470

431

428

430

1024 981 912 877 823 750

622 538

694 655 622 526 505

472 464 445 431

Si–O stretching vibration (out-of-plane) Si–O stretching vibration Si–O stretching vibration (in-plane) Si–O stretching vibration Si–O stretching vibration Inner surface OH deformation AlAlOH bending Inner OH deformation AlFeOH bending AlMgOH bending Si–O–Al vibration Si–O–Al vibration OH translation OH translation Inner surface OH vibration Si–O–Al (out-of-plane) bending (Al in tetrahedral sheet) O–Si–O bending vibration Si–O (in-plane) bending associate with OH Si–O bending vibration Si–O–Mg bending vibration Si–O bending

sitions of most of the absorption bands did not shift. According to previous studies (Eypert-Blaison et al., 2001; Superti et al., 2007) the infrared spectra of magadiite can be divided into three parts. In the

Fig. 6. FTIR spectra of the raw and acid treated illite (9_Illite) (5 M H2SO4 at 80 °C).

Fig. 7. FTIR spectra of the raw and acid treated sepiolite (10_PangelS9) (5 M H2SO4, 1.5 h at 80 °C).

first region (1000–1300 cm- 1, four bands) can be assigned to antisymmetric stretching vibrations of Si–O–Si bridges. The second region 700–1000 cm- 1 (four bands) can be attributed to symmetric stretching vibrations of the Si–O–Si bridges. The third region (400– 700 cm- 1, three bands) can be assigned to Si–O–Si and O–Si–O bending vibrations. The intensity of the symmetric stretching vibrations of the Si–O–Si bridges and the Si–O–Si and O–Si–O bending vibrations decreased after acid attack. The broad absorption band at 1081 cm- 1 was sharper after acid attack, but no shift was observed. In all samples the intensity of the Si–O bands increased with increasing acid attack and new absorption bands appeared.

4.2.4. Morphology and surface After acid attack the particles of illite were agglomerated but the slat-like appearance is still observable (Fig. 11). The fibrous morphology of the sepiolite and the pseudohexagonale habitus of the kaolinite were still observed after acid attack (Figs. 12 and 13). The agglomerated particle of magadiite had an irregular shape, but the grain boundaries had softened against the raw particles after acid attack (Fig. 14). The unreacted illite had a larger specific surface area (AS = 94 m2/g) than vermiculite (AS = 36 m2/g, Steudel et al., 2009) and smectite (AS = 30 m2/g, Steudel et al., 2009), due to its small particle size (Table 8). When reacted 20 h with the acid, AS of illite increased to 194 m2/g, after 96 h AS was 161 m2/g. With prolonged acid attack AE and AS decreased. The volume of micropores as well as the ratio between micropore area (AMP) and specific surface area increased slightly (Table 9). In contrast to the other non-swelling materials sepiolite had a very high specific surface area (300 m2/g) and a high micropore area (AMP = 167 m2/g). Acid attack decreased AS (138 m2/g) and the micropore area (AMP = 18 m2/g). With further acid attack both values increased, but the initial values were not reached (Table 10). The specific surface area values of the kaolins ranged from 10 to 25 m2/g. Acid attack increased AS (Table 11) to 125 m2/g. The ratio between micropore area and specific surface area of 13_Pol was constant up to 20 h and then increased (Table 11). Magadiite had a specific surface area of 30 m2/g which was similar to that of smectites (Steudel et al., in

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Fig. 9. Comparison between the unreacted aluminium content and the area of the OH vibrations. Kaolinite Polwhite (13_Pol) was treated with 5 M H2SO4 at 80 °C at 5 h, 20 h, 72 h, and 96 h.

the initial total Fe2O3 was associated with illite. 24% of Fe2O3 belonged to phlogopite. 70% of the initial total MgO came from illite and 30% from phlogopite. Both Fe2O3 and MgO were no longer detected after 20 h of reaction, while the Al2O3 content was about 5%. Even after 96 h about 2% Al2O3 were observed. The XRD pattern revealed the presence of feldspars. Therefore, the Al2O3 content of feldspar was subtracted from the total amount of Al2O3 (Table 6). The adjusted Al2O3 value (Table 13) can be used to estimate the content of residual illite. In the sepiolite Al mainly were from the octahedral and tetrahedral sheet of the associated muscovite, while Mg was dissolved only from the octahedral sheet of sepiolite. The low Fe2O3 content was related to muscovite and sepiolite, because both minerals may contain only up to 8% Fe2O3 in the structure. The adjustment of both values was not necessary because no other phases in the bulk material contained

Fig. 8. FTIR spectra of the raw and acid treated kaolinite (13_Pol) (5 M H2SO4 at 80 °C).

press), but had a higher amount of micropores (Table 12). The specific surface area and micropore area increased during the reaction with acid. 5. Discussion 5.1. Reaction with H2SO4 5.1.1. Composition Acid leaching of the octahedral cations was indicated by the decreasing content of Mg, Fe and Al in all samples. The structure (dioctahedral or trioctahedral) and the composition of the octahedral sheet played an important role during dissolution. Since pristine materials containing associated Al-minerals were used, adjustment of the amount of Al in octahedral positions to the content of the specific clay mineral is necessary to get information on the stability of kaolinite, illite and sepiolite (Table 3). This was not the case for Mg and Fe since the content of Mg- and Fe-containing phases was rather low in the pristine material and Mg and Fe were fast dissolved. The data reveal that Mg and Fe were removed faster than Al and disappeared completely after 20 h treatment of illite (Table 6). Fe and Mg were dissolved from the octahedral sheet of illite and phlogopite. 76% of

Fig. 10. FTIR spectra of the raw and acid treated magadiite (16_Mag) (5 M H2SO4 at 80 °C).

A. Steudel et al. / Applied Clay Science 44 (2009) 95–104

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Fig. 11. ESEM image of the a) raw and b) acid treated illite (9_Illite) (5 M H2SO4 at 80 °C for 96 h).

Fig. 13. ESEM image of the a) raw and b) acid treated kaolinite Polwhite (13_Pol). (5 M H2SO4 at 80 °C for 96 h).

Fig. 12. ESEM image of the a) raw and b) acid treated sepiolite (10_PangelS9) (5 M H2SO4 at 80 °C for 20 h).

Fig. 14. ESEM image of the a) raw and b) acid treated magadiite (16_Mag). (5 M H2SO4 at 80 °C for 72 h).

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Table 8 Grain size distribution [nm] of illite (9_Illite).

Table 13 Corrected Al2O3 content of the natural and residual illite.

d10

d50

d90

51

144

420

Elements Al2O3 [%]

Table 9 Specific surface area (AS), micro-pore area (AMP) and external specific surface area (AE) of the starting (9_Illite) and acid treated illite (5 M H2SO4, 80 °C). Time [h]

AS [m2/g]

AMP [m2/g]

AE [m2/g]

AMP/AS [%]

0 20 96

94 193 161

13 24 29

81 169 132

14 12 18

Table 10 Specific surface area (AS), micro-pore area (AMP) and external specific surface area (AE) of the starting (10_PangelS9) and acid treated sepiolite (5 M H2SO4, 80 °C). Time [h]

AS [m2/g]

AMP [m2/g]

AE [m2/g]

AMP/AS [%]

0 1.5 5 20 72

300 138 154 166 185

167 18 16 56 36

133 120 138 110 148

56 13 10 34 19

Table 11 Specific surface area (AS) and micro-pore area (AMP) of the starting and acid treated kaolins (5 M H2SO4, 80 °C). Time [h]

13_Pol

14_Kaolex

15_Rogers

AMP AMP/AS AS AMP AMP/AS AS AMP AMP/AS AS [m2/g] [m2/g] [%] [m2/g] [m2/g] [%] [m2/g] [m2/g] [%] 0 1.5 5 20 72 96

11 21 31 46 57 58

0 3 5 6 15 16

0 14 15 13 26 31

24 – – 92 – 120

2 – – 20 – 23

8 – – 17 – 19

24 – – 92 – 111

1 – – 15 – 24

4 – – 16 – 22

MgO and Fe2O3. Here, the Al2O3 content was not adjusted because nearly the whole amount of Al2O3 was associated with muscovite. The low Fe2O3 and MgO contents of the residual sepiolite showed that the sepiolite was completely dissolved after 1.5 h. The measured Al2O3 in the residual solid indicated that the muscovite in the sepiolite sample was not completely dissolved. The presence of Al2O3 in the residual kaolin sample proved that the kaolinite was not completely dissolved after 96 h. The value of the adjusted Al2O3 content can be used to estimate the residual kaolinite (Table 14). This kaolinite content can be compared with the content derived from Xray diffraction analysis. In contrast to smectites (Steudel et al., 2009), there was no correlation between the kaolinite and amorphous silica content determined during Rietveld analysis (Table 14). Starting from these results and from data reported in Steudel et al. (2009), four assumptions were made for the adjustment of the Al2O3 content.

Illite (9_Illite) Untreated

20 h

96 h

19.76

2.81

1.04

and the initial amount of kaolinite. In a sample with 10% kaolinite, the kaolinite was completely dissolved, while in a sample with 80% kaolinite the remaining kaolinite content was between 40 and 60% after 20 h of acid attack. 3) The dissolution of mica depends on its chemical composition such as Fe and Mg content (up to 8.5% Fe can be incorporated into the mica structure, Rösler, 1979). Dioctahedral micas with low Fe content are stable up to 96 h. The kaolin samples 15_Kaolex and 15_Rogers contained Fe-rich dioctahedral micas identified by XRD. After 96 h up to 50% of this mica was dissolved. 4) The dissolution of smectites depends on their chemical composition (Steudel et al., 2009). For samples with smectite as impurity, it was assumed that after 5 h 20% and after 20 h up to 80% smectite were dissolved. 5.1.2. Long range order In all samples decreasing reflection intensity indicated the dissolution of the structure, but different clay minerals showed different dissolution grades. A small 001 reflection still remained after 20 h in the XRD pattern of the 9_Illite i.e. the illite was not completely dissolved. The accessory minerals like phlogopite and calcite were completely dissolved since the (060) reflection of phlogopite at 0.1538 nm (+Ikg60°/2θ) and the (104) reflection of calcite at 0.304 nm (+Ikg29.4°/2θ) disappeared. Sepiolite was completely dissolved after 1.5 h since sepiolite reflections disappeared. In contrast, the basal spacing d001 of muscovite was present, during the whole reaction time, i.e. muscovite was more stable against the acid. All three kaolins showed a small basal reflection after 96 h indicating that kaolinite was not completely dissolved. In all XRD patterns except in that of magadiite decomposition of the structures by leaching of the metal ions increased the background at around 21°/2θ indicating formation of X-ray amorphous silica. In contrast to the main phases, the dissolution of some impurities was much slower. After reaction with acid magadiite was still crystalline. The (001) reflection was the strongest reflection and shifted from 1.56 nm to 1.32 nm. The XRD data were identical with those of Lagaly et al. (1973). The acid treated magadiite, which is described as H+-magadiite, approximately H4Si14O30 xH2O (Eugster, 1967; McAtee and Henslee, 1969; Brindley, 1969; Lagaly and Beneke, 1975; Eypert-Blaison et al., 2001). Magadiite belongs to the phyllosilicates, but it consists of single nets of tetrahedra forming a system of 8-ring channels (Liebau, 1964; Brandt et al., 1988, 1989). Based on the results, we concluded that the layers of SiO4 and SiO3(OH) groups in magadiite are stable against acids and do not transform into amorphous silica.

1) Feldspars like orthoclase, plagioclase albite were stable over the whole period of acid attack. Only Ca-rich feldspars like anorthite start to be dissolved after 20 h. 2) The dissolution of associated kaolinite (in samples other than kaolins) was constant over time and independent of the matrix

5.1.3. Short range order After 96 h a small, broad band at 538 cm- 1 still existed in the spectra of illite. This weak band implies that small amounts of tetrahedral Al still

Table 12 Specific surface area (AS), micro-pore area (AMP) and external specific surface area (AE) of the starting (16_Mag) and acid treated magadiite (5 M H2SO4, 80 °C).

Samples

Time [h]

AS [m2/g]

AMP [m2/g]

AE [m2/g]

AMP/AS [%]

0 72

30 64

14 35

16 29

47 55

Table 14 Kaolinite content estimated by XRD analysis and from the corrected Al2O3 content. Kaolinite content estimated 20 h 13_Pol [%] 14_Kaolex [%] 15_Rogers [%]

96 h

XRD

Al2O3

XRD

Al2O3

55 51 60

54 49 61

12 8 6

10 4 6

A. Steudel et al. / Applied Clay Science 44 (2009) 95–104

103

Table 15 Calculation of the theoretical surface area in relation to the illite and kaolinite content. Sample

a [nm]

b [nm]

A⁎ [nm2]

M⁎⁎ [g/mol]

AS (max)⁎⁎⁎ [m2/g]

m# [%]

AS## [m2/g]

9_Illite 13_Pol 14_Kaolex 15_Rogers

0.5208 0.5160 0.5160 0.5160

0.8995 0.8940 0.8940 0.8940

0.4685 0.4613 0.4613 0.4613

395.05 258.20 258.20 258.20

790 516 516 516

76 71 85 86

541 368 440 444

⁎ Area of the basal plane of the unit cell. ⁎⁎ Molecular mass per unit cell. ⁎⁎⁎ Calculated specific surface area for illite and kaolinite content of 100%. # Illite and kaolinite content obtained from Rietveld quantification. ## Calculated specific surface area related to the illite and kaolinite content.

remained, whereas most of the octahedral Al was dissolved. The OH bands at 694 and 655 cm- 1 disappeared after 1.5 h. This implied the release of Mg from the octahedral sheet. Furthermore, the disappearing of the vibration at 445 cm- 1 after 1.5 h indicated distortion and weakening of the Si–O–Mg bonds between the tetrahedral and octahedral sheets. The Si–O vibrations at 1103 and 1024 cm- 1 disappeared in the asymmetric Si–O band at 1076 cm- 1, which was characterized by a broad shoulder at higher wavenumbers (1195 cm- 1). Also the Si–O band at 505 cm- 1 was no longer well defined. The results revealed that the Si– O stretching vibrations were very sensitive to an acid attack. The dissolution of the octahedral sheet in acid treated sepiolite also caused changes in the Si–O vibration bands. The changes can be related to the conversion of the tetrahedral sheets into silanol groups (McKoewn et al., 2002). The intensity of the OH deformations decreased when increasing acid attack. These data supported the continuous release of Al from the octahedral sheet. After 96 h a small band at 539 cm- 1 (13_Pol) still existed in the three kaolin spectra. This implied that after 96 h small amounts of Al still remained in the tetrahedral sheet, whereas most of the octahedral Al was released. The kaolinite in the kaolin samples was not completely dissolved. 14_Kaolex and 15_Rogers were dissolved to a greater extend than 13_Pol because a small band at 939 and 917 cm- 1 existed in the spectra of 13_Pol after 96 h. The intensity of the Si–O bands increased with increasing acid attack. New absorption bands, which appeared at 798 and 1097 cm- 1, were characteristic of amorphous silica (Komadel et al., 1990; Madejová et al., 1998; Makó et al., 2006). In contrast to the other clay minerals the acid attack of magadiite showed only minor changes in the FTIR spectra. The positions of most of the absorption bands did not shift, but the intensity of few bands decreased slightly after acid attack. The number of symmetric stretching vibrations of the Si–O–Si bridges and Si–O–Si and O–Si–O bending vibrations decreased and more antisymmetric stretching vibrations of Si–O–Si bridges were observed. This indicates that acid attack may have broken some Si–O bonds. 5.1.4. Morphology and surface The changes of the specific surface area and the development of the micropores implied a reduction of the particle size and a partial

Fig. 16. The three stages of dissolution and stepwise splitting of kaolinite particles.

delamination of the illite particles until 20 h. of treatment. The reduction of the particle size was also observed by means of XRD (decrease of the basal reflection). Partial splitting of the particles was also proved by the maximum of AS after 20 h reaction compared with the theoretical AS (541 m2/g; Müller-Vonmoos and Kahr, 1983). The theoretical AS corresponds to the complete delamination of the particles into single layers (Table 15), whereas twice the theoretical AS would imply the existence of single tetrahedral sheets. The increasing micropore surface and the approximate thickness of octahedral sheet (0.3 nm) and interlayer space (0.1 nm) imply that the particles are split within the octahedral sheets. Nitrogen can not penetrate into the interlayer space. The splitting of the octahedral sheet was also proved by the chemical analysis. The release of K+ was slower compared to the leaching of the octahedral cations. The dissolution of the octahedral sheet in illite took place along the OH groups as a consequence of the attack of protons at the edges. The splitting of the octahedral sheet was associated with the formation of tetrahedral sheets, probably SiO3(OH) tetrahedral sheets. The particles did not break into smaller pieces and the initial morphology was still observed. The five stages of dissolution and stepwise particle splitting of illite are displayed schematically in Fig. 15. Stage 5 was not reached. It would be accompanied with a fast formation of three dimensional aggregates of amorphous silica. The AS and AMP of sepiolite decreased, which was associated with the dissolution of small sepiolite particles. At the beginning, there were particles with different diameter and nitrogen could only penetrate completely into the channels of the smaller particles. Reaction with acid dissolved the smaller particles and the larger particles remained after 1.5 h. Nitrogen could penetrate only partially into the longer channels of large particles, which resulted in a lower AS and AMP (Table 10). The increase of both values after 5 h was due to with the dissolution of the larger particles and the slow formation of smaller particles which were then filled with nitrogen completely. The kaolinites of Kaolex (14_Kaolex) and Rogers (15_Rogers) had a higher AS than the kaolinite of Polwhite (13_Pol), due to their different particle size. AS and AMP after acid reaction in connection with the chemical analysis indicated smaller particles by partial dissolution. The remaining layers consisted of mainly intact tetrahedral sheets and residues of octahedra (Fig. 16). Fig. 16 shows schematically the stepwise delamination of kaolinite. The degree of delamination was much lower than for 2:1 clay minerals and only 15 to 27% of the maximum

Fig. 15. The five stages of dissolution and stepwise splitting of illite particles.

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Table 16 Comparison between the theoretical and measured AS and rate of acid attack and the number of layers per stack before and after acid treatment (maximum area calculated according to Müller-Vonmoos and Kahr, 1983). Sample

AS of the crude material [m2/g]

Maximum of theoretical AS [m2/g]

Maximum of measured AS [m2/g]

Degree of delamination [%]

9_Illite 13_Pol 14_Kaolex 15_Rogers

94 11 24 24

541 368 440 444

194 58 120 111

35 15 27 27

possible AS was achieved (Table 16). While the remaining tetrahedral sheets of kaolinite are connected by residual octahedra, the remaining tetrahedral sheets of illite are connected by interlayer potassium ions and partially by the residual octahedral fragments inhibiting the formation of a three dimensional SiO4 structure. Reaction of magadiite increased AS and AMP, indicating a higher porosity probably due to the flexibility of the layers. 6. Conclusions The bulk material of five non swelling clay minerals and magadiite were reacted with H2SO4. We used raw materials, because for most of the industrial applications purification and separation of the clay fraction (b2 μm) is too time consuming and expensive. The investigations showed that the dissolution of the octahedral cations occurred in the following order Mg N Fe N Al. Chemical analyses confirmed that dioctahedral clay minerals resisted acid attack better than trioctahedral clay minerals. The splitting prior to delamination of illite occurred more slowly than smectites and vermiculites (Steudel et al., 2009), because the reactions proceeds along the OH groups of the octahedral sheets (edge attack) whereas the delamination of the swellable clay minerals occurred along the interlayer space and OH groups of the octahedra. Nevertheless, non-swelling clay minerals can be used to produce materials with simple chemical composition and high specific surface area by reaction with acid. In general, the reaction time of non-swellable clay minerals was longer than that of swellable clay minerals of similar composition of the octahedral sheet. Smectitic impurities in kaolins were completely dissolved and therefore separation of those impurities is not mandatory. Based on the presented data, we developed two different models for the dissolution and stepwise delamination of illite and kaolinite. Residuals of the interlayer cations and octahedral sheet inhibited the formation of three dimensional structures. Hence, experimental conditions during acid activation have to be chosen in relation to used raw materials. Acknowledgement The work at TNO (Eindhoven, NL) was supported by a Ph.D. grant (D+ AFw-06+AFw-45522) of the German Academic Exchange Service (DAAD). We are grateful to Nora Groschopf for many of the XRF analysis. The authors acknowledge Marita Heinle for the ICP-OES measurements. For the assistance with FTIR measurements we are thankful to Frank Friedrich and Stefan Heißler. We like to thank Gerhard Lagaly and the two anonymous reviewers for their valuable comments that improved the manuscript. References Abdul-Latif, S., Weaver, C.E., 1969. Kinetics of acid-dissolution of palygorskite (attapulgite) and sepiolite. Clays Clay Miner. 17, 169–178. Aglietti, E.F., Porto Lopez, J.M., Pereira, E., 1988. Structural alteration in kaolinite by acid treatment. Appl. Clay Sci. 3, 155–163. Amman, L., Bergaya, F., Lagaly, G., 2005. Determination of the cation exchange capacity of clays with copper complexes revisited. Clay Miner. 40, 441–453.

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