Adsorption of gaseous SO2 and structural changes of montmorillonite

Applied Clay Science 44 (2009) 251–254

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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

Adsorption of gaseous SO2 and structural changes of montmorillonite C. Volzone ⁎, J. Ortiga Centro de Tecnología de Recursos Minerales y Cerámica (CETMIC), CCT-La Plata (CONICET) – CIC, C.C.49, Cno. Centenario y 506, (1897) M. B. Gonnet, Argentina

a r t i c l e

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Article history: Received 9 September 2008 Received in revised form 1 February 2009 Accepted 4 February 2009 Available online 15 February 2009 Keywords: Montmorillonite SO2 Adsorption

a b s t r a c t Several montmorillonite samples after adsorption of gaseous SO2 were analyzed to evaluate structural and textural changes. The equilibrium adsorption of the SO2 gas was measured at 25 °C and 0.1 MPa. The samples were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), swelling index (SI), pH measurements, and N2 adsorption–desorption isotherms. SO2 adsorption increased with the specific surface area of montmorillonite. SO2 retention decreased pH of the dispersed samples from 6 to 1 and released interlayer and octahedral cations from the structure, which increased the specific BET surface area and specific micropore surface similar to that of acid-activated montmorillonite. © 2009 Published by Elsevier B.V.

1. Introduction

2. Materials and methods

Some industrial activities produce emissions of gases that remain in the atmosphere like pollutants causing serious consequences and even affecting life. Raw clays, or after specific modifications can be used as adsorbents and could remedy atmospheric environmental damages (Sugiera et al., 1990; Galán, 1987, 1996; Volzone, 2007). Sulphur dioxide is mainly formed as a result of the combustion of fossil fuels. It is a corrosive gas which causes acid rain. SO2 in ambient air can be harmful to the environment and human health. Today, many international agencies regulate air quality in the environment. SO2 adsorption has been tested with raw and modified clays (Baksh and Yang, 1992; Volzone, 2007). Generally, smectites retain more gaseous SO2 than kaolinites (Hall Gómez et al., 2002; Volzone and Ortiga, 2006). The capacity of this retention by the clays can be improved by using purified bentonites (Venaruzzo et al., 2000), acidactivated clays (Venaruzzo et al., 2002; Volzone et al., 2003; Volzone and Ortiga, 2006), organic modified bentonites (Vidal et al., 2005; Volzone et al., 2006) and pillared clays (Yang and Baksh, 1991; Yang and Cheng, 1995). Presence of the amorphous silica of the modified clays was mainly responsible for an improved gaseous SO2 adsorption (Volzone and Ortiga, 2006; Volzone, 2007). SO2 retention decreased the pH of bentonite when dispersed in water (Volzone et al., 2006) but structural and textural changes of the montmorillonite where not reported. The aim of this paper is analyzing the structural and textural changes of montmorillonite after stepwise SO2 adsorption. Between each step of SO2 adsorption, the montmorillonite was dispersed in boiling water.

Dioctahedral smectite (SAz-1, Cheto montmorillonite, USA, provided by Clay Mineral Repository, CMS) was used and referred here as B1. The textural and structural characteristics were reported in a previous paper (Volzone and Garrido, 2001). The mineralogical composition of the sample was: montmorillonite with low impurity of cristobalite.

⁎ Corresponding author. Tel.: +54 221 4840247; fax: +54 221 4710075. E-mail address: [email protected] (C. Volzone). 0169-1317/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.clay.2009.02.003

2.1. Gas retention and treatment The equilibrium adsorption of gaseous SO2 was measured using a standard volumetric apparatus, which was connected to a gas flow system. Samples were outgassed at 100 °C during 12 h prior to measurement. SO2 adsorption was measured at 25 °C and 0.1 MPa. A sample of montmorillonite (B1) was used several times for stepwise SO2 adsorption: after the retention of SO2, the sample was dispersed in boiling water in a refluxing system for 2 h. The solid was then washed with distilled water, separated, and dried at 100 °C (sample B1F1). A part of B1F1 was analysed, and another portion was again equilibrated with gaseous SO2. This process was repeated seven times (samples B1F2–B1F7). All samples were ground to pass a sieve M:200 (b74 μm) before characterization.

2.2. pH measurement The pH measurement was obtained by preparing a 5% w/v aqueous dispersion. The value pHi was measured directly after dispersion of the sample in water, pHf after cooling of the boiling dispersion.

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2.3. X-ray diffraction analysis (XRD) The X-ray diffraction equipment was a Philips 3020 Goniometer with PW 3710 Controller, Cu Kα radiation (λ = 1.5405 Å), and Ni filter at 40 kV and 20 mA. d(001) was measured on oriented slide specimens at a relative humidity of 55% (RH) by scanning at 1/ 2°(2θ)/min between 3 and 12°(2θ). The b-dimension of the montmorillonite was derived from (060) reflection (MacEwan, 1961). For measuring the (060) reflection, the samples were randomly oriented.

Table 1 Release of exchangeable cations and pH values of the dispersions (pHi: immediately after dispersion; pHf: after 2 h in boiling water).

pHi pHf CaO (%) Na2O (%)

B1

B1F1

B1F2

B1F3

B1F4

B1F5

B1F6

B1F7

6.0 – 2.41 0.06

3.6 4.5 1.21 0

3.0 4.0 0.58 0

2.5 4.0 0.27 0

1.0 3.0 0 0

1.0 3.0 0 0

1.0 3.0 0 0

1.0 2.0 0 0

specific external surface and specific micropore surface were derived from the t-plots (Gregg and Sing, 1991).

2.4. Infrared absorption spectroscopy (IR) 2.6. Swelling index (SI) The infrared spectra were recorded using an Spectrum One Perkin Elmer equipment from 4000 to 380 cm− 1. The samples were dispersed in KBr (1% mass) and compacted in thin pellet form.

The swelling index (SI) was measured by dispersing 2 g of the sample in 100 mL of distilled water in a graduated cylinder. The sediment height after 24 h was taken as swelling index.

2.5. Nitrogen adsorption–desorption isotherms 3. Results and discussion N2 adsorption–desorption isotherms were carried out at the liquid nitrogen temperature by using a Micromeritics ASAP 2020 equipment. The sample was previously outgassed at 100 °C for 12 h. Specific BET surface area was calculated from the first part of the isotherm. The

The X-ray diffraction diagram of montmorillonite mainly showed changes of the (00 l) and (060) reflections after SO2 adsorption (Fig. 1). The reduction of the basal spacing from 15.6 Å (B1) to 13.5 Å (B1F7) and broadening of the (001) reflection indicated interlamellar disorder after repetitive SO2 adsorption steps. The value of d060 changed only from 1.498 to 1.496 Å, indicating that the structure of the layers was almost unchanged. The pH of B1 in 5% aqueous suspension was 6 (Table 1) and was 4.5 after boiling in water, previous SO2 adsorption. SO2 adsorption reduced pHi to 3.6 (B1F1) and 1 (B1F7) and pHf to 4.5 and 2. Infrared spectra of B1 showed bands at 3624, 3424, 1110, 1035, 915, 840, 519, 464 cm− 1, typical of montmorillonite, and the band at 790 cm− 1 (Farmer, 1974) corresponding to the presence of cristobalite (Si–O stretching vibration). After repeated SO2 adsorption followed by boiling water, the intensity of some infrared bands was reduced and shifted, indicating gradual disturbance of the structure. The OH-stretching vibration (Al– Al–OH, Mg–Al–OH) absorption band at 3624 cm− 1 decreased in intensity in compared to the band at 3424 cm− 1 (stretching vibration of the OH− group of intercalated water). The 3624/3424 intensity ratio changed from 0.74 in B1 to 0.12 in B1F7 samples (Table 2), which is indicative of some cation (Mg–Al) dissolution from octahedral sheets. The Si–O–Si stretching vibration band at 1032 cm− 1 remained unchanged. Nevertheless, the band at 1110 cm− 1, also corresponding to the Si–O–Si stretching vibration was shifted to 1120 cm− 1, indicating a certain influence of oxygen atoms around of silicon atom (Pai et al., 1986). The bands at 915 and 840 cm− 1 are assigned to the deformation vibrations of Al–Al–OH and Mg–Al–OH groups, respectively. The intensity of these deformation bands involving AlVI and MgVI decreased after repeated SO2 adsorption-water treatments (B1F7), (Fig. 2, Table 2). The decrease also indicated dissolution of octahedral cations. Generally, acid-activated clays show similar behaviour (Volzone et al., 1986; Christidis et al., 2003; Foletto et al., 2003).

Table 2 Intensity ratios of IR bands.

Fig. 1. X-ray diffraction of B1–B1F7. A) (001) reflection and d001; B) (060) reflection and d060.

Sample

Mg–Al–OH/H–O–H (intensity ratio of bands 3624/3424)

Al–Al–OH/Si– O–Si (intensity ratio of bands 915/1035)

Mg–Al–OH/Si– O–Si (intensity ratio of bands 840/1035)

Si–O–Al/Si–O–Si (intensity ratio of bands 519/464)

B1 B1F7

0.74 0.12

0.24 0.11

0.30 0.16

0.67 0.40

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Table 3 Textural characteristics. SBET: specific BET surface area; Smic: specific micropore surface; Sext: specific external surface; Vp: specific pore volume; Vmic: specific micropore volume.

Fig. 2. Infrared spectra of B1 and B1F7.

The bands at 519 and 464 cm− 1 are assigned to the deformation vibrations Si–O–AlIV, and Si–O–Si groups, respectively (Farmer, 1974), and a decrease of Si–O–AlIV/Si–O–Si intensity ratio indicated changes of AlIV (Table 2). The changes in the intensity ratios of IR bands (Table 2) indicated that the cations from octahedral sheet are more released than the aluminium from tetrahedral sheet. Fig. 3 shows N2 adsorption–desorption isotherms of the samples. The isotherm shape of B1 was of H3 type, and corresponded to systems with slit-shaped pores (Gregg and Sing, 1991). The shape was slightly changed after SO2 adsorption. The adsorption–desorption isotherms of these samples were located above the B1 isotherm with an increase in adsorbed nitrogen volume at low P/Po. This indicated an increased micropore volume due to the release of octahedral cations (see also Table 2). The specific surface area increased with increasing SO2 adsorption (B1F1–B1F6) reaching the maximum value for B1F6 and decreased for B1F7 (Table 3), similar to acid-activated bentonites at higher concentrations of acids or long reaction periods (Volzone et al., 1986; Venaruzzo et al., 2002; Foletto et al., 2003; Vukovic et al., 2006). The proportion of the specific micropore surface to the specific BET surface area was 28.8% for B1 and reached 52.8% for B1F6 and decreased to 40.6% for B1F7 due to specific micropore area mainly

Fig. 3. N2 adsorption–desorption isotherms of B1 and B1F6.

B1

B1F1

B1F2

B1F3

B1F4

B1F5

B1F6

B1F7

SBET, m2 g− 1 Smic, m2 g− 1 Sext, m2 g− 1 Smic/SBET, % Sext/SBET, %

73.0 21.1 52.2 28.8 71.5

81.0 30.7 50.6 37.9 62.5

93.0 33.5 59.4 36.0 63.9

115.0 49.9 65.1 43.4 56.6

128.0 62.0 66.0 48.4 51.6

135.0 67.0 68.0 49.6 50.4

142.0 75.0 67.0 52.8 47.2

138.0 56.0 82.0 40.6 59.4

Vp, cm3 g− 1 Vmic, cm3 g− 1

0.105 0.009

0.107 0.013

0.115 0.015

0.122 0.022

0.125 0.023

0.119 0.026

0.137 0.028

0.122 0.025

(Table 3). This decrease may be due to structural alteration reported by several authors (Volzone et al., 1986; Venaruzzo et al., 2002; Komadel, 2003; Foletto et al., 2003; Vukovic et al., 2006). The total specific volume (at P/Po = 0.983) also increased and reached a maximum value of 0.137 cm3 g− 1. The specific micropore volume obtained by extrapolating the high-pressure branch to the y-axis in the t-plots (Table 3) increased from 0.009 to 0.028 cm3 g− 1. The reduced swelling index from 7.5 ml (B1) to 4.5 ml (B1F7) indicated that the loss of swelling capacity of the solid in a water medium may be attributed to the presence of protons replacing exchangeable cations and partly octahedral cations (Onal, 2007). Exchangeable cations are more readily replaced than octahedral cations. The adsorption of SO2 by B1 was 0.51 mmol and for B1F1 to B1F7 0.52, 0.54, 0.56, 0.58, 0.60, 0.65, and 0.62 mmol g− 1. The dispersion in boiling water after each SO2 adsorption step released interlayer cations (Table 1) more readily than octahedral cations as indicated by the IR investigation, in a manner similar to the acid-activated bentonites (Volzone et al., 1986). The accompanying increase in the specific surface area in turn increases the uptake of SO2. Retentions of SO2 by montmorillonite could oxidize SO2 into sulphuric acid. The decreasing pH of the aqueous dispersions may be indicative of this reaction. The sulphuric acid may then contribute to the leaching of cations and structural change of the montmorillonite. The normalised gas retention vs. specific surface area (Fig. 4) decreased with increasing number of SO2 adsorption steps up to number four. After that, the normalised values were similar. The adsorption of gaseous SO2 described in this paper (0.51 to 0.65 mmol g− 1) was lower than acid-activated montmorillonite (1.231 mmol g− 1, Venaruzzo et al., 2002), acid-activated kaolinite (0.914–1.242 mmol g− 1, Volzone and Ortiga, 2006) and organomontmorillonites (1.654–1.552 mmol g− 1, Volzone, 2007).

Fig. 4. Normalised SO2 retention vs. specific BET surface area.

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4. Conclusions The structure and texture of montmorillonite were changed by repeated gaseous SO2 retention. pH of the dispersions of the SO2 loaded montmorillonite decreased from pH = 6 to pH = 1. Dispersion of the sample in boiling water released interlayer and octahedral cations accompanied by changes of the basal spacing of interlayer and increasing specific surface area and pore volume. This behavior is comparable to acid-activated bentonites (Venaruzzo et al., 2002; Volzone, 2007) because the SO2 on the surface of montmorillonite is oxidized to sulfuric acid, which is responsible of the changes in the montmorillonite structure. The amount of SO2 retention increased as a function of the specific surface area (mainly specific micropore surface). When the specific surface area decreased due to an increased acid attack, the amount of SO2 retention also decreased. Acknowledgments Financial support by CONICET and CIC Res. 1114/05 are thankfully acknowledged. The authors also wish to thank Prof Dr. G. Lagaly for his fitting suggestions. References Baksh, M.S.A., Yang, R.T., 1992. Unique adsorption properties and potential energy profiles of microporous pillared clays. American Institute of Chemical Engineers Journal 38, 1357–1368. Christidis, G.E., Scott, P.W., Dunham, A.C., 2003. Acid activation and bleaching capacity of bentonites from the islands of Milos and Chios, Aegean, Greece. Applied Clay Science 12, 329–347. Farmer, V.C., 1974. The Infrared Spectra of Mineral. Mineralogical Society, London. Foletto, E.L., Volzone, C., Marques Porto, L., 2003. Performance of an Argentine acidactivated bentonite in the bleaching of soybean oil. Brazilian Journal Chemistry and Physics 20, 139–145. Galán, E., 1987. Industrial applications of sepiolite from Vallacas-Vical, Sapin: a review. Proceedings of the International Clay Conference, pp. 400–404. Galán, E., 1996. Properties and applications of palygorskite-sepiolite clays. Clay Minerals 31, 443–453. Gregg, S.J., Sing, K.S.W., 1991. Adsorption, Surface Area and Porosity, 2nd ed. Academic Press Inc, London. Hall Gómez, D., Volzone, C., Hülsken, A., Ortiga, J., Caro, J.F., 2002. Evaluación de materiales tobáceos modificados como adsorbentes de CO2 y CH. Revista de Ciencia y Tecnología. Facultad de Ciencias Exactas, Químicas y Naturales. Centro de Investigación y Desarrollo Tecnológico. Universidad Nacional de Misiones. Año 4, No. 4b, 26–32.

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