Bentonites impregnated with TiO2 for photodegradation of methylene blue

Bentonites impregnated with TiO2 for photodegradation of methylene blue

Applied Clay Science 48 (2010) 602–606 Contents lists available at ScienceDirect Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s e...

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Applied Clay Science 48 (2010) 602–606

Contents lists available at ScienceDirect

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

Bentonites impregnated with TiO2 for photodegradation of methylene blue Enéderson Rossetto b, Diego I. Petkowicz b, João H.Z. dos Santos b, Sibele B.C. Pergher a, Fábio G. Penha a,⁎ a b

Departamento de Química, Universidade Regional Integrada do Alto Uruguai e das Missões, Campus Erechim, Av. Sete de Setembro, 1621, 99700-000 Erechim, RS, Brazil Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, 91501-970, Porto Alegre, RS, Brazil

a r t i c l e

i n f o

Article history: Received 26 August 2009 Received in revised form 5 March 2010 Accepted 9 March 2010 Available online 19 March 2010 Keywords: Bentonites Diatomites Titanium oxide Photodegradation

a b s t r a c t Four bentonites and one diatomite from Rio Negro (Argentina) were used as supports for titanium oxide (TiO2). The materials were characterized by X-ray diffraction, scanning electron microscopy, infrared spectroscopy, textural analysis by nitrogen adsorption, elemental analysis and diffuse reflectance spectroscopy. The specific surface areas calculated by the BET method were 76, 46, 80, and 31 for the bentonites and 153 m2/g for diatomite and were not changed by impregnation with TiO2. SEM analysis revealed agglomerates, probably due to titania domains on the surface. The properties of the lamellar materials were maintained after TiO2 impregnation, and all materials showed methylene blue photodegradation activity. The bentonites showed a higher activity than the commercial catalyst P25, likely due to the TiO2 distribution and better accessibility. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The treatment of colored wastewaters produced by the textile industry is a problem that has recently been heavily researched (Bergamini et al., 2009; Han et al., 2009; Mahvi et al., 2009). Heterogeneous photocatalysis is one method for treatment of such wastewater; it is a so-called “Advanced Oxidation Process” (AOP) suitable for the oxidation of dyes. The AOP is based on the formation of hydroxyl radicals (HO•), a strong oxidizing agent (E° = 2.8 eV) that can promote the total mineralization of organic pollutants. The process begins with the absorption of a photon by a semiconductor such as titanium oxide, equal to or larger than its band gap. An electron may then be promoted from the valence band (VB) to the conduction band (CB). After the electronic transition, the VB is left with an electron deficiency (VB+) and the CB has an electron excess (CB−). This process affords a redox reaction of the adsorbed species on the semiconductor with the formation of oxided products. It has been proposed that the adsorbed water molecules react with (VB+) to form hydroxyl radicals (OH•) to start the photodegradation of the pollutants (Faisal et al., 2007; Saquib et al., 2008; Singh et al., 2008). Titanium oxide is the most investigated photocatalyst for the degradation of organic pollutants from wastewaters. This catalyst is advantageous over other semiconductors because of its chemical stability, non-toxicity, low cost and commercial availability (Li et al., 2008; Suwanchawalit and Wongnawa, 2008; Wang et al., 2008; Yang et al., 2008).

The efficiency of TiO2 is influenced by its crystal structure, particle size, specific surface area and porosity. Ultrafine powders of TiO2 show a good catalytic activity. However, agglomeration can take place, engendering the production of larger particles and resulting in the reduction or even complete loss of catalytic efficiency. The dispersion of TiO2 particles on clay mineral layers is a potential solution to this problem. Clay minerals impregnated with TiO2 show high thermal stability and larger pore sizes that afford better incorporation of the species without diffusion problems, increased specific surface area or increased acidity (Valverde et al., 2003; Suwanchawalit and Wongnawa, 2008). Photocatalytic degradation of hydrocarbons was studied with Nabentonite, TiO2 and H2O2 as oxidant (Dékány and Pernyeszi, 2004). A synergetic effect of the clay minerals on the photocatalytic degradation due the large specific surface area was observed. Dékány et al. (2008) studied the role of the clay minerals as supports of the TiO2. The authors examined the catalytic properties of the composites for degradation of phenol in aqueous solution and ethanol and toluene vapor. These pollutants were degraded at higher efficiency than by pure TiO2. This paper presents the preparation and characterization of titania supported on bentonite and diatomite and the photodegradation of methylene blue.

2. Experimental 2.1. Materials

⁎ Corresponding author. Tel.: + 55 543520 9000. E-mail address: [email protected] (F.G. Penha). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.03.010

Four bentonites and a diatomite were obtained from Rio Negro (Argentina). Titanium tetrachloride and cyclohexane were purchased

E. Rossetto et al. / Applied Clay Science 48 (2010) 602–606


Table 1 Mineralogical analysis of the natural samples of clays. Samples








C01 C02 C03 C04 C05

X X X X –

– X X – –

X X X X –

X – X – –

– X – – –

– X – – –

– – – – X

from Merck. The standard TiO2 sample P25 was supplied by Degussa. Methylene blue (Basic blue 9) was from Sigma-Aldrich. 2.2. Preparation of the photocatalysts The bentonites and diatomite (Table 1) were used without any treatment. For the impregnation, TiCl4 was dissolved in cyclohexane (Santos et al., 2009). The slurry was shaken for 1 h at 100 °C, and then the solvent was evaporated. The remaining solid was dried at 110 °C and calcinated in air at 450 °C for 4 h. The final Ti content was 10 mass %. The supports were numbered C01 to C05 and C0X–Ti refers to TiO2 impregnated sample COX–Ti. 2.3. Photocatalyst reactions Photodegradation of methylene blue was performed by UV irradiation using a 250 W mercury-vapor lamp (λmax = 365 nm). The photodecomposition reactions were carried out in a quartz reactor, equipped with a cold finger to avoid thermal reactions. In a typical reaction, 0.05 g of the catalyst and 200 mL of dye solution with a concentration of 10 mg L− 1 were stirred and irradiated for 1 h. Aliquots were collected at different times during the irradiation, and the concentration of the residual methylene blue was monitored by UV– visible spectrophotometry (Shimadzu UV-160A) at 665 nm (Santos et al., 2009).

where S is the mean crystallite dimension, K the crystallite shape constant (0.89), θ the Bragg angle, λ the X-ray wavelength, and β is the full width at half maximum (FWHM) of the reflection (in radians). Scanning electron microscopy was performed with a SEM SSZ 550 (Shimadzu). Analyses at the transmittance mode were performed using the KBr pressed technique with 4 cm− 1 of resolution and coadding 32 scans using a FT-IR spectrometer. Elemental analysis was carried on a Noran Instruments EDX system connected to a JEOL JSM 6300F scanning electron microscope with an accelerating voltage of 20 kV. Samples were fixed on carbon tape on a stub and sputtered with gold before measurements. Diffuse reflectance spectra of methylene blue were measured with a UV–visible spectrometer (Varian UV-cary-100) using an integration sphere accessory. Nitrogen adsorption/desorption isotherms were measured with AUTOSORB-1 Quantachrome (Nova-2200e). Before analysis, ca. 100 mg of the samples were heated under vacuum at 300 °C for 3 h. The elemental analyses of the materials were determined by X-ray fluorescence (XRF) spectrometry. The samples were prepared by mixing clay with lithium tetraborate and methaborate flux in a mass ratio of 1:5 (clay:flux). This mixture was homogenized and thereafter 0.3 mL of a 4% mass LiBr solution was added. The crucible containing the mixture was melt in a RIGAKU furnace at 1200 °C for 7 min. The resulting pellets were analyzed in a Philips model PW 148D spectrometer with a rhodium X-ray tube.

2.4. Characterization

3. Results and discussions

Powder X-ray diffraction analysis was performed using a model D5000 Diffractometer (Siemens) with a Ni filter and Cu Kα (λ = 1.54 Å) radiation. The TiO2 particle size (S) was calculated using Scherrer's equation (Sakai et al., 2004):

3.1. Bentonites


Kλ ; β cosθ


According to the X-ray diffractograms (Fig. 1) and to the mineralogical analysis (Table 1), the bentonites C01–C04 contained montmorillonites and some associated minerals. The difference in intensity of the 001 reflection may be due to the different parallel orientation (texture) of the samples. Bentonite C02 seemed to contain a better ordered montmorillonite. The basal spacing of the montmorillonites was 12.8 Å in the air-dried form and 9.8 Å after calcination at 450 °C. Table 2 shows the results of the chemical analysis of the samples. The bentonites were characterized in a previously study (Pergher et al., 2009).

Table 2 Composition of the bentonites (C01–C04) and diatomite.

Fig. 1. X-ray diffractograms of the bentonites and the diatomite.

Composition (%)




Na2O MgO SiO2 Al2O3 SO3 Cl K2O CaO TiO2 Fe2O3 CeO2 Total

3.1 3.0 67.0 17.0 – – 0.8 1.7 0.8 6.3 – 100

3.8 4.5 62.1 21.2 – 0.95 – 0.84 0.84 5.75 – 100

3.41 3.17 67.8 18.2 – – 1.52 – – 5.85 – 100

C04 3.88 4.51 63.4 20.9 1.26 0.34 0.29 0.26 0.22 4.87 – 100

C05 0.48 3.01 77.8 11.4 – – – 2.66 – 4.22 0.38 100


E. Rossetto et al. / Applied Clay Science 48 (2010) 602–606

Fig. 4. Diffuse reflectance spectra of the titania supported samples. Fig. 2. X-ray diffractograms for the TiO2 impregnated bentonites and diatomite.

3.2. TiO2 impregnation The reflections related to the TiO2 phase were characteristic of anatase: 2θ = 25.4, 48.3 and 54.9° (Fig. 2). No reflection of rutile (2θ = 27.4°) was observed (Liqianga et al., 2003). The particle sizes of these samples, calculated according to Scherrer's equation, were between 11 and 15 nm. The specific BET surface areas of the materials, before and after TiO2 impregnation (Table 3) were typical of lamellar materials. Sample C05 presented a higher specific surface area higher due to its porous tubular structure. The infrared spectra of C01 to 04 (Fig. 3) are characterized by the presence of stretching bands associated with the montmorillonite structure (Tichit et al., 1988). The shoulder at 1130 cm− 1 can be attributed to the v(Si–O) apical symmetric stretching vibration of the tetrahedral sheet, while the large band centered at ca. 1038 cm− 1 is Table 3 Specific surface area before and after TiO2 impregnation of TiCl4. Sample

C01 C02 C03 C04 C05

Specific surface area (m2/g) Before impregnation

After impregnation

76 46 80 31 153

71 59 80 39 125

due to the combination of stretching and bending vibrations associated with the Si–O bonds. The band at ca. 930 cm− 1 can be assigned to the deformation of OH groups bound to Al ions. The spectrum of C05 presented a large band between 1050 and 1250 cm− 1, which was assigned to the asymmetric Si–O–Si stretching vibration of a mesoporous diatomite structure (analogous to the band reported for MCM-41) (Bergamini, et al., 2009). The diffuse reflectance spectra in the UV–visible region (Fig. 4) were characterized by a wide band centered at 350 nm, which was assigned to the electron transfer from O2p to Ti3d corresponding to the valence band to conduction band transition of TiO2 (Liqianga et al., 2003). The band centered at ca. 330 nm was due to the presence of anatase (Geobaldo et al., 1992). The determination of the band gap by UV–visible spectroscopy (Table 4) is an alternative method for studying changes in the electronic properties of the TiO2 species (Sanchez and Lopez, 1995; Luo et al., 2002). The band gap is the inflection point of the curves in Fig. 4 (Oliveira et al., 2008). Taking into account the similarities between the supported photocatalysts and the P25 standard TiO2, it seems TiO2 clusters (Zhang et al., 1996) may also be formed in our systems. The amount of the TiO2 impregnated on the samples was monitored by SEM–EDX. (Table 5). A detected amount higher than the impregnated 10 mass% suggests that the TiO2 particles might be enriched on the surfaces, since SEM–EDX records external grain fractions and not the grain bulk. No significant changes of the morphology were observed after impregnation with TiO2 (Fig. 5). Nevertheless, agglomerates could be

Table 4 Values of the band gap energy of the titanium in the supported catalysts. Samples

E° (eV)

C01 C02 C03 C04 C05

3.34 2.98 2.72 3.29 3.32

Table 5 Titanium content in the supported catalysts.

Fig. 3. FT-IR spectra of the samples impregnated with TiO2: (a) C01–Ti; (b) C02–Ti2; (c) C03–Ti; (d) C04–Ti; (e) C05–Ti.


Ti (mass%)

C01–Ti C02–Ti C03–Ti C04–Ti C05–Ti

24.1 27.1 24.1 23.2 17.3

E. Rossetto et al. / Applied Clay Science 48 (2010) 602–606


Fig. 5. Micrographs of the supports before and after impregnation with TiO2. (a) C01; (b) C01–Ti; (c) C02; (d) C02–Ti; (e) C03; (f) C03–Ti; (g) C04; (h) C04–Ti; (i) C05; (j) C05–Ti.

detected, probably due to titania domains generated on the surface. This result corroborates the high TiO2 content detected by EDX analysis. 3.3. Decomposition kinetics of methylene blue The bentonite supported catalysts (except C05) were more active than P25 (Fig. 6), probably due to the better dispersion of the TiO2 particles on the clay mineral particles. These results suggest that the

enrichment of the titania particles on the surface of the support and the better accessibility of the reactants might be more favorable for the photocatalytic reaction. Nevertheless, one cannot neglect that the differences in the aluminum content may have affected the methylene blue decomposition. Diatomite had a larger specific surface area due to the porosity but had low catalytic activity. Previously studies showed that diatomite was a better adsorbent for methylene blue (Pergher et al., 2009).


E. Rossetto et al. / Applied Clay Science 48 (2010) 602–606

Fig. 6. Decomposition kinetics of methylene blue dye: ■ without catalyst; ● P 25; ▲ C01– Ti; ▼ C02–Ti; ♦ C03–Ti; Y C04–Ti; a C05–Ti. Methylene blue concentration (38 mg/L). Irradiation time (60 min).

4. Conclusions Impregnation of bentonites with TiO2 yielded supported anatase catalyst for methylene blue photodegradation. The bentonite supported anatase catalysts were more active than the commercial P-25, probably due to the enrichment of the TiO2 particles on the surface of the supports. Acknowledgement The authors thank the Laboratory of the Institute of Geosciences at UFRGS for XRD analysis. References Bergamini, R.B.M., Azevedo, E.B., Araujo, L.R.R., 2009. Heterogeneous photocatalytic degradation of reactive dyes in aqueous TiO2 suspensions: decolorization kinetics. Chemical Engineering Journal 49, 215–220. Dékány, I., Pernyeszi, T., 2004. Photocatalytic degradation of hydrocarbons by bentonite and TiO2 in aqueous suspensions containing surfactants. Colloids and Surfaces A: Physicochemical and Engineering Aspects 230, 191–199.

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