Synthesis and characterization of TiO2-pillared Romanian clay and their application for azoic dyes photodegradation

Journal of Hazardous Materials 167 (2009) 1050–1056

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Synthesis and characterization of TiO2 -pillared Romanian clay and their application for azoic dyes photodegradation E. Dvininov a,∗ , E. Popovici a , R. Pode b , L. Cocheci b , P. Barvinschi c , V. Nica d a

Department of Chemistry, “Al. I. Cuza” University of Iasi, Bvd. Carol I, no 11, 700506 Romania “Politehnica” University of Timisoara, Victoriei Sq. No. 2, 300006 Timisoara, Romania Faculty of Physics, West University of Timisoara, Bvd. V.Parvan 4, 300223 Timisoara, Romania d Faculty of Physics, “Al.I.Cuza” University of Iasi, Bvd. Carol I, no 11, 700506 Romania b c

a r t i c l e

i n f o

Article history: Received 11 December 2008 Received in revised form 23 January 2009 Accepted 26 January 2009 Available online 6 February 2009 Keywords: Montmorillonite Surfactant Intercalation TiO2 Congo Red Photocatalysis

a b s t r a c t The synthesis and properties of metal oxide pillared cationic clays (PILCs) has been subject to numerous studies in the last decades. In order to obtain TiO2 -pillared type materials, sodium montmorillonite from Romania–areal of Valea Chioarului, having the following composition (% wt): SiO2 -72.87; Al2 O3 -14.5; MgO-2.15; Fe2 O3 -1.13; Na2 O-0.60; K2 O-0.60; CaO-0.90; PC-5.70 and cation exchange capacity, determined by ammonium acetate method, of 82 meq/100 g, as matrix, was used. Sodium form of the clay was modified, primarily, by intercalation of cetyl-trimethylammonium cations between negatively charged layers which will lead to the expantion of the interlayer space. For the preparation of the TiO2 -pillared clay, the alkoxide molecules, as titania precursor, were adsorbed onto/into clay samples (1 mmol Ti/g clay), in hydrochloric acid environment, the resulted species being converted into TiO2 pillars by calcination. The as-prepared materials have been used as catalysts for Congo Red dye photodegradation, under UV. The photocatalytic activity of the pillared clays is a function of TiO2 pillars size, their increase leading to the enhancement of the contact areas between dye solution and photoactive species present in the interlayer space. The structural characteristics and properties of the obtained materials were investigated by X-ray Diffraction, Thermogravimetry Analysis, UV–vis Diffuse Reflectance, Transmission Electron Microscopy and Energy Dispersive X-ray Analysis. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The contamination of rivers and ground water by organic pollutants has acted as stimulus for numerous investigations focused on the effective pollution abatement methods. In aqueous environment, the presence of organic pollutants is of great concern. Most of the industrial and domestic waters are frequently contaminated with organic pollutants such as phenols, VOCs, pesticides, dyes, etc. [1]. The currently used approaches to remove the organic pollutants from the wastewaters are based on the adsorption or chemical oxidation processes. However, these processes have major drawbacks: the adsorption does not lead to pollutants degradation, while the chemical oxidation, in homogeneous phase, is not economically favorable, except the high concentrated pollutants [2]. The clays, such as montmorillonite, vermiculite, kaolinite, mica, hydrotalcite like compounds, etc., have attracted much attention in recent years for their applicability in wastewaters decontamination.

∗ Corresponding author. E-mail address: [email protected] (E. Dvininov). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.01.105

These natural materials possess layered structures, large surface areas and high exchange capacity (CEC), which give them many potential applications in different fields [3]. For environmental uses, Romanian montmorillonite from Valea Chioarului areal, having a 2:1 layered structure, represents a promising material. Because of the isomorphic substitution within the layers (e.g., Al3+ replaced by Mg2+ or Fe2+ , or Mg2+ replaced by Li+ in the octahedral sheet; Si4+ replaced by Al3+ in the tetrahedral sheet), these layers are negatively charged, the charge neutrality being ensured by the presence of cations in the galleries formed by two adjacent sheets. These cations are exchangeable and the sum of these charges determines the cation exchange capacity value (CEC). Generally, the clays can adsorb organic substances either on their external surfaces or within their interlamelar spaces, by interaction with or by substitution of the cations presents in the interlayer spaces [4]. In the last decades, metal oxide pillared clays (PILCs) have been intensely investigated [5–22], their physical and chemical properties finding applications in different fields. These materials contain metal oxide pillars that sustain the clay sheets and lead to the formation of bi-dimensional porous networks. Several

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2.2. Preparation of Ti-pillaring solution 1 mM of titanium tetraisopropoxide (titania precursor) was hydrolyzed with 1 M hydrochloric acid. Briefly, titanium alkoxide was dropped into hydrochloric acid solution, the resulted slurry being vigorously stirred for 3 h, at room temperature, with the obtaining of the titania clear sol solution. The quantities of HCl and titanium tetraisopropoxide were calculated in order to obtain a [Ti]/[H+ ] = 1:4 molar ratio.

Fig. 1. The structure of Congo Red dye.

single and mixed oxide pillars have been prepared using the polycationic species of Al, Zn, Ti and Cr among others [5–10]. Properties such as acidity, surface area and pore size distribution of PILCs lead to new shape-selective catalysts that are similar to zeolites [23]. TiO2 and TiO2 -based materials [24–30] have attracted growing interest because of their low toxicity, such kind of materials being used as photocatalysts for efficient treatment of wastewaters containing toxic organic compounds [1,2,15,16,20,21]. The Romanian montmorillonite from Valea Chioarului areal has the following chemical composition (wt%): SiO2 -72.87; Al2 O3 -14.5; MgO-2.15; Fe2 O3 -1.13; Na2 O-0.60; K2 O-0.60; CaO-0.90; PC-5.70. Its mineralogical composition consists of kaolinite (K), illite (I), montmorillonite (M) and nontronite (N), besides small quantities of feldspar–albite (F) and quartz (Q). The determined cationic exchange capacity was 82 meq/100 g. Romanian montmorillonite pillared with different metal oxides, possessing additional functions originated from both enhanced surface area and pillars identity, is interesting from both academic and industrial point of view [31–33]. The pillaring process can be realized by following two approaches. The first one, and the most intense used, is the direct pillaring which involves the treatment of Na-form of the clay with the pillaring solution, followed by the calcination of the resulted material [2,33]. The second approach uses an intermediary step, the interlayer space being expanded with a swelling agent (indirect method) [31]. This paper reports the synthesis of TiO2 -pillared Romanian clay, the size of pillars being controlled by expanding the clay’s gallery using cetyl-trimethyammonium bromide (CTAB) as expanding agent, and a brief study of their application as photocatalyst for Congo Red (CR) dye degradation (Fig. 1). This dye is the sodium salt of benzidinediazo-bis-1-naphtylamine-4-sulfonic acid (C32 H22 N6 Na2 O6 S2 )2 , its use in cellulose industries (cotton textile, wood pulp and paper) being diminished due to the high toxicity of residual wastewaters resulted from industrial processes.

2.3. Preparation of pillared clay using direct method (TiVC) The monocationic form of Romanian montmorillonite, Na-VC sample, was prepared as follows: the clay was mixed with NaCl aqueous solution (1 M) using a S/L weight ratio = 1/10 and stirred for 3 h at room temperature, this process being repeated three times with the replacing of NaCl solution after each step. The resulted Nacontaining clay was separated by vacuum filtration, washed with deionized water until free of chloride ions (determined with AgNO3 test) and dried at 80 ◦ C over night. In order to obtain the direct pillaring of the clay, we have used the already reported method for montmorillonite [2]. Briefly, the pillaring solution was added, drop-by-drop, to the suspension of Na-clay (1 wt%) in order to have 1 mM Ti per gram of clay. This suspension was stirred for 2 h at room temperature. After vacuum filtration and washing several times with deionized water, the resulted solid was dried at 80 ◦ C for 24 h and calcined at 450 ◦ C for 4 h. 2.4. Preparation of Ti-pillared clay by indirect method (TiVCS1 and TiVCS2) In order to expand its interlayer space, the clay was modified as follows: 1 g Na-clay was first dispersed in 120 mL distilled water and then CTAB was added. The quantities of added CTAB were 0.2 (VCS1 sample) and 0.5 (VCS2 samples) from clay’s cation exchange capacity. The reaction mixture was stirred for 10 h at 80 ◦ C. All products were separated by centrifugation and washed with distilled water until free of bromide and dried at 80 ◦ C for 48 h. In order to obtain a 1% suspension of surfactant-modified clay, the dried solid was dispersed in distilled water. Further, the pillaring solution was added, drop-by-drop, to the resulted suspension, in such manner to obtain 1 mmol Ti per gram of solid, and stirred for 2 h at room temperature, followed by filtration, drying and calcination in similar manner as for the direct pillared compound. 2.5. Adsorption and photodegradation experiments

2. Experimental 2.1. Materials The starting clay was obtained from Valea Chioarului areal, Romania. Chemicals like hydrochloric acid, titanium tetraisopropoxide (titania precursor) and cetyl-trimethylammonium bromide (swelling agent) were purchased from Merck.

The adsorption and photodegradation experiments have used both Na-form of the clay and Ti-pillared clay and the first step was to investigate the pH influence on the material’s Congo Red adsorption capacity. The pH of Congo Red solutions was adjusted by adding diluted HCl and NaOH solutions, using a Inolab pH-meter to control the values. Because of pH influence on sample’s adsorption capacity, the experiments used different concentrations of the

Table 1 Some preparation data and XRD characteristics of the as-synthesized samples. Sample

Surfactant content (gCTAB/g clay)

Aqueous clay suspension (wt%)

mmol TiO2 /g clay

2 [0 0 1]

d0 0 1 (Å)

I [0 0 1] (%)

Ti content (wt%)a

Na-VC TiVC VCS1 TiVCS1 VCS2 TiVCS2

– – 0.059 0.059 0.149 0.149

– 1 – 1 – 1

– 1 – 1 – 1

7.18 6.08 5.85 6.23 4.2 4.63

12.39 14.59 15.03 14.16 20.56 19.04

30.72 84.77 100 100 100 63.3

0.5 6.79 0.41 1.52 0.33 0.96

a

Ti content was determined by Energy Dispersive X-ray Analysis on uncalcined samples.

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Thermal behavior studies were performed using a Netzsch DSC 204 instrument, in the 20–600 ◦ C temperature range. UV–vis Diffuse Reflectance spectra (UV–vis DRS) of the samples were obtained in the range of 200–600 nm on a Schimadzu 2401 UV–vis spectrophotometer with an integrating diffuse reflective sphere and BaSO4 as reference. TEM images were recorded with a JEOL LEM-100CXII transmission electron microscope at an accelerating voltage of 100 kV equipped with an X-ray spectrophotometer. The TEM specimens were prepared by depositing one drop of fresh suspension onto the carbon-coated Cu grids and dried under ambient conditions. The concentration of Congo Red solution was measured using a Varian Carry 50 UV–vis spectrophotometer. 3. Results and discussion 3.1. XRD analysis of materials

Fig. 2. Low angle XRD patterns of Romanian parent clay and the corresponding surfactant-modified material.

adsorbent in the CR aqueous solution which allows us to investigate this influence. The adsorption and photocatalytic experiment have been performed at 25 ◦ C, under magnetic stirring. Prior to irradiation, all the reaction mixtures were allowed to reach the adsorption equilibrium by stirring in dark, for 30 min. The photocatalytic tests used a 12 W Vilber Lourmat UV lamp with Hg 6 W–254 nm tube. The quantification of photodegradation process was investigated by determining the residual concentration of Congo Red in the solution, both under or without UV irradiation and with or without photocatalyst, be measuring the maximum of absorption at 498 nm through UV–vis spectrophotometry. 2.6. Characterization The structure and properties of the obtained materials were investigated by X-ray Diffraction, Thermogravimetric Analysis, UV–vis Diffuse Reflectance Spectroscopy, Transmission Electron Microscopy, Energy Dispersive X-ray Analysis. XRD powder patterns were collected on a BRUKER D8 Advance instrument using Cu K␣1 radiation (Ni filter,  = 0.15401 nm, 40 kV and 50 mA).

Table 1 shows the samples notations, chemical composition and XRD characteristic of these samples. Fig. 2 shows the low angles XRD patterns of the surfactantmodified clay prepared using different amounts of CTAB. It can be observed that the (0 0 1) reflection, characteristic to the parent clay, from 2 = 7.18◦ , is shifted towards lower 2 values in case of the surfactant-modified samples. During the intercalative process, the basal spacing increase from 12.39 Å, corresponding to parent clay, to 15.03 Å for a lower CTA+ loading and to 20.56 Å for a higher loading, respectively. These results clearly indicate an enlargement of the clay’s interlayer space as a consequence of surfactant cations introduction between their anionic sheets, this enlargement being much pronounced as the surfactant content increase. The CTA+ intercalation is taking place through the cationic exchange with Na+ ions, the carbon chains forming different angles with clay’s nanosheets for different loadings. Due to its high adsorptive properties, sodium form of the clay can accommodate varied amounts of CTA+ ions in the interlayer space. With the increase of organic loading, these cations tend to have a more perpendicular orientation on the clay’s layers. This is clearly indicated by the values of the basal spacing observed for these two different loadings. Fig. 3 shows the comparative XRD patterns of the surfactantmodified clay and TiO2 -pillared surfactant-modified clay. In both cases it can be seen that the basal peak shifts to the upper 2 values with the decrease of the basal spacing as a consequence of pillaring process. This shifting is much pronounced in the second

Fig. 3. XRD patterns for surfactant-modified samples and TiO2 -pillared surfactant-modified clay obtained after intercalation of 0.059 g CTAB/g clay (left) and 0.149 g CTAB/g clay (right).

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Fig. 4. UV–vis Diffuse Reflectance spectra of as-synthesized samples: (A) samples generated using 0.059 g CTAB/g clay content ((a) Romanian parent clay (Na-VC), (b) VCS1, (c) TiVCS1, (d) Degussa P-25); (B) samples generated using 0.149 g CTAB/g clay ((a) Romanian parent clay (Na-VC), (b) VCS2, (c) TiVCS2, (d) Degussa P-25).

case (1.52 Å) then in the former one (0.88 Å) and can be correlated with the orientation of CTA+ ions in the interlayer space. As it can be observed in Table 1, the Ti content (wt%) of the uncalcined pillared samples decrease as the initial organic content increases. This suggests that titania loading for TiVC is higher, the pillars being very small and densely dispersed on the surface and between the clay layers. When the interlayer space is expanded by CTA+ cations, the pillaring mechanism involve the replacing of these cations by titania species, the resulted pillars being higher and more isolated, as suggested by XRD and elemental analysis.

Moreover, the titania uptaking from pillaring solution is lower for TiVCS2, the Ti content in the calcined samples (calculated from thermogravimetry and EDS data for uncalcined samples) being 1.84 wt% for TiVCS1 and 1.21 wt% for TiVCS2. These observations suggest that the accessibility of the interlayer space, which is a function of its induced hydrophobicity and the density of the species present at this level, decrease with the increase of CTA+ content, being higher for NaVC and smaller for VCS2. Such behavior will lead to the decreasing of titania intercalation ability in next series: NaVC > VCS1 > VCS2.

Fig. 5. TEM images of (a) Romanian parent clay; (b) TiVC and (c) TiVCS2 (the inset represents an enlarged view of the selected area).

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Fig. 6. Derivative scanning calorimetric curves of the as-synthesized samples.

3.2. Optical properties of the materials The UV–vis diffuse reflectance spectra, obtained comparative for parent clay modified with CTAB and pillared samples are shown in Fig. 4. As demonstrate in Fig. 4A and B, the parent clay is lowest transparent in the wavelength range longer than 400 nm. The pillared clay and Degussa P-25 present absorption edges in the 250–375 nm wavelength range. The absorption edges of the Ti-pillared clays are red-shifted comparatively to parent clay due to the TiO2 nanopillars formation in the interlayer space. The UV–vis diffuse reflectance spectra of the TiO2 nanoparticles (Degusa P-25), presented as reference, with sizes of about 30 nm, shows an adsorption edge at about 375 nm. The blue-shifting of the pillared clays absorption edges comparatively to pure TiO2 suggest that the formed pillars have sizes smaller than 30 nm. According to XRD data this sizes are about 2.2 Å for TiVCS1 and about 7.65 Å for TiVCS2. 3.3. TEM images of the resulted pillared compounds Fig. 5 shows the TEM images of the parent and pillared clay obtained by direct pillaring or by preliminary intercalation of CTA+ ions. It can be observed that, for the former pillaring process, the intercalation of titania species between clay’s layers will be accompanied by the formation of TiO2 particles onto clay surface, too (Fig. 5b). These possess uniform sizes and they are distributed randomly on the particles surfaces. The surfactant intercalation method allows us to obtain a good intercalation of titania clusters between clay sheets through the CTA+ ions replacing. During this intercalative process, the layered structure of the clay is preserved, as it can observe in insert of Fig. 5c which show the magnification of the selected area from the same figure.

content. After 500 ◦ C, the weight loss is attributed to both clay and pillars dehydroxylation. The removing of hydroxide groups, associated with interlayer pillars, began to occur at 150 ◦ C and caused a continuous weight loss up to 600 ◦ C. In this way, a small step to approximately 550 ◦ C is observed. This step is related to the pillars stability, since an important decrease in the basal spacing occurs at this temperature, indicating the collapse of the clay structure. Therefore, the thermogravimetric analyses are in agreement with the well-known thermal stability of the pillared clays up to 500 ◦ C. 3.5. UV assisted removal of azoic dyes It was reported that montmorillonite had a strong capacity to adsorb organic compounds from aqueous solutions [34] and TiO2 can catalyze the Congo Red photooxidation in aqueous solutions. The TiO2 -Romanian pillared clays composites, reported in this work, combine the adsorptive capacity of the Romanian clay and the ability of TiO2 to efficiently photooxidize CR in aqueous solutions. The photocatalytic experiments for CR removing have been performed under UV irradiation, at room temperature. Because of the CR structure, all the experiments have been performed at pH 4, the maximum dye absorption being at pH 1. This extremely low pH value is associated with dye coagulation, which leads us to take the decision of using a higher pH in order to avoid this process.

3.4. Thermogravimetric behavior of the pillared materials The DSC curves of as-synthesized samples are shown in Fig. 6. The main weight loss appears between 50 and 150 ◦ C and corresponds to physically adsorbed water. This weight loss is much important for TiVC sample because its interlayer spaces can accommodate a higher amount of water than the other samples where this space is occupied by voluminous CTA+ ions. Also, the hydrophobicity generated by the organic nature of the expanding agent will contribute to the decreasing of the amount of water that can be incorporated in the interlayer space. The weight loss between 250 and 450 ◦ C, marked by the exothermic (275 ◦ C) and endothermic peak (425 ◦ C), is attributed to CTA+ ions decomposition. These peaks are more intense for VCS2 and TiVCS2 indicating a higher organic

Fig. 7. Time variation of Congo Red concentration for following situations: (䊉) Congo Red solution under UV-irradiation; () Congo Red solution with TiVC content, without UV-irradiation; () Congo Red solution with TiVC content under UV-irradiation. Conditions: Ci —20 mg/L; [TiVC] = 0.4 g/L.

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Fig. 8. Variation of CR removal efficiency as function of time over TiVC, TiVCS1 and TiVCS2.

Moreover, the absorptive studies on the pristine clay indicates that the maximum of CR absorption, in the base environment, is shown at pH 10, but this require high amounts of adsorbent to get a 50% removal. In order to have an overview on photocatalytic properties of the prepared composites a set of kinetic data was realized. This allowed us to estimate the contribution of both CR photolysis and dye absorption by the materials on the final measured values. Fig. 7 shows these measurements for TiVC obtained using a 20 mg/L CR aqueous solution and an S/L ratio of 0.4 g/L. It can be observed that, by stirring a mixture of TiVC and CR in dark, only about ∼10% of the dye is adsorbed by the composite. Moreover, by UV irradiation of the CR solution, in the absence of photocatalyst, 20% of the dye is lost by photolysis. During photocatalytic measurements, ∼40% of the dye is removed in one hour. It can be concluded that the photocatalysis efficiency is relatively low if we are considering the above-mentioned photolysis value. Fig. 8 shows a comparative study of the photocatalytic efficiency of CR removal from solutions over TiVC, TiVCS1 and TiVCS2 using identical experimental conditions like for the previous experiment. After only 10 min, CR removal efficiency increases from 25.4%, in case of TiVC, to 42.7%, in case of TiVCS and to 90% for TiVCS2. After 30 min, all the dye from the solution has been removed over TiVCS2, indicating that this sample shows the highest efficiency for the dye oxidation. Moreover, these results indicates that photocatalytic activity of the resulted materials is not a function of Ti loading of the samples, but it is a function of the contact between catalyst and the dye to be degraded that increase with the adsorptive properties of the materials. 4. Conclusions The Romanian TiO2 -pillared clay and the Romanian TiO2 pilarred organic modified clay were successfully obtained through the classic pillaring process of both Na-form and organic modified form of the clay, respectively. The size of TiO2 pillars vary as a function of CTA+ sample content, by using a higher amount of swelling agent the interlayer space being increased considerably (2.056 nm), comparative to that one corresponding to Na-form of the natural clay (1.239 nm), this increase being similar with that reported by Guo et al. [35] for a Chinese bentonite (CEC80mequiv/100 g), but higher than that reported by Wang and Wang [36] for another Chinese montmorollonite (1.489 nm for a CTAB loading by 0.5 from its CEC −102.8 meq/100 g). The biggest TiO2

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pillars are obtained for TiVCS2 sample, while the smallest sizes are observed for organic unmodified sample (TiVC), as indicated by XRD data. These results suggest that the swelled material possessed better exchange properties than Na-form of the clay, being able to accommodate Ti-polycationic species having sizes direct proportional with the clay’s gallery size. Additionally, by direct pillaring of the clay, the presence of TiO2 nanoparticles on the surfaces can be noticed in TEM images. These particles posses bigger sizes than that ones formed in the interlayer space, as suggested by XRD data. Moreover, the as-obtained pillared materials showed blue-shifted UV absorption edges comparative to TiO2 nanoparticles (Degussa P-25), this indicating a smaller size of the TiO2 nanopillars than 30 nm. The Romanian clay has excellent ability to adsorb CR, while the TiO2 -Romanian pillared clay can combine the adsorptive properties of the clay and photocatalytic abilities showed by TiO2 for CR removing from its aqueous solution. For this purpose, different sized TiO2 -pillared clays has been investigated for photocatalytic oxidation of the mentioned dye, the adsorptive capacity and catalytic activity being function of TiO2 pillars distribution and sizes, and not of TiO2 loading. This study proposes an efficient method for dyes degradation using materials based on natural Romanian clay. References [1] S.V. Awate, K. Suzuki, Enhanced adsorption capacity and photo-catalytic oxidative activity of dyes in aqueous medium by hydrothermally treated titania pillared clay, Adsorption 7 (4) (2001) 319–326. [2] P. Pichat, H. Khalaf, D. Tabet, M. Houari, M. Saidi, Ti-montmorillonite as photocatalyst to remove 4-chlorophenol in water and methanol in air, Environ. Chem. Lett. 2 (4) (2005) 191–194. 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