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Journal of Hazardous Materials 167 (2009) 1050–1056

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

ynthesis and characterization of TiO2-pillared Romanian clay and theirpplication for azoic dyes photodegradation

. Dvininova,∗, E. Popovici a, R. Podeb, L. Cochecib, P. Barvinschic, V. Nicad

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, RomaniaFaculty of Physics, West University of Timisoara, Bvd. V.Parvan 4, 300223 Timisoara, RomaniaFaculty of Physics, “Al.I.Cuza” University of Iasi, Bvd. Carol I, no 11, 700506 Romania

r t i c l e i n f o

rticle history:eceived 11 December 2008eceived in revised form 23 January 2009ccepted 26 January 2009vailable online 6 February 2009

eywords:ontmorillonite

urfactantntercalation

a b s t r a c t

The synthesis and properties of metal oxide pillared cationic clays (PILCs) has been subject to numerousstudies in the last decades. In order to obtain TiO2-pillared type materials, sodium montmorillonite fromRomania–areal of Valea Chioarului, having the following composition (% wt): SiO2-72.87; Al2O3-14.5;MgO-2.15; Fe2O3-1.13; Na2O-0.60; K2O-0.60; CaO-0.90; PC-5.70 and cation exchange capacity, determinedby ammonium acetate method, of 82 meq/100 g, as matrix, was used. Sodium form of the clay was modi-fied, primarily, by intercalation of cetyl-trimethylammonium cations between negatively charged layerswhich 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), inhydrochloric acid environment, the resulted species being converted into TiO2 pillars by calcination. The

iO2

ongo Redhotocatalysis

as-prepared materials have been used as catalysts for Congo Red dye photodegradation, under UV. Thephotocatalytic activity of the pillared clays is a function of TiO2 pillars size, their increase leading to theenhancement of the contact areas between dye solution and photoactive species present in the interlayerspace.

The structural characteristics and properties of the obtained materials were investigated by X-rayDiffraction, Thermogravimetry Analysis, UV–vis Diffuse Reflectance, Transmission Electron Microscopy

ray A

and Energy Dispersive X-

. Introduction

The contamination of rivers and ground water by organic pol-utants has acted as stimulus for numerous investigations focusedn the effective pollution abatement methods. In aqueous environ-ent, the presence of organic pollutants is of great concern. Most

f the industrial and domestic waters are frequently contaminatedith organic pollutants such as phenols, VOCs, pesticides, dyes, etc.

1]. The currently used approaches to remove the organic pollutantsrom the wastewaters are based on the adsorption or chemical oxi-ation processes. However, these processes have major drawbacks:he adsorption does not lead to pollutants degradation, while thehemical oxidation, in homogeneous phase, is not economically

avorable, except the high concentrated pollutants [2].

The clays, such as montmorillonite, vermiculite, kaolinite, mica,ydrotalcite like compounds, etc., have attracted much attention inecent years for their applicability in wastewaters decontamination.

∗ Corresponding author.E-mail address: m1i2m3i4@yahoo.com (E. Dvininov).

304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2009.01.105

nalysis.© 2009 Elsevier B.V. All rights reserved.

These natural materials possess layered structures, large surfaceareas and high exchange capacity (CEC), which give them manypotential applications in different fields [3].

For environmental uses, Romanian montmorillonite from ValeaChioarului areal, having a 2:1 layered structure, represents apromising material. Because of the isomorphic substitution withinthe layers (e.g., Al3+ replaced by Mg2+or Fe2+, or Mg2+ replaced byLi+ in the octahedral sheet; Si4+ replaced by Al3+ in the tetrahedralsheet), these layers are negatively charged, the charge neutralitybeing ensured by the presence of cations in the galleries formed bytwo adjacent sheets. These cations are exchangeable and the sum ofthese charges determines the cation exchange capacity value (CEC).Generally, the clays can adsorb organic substances either on theirexternal surfaces or within their interlamelar spaces, by interac-tion with or by substitution of the cations presents in the interlayerspaces [4].

In the last decades, metal oxide pillared clays (PILCs) havebeen intensely investigated [5–22], their physical and chemicalproperties finding applications in different fields. These materi-als contain metal oxide pillars that sustain the clay sheets andlead to the formation of bi-dimensional porous networks. Several

E. Dvininov et al. / Journal of Hazardous

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Fig. 1. The structure of Congo Red dye.

ingle and mixed oxide pillars have been prepared using the poly-ationic species of Al, Zn, Ti and Cr among others [5–10]. Propertiesuch as acidity, surface area and pore size distribution of PILCsead to new shape-selective catalysts that are similar to zeolites23].

TiO2 and TiO2-based materials [24–30] have attracted grow-ng interest because of their low toxicity, such kind of materialseing used as photocatalysts for efficient treatment of wastewatersontaining toxic organic compounds [1,2,15,16,20,21].

The Romanian montmorillonite from Valea Chioarului areal hashe following chemical composition (wt%): SiO2-72.87; Al2O3-14.5;

gO-2.15; Fe2O3-1.13; Na2O-0.60; K2O-0.60; CaO-0.90; PC-5.70.ts mineralogical composition consists of kaolinite (K), illite (I),

ontmorillonite (M) and nontronite (N), besides small quanti-ies of feldspar–albite (F) and quartz (Q). The determined cationicxchange capacity was 82 meq/100 g.

Romanian montmorillonite pillared with different metal oxides,ossessing additional functions originated from both enhanced sur-

ace area and pillars identity, is interesting from both academicnd industrial point of view [31–33]. The pillaring process can beealized by following two approaches. The first one, and the mostntense used, is the direct pillaring which involves the treatment ofa-form of the clay with the pillaring solution, followed by the cal-ination of the resulted material [2,33]. The second approach usesn intermediary step, the interlayer space being expanded with awelling agent (indirect method) [31].

This paper reports the synthesis of TiO2-pillared Romanianlay, the size of pillars being controlled by expanding the clay’sallery using cetyl-trimethyammonium bromide (CTAB) as expand-ng agent, and a brief study of their application as photocatalystor Congo Red (CR) dye degradation (Fig. 1). This dye is theodium salt of benzidinediazo-bis-1-naphtylamine-4-sulfonic acidC32H22N6Na2O6S2)2, its use in cellulose industries (cotton textile,ood pulp and paper) being diminished due to the high toxicity of

esidual wastewaters resulted from industrial processes.

. Experimental

.1. Materials

The starting clay was obtained from Valea Chioarului areal,omania. Chemicals like hydrochloric acid, titanium tetraiso-ropoxide (titania precursor) and cetyl-trimethylammoniumromide (swelling agent) were purchased from Merck.

able 1ome preparation data and XRD characteristics of the as-synthesized samples.

ample Surfactant content(gCTAB/g clay)

Aqueous claysuspension (wt%)

mmol TiO2/g cla

a-VC – – –iVC – 1 1CS1 0.059 – –iVCS1 0.059 1 1CS2 0.149 – –iVCS2 0.149 1 1

a Ti content was determined by Energy Dispersive X-ray Analysis on uncalcined sample

Materials 167 (2009) 1050–1056 1051

2.2. Preparation of Ti-pillaring solution

1 mM of titanium tetraisopropoxide (titania precursor) washydrolyzed with 1 M hydrochloric acid. Briefly, titanium alkoxidewas dropped into hydrochloric acid solution, the resulted slurrybeing vigorously stirred for 3 h, at room temperature, with theobtaining of the titania clear sol solution. The quantities of HCland titanium tetraisopropoxide were calculated in order to obtaina [Ti]/[H+] = 1:4 molar ratio.

2.3. Preparation of pillared clay using direct method (TiVC)

The monocationic form of Romanian montmorillonite, Na-VCsample, was prepared as follows: the clay was mixed with NaClaqueous solution (1 M) using a S/L weight ratio = 1/10 and stirred for3 h at room temperature, this process being repeated three timeswith the replacing of NaCl solution after each step. The resulted Na-containing clay was separated by vacuum filtration, washed withdeionized water until free of chloride ions (determined with AgNO3test) and dried at 80 ◦C over night.

In order to obtain the direct pillaring of the clay, we have used thealready reported method for montmorillonite [2]. Briefly, the pillar-ing 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 suspensionwas stirred for 2 h at room temperature. After vacuum filtration andwashing several times with deionized water, the resulted solid wasdried at 80 ◦C for 24 h and calcined at 450 ◦C for 4 h.

2.4. Preparation of Ti-pillared clay by indirect method (TiVCS1and TiVCS2)

In order to expand its interlayer space, the clay was modified asfollows: 1 g Na-clay was first dispersed in 120 mL distilled water andthen CTAB was added. The quantities of added CTAB were 0.2 (VCS1sample) and 0.5 (VCS2 samples) from clay’s cation exchange capac-ity. The reaction mixture was stirred for 10 h at 80 ◦C. All productswere separated by centrifugation and washed with distilled wateruntil free of bromide and dried at 80 ◦C for 48 h. In order to obtain a1% suspension of surfactant-modified clay, the dried solid was dis-persed in distilled water. Further, the pillaring solution was added,drop-by-drop, to the resulted suspension, in such manner to obtain1 mmol Ti per gram of solid, and stirred for 2 h at room temperature,followed by filtration, drying and calcination in similar manner asfor the direct pillared compound.

2.5. Adsorption and photodegradation experiments

The adsorption and photodegradation experiments have usedboth Na-form of the clay and Ti-pillared clay and the first step

adsorption capacity. The pH of Congo Red solutions was adjustedby adding diluted HCl and NaOH solutions, using a Inolab pH-meterto control the values. Because of pH influence on sample’s adsorp-tion capacity, the experiments used different concentrations of the

y 2� [0 0 1] d0 0 1 (Å) I [0 0 1] (%) Ti content (wt%)a

7.18 12.39 30.72 0.56.08 14.59 84.77 6.795.85 15.03 100 0.416.23 14.16 100 1.524.2 20.56 100 0.334.63 19.04 63.3 0.96

s.

1052 E. Dvininov et al. / Journal of Hazardous

Fs

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ig. 2. Low angle XRD patterns of Romanian parent clay and the correspondingurfactant-modified material.

dsorbent in the CR aqueous solution which allows us to investigatehis influence.

The adsorption and photocatalytic experiment have been per-ormed at 25 ◦C, under magnetic stirring. Prior to irradiation, all theeaction mixtures were allowed to reach the adsorption equilib-ium by stirring in dark, for 30 min. The photocatalytic tests used a2 W Vilber Lourmat UV lamp with Hg 6 W–254 nm tube.

The quantification of photodegradation process was investi-ated by determining the residual concentration of Congo Red in theolution, both under or without UV irradiation and with or withouthotocatalyst, be measuring the maximum of absorption at 498 nmhrough UV–vis spectrophotometry.

.6. Characterization

The structure and properties of the obtained materials werenvestigated by X-ray Diffraction, Thermogravimetric Analysis,

V–vis Diffuse Reflectance Spectroscopy, Transmission Electronicroscopy, Energy Dispersive X-ray Analysis.XRD powder patterns were collected on a BRUKER D8 Advance

nstrument using Cu K�1 radiation (Ni filter, � = 0.15401 nm, 40 kVnd 50 mA).

ig. 3. XRD patterns for surfactant-modified samples and TiO2-pillared surfactant-modifilay (right).

Materials 167 (2009) 1050–1056

Thermal behavior studies were performed using a Netzsch DSC204 instrument, in the 20–600 ◦C temperature range.

UV–vis Diffuse Reflectance spectra (UV–vis DRS) of the sampleswere obtained in the range of 200–600 nm on a Schimadzu 2401UV–vis spectrophotometer with an integrating diffuse reflectivesphere and BaSO4 as reference.

TEM images were recorded with a JEOL LEM-100CXII trans-mission electron microscope at an accelerating voltage of 100 kVequipped with an X-ray spectrophotometer. The TEM specimenswere prepared by depositing one drop of fresh suspension onto thecarbon-coated Cu grids and dried under ambient conditions.

The concentration of Congo Red solution was measured using aVarian Carry 50 UV–vis spectrophotometer.

3. Results and discussion

3.1. XRD analysis of materials

Table 1 shows the samples notations, chemical composition andXRD characteristic of these samples.

Fig. 2 shows the low angles XRD patterns of the surfactant-modified clay prepared using different amounts of CTAB. It can beobserved that the (0 0 1) reflection, characteristic to the parent clay,from 2� = 7.18◦, is shifted towards lower 2� values in case of thesurfactant-modified samples. During the intercalative process, thebasal spacing increase from 12.39 Å, corresponding to parent clay, to15.03 Å for a lower CTA+ loading and to 20.56 Å for a higher loading,respectively. These results clearly indicate an enlargement of theclay’s interlayer space as a consequence of surfactant cations intro-duction between their anionic sheets, this enlargement being muchpronounced as the surfactant content increase. The CTA+ interca-lation is taking place through the cationic exchange with Na+ ions,the carbon chains forming different angles with clay’s nanosheetsfor different loadings. Due to its high adsorptive properties, sodiumform of the clay can accommodate varied amounts of CTA+ ions inthe interlayer space. With the increase of organic loading, thesecations tend to have a more perpendicular orientation on the clay’slayers. This is clearly indicated by the values of the basal spacingobserved for these two different loadings.

Fig. 3 shows the comparative XRD patterns of the surfactant-modified clay and TiO2-pillared surfactant-modified clay. In bothcases it can be seen that the basal peak shifts to the upper 2� val-ues with the decrease of the basal spacing as a consequence ofpillaring process. This shifting is much pronounced in the second

ed clay obtained after intercalation of 0.059 g CTAB/g clay (left) and 0.149 g CTAB/g

E. Dvininov et al. / Journal of Hazardous Materials 167 (2009) 1050–1056 1053

F enera( oman

clAuipbbta

ig. 4. UV–vis Diffuse Reflectance spectra of as-synthesized samples: (A) samples gc) TiVCS1, (d) Degussa P-25); (B) samples generated using 0.149 g CTAB/g clay ((a) R

ase (1.52 Å) then in the former one (0.88 Å) and can be corre-ated with the orientation of CTA+ ions in the interlayer space.s it can be observed in Table 1, the Ti content (wt%) of thencalcined pillared samples decrease as the initial organic content

ncreases. This suggests that titania loading for TiVC is higher, theillars being very small and densely dispersed on the surface and

etween the clay layers. When the interlayer space is expandedy CTA+ cations, the pillaring mechanism involve the replacing ofhese cations by titania species, the resulted pillars being highernd more isolated, as suggested by XRD and elemental analysis.

Fig. 5. TEM images of (a) Romanian parent clay; (b) TiVC and (c) TiVC

ted using 0.059 g CTAB/g clay content ((a) Romanian parent clay (Na-VC), (b) VCS1,ian parent clay (Na-VC), (b) VCS2, (c) TiVCS2, (d) Degussa P-25).

Moreover, the titania uptaking from pillaring solution is lower forTiVCS2, the Ti content in the calcined samples (calculated fromthermogravimetry and EDS data for uncalcined samples) being1.84 wt% for TiVCS1 and 1.21 wt% for TiVCS2. These observationssuggest that the accessibility of the interlayer space, which is a func-tion 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 willlead to the decreasing of titania intercalation ability in next series:NaVC > VCS1 > VCS2.

S2 (the inset represents an enlarged view of the selected area).

1054 E. Dvininov et al. / Journal of Hazardous Materials 167 (2009) 1050–1056

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performed under UV irradiation, at room temperature. Because ofthe CR structure, all the experiments have been performed at pH4, the maximum dye absorption being at pH 1. This extremely lowpH value is associated with dye coagulation, which leads us to takethe decision of using a higher pH in order to avoid this process.

Fig. 6. Derivative scanning calorime

.2. Optical properties of the materials

The UV–vis diffuse reflectance spectra, obtained comparative forarent clay modified with CTAB and pillared samples are shown inig. 4.

As demonstrate in Fig. 4A and B, the parent clay is lowest trans-arent in the wavelength range longer than 400 nm. The pillaredlay and Degussa P-25 present absorption edges in the 250–375 nmavelength range. The absorption edges of the Ti-pillared clays are

ed-shifted comparatively to parent clay due to the TiO2 nanopillarsormation in the interlayer space. The UV–vis diffuse reflectancepectra of the TiO2 nanoparticles (Degusa P-25), presented as ref-rence, with sizes of about 30 nm, shows an adsorption edge atbout 375 nm. The blue-shifting of the pillared clays absorptiondges comparatively to pure TiO2 suggest that the formed pillarsave sizes smaller than 30 nm. According to XRD data this sizes arebout 2.2 Å for TiVCS1 and about 7.65 Å for TiVCS2.

.3. TEM images of the resulted pillared compounds

Fig. 5 shows the TEM images of the parent and pillared claybtained by direct pillaring or by preliminary intercalation of CTA+

ons. It can be observed that, for the former pillaring process, thentercalation of titania species between clay’s layers will be accom-anied by the formation of TiO2 particles onto clay surface, tooFig. 5b). These possess uniform sizes and they are distributedandomly on the particles surfaces. The surfactant intercalationethod allows us to obtain a good intercalation of titania clusters

etween clay sheets through the CTA+ ions replacing. During thisntercalative process, the layered structure of the clay is preserved,s it can observe in insert of Fig. 5c which show the magnificationf the selected area from the same figure.

.4. Thermogravimetric behavior of the pillared materials

The DSC curves of as-synthesized samples are shown in Fig. 6.he main weight loss appears between 50 and 150 ◦C and cor-esponds to physically adsorbed water. This weight loss is muchmportant for TiVC sample because its interlayer spaces can accom-

odate a higher amount of water than the other samples where thispace is occupied by voluminous CTA+ ions. Also, the hydrophobic-ty generated by the organic nature of the expanding agent will

ontribute to the decreasing of the amount of water that can bencorporated in the interlayer space. The weight loss between 250nd 450 ◦C, marked by the exothermic (275 ◦C) and endothermiceak (425 ◦C), is attributed to CTA+ ions decomposition. These peaksre more intense for VCS2 and TiVCS2 indicating a higher organic

rves of the as-synthesized samples.

content. After 500 ◦C, the weight loss is attributed to both clay andpillars dehydroxylation. The removing of hydroxide groups, asso-ciated with interlayer pillars, began to occur at 150 ◦C and causeda continuous weight loss up to 600 ◦C. In this way, a small step toapproximately 550 ◦C is observed. This step is related to the pillarsstability, since an important decrease in the basal spacing occursat this temperature, indicating the collapse of the clay structure.Therefore, the thermogravimetric analyses are in agreement withthe 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 toadsorb organic compounds from aqueous solutions [34] and TiO2can catalyze the Congo Red photooxidation in aqueous solutions.The TiO2-Romanian pillared clays composites, reported in thiswork, combine the adsorptive capacity of the Romanian clay andthe ability of TiO2 to efficiently photooxidize CR in aqueous solu-tions. The photocatalytic experiments for CR removing have been

Fig. 7. Time variation of Congo Red concentration for following situations: (�) CongoRed solution under UV-irradiation; (�) Congo Red solution with TiVC content, with-out UV-irradiation; (�) Congo Red solution with TiVC content under UV-irradiation.Conditions: Ci—20 mg/L; [TiVC] = 0.4 g/L.

E. Dvininov et al. / Journal of Hazardous

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ig. 8. Variation of CR removal efficiency as function of time over TiVC, TiVCS1 andiVCS2.

oreover, the absorptive studies on the pristine clay indicates thathe maximum of CR absorption, in the base environment, is shownt pH 10, but this require high amounts of adsorbent to get a 50%emoval.

In order to have an overview on photocatalytic properties ofhe prepared composites a set of kinetic data was realized. Thisllowed us to estimate the contribution of both CR photolysis andye absorption by the materials on the final measured values. Fig. 7hows these measurements for TiVC obtained using a 20 mg/L CRqueous solution and an S/L ratio of 0.4 g/L.

It can be observed that, by stirring a mixture of TiVC and CRn dark, only about ∼10% of the dye is adsorbed by the composite.

oreover, by UV irradiation of the CR solution, in the absence ofhotocatalyst, 20% of the dye is lost by photolysis. During photo-atalytic measurements, ∼40% of the dye is removed in one hour.t can be concluded that the photocatalysis efficiency is relativelyow if we are considering the above-mentioned photolysis value.

Fig. 8 shows a comparative study of the photocatalytic efficiencyf CR removal from solutions over TiVC, TiVCS1 and TiVCS2 usingdentical experimental conditions like for the previous experiment.fter only 10 min, CR removal efficiency increases from 25.4%, inase of TiVC, to 42.7%, in case of TiVCS and to 90% for TiVCS2. After0 min, all the dye from the solution has been removed over TiVCS2,

ndicating that this sample shows the highest efficiency for theye oxidation. Moreover, these results indicates that photocatalyticctivity of the resulted materials is not a function of Ti loading ofhe samples, but it is a function of the contact between catalyst andhe dye to be degraded that increase with the adsorptive propertiesf the materials.

. Conclusions

The Romanian TiO2-pillared clay and the Romanian TiO2-ilarred organic modified clay were successfully obtained throughhe classic pillaring process of both Na-form and organic modi-ed form of the clay, respectively. The size of TiO2 pillars varys a function of CTA+ sample content, by using a higher amountf swelling agent the interlayer space being increased consider-bly (2.056 nm), comparative to that one corresponding to Na-form

f the natural clay (1.239 nm), this increase being similar withhat reported by Guo et al. [35] for a Chinese bentonite (CEC-0mequiv/100 g), but higher than that reported by Wang and Wang36] for another Chinese montmorollonite (1.489 nm for a CTABoading by 0.5 from its CEC −102.8 meq/100 g). The biggest TiO2

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Materials 167 (2009) 1050–1056 1055

pillars are obtained for TiVCS2 sample, while the smallest sizesare observed for organic unmodified sample (TiVC), as indicated byXRD data. These results suggest that the swelled material possessedbetter exchange properties than Na-form of the clay, being able toaccommodate Ti-polycationic species having sizes direct propor-tional with the clay’s gallery size. Additionally, by direct pillaringof the clay, the presence of TiO2 nanoparticles on the surfaces canbe noticed in TEM images. These particles posses bigger sizes thanthat ones formed in the interlayer space, as suggested by XRD data.Moreover, the as-obtained pillared materials showed blue-shiftedUV absorption edges comparative to TiO2 nanoparticles (DegussaP-25), this indicating a smaller size of the TiO2 nanopillars than30 nm. The Romanian clay has excellent ability to adsorb CR, whilethe TiO2-Romanian pillared clay can combine the adsorptive prop-erties of the clay and photocatalytic abilities showed by TiO2 forCR removing from its aqueous solution. For this purpose, differentsized TiO2-pillared clays has been investigated for photocatalyticoxidation of the mentioned dye, the adsorptive capacity and cat-alytic activity being function of TiO2 pillars distribution and sizes,and not of TiO2 loading. This study proposes an efficient method fordyes degradation using materials based on natural Romanian clay.

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