Preparation and Characterization of Different Tio₂-Modified Montmorillonite Meso-Microporous Materials with Enhanced Photocatalytic Activity

Preparation and Characterization of Different Tio₂-Modified Montmorillonite Meso-Microporous Materials with Enhanced Photocatalytic Activity

Manel Baizig* (1), Soumaya Khalfallah(1), Bassem Jamoussi(2), Narjès Batis(1) and Raquel Trujillano(3)

1 Unité de recherche de Catalyse dElectrochimie de Nanomatériaux et leurs Applications et de Didactique (CENAD), Institut National des Sciences Appliquées et de Technologie (INSAT), Université de Carthage, Centre Urbain Nord B.P. n° 676-1080 Tunis Cedex Tunisie

2 Institut Supérieur dEducation et de Formation Continue (ISEFC) 43, rue de la Liberté 2019 Le Bardo-Tunisie

3 Departamento de Química Inorgánica, Universidad de Salamanca, 37008 Salamanca, Spain

AbstractA set of TiO2-modified montmorillonite meso- microporous materials, as photocatalysts, was successfully prepared in different ways and evaluated by the photodegradation of 4-nitrophenol (4-NP) in water, since it is a recalcitrant organic pollutant.

The effects of photocatalysts preparation methods on their textural, structural and catalytic properties were tested. To understand the relationship between the photocatalysts structure and performance, these were characterized by X- Ray Diffraction (XRD), UV-spectroscopy, Scanning Electron Microscopy/Energy Dispersive X-ray spectroscopy (SEM/EDX), Fourier Transform Infrared Spectroscopy (FTIR), X-ray fluorescence (XRF), specific surface area and porosity measurements.

A high specific surface area (up to 231 m2/g), a meso- microporous structure and a high stability have been found for the TiCl4-M and TiCl3-M prepared photocatalysts.

In addition, to improve the 4-NP degradation, various parameters effect was studied such as the type of the photocatalyst, its amount loading, the pH of the 4-NP solution and the initial 4-NP concentration. Then, the 4-NP degradation rate has reached 99% only when Anatase Titanium dioxide was supported on montmorillonite, at pH=5, with 0.2 g/100 mL of photocatalyst (TiCl4-M or TiCl3-M) amount and a 4-NP concentration equal to 20 mg.L-1.

Key wordsAnatase Titanium dioxide; TiO2-modified montmorillonite; meso-microporous materials; 4-Nitrophenol photodegradation improvement.

1. INTRODUCTION Montmorillonite clays, being classified as porous

   materials [1], are frequently studied and used as adsorbents and catalysts [2-4]. This is partly due to their exceptional properties such as high cation exchange capacity, good swelling, high surface area and high porosity [1-5]. These properties allow their modification for specific applications. In fact, they are generally used as support of other active species such as inorganic metal oxide particles [6]. Many studies have been reported the utilization of montmorillonite as support of Titania [2, 7, 8]. In fact, Anatase Titanium

   dioxide is the most known and attractive material, having a high photocatalytic activity and it is characterized by a large specific surface [9-11] [12]. It is frequently used in photocatalysis for wastewater treatment of many hazardous organic contaminants such as organic dyes [8], phenol compounds [13], pesticides [14] and herbicides [15].

   However, when Titania is used in slurry form, it presents several practical disadvantages, like particle agglomeration [16] and the recovery of the powder after chemical reaction [12, 17-19] which needs a microfiltration (energetically costly and time consuming).

   The use of clay as support seems to be a good solution to solve these problems. There are different kinds of clays that have been used as a support such as bentonite [20], zeolite [21], kaolinite [22], sepiolite [23], and

   montmorillonite [24].

   In this study, we have chosen a montmorillonite clay as support for titania for its properties (low cost, no toxicity, high surface area and its swelling ability [25-27]).

   Therefore a series of TiO2-supported montmorillonite porous materials has been prepared employing various ways. The effects of titanium source and the preparation methods on the structural, textural and photocatalytic properties of these materials have been investigated. TiO2- modified montmorillonite was prepared firstly by hydrolysis of TiCl4 according to the method presented by Sterte [28]. Secondly, it was made using a solution of TiCl3, as an intercalation solution, based on the method presented by Jianjun Liu et al. [19]. Thirdly, an interaction solution of Titanium isopropoxide is used following the method described by Yamanaka et al [29].

   The performance of different prepared photocatalysts was tested for the photodegradation of 4-nitrophenol in presence of UV-irradiation. The degradation rate was improved by the determination of optimal operating parameters and by the study of the effects of various experimental parameters, essentially the photocatalyst type, its amount loading, the initial concentration and the pH of 4- nitrophenol solution. To understand the relationship

   between the photocatalysts structure and performance, different methods of characterization (X-ray diffraction, UV-spectroscopy, infrared spectroscopy, EDX/MEB, BET specific surface area, X-ray fluorescence) were used.

2. MATERIALS AND METHODS:

   1. Chemicals and reagents:

      Montmorillonite K10, titanium (IV) isopropoxide and titanium (IV) chloride (99.9%) were provided by sigma- aldrich.

      Titanium (III) chloride solution 15% in hydrochloric acid was supplied by Riedel de Haën.

      4-nitrophenol (99%) was purchased from Merck.

   2. Photocatalysts preparation:

      1. The starting Na-montmorillonite:

         The starting Na-montmorillonite was prepared after exchanging the commercial montmorillonite K10 with NaCl solution (1 mol.L-1) and washing the clay several times with distilled water until it becomes free of Cl- ions (proved by the test with AgNO3). Washed clay was centrifuged, dried at 60°C and reffered as Na-mont.

      2. Titanium-modified montmorillonite by hydrolysis of TiCl4 (TiCl4-M):

         TiCl4-M was prepared according to the protocol proposed by Sterte [28], with slight modification by

         hydrolysis of TiCl4 with HCl. Preparation protocol is detailed in fig.1.

         Titanium (IV) Chloride

         HCl (5mol.L-1)

         Distilled water

         Ti-pillaring solution [Ti]=0.82 mol.L-1,

         [HCl] =0.2 mol.L-1, H+/Ti=0.24

         2L of Na-mont suspension Ti/clay= 10 mmol

         Dropwise (Ti /clay=10mmol)

         Slow agitation for 24h

         Clay suspension

         Centrifugation, washing, drying (60°C) and grinding

         Ti- intercalated montmorillonite

         Calcination at 350°C

         Ti- modified montmorillonite reffered as TiCl4-M

         Fig.1 preparation protocol of TiCl4-M

      3. Titanium-modified montmorillonite by hydrolysis of TiCl3(TiCl3-M):

         According to the method presented by Jianjun Liu

         et al. [19], a solvothermal preparation was used to synthesize TiCl3-M, using water and ethanol as solvent, hexamethylene tetramine as precipitant and titanium trichloride as precursor. More details are given in fig.2.

         Titanium (III) chloride solution 15% in HCl

         Ethanol

         Ti-pillaring solution, [Ti3+] =0.8mol.L-1

         Na-montmorillonite suspension Water/Ethanol = 1/1

         Dropwise

         (Ti /clay=10mmol)

         Slow agitation for 4h

         Hexamethylene tetramine (HMT) TiCl3/HMT = 8

         Autoclaving at 90°C for 1h and at 190°C for 2h

         Centrifugation, washing, drying (60°C) and grinding

         Ti-modified montmorillonite reffered as TiCl3-M

         Fig.2 preparation protocol of TiCl3-M

      4. Titanium-modified montmorillonite by hydrolsis of Ti(OC3H7)4(Iso-M):

      Iso-M was obtained by the intercalation from

      Ti(OC3H7)4, hydrolyzed with HCl as described by Yamanaka et al. [29] with slight modifications. Fig.3 illustrates the used protocol.

      D. Photocatalytic Activity:

      Distilled water

      Ti-pillaring solution [Ti]=0.25 mol.L-1,

      [HCl] =1 mol.L-1, H+/Ti=0.4

      HCl (5mol.L-1)

      Titanium (IV) isopropoxide

      The photocatalytic activities of the prepared samples were evaluated by the degradation of 4-nitrophenol aqueous solution under UV light irradiation.

      Centrifugation, washing, drying (60°C) and grinding

      Ti- intercalated montmorillonite

      Calcination at 350°C

      Dropwise

      (Ti /clay=10mmol)

      Slow agitation for 24h

      Na-montmorillonite suspension Ti/clay= 10mmol

      Clay suspension

      The photocatalyst powder was dispersed in a 100 mL of 4-nitrophenol solution and kept under stirring, on a glass beaker equipped with a cooling system as shown in the fig.4. The experiments were conducted at different initial concentrations of 4-NP, at different photocatalyst amounts loading and by varying the pH of the initial solution. The pH of the solution was adjusted by tampon solutions. The suspension was air-oxygen-bubbled and irradiated with UV-light lamp (=253.7nm) which was emerged in the solution. Before irradiation, the suspension was stirred during 30 minutes in order to disperse the photocatalysts and to reach adsorption-desorption equilibrium between the organic molecule and photocatalyst surface.

      Ti-modified montmorillonite reffered as Iso-M

      Fig.3 preparation protocol of Iso-M

   3. Techniques of characterizations:

   The powder X-Ray Diffraction (XRD) patterns were recorded with CuK radiation (=1.5404Å) on a Siemens D-500 diffractometer (2º/min). The diffractograms were treated by Diffract-AT Software.

   The specific surface areas, pore volumes and micropore volumes of the samples were measured by nitrogen physisorption at 77 K using a Gemini VII apparatus from micromeritics, the different photocatalysts were treated under vacuum for two hours at 110°C to evacuate water and gas pore.

   The Fourier-Transform Infra Red (FTIR) spectra of prepared photocatalysts were carried out by KBr pellets technique using a Perkin Elmer FT 1730 instrument.

   The UV-Vis spectra were recorded in a range of

   190 to 800 nm using a HP 8453 Diode Array Spectrophotometer.

   Analyses by Scanning Electron Microscopy with Energy Dispersive X-ray spectroscopy (SEM/EDX) images were taken at Servicio de Microscopía Electrónica (Universidad de Salamanca) on Zeiss microscopy from OXFORD Instruments.

   The Chemical composition of the samples was determined by X-Ray Fluorescence (XRF) using a Magix PW2403 instrument.

   During the reaction, every 10 minutes, a 2 mL of suspension solution was collected and filtered via syringe filter (0.45 µm). Then, the sampling was analyzed by UV- Vis Spectrophotometer Shimadzu 1650PC and by a standard High-Performance Liquid Chromatography apparatus Younglin Acme 9000 (Column type: Eclipse (C-

   18) (5 m, 4.6 × 250 mm), at 20 L injection, mobile phase: acetonitrile:water (40:60), and a fixed wavelength equal to 317 nm).

   The degradation rate was deduced from the formula given by the following equation:

   Degradation rate (%) = (A0-At/A0)*100

   A0: HPLC peak area of 4-NP initial solution.

   At: HPLC peak area of 4-NP after a period of UV- light irradiation of suspension solution.

   Fig.4 Descriptive schema of the photocatalytic reactor

3. RESULTS AND DISCUSSION:

1. photocatalysts characterization:

   1. X-ray analysis:

      Fig.5 shows X-ray powder diffraction patterns of the Na-mont, TiCl3-M, TiCl4-M, and Iso-M photocatalysts. The XRD pattern of Na-mont reveals the presence of diffraction peaks corresponding to Illite impurity (JCPDS file: 00-026-0911) and Quartz impurity (JCPDS file: 00- 005-0490) which are observed at 2= 8.6°, 27.55°, 30.9°,

      45.5° and at 2=20.6°, 26.4°, 49.9°, 59.7° respectively. A diffraction peak also appears at 2~5° which is commonly

      assigned to the (001) Na-Mont diffraction peak corresponding to its basal spacing (d001=15Å) [30]. This peak is weak although it is supposed to show a maximum intensity. This can be explained by a partial disordered structure. Actually, the K10-montmorillonite used is a synthetic clay previously activated by the fabricant with a mineral acid [31] which causes probably the partial delamination of its ordered and periodic structure [32]. Besides, different peaks characteristic of montmorillonite (JCPDS file: 00-013-0135) are detected in this pattern, these are located at 2=17.55° (d003=5.04Å), 19.6° (d100=4.52Å), 29.6° (d005=3.01Å), 34.75° (d110=2.57Å), 36.3°

      (d006=2.47Å), 42.2° (d007=2.14Å) and 61.6° (d0010=1.5Å).

      In TiCl3-M diffraction pattern, the (001) diffraction weak peak is shifted to a lower value of 2 (4.7°). This displacement reveals the increase of the basal spacing to

      18.78 Å after intercalation. This result indicates an expansion in the layer structure; it is a proof of a successful pillaring [33]. New diffraction peaks appear in this pattern compared to the Na-mont pattern, these are observed at 2=25.15°, 37.8°, 47.75°, 54.8° and 62.25°. Referred to JCPDS file 00-21-1272, these diffraction peaks can be assigned to anatase phase. Thus, this result illustrates that the employed process is efficient to immobilize TiO2 onto the clay.

      However, the XRD patterns of TiCl4-M and Iso-M are characterized by the absence of (001) diffraction peak. This reveals the complete loss of periodic structure of montmorillonite, which can be probably caused by the damage of the clay layers by acid conditions of the intercalation process [34]. The characteristic diffraction peaks of anatase phase are identified in the TiCl4-M pattern at 2=25.35°, 37.75° and 48.05° but they are absents in the Iso-M pattern except the weak peak at 25.35°. TiO2 seems to be immobilized in the case of TiCl4 -M but not considerably for Iso-M.

      Fig.5 X-ray patterns of (a) Na-mont (b) TiCl3-M (c) TiCl4-M and (d) Iso- M.

      TABLE.I FTIR BANDS OF NA-MONT

      Band position	3624	3445	1639	1073	797	523	461
      Intensity	medium and thin	Strong and broad	Medium and thin	Strong and broad	Medium and thin	Medium and thin	strong and thin

      The absorption band centered at around 3445 cm-1 could be attributed to the stretching of the hydroxyl groups OH and to the vibration of physisorbed water between the clay layers [35].

      The shoulder observed at about 3624 cm-1 is characteristic of montmorillonite [36].This shoulder corresponds to hydroxyl groups stretching vibration of the octahedral layer (Al and/or MgOH) [37-38]. Absorption band at around 1639 cm-1 is due to bending vibrations of water [37]. A large absorption band between 800 and 1200 cm-1 centered at 1100 cm-1 is generally observed for all silicates. In this spectrum, it is centered at 1061 cm-1 and corresponds to stretching vibration of Si-O [37-38].

      Absorption band observed at around 797 cm-1 is relative to vibration of Hydroxyl groups coordinated to Aluminum (HOAl) or octahedral Magnesium.

      The absorption bands between 400 and 600 cm-1, correspond to bending vibration of Si-O-AlVI, Si-O-MgVI and Si-O-Fe liaisons of octahedral layer [37-38], in fact, two bands at 523 cm-1 and 461 cm-1 are located.

      After the immobilization of TiO2 on Na-mont, a disturbance of the bands located at 797, 523 and 461 cm-1 in the Na-mont spectra, is observed in the spectra of TiCl4-M, TiCl3-M and Iso-M. This disturbance is either explained by broadening of bands and/or by slight shift of their positin and/or by overlap. This is probably due to the vibrations of Ti-O and Ti-O-Ti giving bands in the 400-800 cm-1 region [39-40]. In the spectrum of Iso-M, the band at 1073cm-1 broadens; this may be related to the fact of introduction of more OH groups of the pillar after pillaring [41].

   2. FTIR analysis:

      Fig.6 shows FTIR of the Na-mont spectrum, TiCl4- M, TiCl3-M and Iso-M. The principals bands detected in the spectra of Na-mont are summarized in the table 1.

      Fig.6 FTIR spectra of different samples

   3. UV-Vis analysis:

      The Na-mont UV-Vis spectrum (fig.7) shows that the sample is almost transparent beyond 350 nm, this is similar to the results reported by Oaka et al. [42]. In addition, a characteristic broad band centered at about 250

      nm is detected, which is assigned to charge transfer band for the structural iron that is present in the octahedral layer of the clay mineral (O2- Fe3+; OH- or OH2) [43]. All the spectra of modified montmorillonites show that the samples dont absorb beyond 350 nm. But they present a new band centered at about 320 nm with different intensities. These new bands may be attributed to a charge transfer O2- T4+ of TiO2 in anatase form [44]. The band intensity is probably proportional to the quantity of titanium dioxide fixed on the clay support.

      Fig.7 UV-Vis spectra of different samples

   4. SEM and EDX analysis:

      Fig.8 illustrates the morphology and elemental analysis composition of the modified montmorillonites which were explored using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX). Figure 8 (a-1) shows SEM micrograph of TiCl3 displaying disordered distribution of aggregate particles with different shapes and sizes, there is an absence of ordered crystalline forms. Otherwise, multiple areas are in the forms of stratified layers or sheets which denote a partial ordered structure characteristic of clay materials [45-47].

      Figures 8 (a-2) and 8 (a-3) represent respectively EDX micrograph and EDX spectrum of TiCl3-M, taken on an area randomly chosen. EDX micrograph reveals the presence of titanium uniformly distributed throughout the montmorillonite. This is confirmed by the detection of titanium on the EDX spectra of the sample. Moreover, we denote the predominance of oxygen, silicon and aluminum with coexistence of few amounts of magnesium, calcium, potassium, iron, sodium and copper. These results indicate that even after modification the sample retains a characteristic chemical composition of the clay.

      SEM images 8(b-1) and 8(c-1) of TiCl4-M and Iso- M respectively present two irregular and disordered structures. The structure of the two modified clays appears to be strongly altered and disintegrated. TiCl4-M particles are in tatters and between them we observe spheroid grains corresponding probably to TiO2. These particles are

      a)TiCl3-M	b)TiCl4-M	c)Iso-M
      ( a-1)	(b-1)	(c-1)
      (a-2)	( b-2)	(c-2)
      (a-3)	(b-3)	(c-3)

      Fig.8 SEM micrographs and EDX micrographs and spectra of different samples

      smaller and thinner than Iso-M particles which are in aggregate forms with different shapes and sizes.

      We note the uniformity dispersion and the abundance of titanium in the TiCl4-M micrograph shown in figure 8 (b-2). However the titanium is almost absent in the Iso-M micrograph presented in figure 8 (c-2). These results are also illustrated by the EDX spectra shown in figures 8 (b-3) and 8 (c-3). In fact, the TiCl4-M spectrum reveals the presence of titanium unlike the Iso-M spectrum which titanium is detected in trace amounts. Results are similar to those found by DRX.

      In accordance with the EDX spectrum of TiCl3-M, the spectra of TiCl4-M and Iso-M indicate the predominance of silicon, oxygen and aluminum and the presence in very low quantities of elements such as magnesium, iron, calcium, sodium, potassium and copper. So the basic structure of the montmorillonite is maintained after modification.

   5. Textural analysis:

      Fig.9 indicates the nitrogen adsorption- desorption isotherms of Na-Mont and TiO2-modified montmorillonites. All isotherms can be considered as type II according to IUPAC classification, and they present hysteresis loops type H4 characteristic of materials with layered structure [48] and typical of slit-like pores [49]. Moreover, these isotherms shapes suggest the existence of both micropores and large pores in the samples. Actually, an increase in adsorbed volume at higher partial pressures was the result of larger mesopores presence [50].

      TABLE II. RESULTS OF ADSORPTION- DESORPTION MEASUREMENT

      Type of catalysts	m2/g			cm3/g		nm
      	BET Surface Area	t-plot Micropore Area	t-plot External Surface Area	t-plot Micropore Volume	BJH Desorption pore volume	BJH Desorption pore Diameter
      Na-mont	170	10.75	160.04	0.005	0.2616	5.12
      TiCl3-M	205	20.78	184.25	0.01	0.235	4.2
      TiCl4-M	231	0.9	230.48	0.002	0.205	3.65
      Iso-M	183	18.26	165	0.01	0.238	4.98

      Sodium montmorillonite has a specific surface area equal to 170 m2/g (table.2), an average pore diameter equal to 5.12 nm typical of mesoporous materials; we also note the presence of micropores which occupy a surface of 10.75 m2/g. This material is so specified by a bimodal porous distribution with mesopores dominance.

      An increase of the specific surface area of TiCl3-M is noted (table.2) compared to Na-Mont surface (from 170 m2/g to 205 m2/g) and a development of its microporous surface after insertion of TiO2. This is possibly due to its insertion between the clay layers [19]. These results corroborate those obtained by DRX; in fact, we observed an increase in the basal spacing of clay from 15 Ã… to 18.78 Ã… which is probably caused by the formation of titanium dioxide pillars between the sheets [33]. These pillars generate space between adjacent layers [51]. The average pore diameter and the pore volume decreased after modification. This is due to the introduction of TiO2 in the pores [52].

      TiCl4-M has the highest specific surface (231 m2/g) and its pore average diameter is about 3.95 nm (table.2) which is typical of mesoporous materials. The decrease of pore average diameter indicates probably the incorporation of TiO2 into the pores. Micropore surface decrease from 10.45 m2/g to 0.9 m2/g, consequently, we can conclude that TiO2 insertion blocks the micropores, [52] which makes them inaccessible to nitrogen molecules N2[51]. The increase of specific surface of TiCl4-M is induced essentially by the increase of mesopores number, concluding that the sample is a unimodal pore size distribution.

      A little increase of Iso-M specific surface and an insignificant decrease of the pore average diameter are noticed after modification as reported on the table 2. Micropores surface extends from 10.75 m2/g to 18.26 m2/g. Indeed, this sample is characterized by a bimodal pore distribution dominated by mesopores. Textural change is probably due to structure delamination as previously demonstrated by X-ray and SEM results; however, these results dont show the TiO2 immobilization on the material and suggest probably its failure. Perhaps, it is due to the formation f Ti-polymeric species (Ti-polyoxycations) in the pillaring solution [53-54], which are voluminous and could not be inserted onto the pores of the clay.

      Fig.9 N2 adsorption-desorption isotherms of different samples

   6. Chemical analysis:

   The results obtained previously were confirmed by chemical analysis shown in Table 3. Compared to Na-mont, a significant increase of TiO2 content is observed in TiCl4- M and TiCl3-M, which indicates the immobilization of TiO2, while the increase of TiO2 content in Iso-M is slight. The increase of TiO2 content in modified montmorillonites seems to be at the expense of Silicon, Aluminium and Iron contents.

   TABLE III. CHEMICAL COMPOSITION OF NA-MONT AND TiO2-MODIFIED MONTMORILLONITES

   Oxyde %wt	SiO2	Al2O3	Fe2O3	K2O	MgO	P2O5	TiO2	SO3	CaO	ZrO2
   Na-mont	71.68	16.18	5.07	2.72	1.50	1.18	0.82	0.25	0.18	0.06
   TiCl3-M	51.47	10.44	2.74	2.3	2.10	1.49	28.9	0.3	0.18	0.07
   TiCl4-M	51.09	11.47	3.84	1.91	2.10	0.88	27.87	0.26	0.17	0.07
   Iso-M	65.3	14.3	4.29	2.58	2.6	0.88	9.01	0.42	0.22	0.06

2. photocatalytic activity improvement:

   1) Effect of UV-light irradiation and photocatalyst addition:

   100

   80

   60

   40

   20

   0

   100 mL of 4-NP solution (15 mg.L-1) was prepared and 0.3g of TiCl4-M (or TiCl3-M) was added to evaluate the photocatalytic process of 4-NP degradation. Fig.10 shows the effect of the UV light irradiation and photocatalyst addition on the 4-NP degradation rate. In fact, we note that the 4-NP degradation did not occur in the presence of UV- light irradiation only. Adding the photocatalyst in the darkness, the 4-NP degradation rate did not exceed 11.46 %. This obtained rate is probably due to the adsorption of 4-NP into the clay [13]. The use of photocatalyst and the UV-light irradiation allows the 4-NP degradation at about 97% after 220 minutes. These results reveal that the 4-NP degradation can be achieved only in the presence of photocatalyst and UV-light irradiation and indicate that the decomposition of 4-NP is essentially caused by photocatalytic degradation but not adsorption [55].

   Only UV-light irradiation

   TiO2-Mont

   UV/TiO2-Mont

   Fig.10 Effect of UV-light irradiation and photocatalyst add

   2) Effect of photocatalyst type:

   The photocatalytic efficiency of prepa

   Degradation rate (%)

   ition

   0

   50

   100

   Time (min)

   150

   200

   red photocatalysts was tested by the degradation of a 4-NP solution (15 mg.L-1) at 0.3g photocatalyst loading.

   Fig.11 presents the effect of photocatalyst type on 4-NP degradation. In these curves we denote that Na-mont is relatively not photoactive and the low degradation rate obtained in this case and which did not exceed 8.6%, is caused probably by the adsorption of 4-NP onto the sample.

   The degradation rate acquired over the Iso-M did not reach 40% after 220 minutes, which is low, this may be ascribed to its different structural and textural properties, actually, it present the lowest specific surface compared to other modified montmorillonite. In addition characteristic diffraction peaks of anatase phase were absent in its x-ray spectrum, also in its EDX spectrum titanium was detected in trace amounts.

   Moreover, the TiCl4-M and TiCl3-M curves show that these two samples are efficient to degrade over 96% the 4-NP after 220 minutes. This is due to their interesting proprieties and morphologies. Their surfaces are porous and developed moreover characteristic diffraction peaks of anatase phase were detected in x-ray spectra and confirmed by the results obtained by SEM/EDX and chemical analysis.

   100

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   0

   Na-Mont

   Iso-M

   TiCl -M

   4

   TiCl3-M

   0

   50

   100

   Time (min)

   150

   200

   Fig.11 Effect of photocatalyst type

   Degradation rate (%)

   1. Effect of photocatalyst amount loading:

      Photocatalyst loading is one of the major parameters which influence the degradation rate of organic compounds, thats why many investigations have been conducted in this regard [56-59].

      In our study, we have followed, the influence of the photocatalyst amount increase added to a 4-NP solution (15 mg.L-1), and UV-light irradiation during 120 minutes.

      We found that the degradation rate is proportional to the photocatalyst loading until the value of 0.2 g/ 100 mL as shown in fig.12. These results can be explained by an increase of the active sites on the photocatalyst surface due to an increase in the photocatalyst amount, generating the hydroxyl radicals. However, when the photocatalyst amount exceeds the optimum value (0.2g/100 mL), it prevents the UV-light illumination, so the creation of HO. radicals decreases and the efficiency of the degradation reduces [60- 61].

      80

      Degradation rate (%)

      Moreover, the excess of photocatalyst loading can induce the photocatalyst particles agglomeration which provokes the decrease of degradation rate [59].

      100

      0,1

      0,2

      0,3

      0,4

      Amount of photocatalyst (g/100mL)

      60

      Fig.12 Effect of photocatalyst amount loading

   2. Effect of the 4-NP solution pH:

      Degradation rate (%)

      The pH effect on the photocatalytic efficiency of organic pollutants has been studied by many researchers [62]. It was demonstrated that pH influences greatly the photocatalytic degradation of organic compounds. Thats why we have investigated the effect of varying pH of 4-NP solution from 2 to 9 on the 4-NP degradation over TiCl4-M (0.2 g/100mL) after 120 minutes light irradiation. As shown in fig.13, the increase of the pH from 2 to 5 raises the rate degradation from 46% to 84%. Beyond a value of pH=6, the rate degradation decreases.

      A value of pH fixed to 5 should be chosen to achieve an ideal degradation rate. Therefore, the photocatalytic degradation is promoted in acidic medium since photocatalyst surface is positively charged while it is negatively charged in alkaline medium (pH>7). This can be explained by the ionization state of the surface corresponding to the following reactions [62]:

      TiOH + OH- = TiO- + H2O TiOH + H+ = TiOH2+

      Degradation rate (%)

      In fact, in alkaline solution, a coulombic repulsion can be established between the negative charged surface of photocatalyst and the hydroxide anions which prevent the formation of HO. and provoke the degradation decrease, whereas, the positive holes are the major oxidation species at low pH, that react with hydroxide ions to form hydroxyl radicals [62-64].

      100

      90

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      70

      60

      50

      40

      30

      1

      2

      3

      4

      5 6

      pH

      7

      8

      9

      10

      Fig.13 Effect of pH of the 4-NP solution pH

   3. Effect of the 4-NP initial concentration:

   Fig.14 shows the effect of the initial concentration of 4-NP on the 4-NP degradation over TiCl4-Mont after 120 minutes, at pH 5 and 0.2 g/100 mL photocatalyst loading. Increasing the 4NP concentration seems to decrease the 4- NP degradation rate. This can be explained by the fact of the increase of concentration which attenuates the UV-light that cannot pass through the solution and decreases the photocatalytic activity of the catalyst [55].

   100

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   40

   4-Nitrophenol concentration (mg/L)

   Fig.14 Effect of the 4-NP initial concentration

3. Conclusion:

The improvement of the photodegradation of the 4- NP has required the preparation of photocatalysts by immobilization of TiO2 on montmorillonite nanocomposite by different methods.

The third method consisted to hydrolyze Titanium

(IV) isopropoxide with HCl. Also, the acidic conditions employed in this process destroyed the ordered structure of the montmorillonite and allowed getting a delaminated meso-microporous structure. The immobilization of TiO2 on the clay by this process did not took place efficiently because there was not a significant development of its specific surface area, the X-ray spectrum of this prepared sample did not reveal the presence of anatase phase and the EDX spectrum shows the presence of titanium in trace amounts. This photocatalyst was referred as Iso-M; the 4- NP degradation rate obtained did not exceed 40 %.

The first method was based on the reaction between Na-montmorillonite and acidic solution of hydrolyzed TiCl4 with HCl. The resulting photocatalyst is named TiCl4-M, it is characterized by a delaminated mesoporous structure. The acidic conditions of this process destroyed totally the periodic structure of the clay. But a developed specific surface area equal to 231 m2/g was obtained, also characteristic peaks of anatase was detected on its x-ray spectrum and confirmed by the EDX and chemical analysis. This sample seems to be efficient to obtain a degradation rate over 96%.

The second method was a solvothermal preparation employing a TiCl3-M as precursor, water and ethanol as solvent, and hexamethylene tetramine as precipitant. This method allowed obtaining a semi-ordered and meso- microporous structure, with a specific surface area equal to 205 m2/g and with a basal spacing equivalent to 18.78 Ã…. It is characterized by the formation of anatase TiO2. This sample was referred as TiCl3-M, and by using it, a 4-NP degradation rate reached 99 %.

The study of the effect of various experimental parameters, essentially photocatalyst type, photocatalyst amount loading, 4-nitrophenol concentration, pH of 4- nitrophenol solution on 4-NP photocatalytic degradation, shows that to obtain an ideal degradation rate, the 4-NP degradation must be performed under UV-light irradiation, over 0.2g/100mL of TiCl3-M or TiCl4-M at pH=5.

ACKNOWLEDGEMENTS:

This work was financially supported by the Tunisian Ministry of High Education and Scientific Research. Authors thank the Departamento de Química Inorgánica de la Universidad de Salamanca (Spain) for its rich cooperation. N. Ben Messouad is gratefully acknowledged for her help in revising the English language of the paper and for XRF chemical analysis.

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