Hydrothermal Sol-gel TiO2 Nanoparticles fixed to Clay and its Photocatalytic Application for the Degradation of Methyl Orange

In this paper, Clay-TiO2 nanomaterials were synthesized via hydrothermal sol-gel method using Ivorian clays and titanium tetra-isopropoxide as precursors. The synthesized composite was characterized through XRD, SEM/EDX, FTIR and BET surface measurements. The photocatalytic activity of composite towards the degradation of methyl orange (MO), a model pollutant, has been investigated under UV and simulated solar radiation. Comparative experiments done in a solution with and without H2O2 indicate an increase in efficiency for methyl orange removal from the polluted water in the presence of H2O2. The optimized parameters (contact time, amount of nanocomposite and amount of Fe/H2O2) allowed to reach removal efficiency up to 60 % of MO. Immobilization of TiO2 on clay facilitate repeated using of nanomaterials, two times.


Introduction
Textile and other industrial dyes cause an increased environmental threat to the ecosystem and its preservation. These dyes mainly include azo groups (N=N) combined with benzene or naphthalene rings 1,2 . More than 10% of the total dyestuff is used for the dyeing of cotton products, whereas up to 30% of the used azo dyes discarded into the environment after they have been utilized in dyeing 3, 4,5 . For this purpose, dye removal or degradation from wastewater has been extensively studied to reduce its adverse impact on the environment. The conventional methods of removing these pollutants are chemical coagulation 6 , electro-flocculation and biological treatment 7 , photocatalysis 8 , Fenton 9 , nanofiltration membranes 10,11 and adsorption 12 .
Photocatalysis is being focused largely from the last three decades and is expected to be a promising alternative to the reduction of environmental pollutants 13 . This technique is defined as a process that occur at normal temperature conditions involving the generation of highly reactive species (the hydroxyl radical) in an amount sufficient to be effective in water purification processes. The generation of free radicals is initiated by the interaction of photons with the molecules or chemical species present in solution or with a catalyst 8 . High photocatalytic properties of certain semiconducting materials have been used to convert solar energy into chemical energy to remove pollutants and bacteria 14 on wall surfaces, in air and water 15. Among different types of photocatalysts used for degradation of pollutants 16 , titanium dioxide (TiO2) is usually chosen due to its strong oxidizing and antibacterial abilities 17 , its superhydrophilicity 18 , chemical stability, long durability, nontoxicity, low cost and easy availability. Although TiO2 was excellent photocatalyst, his practical application is hindered by a lack of visible-light absorption and the low quantum efficiency 19 . In addition, his recycling for repeated use is difficult 20,21 . A straightforward method to solve the problem is to immobilizing TiO2.
As a response, several workers have coated photocatalysts (TiO2) onto a variety of surfaces, such as glass silica gel, metal, ceramics, polymer, activated carbon 22,23 . In this study, the application of TiO2 immobilized on clay from Côte d'Ivoire for wastewater treatment is reported. An aqueous solution containing methyl orange (MO) was selected as a model compound to simulate the industrial wastewater. The obtained results suggest that TiO2 nanoparticles immobilized on Clay can be used as a photocatalytic agent for the degradation of organic pollutants in the environment, especially in aqueous media. This work is a novelty in order to contribute to the valorisation of clays, especially those of Côte d'Ivoire.

Raw material
The clay material used in this work was collected from the Hambol region (North of Côte d'Ivoire), precisely in the town of Katiola. The clay was named according to their color in French (Rouge (R)). These clays are extracted from quarries located around the geographical coordinates: 08˚09.030'North, 005˚05.850'West and altitude 996 m extend over about 10 Km 2 at a depth of 1.5m.

Synthesis of TiO2 powder by sol-gel technique
The TiO2 sol was synthesized by acid catalyzed solgel formation method using 60 mL of 1 M HNO3 and 15 mL of titanium tetra-isopropoxide following by the hydrolysis reaction 24 . Briefly, titanium tetraisopropoxide was added gradually to the aqueous solution of HNO3 solution under continuous stirring for 3 hours to produce a transparent sol. Subsequently, the pH of the colloid solution was adjusted to 3 with the addition of 1 M NaOH, which results in a turbid colloid. The mixed suspension was then stirred for another 2 hours at room temperature, followed by filtration (Millipore 0.45 µm) and repeatedly washing with de-ionized water until the pH of the supernatant was about 6. The resulting TiO2 was dried in a furnace for 1 hour at 100°C and finally, the powder was annealed at 500°C for 3 hours.

Preparation of Clay-TiO2 nanomaterials
For repeated use of nanocomposite, TiO2 nanoparticles were immobilized on clay. Clay-TiO2 was synthesized by simple mixing of TiO2 sol obtained by the process described above and clay previously washed and activated 25,26 noted RW. The mixture is realized before the final calcination at 500°C for 3 hours, according to mass ratio 1:1. The nanocomposite obtained has been noted R-TiO2.

Characterization of the material substrate 2.4.1. X-ray diffraction
X-ray diffraction (XRD) spectra were obtained using a Bruker multifunctional D8 ADVANCE model diffractometer. XRD experiments were achieved on powdered samples in the step-scan mode in the range 2° ≤ 2θ ≤ 60° with a step size of 0.01° (2θ) and a counting time of 0.25s. Phases identification was performed by comparing the X-ray diffraction patterns with the International Center for Diffraction Data (ICDD) using the EVA (Bruker AXS) software.

Specific surface area measurements
The Brunauer Emmett and Teller (BET) method 27 was used to determine the specific surface area of the samples, using a TriStar II equipment from Micromeritics. Measurements were carried out after a 16 h degassing step at 200°C.

Scanning Electron Microscopy (SEM)
The microstructural observation of the samples was done using a Scanning Electron Microscopy of the "FEI, Quanta FEG 450, Environmental" type. SEM images were performed on powered samples. Prior to their observation, a small amount of powder from each sample was suspended in distilled water. After stirring with ultrasound for 5 minutes, a drop of each suspension was placed on a sample holder and then dried.

FTIR analysis
Structural characterization of prepared nanocomposites was analyzed by using Fourier transform infrared (FTIR) spectroscopy, coupled with smart ATR accessory and photoacoustic sampling cells. Spectra were recorded within the wavelength range of 4000-400 cm -1 , with average 256 numbers of scans at 8 cm -1 resolution on a Thermo-Nicolet 6700P FTIR Spectrometer (USA)

Photocatalysis experiments
Batch photocatalysis experiments were done at room temperature, under stirring (using magnetic stirrer VELP Scientifica ARE-6, at 100 rpm). In all the experiments, a constant ratio substrate mass: solution volume was set at 0.1 g (TiO2 sol mixing to clay: R-TiO2):50 mL. The adsorption mechanisms and the kinetic data were evaluated on two types of suspensions: (1) R-TiO2 + MO (CMO= 0.0125 mM); this concentration was chosen as it is a usual upper limit of the industrial wastewaters subject of investigations 28 .
In the kinetic studies, aliquots were taken at pre-set moments (up to 360 min), when stirring was briefly interrupted and, after filtration on a 0.45 mm filter, the supernatant was analysed. Preliminary experiments proved dye losses (adsorption on the beaker walls and the filtering paper) were negligible. The photocatalysis experiments have been repeated two times by reusing the photocatalyst, previously washed (in 0.1 N NHO3) and dried.
Photodegradation studies were done on R-TiO2 suspensions with an identical overall concentration of the dispersed phase, with/without Fenton reactive and hydrogen peroxide (30%) under UV irradiation using quartz beakers. The homemade reactor is equipped with three F18 W/T8 blacklight tubes (Philips), emitting UV light, typically 340-400 nm, with λmax (emission) = 365 nm 29 . The mean value of the radiation flux intensity, reaching the middle of the reacting suspension was 3 Lx(Mavolux5032B/USM), and the irradiance is 846 W m −2 . During the investigations the radiation flux intensity was in the range of 0.7-1.6 Lx with average irradiation of 215 W m −2 .

Pollutants analysis
The MO was analysed by UV-vis spectrophotometry (Perkin Elmer Lambda 25), on the calibration curve registered at λ = 460 nm. The amount of pollutants uptake by adsorbents (adsorption capacity), qt, the removal efficiency, η, percentage were calculated using the following Eqs. (1) and (2).
where c0 represents the initial concentration of the pollutants; ct represents the concentration of the pollutants at the time t; V the solution volume (L) and m, the amount of substrate (g).

Substrate characterization
The structure of the synthesized composite was investigated using X-ray diffraction. The diffraction patterns are shown in Figure 1 A and B for RW and R-TiO2, respectively. For the RW (Figure 1A), the X-ray spectrum shows two peaks located at 10.23° and 24.95°, which are characteristic of kaolinite [30][31][32] . Also, high and low intensity of quartz peaks located at 2θ° 21°, 39.2°, 40° and 42.3° are observed 33 . Quartz is a commonly found mineral in the earth's continental crust, which satisfy the appearance of these peaks. Further, the presence of iron is confirmed by goethite peaks at 34.1° and 36°. The presence of iron ions could explain the red color of the clays used.
After the addition of TiO2, the anatase diffraction peaks were detected in the X-ray spectrum of R-TiO2 ( Figure 1B). The sharpest peak of anatase appeared at 12.0°, and other peaks are detected at 36.0°, 38.1° and 46.0°. Thus, during the synthesis of TiO2 by the sol-gel process, the calcination at 500 °C produce mainly the anatase phase of TiO2. The specific surface area of RW and R-TiO2 was obtained as 11.48 and 19.12 m 2 . g -1 respectively as measured through BET pore size and surface area analyzer. These values of the specific surface are in concordance with the values found in literature 35,36 . In the presence of TiO2, the surface area increases (from 11.48 to 19.12 m 2 g -1 ). This result was also observed by Soro et al. 37 during their works. To better access, the structure of the synthesized compounds, SEM and FTIR techniques were done. The results are shown in Figure 2 and Figure 3, respectively. Figure 2A show platelets of kaolinite and grains of quartz. These results are in correlation with X-Ray diffraction analysis. In Figure 2B, we observe a modification of surface morphology of clay. The micrograph of R-TiO2 shows aggregates formation.   Figure 4. MO is an azo (-N=N-) group compound with a diethylamine group side substituent on one of the aromatic rings. When ionized in water, a negatively charged chromophore is produced, so that MO is also known as an anionic dye or acidic dye. After the 30min dark adsorption, is 5.45 %.
The photocatalytic properties of the synthesized material were evaluated using MO as a pollutant in aqueous media. Figure 5 shows the time-dependent UV/Vis absorption measurement of methyl orange degradation. At t = 0 min, the curve is associated with the absorption spectrum of methyl orange solution without R-TiO2 nanocomposite treatment. After the addition of R-TiO2, a decrease in the intensity of the absorption peak at 460 nm and the formation of a weak peak at 650 nm, were observed. This decrease indicates the degradation of the methyl orange molecules by R-TiO2 and possible cleavage of the azo bond. The formation of aromatic products is indicated by the increase in the peak∼280nm and formation of a weak peak at 650 nm. This result suggests the successful decomposition of methyl orange dye by the reduction of the azo bond to two or more possible chemical structures with amines (−NH2) 39 . In the photodegradation process, the concentration of MO decreases by 9.54% of the initial value within a short period of 60 min, indicating the good photocatalytic activity of the R-TiO2. In this work, the TiO2 nanoparticles are uniformly dispersed onto the washed clay (Figure 2) in the as-prepared composite catalyst, leading to high specific surface area and increased reaction sites. Therefore, both the adsorption and photochemical reactions are accelerated in the composite catalyst (R-TiO2), which is responsible for its good photocatalytic activity in the degradation of MO.
The degradation of MO by photocatalysis and photo-Fenton are presented in Figure 6. As can be seen, MO removal efficiency increase using the Fenton system (H2O2/Fe 2+ /Fe 3+ ). After 60 min, 15.02 % of MO was removed and removal efficiency up to 60 % after 360 min. The mixture of H2O2 and Fe 2+ in medium leads to the promotion of hydroxyl radicals which efficiently oxidize the organic pollutants 40    The combination of hydrogen peroxide and UV radiation with a Fe 2+ ion produces more hydroxyl radicals, and in turn, it increases the rate of degradation of organic pollutants. Fenton reaction accumulates Fe 3+ ions in the system, and the reaction does not proceed, once all Fe 2+ ions are consumed. The photochemical regeneration of ferrous ions by photo-reduction of ferric ions occurs in the photo-Fenton reaction. The newly generated ferrous ions react again with H2O2 generating hydroxyl radical and ferric ion, and in this way, the cycle continues 41 .

Degradation mechanism
On the new composite (R-TiO2), TiO2 is activated when irradiated with energy higher than the bandgap value (Eg = 3.1 eV, corresponding to λ = 399.9 nm). Under irradiation, electron-hole pairs are photogenerated (Eq.3); the holes will most likely react with water to form active oxidant species HO • (Eq.4) that may degrade the organic dye (MO) 28 .
Thus, the mechanism for methyl orange degradation using R-TiO2 can be proposed as below: When the photocatalyst is introduced, the methyl orange molecule was first adsorbed on the surface of the nanocomposite, then the TiO2 present on the surface reacted with the -N=N-double bond and broke it to single phenyl ring compounds with amine group as possible products. In addition, HO • products by photo-Fenton acts directly with MO in the medium.

Kinetic study
In the photodegradation process, the kinetics of the photocatalytic degradation is described using the pseudo-first-order model (Eq.8), and the results are displayed in Figure 7.
where C0 is the initial concentration of MO; Ct is the concentration of MO at time t, and ka (min -1 ) is the apparent pseudo-first-order decay rate constant which can be obtained by plotting ln(Ct/C0) vs. t as shown in Figure 6 42 . The values of rate constants confirm that HO • in solution increases the kinetic of MO degradation on R-TiO2.

Conclusion
TiO2 nanoparticles were successfully prepared by solgel method and immobilized on clay via hydrothermal method. This immobilization of TiO2 on clay facilitate repeated using of nanomaterials. From the R-TiO2 samples, only anatase phase was confirmed from the XRD results. The SEM and FTIR analysis results confirmed the modification of morphology surface of clay by TiO2 incorporation. In this work, the presence of TiO2 on the clay raise weakly its surface specific area from 11.48 to 19.12 m 2 g -1 . The photocatalytic degradation of mono azo dye MO was investigated using R-TiO2 nanocomposite in aqueous solution under sunlight. This study demonstrated the effectiveness of nanomaterials in methyl orange degradation. The presence of the anatase phase of TiO2 nanoparticles should be responsible for the best catalytic activity of R-TiO2. However, the kinetics of degradation of MO increases in the presence of HO • in the media. These results suggest that R-TiO2 can be a powerful tool for organic pollutants remover in aqueous media.

Acknowledgments
This work is supported by Agence Universitaire de la Francophonie (Eugen Ionescu scholarship)