Preparation of TiO2 particles in reverse microemulsion and their use in elimination of methylene blue used as reactive dye model

ZUBIETA C.E; SCHULZ P. C; MESSINA P. V

España

Congreso; III Reunión Ibérica de Coloides e Interfases. VIII Reunión del grupo especializado de coloides e interfases; 2009

Universidad de Granada

The textile industry requires the use of vast amounts of water and chemicals, and their effluents have important effects on environmental quality.  Over 100.000 commercially available dyes exist, and more than 7x105 tons are produced annually, with a considerable fraction of them being discharged directly in aqueous effluents [1].  It is estimated that 400 tons daily find their way into the environment, primary dissolved or suspended in water. However, an important part of dyes are not directly toxic for aquatic life, but the strong coloration that remain in wastewater can disturb the photosynthesis process in water causing biological problems. Then, the presence of these substances must be controlled [2] and dyes need to be eliminated from water. In general the dyes used nowadays have several and complex structures. Most of them are synthetic, highly soluble in water, highly resistant to the chemical agents and poorly biodegradable.              We studied titania as a possible de-contaminant of dye-contaminated wastewaters. With this aim, Methylene Blue was used as a model dye. This cationic dye is extensively used for cotton dying and colouring papers, wool and silk. This dye may cause burns effect of eye, nausea, vomiting and diarrhea [3]. Nowadays the most widely used method for the reactive dyes elimination is degradation. The conventional methods for colour removal, like biological oxidation and chemical precipitation are not always effective and economic when the solute concentration is very low [4]. In recent years, photocatalytic TiO2 was extensively studied due to its excellent stability, low cost, easy availability and the capacity to provide a destructive oxidation process using solar energy. In such process the oxidant is the atmospheric oxygen and the catalyst is non- hazardous. The main advantage of using the water-in-oil (W/O) microemulsions in synthesis is low reaction temperature, uniform nucleation, and short processing time that avoid the reactants supersaturation. Additionally, reverse micelles can protect the nanoparticles against excessive aggregation. Moreover, there are no organic impurities in the final product. The reverse microemulsions where composed by Aerosol OT (AOT) (C20H37NaO7S)/hexane/water using different [water]/[surfactant] (=R) ratios, we obtained three materials called R10, R20 and R30. Preparation of catalysts Reverse microemulsion solution was prepared by dissolving 1.1276 g OF Sodium dioctyl sulfosuccinate (Aerosol OT; AOT, 99%,  Sigma) in 0.46 g of water. The sample was left 3 hours to produce the surfactant hydration. In this case the water/ surfactant ratio (R) was R: 10. Then 80 mL of n- hexane (Carlo-Erba, p.a.) were added and the system was sonicated to produce the microemulsion. After that 1.4 mL of TiCl4 (Carlo Erba, 99%, ƒÂ= 1.722 g/cm3) were added and left three days to react following the reaction: 2H2O(l)  + TiCl4(l) ¨ TiO2(s) + 4HCl(g)                                  (1) Subsequently the excess of HCl and n-hexane were eliminated by evaporation under vacuum. The resulting material was left 3 hour in hydrothermal treatment at 70 ‹C in autoclave. A white powder composed by titania nanoparticles covered by AOT was obtained. The material was calcined during 7 hours at 540 ‹C with air flux. In other synthesis, R was 20 and 30. The materials were characterized using X-ray powder diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Nitrogen adsorption isotherms.   Characterization of Titania samples X-Ray diffraction Powder X- ray diffraction (XRD) was performed with a Philips PW 1710 diffractometer with Cu Kƒ¿ radiation (ƒÉ= 1.5418 A) and graphite monochromator operated at 45 kV; 30 mA and 25 ‹C. Results are shown in Figure 1. Figure 1: X- ray diffraction patterns of TiO2 powders prepared with different water/surfactant ratios (R). A: anatase; R: rutile The sample with R: 10 shows the presence of anatase (2ƒÆ: 25.2‹, 54.2‹) and a small amount of rutile (2ƒÆ: 27‹, 35.9‹). In R: 20, the presence of a small peak of anatase (2ƒÆ: 25.2‹) can be seen together with increased rutile peaks (2ƒÆ: 27‹, 35.9‹). In R: 30 the two peaks in (2ƒÆ: 27‹, 35.9‹) indicate that the TiO2 was pure rutile.  So, the water proportion plays an important role in the proportion of the different crystalline structures of TiO2 in the materials obtained from reverse microemulsion. An increase in R causes a decrease in anatase and an increases in rutile proportions. Also the crystallinity increases with R, as it can be seen by the narrowing of the peaks. These effects may be caused by an increase in acidity in the solution during the TiCl4 hydrolysis (see Equation (1)). The rutile formation is favoured by low pH values and high chloride concentrations, while at higher pH values the anatase phase is present. Anatase is unstable in strong acid solution [5], and disappeared when the quantity of water is increased. Nitrogen adsorption isotherms             The nitrogen adsorption isotherms at 77.6 K were determined with a Micrometrics Model Accelerated Surface Area and Porosimetry System (ASAP) 2020 instrument. Each sample was degassed at 373 K for 720 min at pressure of 10-4 Pa. Specific surface areas of the samples were determined from the nitrogen adsorption data using Brunauer-Emmet-Teller technique. The adsorption-desorption isotherms for all samples are shown in Figure 2. Fig 2. Nitrogen Adsorption-desorption isotherms for all studied titania catalyst. The full points represents adsorption (ads). The hole points indicates desorption (des) All materials showed the typical type II isotherm, which represents unrestricted monolayer-multilayer adsorption. It is generally associated with non-porous or macroporous materials [6]. The t-plots in Figure 3 showed are typical of the materials having slit shaped pores. Figure 3. t-plot of TiO2 samples The total area (Attot), the exterior area (Atext), the pore surface area (estimated from the difference between A ttot and A text), the pore and the core volume (Vtp, Vc) were computed from the t-plot and the results were summarized in Table 1. Table 1 Nitrogen adsorption data of the TiO2 samples prepared by inverse microemulsion. Asp: single point surface area at P/Po=0.2002, ABET: BET surface area, Atext: t-plot external surface area, ABJHac: BJH adsorption cumulative surface area of pores between 1700 nm and 300,000 nm, ABJHdc: desorption cumulative surface area of pores between 1700 and 300,000 nm, Daap: adsorption average pore diameter by BET (4V/A), DaBJH: BJH adsorption average pore diameter (4V/A), DdBJH: BJH desorption average pore diameter (4V/A), Vspat: single point adsorption total pore volume of pores, VBJHacvp: BJH adsorption cumulative volume of pores, V BJHdcpv: desorption cumulative volume of pores. Sample A sp m2/g A BET m2/g A text m2/g ABJHac m2/g ABJHdc m2/g Daap  nm DaBJH  nm DdBJH  nm Vspat cm3/g VBJHacvp cm3/g VBJHdcvp cm3/g R: 10 3.515 3.884 6.179 2.002 2.923 9.258 19.18 13.51 0.00899 0.009610 0.009877 R: 20 7.954 8.212 10.95 8.149 11.98 23.49 29.41 20.21 0.04823 0.059911 0.06055 R: 30 3.642 3.830 4.824