265486 Low-Temperature Preparation of Titania-Pillared Montmorillonite with Complex Layered Structure and the Application in Arsenic Deep Removal
Low-temperature preparation of titania-pillared montmorillonite with complex layered structure and the application in arsenic deep removal
Ping Na, Jingwei Guo, Yuan Li, Shimin Zhou, Xiaojiao Cai,
School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China
The Arsenic (As) contamination in environment which is notoriously toxic to man and other living organisms has draw worldwide concern [1-4] and consequently the world health organization (WHO) has recommended that the value of arsenic remain below 10µg/L in drinking water. In the present work, titania pillared montmorillonite (Ti-MMT) was prepared by hydrothermal method. Freeze-drying method instead of drying method was used to fabricate Ti-MMT with complex layered structure and homogeneous pore distribution. The removal of As (V) and As (III) in aqueous solutions was investigated.
2. Materials and methods
Titanium-pillared montmorillonite was prepared by a sol¨Cgel method. The product was freeze-dried: the slurry was cooled at -50°ć for 4 h, and directly sublimated at room temperature under reduced pressure of 30 Pa. The product was sieved using a sieve of 100-180 mesh. Adsorption experiments were carried out by adding 0.10g of Ti-MMT to 500 mL of solutions containing 1 mg L-1 of arsenite or arsenate. Under UV irradiation or in dark, As(III) or As(V) adsorption experiments were carried out in a 1L quartz glass vessel under vigorous stirring, which contained 500 mL of As(III) or As(V) solution. The pH of solution was adjusted with 0.1mol L-1 HCl or 0.1mol L-1 NaOH. The reactor was located above the magnetic stirrer with a constant-temperature water bath set to 298 K. A 500 W high pressure mercury lamp (365nm, Beijing Lighting Research Institute, China) was placed outside the reactor as the UV light source. The samples were filtered using a 0.45 µm membrane syringe and analyzed by atomic fluorescence spectrometry (AFS, Beijing Rayleigh, China). The detection limit was determined to be 0.02 µg L-1.Transmission electron microscopy (TEM) images were taken with a Japan Electronics JEM-2100F microscope (Japan). Nitrogen adsorption¨Cdesorption isotherms of Ti-MMT were obtained at 77 K (Quantachrome Corporation, NOVA-2000, USA). The adsorption and desorption branches of the isotherms were determined. The BET surface area and pore size distribution curve were calculated from the isotherms. The samples were pretreated in a vacuum (ca. 0.10 Torr) at 423 K for 2 h.
3. Results and discussion
Figure 1. pore size distribution of Ti-MMT obtained by different dry methods: freeze-dried and dried in oven at 80°ć.
The BET surface area of Ti-MMT prepared by drying at 80°ć and freeze-drying are 89.7 and 201.6 m2 g-1 respectively. The pore size distribution was shown in figure 1 and calculated using the density functional theory (DFT) method. The samples freeze-dried had trimodal pore-size distributions. One pore-size peak was above 4.5 nm, and the second was around 3.2 nm, the third was about 1.76 nm (the former two pores and the third are denoted as mesopores and micropores, respectively, according to International Union of Pure and Applied Chemistry (IUPAC)). However, for the samples dried at 80°C in an oven, the size of micropores and mesopores decreased. Materials containing macropores and micropores are expected to be used as support of catalysts and absorbents and for other applications. Freeze-drying method is effective in preparing materials with high porosity (90%). It was proposed that higher surface area benefit from small sintering shrinkage.
Figure 2. Schematic representation of the preparation of Ti-MMT: L-Layer (titania pillared in large spacing of layer); S-Layer (pillared in small spacing of layer); U-Layer (the un-pillared layer); Surface (titania on the outer surface of layer)
Four types of layers exist in Ti-MMT: S-Layer, L-Layer and U-Layer and surface (exfoliated layer) as described in schematic representation (figure 2) and TEM photographs (figure 3), especially to be mentioned, they are one-to-one relationship. Titania (anatase) was pillared in S-Layer, L-Layer and also exfoliated layer; as for U-Layer, no lattice of TiO2 was observed (figure 3a), the spacing of layers changed to less than 1.42 nm (measured with software of DigitalMicrograph (Gatan, America)). The lattice fringes with a spacing of 0.35 nm are due to the spacing of the (101) planes of the anatase (figure 3b). Some of anatase particles were pillared in the space of multilayer (L-Layer), it was consistent with energy dispersive X-ray (EDX) analysis (Fig. 3c, inset). Figure 3d showed the photograph of S-Layer, the layer space is in the range 1.42-6nm. These kinds of layers have good stability and small particles of anatase exist in the interlayer, which created large number of
Figure 3 TEM photographs and X-ray (EDX) spectroscopic analysis of as prepared Ti-MMT
mesopores and micropores, they are highly insistent with the result in figure 3. During the preparation process, some of the ordered layers exfoliated, titania deposited on the outer surface. From the TEM photographs, it was concluded that titania formed at low temperature in the layers of MMT, and if we could turn all the U-layer and L-layer to S-Layer, the surface area would increase much more.
Figure 4 the adsorption of As(III) and As(V) under UV and in dark
In order to evaluate the efficiency of as-obtained product for arsenic removal in water intuitively, arsenic residual concentration was set as y in figure 4. We carried out the experiments under UV and dark conditions respectively. The uptake of As(V) and As(III) in dark system reached equilibrium approximately within 3h. The equilibrium concentration of As(III) is much higher than As(V), indicating that the strong affinity of As(V) with Ti-MMT. As to the removal of arsenic in UV system, excellent results can be observed that the residual concentration is 0.009 and 0.006 mg L-1of As(V) and As(III) systems respectively, meanwhile, the adsorption capacity reached 4.96 and 4.97mg g -1. It reached adsorption equilibrium within 1h. UV light greatly promoted the adsorption ability of As(V) and As(III). We attribute the good performance of Ti-MMT to three features: more active sites due to the higher specific surface area; facile diffusion of arsenic during the reaction because of the highly porous structure; and the pillared titania (anatase) prepared at low temperature. Under UV irradiation, the oxidation of As(III) to As(V) enhanced the removal of As(III), this principle is similar to arsenic removal with TiO2 .
Titania (anatase) pillared MMT with homogeneous pore distribution and a surface area (201.6 m2 g-1) was obtained using hydrothermal method. According to the analysis of TEM photographs, it is the first time to confirm that the as prepared Ti-MMT has four types of layers, U-layer, S-Layer, L-layer and the exfoliated layer; titania was pillared in S-Layer, L-layer and deposited on the surface of the exfoliated layer. Under UV irradiation, the as prepared Ti-MMT has an adsorption capacity of 4.96 mg g-1 and 4.97 mg g-1 for As(V) and As(III), the residual concentration was 0.006 mg L-1 (As(III)) and 0.009 mg L-1 (As(V)). The high efficiency in arsenic removal resulted from the higher specific surface area, porous structure and the active anatase phase of TiO2 prepared at low temperature.
Financial support from the Social Development of Science and Technology Projects of Yunnan Province (No. 2009CA038) is gratefully acknowledged.
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