comparative structural characterization of natural and Ag - doped zeolite were .... Figure 3a and 3b illustrates the SEM images of natural and thermally treated ...
OBTAINING AND CHARACTERIZATION OF ROMANIAN ZEOLITE SUPPORTING SILVER IONS Corina ORHA1, Florica MANEA1, Cornelia RATIU2, Georgeta BURTICA1*, Aurel IOVI1 1 2
”Politehnica” University of Timisoara, P-ta Victoriei no. 2, 300006, TIMISOARA (Romania)
Institutul National de Cercetare-Dezvoltare pentru Electrochimie si Materie Condensata, Str.Plautius Andronescu, Nr.1, Cod 300224, TIMISOARA (Romania)
Abstract The aim of this work was to obtain Ag - doped zeolite as antibacterial material using natural and sodium form of zeolite from Mirsid Romania with the dimensions ranged between 0.8-1.2 mm. The comparative structural characterization of natural and Ag - doped zeolite were performed using Laser Induced Breakdown Spectroscopy (LIBS), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and Infrared Spectroscopy (IR). In addition, the ion exchange total capacities for silver of natural and sodium forms of zeolite were determined. The quantitative assessment of silver amount incorporated into zeolite lattice was achieved by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). Keywords: characterization.
Ag - doped zeolite, qualitative assessment, quantitative assessment, structural
1. Introduction In general, the sorption and ion exchange properties of zeolites, as well as, their inexpensive from the economical aspect can be advantageously used in drinking water technology (Cooney et al., 1999; Woinarski et al., 2006). There are several studies concerning the use of synthetic and natural zeolites, e.g., A, X, Y, Z and clinoptilolite supporting metal ions (Ag, Cu, Zn, Hg, Ti) as antibacterial material in water disinfection (Inoue et al., 2002; Top et al., 2004; Rivera-Garza et al., 2000). The mineral properties of natural zeolite from Mirsid, Romania, with 68 %, wt. clinoptilolite make it suitable for the obtaining of Ag-doped zeolite, with antibactericidal activity. It has been found that for the uniform retaining of the Ag ion with antibactericidal property, the zeolite must exhibit a
SiO2/Al2O3 molar ratio at most 14, this molar ratio being ranged between 8.5 to 10.5 for clinoptilolite (Hagiwara et al., 1990). The antibactericidal activity of Ag-doped zeolite depends on the amount of Ag ions incorporated into zeolite lattice. If the Ag ions amount is the same with the ion-exchange total capacity the bactericidal effect of the modified zeolite is very poor, due to the other form of Ag as silver oxide can be deposited on the zeolite surface. Under these conditions, it is required to prevent the deposition of such excessive silver onto the solid phase of zeolite. The aim of our present study was the preparation and characterization of Ag – doped zeolite, with the dimensions ranged between 0.8-1.2 mm, which will be used further for drinking water disinfection. The ion-exchange total capacities of natural and Na form of zeolite from Mirsid, Romania were determined. The structural characterizations of Ag-doped zeolite with the Ag amount lesser than ion-exchange total capacity of zeolite for Ag ions were investigated relating its further utilization as antibacterial material.
2. Experimental For this study, it was used the natural zeolite from Mirsid (Romania), which contains 68%, wt. natural clinoptilolite. The preparation of Ag-doped zeolite requires two stages, i.e., in the first stage the natural zeolite was chemically treated with 2M HCl and 2M NaNO3 to obtain the sodium form. The second stage consists of the obtaining of the Ag - doped zeolite by the mixing of the sodium form of the zeolite with 0.1 N AgNO3 solution for 3 hours (Hagiwara et al., 1990). As a previous stage for the preparation of Ag-doped zeolites the ion exchange total capacity of the zeolite for silver was determined. 1.000 g of zeolite with the dimension of pores between 0.8-1.2 mm was shaken with 25 mL of 0.1 N AgNO3 during 3, 5, 7, 12 and 14 days (Cerjan-Stefanovic et al., 2004). Once equilibrium was established water solution was separated by filtration from the zeolite phase and the Ag amount into zeolite was determined by using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) after sample mineralization. The ICP-AES analysis was made with an ICP-AES SpectroFlame spectrometer. The thermal treatment of Ag – modified zeolite was realized in reducer environment at 500°C, when Ag - doped zeolite (Z-Ag) was formed and the thermal treatment of natural zeolite was not performed.
To determine the presence of silver within zeolite lattice, the samples was analyzed by Laser Induced Breakdown Spectroscopy (LIBS). The material ablation and the excitation were performed with Q-switched Nd:YAG laser with an energy of 12-15 mJ, using an Ag standard. The morphology and the composition of the unmodified/modified zeolite were characterized by using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Infrared Spectroscopy (IR). XRD spectra were recorded at room temperature on a BRUKER D8 ADVANCE X-ray diffractometer using Cu Kα radiation (λ = 1.54184 Å, Ni filter) in a θ: 2θ configuration. The peaks of the XRD patterns were identified using the PCPDFWIN Database of JCPDS, version 2.02 (1999). The SEM images were made in a Jeol JSM-6300LV scanning electron microscope. The AFM images were made in a NanoSurf EasyScan 2.0 atomic force microscope. The IR spectra were recorded in KBr pellet for solid compounds on a Jasco FT/IR-430 instrument. 3. Results and discussion The results of the ion exchange total capacity of zeolite for silver that was determined by equilibrium of zeolite samples with 0.1 N AgNO3 are shown in table 1 . Table 1. The ion exchange total capacity of the zeolite samples with 0.1 N AgNO3 solution Zeolite type Z-N Z-N Z-N Z-N Z-N Z-Na Z-Na Z-Na Z-Na Z-Na
Contact time [days] 3 5 7 12 14 3 5 7 12 14
mg Ag/g zeolite 0.115 0.108 0.102 0.101 0.101 0.115 0.107 0.102 0.101 0.099
The form type of zeolite, e.g., natural and sodium form did not influence the ion exchange total capacity for silver ion. The Ag amount retained into zeolite as Ag-doped zeolite was 0.0065 mg/g zeolite almost twenty times smaller than the total exchange capacity of zeolite for silver, versus 0.008 mg/g zeolite (with the dimension of pores between 315-500 μm). Figure 1 shows the laser spectrum of Ag-doped zeolite versus silver standard one to identify qualitatively the presence of silver into zeolite.
328.16; 330.5 * - Ag 6000
6500 Ag standard Z-Ag
Relative Intensity
6000
5500
5500
5000
5000
4500
4500
4000
*
4000
3500
3500
3000
3000 2500 2500 2000
2000
1500
1500
1000
1000 500
500
*
0 320
322
324
326
328
330
332
334
0 336
Wavelength(nm)
Fig.1. The laser spectrum of Ag-doped zeolite (Z-Ag) The comparative XRD spectra of natural and Ag-doped zeolite (Z-Ag) are shown in Figure 2. * 21.5; 30.5 AgAlO2 1400 Z-Ag
natural zeolite
1200
1200 1000
Intensity
1000 800 800 600
600
* *
400
400
200
200
0
0 6
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
2 theta
Figure 2. X-ray diffraction patterns of the natural zeolite and of the Ag-doped zeolite The intensities of reflection observed at about 10°, 23°, and 30° corresponding to the clinoptilolite amount decreased due to the thermal treatment of Ag - doped zeolite (Rivera-Garza et al., 2000). No major differences in the diffraction patterns were observed due to de presence of the low amount of silver into zeolite. However, traces of AgAlO2 (21.5°; 30.5°) were identified. Figure 3a and 3b illustrates the SEM images of natural and thermally treated Ag-doped zeolite.
Figure 3a SEM image of the natural zeolite
Figure 3b SEM image of Z-Ag
Significant changes of the morphology of Ag-doped zeolite in relation to the natural zeolite were shown in Figures 3a and 3b, some small particles agglomerated on the zeolitic material were observed (Figure 3b). AFM was used to provide complementary information from both the internal and external structure of natural and Ag – doped zeolite and to image Ag particles located within the structure of zeolite. The natural zeolite consisted of particles with diameters ranged between 102.7 and 307.9 nm (Orha C. et al., 2007) and AFM image of thermal untreated/treated Ag - doped zeolite showed the existence of smaller particles ranged between 106.9 and 160.9 nm (Figures 4a and 4b). With the existence of smaller particles (nanocrystals) the specific surface area increased, which means that the precipitation of silver oxide on the zeolite surface was avoided.
Figure 4a. AFM image of the Ag – modified zeolite, without thermal treatment
Figure 4b AFM image of the Ag – doped zeolite treated thermally at 500°C The IR spectra of the thermally treated Ag-doped zeolite sample (Z-Ag) and untreated (Agmodified Z) were compared with the IR spectrum of natural zeolite (Z), and are presented in figure 5.
Figure 5. IR spectra of natural zeolite (Z), silver doped zeolite (Z-Ag) and silver modified zeolite (Ag modified-Z)
The influence of thermal treatment on Ag - doped zeolite was clearly observed especial for the vibration bands in the range between 3000 and 4000 cm-1. Taking into account the literature data (Rivera-Garza et al., 2000; Rodriguez-Fuentes et al., 1998) the assignation of vibration bands shown in Table 2 can be proposed for the Romanian zeolite from Mirsid. The influence of both Ag cation and thermal treatment on the IR vibration is gathered in Table 3, and it can be underlined that the shoulder at 1205 cm-1 corresponding the internal tetrahedral asymmetric stretching disappeared in the presence of silver. Also, small changes related to the transmittance intensity of the vibration bands at 456 cm-1, 1053 cm-1 in the presence of Ag cation were found out. The changes of the vibration bands, i.e., internal tetrahedral bending and internal tetrahedral asymmetric stretching in the presence of Ag gave information about the Ag incorporation into zeolite lattice. Table 2. The vibration bands for Romanian natural zeolite from Mirsid Vibration modes
Wavenumber [cm-1]
Intensity
Internal tetrahedral bending External tetrahedral double ring External tetrahedral linkage symmetric stretching External tetrahedral linkage asymmetric stretching Internal tetrahedral asymmetric stretching O-H bending
456 604
strong medium
791
weak
1053
strong
1205
shoulder
1650
wide
Table 3.The transmittance values at each wave number for unmodified/modified zeolite Sample Natural zeolite Ag doped zeolite Z-Ag
T1650 (%) 60.7356 58.6873 81.0015
T1205(%) 25.014 -
T1053(%) 6.72181 1.97174 23.1584
T791(%) 68.6638 63.8796 81.0711
T604(%) 42.8188 36.7916 71.9144
T456(%) 18.0681 8.58711 38.5474
4. Conclusions The ion exchange total capacities of natural and sodium forms of zeolite from Mirsid, Romania with the dimension of pores ranged between 0.8-1.2 mm for silver depended slightly on the zeolite form (natural or Na-form). The presence of Ag into zeolite was identified by LIBS and the quantitative assessment of Ag modified zeolite obtained for its use in water disinfection was achieved by ICP-AES, the Ag amount incorporated into zeolite lattice was 0.0065 mg/g zeolite. Ag amount incorporated into zeolite was about twenty times smaller than ion exchange total capacity of the zeolite for silver, this aspect being suitable for its use as antibacterial material due to the avoiding of undesired precipitation of silver oxide on zeolite surface.
The comparative characterization of natural and Ag - doped zeolite performed by XRD proved the trace existence of AgAlO2 form into the zeolite lattice. The existence of smaller particles in the presence of Ag proved by the structural characterization provided by SEM and AFM showed the specific surface area increasing of the Ag doped zeolite. The changes of internal tetrahedral bending and internal tetrahedral asymmetric stretching vibration bands of IR spectrum supported the Ag incorporation into zeolite lattice. Acknowledgements The structural analyses were made by co-operation with University of Pecs, Institute of Physics and Laser Spectroscopy within the framework of the Romanian - Hungarian bilateral scientific research Project RO-Hu-20/2002 – 2005. This study was supported by the Romanian National Research of Excellence Programs - CEEX, Grant 631/03.10.2005 - PROAQUA and 115/01.08.2006 SIWMANET.
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