VIETNAM JOURNAL OF CHEMISTRY VOL. 51(5) 534-538 OCTOBER 2013
DEGRADATION OF METHYLENE BLUE IN WATER OVER Mg-Fe-AI HYDROTALCITES
Nguyen Tien Thao*, Nguyen Thi Tuoi, Do Thi Trang
Faculty of Chemistry, VNU University of Science. Vietnam National University-Hanoi Received 19 September 2012
Abstract
A set of (Mgo7-xFe,:)Alo3(OH)2(C03)o.i5.xH20 hydrotalcites with different ratios of Mg/Fe/Al has been successfully prepared. These solids are characterized by several physical means including XRD, BET, SEM, and TEM. The characteristic results indicated that Mg^"^ ions were replaced by Fe^* in the bmcite layers. The synthesized hydrotalcites have uniform particles and layered stmctures. They have been treated methylene blue in water. The preliminary experimental data showed that the synthesized materials have a good activity in the degradation of methylene blue.
Keywords Hydrotalcite, methylene blue, layered structure, degradation, decoloration.
1, INTRODUCTION
Organic dyes are used m many industries such as paper, plastics, food, cosmetics, textile.,, to color their products. A tiny amount of such compounds m water is highly visible and undesirable. The existence of the dyes in water is very harmful to humans and also causes environmental pollution [1]. Thus, they must be removed through the adsorption or oxidation into less toxic compounds such as H2O and CO2. A larger number of heterogeneous catalysts and adsorbents have been investigated to remove these dyes in water [2-4], but many of them would not degrade the organic dyes to a limiting concenfration.
Layered double hydroxides (LDH) are known to be great potential as inexpensive and environmental friendly sorbents due to their large quantities, chemical and mechanical stability, high surface area and structural properties [5]. They have shown good ability to adsorb many organic/inorganic compounds in water [6-8]. In some cases, it was found that layered double hydroxides act as catalysts to oxidize the organic adsorbates if they are designed to have active sites [5, 9, 10]. The purpose of the present work is to prepare (Mgo 7-.Fe^Alo 3{OH)2(C03)o is-xHjO hydrotalcites for the degradation of methylene blue in water.
2. EXPERIMENTAL
2.1 Preparation and characterization of the catalysts
A stoichiometnc amount of sodium carbonate was dissolved in 25 ml of water in a 500 ml -beaker.
The solution was heated to 60-65°C. Then, a quantity of aluminum nitrate nonahydrate and magnesium nifrate hexahydrate, iron (III) nitrate nonanhydrate were dissolved in 150 ml of distilled water. The pH of the solution was adjusted to approximately 9.5 using 1.5 M NaOH and was kept for 24 h. The precipitate was filtered, washed and dried at 80°C.
Powder X-ray diffraction (XRD) patterns were recorded on a D8 Advance-Bruker instrument using CUKQ radiation (X. = 1.59 nm). Fourier transform infrared (FT-IR) spectrum was obtained in 4000 - 400 cm"' range on a FT/ER, specfrometer (DX-Perkin Elmer, USA). The scanning electron microscopy (SEM) microphotographs were obtained in a JEOS JSM-5410 LV. TEM image was collected on a Japan Jeol.Jem.lOlO. The nitrogen physisorption was measured at 77 K on an Autochem II2920 (USA).
2.2. Degradation of aqueous methylene blue 0.20 grams of hydrotalcite catalyst and 2 mL of H2O2 30% were added into 200 mL methylene blue
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solution with 500 ppm. The susjjension was stirred at room temperature under solar light for a penod of time. Then, the resultant was filtered. The leaching solution was used for absorbance measurement at the charactenstic absorption wavelength of methylene blue, 664 nm. Removal percent of methylene blue was calculated from initial and remaining concenfration of reactant using the absorbance recorded on a UV-Vis spectrophotometer (72 - Shanghai, China) before and after the reaction. The removal percent was calculated as following equation:
where [MBJBefote and [MB]Afier stand for the
Nguyen Tien Thao, et al.
concentration of methylene blue solution (ppm) corresponding to the initial and tested sample, respechvely.
3. RESULTS AND DISCUSSION 3.1 X-ray diffraction
Two Mg-Fe-Al based hydrotalcites were prepared through co-precipitahon methods. Their nominal composition and XRD patterns are reported in Figure 1. It IS observed that sharp, intense peaks at low diffrachon angles (2-theta of 23.5, 34.8° are ascribed to diffraction by basal planes (0 0 6), (1 0 2, respectively) while lower intense reflechon signals at higher angles of 39.6, 47.1, 47.1, 60,8, 62,3°are indexed to the diffraction by (1 0 5), (1 0 8), and (1 1 0) planes, in good accordance with literature [10, 11].
MgO. SFeO.ZAlO. 3 ( O H ) 2 ( C O 3 ) 0 . 1 S . x H 2 0
— • - M g 0 . 6 F e 0 . 1 A f 0 . 3 ( O H ) 2 ( C O 3 ) 0 . 1 5 . x H 2 O
0 • — 1 - - - f — ) - - . I , , 1- .
0 55 60 I 65
Figure 1: XRD patterns for Mg-Fe-Al hydrotalcites The appearance of these peaks confirms the
formation of a well-crystallized hydrotalcite -like phase. No other phases such as bmcite, magnesite, and hydroxides... were observed [9, 10].
3.2. Catalyst morphology
The morphology of two prepared LDH samples was investigated by scanning elechron microscopy (SEM), SEM images of such matenals are displayed in Figure 2. In general, it can be appreciated that the samples are homogeneity evidenced by their uniform contrasts in SEM pictures [11]. The iron-
low-hydrotalcite-like sample (MgofiFeo ,Alo3(OH)2(C03)o 1S.XH3O) is composed of
regularly rhombohedra-Iike particles with length x width of 0.3 X 0.5 ^mi (Fig. 2A). Meanwhile, the smaller particles were not apparent m the iron-high- sample (Mgo sFco 2AI0 3(OH)2(C03)o is-xHsO), implying that the catalyst morphology may be
correlated with the chemical composition [6, 10].
Indeed, the latter sample compnses numerous platelet particles with the average particle sizes in the range of 1-4 jim as illustrated in Fig. 2B, Careful observation indicates that these particles are aggregates of smaller platelets. Examination of a TEM image will elucidate this issue (Fig. 2C) [10,11].
The TEM microphoto of Mgo sFco 2AI0 3(OH)2(C03)o i5.xH,0 hydrotalcite-like sample shows the formation of nearly hexagonal- shaped particles with layered sfructure (Fig. 2C).
The pnmary nanoparticles agglomerate to larger palettes (Fig. 2B) [11], resulhng in the appearance of voids between nanoparticles which fonn openly micropores [10]. Moreover, the regular stacks of layered hexagonal platelets result in the formation of numerous empty spaces [11, 12]. In other words, smaller mesopores probably stem from voids among the crystallite agglomerates.
VJCVol. 51(5), 2013 Degradation of methylene blue in .
Figure 2- SEM micrographs of Mg„ „Feo lAlo 3(OH).(CO.)o ,i.xH.0(A) and Mgo;Feo2Alo3(OH)2(C03)ni5.xH20(B)and
TEM image of Mgo sFe, :AI„ 3(OH)2(COj)o ,5-xH20(C) 3.3 Nitrogen physisorption
The textural properties of Mgo7.xFe,Alo3(OH)2(C03)o,is.xH20 hydrotalcites were slightly changed from x = 0.1 to 0.2. In practical, the BET specific surface areas of the iron- low-sample (x = 0.1) is only 83.2 m^/g while that of the other (x = 0,2) reaches to 198.2 m^/g. Their nitrogen adsorption-desorption isotherms in this study are fallen in the intermediate type between types II and IV adsorption isotherms (lUPAC classification), interpreting the presence of both
mesopores and micropores in the materials. The Mgo sFco 2AI0 3(OH)2(C03)o 1S.XH2O hydrotalcite exhibits a H3 hysteresis loop over the relative pressure range 0.20-0.95, which is attributed to aggregates of plate-like particles, leading to slit-like mesopores [12, 13]. Moreover, the solid contains not only mesopores, but also some micropores which are corroborated by a sharp increase in the amount of nifrogen adsorption in the range of very relahve low pressure (Fig. 3) [14]. The formation of the porous system will improve the adsorptive and catalytic ability of hydrotalcites [5, 10].
Relative Presure (P/Po)
Figure 3: Nifrogen adsorption/desorphon curves on Mg-Fe-Al hydrotalcites
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3.4 Degradation of methylene blue over M g - F e - A l hydrotalcite catalysts
The decolonzation of aqueous methylene blue was investigated over two prepared Mg-Fe-Al hydrotalcites in absence/presence of hydrogen peroxide oxidant. Fig. 4 shows the methylene blue degradation percent in the absence of H2O2. For blank test, it is observed that approximately 24% of the methylene blue was degraded within 150 minutes. In contrast, the removal percent gradually
Nguyen Tien Thao, et al.
increases and then approaches almost constant with increasing retention time after 80 minutes over both studied hydrotalcite catalysts (Fig. 5). This profile mdicates that methylene blue adsorbed on porous Mg-Fe-Al hydrotalcites [2, 15, 16]. To investigate the catalytic ability of hydrotalcite, we have added 2.0 mL of H2O2 oxidant mto 200 mL of an aqueous methylene blue solution containing 200 mg of catalyst. The changes in removal percent of methylene blue is plotted in figure 5.
Mettytene blue + Catalysts Metiylene blue+ Hydratabtes+H202
• Mg0,6FeD,1M0,3(OM)2(CO3)D.1S.iH2D a MgO SFeD 2M0,3{OM)2(CO3)l).1S,xHZO
• Blank Tett
: 70 -I- /
* Mg0,EFe01AI03|OH)2(CO3)0.15,xH2O
= MgO SFe0,2AI0 3(OH):(CO3l0,1S xHaO
• Blank test
Time (mjn)
Figure 4: Removal of methylene blue on Mg-Fe-Al hydrotalcite catalysts (catalyst:
0,20 grams, methylene blue: 500 ppm; under solar light)
Figure 5- Removal of methylene blue in presence of H2O2 over Mg-Fe-Al hydrotalcite catalysts (Catalyst: 0.20 grams, methylene blue:
500 ppm; H2O2 30 %: 2 mL under solar light)
Clearly, methylene blue is oxidized in the presence of hydrogen peroxide. Comparison between figures 4 and 5 indicates that the removal percent of methylene blue in the absence of H2O2 (Fig, 4) is much lower than that in the presence of oxidant agent (Fig. 5) within 60 minutes. It is well known that hydrogen peroxide is a potential source for the production of hydroxyl radicals ('OH) [4,"
15]. In the presence of Mg-Fe-Al hydrotalcite, iron ions immobilized layered double hydroxides presumably play as achve sites for the decomposihon of H2O2 into hydroxyl radicals, analogous to a fraditional Fenton catalyst [13].
Therefore, the rate of degradation of methylene blue over porous Mg-Fe-Al hydrotalcites was photocatalytically accelerated by addiUon of H2O2.
The removal percent of methylene blue achieves about 97-99 % after 15 minutes [16]. These preliminary experimental results open a very promising way to use Mg-Fe-Al hydrotalcites as efficient adsorbents/catalysts for the removal of methylene blue in water.
4. CONLCUSIONS
Two (Mgo7..Fe,)Alo3(OH)2(C03)oi5xH20 hydrotalcite-like matenals prepared by coprecipitation method were characterized and used as adsorbents/catalysts for the degradation of aqueous methylene blue. All synthesized catalysts have well-crystallized hydrotalcite phases and uniform particles, micropores and mesopores. The catalyst morphology is probably associated with the chemical compositions of hydrotalcite. The prepared hydrotalcites were not only efficient adsorbents, but also played a role of active catalysts for the degradahon of aqueous methylene .blue in the presence of H2O2 oxidizing reactant. The removal percent reached about 97-99% after 15 minutes as a small amount of H2O2 added.
Acknowledegements: This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.99-2011.50.
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Corresponding author. Nguyen Tien Thao
Department of Pefrochemistry, Faculty of Chemistry,
VNU University of Science, Vietnam National University-Hanoi (VNU).
19 Le Thanh Tong ST, Hoan Kiem, Hanoi, Viefriam Tel.: + 844 3933 1605; Fax: + 844 3824 1140; Cell: 0937898917 Email address: ntthao(§vnu.edu.vn /nguyentienthao(§gmail.com.
