Photodegradation of Methylene Blue by TiO 2 -Fe 3 O 4 -Bentonite Magnetic Nanocomposite

Abstract

[subscript] Fe 3 [/subscript] [subscript] O 4 [/subscript] -bentonite nanoparticles have been prepared by a coprecipitation technique under a nitrogen atmosphere. An aqueous suspension of bentonite was first modified with FeCl2 and FeCl3. TiO2 was then loaded onto the surface of the [subscript] Fe 3 [/subscript] [subscript] O 4 [/subscript] -bentonite by a sol-gel method. After sufficient drying, the colloidal solution was placed in a muffle furnace at 773 K to obtain the [subscript] TiO 2 [/subscript] - [subscript] Fe 3 [/subscript] [subscript] O 4 [/subscript] -bentonite composite. The material has been characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, and vibrating sample magnetometry (VSM). Morphological observation showed that [subscript] Fe 3 [/subscript] [subscript] O 4 [/subscript] and TiO2 nanoparticles had been adsorbed on the surface of bentonite nanoneedles. The material was then applied for the photodegradation of the azo dye methylene blue (MB). It was found that the removal efficiency of MB exceeded 90% under UV illumination, and that only a 20% mass loss was incurred after six cycles. The composite material thus showed good photocatalytic performance and recycling properties.

[ProQuest: [...] denotes non US-ASCII text; see PDF]

Wei Chen 1, 2 and Hongyao Xiao 1, 2 and Hang Xu 1, 2 and Tonggang Ding 1, 2 and Yanmei Gu 2

Academic Editor:Yasser Shaban

1, Key Laboratory of Integrated Regulation and Resource Development on Shallow Lake of Ministry of Education, College of Environment, Hohai University, Nanjing 210098, China
2, College of Environmental Science, Hohai University, Nanjing 210098, China

Received 20 September 2014; Revised 9 January 2015; Accepted 12 January 2015; 2 March 2015

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Among environmental contaminants, dyestuffs constitute a major component and are of particular concern [1]. Azo dyes are widely applied in the printing and dyeing industries [2-4]. Some azo dyes can be reduced to aromatic amines, which are potent carcinogens [5]. Therefore, they need to be completely removed from wastewater from printing and dyeing. Due to its good catalytic effect, stable chemical performance, ready availability, and nontoxicity, TiO₂ has been regarded as an ideal kind of catalyst since the 1940s [6]. Under illumination, the crystal surface of TiO₂ generates electrons (e^- ) and holes (h⁺ ), allowing the production of reactive oxygen species (ROS) such as hydroxyl free radicals or oxygen free radicals [7, 8]. Such species react with the nitrogen double bonds in azo dyes, and the dyes are thereby decomposed into nontoxic small-molecule chemicals [9]. However, pure TiO₂ is of limited use because of its low specific surface area and low adsorption rate [10-13]. Various catalyst supports have been applied for loading with TiO₂ . Xia et al. examined TiO₂ supported on layered double hydroxides that shows over 85% removal rate of azo dyes [14]. Janus et al. loaded TiO₂ on the surface of powder activated carbon and used it to remove azo dyes [15-18]. Xu et al. synthesized a new composite material, hydroxy-iron-aluminum pillared bentonite (H-Fe-Al-Ben), by ion exchange, and studied its 99.5% removal rate of azocarmine B [19].

Due to their adsorption abilities and catalytic activities, natural clays have been widely used as adsorbents and catalyst supports. Bentonite is a clay mineral, the main component of which is montmorillonite [20, 21]. Due to its unique structure, bentonite has good mechanical properties, high thermal stability and acid and alkali resistance, large surface area, and other favorable characteristics. Hence, it has been widely used in adsorbents, catalysts, adhesives, catalyst supports, and so on [22]. Bentonite is abundant in China, notably in Wuxi, Jiangsu, and Zhejiang provinces [23]. Researchers have modified the surface of bentonite to enhance its dispersibility and selectivity. Toor et al. studied acid and thermal activation that leads to approximately 25% increase in the Congo-red adsorption capacity of natural bentonite [24]. Li et al. synthesized Fe-pillared bentonite and Al-Fe-pillared bentonite and found that these composite materials were effective in removing azo dye X-3B [25]. Liu et al. found that the adsorption capacity of bentonite was greatly increased by 90.55% after interaction with attapulgite [26]. Xu et al. studied that when ZnO/bentonite particles catalyst was used, the removal rate of Acid Yellow 11 reached to 90% under UV irradiation, which was remarkably effective [27].

In practice, such adsorbents are difficult to recycle and are liable to cause secondary pollution and a waste of resources [28]. Iron oxide, with the formula Fe₃ O₄ , consists of magnetic black crystals. Its good magnetic properties are often applied to modify materials, allowing for improved recycling performance. Pan et al. studied the recovery performance of Fe₃ O₄ -attapulgite used for the degradation of 2,4-dichlorophenol and found the mass loss of adsorbent to be just 7.53% after five cycles [29]. Wu et al. used a solution copolymerization technique to synthesize a bentonite-Fe₃ O₄ PSA magnetic nanocomposite, which proved that a small amount of bentonite could improve the swelling ability for the removal of Th(IV) [30].

Using bentonite as a catalyst support, iron oxide for enhanced recovery performance, and TiO₂ for catalytic decomposition, we have prepared a TiO₂ -Fe₃ O₄ -bentonite composite photocatalyst material by a simple combined precipitation-coating and sol-gel method. This composite has been investigated by SEM, XRD, and VSM.

2. Materials and Methods

2.1. Materials

Bentonite (average size 200 mesh) was purchased from Oilbetter (China). FeCl₃ ·6H₂ O and FeSO₄ ·7H₂ O were purchased from Sinopharm Chemical Reagent Co. (China); the latter was stored under a nitrogen atmosphere to avoid the oxidation of Fe(II). Azo dyes were procured from Dye Company (Dystar). The test solution was prepared by dissolving azo dye (30 mg) in distilled water (1 L). Methanol, ethanol, acetylacetone, tetrabutyl titanate, and sodium hydroxide were purchased from Sigma-Aldrich Corporation. All experiments were performed using deionized ultrapure water from an ultrapure water preparation device. All reagents were of analytical grade purity and were used directly without further purification.

2.2. Preparation of the Catalyst

2.2.1. Pretreatment of Bentonite

Bentonite clay powder (particle size 0.074 mm) was finely sieved, a 5% suspension in water was prepared, and this was allowed to age for 24 h. The bentonite powder was then recovered by centrifugation and placed in an oven at 378 K until it completely dehydrated. It was kept dry until its subsequent loading with TiO₂ .

2.2.2. Loading of Bentonite with Fe₃ O₄

Fe₃ O₄ -bentonite was prepared by a coprecipitation method.

Step 1. At 343 K under nitrogen atmosphere, bentonite powder (1.0 g) was added to 200 mL of aqueous solution containing FeCl₃ ·6H₂ O (5.0 g) and FeSO₄ ·7H₂ O (2.5 g). Aqueous NaOH solution (10 mL, 8 mol/L) was then added dropwise to adjust the pH to 11 and the mixture was stirred continuously for 1 h.

Step 2 . The mixture was aged at 343 K for 4 h. The particles were then washed with distilled water until neutral and dried in an oven at 373 K for 3 h to afford Fe₃ O₄ -bentonite crystals.

2.2.3. Loading of Fe₃ O₄ -Bentonite with TiO₂

Step 1 . Fe₃ O₄ -bentonite (2.5 g) was placed in a beaker, and ethanol (80 mL) was added. The mixture was sonicated for 20 min and then left to stand.

Step 2. Ethanol (80 mL) and tetrabutyl titanate (20 mL) were mixed in a beaker and sonicated for 20 min. Further ethanol (80 mL) and aqueous nitric acid (obtained by diluting 10 m HNO₃ (1 mL) with distilled water (16 mL)) were then added to form a colloidal solution. The mixture was vigorously stirred for 30 min and then left to stand for 24 h.

Step 3. The mixture was placed in an oven at 353 K until completely dry and then transferred to a muffle furnace and heated at 773 K for 3 h to afford TiO₂ -loaded magnetic bentonite.

2.3. Analytical Methods

The crystal structure of the product was determined on a Thermo X'TRA type X-ray diffractometer. The crystal size and morphology of the samples were examined using a scanning electron microscope (Hitachi Ltd., S-4800). The magnetic properties of the samples were determined using a vibrating sample magnetometer (VSM) (Lake Shore VSM7410).

2.4. Photocatalytic Reactions

Experiments on the degradation of methylene blue were performed to investigate the photocatalytic effect of the TiO₂ -Fe₃ O₄ -bentonite. A 350 W xenon lamp equipped with a 288 K constant temperature circulator was used. TiO₂ -Fe₃ O₄ -bentonite (30 mg) and 30 mg/L methylene blue solution (100 mL) were mixed for 30 min in a dark environment. After the solution had reached equilibrium, it was placed in a batch stirrer for several minutes under UV light irradiation. Aliquots (2 mL) of the supernatant were withdrawn at intervals to measure the methylene blue concentration by UV spectrophotometry at a wavelength of 664 nm. No catalyst was added in a blank test.

3. Results and Discussion

3.1. SEM Analysis of TiO₂ -Fe₃ O₄ -Bentonite

Figure 1 shows the XRD patterns of different samples. From the figure, it is evident that the modification of bentonite did not change its structure. Comparing Figures 1(a) and 1(b), it can be seen that the bentonite surface had been successfully loaded with Fe₃ O₄ and TiO₂ by the concerted precipitation-coating and sol-gel method.

Figure 1: XRD patterns of (a) bentonite, (b) TiO₂ -Fe₃ O₄ -bentonite. [open diamond] Bentonite; [white triangle up] Fe₃ O₄ ; [...] TiO₂ (rutile).

[figure omitted; refer to PDF]

Figure 2 shows SEM images of the samples. Figure 2(a) shows a bentonite particle of diameter about 2-3 μ m. When 5% aqueous suspensions of bentonite were aged for 24 h, the surface area of the crystals was significantly increased, providing a good surface for the loading of TiO₂ and Fe₃ O₄ [31]. From Figures 2(a) and 2(b), it can be seen that the Fe₃ O₄ -bentonite surface was more rough and that the crystal size was smaller after loading with Fe₃ O₄ . Many Fe₃ O₄ particles were adsorbed on the bentonite surface. Figure 2(c) shows a surface image of TiO₂ -bentonite, from which it is evident that the bentonite particles had a good surface condition for the loading of TiO₂ . Figure 2(d) shows that the crystal surface morphology was not significantly changed when a large number of TiO₂ particles of diameter 10-20 nm were adhered on the surface of the Fe₃ O₄ -bentonite.

SEM images of (a) bentonite, (b) Fe₃ O₄ -bentonite, (c) TiO₂ -bentonite, and (d) TiO₂ -Fe₃ O₄ -bentonite.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

(c) [figure omitted; refer to PDF]

(d) [figure omitted; refer to PDF]

3.2. Magnetic Properties of TiO₂ -Fe₃ O₄ -Bentonite

Figure 3 shows the magnetic hysteresis loop of TiO₂ -Fe₃ O₄ -bentonite. The magnetic properties of the composite were measured by vibrating sample magnetometry (VSM) at room temperature, at magnetic field strengths in the range -20 kOe <= H <= 20 kOe. The magnetic saturation (Ms), residual magnetization (Mr), coercivity (Hc), and Mr/Ms ratio (Sr) of the composite were 33.29 emu/g, 0.87 emu/g, 7.973 G, and 0.026, respectively. Low residual magnetism and coercivity showed the sample to have ferromagnetic behavior at room temperature. Weak hysteresis attributed to a single magnetic domain of nanocrystalline nature was observed, which indicates that TiO₂ -Fe₃ O₄ -bentonite is a strongly paramagnetic material. The magnetism of Fe₃ O₄ was weakened after loading with TiO₂ , but was still 33.29 emu/g, so that it could be attracted by a commercial magnet. Thus, the TiO₂ -Fe₃ O₄ -bentonite composite can be recovered by a simple magnetic separation technique, greatly reducing secondary pollution and the material cost of wastewater treatment.

Figure 3: Magnetic hysteresis loop of TiO₂ -Fe₃ O₄ -bentonite at 298 K.

[figure omitted; refer to PDF]

3.3. Adsorption Kinetics of TiO₂ -Fe₃ O₄ -Bentonite

The Langmuir-Hinshelwood kinetic model is commonly used to describe the photocatalytic degradation of organic compounds. The relationship between the reaction rate [figure omitted; refer to PDF] and the concentration [figure omitted; refer to PDF] is expressed as follows: [figure omitted; refer to PDF] where [figure omitted; refer to PDF] is the intrinsic rate constant and [figure omitted; refer to PDF] is the adsorption equilibrium constant. When the adsorption is weak or the adsorbate concentration is low, [figure omitted; refer to PDF] is negligible and the adsorption kinetic equation can be simplified to a first-order model: [figure omitted; refer to PDF] where [figure omitted; refer to PDF] is the apparent adsorption constant.

In the initial condition ( [figure omitted; refer to PDF] , [figure omitted; refer to PDF] ), [figure omitted; refer to PDF] where [figure omitted; refer to PDF] and [figure omitted; refer to PDF] are the initial dye concentration and the concentration at time [figure omitted; refer to PDF] , respectively. The values of [figure omitted; refer to PDF] can thus be calculated from regression analysis of the reaction time [figure omitted; refer to PDF] and [figure omitted; refer to PDF] .

Figure 4 shows the linear relationship between [figure omitted; refer to PDF] and [figure omitted; refer to PDF] for the degradation of methylene blue at 298 K, as befits the first-order adsorption model (linear correlation constant > 0.98). The apparent adsorption constant and [figure omitted; refer to PDF] value are shown [...]in Table 1. The degradation of methylene blue was 90% complete after 90 min of reaction. Specifically, the rate of degradation of methylene blue adsorbed on the TiO₂ -Fe₃ O₄ -bentonite composite material was 0.0324 min^-1 ( [figure omitted; refer to PDF] = 21.4 min); that is, it was 50% degraded in just 21.4 min. This synthetic clay composite material thus showed considerable photocatalytic degradation efficiency.

Table 1: Pseudo-first-order kinetic parameters for the photodegradation of MB dye.

Sample	MB
[figure omitted; refer to PDF] [min-1 ]	[figure omitted; refer to PDF] [min]	[figure omitted; refer to PDF]
TiO2 -Fe3 O4 -bentonite	0.03236	21.4	0.9848

Figure 4: Pseudo-first-order kinetics for the degradation of methylene blue.

[figure omitted; refer to PDF]

3.4. The Photocatalytic Properties of TiO₂ -Fe₃ O₄ -Bentonite

We investigated the catalytic effects of bentonite, Fe₃ O₄ -bentonite, TiO₂ -bentonite, and TiO₂ -Fe₃ O₄ -bentonite on the degradation of methylene blue under xenon lamp light, and the results are shown in Figure 5. It can be seen that the reaction almost reached completion after 90 min with TiO₂ -bentonite and TiO₂ -Fe₃ O₄ -bentonite; the removal of methylene blue was 90% complete after 90 min. Comparison of these four adsorbents for the removal of methylene blue indicates that TiO₂ -Fe₃ O₄ -bentonite is better than bentonite. The TiO₂ crystals loaded on the surface of bentonite are mainly responsible for the decomposition of methylene blue. Studies have indicated that azo dyes are first adsorbed and then a surface-catalyzed reaction ensues, which is favorable for their removal. From Figure 5, it is also evident that TiO₂ -Fe₃ O₄ -bentonite and TiO₂ -bentonite removal rate shows superior removal efficiency compared to Fe₃ O₄ -bentonite and bentonite, and that the little difference between TiO₂ -Fe₃ O₄ -bentonite and TiO₂ -bentonite is led by little contribution of Fe₃ O₄ to photocatalytic removal efficiency.

Figure 5: Photocatalytic degradation of methylene blue by bentonite, Fe₃ O₄ -bentonite, TiO₂ -bentonite, and TiO₂ -Fe₃ O₄ -bentonite under Xe light.

[figure omitted; refer to PDF]

The regeneration performance of TiO₂ -Fe₃ O₄ -bentonite was tested by carrying out six cycles of adsorption and desorption. The eluent consisted of methanol and acetic acid (9.5 : 0.5). After separating the catalyst from the aqueous solution by means of a magnet at the end of each experiment, the catalyst was washed for 30 min with 10 mL of eluent under ultrasonication. The experimental results are shown in Figure 6. It can be seen that the mass loss of the composite material over the six cycles was only 20%. The composite material retained its stable structure during the process, indicating good regeneration properties.

Figure 6: TiO₂ -Fe₃ O₄ -bentonite regeneration experiment.

[figure omitted; refer to PDF]

4. Conclusions

A TiO₂ -loaded magnetic clay composite material has been synthesized by a simple ion coprecipitation and sol-gel method. The catalyst has proved to be effective for the removal of methylene blue under irradiation with a xenon lamp. The absorption of methylene blue on the composite material showed pseudo-first-order kinetics, with an adsorption rate [figure omitted; refer to PDF] of 0.0324 min^-1 . In methylene blue degradation experiments, the removal efficiency of a 30 mg/L methylene blue solution reached 90% after 90 min at 298 K and pH 7.0. In addition, the composite material incurred only 20% mass loss after six cycles of repeated use, which shows good recycling properties. Therefore, the described TiO₂ -loaded magnetic clay material showed not only good adsorption decomposition performance for azo dyes under visible light, but also excellent recycling properties. It thus has great potential application in the azo dye treatment industry and is worthy of further study.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51308185), National Innovation Training Program (201310294024), and the open research fund of the Jiangsu Key laboratory of industrial water conservation abatement (IWCER201203).

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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