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Photocatalytic degradation of malachite green using nano-sized cerium-iron oxide.

1. INTRODUCTION

Photocatalysis has emerged as a probable solution to some of the worldwide problems like energy crisis, environmental pollution, waste water treatment, etc. Extensive researches in the field of photocatalysis have revealed various fascinating applications of photocatalytic reactions and semiconductor photocatalysts [1, 2]. In this process a photoactive material is required to initiate the catalytic reaction. Semiconductor particulate system provides low cost and convenient way of treating several undesirable chemicals [3, 4].

A lot of work has been done on various photocatalytic materials such as Ti[O.sub.2], ZnS, ZnO, W[O.sub.3], [V.sub.2][O.sub.5], CdS, etc. [5-10] but very less attention is being given to the mixed oxide nanoparticles. The mixed oxide particle has the ability to obtain structures in combination with the properties that neither individual oxide possesses [11].

Ceria has recently been attracting much attention in the oxidative catalysis research due to its high oxygen storage capacity [12] and redox properties [13]. Cerium compounds has been used in applications like optical coating and fuel cells [14], various industrial heterogeneous catalysts and in automotive Three-Way Converters (TWC) [15] because of its high ultraviolet absorbance [16], high oxygen ion mobility [17] and its multiple valence states [18]. Wide spread use of cerium compounds in industrial applications is spurred by its large abundance on earth's crust more than that of copper [19, 20]. However, work that shows the potential use of ceria with other metal oxide in degradation of dyes is scarce in literature.

In this study, the removal of malachite green dye with cerium-iron oxide (CeFeO3) was investigated. CeFeO3 nanoparticles were synthesized and characterization of the synthesized oxide was carried out. The kinetics of the absorption process was evaluated to study the absorption mechanism of the dye molecules.

2. MATERIAL AND METHODS

Materials

Chemicals and reagents

Cerium (III) nitrate hexahydrate, iron (III) nitrate nonahydrate, sodium hydroxide, sulphuric acid and doubly distilled water were used to conduct the experiments. These were purchased from Sigma-Aldrich Chemicals Pvt. Ltd. India, SD Fine, CDH and Merck, India. All the chemicals and reagents used were of analytical grade and used without further purification.

Dye

Malachite green ([C.sub.23][H.sub.25][N.sub.2]Q), dye which is used for dyeing cloth and leather and as a histological stain was procured from CDH, India. The molecular weight of malachite green is 364.90 g/mol and maximum wavelength is 620 nm. The chemical structure of the dye is shown in Fig. 1.

Methods

Synthesis of cerium-iron oxide (CeFe[O.sub.3]) nanoparticles

The synthesis of mixed cerium iron oxide was achieved by co-precipitation method. The mixed oxide was prepared by adding aqueous solution of 1M NaOH drop-wise to the aqueous solution of 0.1M of both Ce[(N[O.sub.3]).sub.3]. 6[H.sub.2]O and Fe[(N[O.sub.3]).sub.3]. 9[H.sub.2]O with concurrent vigorous stirring. The pH of the mixed solution was adjusted at different pH in alkaline range but at 10 pH complete precipitation was observed so the pH was maintained at 10. After 4 hours of continuous stirring, the precipitate was filtered and repeatedly washed with deionized water. The residue was dried in an oven at 110[degrees]C overnight and then grounded in acetone with mortar and pestle. The powder received was then calcined at 500[degrees]C for 4 hours under static air in muffle furnace.

Characterization of the synthesized nanoparticles

X-ray powder diffraction study was performed to establish the phase purity and crystallinity of the prepared bimetal oxide by X-ray diffractometer. The nanoparticles size was determined by the Scherrer equation.

Photocatalytic degradation of dye

A stock solution of malachite green of 1.0 x [10.sup.-3] M concentration was prepared by dissolving 0.365 g of malachite green in 1000 mL of doubly distilled water. The absorption maximum of the dye was determined with the help of a spectrophotometer (Systronics Model 106). Photocatalytic degradation of malachite green was studied by taking 50 mL reaction mixture which contains 2.0 x [10.sup.-5] M of malachite green and 0.05 g of CeFe[O.sub.3]. The reaction mixture was exposed to light. For irradiation purpose, 200 W tungsten lamp (Philips) was used. The intensity of light was measured by solar power meter (TENMARS Model TM 207). A water filter was used to cut off thermal radiation. The pH of the solution was measured by a digital pH meter (Systronics Model 324). The desired pH of the solution was adjusted by the addition of 0.1N sodium hydroxide and 0.1N hydrochloric acid solutions. To measure the degradation of dye, optical density was taken at regular time intervals.

3. RESULTS AND DISCUSSION

Characterization of the synthesized nanoparticles

The XRD pattern for CeFe[O.sub.3] is shown in Fig. 2. Graph has been plotted between intensity (cycles per second) and 2[theta] values (in degrees). The nanoparticles size was 23.28 nm as determined by Scherrer equation.

Photocatalytic degradation of dye

A 2.0 mL of the solution was taken out from the reaction mixture at regular time intervals and absorbance was measured spectrophotometrically at [[lambda].sub.max] 620 nm. It was observed that the absorbance of the solution decreases with increasing time intervals showing thereby that the concentration of the dye decreases with increasing time of exposure. A plot of 2 + log O.D. versus time was linear and follows first order kinetics. The rate constant was determined by using the expression, k = 2.303 x slope.

The typical run for dye degradation is given in Table 1 and graphically represented in Fig. 3.

Effect of different variables on photocatalytic degradation of dye

Effect of pH

The pH of the solution is likely to affect the degradation of dye and hence, the effect of pH on the rate of degradation of the dye was investigated in the pH range 6 to 10. The results are reported in Table 2. As observed, the rate of reaction increased with increasing pH of the solution up to pH 8.5. However, a further increase in pH of solution resulted in decreased reaction rate. An increase in the rate of photocatalytic degradation of malachite green with increase in pH may be due to generation of more x OH radicals, which are produced from the reaction between -OH ions and hole ([h.sup.+]) of the semiconductor. Above pH 8.5, a decrease in the rate of photocatalytic degradation of the dye was observed, which may be due to the fact that cationic form of malachite green converts in its neutral form, which faces no attraction towards the negatively charged semiconductor surface due to absorption of -OH ions.

Effect of dye concentration

The effect of dye concentration on the rate of the reaction of photocatalytic degradation was studied by taking different concentrations of dye. The results are reported in Table 3. It has been observed that the rate of photocatalytic degradation increases with an increase in the concentration of the dye up to 2.0 x [10.sup.-5] M. Further increasing in the concentration of dye, the rate of photocatalytic bleaching decreases. It may be due to the fact that as the concentration of dye was increased, more dye molecules were available for excitation and energy transfer and, hence an increase in the rate was observed but on further increase of dye concentration, dye starts acting as a filter for the incident light and will not permit the light intensity to reach the semiconductor surface and as a result rate decreases.

Effect of amount of CeFe[O.sub.3]

The amount of CeFe[O.sub.3] is also likely to affect the rate of photocatalytic degradation of dyes and therefore, different amounts of CeFe[O.sub.3] were used. The results are reported in Table 4. These results showed that an increase in catalyst amount from 0.01 g to 0.05 g increased the photodegradation efficiency, as the exposed surface area of the semiconductor also increases and after that the further increase in catalyst above 0.05 g has negligible effect on the photodegradation efficiency.

Effect of light intensity

The effect of light intensity on the rate of the reaction was also observed and the observations are summarized in Table 5. It has been observed that on increasing the intensity of light up to 600[Wm.sup.-2], the rate of reaction also increases because on increasing the intensity, the number of photons striking per unit area of reaction mixture will also increase. This will result in a corresponding increase in the rate of degradation of malachite green. Small decrease in the rate on further increasing light intensity may be due to some thermal or side reactions.

Mechanism

On the basis of these observations, a tentative mechanism for photocatalytic degradation of malachite green may be proposed as

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] ...(1)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] ...(2)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] ... 3)

[e.sup.-] + [O.sub.2] [right arrow] [O.sup.-x.sub.2] ...(4)

[O.sup.-x.sub.2] [+.sup.3] M[G.sub.1] [right arrow] ...(5)

Leuco MG [right arrow] Products ...(6)

Malachite green (MG) absorbs radiations of suitable wavelength and gives rise to its first excited singlet state. Then it undergoes intersystem crossing (ISC) to give the triplet state of the dye. On the other hand, the semiconducting CeFe[O.sub.3] (SC) also utilizes the radiant energy to excite its electron from valence band to the conduction band. This electron will be abstracted by oxygen molecule (dissolved oxygen) generating superoxide anion radical ([O.sup.-x.sub.2]). This anion radical will reduce the dye malachite green to its leuco form, which may ultimately degrade to products. It was also confirmed that this degradation proceeds through reduction and not oxidation as the rate of degradation was not affected appreciably in presence of hydroxyl radical scavenger (2-propanol).

4. CONCLUSION

The synthesized bimetal oxide presented a pure phase and the crystal size in nanometric scale. Results indicate that cerium-iron oxide can be employed for the degradation of malachite green dye. The absorption kinetics of the dye followed the pseudo-first order model. The absorption process was found to be controlled by both the external surface and the interparticle diffusion with surface diffusion at the earlier stage followed by interparticle diffusion at the later stage. Experimental results showed that various variables such as the pH of the reaction mixture, concentration of dye, light intensity and amount of semiconductor had their effect on dye degradation.

5. ACKNOWLEDMENTS

The authors are thankful to the Department of Science & Technology (DST, Rajasthan Government, India) for sanctioning the student project for Ph.D. and SAIF Chandigarh, India for the instrumental facility.

6. REFERENCE AND NOTES

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[8] Neppolian, B.; Choi, H.C.; Sakthivel, S.; Arabindoo, B.; Murugesan, V. J. Hazard. Mater. 2002, B89, 303. [CrossRefl

[9] Li, B.; Xu, Y.; Rong, G.; Jing, M.; Xie, Y. Nanotechnology 2006, 17, 2560. [CrossRefl

[10] Ma, L. L.; Sun, H. Z.; Zhang, Y. G.; Lin, Y. L.; Li, J. L; Wang, E.; Yu, Y.; Tan, M.; Wang, J. B. Nanotechnology 2008, 19, 115709. [CrossRefl

[11] Fang, J.; Xu, L.; Zhang, Z.; Yuan, Y.; Cao, S.; Wang, Z.; Yin,L.; Liao, Y.; Xue,C. Appl. Mater. Interfaces 2013, 5, 8088. [CrossRefl

[12] Campbell, C. T.; Peden, C. H. F. Science 2005, 309, 713. ICrossRefl

[13] Dong, F.; Suda, A.; Tanabe, T.; Sobukawa, H. Y. N.; Shinjoh, H.; Sugiura, M.; Descorme, C.; Duprez, D. Catal. Today 2004a, 90, 223. [CrossRefl

[14] Murray, E. P.; Tsai T.; Barnett, S. A. Nature 1999, 400, 649. [CrossRefl

[15] Mai, H.; Mengfei, L.; Ping, F. J. Rare Earth. 2006, 24, 188. [CrossRefl

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[19] Hickey, N.; Arneodo Larochette, P.; Gentilini, C.; Sordelli, L.; Olivi, L.; Polizzi, S.; Montini, T.; Fornasiero, P.; Pasquato, L.; Graziani, M. Chem. Mater. 2007, 19, 650. [CrossRef]

[20] Lide, D. R. CRC Handbook of Chemistry and Physics: CRC, 88th edn, 2007.

K. L. Ameta (a) *, Neema Papnai (a) and Rakshit Ameta (b)

(a) Department of Chemistry, Faculty of Arts, Science and Commerce, Mody Institute of Technology and Science, Lakshmangarh-332311, Rajasthan, India.

(b) Department of Chemistry, Pacific College of Basic & Applied Sciences, PAHER University, Udaipur 313024, Rajasthan, India.

* Corresponding author. E-mail: klameta77@hotmail.com

Article history: Received: 06 December 2013; revised: 13 January 2014; accepted: 26 January 2014. Available online: 02 April 2014.

Table 1. A typical run of malachite green dye using
CeFe[O.sub.3]. (a)

Time (min.)                        Optical Density   2 + log O.D.
                                       (O.D.)

0                                      1.1479           2.0599
20                                      0.685           1.8357
40                                      0.413           1.616
60                                      0.277           1.4425
80                                      0.192           1.2833
100                                     0.123           1.0899
120                                     0.095           0.9777
k= 3.45 x [10.sup.-4] [s.sup.-1]

(a) Reaction Conditions: Dye concentration = 2.0 x [10.sup.-5] M,
Light intensity = 600 [Wm.sup.-2], CeFe[O.sub.3] = 0.05 g in 50
mL dye solution, pH = 8.5.

Table 2. Effect of pH on degradation of malachite
green. (a)

pH     k x [10.sup.4] [s.sup.-1]

6.0              0.26
6.5              0.42
7.0              0.71
7.5              1.61
8.0              2.76
8.5              3.45
9.0              2.52
9.5              2.17
10.0             1.76

(a) Reaction Conditions: Dye Concentration = 2.0 x [10.sup.-5] M,
Light intensity = 600 [Wm.sup.-2], CeFe[O.sub.3] = 0.05 g in 50
mL dye solution.

Table 3. Effect of dye concentration on degradation
of malachite green. (a)

[Malachite green]   k x [10.sup.4] [s.sup.-1]
x [10.sup.5] M

1.5                           3.19
2.0                           3.45
2.5                           2.78
3.0                           2.25
3.5                           1.84
4.0                           1.62
4.5                           1.33

(a) Reaction Conditions: CeFe[O.sub.3] = 0.05 g in 50 mL dye
solution, pH = 8.5, Light intensity = 600 [Wm.sup.-2].

Table 4. Effect of amount of CeFe[O.sub.3] on degradation of
malachite green. (a)

CeFe[O.sub.3] (g/ 50 mL   k x [10.sup.4]
dye solution)               [s.sup.-1]

0.01                           1.59
0.02                           2.09
0.03                           2.37
0.04                           2.99
0.05                           3.45
0.06                           3.13
0.07                           3.02

(a) Reaction Conditions: Dye Concentration = 2.0 x [10.sup.-5] M,
Light intensity = 600 [Wm.sup.-2], pH = 8.5.

Table 5. Effect of light intensity on degradation of
malachite green.

Light intensity ([Wm.sup.-2])   k x [10.sup.4] [s.sup.-1]

200                                       1.89
300                                       1.99
400                                       2.07
500                                       2.58
600                                       3.45
700                                       3.01
800                                       2.78

(a) Reaction Conditions: Dye Concentration = 2.0 x [10.sup.-5] M,
CeFe[O.sub.3] = 0.05 g in 50 mL dye solution, pH = 8.5.
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Title Annotation:Full Paper
Author:Ameta, K.L.; Papnai, Neema; Ameta, Rakshit
Publication:Orbital: The Electronic Journal of Chemistry
Article Type:Report
Date:Jan 1, 2014
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