Photocatalytic Decolorization of Methylene Blue Dye with Zinc Oxide Powder

Journal of Babylon University/Pure and Applied Sciences/ No.(9)/ Vol.(22): 2014

Photocatalytic decolorization of methylene blue dye by zinc oxide powder

Amjed Mirza Oda University of Babylon, College of Basic Education, Science Department. [email protected]. Ahmed Salih , Salih Hadi , Ahmed Jawad , Ahmed Sadoon , Yahya Fahim University of Babylon, Science College, Chemistry department Anfal Fadhil University of Babylon, Science College of Women, Physics Department. Abstract: Photocatalytic decolorization of methylene blue dye (MB) in water has been investig -ated in an immersed UV light irradiation slurry photoreactor using zinc oxide (ZnO) as a semiconductor photocatalyst. The effects of different parameters such as initial dye concentration, photocatalyst loading, and pH of solution on the decolorization rate of MB have been systematically investigated. A two-stage photocatalytic decolo -rization of MB, the first stage of fast decolorization rate and the subsequent second stage of rather slow decolorization rate, was found. The efficiency of decolorization of MB increased as initial MB concentration decreased. There was the optimal ZnO concentration being around 1500 mg L−1. The optimal pH was around 7 where reaction rate decreased above and under this pH value. Key words: methylene blue dye, zinc oxide powder. الخلصة الزالة اللونية المحفزة ضوئيا لصبغة المثيلين الزرق في الماء تم اختبارها بواسطة مفاعل ذو إشعاع فوق بنفسجي باستعمال اوكسيد الزنك كشبة موصل محفز ضوئيا. تأثير العوامل في العملية كتركيز الصبغة البتدائي ، وتركيز العامل المساعد ، والدالة الحامضية على الزالة اللونية تمت دراستها باستخدام هذا المفاعل .وجدت مرحلتين للزالة اللونية المحفزة لصبغة المثيلين الزرق ، الولى هي الزالة اللونية السريعة ويعقبها المرحلة الثانية نوعا ما تكون بطيئة. ان كفاءة الزالة اللونية للصبغة تزداد كلما قل التركيز البتدائي لها . ووجد أن أفضل تركيز لوكسيد الزنك هو 1500 ملغم لتر-1 . ووجد أن أفضل دالة حامضية حوالي 7 حيث تقل سرعة التفاعل فوق واقل من هذه القيمة . الكلمات المفتاحية: صبغة المثيلين الزرق , مسحوق اوكسيد الزنك . Introduction Photocatalysis using semiconductor oxides have been used to solve pollution problems, and its proves to be effective degradation of great number of contaminants (Bahnemann et al 2010) . The conventional technologies like primary (adsorption, flocculation), secondary (biological methods), and chemical processes (chlorination, ozonization) used to degrade the color of the dye contaminated water. Techniques like these are non-destructive, they only transfer non-biodegradable matter into sludge, giving rise to new type of pollution, which needs further treatment

(Bansal et al 2010) ZnO appears to be a suitable alternative to TiO2 since its photodegradation mechanism has been proven to be similar to that of TiO2 . ZnO has been reported, sometimes, to be more efficient than TiO2 (Daneshvar et al 2004). In heterogeneous photocatalysis system, when a suspension of a particular semiconductor is irradiated with a natural or artificial light, with an energy equals to or larger than the band gap. The electrons will be promoted to the conduction band (C.B.), leaving positive holes in the valence band (V.B.). If the electrons and holes are in a reaction, a steady state will be reached when the removal of electrons and holes equal the rate of generation by illumination (Hurum et al 2003, Zhang et al 2005, Fujishima and Honda 1972). In photocatalysis systems a combination of semiconductors (such as TiO2, ZnO, F2O3, CdS and ZnS) and UV or visible lights can

204 Journal of Babylon University/Pure and Applied Sciences/ No.(9)/ Vol.(22): 2014

be used. Upon irradiation, valence band electrons are promoted to the conduction band leaving a hole behind. These electron-hole pairs can either recombine or interact separately with other molecules. The holes at the valence band, having an oxidation potential of +2.6 V versus normal hydrogen electrode (NHE) at pH=7. These species can oxidize water or hydroxide to produce hydroxyl radicals (Behnajady et al 2006). Zinc Oxide is an excellent photocatalytic oxidation material. It has been widely used to deal with wastewater, such as pharmacy wastewater, printing and dyeing wastes, papermaking wastewater, and so on (Vora et al 2009). Experimental ZnO powder (Fluka Co.) and methylene blue dye powder (BDH) were used without any further purification. Experiments were conducted in a steeliness steel cylindrical reactor of 3.5 cm inside diameter and 25 cm height. The working volume was 130 ml at room temperature. The UV irradiation source was 5 Watt near-UV covered with Pyrex glass cylinder immersed in the photoreactor. The slurry magnetically stirred in a dark condition for 30 min to disperse fully and establish an adsorption/ desorption equilibrium. Photodecolorization was initiated by turning on the UV lamp. The samples were collected withdrawing syringe and centrifuged doubly (recommended). A UV–vis spectrophotometer (1200 Shimadzu Co., Japan) was used to determine the absorbance of methylene blue at a wavelength of 660 nm. With this method the decrease in the absorbance was directly proportional to the dye concentration reduction. Prior to the measurement,a calibration curve was obtained using standard methylene blue solution with the known concentrations. Distilled water was used for all solutions. Results and discussion Photodecolorization of methylene blue was carried out on ZnO clearly appears in the uv. vis. spectrum Fig 1. All the peaks are decreased during the degradation and the color disappeared. Typical data for methylene blue decolorization by ZnO photocatalyst with the initial methylene blue concentration of 10 mg L−1 at pH 7.7 and the ZnO concentration of 1500 mg L−1. In the presence of ZnO photocatalyst and UV light, the decolorization of methylene blue increased and most of methylene blue

Figure (1): uv. vis spectra of methylene blue for dark and light reaction at 25 min

degraded after 25 min of irradiation. Blank experiments were carried out with UV light alone to verify that the color removal of methylene blue was indeed due to

1.2 ZnO + uv 1 uv 0.8 o C

/ 0.6 C 0.4 0.2 0 0 5 10 15 20 25 30 Time min

Figure 2: Photocatalytic degradation and photolysis of methylene blue concentration with time. photocatalytic reaction. Since some dyes are degraded by direct UV irradiation, it should be examined to what extent methylene blue is photolyzed if no photocatalyst was used. In the absence of the catalyst, as shown in Fig. 2, methylene blue was photolyzed by direct UV radiation only up to 30% in 25 min. It is obvious that simultaneous utilization of UV irradiation with ZnO could increase the decolorization rate of methylene blue with efficiency of 98% in 25 min. The effect of the amount of ZnO on the photodegradation efficiency was shown in Fig. 3. Experiments performed with different concentrations of ZnO showed that the photodegradation efficiency increases with an increase in ZnO concentration up to 150 ppm, and is then decreased. This observation can be explained in terms of availability of active sites on the catalyst surface and the penetration of UV light into the suspension. The total active surface area increases with increasing catalyst dosage. At the same time, due to an increase in the turbidity of the suspension, there is a decrease in UV light penetration as a result of increased scattering effect and hence the photoactivated volume of suspension decreases (Shankar 2004) . Since the most effective decomposition of methylene blue was observed with 150 mg. of ZnO per 100 ml of dye solution.

0.15 n

i 0.1 m / 1

K 0.05

0 0 0.05 0.1 0.15 0.2 0.25 Wt of ZnO gm/dL

Figure 3: Effect of ZnO amount on the photodegradation of methylene blue

0.14 0.12 0.1 n i 0.08 m / 1 0.06 K 0.04 0.02 0 4 5 6 7 8 9 10 pH

Figure 4: Effect of pH on the photodegradation of methylene blue The effect of pH on the rate of photocatalytic degradation of methylene blue was investigated in pH range 5 to 9. The results are shown in Fig. 4. In the acidic region, when pH was raised, the rate constant value increased and at pH 7.7, the k value was highest. On still increasing pH in the alkaline region, the k value decreased. It seems that neutral species play an important role in the degradation process. It was observed that the products of photocatalytic degradation of methylene blue in presence of ZnO were colorless gases with virtually no solid residue left in the solution after almost complete degradation. The results showed that a direct influence the pH of the solution on the heterogeneous photocatalysis process (Daneshvar et al 2004, Vora et al 2009).

n 0.7 o i t 0.6 c n a i e r m 0.5

e 0.4 d K

r t o n

t 0.3 a s t r s i

f 0.2 n - o o c d 0.1 u e

s 0 p 2 4 6 8 10 conc. of methylene blue ppm

Figure 5: The relationship of pseudo-first order reaction constant with initial concentration of methylene blue When the initial concentration of organic pollutants is not high, the photocatalytic degradation or decolorization rate of the most organic compounds is described by the pseudo-first-order model: −dC/ dt= kC------1 Where k is pseudo-first-order decolorization or reaction rate constant and C is methylene blue concentration and the relationship shows in Fig. 5. It should be noted here that k represents the apparent decolorization rate constant. A two-stages photodecolorization of methylene blue was found. During the first stage of photodecolorization, methylene blue was decayed with fast decolorization rate, followed by the second stage which was characterized by a rather slow decolorization rate. As shown in Fig. 6 plots of (C/Co ) versus t in the first stage

gave straight lines. This indicates that in the first stage the majority of methylene blue was rapidly degraded by following pseudo-first-order kinetics, Eq. (1). Its intermediates might be generated in this stage. In the subsequent second stage, the intermediates which were competitive with parent dye molecules in the photocatalytic decolorization process were accumulated and as a result the decolorization rate decreased. It is clear from the figure 6 that in the second stage the experimental data deviated from the straight lines and the actual decolorization rates were rather slower than the first order reaction kinetics. The slow kinetics of methylene blue decolorization in the second stage of decolorization might be due to the difficulty in converting the N-atoms of the dye into oxidized nitrogen compounds (Sakthivel et al

Figure 6: The two stages of methylene blue photodecolorization by ZnO in different concentrations. The photocatalytic reaction occurs on the surface of the catalyst and therefore a good adsorption capacity is expected to favor the reaction kinetics. Adsorption-kinetic models, and specifically the Langmuir-Hinshelwood (L-H) model, are the most commonly applied to describe photocatalytic mineralization reactions. The L-H based kinetic models relate the rate of surface-catalyzed reactions to the surface covered by the substrate. According to the L-H model, the rate of a unimolecular surface reaction is proportional to the surface coverage.( Bahnemann et al 2010, Vora et al 2009). As the concentration of the reactants increases above a certain level however, the catalyst surface becomes saturated, and this may even lead to a decrease in the observed rates. In some cases a difficult degradable intermediates and their continued presence on the surface can also have a negative effect on the degradation rate of their parent compounds(Bahnemann et al 2010). The relationship between rate of reaction constant and initial concentration of methylene blue Co can be expressed as a linear equation represents langmuire – henshilhood relationship: 1/ kapp = 1/ k K + Co / k ------2 In Fig. 7 is shown a plot of 1/ kapp versus Co. The values of the adsorption equilibrium constant, K, and the rate constant, k, were obtained by linear regression of the points calculated by Eq. (2).

Figure 7: Langmuire – Henshilhood relationship of methylene blue concentration with kapp. Conclusion MB dye was poorly degraded in uv alone and a high degradation in presence of uv and ZnO.Weight of photocatalyst, pH of slurry and MB concentration were effective parameters on photocatalysis. Kinetic of reaction obeyed Langmuir- Hhanshilhood. References .Behnajady M.A ,N.Modirshahla andR.Hamzavi2006.Kinetic study on photocatalytic degradation ofC.I.Acid Yellow23by ZnO photocatalyst,JHazard.Mater.B, 133,. Hurum,D.C A.G.Agrios,and A.Kimberly2003.GrayExplaining the Enhanced Photocatalytic Activity of Degussa P25 Mixed-Phase TiO2 Using EPR,J.Phys. Chem.B,107,. Bansal P.,D.Singh,D.Sud(2010),Photocatalytic degradation of azo dye in aqueous TiO2 suspension: Reaction pathway and identification of intermediates products by LC/MS, Separation and Purification Technology, 72. Daneshvar N.,D.Salari,A.R.Khataee2004.Photocatalytic degradation of azo dye acid red14 in water on ZnO as an alternative catalyst to TiO2 J. Photochemistry and Photobiology A:Chemistry 162. Friedmann D.,C.Mendive,D.Bahnemann(2010).TiOD2for water treatment:Parameters affecting the kinetics and mechanisms of photocatalysis.Applied Catalysis B: Environmental 99. Fujishima, A. K.Honda(1972),Electrochemical photolysis of water at a semiconductor electrode. Natur,238. Gomes C. da Silva,J.Lu´ıs Fari (2003), Photochemical and photocatalytic degradation of an azo dye in aqueous solution by UV irradiation J. Photochem Photobio A: Chem 155 . Sakthive.S.2003,B.Neppolian,M.V.Shankar,B.Arabindoo,M.Palanichamy,V. Murugesan,Solar photocatalytic degradation of azo dye:comparison of photocatalytic efficiency of ZnO and TiO2,Solar Energy Mater. Solar Cells 77. Shankar,M.V.K.K.Cheralathan,B.Arabindoo,M.Palanichamy,V.Murugesan(2004), Enhanced photocatalytic activity for the destruction of monocrotophos pesticide by TiO2/H. J. Molecular Catalysis A: Chemical 223. Vora,S.KJ.J..Chauhan,K.C.Parmar,S.B.Vasava,S.Sharma and L.S.Bhutadiy (2009).Kinetic Study of Application of ZnOas a Photocatalyst in Heterogeneous Medium. E-Journal of Chemistry, 6(2),.

Zhang M.,G.Sheng,J.Fu,T.An,X.Wang,X.Hu(2005).Novel preparation of nanosized ZnO–SnO2with high photocatalytic activity by homogeneous co-precipitation method. Materials Letters 59