CC BY-NC-ND
0Journal of Saudi Chemical Society (2012) 16, 423-429
ORIGINAL ARTICLE
Photocatalytic decolorization of Brilliant Golden Yellow in TiO2 and ZnO suspensions
Md. Ahsan Habib a *, Iqbal Mohmmad Ibrahim Ismail c, Abu Jafar Mahmood a, Md. Rafique Ullah b
a Department of Chemistry, Faculty of Science, University of Dhaka, Dhaka 1000, Bangladesh b Department of Chemistry, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh c Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
Received 8 August 2010; accepted 16 February 2011 Available online 22 February 2011
KEYWORDS
Dye pollution; Brilliant Golden Yellow; Photocatalysts; Decolorization
Abstract Photocatalytic decolorization of BGY, an anionic dye, has been investigated in TiO2 and ZnO aqueous dispersions under UV-light irradiation. Spectrum of the dye has been found unaffected in the pH range 4.21-11.30. Adsorption is a prerequisite for the metal oxide-mediated photodegradation/photodecolorization and the extent of decolorization has been discussed in terms of the Langmuir-Hinshelwood model. Complete decolorization was achieved in case of UV irradiation whereas degradation of BGY was found to be about ca.75%. ZnO-mediated decolorization has appeared to be better and faster. The effects of various parameters, such as catalyst loading, pH and initial concentration of the dye on decolorization have been investigated.
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1. Introduction
From long time, environmental contamination has been considered as one of the greatest problems of modern society,
* Corresponding author. Tel.: +880 731926504; fax: +880 2 8615583.
E-mail address: mahabibbit@yahoo.com (Md. Ahsan Habib).
1319-6103 © 2011 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.
Peer review under responsibility of King Saud University. doi:10.1016/j.jscs.2011.02.013
mainly due to population explosion and increased industrial activities (Byrne et al., 1998; Ollis et al., 1991; Passos et al., 1994; Rivera et al., 1993; Zhang et al., 1998). In this state of affairs, the dyeing and textile industries are pointed out as they produce a large amount of effluents which can cause serious environmental problems as they contain colored compounds resulting from dyes unfixed to fibers during the dyeing process (Konstantinou and Albanis, 2004; Lachheb et al., 2002; Neppolian et al., 2002; Ozkan et al., 2004; Qamar et al., 2005). Wastewater from the dyeing/textile industries has been let into nearby water bodies like river, canals, ponds etc. without any treatment in the developing countries (e.g., Bangladesh, West Bengal (India), Sri Lanka etc.) that causes severe environmental problems. Dyes make up an abundant class of organic compounds characterized by the presence of unsaturated groups (chromophores), such as —C=C—, —N=N— and —C„N—, which are responsible for the dye color, and of functional
groups responsible for their fixation to fibers as for example, —NH2, -oh, -COOH and -SO3H (Molinari et al., 2004).
To de-pollute dye-contaminated wastewater, several methods have been investigated, including chemical oxidation, reduction, precipitation and flocculation, photolysis, adsorption, ion pair extraction, electrochemical treatment and advanced oxidation (Peng and Fan, 2005). Advanced oxidation is one of the most promising technologies for the removal of dye-contaminated wastewater due to its high efficiency. It is based mainly on the oxidative reactivity of hydroxyl free radicals (HO') generated by various methods, such as O3/UV, H2O2/UV, H2O2/Vis, O3/H2O2/UV photolysis, photoassisted Fe3+ or Fe2 + /H2O2, and TiO2-mediated photocatalysis processes. Since pollutants could be completely degraded to harmless compounds by photocatalysis under normal temperature and air pressure, it is predicted that photocatalysis will soon become one of the most effective means of dealing with various kinds of industrial wastewater (Chen and Cao, 2005). TiO2 is used as a photocatalyst because of its nontoxic-ity, photochemical stability and cost effective (Duxbury, 1993; Hoffman et al., 1995). Accordingly, the TiO2-mediated photo-catalysis process has been successfully used to degrade pollutants in recent years (Chen et al., 2002, 2006a,b; Dimitrijevic et al., 2005; Konstantinou and Albanis, 2004; Kyung et al., 2005; Mestankova et al., 2005; Tang and Chen, 2004). The initial step in TiO2-mediated photocatalysis involves the generation of an electron-hole pair (e"/h+), leading to the formation of hydroxyl radicals, superoxide radical anions (O2~), and hydroperoxyl radicals ('OOH) as shown below (Daneshvar et al., 2003; Da Silva and Faria, 2003; Dionysiou et al., 2000):
TiÜ2 + hv ! TiÜ2(h+ + e") (1)
H2O(ads) + h+ ! -OH + H+ (2)
OH" + h+ ! -OH (3)
O2 + e" ! -O" (4)
O2(ads)+ e" + H+! HO2 (5)
Organic pollutants are attacked and oxidized by the radicals formed through the above mechanisms. In addition to hy-droxyl radicals, superoxide radical anions and, in some cases, the positive holes are also suggested as possible oxidizing species that can attack organic contaminants present at/or near the surface of TiO2 (Zang and Farnood, 2005). Accordingly, much work has been done on photocatalytic degrada-
tion of common azo dyes in the presence of TiO2 (anatase) or Degussa P-25 TiO2, however, so far we know there is no report on the degradation of BGY in the presence of TiO2 or any other metal oxide semiconductor, which has been used as a common dye in textile/dyeing industries in Bangladesh, West Bengal (India) etc. In this paper, TiO2 (rutile) and ZnO have been used as photocatalysts in the decolorization of the BGY in aqueous suspensions of the metal oxides. Further, attempt has been made to compare the photocatalytic activities of the two metal oxides in decolorization of BGY as well.
2. Experiment
2.1. Materials
Brilliant Golden Yellow (BGY) as shown in Table 1, was purchased from BDH. The photocatalysts, TiO2 (rutile) (surface area 50 m2/g) and ZnO (surface area 100 m2/g) were purchased from Fluka. Stock solution containing 1 gL"1 of the BGY was prepared, protected from light by covering with aluminum foil and kept at room temperature. Reagent grade sodium hydroxide, hydrochloric acid and hydrogen peroxide were purchased from Merck. All the chemicals were used without further purification. Double distilled water was used throughout the experiment.
2.2. Procedure
An aqueous TiO2 dispersion was prepared by adding an requisite amount of TiO2 powder to a 50 mL solution containing the dye at an appropriate concentration. For reactions in different pH media, the initial pH of the suspensions was adjusted by the addition of either dilute aqueous HCl or NaOH. Prior to irradiation, the dispersions were magnetically stirred in the dark for about 20 min to insure the establishment of adsorption/desorption equilibrium. The zero time reading was obtained from blank solution kept in the dark but otherwise treated similarly as the irradiated solution. The suspensions were continuously purged with air throughout in each experiment. Irradiations were carried out using a low-pressure mercury lamp (PASCO scientific light source, Hg Light Source OS-286) of 125 W. Samples (5 mL) were collected before and at regular intervals during the irradiation. They were centrifuged before measuring absorption spectra. Similar batch wise experiments were carried for ZnO.
Table 1 Chemical structure of BGY.
Chemical structure
kmax (nm)
Brilliant Golden Yellow (BGY)
NaO3S-O-CH2CH2O2S
NHCOCH3
2.3. Analytical methods
The pH of the solution was measured using a pH meter (digital, Orion, Japan). Sample solutions were withdrawn at certain time intervals for spectrophotometric analysis. The decoloriza-tion of BGY was followed by a UV-visible spectrophotometer (Shimadzu 160 A, Japan) at 416 nm. Chemical structure and absorption maxima of the BGY are shown in Table. 1. Calibration plot based on Beer-Lambert's law was established by relating the absorbance to the concentration. The absorbance at 416 nm is due to the color of the dye solution used to monitor the decolorization of the dye. Mineralization was investigated by monitoring the evolving of CO2 gas (turned transparent lime water to milky white). Moreover, a peak was observed at 224 nm when water was used as the reference but this peak disappeared when nitrate solution was the reference indicating the formation of nitrate ion after degradation. All the experiments were performed at 29 0C.
3. Results and discussion
3.1. Adsorption isotherm
Adsorption of the dye pollutants on the semiconducting metal oxides surface is a significant parameter in heterogeneous pho-tocatalysis. The adsorption experiments have been carried out in order to evaluate the equilibrium constants of adsorption. The adsorption of the BGY on TiO2 and ZnO surfaces at various pH is shown in Fig. 1. Adsorption isotherm expresses the relationship between the mass of dye adsorbed per unit weight of the semiconductors and equilibrium concentration of the dye in solution. The zero point charge (zpc) of TiO2 (rutile) is 5.1 and of ZnO is 8.9, therefore, the surface of the TiO2 becomes positively charge at pH <5.1 and the ZnO surface becomes positively charged at pH < 8.9, on the other hand, if the pH of the suspended solution is higher than their respective zpc, surface of both the semiconductors becomes negatively charged. The extent of adsorption of the dye on the TiO2 and ZnO surfaces under different pH values are found in the following decreasing order: For TiO2, pH 5.5 > 3.5 > 7.5; for ZnO, pH 7.5 > 6.0 > 8.5 > 10.0. The average value of the dye adsorbed per unit weight of the photocatalysts is as follows: for TiO2, 7.1, 2.35 and 2.05 x 10~4mol/g at pH 5.5, 3.5 and 7.5, respectively; for ZnO, 7.4, 7.2, 6.7 and 2.2 x 10~4 mol/g at pH 7.5, 6.0, 8.5 and 10.0, respectively. As can be seen from Fig. 1, all the isotherms showed the L-shape (Giles et al., 1974). In such a case, it is assumed that there is no strong competition between the solvent and the dye molecules to occupy the TiO2 as well as the ZnO surface sites. The data obtained from the adsorption experiments were fitted to the linear form of the rearranged Langmuir equation as shown in Eq. (6).
Ce/qe = 1/Qob + Ce/Qo
where Ce is the concentration of the dye in solution at equilibrium, qe is the amount of dye adsorbed per unit weight of the photocatalyst at equilibrium, b is the Langmuir adsorption constant and Qo is the amount of dye adsorbed corresponding to the monolayer coverage.
The dimensionless separation factor (RL) as shown in Eq. (7) indicates the shape of the Langmuir isotherm to be
either favorable (0 < RL < 1), unfavorable (RL > 1), linear (Rl = 1) or irreversible (RL = 0).
Rl = 1/1 + bCo
where Co is the initial concentration.
Table 2 shows the Langmuir equilibrium constant and RL factor. The adsorption of BGY on TiO2 at pH 5.5 is fairly good and it decreases on both sides, on the other hand, that of BGY on ZnO was also fairly good at pH 7.5 but slightly decreases in the range of pH from 6.0 to 8.5. The adsorption of BGY on both the photocatalysts surfaces at different pH is influenced by the surface charge. This charge becomes positive when pH of the medium is less than the pHZPC of the photo-catalyst, but negative when the medium pH > pHZPC.
Although adsorption of BGY is higher at pH < 6, ZnO dissolves through photo-corrosion by UV irradiation under this condition. Lowering of pH (pH 3 or below) of the medium causes aggregation of TiO2 particles and reduces the adsorption capacity of the photocatalyst for the dye.
Table 2 Langmuir equilibrium constants and RL factor of
BGY adsorption on TiO2 and ZnO.
pH Ka (mol/L) Rl
TiO2 ZnO TiO2 ZnO
3.5 1.30 - 0.42 -
5.5 0.27 - 0.71 -
7.5 0.82 - 0.53 -
6.0 - 0.28 - 0.70
7.5 - 0.24 - 0.86
8.5 - 0.35 - 0.64
10.0 - 1.10 - 0.38
3.2. pH effect
Solution pH is an important parameter for metal oxides to act as photocatalyst, since it influences the surface charge properties of the photocatalysts (Zhu et al., 2005). Therefore, the decolorization of the dye, BGY, has been studied at different pH values (pH = 3.5-7.5 for TiO2 and pH = 6-10 for ZnO) as shown in Fig. 2. The results show that about 99% decolorization of BGY was observed at pH 5.5 and 7.5 in the presence of TiO2 (90 min) and ZnO (60 min), respectively. This is because the photocatalysts surface becomes positively charged at pH <5.1 for TiO2 and at pH < 8.9 for ZnO since the zero point charge (zpc) of TiO2 is 5.1 and that for ZnO is 8.9 (Zhu et al., 2005), making the electrostatic interaction between the dye anion and the catalyst surfaces predominant adsorption process. Consequently, enhanced adsorption leads to favorable decolorization of the dye (Figs. 1 and 2). On the other hand, the surface charges of the photocatalysts become negative at the medium pH greater than the respective pHzpc values and the decolorization efficiency decreases with solution pH as shown in Fig. 2. Protonation and deprotonation of the photo-catalysts surface by considering TiO2 as an example, can be shown as follows:
TiOH + H+ ! TiOHj in acidic condition TiOH + OH" ! TiO" + H2O in alkaline condition
3.3. Effect of initial dye concentration
The pollutant concentration is a very important parameter in wastewater treatment. The effects of initial dye concentrations
on the photocatalytic decolorization have been investigated from 5 to 60 mg/L. The results are shown in Fig. 3. It is found that the increase in the dye concentration decreases the decolorization efficiency. Comparable results have been reported for the photocatalytic oxidation of other dyes (Davis et al., 1994; Poulios and Aetopoulou, 1999; San et al., 2001). Increasing the dye concentration from 20 to 60 mg/L decreases the decolorization efficiency from 99% to 60% in 90 and 60 min in the presence of TiO2 and ZnO, respectively. The amount of the dye adsorbed onto the catalytic surface increases with increasing dye concentration. This affects the catalytic activity of the both catalysts. The increase in dye concentration also decreases the path-length of photon entering into the dye solution. At high dye concentration, a significant amount of UV-light may be absorbed by the dye molecules rather than the catalyst and this may also reduce the catalytic efficiency (Mills et al., 1993). The photocatalytic decolorization of the BGY in the presence of the both photocatalysts follows apparently pseudo-first order kinetics at low initial dye concentration and the rate expression is given by Eq. (8).
—dc/dt — kac
where ka is the apparent first order rate constant. The BGY is adsorbed onto TiO2 as well as ZnO surfaces and the adsorption-desorption equilibrium is reached in 90 and 60 min, respectively. Integration of Eq. (8) (with the limit C = Co at t = 0) gives
ln[Co/C] — kat
where Co is the initial concentration of BGY, [BGY]o, and C is its concentration at any time, t.
Figure 3 Effect of initial dye concentration on decolorization of BGY in the presence of (a) TiO2 and (b) ZnO. pH 5.5 (TiO2), 7.5 (ZnO); [Catalyst] = 1 g/L.
Table 3 Parameters k and K of Langmuir-Hinshelwood equation for photodecolorization of BGY.
Photo-catalysts K (L moP1) x 105 k (mol min ')
TiO2 1.28 1.19 x 103
ZnO 1.01 0.32 x 105
Langmuir-Hinshelwood (L-H) kinetic model has been used to analyze the heterogeneous photocatalytic reaction successfully by a group of researchers (Alaton and Balcioglu, 2001; Galindo et al., 2000; Wenhua et al., 2000). Generally the initial rate (ri) of photocatalytic decolorization of the dye in aqueous solution matches L-H kinetic model, thus, the initial rate equation can be expressed as follows:
r = kK[BGY]o/(1 + K[BGY|o) (10)
ri was determined by obtaining ka from ln(Co/C) vs t plot (Eq. (9)) and using the expression, ri = ka [BGY]o. K is the Langmuir-Hinshelwood constant. k is the proportionality constant suggesting a measure of the intrinsic reactivity of the photoactivated surface with BGY. k has been termed zero order surface reaction constant. Re-arranged form of Eq. (10) is
1/ri = 1/k + 1/(kK[BGYo]) (11)
k and K values are obtained by plotting of 1/ri vs 1/[BGY] (Fig. not shown). The values of k and K are shown in Table 2. The relatively strong bound BGY on TiO2 makes the progress of its photocatalytic decolorization comparatively slower Table 3.
3.4. Effect of catalysts concentration
Fig. 4 shows the effect of catalyst concentration ranging from 0.1 to 3.0 g/L on the decolorization of BGY in the presence of TiO2 and ZnO. The results indicated that the percent of decol-orization increases with increasing the amount of catalyst and are effective to decolorize the BGY. For economic removal of dye effluent from wastewater, it is necessary to find the optimum amount of catalyst for efficient degradation. Several authors have investigated the reaction rate as a function of catalyst dosage in photocatalytic oxidation process (Gouvea et al., 2000; San et al., 2001). As the concentration of the catalyst is increased from 0.1 to 1.0 g/L the initial rate of decolorization of dye increases sharply from 62% to 99% decolorization at 90 and 60 min irradiation time in the presence of TiO2 and ZnO, respectively. This is due to increase in the number of photocatalysts particles, which increase the adsorption of the number of BGY molecules. An increase in the catalyst dosage from 1.0 to 3.0 g/L makes the initial rate remain almost constant. Increasing the catalyst dosage beyond 1.0 g/L may cause light scattering and screening effects. These reduce the specific activity of the catalyst (Lea and Adesina, 1998). At high concentrations of catalysts, particle aggregation may also reduce the catalytic activity. The optimum amount of catalyst dosage is found to be 1 g/L for the decolorization of BGY.
3.5. Decolorization and degradation
Dyes or pigments exhibit color and this color is due to the presence of one or more chromophoric groups in the dye
50 100
Irradiation time/mill
Figure 4 Effect of catalysts concentration on decolorization of BGY in the presence of (a) TiO2 and (b) ZnO. [BGY] = 20 g/L; pH 5.5 (TiO2), 7.5 (ZnO).
molecules. The present dye, BGY, contains an azo (—N=N—) and —C=C— groups as chromophores which are responsible for the color of the dye. In photodecolorization or photodegradation process (e.g., Fenton- or photoFenton or photocata-lytic processes), the generated reactive oxygen species (ROS) such as hydroxyl free radicals or superoxide oxygen anion radicals attack first the azo group resulting in the decolorization of the dye which is indicated as a decrease in the absorbance at 416 nm (absorption maxima of BGY). In comparison, absorbance at 224 nm also decreases with time but with a lower rate. This indicates that decolorization occurs with higher rate than that of degradation. As can be seen from Fig. 5, the decolorization of BGY observed is 99% in presence of ZnO, while degradation is only 75% in the presence of TiO2. The peak at 224 nm is shifted to lower wavelength (ca. 212 nm) with a considerable intensity indicating the degradation of BGY (i.e., degradation of benzene or naphthalene rings to CO2, nitrate, sulfate and other smaller ionic species).
3.6. Effect of electron acceptors
In heterogeneous photocatalytic reactions, electron-hole recombination is considered as a major energy wasting step which leads to low quantum yield. Consequently, the prevention of electron-hole recombination becomes very important. This can be achieved by adding proper electron donor (or) acceptor to the system. Generally molecular oxygen is used as an electron acceptor in heterogeneous photocatalyzed reaction. Besides the addition of molecular oxygen, the electron-hole recombination can be reduced by the addition of
Figure 5 Decolorization pattern of BGY in the presence of ZnO (1 g/L). [BGY] = 20 g/L; pH 7.5.
irreversible electron acceptors such as H2O2, (NH4)2S2O8, and KBrO3. The addition of these electron acceptors enhanced the decolorization as well as the degradation rate by several ways. These are as follows: (i) preventing the electron-hole recombination by accepting the conduction band electron, (ii) increasing the hydroxyl radical concentration, and (iii) generating other oxidizing species e.g., SO4 , to accelerate the intermediate compound oxidation rate. It has been found that addition of requisite amount of the electron acceptors in photocatalytic reactions enhance the decolorization efficiency but addition of excess inhibits the decolorization as well as the degradation of the BGY (data not shown). The enhancement of decoloriza-tion as well as degradation by addition of H2O2 is due to increase in the hydroxyl radical concentration but at high concentration of H2O2, it acts as scavenger for hydroxyl free radical resulting in the decrease of the decolorization as well as the degradation rate. Similarly, addition of persulfate ion in heterogeneous photocatalytic reactions enhances the decol-orization of BGY as well through formation of sulfate radicals which generates hydroxyl free radicals but excess amount of persulfate inhibits photocatalytic activities of the TiO2 and ZnO through adsorption of sulfate ions on the catalysts surface. The addition of BrO^ ion in the photocatalytic reactions also increases the decolorization efficiency of the BGY in different mechanism from H2O2 or persulfate. Here, the BrO^ ion reacts with conduction band electron, therefore, the decol-orization rate of the BGY increases.
4. Conclusion
The azo-dye, BGY is easily decolorized by the TiO2 and ZnO assisted photocatalysis in aqueous dispersion under irradiation by UV light (predominantly 320 nm). The dye is resistant to direct photolysis. The adsorption of BGY on both the metal oxide semiconductors follows the Langmuir pattern. The adsorption is maximum at pH 5.5 for TiO2 and at 7.5 for ZnO. For optimum decolorization, the suitable initial pH of
20 mg/L BGY solution at 1.0 g/L photocatalyst load at 29 0C seems to be 5.5 for TiO2 and 7.5 for ZnO. The initial photocatalytic decolorization follows apparent first order kinetics. The initial decolorization rates fit the Langmuir-Hinshelwood model up to 20 mg/L BGY solution. The electron acceptors like H2O2, persulfate ion or BrO^ ion enhance the decolorization rate of the BGY. The evolution of CO2 during the decolorization process, which turns transparent lime water to white milky, indicates the degradation of BGY as well. The presence of nitrate ions after decolorization also suggests the degradation of the BGY. According to the Langmiur-Hinshelwood kinetic model, it is concluded that ZnO acts as a better photocatalysts to decolorize/degrade the BGY under the present experimental conditions.
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