Chemical Engineering Communications PHOTOCATALYTIC

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Chemical Engineering Communications

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PHOTOCATALYTIC DEGRADATION OF PATENT BLUE V BY SUPPORTED TiO2: KINETICS, MINERALIZATION, AND REACTION PATHWAY

N. Barkaa; S. Qourzalb; A. Assabbaneb; A. Nounahc; Y. Ait-Ichoub a Faculté Polydisciplinaire de Khouribga, Université Hassan 1er, Khouribga, Morocco b Faculté des Sciences d'Agadir, Département de Chimie, Equipe de Matériaux, Photocatalyse et Environnement, Agadir, Morocco c Laboratoire des Sciences de l'Environnement, Ecole Supérieure de Technologie de Salé, Salé-Médina, Morocco Online publication date: 09 June 2011 To cite this Article Barka, N. , Qourzal, S. , Assabbane, A. , Nounah, A. and Ait-Ichou, Y.(2011) 'PHOTOCATALYTIC

DEGRADATION OF PATENT BLUE V BY SUPPORTED TiO2: KINETICS, MINERALIZATION, AND REACTION PATHWAY', Chemical Engineering Communications, 198: 10, 1233 — 1243 To link to this Article: DOI: 10.1080/00986445.2010.525206 URL: http://dx.doi.org/10.1080/00986445.2010.525206

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Chem. Eng. Comm., 198:1233–1243, 2011 Copyright # Taylor & Francis Group, LLC ISSN: 0098-6445 print=1563-5201 online DOI: 10.1080/00986445.2010.525206

Photocatalytic Degradation of Patent Blue V by Supported TiO2: Kinetics, Mineralization, and Reaction Pathway N. BARKA,1 S. QOURZAL,2 A. ASSABBANE,2 A. NOUNAH,3 AND Y. AIT-ICHOU2

Downloaded By: [Barka] At: 11:17 9 June 2011

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Faculte´ Polydisciplinaire de Khouribga, Universite´ Hassan 1er, Khouribga, Morocco 2 Faculte´ des Sciences d’Agadir, De´partement de Chimie, Equipe de Mate´riaux, Photocatalyse et Environnement, Agadir, Morocco 3 Laboratoire des Sciences de l’Environnement, Ecole Supe´rieure de Technologie de Sale´, Sale´-Me´dina, Morocco The photocatalytic degradation of patent blue V (sodium salt of [4-(a-(4-diethylaminophenyl)-5-hydroxy-2, 4-disulfophenyl-methylidene)-2, 5-cyclohexadien-1ylidene] diethylammonium hydroxide) in aqueous solutions has been investigated by using TiO2-coated nonwoven fibers as the photocatalyst. Before the study began, adsorption in the dark was done, and the effect of the initial concentration of dye in the solution was determined. The mineralization was monitored by chemical oxygen demand (COD), and the HPLC-MS method was used to identify reaction intermediates. The experimental results show that adsorption is an important parameter, controlling the apparent kinetic constant of degradation. The rate of fading was favored by a high concentration of dye in the solution with respect to the LangmuirHinshelwood model. The COD abatement was slower than the discoloration of the solution; this indicates that the dye was not directly mineralized, but transformed to intermediate photoproducts. These intermediate photoproducts lead to other cycles of degradation, to the point of total mineralization. Several by-products were detected and identified by HPLC-MS. On the basis of these findings, we propose a probable degradation pathway as critical for assessing the suitability of detoxification procedures for the degradation of particular contaminant classes. Keywords Mineralization; Patent blue V; Photocatalysis; Reaction pathway; Supported TiO2

Introduction Triphenylmethane dyes are used extensively in the textile industry for dyeing nylon, wool, cotton, and silk, as well as for coloring oil, fats, waxes, varnishes, and plastics. The paper, leather, cosmetic, and food industries are also major consumers of various types of triphenylmethane dyes (Zollinger, 1991; Feller, 1994). Additionally, these dyes are used as staining agents in bacteriological and histopathological applications. The photocytotoxicity of triphenylmethane dyes resulting from the Address correspondence to N. Barka, Faculte´ Polydisciplinaire de Khouribga, Universite´ Hassan 1er, BP: 145, Hay Ezzaitoune, Khouribga, Morocco. E-mail: [email protected]

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production of reactive oxygen species has been tested intensively with regard to their photodynamic therapy (Baptista and Indig, 1998; Kowaltowski et al., 1999; Bonnett and Martinez, 2001; Bhasikuttan et al., 2002). In the dyeing section of the textile industry, 10 to 20% of dyes is lost to wastewater as a result of inefficiency in the dyeing process. The disposal of dyes into receiving waters causes several kinds of damage to the environment because of their refractory carcinogenic nature (Reife, 1993). To purify dyeing wastewater, a number of methods, including chemical oxidation and reduction, chemical precipitation and flocculation, photolysis, adsorption, ion pair extraction, electrochemical treatment, and advanced oxidation have been investigated. Photocatalytic treatment is an attractive alternative for removing soluble organic compounds. Unlike physical processes such as coagulation and adsorption, which merely transfer the pollutants from wastewater to other media and cause secondary pollution, photocatalytic treatment can completely mineralize organic compounds to form carbon dioxide, water, and mineral acids. Moreover, photocatalysis does not require expensive oxidants and can be carried out at mild temperatures and pressures. However, the need to separate the small TiO2 particles from the suspension after treatment limits use of the process. Alternatively, the catalyst can be immobilized onto a suitable solid inert material. This eliminates the step of removing the catalyst (Fernandez et al., 1995; Lachheb et al., 2002; Guillard et al., 2003) and permits the reuse of the photocatalyst several times. The purpose of this work is twofold: (i) to investigate the factors that influence the photocatalytic discoloration and mineralization of the most useful dye of the triphenylmethane class, patent blue V (C27H31N2NaO7S2), by using TiO2-coated nonwoven fibers, and (ii) to determine the main by-products by the HPLC-MS method and then propose a pathway for degradation.

Materials and Methods Materials The photocatalyst used in this study is a commercial product from Ahlstrom (France). It consists of nonwoven natural and synthetic fibers, 254 mm thick, coated with a mixture of TiO2 (PC500 Titania, supplied by Millennium Inorganic Chemicals), SiO2 (EP1069950B1 European patent), and zeolite (UOP, 2000 m2 g1). After washing, the surface load of the photocatalyst was 18 g m2 of PC500, 20 g m2 of SiO2, and 2 g m2 of zeolite. The patent blue V was obtained from Clariant (color index: 42051) and used as received without further purification. Its molecular structure is shown in Figure 1. Solutions were prepared by dissolving the requisite quantity of the dye in distilled water. Photocatalytic Reactor Photocatalytic experiments were carried out in a cylindrical batch reactor, 8 cm in diameter and 12 cm in working height. The water jacket has a diameter of 5 cm, contains a UV lamp, and permits water circulation (Figure 2). The photocatalytic reactor was covered inside with (11 cm 25 cm) of the photocatalyst and was exposed to a luminous source (an HPK 125 W Philips ultraviolet lamp with a

Photocatalytic Degradation of Patent Blue V

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Figure 1. Molecular structure of patent blue V.


wavelength maximum of 365 nm), placed in axial position inside the water jacket. The reactor was stirred continuously at a low setting, 100 rpm, by a magnetic stirrer. Procedure and Analysis Adsorption experiments were carried out by immersing 11 cm2 of the photocatalyst in 20 mL of the dye solutions for one hour. The adsorption isotherm was obtained for each of several different initial dye concentrations, from 4 to 20 mg L1. The adsorbed quantity was calculated by measuring the concentration of the solution before and after adsorption, using the following equation: qe ¼

ðC0 Ce Þ S

ð1Þ

where qe (mg m2) is the quantity of patent blue V adsorbed at equilibrium per unit surface of the photocatalyst, C0 (mg L1) is the initial concentration of dye in the solution, Ce (mg L1) is the concentration of dye at equilibrium, and S (m2 L1) is the ratio of the surface of the photocatalyst per liter of aqueous solution. Photocatalytic degradation experiments were carried out by loading 500 mL of dye solutions in the photocatalytic reactor. The effect of initial concentration was obtained with different initial dye concentrations, from 4 to 20 mg L1. All photocatalytic experiments were carried out after one hour of adsorption in the dark.

Figure 2. The photocatalytic converter.


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The dye aqueous solutions were filtered by use of a Millipore membrane filter, type 0.45 mm HA, and the concentrations were determined from the UV-vis absorbance characteristic by the calibration curve method. A Jenway 6405 UV= visible spectrophotometer was used. The maximum absorption wavelength (kmax) was 635 nm. Chemical oxygen demand (COD) was determined by using the acidic dichromate micro-method with a Hach DRB 200 analyzer. The organic intermediates were analyzed by the high-pressure liquid chromatography-mass spectrometry (HPLC-MS) method. The chromatographic conditions were adjusted to make the mobile phase compatible with the working conditions of the mass spectrometer. Solvent A was 25 mM aqueous ammonium acetate buffer (pH 6.9), and solvent B was methanol. HPLC was carried out on a Hypersil BDS C18 column (150 mm 4.6 mm, dp ¼ 5 mm). The mobile phase flow rate was 1 mL min1. A linear gradient was run as follows: t ¼ 0, A ¼ 95, B ¼ 5; t ¼ 20, A ¼ 50, B ¼ 50; t ¼ 35–40, A ¼ 10, B ¼ 90; t ¼ 45, A ¼ 95, B ¼ 5. The column effluent was introduced into the electrospray ionization (ESI) source of the mass spectrometer. A Thermo Electron (LC Surveyor) HPLC=MS system equipped with a photodiode array detector was used.

Results and Discussion Adsorption Study in the Dark Adsorption is important in determining the rate of photocatalytic degradation of organic molecules. The dye adsorbed on the surface of the semiconductor particles acts as an electron donor, injecting electrons from the excited state of the semiconductor under UV irradiation to its conduction band. Figure 3 shows an isotherm of L-shape according to the classification of Giles et al. (1974). The L-shape means that there is no strong competition between the solvent and the adsorbate to occupy the

Figure 3. Adsorption isotherm of patent blue V on the photocatalyst.


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adsorbent surface sites. The Langmuir equation (Equation (2)) was used to fit the experimental data: qe KCe ¼ qm 1 þ KCe

ð2Þ

where qm (mg m2) is the maximal monolayer adsorption capacity per unit surface of the photocatalyst, qe (mg m2) is the amount of the dye adsorbed per unit surface of the photocatalyst at equilibrium, K (L mg1) is the Langmuir equilibrium constant, and Ce (mg L1) is the concentration of the dye in aqueous solution at equilibrium. The linear transformation of Equation (2) can be expressed by the following equation:


1 1 1 1 ¼ þ : qe qm K qm C e

ð3Þ

The ordinate at the origin is equal to the reciprocal of qm, whereas K can be calculated from the slope (1=qm K). From the data obtained, the maximal adsorption quantity is 13.37 mg m2, and the Langmuir adsorption constant is 0.129 L mg1. Kinetics of Degradation as a Function of Initial Concentration The kinetics of the photocatalytic degradation of patent blue V at different initial concentrations is plotted in Figure 4. It is evident that the degradation rate depends on the initial concentration of the dye. Since hydroxyl radicals have a very short lifetime (only a few nanoseconds), they can react only at or near the location where they are formed. A high dye concentration increases the probability of collision between organic matter and oxidizing species, leading to an increase in the discoloration rate.

Figure 4. Kinetics of photocatalytic degradation of patent blue V for different intitial concentrations.

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According to numerous works (Galindo et al., 2001; Bouzaida et al., 2004; Bizani et al., 2006; Sleiman et al., 2007; Barka et al., 2008), the kinetics of the photocatalytic degradation rate of most organic compounds is described by pseudo-first kinetic order according to the linear form: C0 ln ¼ kap t C

ð4Þ


where kap (h1) is the apparent rate constant of degradation. The values of ln(C0=C) versus t with different initial concentrations are plotted in Figure 5. The figure shows that the photocatalytic degradation follows perfectly the pseudo-first kinetic order with respect to patent blue V concentration. The effect of initial dye concentrations on the initial rate of degradation is shown in Figure 6. The figure shows that the rate of discoloration increases with increasing initial concentration of patent blue V. This corresponds to the LangmuirHinshelwood model in accordance with the following equation: r0 ¼ kap C0 ¼

kc KC0 1 þ KC0

ð5Þ

where r0 (mg L1 h1) is the initial rate of the photocatalytic degradation, kap (h1) is the apparent rate constant, K (L mg1) is the adsorption equilibrium constant, and kc is a constant depending on the other factors influencing the process. A linear expression can be obtained conventionally by plotting the reciprocal initial rate constant against the initial concentration: 1 1 ¼ kap kc

C0 þ

1 kc K

ð6Þ

The plot of 1=kap against C0, shown in Figure 7, gives a linear relation. For the values of the slope (1=kc) and the intercept (1= kc K), the values for the photocatalytic degradation of patent blue were 8.74 mg L1 h1 for kc and 0.066 L mg1 for K.

Figure 5. Plot of ln(C0=C) vs. t with different initial concentrations.


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Figure 6. Effect of initial patent blue V concentration on initial rate of degradation.

The constant of adsorption calculated for the Langmuir-Hinshelwood model was found to be different from that for the adsorption isotherm. This difference may be due to the fact that, in the photocatalytic kinetic model, the adsorption constant depends on the reactive sites on the TiO2 surface, and the degradation level is high when the number of reactive sites is greater. However, not only the dye compound molecules but also their degradation products were adsorbed on the TiO2 surface; therefore, the number of photoreactive sites on the catalyst decreased. As a result, the adsorption constant in the kinetic model was lower than that calculated for adsorption in the dark. Kinetics of COD Disappearance To investigate the efficiency of photocatalytic treatment in mineralizing patent blue V, the kinetics of the removal of the chemical oxygen demand (COD) was observed

Figure 7. Reciprocal of apparent rate of degradation against initial concentration.

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as a function of irradiation time. The results are shown in Figure 8. Interestingly, the COD decreases more slowly than the discoloration of the solution. Initially, COD decreases linearly as a function of the irradiation time. The reason is that the dye was not directly mineralized, but transformed in the intermediate photoproducts. This may cause other cycles of degradation to complete the mineralization. After total discoloration of the patent blue V solution, obtained after 10 h of UV irradiation, about 30% of the COD remains present. This means that discoloration can provide an aesthetic aspect to water, but falls far short of purifying and detoxifying it. Total mineralization occurs after more than 20 h of UV irradiation.


Identification of Intermediate Products and Reaction Pathway After 3 h of irradiation, we stopped the reaction to identify intermediate products by the HPLC-MS method. The chromatograms in Figure 9 show a photocatalytic degradation of patent blue V. Under the experimental conditions used, patent blue has a retention time of 25.96 min. The main products were observed at retention times of 2.33, 5.48, 11.24, 12.80, 16.38, 18.00, 31.18, 35.91, and 48.00 min. The products identified in HPLC=MS present peaks at 94, 107, 257, 278, 300, 344, 390, 417, and 474 m=z. These could be assigned to the chemical structures I, H, G, E, F, C, D, B, and A, as shown in Figure 10. On the basis of these results, we propose a general reaction pathway of the photocatalytic degradation of patent blue, as presented in Figure 10. This reaction pathway has several stages: hydroxylation, desulfonation, deamination, desalkylation, and deshydroxylation. The great number of compounds detected during the degradation of patent blue V shows the complexity of the photocatalytic process and suggests the existence of various degradation routes, resulting in multistep and interconnected pathways. Cost-effective treatments to mineralize the compounds completely are usually not practicable, and the presence of by-products during and at the end of water treatment appears to be unavoidable. Therefore, the evaluation of by-products is the key to optimizing each treatment and maximizing the overall process. The detection

Figure 8. Kinetics of COD removal.


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Figure 9. HPLC chromatogram of patent blue V after 3 h of irradiation.

Figure 10. Proposed reaction pathway of the photocatalytic degradation of patent blue V.

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of any highly toxic compounds, even at low concentrations, is essential for the assessment of treated water.

Conclusion


A method for the photocatalytic degradation of patent blue V has been successfully developed, with TiO2-coated nonwoven fibers as the photocatalyst. A correlation between the adsorption of the dye onto the photocatalyst and the rate of photocatalytic degradation has been found. The photocatalytic degradation was obviously affected by the initial concentration in accordance with the Langmuir-Hinshelwood model. The chemical oxygen demand decreases more slowly than the discoloration of the solution. This indicates that the discoloration improves the appearance of water, but falls far short of purifying and totally detoxifying it. The great number of compounds detected and identified by HPLC-MS during the photocatalytic degradation shows the complexity of the photocatalytic process and suggests the existence of various degradation routes, resulting in multistep and interconnected pathways.

Acknowledgments The authors would like to gratefully thank Mr. J. Dussaud from Ahlstrom Research & Services (France) for generously supplying the photocatalyst used in the work reported here.

Nomenclature C0 Ce COD HPLC-MS kap K qe qm r0 S UV kmax

initial dye concentration in solution, mg L1 dye concentration at equilibrium, mg L1 chemical oxygen demand, mg L1 high-pressure liquid chromatography coupled mass spectroscopy apparent rate constant, h1 Langmuir equilibrium constant, L mg1 quantity of dye adsorbed at equilibrium, mg m2 maximum adsorption capacity, mg m2 initial rate of photocatalytic degradation, mg L1 h1 surface of the photocatalyst per liter of aqueous solution, m2 L1 ultraviolet irradiation maximum absorption wavelength, nm

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