Article pubs.acs.org/IECR
Heat Attachment Method for the Immobilization of TiO2 on Glass Plates: Application to Photodegradation of Basic Yellow Dye and Optimization of Operating Parameters, Using Response Surface Methodology L. Yahia Cherif,*,† I. Yahiaoui,† F. Aissani-Benissad,† K. Madi,† N. Benmehdi,† F. Fourcade,‡,§ and A. Amrane‡,§ †
Laboratoire de Génie de l’Environnement (LGE), Faculté de Technologie, Université de Bejaia, 06000 Bejaia, Algeria Ecole Nationale Supérieure de Chimie de Rennes, Université Rennes1, CNRS, UMR 6226, Avenue du Général Leclerc, CS50837 , 35708 Rennes Cedex 7, France § Université Européenne de Bretagne, 35000 Rennes, France ‡
S Supporting Information *
ABSTRACT: The UV/TiO2/H2O2 degradation of Basic Yellow 28 dye (BY28) in aqueous solutions was investigated with immobilized P-25 TiO2 powder on a glass plate by a heat attachment method. A central composite design (CCD) was employed for the screening of the significant parameters (flow rate, initial dye concentration, solution pH, and initial H2O2 concentration) and to identify the most relevant interactions between the operating parameters. Results showed that solvent type and thickness of the coating are very effective on the photoactivity of immobilized TiO2. The model equation obtained led to a classification of the parameters based on their level of significance. In addition, a relevant interaction between the initial dye concentration and the initial H2O2 concentration was highlighted. After performing a screening of the various operating parameters, response surface analysis led to the optimal conditions for the yield of BY28 degradation, resulting in 96% decolorization yield. nontoxic, and photochemically stable.13 However, TiO2 can be used in suspension, with two drawbacks: the scattering of UV light by particles and the difficulty involved in recycling the photocatalyst. Thus, TiO2 can be immobilized on a glass plate to solve these problems.14 The process can easily decolorize and considerably reduce the organic load of textile wastewaters and similar effluents. Moreover, according to Garcia et al., among processes such as UV/TiO2, UV/H2O2, UV/TiO2/H2O2, and UV/Fe2+/H2O2, the association of TiO2 and H2O2 was the most efficient treatment for removing organic compounds from textile effluents.15 Response surface methodology (RSM) is widely accepted in manufacturing industry to improve product performance and reliability, as well as process capability and yield. In the statistical design of experiments, the factors involved in a given experiment and their respective levels can be simultaneously varied. Much information can be collected, and the design of experiments is a useful tool to study the interactions between two or more variables at a reduced number of experimental trials. Many studies have reported the effectiveness of RSM for the modeling and the optimization of processes. Using RSM, Yahiaoui et al.16−18 optimized the degradation of tetracycline and dyes (Basic Yellow 28 and Methylene Blue) by an electrochemical process coupled to a biological treatment. The electrochemical
1. INTRODUCTION Wastewaters generated by textile industries are known to contain considerable amounts of toxic aromatic dyes.1,2 The colored wastewater released into the ecosystem is a dramatic source of pollution and perturbation in the aquatic life.3 Because of the high stability of textile dyes and their toxicity, conventional biological processes involving activated sludge do not allow complete removal. Physical processes are nondestructive, since they only consist of a transfer of the pollution to another phase, and hence pollution post-treatment is then necessary to perform degradation.4,5 Moreover, regeneration of the adsorbent materials and post-treatment of solid wastes, which are expensive operations, are needed. Recently, there has been a considerable interest in the utilization of advanced oxidation processes (AOPs) to remove organic compounds. AOPs are based on the production of hydroxyl radicals as oxidizing agent to mineralize organic compounds.6 Many efforts have been directed to the photodegradation of dyes by UV irradiation systems. Heterogeneous photocatalysis is a process in which the illumination of a semiconductor produces photoexcited electrons (e−) and positively charged holes (h+).7 The photoexcitation of the semiconductor particles by UV light changes the energy state of the electrons from the valence band of the solid to the conduction band. In heterogeneous photocatalysis, TiO2 is extensively used as a photocatalyst8 with advantages such as a lack of mass-transfer limitations, ambient operating conditions, and the possible use of solar irradiation.9−12 The catalyst itself is inexpensive, commercially available in various crystalline forms and particle characteristics, © 2014 American Chemical Society
Received: Revised: Accepted: Published: 3813
November 26, 2013 February 13, 2014 February 14, 2014 February 27, 2014 dx.doi.org/10.1021/ie403970m | Ind. Eng. Chem. Res. 2014, 53, 3813−3819
Industrial & Engineering Chemistry Research
Article
oxidation of tetracycline and an herbicide, atrazine, in aqueous medium has been studied using a central composite design by Jie et al.19 and Zaviska et al.,20 respectively. The main objective of this work was, on the one hand, to increase the total surface of the catalyst and, on the other hand, to investigate the individual and the interactive effects of four operating parameters, namely, the flow rate (Qv), the initial dye concentration ([BY28] 0 ), the pH and the initial H 2 O 2 concentration ([H2O2]0), on the yield of degradation of Basic Yellow 28 in the presence of immobilized TiO2 P-25 on glass plate using RSM, according to a central composite design (CCD).
2. METHODOLOGY 2.1. Chemicals and Materials. All reagents and materials used in this study were of analytical grade and distilled water was used to prepare the synthetic dye solution. Basic Yellow 28 (BY28), which is a cationic dye, was supplied by Textile Factory (Alfaditex Remila, Bejaia, Algeria). Its chemical structure is given in Figure 1. The photocatalyst used was a Degussa P-25 (99%), Figure 2. Scheme of the heat attachment method for immobilization of TiO2 P-25 on glass plate.14
Figure 1. Chemical structure of Basic Yellow 28 (BY28).
anatase/rutile ca. 80/20 with a surface area of 55 ± 15 m2/g. It was purchased from BIOCHEM (Montreal, Canada). Absolute ethanol, HNO3 (52.5%) and HF (40%) were purchased from PROLABO (Paris, France). H2SO4 (98%), NaOH (98%), and H2O2 (30%) were purchased from BIOCHEM (Montreal, Canada). 2.2. Immobilization of TiO2 P-25 on Glass Plate. A heat attachment method was used to immobilize TiO2 P-25 on glass plate (9 cm × 35 cm). This material was of soda-lime type, the most common form of produced glass. Soda-lime glass is inexpensive and chemically stable, but it is sensitive to thermal shock, with a softening temperature of 700 °C. The glass plate was pretreated with HF solution (0.1 M) overnight and washed with NaOH solution (0.01 M) for 2 h in order to increase the number of OH groups14 and to ensure a good fixing of TiO2 P-25 on glass plate. Thirty milliliters (30 mL) of suspensions containing 4 g/L TiO2 P-25 were prepared using water or ethanol as solvents. The suspension was sonicated in an ultrasonic bath (J. P. SELECTTA, Barcelona, Spain) at a frequency of 30 kHz for 30 min, in order to improve the dispersion of TiO2 P-25 in the solvent. The sonicated suspension was poured on a glass plate and then placed in a drying oven at 30 °C. After drying, the glass plate was fired at 475 °C for 60 min and washed with distilled water for the removal of weakly attached TiO2 P-25 particles. This thermal treatment ensured the removal of the organic (ethanol solvent) load and facilitated interconnection (sintering) of titanium dioxide nanoparticles.21 Deposition process was carried out 2−4 times to increase the amount of TiO2 P-25 loaded on the surface of the glass plate (see Figure 2). 2.3. Experimental Setup and Procedure. The homemade photocatalytic reactor is shown in Figure 3. The installation prototype is mainly composed of a glass plate loaded with TiO2
Figure 3. Experimental device.
P-25 (denoted as “1” in the figure), one UV lamp (2) (30 W, UVA, λmax = 360 nm, manufactured by Philips, Netherlands) located 15 cm above the catalyst, a centrifugal pump (3), and a vessel (4) fitted with a Pyrex glass jacket (5) and connected to a thermostatted bath (6). A stirrer with four blades in stainless steel (7) was used for the homogenization, and an air pump (8) was used to provide the solution with dissolved oxygen required for the experiment. The UV lamp was lighted after 15 min of contact between the treated solution and a glass plate loaded with TiO2 P-25 to ensure that equilibrium was attained between the dye in the solution and on the sorbent surface; experiments were monitored for 8 h at a temperature of 25 °C. The sample of the treated solution was collected at the end of the experiment. The concentration of BY28 in the aqueous solution was spectrophotometrically determined at the maximum absorption wavelength (412 nm) using an ultraviolet-visible light (UV−vis) system (A SAFAS SP2000, Monaco, Principality of Monaco) and calibration curve. pH measurements were made using a HANNA Model 211R pH meter. 3814
dx.doi.org/10.1021/ie403970m | Ind. Eng. Chem. Res. 2014, 53, 3813−3819
Industrial & Engineering Chemistry Research
Article
3. RESULTS AND DISCUSSION 3.1. Optimization of the Heat Attachment Method for TiO2 P25 Immobilization on a Glass Plate. 3.1.1. Effect of the Solvent. To investigate the effect of the solvent on the photoactivity of the immobilized TiO2, two glass plates with three repetitions of the procedure of immobilizations (4 g/L of TiO2 slurry), at 475 °C were prepared using water or ethanol as solvents. The yields of BY28 degradation were 69.6% and 83.7%, respectively, for water and ethanol (see Figure 4a). The high
Figure 5. SEM micrographs of immobilized TiO2 on glass plates prepared with (a) two, (b) three, and (c) four immobilizations.
repetitions. Figure 5a indicates that the second coat did not cover the entire surface; however, an additional coat was needed to complete coverage, with small cracks and aggregates (see Figure 5b). As shown in Figure 5c, the surface of four coats of TiO2 revealed microfractures, which are probably due to the different thermal expansions among the various coats induced by the thermal treatment. 3.1.3. Effect of Adsorption and Photolysis. pH is an important operational variable in practical wastewater. It is wellknown that it influences the rate of photocatalytic degradation of some organic compounds, because, at the same time, it acts on the surface state of TiO2 and on the ionization state of ionizable organic molecules. In this study, dark adsorption experiments of BY28 on TiO2 under different pHs (3, 6.5, and 10) in the absence of UV irradiation were performed with an initial BY28 concentration of 20 mg/L for 60 min (see Figure 6). From the results, the adsorption yields of BY28 were found to be dependent on the initial pH solution and the best adsorption was obtained at pH 10, as shown in Figure 6a. It was noticed that the adsorption equilibrium of dye was reached at ∼10 min for all pHs. The TiO2 particle surface is negatively charged in alkaline medium (pH >6.5), the collision of the charged BY28 cation and TiO2 will be significantly increased when the solution pH approaches to 10. The electrostatic attraction occurring between the TiO2 surface and BY28 cation
Figure 4. Effect of (a) the solvent nature and (b) the number of immobilizations on the photodegradation of YB28 for an initial concentration of 10 mg/L.
photoactivity of the immobilized TiO2 prepared with ethanol as solvent can be related to the easier evaporation of ethanol than water. Indeed, the faster evaporation of ethanol did not leave time for TiO2 particles to agglomerate. The active surface of the photocatalyst obtained with alcohol was higher than that obtained with water. These results were in agreement with other findings.14 3.1.2. Effect of the Repetition of the Immobilization Procedure. The effect of the repetition of the immobilization procedure on the photoactivity of immobilized TiO2 on glass plate was investigated. Several plates with 2, 3, or 4 layers of TiO2 P-25 dispersed in ethanol were prepared. Figure 4b shows that repeating the procedure of immobilization two or three times caused an increase of the BY28 degradation yield, from 62.5% to 83.7%. Therefore, three repetitions of the immobilization procedure appeared to be optimal. Figure 5 represents micrographs taken by scanning electron microscopy (SEM) of surface deposits of TiO 2 P-25 immobilized on glass plates and obtained after 2, 3, or 4 3815
dx.doi.org/10.1021/ie403970m | Ind. Eng. Chem. Res. 2014, 53, 3813−3819
Industrial & Engineering Chemistry Research
Article
rotatability of the design, α = (2k)1/4 (where k is the number of studied parameters). This characteristic provides good predictions at points equidistant from the center.26−28 The response (yield degradation) is expressed as the percent of Basic Yellow 28. The coded values of xj were obtained from the following relationship:26−30 xj =
Zj − Z j0
j = 1, 2 , ..., k
ΔZj
(1)
with Z j0 =
Zj max + Zjmin 2
and
ΔZj =
Zjmax − Zjmin 2α
where xj is the coded value of variable j, Zj is the encoded value of variable j, Zj0 is the value of Zj at the center point of the investigation domain, and ΔZj is the step size. Here, Zjmax and Zjmin represent the maximum and the minimum level of factor j in natural unit, respectively. The original values of each factor and their corresponding levels are showed in Table 1 (given in the Supporting Information). Other variables such as the duration of experiments, the volume of the solution, and the temperature were set to 8 h, 1 L, and 25 °C, respectively. Before each run, the glass plate loaded with TiO2 P-25 was regenerated by washing with distilled water. The central composite design was composed of 24 experiments of factorial design (see Table 2 (given in the Supporting Information)), three experiments realized at the center work domain (see Table 3 (given in the Supporting Information), and eight-star points (see Table 4 (given in the Supporting Information)). The experiments were performed according to the central composite design. The stars points and the replicates runs are added to the factorial design to provide an estimation of the curvature of the model and to allow an estimation of the experimental error. The correlation of the independent variables and the response were estimated by a second-order polynomial (eq 2), using the least-squares method, as shown below:
Figure 6. pH effect on (a) the dye adsorption on TiO2 and (b) photolysis. Conditions: [BY28]0 = 20 mg/L, Qv = 224.4 mL/min, T = 25 °C.
tends to improve the adsorption effect and, therefore, photodegradation efficiency will be favored by high pH. Under the acidic conditions, (pH
