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Optimized photocatalytic degradation of caffeic acid by sol–gel TiO2 Xiomara L. García-Montelongo, Azael Martínez-de la Cruz, David Contreras and Héctor D. Mansilla
ABSTRACT TiO2 anatase powder was prepared by means of the sol–gel method with titanium(IV) butoxide as W
precursor. The formation of a tetragonal crystal structure of TiO2 anatase at 500 C was confirmed by Xray powder diffraction. The characterization of the samples synthesized was complemented by scanning electron microscopy, diffuse reflectance infrared Fourier transform spectroscopy, nitrogen adsorption–desorption isotherms (Brunauer–Emmett–Teller) and diffuse reflectance spectroscopy. The photocatalytic activity of the TiO2 anatase powder was evaluated in the degradation of caffeic acid in aqueous solution under ultraviolet radiation. A central composite circumscribed design was used to assess the weight of the experimental variables, pH and amount of catalyst in the percentage of caffeic acid degraded and the optimal conditions. The optimized conditions were found to be pH ¼ 5.2 and a load of TiO2 of 1.1 g L1. Under these conditions more than 90% of caffeic acid degradation was
Xiomara L. García-Montelongo Azael Martínez-de la Cruz (corresponding author) CIIDIT-Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, Ciudad Universitaria, C.P. 66451, San Nicolás de los Garza, Nuevo León, Mexico E-mail: [email protected] David Contreras Héctor D. Mansilla Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción, Chile
achieved after 30 min of lamp irradiation. At this time the mineralization reached was almost 60%. Key words
| caffeic acid, heterogeneous photocatalysis, response surface, TiO2
INTRODUCTION In recent decades heterogeneous photocatalysis has attracted attention due to its possible application in generating clean energy from water splitting (Maeda ; Preethi & Kanmani ), as well as being one of the most promising technologies for the removal of organic pollutants from water, air and soil (Sekiguchi et al. ; Sá da Rocha et al. ; Torres-Martínez et al. ). In particular, the photocatalytic degradation of complex molecular structures of organic compounds in aqueous media is a technology that is rapidly expanding as a complementary technique for the treatment of wastewater (Malato et al. ). The polymorph of TiO2 with the crystalline structure of anatase is the semiconductor oxide with major commercial applications as a photocatalyst, due to its high photocatalytic activity under ultraviolet (UV) radiation, low cost and stability to photocorrosion processes (Sánchez-Martínez et al. ). On the basis of these physical properties, TiO2 has been used as an effective photocatalyst for the degradation of herbicides and insecticides, as an antibacterial test, and in drugs and dyes (Nakata & Fujishima ). For this reason, numerous routes of synthesis of TiO2 have been proposed, with the aim of achieving control over the doi: 10.2166/wst.2015.039
chemical composition, morphology and particle size, among other properties (Chen & Mao ; Nakata & Fujishima ). Organic compounds such as substituted phenols, nonbiodegradable chlorinated solvents, pesticides and surfactants are recognized as typical substances that are difficult to remove from wastewater. In particular, phenolic compounds are suspected to contribute to the toxicity and antibacterial activity of agricultural wastewater. Among this family of organic compounds, phenolic acids such as hydroxycinnamic acids (p-coumaric acid, ferulic acid and caffeic acid (CA)) are compounds frequently present in agro-industrial effluents (Baransi et al. ; Nguyen & Doherty ; Prieto-Rodríguez et al. ). The CA is representative of the phenolic acids present in wastewater from the olive oil industry. The high content of polyphenolic compounds represents a serious environmental contamination problem, whose remediation has been the target of several recent investigations by different methods, including adsorption and heterogeneous photocatalysis (Grimes et al. ; Amadelli et al. ; Baransi et al. ; Barreto et al. ; Nguyen & Doherty ).
X. L. García-Montelongo et al.
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Optimized photocatalytic degradation of caffeic acid by TiO2
The efficiency of the photocatalytic systems can be enhanced by optimizing the process parameters, such as photocatalyst loading, pollutant concentration, pH, light intensity and flow rate. The selection of optimal operating conditions is case-specific and may lead to complete removal of pollutants (Kumar & Bansal ). The classical one-variable-at-a-time methodology does not enable the study of combined effects of two or more variables on a measured response. In the same way, probing each variable independently is labor-intensive and time-consuming (Jiang et al. ). Thus, response surface methodology (RSM) is one of the experimental design techniques that is commonly used for process analysis and modeling. The interactions of possible influencing factors on desired responses can be evaluated with RSM just by using a minimum number of designed experiments and then an optimal reaction condition can be much more easily achieved (Miranda et al. ). This paper reports the activity of a TiO2-anatase polymorph, synthesized by the sol–gel method, in the photocatalytic degradation reaction of CA, evaluating the effect of initial pH and the load of photocatalyst. An experimental central composite circumscribed design was used to optimize the reaction conditions to obtain the highest degradation of CA.
EXPERIMENTAL Synthesis of TiO2 anatase TiO2 anatase powder was prepared by the sol–gel method with titanium(IV) butoxide as precursor, according to the following procedure. In a flask, 0.2572 mole of ethanol and 0.5555 mole of deionized water were put under reflux at 70 C for 10 min under continuous stirring. Then, 0.0646 mole of titanium(IV) n-butoxide (Aldrich, 97 wt. %) was added drop-wise causing gel formation. Once obtained, the gel remained under reflux for 2 h and it was then dried at 70 C under atmospheric conditions. Titania xerogel was then heated to 500 C for 4 h. W
W
W
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the 2θ range of 10–70 using a scan rate of 0.05 by 0.05 s1. The morphology of the synthesized TiO2 samples was analyzed using a scanning electron microscope (FEI Nova NanoSEM). Prior to analysis, the powder was deposited onto a silicon wafer, which was subsequently placed into the chamber of the scanning electron microscope (SEM). Diffuse reflectance infrared Fourier transform (DRIFT) spectra of samples were recorded on a Perkin Elmer System 2000 spectrometer equipped with a diffuse reflectance attachment. It was used to confirm the elimination of the solvent used in the synthesis. The energy band gap (Eg) of the photocatalyst was determined via the Kubelka–Munk function using a UV-vis spectrophotometer (Perkin Elmer Lambda 35) coupled with an integrating sphere. The Brunauer–Emmett–Teller (BET) specific surface area, pore size and pore volume of the sample were measured from adsorption-desorption nitrogen isotherms acquired with a Belsorp Mini-II surface area and pore size analyzer. The isotherms were evaluated at 196 C after pretreating the sample at 150 C for 24 h. W
W
Photocatalytic evaluation The photocatalytic reactions were carried out in a borosilicate glass bottle (300 mL) containing 150 mL of CA solution (Aldrich, >95%). The reaction was always maintained under magnetic stirring. The CA solution (approximately 0.01 g L1) was kept in the dark for 20 min with the photocatalyst in order to reach the adsorption– desorption equilibrium between the surface of the photocatalyst and the solution. Then, the reactor was illuminated by a commercial Solarium Philips HB311 equipped with 6 × 20 W lamps (λ ¼ 300–400 nm) placed 15 cm in front of the reactor. Sampling was carried out using a syringe connected to the reactor by a Teflon tube. Samples were immediately filtered in a Millipore filtration system using 0.22 µm nitrocellulose filters. Filtered samples were analyzed in a UV1800 Shimadzu spectrophotometer (315.9 nm). The photocatalytic experiments were performed at a pH ranging from 3 to 7 and loads of TiO2 ranged from approximately 0.4 to 1.2 g L1. Total organic carbon (TOC) was determined in a Vario TOC Select Elementar analyser.
Sample characterization Optimization and response surface modeling The crystal structure and phase formation of TiO2 were determined by X-ray powder diffraction (XRD) using a Bruker D8 Advance diffractometer equipped with a CuKα radiation (λ ¼ 1.5406 Å) source, a Vantec high-speed detector and a Ni filter. X-ray diffraction data were collected in
A response surface modeling of the photocatalytic reaction was performed based on a central composite circumscribed design, composed of a factorial model, star points and three replicated central points. In a first approach, the variables
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Optimized photocatalytic degradation of caffeic acid by TiO2
pH (ranging from 3 to 7) and load of catalyst (ranging from 0.4 to 1.2 g L1) were evaluated by a 2n model. For n variables and two levels (low and high, or 1 and þ1) the total number of experiments is determined by the expression: 2n þ 2n þ 3, where 2n represents the four factorial experiments, 2n denotes the star points, and finally three central points are added in order to provide statistical validation to the model. It is important to point out that the levels are defined by the factorial design. The response factor was defined as the percentage of CA degradation after 30 min of lamp irradiation. The data were analyzed using the software Modde 7.0™ in order to get the associated polynomial for the reaction system and to build the response surface plot. The model was statistically validated with the same software using ANalysis Of VAriance (ANOVA) (95% confidence level).
RESULTS AND DISCUSSION Structural characterization The formation of powder with a TiO2 anatase crystal structure was followed by XRD. Figure 1 shows the diffractogram of the samples obtained by the sol–gel method over the course of TiO2 formation, i.e. before and after the thermal treatment at 500 C. The weak and broad peaks observed in the diffractogram of the uncalcined sample reveal a low crystallinity, but the presence of the main diffraction peaks W
Figure 1
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X-ray diffraction patterns of samples obtained by sol–gel in the course of TiO2 formation and without heat treatment.
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of the anatase polymorph are in agreement with JCPDS card no. 21-1272. When the sample was treated at 500 C (hereafter labeled as TiO2-500 C), a significant increase in the crystallinity of the sample was observed, maintaining the main diffraction peaks of anatase. The diffraction peaks observed correspond to (101), (004), (200), (105), (211) and (204) planes at 2θ ¼ 25.33, 37.87, 48.03, 54.01, 55.12 and 62.76, respectively. Figure 2 reports the DRIFT spectra in the range 400–4,000 cm1 of both the fresh and the 500 C (for 4 h) treated TiO2. This plot reports normal transmittance infrared (IR) spectra, converted to KubelkaMunk-type spectra by computer software. As expected, only the fresh sample showed signals corresponding to the organic functional groups associated with precursors used during the synthesis. Figure 2 shows a broad band in the region of 2,500–3,600 cm1 corresponding to the stretching vibrations of the O-H functional group. At lower wavenumbers, between 1,500 and 1,700 cm1, the stretching vibration of the carbonyl group can be identified, while the range of 1,300–1,500 cm1 shows the stretching vibration of the C-C group. In both samples it was possible to detect the vibration associated with the Ti-O bond, which confirms the results obtained by X-ray diffraction, where the formation of TiO2 was observed even without heat treatment. The morphology of the TiO2 sample obtained after 4 h at 500 C was analyzed by SEM and some representative images are shown in Figure 3. TiO2 particles tend to form large agglomerates, due to the physical attraction between them, with nanometric size and irregular shapes. The W
W
W
W
Figure 2
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DRIFT spectra in the range 400–4000 cm W
1
of TiO2 samples prepared with
thermal treatment at 500 C for 4 h and without heat treatment.
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Figure 3
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SEM images of TiO2 obtained by sol–gel at 500 C for 4 h.
latter can be attributed to the nature of the route of synthesis used and the absence of an agent that allows control of the morphology. Band gap energy and specific surface area W
The optical properties of the TiO2-500 C sample were analyzed by UV-vis diffuse reflectance spectroscopy. The value of Eg obtained (3.28 eV) shows that the material is able to absorb radiation in the UV region. This value is similar to the commercial Degussa-P25 and other values as reported in the literature. The TiO2-500 C sample showed the typical behavior of a material with mesoporosity, with a low energy of adsorption (Chen & Mao ) as was inferred from the profile of N2 adsorption-desorption isotherms at 196 C, shown in Figure 4. The specific surface area calculated using the multipoint BET method was 74 m2 g1 (50% more than the commercial Degussa P25), and the average pore diameter was estimated as 12.7 nm. W
W
Preliminary photocatalytic results Preliminary experiments were performed to determine the effectiveness of TiO2 to act as a photocatalyst in the degradation of CA in aqueous solution. Figure 5 shows the decay profile of CA concentration in solution during UV irradiation in experiments under non-optimized conditions. Experiments in the dark indicate that the adsorption of CA
Figure 4
|
W
Adsorption-desorption N2 isotherms taken at 196 C for TiO2 obtained by W
sol–gel at 500 C for 4 h.
on the catalyst surface is negligible at normal pH of solution, which was pH ¼ 4.54. Conversely, when a solution of CA was irradiated in the absence of photocatalyst, a slight photolysis was observed. The degradation of CA by action of the irradiation source reached a maximum value of 15% after 10 min, and this percentage remained constant for longer irradiation times. A notable improvement in the percentage of degradation of CA was observed when the photocatalyst was added, indicating the high activity of the material. Under this condition, a
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Figure 5
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Optimized photocatalytic degradation of caffeic acid by TiO2
Profile of CA degradation by TiO2 photocatalysis irradiated with UV light at pH 4.5. (○) Photolysis; (●) TiO2 photocatalysis (0.02 g of TiO2 in CA solution [0.01 g L
1
]).
value of 50% of photocatalytic degradation was reached after 60 min of lamp irradiation.
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number of experiments. Optimization was carried out in a wide domain of variables, generating 11 experiments, which are shown in Table 1. The percentage of CA degraded after 30 min of lamp irradiation was chosen as the response factor. In Table 1, real and codified values are shown. Using the response factor (Yexp), a polynomial was created to describe the reaction system (Equation (1)). The validation of the mathematical model was performed by ANOVA for regression test. In this test, there was a significant difference between the variances of regression and residuals (p ¼ 0.004), but there was not a significant difference between the variances of lack of fit and pure error (p ¼ 0.090). Also the validation of the mathematical model was demonstrated comparing the experimental (Yexp) and predicted results (Ycalc) calculated by using the polynomial. Values in parentheses in the polynomial represent the standard deviation of each codified coefficient. Clearly, both variables, the pH and load of TiO2, have a positive and similar effect on the percentage of CA degradation: Y ¼ 80:7( ± 3:4) þ 10:1( ± 2:2)pH þ 11:2( ± 2:1)TiO2 13:6( ± 2:8)pH2 6:8( ± 2:4)TiO22 9:3( ± 2:9)pH TiO2
Optimization of the reaction conditions
(1) To identify the optimal values for the most important parameters associated with photocatalytic processes, an experimental design considering pH and load of TiO2 was carried out. This procedure consists of the simultaneous change in the variables, identifying the conditions for the optimal degradation of CA by performing a limited Table 1
|
A 3D representation of the polynomial is shown in Figure 6, where it can be seen that the percentage degradation of CA is improved at pH values between 5.0 and 5.5 and high TiO2 photocatalyst loads (0.8–1.2 g/L). Based on the
Experimental results from factorial design of caffeic acid photocatalytic degradation assisted by TiO2 and ultraviolet irradiation % degradation
Experiment
1
pH
3 ( 1)
TiO2
0.4 ( 1)
Yexp
Ycalc
32.54
29.78
2
7 (1)
0.4 ( 1)
75.43
68.51
3
3 ( 1)
1.2 (1)
65.73
70.65
7 (1)
1.2 (1)
71.56
72.33
0.8 (0)
54.14
51.47
4 5
2.2 ( 0.7)
6
7.8 (0.7)
7
5 (0)
8
5 (0)
1.36 (0.7)
87.32
82.91
9
5 (0)
0.8 (0)
82.68
80.71
10
5 (0)
0.8 (0)
80.38
80.71
11
5 (0)
0.8 (0)
78.10
80.71
0.8 (0)
63.99
67.71
0.23( 0.7)
44.69
51.06
Figure 6
|
Response surface of CA degradation by TiO2 photocatalyst as a function of pH and load of catalyst.
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Optimized photocatalytic degradation of caffeic acid by TiO2
maximum value marked on the surface, the optimized reaction conditions were located as pH 5.2 and a load of photocatalyst of 1.1 g/L. Further experiments were carried out under such conditions, obtaining a CA degradation of 90% after 30 min irradiation (5% above that predicted), showing a considerable improvement compared with previous studies in which a degradation of 50% was reached in double the time. Photocatalytic tests performed under optimized conditions allowed determination of the degree of mineralization of CA, finding that a mineralization degree of 60% was reached after 40 min of irradiation. These results reveal a high activity of TiO2-500 C, leading to mineralization of the CA molecule, rather than a simple conversion of CA in other, possibly still harmful, chemical intermediates. Figure 7 shows the plot corresponding to the analyses. W
CONCLUSIONS Highly crystallized TiO2 powder with anatase structure was synthesized by means of the sol–gel method. Analyses by XRD, SEM and BET were carried out in order to characterize the macrostructure, morphology, surface area and particle size of the sample. The material obtained exhibited high photocatalytic activity for CA degradation. It was found that some variables, such as photocatalyst dosing and pH, have a positive effect on the degradation percentage of CA. The optimized conditions found were pH ¼ 5.2 and a TiO2 photocatalyst dosing of 1.1 g/L. The TOC analysis confirmed the potential of TiO2 synthesized by the sol–gel method for the mineralization of CA.
Figure 7
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Course of TOC (●) evolution in photocatalytic degradation of CA by TiO2.
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ACKNOWLEDGEMENTS A. M. C. and X. L. G.-M. thank CONACYT for its financial support through project CIAM 148138 and PhD scholarship 251562. D. C. and H. D. M. thank FONDAP/CONICYT/ 15110019 and CIAM/CONICYT 2010-251 projects.
REFERENCES Amadelli, R., Maldotti, A. & Samiolo, L. Adsorption and photo-oxidation of 3,4-dihyroxy-cinnamic acid on TiO2 films. Catal. Today 144, 149–153. Baransi, K., Dubowski, Y. & Sabbah, I. Synergetic effect between photocatalytic degradation and adsorption processes on the removal of phenolic compounds from olive mill wastewater. Water Res. 46, 789–798. Barreto, W., Ando, R., Estevão, B. & Da Silva, K. Adsorption of caffeic acid on titanium dioxide: A spectroscopic study. Spectrochim. Acta Part A 92, 16–20. Chen, X. & Mao, S. Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications. Chem. Rev. 107, 2891–2959. Grimes, S., Mehta, L. & Ngwang, H. Chemical kinetics of the photocatalytic degradation of trans-cinnamic, dihydrocinnamic, trans-caffeic, and dihydrocaffeic acids. J. Environ. Sci. Health Part A 36, 1891–1904. Jiang, W., Joens, J., Dionysiou, D. & ÓShea, K. Optimization of photocatalytic performance of TiO2 coated glass microspheres using response surface methodology and the application for degradation of dimethyl phthalate. J. Photochem. Photobiol. A 262, 7–13. Kumar, J. & Bansal, A. Photocatalytic degradation in annular reactor: Modelization and optimization using computational fluid dynamics (CFD) and response surface methodology (RSM). J. Environ. Chem. Eng. 1, 398–405. Maeda, K. Photocatalytic water splitting using semiconductor particles: History and recent developments. J. Photochem. Photobiol. C, Photochem. Rev. 12, 237–268. Malato, S., Fernández-Ibáñez, P., Maldonado, M. I., Blanco, J. & Gernjak, W. Decontamination and disinfection of water by solar photocatalysis: Recent overview and trends. Catal. Today 147, 1–59. Miranda, C., Yáñez, J., Contreras, D. & Mansilla, H. D. Phenylmercury degradation by heterogeneous photocatalysis assisted by UV-A light. J. Environ. Sci. Health A 48, 1642– 1648. Nakata, K. & Fujishima, A. TiO2 photocatalysis: Design and applications. Photochem. Photobiol. C, Photochem. Rev. 13, 169–189. Nguyen, D. & Doherty, W. Optimisation of process parameters for the degradation of caffeic acid in sugar solutions. Int. J. Food Sci. 47, 2477–2486. Preethi, V. & Kanmani, S. Photocatalytic hydrogen production. Mater. Sci. Semicond. Process. 16, 561–575.
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Prieto-Rodríguez, L., Miralles-Cuevas, S., Oller, I., FernándezIbáñez, P., Agüera, A., Blanco, J., Ruiz-Gómez, M., TorresMartínez, L., Figueroa-Torres, M., Moctezuma, E. & JuárezRamírez, I. Hydrogen evolution from pure water over a new advanced photocatalyst Sm2GaTaO7. Int. J. Hydrogen. Energy 38, 12554–12561. Sá da Rocha, O., Dantas, R., Menezes, M., Lima, M. & Da Silva, V. Oil sludge treatment by photocatalysis applying black and white light. Chem. Eng. J. 157, 80–85. Sánchez-Martínez, D., Martínez-de la Cruz, A. & López-Cuellar, E. Synthesis of WO3 nanoparticles by citric acid-assisted
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precipitation and evaluation of their photocatalytic properties. Mater. Res. Bull. 48, 691–697. Sekiguchi, K., Morinaga, W., Sakamoto, K., Tamura, H., Yasui, F., Mehrjouei, M., Müller, S. & Möller, D. Degradation of VOC gases in liquid phase by photocatalysis at the bubble interface. Appl. Catal. B 97, 190–197. Torres-Martínez, L., Ruiz-Gómez, M., Figueroa-Torres, M., JuárezRamírez, I. & Moctezuma, E. Sm2FeTaO7 photocatalyst for degradation of indigo carmine dye under solar light irradiation. Int. J. Photoenergy vol. 2012, Article ID 939608, 7 pages, doi:10.1155/2012/939608.
First received 19 November 2014; accepted in revised form 15 January 2015. Available online 29 January 2015
© IWA Publishing 2015 Water Science & Technology
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71.6
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2015
Optimized photocatalytic degradation of caffeic acid by sol–gel TiO2 Xiomara L. García-Montelongo, Azael Martínez-de la Cruz, David Contreras and Héctor D. Mansilla
ABSTRACT TiO2 anatase powder was prepared by means of the sol–gel method with titanium(IV) butoxide as W
precursor. The formation of a tetragonal crystal structure of TiO2 anatase at 500 C was confirmed by Xray powder diffraction. The characterization of the samples synthesized was complemented by scanning electron microscopy, diffuse reflectance infrared Fourier transform spectroscopy, nitrogen adsorption–desorption isotherms (Brunauer–Emmett–Teller) and diffuse reflectance spectroscopy. The photocatalytic activity of the TiO2 anatase powder was evaluated in the degradation of caffeic acid in aqueous solution under ultraviolet radiation. A central composite circumscribed design was used to assess the weight of the experimental variables, pH and amount of catalyst in the percentage of caffeic acid degraded and the optimal conditions. The optimized conditions were found to be pH ¼ 5.2 and a load of TiO2 of 1.1 g L1. Under these conditions more than 90% of caffeic acid degradation was
Xiomara L. García-Montelongo Azael Martínez-de la Cruz (corresponding author) CIIDIT-Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, Ciudad Universitaria, C.P. 66451, San Nicolás de los Garza, Nuevo León, Mexico E-mail: [email protected] David Contreras Héctor D. Mansilla Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción, Chile
achieved after 30 min of lamp irradiation. At this time the mineralization reached was almost 60%. Key words
| caffeic acid, heterogeneous photocatalysis, response surface, TiO2
INTRODUCTION In recent decades heterogeneous photocatalysis has attracted attention due to its possible application in generating clean energy from water splitting (Maeda ; Preethi & Kanmani ), as well as being one of the most promising technologies for the removal of organic pollutants from water, air and soil (Sekiguchi et al. ; Sá da Rocha et al. ; Torres-Martínez et al. ). In particular, the photocatalytic degradation of complex molecular structures of organic compounds in aqueous media is a technology that is rapidly expanding as a complementary technique for the treatment of wastewater (Malato et al. ). The polymorph of TiO2 with the crystalline structure of anatase is the semiconductor oxide with major commercial applications as a photocatalyst, due to its high photocatalytic activity under ultraviolet (UV) radiation, low cost and stability to photocorrosion processes (Sánchez-Martínez et al. ). On the basis of these physical properties, TiO2 has been used as an effective photocatalyst for the degradation of herbicides and insecticides, as an antibacterial test, and in drugs and dyes (Nakata & Fujishima ). For this reason, numerous routes of synthesis of TiO2 have been proposed, with the aim of achieving control over the doi: 10.2166/wst.2015.039
chemical composition, morphology and particle size, among other properties (Chen & Mao ; Nakata & Fujishima ). Organic compounds such as substituted phenols, nonbiodegradable chlorinated solvents, pesticides and surfactants are recognized as typical substances that are difficult to remove from wastewater. In particular, phenolic compounds are suspected to contribute to the toxicity and antibacterial activity of agricultural wastewater. Among this family of organic compounds, phenolic acids such as hydroxycinnamic acids (p-coumaric acid, ferulic acid and caffeic acid (CA)) are compounds frequently present in agro-industrial effluents (Baransi et al. ; Nguyen & Doherty ; Prieto-Rodríguez et al. ). The CA is representative of the phenolic acids present in wastewater from the olive oil industry. The high content of polyphenolic compounds represents a serious environmental contamination problem, whose remediation has been the target of several recent investigations by different methods, including adsorption and heterogeneous photocatalysis (Grimes et al. ; Amadelli et al. ; Baransi et al. ; Barreto et al. ; Nguyen & Doherty ).
X. L. García-Montelongo et al.
879
|
Optimized photocatalytic degradation of caffeic acid by TiO2
The efficiency of the photocatalytic systems can be enhanced by optimizing the process parameters, such as photocatalyst loading, pollutant concentration, pH, light intensity and flow rate. The selection of optimal operating conditions is case-specific and may lead to complete removal of pollutants (Kumar & Bansal ). The classical one-variable-at-a-time methodology does not enable the study of combined effects of two or more variables on a measured response. In the same way, probing each variable independently is labor-intensive and time-consuming (Jiang et al. ). Thus, response surface methodology (RSM) is one of the experimental design techniques that is commonly used for process analysis and modeling. The interactions of possible influencing factors on desired responses can be evaluated with RSM just by using a minimum number of designed experiments and then an optimal reaction condition can be much more easily achieved (Miranda et al. ). This paper reports the activity of a TiO2-anatase polymorph, synthesized by the sol–gel method, in the photocatalytic degradation reaction of CA, evaluating the effect of initial pH and the load of photocatalyst. An experimental central composite circumscribed design was used to optimize the reaction conditions to obtain the highest degradation of CA.
EXPERIMENTAL Synthesis of TiO2 anatase TiO2 anatase powder was prepared by the sol–gel method with titanium(IV) butoxide as precursor, according to the following procedure. In a flask, 0.2572 mole of ethanol and 0.5555 mole of deionized water were put under reflux at 70 C for 10 min under continuous stirring. Then, 0.0646 mole of titanium(IV) n-butoxide (Aldrich, 97 wt. %) was added drop-wise causing gel formation. Once obtained, the gel remained under reflux for 2 h and it was then dried at 70 C under atmospheric conditions. Titania xerogel was then heated to 500 C for 4 h. W
W
W
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W
the 2θ range of 10–70 using a scan rate of 0.05 by 0.05 s1. The morphology of the synthesized TiO2 samples was analyzed using a scanning electron microscope (FEI Nova NanoSEM). Prior to analysis, the powder was deposited onto a silicon wafer, which was subsequently placed into the chamber of the scanning electron microscope (SEM). Diffuse reflectance infrared Fourier transform (DRIFT) spectra of samples were recorded on a Perkin Elmer System 2000 spectrometer equipped with a diffuse reflectance attachment. It was used to confirm the elimination of the solvent used in the synthesis. The energy band gap (Eg) of the photocatalyst was determined via the Kubelka–Munk function using a UV-vis spectrophotometer (Perkin Elmer Lambda 35) coupled with an integrating sphere. The Brunauer–Emmett–Teller (BET) specific surface area, pore size and pore volume of the sample were measured from adsorption-desorption nitrogen isotherms acquired with a Belsorp Mini-II surface area and pore size analyzer. The isotherms were evaluated at 196 C after pretreating the sample at 150 C for 24 h. W
W
Photocatalytic evaluation The photocatalytic reactions were carried out in a borosilicate glass bottle (300 mL) containing 150 mL of CA solution (Aldrich, >95%). The reaction was always maintained under magnetic stirring. The CA solution (approximately 0.01 g L1) was kept in the dark for 20 min with the photocatalyst in order to reach the adsorption– desorption equilibrium between the surface of the photocatalyst and the solution. Then, the reactor was illuminated by a commercial Solarium Philips HB311 equipped with 6 × 20 W lamps (λ ¼ 300–400 nm) placed 15 cm in front of the reactor. Sampling was carried out using a syringe connected to the reactor by a Teflon tube. Samples were immediately filtered in a Millipore filtration system using 0.22 µm nitrocellulose filters. Filtered samples were analyzed in a UV1800 Shimadzu spectrophotometer (315.9 nm). The photocatalytic experiments were performed at a pH ranging from 3 to 7 and loads of TiO2 ranged from approximately 0.4 to 1.2 g L1. Total organic carbon (TOC) was determined in a Vario TOC Select Elementar analyser.
Sample characterization Optimization and response surface modeling The crystal structure and phase formation of TiO2 were determined by X-ray powder diffraction (XRD) using a Bruker D8 Advance diffractometer equipped with a CuKα radiation (λ ¼ 1.5406 Å) source, a Vantec high-speed detector and a Ni filter. X-ray diffraction data were collected in
A response surface modeling of the photocatalytic reaction was performed based on a central composite circumscribed design, composed of a factorial model, star points and three replicated central points. In a first approach, the variables
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Optimized photocatalytic degradation of caffeic acid by TiO2
pH (ranging from 3 to 7) and load of catalyst (ranging from 0.4 to 1.2 g L1) were evaluated by a 2n model. For n variables and two levels (low and high, or 1 and þ1) the total number of experiments is determined by the expression: 2n þ 2n þ 3, where 2n represents the four factorial experiments, 2n denotes the star points, and finally three central points are added in order to provide statistical validation to the model. It is important to point out that the levels are defined by the factorial design. The response factor was defined as the percentage of CA degradation after 30 min of lamp irradiation. The data were analyzed using the software Modde 7.0™ in order to get the associated polynomial for the reaction system and to build the response surface plot. The model was statistically validated with the same software using ANalysis Of VAriance (ANOVA) (95% confidence level).
RESULTS AND DISCUSSION Structural characterization The formation of powder with a TiO2 anatase crystal structure was followed by XRD. Figure 1 shows the diffractogram of the samples obtained by the sol–gel method over the course of TiO2 formation, i.e. before and after the thermal treatment at 500 C. The weak and broad peaks observed in the diffractogram of the uncalcined sample reveal a low crystallinity, but the presence of the main diffraction peaks W
Figure 1
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X-ray diffraction patterns of samples obtained by sol–gel in the course of TiO2 formation and without heat treatment.
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of the anatase polymorph are in agreement with JCPDS card no. 21-1272. When the sample was treated at 500 C (hereafter labeled as TiO2-500 C), a significant increase in the crystallinity of the sample was observed, maintaining the main diffraction peaks of anatase. The diffraction peaks observed correspond to (101), (004), (200), (105), (211) and (204) planes at 2θ ¼ 25.33, 37.87, 48.03, 54.01, 55.12 and 62.76, respectively. Figure 2 reports the DRIFT spectra in the range 400–4,000 cm1 of both the fresh and the 500 C (for 4 h) treated TiO2. This plot reports normal transmittance infrared (IR) spectra, converted to KubelkaMunk-type spectra by computer software. As expected, only the fresh sample showed signals corresponding to the organic functional groups associated with precursors used during the synthesis. Figure 2 shows a broad band in the region of 2,500–3,600 cm1 corresponding to the stretching vibrations of the O-H functional group. At lower wavenumbers, between 1,500 and 1,700 cm1, the stretching vibration of the carbonyl group can be identified, while the range of 1,300–1,500 cm1 shows the stretching vibration of the C-C group. In both samples it was possible to detect the vibration associated with the Ti-O bond, which confirms the results obtained by X-ray diffraction, where the formation of TiO2 was observed even without heat treatment. The morphology of the TiO2 sample obtained after 4 h at 500 C was analyzed by SEM and some representative images are shown in Figure 3. TiO2 particles tend to form large agglomerates, due to the physical attraction between them, with nanometric size and irregular shapes. The W
W
W
W
Figure 2
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DRIFT spectra in the range 400–4000 cm W
1
of TiO2 samples prepared with
thermal treatment at 500 C for 4 h and without heat treatment.
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Figure 3
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Optimized photocatalytic degradation of caffeic acid by TiO2
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W
SEM images of TiO2 obtained by sol–gel at 500 C for 4 h.
latter can be attributed to the nature of the route of synthesis used and the absence of an agent that allows control of the morphology. Band gap energy and specific surface area W
The optical properties of the TiO2-500 C sample were analyzed by UV-vis diffuse reflectance spectroscopy. The value of Eg obtained (3.28 eV) shows that the material is able to absorb radiation in the UV region. This value is similar to the commercial Degussa-P25 and other values as reported in the literature. The TiO2-500 C sample showed the typical behavior of a material with mesoporosity, with a low energy of adsorption (Chen & Mao ) as was inferred from the profile of N2 adsorption-desorption isotherms at 196 C, shown in Figure 4. The specific surface area calculated using the multipoint BET method was 74 m2 g1 (50% more than the commercial Degussa P25), and the average pore diameter was estimated as 12.7 nm. W
W
Preliminary photocatalytic results Preliminary experiments were performed to determine the effectiveness of TiO2 to act as a photocatalyst in the degradation of CA in aqueous solution. Figure 5 shows the decay profile of CA concentration in solution during UV irradiation in experiments under non-optimized conditions. Experiments in the dark indicate that the adsorption of CA
Figure 4
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W
Adsorption-desorption N2 isotherms taken at 196 C for TiO2 obtained by W
sol–gel at 500 C for 4 h.
on the catalyst surface is negligible at normal pH of solution, which was pH ¼ 4.54. Conversely, when a solution of CA was irradiated in the absence of photocatalyst, a slight photolysis was observed. The degradation of CA by action of the irradiation source reached a maximum value of 15% after 10 min, and this percentage remained constant for longer irradiation times. A notable improvement in the percentage of degradation of CA was observed when the photocatalyst was added, indicating the high activity of the material. Under this condition, a
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Figure 5
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Optimized photocatalytic degradation of caffeic acid by TiO2
Profile of CA degradation by TiO2 photocatalysis irradiated with UV light at pH 4.5. (○) Photolysis; (●) TiO2 photocatalysis (0.02 g of TiO2 in CA solution [0.01 g L
1
]).
value of 50% of photocatalytic degradation was reached after 60 min of lamp irradiation.
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number of experiments. Optimization was carried out in a wide domain of variables, generating 11 experiments, which are shown in Table 1. The percentage of CA degraded after 30 min of lamp irradiation was chosen as the response factor. In Table 1, real and codified values are shown. Using the response factor (Yexp), a polynomial was created to describe the reaction system (Equation (1)). The validation of the mathematical model was performed by ANOVA for regression test. In this test, there was a significant difference between the variances of regression and residuals (p ¼ 0.004), but there was not a significant difference between the variances of lack of fit and pure error (p ¼ 0.090). Also the validation of the mathematical model was demonstrated comparing the experimental (Yexp) and predicted results (Ycalc) calculated by using the polynomial. Values in parentheses in the polynomial represent the standard deviation of each codified coefficient. Clearly, both variables, the pH and load of TiO2, have a positive and similar effect on the percentage of CA degradation: Y ¼ 80:7( ± 3:4) þ 10:1( ± 2:2)pH þ 11:2( ± 2:1)TiO2 13:6( ± 2:8)pH2 6:8( ± 2:4)TiO22 9:3( ± 2:9)pH TiO2
Optimization of the reaction conditions
(1) To identify the optimal values for the most important parameters associated with photocatalytic processes, an experimental design considering pH and load of TiO2 was carried out. This procedure consists of the simultaneous change in the variables, identifying the conditions for the optimal degradation of CA by performing a limited Table 1
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A 3D representation of the polynomial is shown in Figure 6, where it can be seen that the percentage degradation of CA is improved at pH values between 5.0 and 5.5 and high TiO2 photocatalyst loads (0.8–1.2 g/L). Based on the
Experimental results from factorial design of caffeic acid photocatalytic degradation assisted by TiO2 and ultraviolet irradiation % degradation
Experiment
1
pH
3 ( 1)
TiO2
0.4 ( 1)
Yexp
Ycalc
32.54
29.78
2
7 (1)
0.4 ( 1)
75.43
68.51
3
3 ( 1)
1.2 (1)
65.73
70.65
7 (1)
1.2 (1)
71.56
72.33
0.8 (0)
54.14
51.47
4 5
2.2 ( 0.7)
6
7.8 (0.7)
7
5 (0)
8
5 (0)
1.36 (0.7)
87.32
82.91
9
5 (0)
0.8 (0)
82.68
80.71
10
5 (0)
0.8 (0)
80.38
80.71
11
5 (0)
0.8 (0)
78.10
80.71
0.8 (0)
63.99
67.71
0.23( 0.7)
44.69
51.06
Figure 6
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Response surface of CA degradation by TiO2 photocatalyst as a function of pH and load of catalyst.
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maximum value marked on the surface, the optimized reaction conditions were located as pH 5.2 and a load of photocatalyst of 1.1 g/L. Further experiments were carried out under such conditions, obtaining a CA degradation of 90% after 30 min irradiation (5% above that predicted), showing a considerable improvement compared with previous studies in which a degradation of 50% was reached in double the time. Photocatalytic tests performed under optimized conditions allowed determination of the degree of mineralization of CA, finding that a mineralization degree of 60% was reached after 40 min of irradiation. These results reveal a high activity of TiO2-500 C, leading to mineralization of the CA molecule, rather than a simple conversion of CA in other, possibly still harmful, chemical intermediates. Figure 7 shows the plot corresponding to the analyses. W
CONCLUSIONS Highly crystallized TiO2 powder with anatase structure was synthesized by means of the sol–gel method. Analyses by XRD, SEM and BET were carried out in order to characterize the macrostructure, morphology, surface area and particle size of the sample. The material obtained exhibited high photocatalytic activity for CA degradation. It was found that some variables, such as photocatalyst dosing and pH, have a positive effect on the degradation percentage of CA. The optimized conditions found were pH ¼ 5.2 and a TiO2 photocatalyst dosing of 1.1 g/L. The TOC analysis confirmed the potential of TiO2 synthesized by the sol–gel method for the mineralization of CA.
Figure 7
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Course of TOC (●) evolution in photocatalytic degradation of CA by TiO2.
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ACKNOWLEDGEMENTS A. M. C. and X. L. G.-M. thank CONACYT for its financial support through project CIAM 148138 and PhD scholarship 251562. D. C. and H. D. M. thank FONDAP/CONICYT/ 15110019 and CIAM/CONICYT 2010-251 projects.
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Prieto-Rodríguez, L., Miralles-Cuevas, S., Oller, I., FernándezIbáñez, P., Agüera, A., Blanco, J., Ruiz-Gómez, M., TorresMartínez, L., Figueroa-Torres, M., Moctezuma, E. & JuárezRamírez, I. Hydrogen evolution from pure water over a new advanced photocatalyst Sm2GaTaO7. Int. J. Hydrogen. Energy 38, 12554–12561. Sá da Rocha, O., Dantas, R., Menezes, M., Lima, M. & Da Silva, V. Oil sludge treatment by photocatalysis applying black and white light. Chem. Eng. J. 157, 80–85. Sánchez-Martínez, D., Martínez-de la Cruz, A. & López-Cuellar, E. Synthesis of WO3 nanoparticles by citric acid-assisted
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precipitation and evaluation of their photocatalytic properties. Mater. Res. Bull. 48, 691–697. Sekiguchi, K., Morinaga, W., Sakamoto, K., Tamura, H., Yasui, F., Mehrjouei, M., Müller, S. & Möller, D. Degradation of VOC gases in liquid phase by photocatalysis at the bubble interface. Appl. Catal. B 97, 190–197. Torres-Martínez, L., Ruiz-Gómez, M., Figueroa-Torres, M., JuárezRamírez, I. & Moctezuma, E. Sm2FeTaO7 photocatalyst for degradation of indigo carmine dye under solar light irradiation. Int. J. Photoenergy vol. 2012, Article ID 939608, 7 pages, doi:10.1155/2012/939608.
First received 19 November 2014; accepted in revised form 15 January 2015. Available online 29 January 2015
