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Titania-Coated Magnetic Composites as Photocatalysts for Phthalate Photodegradation Chiung-Fen Chang* and Chang-Yi Man Department of Environmental Science and Engineering, Tunghai University, P.O. Box 818, Taichung 407, Taiwan ABSTRACT: Bifunctional photocatalysts, namely, magnetic TiO2 composites (TiO2/SiO2/Fe3O4, TSM), were prepared and used to degrade dimethyl phthalate (DMP), which is suspected of having endocrine-disrupting effects and a high potential to threaten aquatic environments. The main factor affecting the deposition of titania on the magnetic carrier (SiO2/Fe3O4) for the preparation of TSM composites investigated in this study was the water-to-titanium ratio (WTR) in the range of 60250. The TSM composites were characterized by SQUID, TEM, XPS, XRD, UV absorption, and zeta potential analysis. The effects of the dosage of TSM composite and solution pH on the photodegradation of DMP were also investigated. The best water-to-titanium ratio for the synthesis of the thickest film of TiO2 on the magnetic carrier in a homogeneous nucleation system was determined to be WTR = 130. The obtained TSM composites were superparamagnetic and mainly exhibited the crystalline structure of the anatase phase. The energy gap and pHPZC of TSM130 were found to be 3.2 eV and 5.6, respectively. The degradation of DMP using TSM composites was found to follow pseudo-first-order kinetics, and the calculated values of the rate constant were in the range of 0.00140.0037 min1. Furthermore, the adsorption and the photodegradation efficiency of DMP were significantly affected by the solution pH value. A 1.2 g L1 dosage of TSM130 in the photocatalytic system was found to ensure the most effective utilization and minimal detrimental scattering of radiation. The results led to the conclusion that TSM composites, which can be easily separated from solution by magnetic field, exhibit good photocatalytic activity in the degradation of DMP.
1. INTRODUCTION Endocrine-disrupting chemicals (EDCs) draw the special attention of global researchers because of their great potential to interfere with the hormonal control systems of humans and other organisms.14 Hundreds of chemical compounds are considered to be possible ECDs, such as pesticides, metals, natural plant materials, detergents, pharmaceuticals, combustion byproducts, and flame retardants. Phthalic acid esters (PAEs; also called phthalate esters) suspected of having endocrine-disrupting effects are widely used in a large variety of products, especially as plasticizers in soft plastic manufacturing.5,6 Furthermore, PAEs are almost ubiquitous in the environment, such as sediments and fresh and marine waters, because of their chemical stability.7,8 Dimethyl phthalate (DMP), the plasticizer with the shortest-chain ester, is among the most frequently identified and used PAEs. As a result, it poses a high potential threat to the aquatic environment. Furthermore, previous studies have reported that the elimination of DMP from aqueous solution demands an effective physicochemical process rather than biodegradation of industrial effluents.915 In recent years, there has been a dramatic increase in research concerned with the preparation and modification of TiO2-type semiconductor photocatalysts because of their advantages in terms of high physicochemical stability and potential applications in the elimination of environmental pollutants from aquatic solutions.16,17 Photochemical reactors can be classified into two types according to the state of the photocatalyst: immobilized and suspended. The latter type, in a corresponding slurry reactor, is employed in water and wastewater treatments because of the larger photocatalytic activity.18 Nevertheless, a major drawback of slurry photocatalytic systems is the difficulty of achieving solidliquid separation of nanosize particles. To overcome the r 2011 American Chemical Society
separation problem encountered by slurry reactor systems, magnetic particles modified with specific functional groups have been shown to have a high potential for use in heterogeneous systems with their easily, effectively, and magnetically controllable property.1922 The direct coating of TiO2 onto a magnetic core has been reported to be detrimental to the catalytic activity of the photocatalyst because the magnetic core acts as a sink for its charge carriers, which accounts for the enhancement of electronhole recombination.2326 Furthermore, the strong UV absorption of the magnetic core can lower the photoactivity of TiO2.27 To overcome the disadvantage of the unfavorable electronic interaction between the magnetic core and the TiO2 shell, an inert silica film coated on magnetite as an insulator is anticipated to prevent the detrimental electron transfer, as several studies have confirmed.26,2833 Furthermore, the silica film inhibits the sintering of the magnetic core so as to promote the high coercivity of the magnetic material as well.3437 Among the methods of wet chemistry, the solgel route is reported to be the best-controlled method for obtaining TiO2 nanocrystals of uniform size at low temperatures. The admixture of precursors of the simplest system to synthesize TiO2 nanocrystals is made up of titanium alkoxides, water, a base/acid solution, and other additives for alkoxide-derived TiO2 particles.38 Although a previous study reported methods for producing titania particles with a narrow size distribution using acid and electrolytes,39 the TiO2 Received: July 8, 2011 Accepted: September 1, 2011 Revised: September 1, 2011 Published: September 01, 2011 11620
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Industrial & Engineering Chemistry Research nanocrystals were mostly prepared under strict conditions by trial and error.40 To produce magnetic titania, the coating process is normally employed to deposit titania onto a magnetic material. Therefore, more conditions of the synthesis, such as the number of magnetic particles, the pH, and the temperature, need to be further considered. Because of the lack of previous reports on the use of the waterto-titanium ratio (WTR) to control the morphology of titania precipitated on magnetic materials, in this article, we present experimental results on the physicochemical properties of magnetic titania-coated composites (TiO2/SiO2/Fe3O4, TSM) prepared at various WTR values and the best conditions for degrading DMP using TSM.
2. MATERIALS AND METHODS 2.1. Preparation of Magnetic Titania-Coated Composites (TSM). The preparation of the magnetic core (Fe3O4) and
magnetic carrier (SiO2/Fe3O4, SM) followed the method described elsewhere.20 Previous studies confirmed that the silica film can well protect the magnetic core. The TiO2 gel was synthesized by hydrolysis of titanium tetraisopropoxide (TTIP, 97%, reagent grade, supplied by Aldrich, St. Louis, MO) in a solution composed of water (resistance of 18 MΩ cm, Barnstead, Nanopure) and absolute alcohol (99.5%, reagent grade, supplied by Shimakyu’s Pure Chemicals, Osaka, Japan). Solution A was prepared by mixing 2.5 g of SM with 50 mL of absolute alcohol. Solution B was prepared by adding 17 mL of TTIP slowly to 40 mL of absolute alcohol. Solution C was made of 50 mL of absolute alcohol and the desired amount of water. The water amount used in each case was determined to provide a WTR in the range of 60250. After ultrasonic treatment of solution A for 30 min, solution B was added rapidly into solution A. Then, the combination of solutions A and B was well-mixed under ultrasonic treatment for 30 min. Finally, solution C was added dropwise into the mixture of solutions A and B. Then, the resulting solution was aged at a constant temperature of 293.15 K under ultrasonic treatment. After 5 h, the resulting solution was dried under an infrared lamp for 12 h and subsequently processed by calcination. All of the as-prepared particles were heated at a rate of 5 K/min to 823.15 K and held for 2 h in an atmosphere of air. Finally, the TSM composites were washed several times with doubly distilled water to remove unwanted contaminants. Separation of the magnetic particles from aqueous solution in the heterogeneous system was done simply by means of a magnet. 2.2. Characterization of Physicochemical Properties. The magnetic properties of the particles were measured on a SQUID (superconducting quantum interference device) magnetometer (model MPMS7, Quantum Design Company, San Diego, CA). Transmission electron microscopy (TEM) was performed on a JEM 1400 microscope from JEOL (Tokyo, Japan). These analyses were used to provide insight into the microstructure of the various magnetic particles. The chemical analysis of TSM composites was performed by X-ray photoelectron spectroscopy (XPS; ESCA PHI 5000 VersaProbe/Scanning ESCA Microprobe, ULVAC, Tokyo, Japan), and the data were analyzed using Portable XPSPeak 4.1 software. Energies of 58.7 and 187.85 eV were used for survey and high-resolution spectra, respectively, under monochromatic Al Kα radiation. The crystal structure of the particles was classified by powder X-ray diffraction (XRD; Philips X’PERT Pro MPD, Philips Company, Eindhoven, The
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Netherlands) with Cu Kα radiation (λ = 1.5418 Å) and the Joint Committee on Powder Diffraction Standards (JCPDS) database. UV absorption was measured on a UVvisNIR spectrometer (V-630, Jasco Co., Tokyo, Japan). The zeta potentials of the TSM composites at various pH values under ionic strengths of 0.01 and 0.1 N NaCl were determined on a Zetasizer 3000 apparatus (Malvern Instruments, Malvern, U.K.). To determine the proportion of individual layers in TSM composites, TSM composites of 0.5 g was digested with 9 mL of HNO3 (70%), 3 mL of HF (40%), and 2 mL of HCl (37%) in a microwave oven (Berghof Speedwave MWS-2, Eningen, Germany). After the solution had been filtered through a 0.22-μm membrane filter, the concentrations of Ti, Si, and Fe ions were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES; Optima 3000DV, Perkin-Elmer, Wellesley, MA). 2.3. Photocatalytic Activity. The batch-type annular photocatalytic reactor used was made of pyrex glass and had a diameter and height of 3.6 and 29 cm, respectively, as shown in Figure 1. A 14-W UV lamp (APUV-12, Aquapro Industrial Co. Ltd., Taipei, Taiwan) was placed at the center of the inner tube (i.e., a quartz jacket). The UVvis irradiation intensity under various conditions was measured with a diffraction grating spectrometer (model EPP 2000, StellarNet, Oldsmar, FL). DMP in aqueous solution was selected as the target compound, and its degradation efficiency was used as an indicator to judge the photocatalytic activity of the TSM composites. An initial concentration of 100 mg L1 DMP was used for all photocatalytic experiment to compare the degradation efficiencies of the various TSM composites. The flow rate of oxygen (FO2) was fixed at 500 cm3 min1 with a mass flow controller (MFC; model 5850E, Brooks, Hatfield, PA). Prior to illumination by a UV lamp, the adsorption of DMP on the TSM composites was measured in the dark for a contact time of 30 min. The concentration of DMP was determined by high-performance liquid chromatography (HPLC) with UV detection (UDV 170U, Dionex Corporation, Sunnyvale, CA).
3. RESULTS AND DISCUSSION 3.1. Morphologies and Magnetization of TSM Composites. The TiO2 shell of TSM was prepared by the hydrolysis of
TTIP so that the precipitation of TiO2 could be obtained only when the WTR was greater than 2.5.38 Furthermore, the lower the WTR, the larger the molecular species formed in the system. The values of WTR investigated in this study were 60, 100, 130, 200, and 250. The results showed that, when the WTR was less than or equal to 130, the formation of TiO2 occurred only on the SM particles, so that a clear supernatant solution was obtained after the aging process. Only when the WTR value was greater than or equal to 200 did the solution become turbid. The magnetization characteristics of TSM composites prepared at various values of the WTR are reported in Table 1 and Figure 2. The coating of TiO2 on the parent SM particles resulted in lower saturation magnetization (MS) values for the TSM composites compared to the SM particles, because the magnetic properties of TSM and SM derive from only the magnetic core. Therefore, the MS values of the TSM composites clearly reflect the relative thicknesses of the TiO2 films deposited on the SM particles. The results indicate that the synthesized TSM composites were all superparamagnetic and that the thickest TiO2 film on the SM particles was formed at WTR = 130, because the resulting composite had the lowest value of MS. Furthermore, the results suggest that the TiO2 layer became thicker with increasing 11621
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Figure 1. Schematic diagram of the experimental apparatus used in this study: (1) UV lamp, (2) mass flow controller, (3) gas cylinder, (4) solution pH controller, (5) pump, (6) air stripping head, (7) sampling syringe.
Table 1. Saturation Magnetizationsa of Various TSM Composites Prepared at Various Water Concentrations water-to-titanium
saturation magnetization
ratio (WTR)b
(MS) (emu g1)
60 (TSM60)
15.6
100 (TSM100)
10.0
130 (TSM130) 200 (TSM200)
8.9 9.1
250 (TSM250)
10.5
For parent SM particles, MS = 17.3 emu g1. b TSM composite notation in parentheses. a
water concentration until it reached its maximum thickness at WTR = 130. For WTR > 130, the layer became thinner with increasing WTR, and finally, free TiO2 particles precipitated in the solution, resulting in a colloidal system. Figure 3 illustrates the morphologies of various TSM composites as observed by TEM. The agglomeration of TiO2 occurred in the samples prepared at low WTRs (e.g., WTR = 60). Discrete TiO2 grains with diameters of around 20 nm were clearly observed to form on the SM particles at WTR = 130. At higher water concentrations, the particle size distribution seemed to become wider because of the faster hydrolysis and condensation reactions. Therefore, judging from the above data, we conclude that a critical range of water concentrations exists for which the thickest coating of TiO2 on the surface of SM particles can be obtained in a homogeneous nucleation system. 3.2. Structural and Chemical Analyses of TSM Composites. The XPS results provide insight into the nature and structure of the various TSM composites and the interactions between various layers, as shown in Figure 4. The O 1s XPS patterns of various TSM composites showed two main peaks at 529.6 and 532.6 eV. Curve-fit analysis indicated that they represent TiOTi and SiOSi linkages, respectively.4145 The results of curve-fit analysis of Si 2P peaks revealed that the main peak at
Figure 2. Magnetization curves of various TSM composites.
102.9 eV represents SiOSi bonding.45 The presence of the weak low-energy shoulder of the broad signal at 101.3 eV indicates the structure of SiOTi bonding possibly exists in the TSM composites as well. It is noteworthy that the series of characteristic signals differed by up to 0.4 eV from the values mentioned above because of the nature of TSM materials. Table 2 reports the elemental compositions of various TSM composites determined by XPS analysis. To compare the amounts of TiO2 coating the SM particles obtained at various water concentrations, the Si/Ti ratio was employed, because it reflects the relative abundances of silicon and titanium on the surface of the composites. Because the silica film on the magnetic 11622
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core was fixed for the SM composites, greater Si/Ti ratios indicated thinner TiO2 coatings on the SM composites. A good
agreement was observed between the XPS and SQUID results, indicating that the maximum coating of TiO2 was obtained for sample TSM130. Therefore, the characterization of TSM130 was further investigated, as reported in the remainder of this section. The crystal structure of the representative TSM130 composite was characterized by XRD spectra and confirmed by comparison with the JCPDS database, as illustrated in Figure 5. Commercial TiO2 (i.e., Degussa P25) was also analyzed for comparison. The characteristic diffraction peaks of SM indicate the main formation of short-range order in the silica and magnetite. With respect to the characteristic peaks in the sample of TSM130, the principal crystal structure was anatase phase with rutile phase at a negligible level, whereas Degussa P25 was found to contain both anatase (80%) and rutile (20%) phases. The UV/vis absorption spectra of various materials synthesized in this study are presented in Figure 6. The magnetite exhibited great absorptivity in the investigated wavelength range of 2001000 nm; on the contrary, silica had poor absorptivity throughout the whole range. The absorption of TSM130 can be divided into two sections: The sharp decrease before 400 nm is attributable to absorption by TiO2, whereas the gradual decrease is due to absorption by magnetite in the visible range. The absorption spectra of TSM130 at wavelengths less than 400 nm
Figure 3. TEM images of various TSM composites.
Figure 4. (A) O 1s and (B) Si 2p XPS spectra of various TSM composites. 11623
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Table 2. Elemental Compositions of Various TSM Composites Determined by XPS Analyses element (at. %)
a
Ti
Fe
Si/Ti ratioa
16.1
9.7
0.5
1.66
8.1
18.9
0.2
0.43
9.4
19.3
0.3
0.49
composite
O
Si
TSM60
73.7
TSM130
72.9
TSM250
71.0
Si/Ti ratio determined by dividing Si (at. %) by Ti (at. %).
Figure 7. Zeta potentials of TSM130 as a function of solution pH value at ionic strengths of (O) 0.01 N and (]) 0.1 N NaCl.
Figure 5. XRD spectra of Degussa P25, TSM130, and SM particles.
Figure 8. Photodegradation of DMP using various TSM composites under UV irradiation. Conditions: O2 flow rate (FO2), 500 mL min1; volume of solution, 0.8 L; initial concentration (C0) of DMP, 100 mg L1; and dosage of TSM composite, 2 g L1.
Figure 6. Absorption spectra of various materials.
were also calculated to obtain the energy gap and beginning absorption wavelength, which are equal to 3.2 eV and 387.8 nm, respectively.
The zeta potentials of the TSM130 composites are reported in Figure 7. The pHPZC (i.e., the point of zero charge) of TSM130 is 5.6 and is very sensitive to the pH value of the solution in the pH range of 47. In addition, the analysis of chemical composition by ICP-AES revealed that the mass percentages of TiO2, SiO2, and Fe3O4 contents for the TSM130 composite were 59%, 23%, and 18%, respectively. 3.3. Photocatalytic Activity of TSM Composites. The photocatalytic activities of the TSM composites for the degradation of DMP are illustrated in Figure 8. It was observed that the photodegradation of DMP follows first-order kinetics based on the LangmuirHinshelwood model, so that the rate constant (kobs, min1) can be used to judge the photocatalytic activities of 11624
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Table 3. Kinetic Parameters and DMP Degradation Efficiencies Obtained Using Various TSM Composites composite
kobsa (min1)
r2
Eb (%)
TSM60
0.0018
0.93
28
TSM100 TSM130
0.0034 0.0037
0.96 0.99
47 50
TSM200
0.0018
0.93
30
TSM250
0.0014
0.96
26
Calculated by the pseudo-first-order rate equation r = (dC/dt) = KobsC. b Calculated as E = [(C0 Ct)/C0] 100 with t = 3 h. a
Figure 10. Absorption spectra of UV radiation measured at the periphery of the reactor at various TSM130 dosages. Conditions: radiation source, 14-W UV lamp; O2 flow rate (FO2), 500 mL min1; volume of solution, 0.8 L; and initial concentration (C0) of DMP, 100 mg L1.
Figure 9. Photodegradation of DMP at various TSM130 dosages. Conditions: O2 flow rate (FO2), 500 mL min1; volume of solution, 0.8 L; and initial concentration (C0) of DMP, 100 mg L1.
the various TSM composites, as summarized in Table 3. The values of kobs in sequential order were TSM130 > TSM100 > TSM200 = TSM60 > TSM250. The best degradation efficiency was observed for the sample TSM130. The most likely explanation for this finding is that the TiO2 coating of TSM130 was the maximum among all of the samples prepared. Therefore, the photodegradation of DMP using TSM130 composite was further investigated. In particular, the photodegradation of DMP using Degussa P25 was also investigated to judge the degradation efficiency of DMP by the TSM composites. Obviously, Degussa P25 was superior to TSM130 because of its greater DMP degradation rate (i.e., 0.0159 min1) and efficiency (E = 94%). Furthermore, because the reaction takes place on the surface of the catalyst, the reaction rate based on the mass of TiO2 was also calculated, with values of 0.0080 and 0.0031 min1 g1 being obtained for TSM130 and Degussa P25, respectively. Despite the lower photodegradation efficiency of DMP by TSM130, the TSM130 composite has the advantages of easy control and recovery. The effects of TSM130 dosage on the degradation of DMP were investigated, as shown in Figure 9. The degradation efficiency increased with increasing dosage up to 1.2 g L1 because the generation of electronhole pairs was effectively enhanced. However, when the dosage was higher than this optimum value, the degradation efficiency decreased with increasing dosage, possibly due to agglomeration resulting in lower active surface area and higher turbidity resulting in greater shielding of UV radiation.4648 Figure 10 shows
the UV absorption spectra measured at the periphery of the reactor under various dosages of TSM130. The penetration of the radiation was almost negligible at dosages greater than 1.2 g L1, indicating that the most effective absorption of UV radiation was at a dosage of 1.2 g L1. This clearly demonstrates that the dosage of 1.2 g L1 ensured maximal utilization and minimal detrimental scattering of radiation. The pH of an aqueous solution has a significant impact on the photodegradation of dissolved organic compounds using semiconductor photocatalysts on account of the surface charge, the aggregation of particles, and the shift in the valence and conduction band edges.49 Even though acid dissociation constants are normally used to describe the acidic or basic properties of a solid surface, the acidbase equilibrium on the amphoteric surface of TSM130 can be easily described using the pHPZC, as shown in the equations TiOH þ Hþ f TiOH2 þ TiOH þ OH f TiO
at solution pH < pHPZC at solution pH > pHPZC
ð1Þ ð2Þ
Consequently, the surface charge of TSM130 was significantly affected by the solution pH. Furthermore, the solution pH also plays an important role in solute ionization. Therefore, the amount of adsorption of solutes on the surface of TSM130 is primarily determined by the solution pH. The relationship between the amount of DMP adsorbed on TSM130 and the photodegradation of DMP using TSM130 at various solution pH values was examined, as illustrated in Figure 11. The results show that the adsorption and photodegradation efficiency of DMP both increased with decreasing pH. DMP is in its nonionic form throughout the entire investigation range, so that the dispersion forces and polarization of π electrons dominate the adsorption capacity of DMP on the surface of TSM130. When the solution pH is lower than pHPZC of 5.6, the TSM130 surface is positively charged so as to promote the attraction between electronrich aromatic nuclei and the oxygens of DMP chains and positive sites on the adsorbent surface. Accordingly, acidic solution is beneficial 11625
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’ REFERENCES
Figure 11. Dependence of adsorbed amount on the photodegradation efficiency of DMP using TSM130 as the photocatalyst at various solution pH values. Conditions: O2 flow rate (FO2), 500 mL min1; volume of solution, 0.8 L; initial concentration (C0) of DMP, 100 mg L1; and dosage of TSM composite, 1.2 g L1.
to the coupling of DMP and the TSM130 surface. It was also found that greater adsorption amounts indeed enhanced the photodegradation efficiency. Basic solution not only caused weaker interactions between DMP and the TSM130 surface but also inhibited the formation and lowered the oxidation potential of hydroxyl radicals.50,51 A previous study52 even indicated that the reaction of carbonate ions, which are the byproduct generated during the oxidation process, and hydroxyl radical decrease the oxidation efficiency of organic materials in solutions with pH values greater than 7.
4. CONCLUSIONS Using the wet solgel method, we successfully prepared several magnetic TiO2/SiO2/Fe3O4 photocatalysts that can be easily separated by magnetic field. The best water-to-titanium ratio for the synthesis of the thickest film of TiO2 on the magnetic carrier in a homogeneous nucleation system was determined to be WTR = 130. The characterization of TSM130 by SQUID, UV/vis absorption, and zeta potential analysis demonstrated the physicochemical property of superparamagnetism, the crystal structure of the anatase phase of TiO2, an energy gap of 3.2 eV, and a pHPZC of 5.6. TSM130 presented good photocatalytic activity for DMP, which is suspected of having endocrine-disrupting effects and has a high potential to threaten aquatic environments. The dosage of TSM130 used in the photocatalytic system to ensure the most effective utilization and minimal detrimental scattering of radiation was found to be 1.2 g L1. Furthermore, the photodegradation efficiency was significantly affected by the solution pH. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: +886 4 23590121 ext. 33622. Fax: +886 4 23594276. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank the National Science Council of Taiwan for financial support under Grant NSC 95-2221-E-029-013-MY3.
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