Ag

Environ Sci Pollut Res DOI 10.1007/s11356-014-2678-1

ADVANCED OXIDATION TECHNOLOGIES: ADVANCES AND CHALLENGES IN IBEROAMERICAN COUNTRIES

Synthesis and characterization of TiO2 and TiO2/Ag for use in photodegradation of methylviologen, with kinetic study by laser flash photolysis Dayana Doffinger Ramos & Paula C. S. Bezerra & Frank H. Quina & Renato F. Dantas & Gleison A. Casagrande & Silvio C. Oliveira & Márcio R. S. Oliveira & Lincoln C. S. Oliveira & Valdir S. Ferreira & Samuel L. Oliveira & Amilcar Machulek Jr.

Received: 6 November 2013 / Accepted: 19 February 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract This paper reports the synthesis, characterization, and application of TiO2 and TiO2/Ag nanoparticles for use in photocatalysis, employing the herbicide methylviologen (MV) as a substrate for photocatalytic activity testing. At suitable metal to oxide ratios, increases in silver surface coating on TiO2 enhanced the efficiency of heterogeneous photocatalysis by increasing the electron transfer constant. The sol–gel method was used for TiO2 synthesis. P25 TiO2 was the control material. Both oxides were subjected to the same silver incorporation process. The materials were characterized by conventional spectroscopy, SEM micrography, Xray diffraction, calculation of surface area per mass of catalyst, and thermogravimetry. Also, electron transfers between TiO2 or TiO2/Ag and MV in the absence and presence of sodium formate were investigated using laser flash photolysis. Oxides

Responsible editor: Philippe Garrigues D. D. Ramos : P. C. S. Bezerra : G. A. Casagrande : S. C. Oliveira : M. R. S. Oliveira : L. C. S. Oliveira : V. S. Ferreira : A. Machulek Jr. (*) Instituto de Química, Universidade Federal de Mato Grosso do Sul, Av. Senador Filinto Muller, 1555, CP 549, Campo Grande, MS 79074-460, Brazil e-mail: [email protected] F. H. Quina Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748, CP 26077, São Paulo, SP 05513-970, Brazil R. F. Dantas Departament d’Enginyeria Química, Universitat de Barcelona, Martí i Franquès, 1, 08028 Barcelona, Spain S. L. Oliveira Instituto de Física, Universidade Federal de Mato Grosso do Sul, Av. Senador Filinto Muller, 1555, CP 549, Campo Grande, MS 79074-460, Brazil

synthesized with 2.0 % silver exhibited superior photocatalytic activity for MV degradation. Keywords Heterogeneous photocatalysis . TiO2 . TiO2/Ag . Methylviologen . Laser flash photolysis

Introduction Given the crucial role of water in human life—whether in agriculture, power generation, industrial production, transportation, or other areas—water pollution has become a pressing social issue (Baird 1998). The pursuit of quality of life has often translated into increased consumption, with the accompanying generation of vast amounts of waste, including a wide range of residues. The persistence of residues in surface water is particularly worrying, given its impact on ecosystems and public health (Romero et al. 2004; Yang et al. 2010). Concern over the conservation of aquatic ecosystems has prompted studies for the development of efficient residue removal methods. Sophisticated technologies have recently been used in the treatment of effluents containing multiple compounds. Advanced oxidation processes (AOPs) have gained particular attention for their ability to successfully remove several organic compounds (Basha et al. 2010; Cavalcante et al. 2013; Gozzi et al. 2012; Machulek et al. 2007, 2009; Melo et al. 2009). Environmental decontamination methods based on generation of hydroxyl radicals as oxidizing agents are known as advanced oxidation processes. Hydroxyl radicals can be generated by homogeneous or heterogeneous processes. Heterogeneous photocatalysis, for instance, is based on activation of a semiconductor, such as titanium dioxide (TiO2), by sunlight or artificial light (Andreozzi et al. 1999; Basha et al. 2010; Derbalah et al. 2004; Legrini et al. 1993).

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Among the variety of known semiconductors (including ZnO, ZnS, WO3, CdS, and Fe2O3), TiO2 is the most extensively used, owing to advantageous properties such as chemical stability in a wide pH range, low cost, low toxicity, insolubility in water, and activation by sunlight (Augugliaro et al. 2006; Basha et al. 2010; Bessa et al. 2001; Marcone et al. 2012). TiO2 exhibits the highest photoactivity among semiconductors, yet recombination of the electron/hole pair is described as the chief factor limiting its photoactive efficiency. Attempts have been made to minimize recombination, including the incorporation of metal to the crystal structure (Ahmed 2012; Chen et al. 2010; Matthews 1991; Zaleska 2008). Decreased electron/hole recombination by incorporation of metals into an oxide has been shown to effectively increase oxide photoactivity (Ahmed 2012; Bumajdad et al. 2014; Wang et al. 2008; Zaleska 2008). Noble metal coating of a titanium dioxide surface or embedding into the structure of this material can improve its photocatalytic activity by modifying, for instance, its surface properties (such as surface area per mass and porosity) and by extending the radiation absorption range to the visible electromagnetic spectrum (Linsebigler et al. 1995). Studies on surface modification of titanium oxide report that the percentage of added metal on a semiconductor surface is a key parameter, since metal species participate in the capture of electrons photogenerated during electron excitation in the semiconductor. Evaluating the optimal concentrations of these metals can therefore lead to improvements in the photocatalytic activity of oxides. Investigations on oxide coating have shown that silver significantly improves photocatalytic activity (El-Kemary et al. 2011; Pathak et al. 2005; Zhang et al. 2003; Wang et al. 2008). However, to the best of our knowledge, there is no study describing the electron transfer improvement and consequent photocatalytic improvement of TiO2 particles by doping TiO2 with Ag. The purpose of this study was to synthesize and characterize TiO2 and TiO2/Ag nanoparticles for use in photocatalysis, employing the herbicide methylviologen (MV) as a substrate.

Fig. 1 Thermogravimetric analysis of P25 TiO2 (a) and SG TiO2 (b)

A kinetic study of electron transfer between TiO2 or TiO2/Ag and MV in the presence and absence of sodium formate was performed using laser flash photolysis.

Experimental Synthesis and characterization of TiO2 and TiO2/Ag Eight TiO2 samples were prepared from two matrices—sol– gel (SG) TiO2 and P25 TiO2—with varying concentrations of silver (0.05 to 0.2 mM). Two silver-free samples were used as controls. Using the sol–gel method, a solution of 100 mL of water and 1.5 mL of HNO3 (65 %) was prepared under stirring at 50 °C, followed by slow addition of 16.5 mL of titanium IV isopropoxide. The system was maintained at 50 °C for about 12 h. The resulting TiO2 solution was dried in an oven at 80 °C. The remaining material was macerated in a mortar and calcined in a muffle oven at 450 °C for 1 h to promote crystal phase transition. For silver incorporation, 1.0 g of P25 TiO2 or SG TiO2 was added to 100 mL of water, followed by 2 mL of methanol and silver nitrate at the desired proportions, namely, 0.5 % (0.0078 g), 1.0 % (0.0157 g), 2.0 % (0.0315 g), and 4.0 % (0.0630 g). To prevent the silver from precipitating, the pH was adjusted to 3.0 with nitric acid. This was followed by ultraviolet irradiation (for 8 h under nitrogen bubbling for metal reduction, with formation of zero-valent silver on the photocatalyst surface (El-Kemary et al. 2011)). Finally, the solutions were centrifuged and dried in an oven at 100 °C. Thermogravimetric analysis was performed on a thermobalance, with samples placed in a platinum crucible, at a heating rate of 20 °C min−1 and scan temperatures from 25 to 900 °C. Flow rate of the nitrogen atmosphere in the balance was 40 mL min−1 and for the argon atmosphere in the oven, 60 mL min−1.

Environ Sci Pollut Res Fig. 2 X-ray diffractions of P25 TiO2 (a) and SG TiO2 (b). Filled circles indicate (●) peaks related to anatase crystal phase and filled squares (■), peaks related to rutile crystal phase

X-ray diffractograms of bare and silver-coated oxides were obtained on a Rigaku Miniflex diffractometer, using the powder diffraction method. Surface areas per mass of photocatalyst were determined by analyzing nitrogen adsorption based on Brunauer– Emmett–Teller (BET) isotherms. In the present case, the samples were vacuum-treated at room temperature for about 24 h to remove water and any organic compounds adsorbed onto particle surfaces. The samples were subsequently evaluated for N2 adsorption at 77 K. Diffuse UV–vis reflectance spectroscopy of the solid matrices was performed using a PX2 pulsed xenon lamp and USB-4000 detector (both from Ocean Optics), optical fibers, and Ocean Optics SpectraSuite software. Barium sulfate was used as the reference for total reflection. Scans were performed at 200 and 800 nm. Fourier transform infrared spectra were recorded on a PerkinElmer 100 spectrophotometer in the 4,000–400-cm−1 range. The samples were shaped into pellets using potassium bromide. Scanning electron microscopy (SEM) images were recorded using a JSM-6380 LV scanning electron microscope (Jeol). The samples were mounted on carbon tape and sputter-coated with gold.

Table 1 Surface areas per mass of photocatalysts investigated

Photocatalyst

Surface area per mass (m2/g)

P25 TiO2 P25 TiO2 + 2.0 % Ag SG TiO2 SG TiO2 + 2.0 % Ag

46 43 71 70

Laser flash photolysis for determination of electron transfer constants The kinetic study of electron transfer between TiO2 or TiO2/ Ag and MV in the presence and absence of sodium formate was performed using laser flash photolysis. Presence of this organic additive has been reported to increase the electron transfer constant (Tachikawa et al. 2004). MV, an often overused herbicide in many countries, was selected for the present study for its notable ability as an electron acceptor. The photolysis experiments (Machulek et al. 2006) were performed using an LFP-111 laser flash photolysis spectrophotometer (LuzchemResearch). Excitation in the third harmonic (355 nm, 25 mJ per flash) was provided by a Continuum II-10 Nd: YAG laser (Surelite, Santa Clara, CA, USA).

Fig. 3 Diffuse reflectance (a), absorbance (b), and linear regression (c) as a function of energy (eV) for determining the energy bandgap value of P25 TiO2

Environ Sci Pollut Res Table 2 Energy bandgaps of the photocatalysts investigated Photocatalyst (P25)

Egap (eV)

Photocatalyst (SG)

Egap (eV)

TiO2 TiO2 + 0.5 TiO2 + 1.0 TiO2 + 2.0 TiO2 + 4.0

3.1 2.9 2.8 2.8 2.1

TiO2 TiO2 + 0.5 TiO2 + 1.0 TiO2 + 2.0 TiO2 + 4.0

2.8 2.5 2.3 2.3 1.8

% Ag % Ag % Ag % Ag

% Ag % Ag % Ag % Ag

Four photocatalyst suspensions were prepared—P25 TiO2, P25 TiO2/Ag (2.0 %), SG TiO2, and SG TiO2/Ag (2.0 %)— and MV was added at the following concentrations: 0.05, 0.07, 0.10, 0.15, and 0.2 mM. The pH values of the samples were adjusted to 2 with HClO4 and were subsequently subjected to ultrasonic agitation and placed in the fluorescence cuvettes of the laser flash photolysis device.

Photodegradation procedures Photocatalyst efficiencies were evaluated by the rate of MV photodegradation. The experiments were conducted in an annular glass photochemical reactor of 1-L capacity, with a quartz casing for insertion of the radiation source. Water bath temperature was maintained at around 15 °C by a thermostatic circulator. UV radiation was provided by a 125-W mediumpressure HPL-N mercury vapor lamp (Philips) of 4.59× 1021 s−1 photon flux (Scaiano 1989). The lamp, without its protective casing, was positioned lengthways to the reactor, within a quartz tube. A magnetic stirrer homogenized the solution throughout the experiment. In the degradation assays, 0.5 g of TiO2 or TiO2/Ag was placed in 500 mL of water amended with 10 mL of the MV solution, resulting in a 0.2 mM solution of the herbicide. Reaction time was 90 min, and aliquots of approximately 4 mL were collected at predetermined intervals and filtered. Degradation was monitored by absorption spectrophotometry in the UV–vis range (Hitachi UV–vis spectrophotometer, model U-3000).

Fig. 4 Fourier transform infrared spectra for P25 TiO2 (a) and SG TiO2 (b)

Fig. 5 SEM image of P25 TiO2

Results and discussion Characterization of TiO2 and TiO2/Ag nanoparticles The purpose of incorporating silver onto the SG TiO2 and P25 TiO2 matrices was to increase their photocatalytic efficiency. The titanium oxides obtained were subjected to thermogravimetric analysis in order to evaluate stability or thermodecomposition. P25 TiO2 and SG TiO2 mass losses as a function of temperature and silver concentration are shown in Fig. 1. From 25 to 300 °C, both materials suffered mass loss, probably related to loss of adsorbed water molecules. As shown by the precalcination thermogravimetric curve, mass loss was greater for SG TiO2 (Fig. 1b) than for the other tested materials and more pronounced between room temperature and around 400 °C. In this range, roughly 20 % of its original mass was lost, which may be related to loss of NO3− ions, one of the species used in the synthesis of this material. No thermodecomposition was observed for either oxide in the range of temperatures investigated, irrespective of silver presence. The oxides proved stable at temperatures up to 900 °C. The crystal structures of uncoated and silver-coated oxides were analyzed by X-ray diffraction (Fig. 2). Owing to oxide heterogeneity, rutile and anatase compositions in P25 TiO2 can range from 20:80 to 30:70, respectively

Environ Sci Pollut Res Fig. 6 SEM images. a P25 TiO2 + 0.5 % Ag, b P25 TiO2 + 1.0 % Ag, c P25 TiO2 + 2.0 % Ag, and d P25 TiO2 + 4.0 % Ag

(Ohtani et al. 2010). Experimental and published data were therefore in agreement (Fig. 2a). The characteristic peak of anatase was found at 2θ=25.3 (highest intensity peak for this phase) and that of rutile, at 2θ=27.9. SG TiO2 diffractograms (Fig. 2b) were similar to those of P25 TiO2 (Fig. 2a), but differed in the ratio between rutile and anatase peak intensities, indicating lower crystallinity and increased surface area per mass in SG TiO2 (Table 1). In both oxides, silver-related peaks were of low intensity, blending with the spectral background noise (González et al. 2001). The low concentration of silver crystals observed at the end of the process precludes a more thorough interpretation of these results, yet it possibly indicates that silver is not present in the structure of the material but coats the oxide instead. Both cubic and hexagonal silver nanoparticles might coexist as a result of the process. Metal coating of oxides affects area-per-mass values, as shown by the fact that P25 TiO2 coated with 2.0 % silver

Fig. 7 SEM image of SG TiO2

exhibited smaller surface area per mass of catalyst (43 m2/g) than the bare material. The same pattern was observed for silver-coated SG TiO2 (70 m2/g). To determine the energy bandgap (Egap) values of the samples, diffuse reflectance spectroscopy experiments were performed (Fig. 3). Extrapolation of the absorption curve provided the Egap value of each oxide, a relevant parameter for the photocatalytic process—the smaller the Egap value of a material, the higher its photocatalytic potential, as it will need lower-energy photons to be activated. (Visible light can occasionally be used for this purpose.) Figure 3 shows the behavior of absorbance and diffuse reflectance, from which the Egap value was determined for P25 TiO2. Egap values of the other photocatalysts were determined in the same manner. Table 2 lists the Egap values of the photocatalysts investigated. The Egap value of 3.1 eV experimentally obtained for P25 TiO2 (Table 2) is close to that reported in the literature (3.2 eV) (Sobczyński and Dobosz 2001). For silver-coated photocatalysts, Egap was lowest for bare P25 TiO2. Egap of the oxide coated with 4.0 % silver was around 32 % lower than that for bare TiO2—the greater the amount of added silver, the lower the energy bandgap, indicating that coating reduces Egap, extending radiation absorption to the visible, longer-wavelength, lower-energy region. The same pattern was observed for SG TiO2, with a more pronounced difference of 36 % between the bare oxide and oxide coated with 4.0 % silver. Figure 4 depicts the curves corresponding to the infrared spectrum of P25 TiO2 and SG TiO2. The infrared spectra of both oxides showed bands in the 3,000–3,500-cm−1 range, probably related to stretching vibration by OH groups from adsorbed water. Absorption bands were also found at 600 cm −1 , indicating the presence of O–Ti–O bonds

Environ Sci Pollut Res Fig. 8 SEM images. a SG TiO2 + 0.5 % Ag, b SG TiO2 + 1.0 % Ag, c SG TiO2 + 2.0 % Ag, and d SG TiO2 +4.0 % Ag

(Silverstein et al. 1979). Figure 4b depicts the curve corresponding to uncalcined SG TiO2, with a peak in the vicinity of 1,300 cm−1, probably related to adhered nitrate groups from nitric acid used in the synthesis process. Presence of these groups was also observed in thermogravimetric analysis. SEM images showed that the oxides constituted particle aggregates of large size and rounded morphology (Figs. 5 and 6). The effect of silver on the morphology of particles can be observed by comparing SEM images of the modified oxides with those of the control sample. Silver coating did not alter TiO2 morphology, relative to the bare material, yet caused TiO2 particles to grow in size. SEM images of SG TiO2 are shown in Fig. 7. Note the lack of uniformity in the material, with some larger particles than those seen in P25 TiO2, dispersed on a submicrometer surface. As shown in Fig. 8, the amount of silver affects oxide morphology. In materials with 0.5 % (Fig. 8a) and 1.0 % silver (Fig. 8b), particles are larger than in those with 2.0 % (Fig. 8c) or 4.0 % silver (Fig. 8d), whose particles are more rounded, similar to those of P25 TiO2. This indicates that higher amounts of silver were responsible for the more pronounced surface uniformity of SG TiO2 Figure 9 shows energy-dispersive X-ray (EDS) spectra generated under the same conditions as the SEM images. The spectra served to identify the constituents of samples and to confirm the presence of silver in the coated materials. Investigation of electron transfer between TiO2 or TiO2/Ag and methylviologen, based on laser flash photolysis In photochemical reactions, electron transfer mechanisms can be advantageously investigated by means of transient

absorption spectroscopy. In the present study, this technique was employed to elucidate MV reduction during the photocatalytic reaction on TiO2 in the presence and absence of sodium formate, as well as on TiO2 coated with 2.0 % silver. To elucidate the kinetics of MV2+ photocatalysts, formation of MV+ was estimated at different concentrations of the former (0.05, 0.07, 0.1, 0.15, and 0.2 mM). The transient absorption spectra of MV + were observed at 605 nm (Tachikawa et al. 2004). Figures 10 and 11 show the MV+ absorption spectra generated by laser flash photolysis for bare and silver-coated SG TiO2 in the presence and absence of sodium formate. The same procedure was performed for P25 TiO2. The rate constants for electrons transfer (kETobs) were extracted by fitting the rate of decay of the transient, which are shown in Figs. 10 and 11. kET values were determined from the linear relationship between MV2+ concentrations (0.05– 0.2 mM) and kETobs values (see Fig. 12). These values are near the kinetics of MV2+ reduction (6.4×109 M−1 s−1) by the CO2•− generated from the technique of pulse radiolysis (Das et al. 2003). No absorption of transient MV2+ was observed after the laser pulse at 355 nm of the MV2+ aqueous solution in the absence of TiO2 particles, suggesting that absorption of transient MV2+ observed at 605 nm in the presence of TiO2 particles came from electron transfer from the TiO2 directly to MV2+ or was also mediated by the CO2•− radical when HCO2Na is present in the solution (Tachikawa et al. 2004). Decay of these transients obeys first-order kinetics (Tachikawa et al. 2004). The electron transfer constants were obtained through a graph expressing the linear relationship of MV2+ initial concentrations versus decay constants of transients. Figure 12 shows the rate constant of this process for SG TiO2

Environ Sci Pollut Res Fig. 9 Energy-dispersive X-ray (EDS) spectra of P25 TiO2 (a) and SG TiO2 (b)

in the absence of sodium formate. The same procedure was performed for the other photocatalysts (Fig. 3). Presence of sodium formate increased the electron transfer constants of all the photocatalysts investigated, probably because initial oxidation of organic additives such as sodium Fig. 10 Laser flash photolysis for SG TiO2 in the presence (a) and absence (b) of sodium formate (HCO2Na)

formate generates CO2− radicals with strong reducing power, capable of promptly reducing other substrates (Tachikawa et al. 2004). In the absence of sodium formate, a direct process takes place: as light reaches the particle, the valence band electron

Environ Sci Pollut Res Fig. 11 Laser flash photolysis for SG TiO2 + 2.0 % Ag in the presence (a) and absence (b) of sodium formate (HCO2Na)

Fig. 12 Constants of electron transfer from SG TiO2 to MV2+ in the absence of sodium formate

of CO2−, a strong reductant. The probable mechanism is depicted in Scheme 4. The electron transfer rates for the silver-coated semiconductor are higher than those for bare counterparts (Table 3). To e s t i m a t e p h o t o c a t a l y s t e ff i c i e n c y f o r M V photodegradation, the process was monitored using UV–vis absorption spectroscopy. Absorption was highest around 250 nm, corresponding to the absorption bands of chromophore groups in the herbicide. Spectroscopic analysis revealed that the photodegradation process causes a significant decrease in the herbicide’s absorption band with time. Investigation of photocatalyst efficiency using methylviologen photodegradation

migrates to the conduction band, forming an electron/hole pair. This electron is transferred to MV2+, forming MV+ (Scheme 1). In the presence of sodium formate, the following mechanism unfolds: given its availability in greater amounts than MV, the electron at the conduction band is first transferred to a formate ion, generating a CO2− radical, which, as a strong reductant, reacts with MV2+ forming MV+ (Scheme 2). As shown in Table 3, presence of silver also increases the electron transfer constant even in the absence of sodium formate because, in addition to direct electron transfer to MV2+, the metal is also transferred to this species (Scheme 3). In the presence of formate, the constant is increased, possibly because, in addition to direct electron transfer to the formate ion, the metal is also transferred, further promoting formation

Figure 13 shows the results of photodegradation experiments using P25 TiO2 and SG TiO2 with different proportions of silver. The rate of herbicide degradation in the presence of oxides was higher than in the photolysis procedure, except for SG TiO2 with 4.0 % Ag (Fig. 13). Irrespective of silver coating, SG TiO 2 exhibited a lower degradation rate (Fig. 13b) than P25 TiO2 (Fig. 13a), a feature that may result, for instance, from preparation method, crystal structure, surface area per mass, particle size distribution, or porosity— factors that play a role in the formation of electron/hole pairs in adsorption and redox processes. Despite the larger surface area per mass of SG TiO2, its morphology (nonuniform particles, more uneven size

Scheme 1 Proposed mechanism for reduction of MV2+ in the absence of sodium formate

Scheme 2 Proposed mechanism for MV2+ reduction in the presence of sodium formate

Environ Sci Pollut Res Table 3 Electron transfer constants obtained for photocatalysts in the absence and presence of sodium formate Photocatalyst

Absence of HCO2Na

P25 TiO2 P25 TiO2 + 2.0 % Ag SG TiO2 SG TiO2 + 2.0 % Ag

5.4×109 6.5×109 3.2×109 4.5×109

M−1 M−1 M−1 M−1

s−1 s−1 s−1 s−1

Presence of HCO2Na 6.0×109 8.0×109 3.6×109 6.6×109

M−1 M−1 M−1 M−1

s−1 s−1 s−1 s−1

distribution, and possibly lower porosity) differed from that of P25 TiO2. Also, these materials differed in phase ratios, with P25 TiO2 exhibiting anatase to rutile ratios that are more suitable for photocatalytic efficiency. These differences were also observed in the silver-coated oxides, proportionally to their silver contents. For both SG TiO2 and P25 TiO2, photocatalytic performance was higher in oxides coated with 2.0 % silver, a result possibly influenced by surface area, energy bandgap, and silver concentration. Photocatalytic performance was lower for oxides with a 4.0 % silver coating, even though they exhibited lower Egap values than other oxides, possibly because maximum saturation was achieved on the surface of semiconductor particles and excess silver occupied the active sites of the catalyst, decreasing the catalytic activity by lowering the incidence of radiation on TiO2 particles and decreasing the number of active sites for MV adsorption.

Conclusions 2+

Scheme 3 Proposed mechanism for MV silver and absence of sodium formate

reduction in the presence of

Scheme 4 Proposed mechanism for MV2+ reduction in the presence of silver and sodium formate

Fig. 13 Photodegradation of P25 TiO2 (a) and SG TiO2 (b)

This investigation of the synthesis, characterization, and application of silver-coated titanium oxides to degradation of MV revealed that surface modifications of TiO2 with suitable amounts of metal enhance the efficiency of heterogeneous photocatalysis. This effect was also observed in laser flash photolysis experiments, with the presence of silver increasing electron transfer constants. With higher surface area per mass and lower energy bandgap, SG TiO2 facilitated photocatalysis and photodegradation, compared with P25 TiO2. In contrast with P25 TiO2, however, SG TiO2 exhibited a nonhomogeneous structure and lower degradation rate, irrespective of silver coating. The presence of silver enhanced the photocatalytic efficiency of the oxides, more markedly in the material coated with 2.0 % silver. This pattern was also observed in

Environ Sci Pollut Res

laser flash photolysis experiments, with increased electron transfer constants in the presence of silver or sodium formate. Acknowledgments The authors wish to thank the Brazilian funding agencies CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), and FUNDECT (Fundação de Apoio ao Desenvolvimento do Ensino, Ciência e Tecnologia do Estado de Mato Grosso do Sul) for their financial support. A.M.Jr. is associated with NAP-PhotoTech, the USP Research Consortium for Photochemical Technology, and INCT-EMA (Instituto Nacional de Ciência e Tecnologia de Estudos do Meio Ambiente).

References Ahmed S (2012) Impact of operating conditions and recent developments in heterogeneous photocatalytic water purification process. Crit Rev Environ Sci Technol 42(6):601–675 Andreozzi R, Caprio V, Insola A, Marotta R (1999) Advanced oxidation processes (AOP) for purification and recovery. Catal Today 53:51– 59 Augugliaro V, Litter M, Palmisano L, Soria J (2006) The combination of heterogeneous photocatalysis with chemical and physical operations: a tool for improving the photoprocess performance. J Photochem Photobiol C 7:127–144 Baird C (1998) Environmental chemistry, 2nd edn. WH Freeman and Company, New York Basha S, Keane D, Morrissey A, Nolan K, Oelgemoller M, Tobin J (2010) Studies on the adsorption and kinetics of photodegradation of pharmaceutical compound, indomethacin using novel photocatalytic adsorbents (IPCAs). Ind Eng Chem Res 49:11302–11309 Bessa E, Sant’anna GL Jr, Dezotti M (2001) Photocatalytic/H2O2 treatment of oil field produced waters. Appl Catal B 29:125–134 Bumajdad A, Madkour M, Abdel-Moneam Y, El-Kemary MRP (2014) Nanostructured mesoporous Au/TiO2 for photocatalytic degradation of a textile dye: the effect of size similarity of the deposited Au with that of TiO2 pores. J Mater Sci 49:1743–1754 Cavalcante RP, Sandim LR, Bogo D, Barbosa AMJ, Osugi ME, Blanco M, Oliveira SC, Matos MFC, Machulek A Jr, Ferreira VS (2013) Application of Fenton, photo-Fenton, solar photo-Fenton, and UV/ H2O2 to degradation of the antineoplastic agent mitoxantrone and toxicological evaluation. Environ Sci Pollut Res 20:2352–2361 Chen X, Shen S, Guo L, Mao SS (2010) Semiconductor-based photocatalytic hydrogen generation. Chem Rev 110:6503–6570 Das TN, Ghanty TK, Pal H (2003) Reactions of methyl viologen dication (MV2+) with H atoms in aqueous solution: mechanism derived from pulse radiolysis measurements and ab initio MO calculations. J Phys Chem A 107(31):5998–6006 Derbalah AS, Nakatani N, Sakugawa H (2004) Photocatalytic removal of fenitrothion in pure and natural waters by photo-Fenton reaction. Chemosphere 57:635–644 El-Kemary M, Abdel-Moneam Y, Madkour M, El-Mehasseb I (2011) Enhanced photocatalytic degradation of safranin-O by heterogeneous nanoparticles for environmental applications. J Lumin 131: 570–576 González VR, Pineda FMM, Angel PD, Cuchillo OV, Gómez R (2001) Sol–gel and impregnated prepared silver TiO2 semiconductors as photocatalysts for the UV decomposition of 2,4-D: a comparative study of the preparation method. J Sol-Gel Sci Technol 59:57–62

Gozzi F, Machulek A Jr, Ferreira VS, Osugi ME, Santos APF, Nogueira JA, Dantas RF, Esplugas S, Oliveira SC (2012) Investigation of chlorimuron-ethyl degradation by Fenton, photo-Fenton and ozonation processes. Chem Eng J 210:444–450 Legrini O, Oliveros E, Braun AM (1993) Photochemical processes for water treatment. Chem Rev 671–698 Linsebigler AL, Lu G, Yates JT (1995) Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results. Chem Rev 95:735– 758 Machulek A Jr, Vautier-Giongo C, Moraes JEF, Nascimento CAO, Quina FH (2006) Laser flash photolysis study of the photocatalytic step of the photo-fenton reaction in saline solution. Photochem Photobiol 82:208–212 Machulek A Jr, Moraes JEF, Vautier-Giongo C, Silverio CA, Friedrich LC, Nascimento CAO, Gonzalez MC, Quina FH (2007) Abatement of the inhibitory effect of chloride anions in the photo-Fenton process. Environ Sci Technol 41:8459–8463 Machulek A Jr, Moraes JEF, Silverio CA, Okano LT, Quina FH (2009) Photolysis of ferric ion in the presence of sulfate or chloride ions: implications for the photo-Fenton process. Photochem Photobiol Sci 8:985–991 Marcone GPS, Oliveira AC, Almeida G, Umbuzeiro GA, Jardim WF (2012) Ecotoxicity of TiO2 to Daphnia similis under irradiation. J Hazard Mater 211–212:436–442 Matthews RW (1991) Photooxidative degradation of coloured organics in water using supported catalysts. TiO2 on sand. Water Res 25:1169– 1176 Melo SAS, Trovó AG, Bautitz IR, Nogueira RFP (2009) Degradação de fármacos residuais por processos oxidativos avançados. Quim Nova 32:188–197 Ohtani B, Prieto-Mahaney OO, Li D, Abe R (2010) What is Degussa (Evonik) P25? Crystalline composition analysis, reconstruction from isolated pure particles and photocatalytic activity test. J Photochem Photobiol A 216:179–182 Pathak P, Meziani MJ, Castillo L, Sun Y (2005) Metal-coated nanoscale TiO2 catalysts for enhanced CO2 photoreduction. Green Chem 7: 667–670 Romero AHH, Hernández CT, Malo EA, Mendoza RB (2004) Water quality and presence of pesticides in a tropical coastal wetland in southern Mexico. Mar Pollut Bull 48:1130–1141 Scaiano JC (1989) CRC handbook of organic photochemistry. CRC Press, Boca Raton Silverstein RM, Bassler GC, Morrill TC (1979) Identificação espectrofotométrica de compostos orgânicos. Editora Guanabara, Rio de Janeiro Sobczyński A, Dobosz A (2001) Water purification by photocatalysis on semiconductors. Pol J Environ Stud 10:195–205 Tachikawa T, Tojo S, Fujitsuka M, Majima T (2004) Direct observation of the one-electron reduction of methyl viologen mediated by the CO2 radical anion during TiO2 photocatalytic reactions. Langmuir 20:9441–9444 Wang HW, Lin HC, Kuo CH, Cheng YL, Yen YC (2008) Synthesis and photocatalysis of mesoporous anatase TiO2 powders incorporated Ag nanoparticles. J Phys Chem Solids 69:633–636 Yang H, An T, Li G, Song W, Cooper WJ, Lu H, Guo X (2010) Photocatalytic degradation kinetics and mechanism of environmental pharmaceuticals in aqueous suspension of TiO2: a case of βblockers. J Hazard Mater 179:200–207 Zaleska A (2008) Doped-TiO2: a review. Recent Patents Eng 2:157–164 Zhang L, Yu JC, Yip HY, Li Q, Kwong KW, Xu A, Wong PK (2003) Ambient light reduction strategy to synthesize silver nanoparticles and silver-coated TiO2 with enhanced photocatalytic and bactericidal activities. Langmuir 19:10372–10380