Malachite green degradation in simulated wastewater using Nix:TiO2 thin films

Fuel 110 (2013) 17–22

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Malachite green degradation in simulated wastewater using Nix:TiO2 thin films D. Solís-Casados a,b,⇑, L. Escobar-Alarcón c, M. Fernández d, F. Valencia a a

Centro Conjunto de Investigación en Química Sustentable UAEM-UNAM, Carretera Toluca-Atlacomulco Km 14.5, Unidad San Cayetano, Toluca, Estado de México, C.P. 50200, Mexico Personal académico de la Facultad de Química, UAEM, Mexico c Departamento de Física, Instituto Nacional de Investigaciones Nucleares Apdo, Postal 18-1027 México D.F. 1180, Mexico d Departamento de Aceleradores, Instituto Nacional de Investigaciones Nucleares Apdo, Postal 18-1027 México D.F. 1180, Mexico b

h i g h l i g h t s " Ni incorporation into TiO2 narrows the band gap from 3.0 eV to 2.6 eV. " Nix:TiO2 thin films were used as catalysts in simulated wastewater. " Escherichia coli and sucrose dissolved in the MG solution affect the degradation performance.

a r t i c l e

i n f o

Article history: Received 15 April 2012 Received in revised form 3 October 2012 Accepted 19 October 2012 Available online 27 November 2012 Keywords: Nix:TiO2 Thin films Photocatalysis Malachite green dye

a b s t r a c t Growing problems associated with the increased production of wastewater have encouraged studies on the development of photocatalytic materials for wastewater treatment to decrease its impact on the environment and on human health. In particular, one important issue is related to the presence of pollutants, such as bacteria and dissolved organic constituents, in wastewater. There have been various approaches for wastewater treatment using semiconductors as photocatalytic materials. Among them, the most widely used is based on titanium dioxide (TiO2) thin films; however, the applications of TiO2 have been limited by its low quantum yield and relatively large band gap. Therefore, research has been focused on the improvement of the photocatalytic properties of TiO2 by doping it with metals such as Fe, Co, Ni, Au, Ag, Pt and oxides such as WO3. The main aim of this work is to report the preparation of TiO2 thin films modified with different nickel content (Nix:TiO2) using the sol–gel technique and their application as catalysts in the degradation of a dye solution unpolluted and polluted with some easier organic material, such as sucrose and Escherichia Coli (E. coli) microorganisms, which were used as a first approximation of wastewaters pollutants to assess whether the dye is degraded in presence of pollutants. From EDS and XPS analysis, was determined atomic nickel content of about 1, 5, 10 and 20 at.%. The results of the catalytic performance showed that Ni1:TiO2 was able to obtain the same percent of degradation of malachite green (MG) when both polluted and unpolluted dye solutions were employed. The Ni10:TiO2 and Ni20:TiO2 catalysts showed a decrease in their catalytic performance when pollutants were present in the MG solution. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The degradation of organic compounds contained in wastewater is an important environmental issue because the degradation process of organic compounds occurs slowly under solar irradiation, often taking several days to reach complete mineralization. When the natural process (photolysis) is converted into a catalytic process using a catalyst that is subjected to an irradiation source (photoca-

⇑ Corresponding author at: Personal académico de la Facultad de Química, UAEM, Mexico. E-mail addresses: [email protected] (D. Solís-Casados), luis.escobar@inin. gob.mx (L. Escobar-Alarcón). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.10.042

talysis), the degradation process can be enhanced, mineralizing the organic compounds in a fast and efficient manner [1]. The photocatalytic process is currently recognized as an efficient method for the removal of organic pollutants, such as pesticides, dyes or organic compounds present in the aqueous phase [2]. There have been various approaches for wastewater treatment using semiconductors as photocatalytic materials. The most widely used are based on titanium dioxide (TiO2), mainly in the powder and thin film forms due to its ease of recovery from the treated solution. However, the applications of TiO2 have been limited by its low quantum yield and relatively high band gap, therefore the degradation process requires activation by UV light. Further research has been focused on the improvement of the photocatalytic properties of TiO2 by increasing the fraction of the solar spectrum used (UV and visible

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light) to optimize and to enhance its catalytic performance. The main approach to improve the photocatalytic properties has relied on doping TiO2 with metals such as Fe, Co, Ni, Au, Ag, Pt and oxides such as WO3 [3–6]. In general, doping with transition metal nanostructures induces a modification in the TiO2 electronic state, leading to an increase in the electron–hole separation and extending the light absorption into the visible range by narrowing the band gap, resulting in enhanced photocatalytic activity. The incorporation of doping agents into catalysts, in powder and/ or thin film form has been performed by several techniques, including with evaporation induced self assembly (EISA) [7], deposition– precipitation [8], solvothermal and hydrothermal methods [9], sol–gel [10], micro-emulsion [11], chemical vapor deposition [10], laser ablation [12], spray pyrolysis [13], and sputtering [14], among others. It is worth mentioning that catalysts in thin film form are of great interest because the issue of catalyst separation from the treated solution is avoided. Until now, TiO2-based catalysts have shown good results in the photodegradation of several dyes used as organic model compounds. It has been reported that the presence of Ni within the anatase lattice increases the effectiveness of the electrophotocatalytic process as evidenced by a faster color removal rate of the dye Orange II when it is compared to the undoped TiO2 [15]. Some researchers observed that the photocatalytic activity reforming glucose using TiO2–SiO2 catalyst increased with the addition of Ni, because a decrease in band gap energy, which is attributed to the lower energy needed to transfer an electron from the valence to the conduction band [16]. Additionally, there are also reports that doping TiO2 with foreign elements is one of the most promising ways to activate TiO2 under visible light irradiation and also to inhibit the recombination of electron–hole pairs [17]. N-doped TiO2 films have been prepared to study the photocatalytic activity in the degradation of methylene blue (MB) dye. The results showed that the Ndoped TiO2 films exhibit a higher photocatalytic activity in the visible range, reaching a greater MB degradation in comparison with undoped samples [18]. One of the most interesting dyes is malachite green, which has previously been reported as harmful pollutant in wastewaters from textile and fish industries. Malachite green is widely used in the aquaculture industry world-wide as a topical treatment; it is also used as a food coloring agent, food additive, a medical disinfectant and anthelminthic as well as a dye in silk, wool, jute, leather, cotton, paper and acrylic industries [19]. However, malachite green is environmentally persistent and acutely toxic to a wide range of aquatic and terrestrial animals causing serious public health hazards and also poses potential environmental problem [20]. Most catalysts have been studied under laboratory conditions, and their catalytic performance is often diminished, or even lost, when they are tested under more real wastewater conditions, reason to study the catalytic performance of catalysts in less ideal systems. Therefore, great interest has been generated in the last few years in the photodegradation of organic compounds as well as the photocatalytic inactivation of microorganisms in water to clean and purify wastewater [21–24]. Although inactivation of microorganisms has been recently studied, there are few reports on the photocatalytic inactivation of Escherichia coli in wastewater. Disinfection of water by photocatalysis has been reported using a solid catalyst, such as TiO2 (P25) and TiO2 modified with transition metals [25]. However, to the best of our knowledge, there have not been any studies on the effect of microorganisms and dissolved organic matter on the photocatalytic performance of this catalyst in simulated real wastewater. The main aim of this work was to study the effect of add pollutants to the malachite green solution, particularly E. coli (representing bacteria in wastewaters) and sucrose (representing other dissolved organics), to gain insight into the suitable experimental conditions required for the preparation of a catalyst with improved catalytic performance in aqueous polluted systems to assess whether the dye is preferentially degraded over E. coli and sucrose.

2. Experimental procedures 2.1. Thin film preparation Nix:TiO2 thin films were obtained on glass substrates (1  1 in.) by spin coating. The sol–gel method was used to obtain the precursor gel to prepare the films. These solutions were prepared using 2 ml of titanium isopropoxide (i-PrO, 97% Aldrich) and 0.0218, 0.1150, 0.2425 and 0.5464 g of nickel nitrate (Ni(NO3)2 6H2O, Baker). To obtain Nix:TiO2 films with different amounts of Ni, the starting materials were mixed in different proportions. Precursors were dissolved in 10 ml of 2-propanol (Fermont) under vigorous stirring for 4 h to obtain a sol; then, 1 ml of nitric acid was added drop wise until the sol began forming a gel, avoiding the hydrolysis step. The resulting transparent gel was spun onto glass substrates at 3000 rpm to deposit the thin films. As a final step, the films were subjected to annealing at 400 °C for 2 h to obtain a crystalline material; the annealing temperature was chosen lower than the melting point of the glass used as substrate. 2.2. Thin film characterization Raman spectroscopy (RS) was used to study the microstructure of the films. The Raman spectra were acquired using an HR LabRam 800 system equipped with an Olympus BX40 confocal microscope. A Nd:YAG laser beam (532 nm) was focused by a 100 objective onto the sample surface (1 lm diameter spot). The laser power delivered to the sample was regulated by a neutral density filter (OD = 1) to prevent sample heating and structural changes in the sample. The crystalline structure of the deposited thin films was determined by X-ray diffraction (XRD) using a Bruker D8 Advance Diffractometer, using the Cu Ka radiation line (kk = 1.5406 Å). The diffraction patterns were recorded from 2° to 80°. Elemental chemical composition of the thin films was determined by X-ray Photoelectron Spectroscopy (XPS) using a Jeol JPS 9200 XPS using an Mg Ka X-ray source (1253.6 eV), this technique provides also detailed chemical information about the surface-near surface regions of almost any type of solid material. Spectra were acquired in both low and high-resolution to determine the nickel interaction with the TiO2 crystalline lattice. Energy dispersive X-ray spectroscopy (EDS) was used to determine the atomic nickel content on an Oxford EDX probe with a resolution of 137 eV coupled to a JEOL JSM 6510LV microscope. The band gap was determined by the Tauc plot method from the UV–Vis spectra, which were recorded on a Varian Cary 5000 spectrophotometer. From the UV– Vis spectra, and applying the Goddman’s method [26] for transparent films, the thicknesses of the deposited materials were calculated. 2.3. Cultivation of the microorganisms The bacterial strain used in the present experiments was obtained from the American Type Culture Collection (ATCC). The microorganism solutions were prepared by taking an aliquot of microorganisms from the ATCC tube and inoculating them in an assay tube containing an agar culture. Then, the microorganisms were incubated for 24 h at 38 °C, and an aliquot was taken from this assay tube and dissolved in water to form a suspension. The turbidity of the prepared microorganism suspensions was adjusted to the 0.5 McFarland scale to obtain suspensions with a similar number of bacteria. This corresponds to an approximate initial concentration of 1.5  108 colony-forming units (CFU). 2.4. Photocatalytic experiments Catalytic performance of the thin films was studied via the degradation of a 25 ml of polluted or unpolluted solution of 10 lmol/l

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Raman intensity (arb. units)

of malachite green in form of hydrochloride with empirical formula C23H25ClN2. The catalytic system was illuminated using a 6 Watts UV lamp with emission at 254 nm (model UVGL-58, multiband UV–254/366 nm) at a height of 15 cm from the solution surface. A photolysis reaction was performed as a reference to compare the conversion percent obtained for the photocatalytic systems. Dye degradation was followed by a decrease in the characteristic absorption band of the MG solution at 618 nm over the reaction time. Afterward, the thin films were evaluated in polluted systems to demonstrate their catalytic response in the presence of contaminants. This step was performed by adding organic dissolved matter (0.03, 0.06 and 0.12 g sucrose) and (1, 2, 3, 4 and 5 ml of an E. coli solution) bacteria to the MG solution with an initial concentration of 1.5  108 CFU (Mc Farland scale 0.5).

Ni 20:TiO2 Ni 10:TiO2 Ni 5:TiO2 Ni 1:TiO2 Ni 0:TiO2

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-1

3. Results and discussion

Raman shift (cm )

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3.1. Catalyst characterization

Raman intensity (arb. units)

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Ni20 :TiO2 Ni10 :TiO2 Ni5 :TiO2 Ni1 :TiO2 Ni0 :TiO2

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-1

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Thin films with a mean thickness of 225 ± 14 nm were obtained using the Goodman method. Elemental composition results, as determined by XPS and EDS analysis, reveal that the nickel contents in the catalytic formulations prepared were close to 0, 1, 5, 10 and 20 at.%. The Raman spectra of all films showed peaks at 146, 195, 400, 521 and 640 cm 1 (Fig. 1a), which are characteristic of crystalline TiO2 in its anatase phase [27]. Additionally, as the quantity of Ni added was increased in the films, a new feature appears at approximately 556 cm 1 (Fig. 1b). This new signal is attributed to the formation of NiO [28] suggesting that in case of high Ni content in the film, a mixture of TiO2 and NiO phases are formed. Additionally, Fig. 1c reveals that as more nickel is added to the material, a slight shift in the 146 cm 1 Raman peak toward lower frequencies is observed. This shift indicates that the Ni is incorporated into the anatase lattice, occupying Ti sites [15]. The X-Ray diffraction results showed low-intensity broad diffraction lines located at 25.7°, 38.1° and 48.4° (patterns not presented), confirming the presence of the anatase crystalline phase (JCPDS 00-001-0562). However, it was not possible to observe any change in these diffraction peak locations due to the nickel addition. In contrast, the absence of diffraction lines corresponding to the NiO crystalline phase could be attributed to a dilution effect or to very small crystallite sizes. Fig. 2 shows the high-resolution XPS spectra of the (a) Ti 2p3/2, (b) Ni 2p3/2, and (c) O 1s regions. Fig. 2a shows the Ti peaks at 457.8 and 462.0 eV, that are attributed to the binding energy of Ti 2p3/2 and Ti 2p1/2 in TiO2 in the anatase phase, according to the NIST database [29]. The peak at 457.8 has a slight shift towards lower binding energies (457.5 eV) with the increase of the nickel amount in the films. This shift could attributed to the introduction of Ni into the TiO2 lattice. All peak positions were charge corrected relative to the C 1s signal at 285.0 eV. Fig. 2b, corresponding to the Ni 2p3/2 region, shows the doublet assigned to the Ni 2p1/2 signal. The peak located at 851.2 eV clearly increases in intensity as Ni is added to the film, reaching a maximum for the sample with Ni20:TiO2. This peak can be attributed to the formation of NiO on the surface of the films with the increase of nickel content [30]. However, an increase in the nickel content produces a greater quantity of nickel oxide deposited on the surface of the films, as observed by an increase in the 851.2 eV signal intensity. These data support the results obtained by Raman spectroscopy. The O 1s region, shown in Fig. 2c, reveals a slight shift in the peak located at 529.2 eV towards 528.7 eV. This shift to lower binding energies appears to indicate a gradual change in the binding interactions in TiO2 and supports the idea of the introduction of Ni2+ into the TiO2 lattice. The differences in measured binding energies are direct registration of the degree of ionicity in the corresponding MO bonds.

640

521

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Ni 20:TiO2

Ni 10:TiO2 Ni 5 :TiO2 Ni 1 :TiO2 Ni 0 :TiO2

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Raman shift (cm )

(c) Fig. 1. Raman spectra of the prepared Nix:TiO2 thin films.

Fig. 3a shows the UV–Vis transmittance spectra of the Nix:TiO2 films acquired in the 200–900 nm wavelength range. Two main differences are clearly observed. First, the absorption edges are shifted to higher wavelengths depending on the Ni content. Second, the transmittance is reduced from approximately 90% for the sample without Ni to approximately 60% for the sample with 20 at.% Ni. From these transmittance spectra, the band gap was determined according to the Tauc plot method, which was performed by

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Ti 2p 3/2 457.5

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Ti 2p1/2 463.1

80 Ni20 :TiO2 Ni10 :TiO2 Ni5 :TiO2 Ni1 :TiO2

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Intensity (arb. units)

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Fig. 3. (a) UV–Vis spectra of the Nix:TiO2 deposited thin films, and (b) band gap energy as a function of the nickel content.

Intensity (arb. units)

3.2. Catalytic performance Ni 20:TiO2 Ni 10:TiO2

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Ni 5 :TiO2 Ni 1 :TiO2

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536

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(c) Fig. 2. XPS high resolution spectra of the Ti (a), Ni (b) and O (c) regions.

plotting (ahv)1/2 as a function of the photon energy and then extrapolating the straight line part of the curves (ahv)1/2 to 0 to evaluate the indirect band gap of the films. The results indicate that the band gap decreases monotonically with the increase of nickel content, from 3.0 eV to 2.6 eV, as observed in Fig. 3b. This decrease can be attributed to the introduction of Ni into the TiO2 lattice, as inferred from the XPS and Raman results. It can be concluded that Ni2+ doping results in a shift in the absorption edge of TiO2 films, causing a narrowing of the band gap and potentially becoming active under visible light illumination.

The catalytic performance of the thin films was studied by following the degradation of 25 ml of unpolluted solution (10 lmol/l of malachite green, MG), which was performed by monitoring the decrease in the characteristic absorption band of MG, located at 618 nm [31], as a function of the irradiation time. The MG degradation percent of the unpolluted reaction systems can be observed in Fig. 4. An improvement in the catalytic performance is observed when the Nix:TiO2 thin films are used. It is worth noting that for times greater than 60 min, the nickel incorporated in the films appears to improve the catalytic performance by approximately 40%. For the films with higher Ni content, it is clearly observed that the degradation percent diminishes as more Ni is incorporated into the film; this is most likely due to the formation of NiO reducing the number of active sites. The sample with 5 at.% Ni showed better efficiency for MG degradation, suggesting that the optimal quantity of nickel incorporated into TiO2 thin films should be approximately 5 at.%. Additionally, an important increase in performance is observed from 28% in the photolysis reaction to 60% in the photocatalytic reaction using the Ni5:TiO2 thin films. These results seem to indicate that Ni2+ needs to be introduced into the lattice of the anatase phase to decrease the recombination of electron–hole pairs, increasing the photodegradation activity. Further addition of Ni results in the formation of NiO, which does not seem to be the most suitable phase for the degradation of organic compounds.

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90

Ni 1 :TiO2

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Reaction time (minutes) Fig. 4. Degree of malachite green degradation using the Nix:TiO2 films in an unpolluted solution.

Fig. 5 shows the degradation conversion percent in case of the MG solution polluted with organic dissolved matter (0.12 g sucrose) and E. coli microorganisms (5 ml). It is worth noting that the quantities for the organic matter and E. coli microorganisms were chosen because they are an approach in representation of wastewater in terms of turbidity. Compared with the catalytic response without contaminants, the degradation conversion was observed to decrease by nearly 30% for each catalyst. These results reveal that the presence of microorganisms and organic matter dissolved in the MG solution noticeably affect the conversion percent of each system, which could be attributed, as a first approximation, to the solution turbidity, causing an impedance due to the reduced light intensity at the photocatalytic film. A competitive adsorption is not considered since previous reports. For reaction times greater than 60 min, could be observed an increase in the percent conversion to approximately 15% for the sample with 10 at.% Ni (Ni10:TiO2) and diminishes for samples with quantities of Ni higher. Several additional experiments were performed to evaluate the catalytic performance of these catalysts in the presence of different turbidity systems (bacteria and dissolved organics). The Ni1:TiO2 catalyst (film with 1 at.% Ni) was selected for this purpose. Fig. 6

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Fig. 6. Degree of MG degradation in a solution polluted with different contents of microorganisms (E. coli) using the Ni1:TiO2 catalyst.

shows the catalytic performance of the Ni1:TiO2 catalyst when different quantities, from 1 to 5 ml, of an E. coli solution were added to the reaction system. It can be seen that the addition of the lowest quantity (1 ml) of the E. coli solution produces a drastic decrease in the catalytic conversion; however, for greater quantities of E. coli, an improvement is observed in the percent conversion. The percent conversion obtained by the Ni1:TiO2 catalyst in the reaction system polluted with 5 ml of the E. coli solution is approximately 25% greater than the obtained in the reaction system polluted with both contaminants indicating that the presence of organic matter has an important effect in the combined polluted system. In general, the addition of E. coli to the MG solution diminishes the catalytic performance of the Ni1:TiO2 catalyst by approximately 50%. Further experiments are currently underway to elucidate this behavior. Fig. 7 shows the catalytic performance of the Ni1:TiO2 catalysts in the reaction systems polluted with organic matter, or 0.03, 0.06 and 0.12 g of sucrose. It is evident that the presence of dissolved organic matter drastically decreases the catalytic activity (by more than five times). The catalytic performance is lower than that of the photolysis reaction, which indicates an inhibition of the degradation reaction by the presence of organic matter. This is most likely due to the turbidity of the reaction solution, which prevents light from reaching the catalyst, thus inhibiting the reaction. These results indicate that the organic matter more strongly affects the catalytic response. Fig. 8 shows the catalytic performance of the most polluted systems.

Unpolluted Ni 0 :TiO2

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0 0

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Reaction time (minutes) Fig. 5. Degree of malachite green degradation using the Nix:TiO2 films in a solution composed of 5 ml of E. coli +0.12 g of sucrose (the sample Ni1:TiO2 was used as reference in unpolluted solution).

Fig. 7. Degree of MG degradation in a solution polluted with different contents of organic matter (sucrose) dissolved using the Ni1:TiO2 catalyst.

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Photolysis 5 ml E.Coli 0.12 g Sucrose Unpolluted Unpolluted (TiO2)

Degree of MG degradation (%)

80 70 60 50 40 30 20 10 0 0

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Reaction time (minutes) Fig. 8. Efficiency of the Ni1:TiO2 catalyst in the different systems tested for MG degradation.

It is interesting to note that the combination of pollutants results in better performance than the organic matter alone, indicating a recovery of the catalytic activity.

4. Conclusions The sol–gel technique was a suitable method for the incorporation of nickel into the TiO2 lattice according to Raman and XPS results. There is an optimal nickel content of approximately 1–5 at.% that allows diffusion of Ni2+ ions into the lattice, and increasing the nickel content results in the formation of NiO on the catalyst surface. The Nix:TiO2 thin films were used as catalysts for the degradation of Malachite Green solutions that were unpolluted or polluted with E. coli microorganisms and dissolved organic matter. From the unpolluted reaction, it was found that introduction of Ni2+ into the anatase lattice enhanced the catalytic performance of the TiO2. In addition, the presence of NiO appeared to inhibit the photocatalytic activity. The band gap was narrowed when Ni is incorporated into the TiO2 from 3.0 eV to 2.6 eV. This is a key issue in photocatalytic applications because decreasing the band gap of a semiconductor is one strategy towards increasing the adsorbed fraction of solar radiation. The presence of E. coli microorganisms in the MG solution decreases the photocatalytic performance compared with the unpolluted system; however, the catalytic performance improves with an increase in the E. coli quantity of up to 5 ml of the solution in the reaction system. The presence of organic matter in the MG solution significantly inhibited the degradation performance, which is most likely due to the absorption of the irradiated light caused by turbidity, diminishing the intensity of the light that reached the catalyst.

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