Photocatalytic application of TiO2/SiO2-based magnetic nanocomposite (Fe3O4@SiO2/TiO2) for reusing of textile wastewater

 INTRODUCTION

       The foremost application of nanomaterials is in wastewater and air treatment. Recent growing ecological problems related to air and water pollutions have increased necessitate to new methods for pollution removing [1]. For purification of wastewater, several methods such as reverse osmosis [2], photocatalytic [3], ultrafiltration, photo-Fenton [4], oxidation, ozonation [5,6], biological [7,8] and electrochemical methods [9] were developed. Among of these protocols, treatment based on nanoparticles offer excellent prospects for chemical and biological sensing [10].

      The estimated total wastewater discharge from textile plants engaged in wet-finishing is noticeable and needs to be treated through economical cost-effective methods [11]. Biological treatment reduces soluble organics and other contaminants  in  wastewater  which are   not   removed  in primary treatment [12]. The progress of treatment processes is followed by various nanostructures of titan dioxide. TiO₂ photocatalyst has  attracted much attention for the last decade because of its high efficiency [13-16] and also several advantages including inexpensivity, chemically stability, non-toxicity, photogenerating holes and electrons [17,18]. Based on these properties, TiO₂ has a wide application in the degradation of pollutants in wastewater, photovoltaic cells, catalysis, photocatalysis, separation, sensors, optical devices and selective absorption [19-21]. The band gap of TiO₂ is slightly more than 3.0 eV which means a very small part in the solar radiation could be suitable for the excitation of electrons from the valence band to the conduction band (the ultraviolet part only) [22]. TiO₂ as the photocatalyst has recently attracted great interest for using solar radiation as a light source for water and wastewater treatment [18]. On the other hand, magnetic photocatalyst containing a core of Fe₃O₄ and a shell of TiO₂ has been developed to avoid photodissolution of iron and to prevent Fe₃O₄ from acting as an electron hole recombination center. And using a SiO₂ layer between Fe₃O₄ and TiO₂ improves the photocatalytic properties of the composites [18].

      Photocatalytic treatment with TiO₂ is an emerging wastewater treatment technology with key advantages counting on the lack of mass transfer limitations, operation at ambient conditions and the feasible use of solar irradiation. Photolysis and photocatalytic treatments were carried out over a suspension of titanium dioxide or zinc oxide under artificial irradiation [22]. Yu et al. used TiO₂ nanoparticles for selective photocatalytic degradation anionic dyes [23]. Krusic's research group developed a method using TiO₂/hydrogel nanocomposite for treatment of some textile azo dyes [1]. Lucas et al. used Fe₃O₄@SiO₂@TiO₂ photocatalyst in combination with UV irradiation for Reactive Blacks [17]. TiO₂ photocatalyst, in anatase phase, was used by Hu's group for decoloration of some dyes such as methylene blue, orange G and rhodamine B [24]. Reyens-Gil's group developed WO₃-enhanaced TiO₂ nanotube photoanodes for organic and oxidized pollutants [25]. Cyclodextrin- functionalized Fe₃O₄/TiO₂ core-shell nanocrystal was used for photocatalytic endocrine-disrupting chemical in water by Vasudevan's group [26].

      In this study, we have synthesized and used TiO₂/SiO₂-based nanocomposite (Fe₃O₄@SiO₂/TiO₂) in presence of UV irradiation for removing some dyes from the textile wastewater. For this research, several cationic, anionic and natural dyes including methyl orange, methylene blue, neutral red, bromocresol green and methyl red were applied. Neutral red and bromocrosol green have good results in reuse treatment. By using magnetic particles, the TiO₂ composite could be recovered and used three times for treatments. To the best of our knowledge, the simple synthesized Fe₃O₄@SiO₂/TiO₂ nanocomposite (Fe₃O₄@SiO₂/TiO₂-NC) is able to remove various dyes and UV-Vis spectroscopy as an easy and accessible method can be used for treatment monitoring. The synthesized Fe₃O₄@SiO_2/TiO₂ was characterized using FT-IR, UV-Vis and TEM instruments.

MATERIAL AND METHODS

 Chemicals

      Bromocresol green, methyl orange,  methyl red,  natural red, methylene blue, hydrochloric acid, iron(II) sulfate hepta-hydrate, iron(III) chloride hexa-hydrate, tetraethylammonium hydroxide (Et₄NOH), tetraethoxysilane (TEOS) and sodium hydroxide were purchased from commercial sources (Merck, Fluka or Sigma) and used without further purification. The distilled-deionized water was used in all solution preparation (18 MΩ). The stock solutions of the dyes (50 ppm) were prepared freshly in distillated water. 

      Transmission electron microscopy (TEM) (Phillips-CM10 operating at 100 kV) was used for TEM image. The FTIR spectra of the nanoparticles were obtained by Bruker Vector 33 (Made in Germany) driven by OPUS software version 3.1. UV-Vis spectrophotometer (Cary 1E, Varian) and software version 3.00L182 was used for treatment monitoring. The pHs of the solutions were adjusted and measured with a pH meter 744 (Metrohm, Switzerland)

Preparation of the TiO₂ Coated Magnetic (Fe₃O₄@SiO_2/TiO₂-NC)

      Preparation of the Fe₃O₄ nanoparticles. Hydrophilic magnetic-nanoparticles were prepared according to Sun’s method [27]. Briefly, 3 ml of iron(II) sulfate solution (2 M) and 10 ml of an iron(III) chloride solution (1 M) were mixed under vigorous mechanical stirring at room temperature and 15 ml of HCl solution (2 M) was used to dissolve the iron salts. An aliquot of 50 ml of tetraethylammonium hydroxide (Et₄NOH) was added to the above solution until the solution reached a pH of 13. Immediately a black solution was formed. Figure 1 shows the TEM image of Fe₃O₄ nanoparticles. Based on TEM image, the ultrafine particle diameter size of the synthesized Fe₃O₄ is about 15-25 nm.

      Preparation of TiO₂ coated Fe₃O₄. An inertial layer (SiO₂) between TiO₂ coating and magnetic material was proposed for better coating of TiO₂ layer and to avoid photodissolution of iron. 50 mg of magnetite-nanoparticles was dispersed in 20 ml of deionized water containing (TEOS) under sonication for 20 min [28].  The aggregated magnetic particles were separated and the suspension was transferred to another beaker. The separated Fe₃O₄@SiO₂ nanoparticles were washed with water and ethanol. For TiO₂ coating purpose Fe₃O₄@SiO₂ nanoparticles were dispersed in TiCl₄  solution  [50 ml  ethanol  containing 2 ml of TEOS] solution for 30 min under sonication and relaxed for 2 h. After 4 steps washing with ethanol and distillated water, the Fe₃O₄@SiO_2/TiO₂ composite was dried and stored [28].

      The synthesized nanocomposite was characterized using FT-IR and UV-Vis (Figs. 2a and 2b). The background corrected FT-IR spectra of MNPs are shown in Fig. 2a. The broad band around 3400 cm^-1 can be assigned to O-H stretching vibration which is assigned to the surface OH groups of Fe₃O₄ NPs. The absorption bands around 580-610cm^-1 is attributed to the vibration of Fe-O bond in Fe₃O₄. In the FT-IR spectrum of Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂/TiO₂-NC (Figs. 2a and 2b),the band at about 1100 cm^-1 is assigned to stretch of Si-O bond and the SiO-H/TiO-H groups are appeared by the  very  broad  IR absorption band in the 2800-3700 cm^-1 region. In the Fig. 2c, the ratio of 580-610 cm^-1 to the 1100 cm^-1 show that the coating of TiO₂ is not core-shell nanoparticle. The structure was synthesized as nanocomposite format (Fe₃O₄@SiO₂/TiO₂).

      For refinement process the Fe₃O₄@SiO₂/TiO₂ composite was added to wastewater and mixed by shaker for an appropriate minutes (Table 1), inside of UV irradiation chamber (254 nm) and then test tube was removed from UV chamber and nanocomposite was removed by external magnetic field. Some parameters such as mixing time, amount of Fe₃O₄@SiO_2/TiO₂, pH and mixing method were optimized. Figure 3 shows the steps of wastewater treatment with Fe₃O₄@SiO₂/TiO₂.

 Fig. 1.TEM image of Fe₃O₄ magnetic nanoparticle.

Fig. 2. (a) FT-IR and (b) UV-Vis spectra of a:Fe₃O₄ nanoparticles, b: SiO₂ modified Fe₃O₄ nanoparticles  and c: TiO₂ modified particle.

RESULTS

       TiO₂ photocatalysis is an emerging wastewater treatment technology with key advantages including the lack of mass transfer limitations. Immobilizing TiO₂ on Fe₃O₄ magnetic nanoparticles is a good strategy enabling the scientists to recover the photocatalysis for several treatment processes [18]. This type of nanoparticle in aqueous solution under UV irradiation is better than that of Fe₃O₄@TiO₂ because insulation SiO₂ layer prevents photodissolution of Iron. Fe₃O₄@SiO₂/TiO₂-NCcould be easily separated from wastewater by applying a magnetic field. In this research for achieving a good performance in treatment process several parameters were optimized as will be mentioned in below such as UV irradiation, pH, amount of Fe₃O₄@SiO_2/TiO₂-NCand treatment time.

Table 1. Comparison Between the Proposed Method and other Reported  Techniques for  the Photocatalytic treatment method

Fig. 3. Steps of the proposed wastewater treatment.

 Discussions

 Effect of UV Irradiation on Treatment

      When solution of dye, neutral red or bromocresol green, was exposed to light in presence of TiO₂; initially, the dye molecules, present in singlet ground state, absorb the radiation of suitable wavelength, and are excited to their first excited singlet state. These excited singlet molecules are converted  to  the  triplet  excited  state  [29]. When TiO₂ absorbs a photon with equal or greater energy than its band gap width, an electron may be promoted from the valence band to the conduction band leaving behind a hole in the valence band. The electron and hole may migrate to the catalyst surface where they participate in redox reactions with absorbed species. The electron, released from TiO₂, will reduce the dye molecule to its colour less products [30].   (Eq. (1)).

 TiO₂ + hυ  → e^- + h⁺          (λ ˂ 300 nm)                         (1)

For determination the effect of UV irradiation on the degradation process two solutions containing 8 ml of 10 ppm of determined dye and 2 mg of Fe₃O₄@SiO_2/TiO₂-NCin different conditions were compared, with and without UV irradiation condition. The Fig. 4 shows the UV-Vis spectrum of these samples. The UV irradiation in λ_max (254 nm for) improved the treatment up to 65% (decrease percentage of dye absorption) and without UV irradiation no significant treatment was observed.

Optimization of pH and Amount of Fe₃O₄@SiO_2/TiO₂-NC

      pH is an important factor for the interaction of target compounds with mineral oxide surface  due to the charge density of nanoparticle which varies strongly with pH value. Therefore pH is very important  in wastewater treatment process with nanomaterial. Also pH changes can influence the adsorption of dye molecules onto the TiO₂ surfaces, a key step for the photocatalytic oxidation. To obtain a stable colloidal solution of Fe₃O₄@SiO₂/TiO₂ a confident amount of acid (HCl) is crucial and this is necessary to preserve the particles in nano-sized structure. The effect of pH on the degradation was determined by using different pH including 2, 3, 4 and 8. In basic condition especially pH 8, there is not any effect and degradation. In acidic condition a good performance for Fe₃O₄@SiO₂/TiO₂ was observed. The efficiency of treatment process in different pH values is shown in Fig.4. UV-Vis absorption peak at λ_max 543 nm for acidic pH was revealed. The pH 3.7 was selected as optimum pH for wastewater treatment process (Fig. 5a).

      The amount of Fe₃O₄@SiO_2/TiO₂-NC can affect the kinetic of the treatment processing. For various amounts of Fe₃O₄@SiO_2/TiO₂-NC,the intensity  of  UV-Vis  absorption peak (at wavelength of 543 nm) was monitored for 10 ppm textile color solution (Fig. 5b). In next experiments 2 mg of  Fe₃O₄@SiO_2/TiO₂-NC, as optimum value, and 8 ml of sample were used for time optimization.

 Fig. 4. The   effect  of   treatment  efficiency of neutral  red solution (5 ppm) after 3 h in different conditions, (a) without UV  irradiation, (b) with  UV and  (c) with UV and Fe₃O₄@SiO₂/TiO₂ for neutral red.

Fig. 5. The effect of different (a) pH values and (b) amounts of Fe₃O₄@SiO₂/TiO₂ on treatment efficiency.

 Optimization of Processing Time

      For determination the kinetic of treatment process with Fe₃O₄@SiO_2/TiO₂-NCin presence of UV irradiation, different times were checked. For treated wastewater after 3 h, a significant  decrease in UV-Vis absorption intensity was observed.  UV-Vis absorption intensity of wastewater after different times is shown in Fig. 6. The optimum time was used for next experiments.

Efficiency of the Proposed Treatment for Different Concentrations

      Different concentrations of dyes were used for investigation of treatment efficiency. In Fig. 7 the efficiency of treatment on bromocrosol green is shown. The UV-Vis absorbance of different concentrations of bromocrosol green before and after treatment is shown in Fig. 7.  For 2 mg of Fe₃O₄@SiO_2/TiO₂-NC, 10 ppm and lower concentration showed excellent efficiency.

Fig. 6. Time optimization for  treatment  processing (a) UV- Vis absorbance of treated solution, (b) the difference of UV-Vis absorbance between non-treated solution and treated solution for different times.

Fig. 7. UV-Vis  absorbance  of  wastewater  (left) before treatment and (right) after treatment for different concentrations of Bromocresol green.

 Fig. 8. Efficiency of the wastewater  treatment  for  real sample spiked with BCG and bromocrosol green (UV-Vis   absorbance   a:  before   and   b:  after Treatment).

CONCLUSIONS

       In this research, efficiency of wastewater treatment for different dyes was determined. Several dyes were used for treatment investigation including methyl orange, ethylene blue, neutral red, bromocrosol green and methyl red. Neutral red and bromocrosol green have shown good results in reusing. In Table 1, Comparison of the proposed methods with other reported techniques is shown. The UV-Vis absorbance of these dyes before and after treatment with Fe₃O₄@SiO_2/TiO2-NC and UV irradiation are shown in Fig. 8. Fe₃O₄@SiO_2/TiO₂-NChas shown a good efficiency for thetreatment of textile wastewater. The efficiency of this method for neutral red and bromocrosol green is 100% and 65% for low concentration (about 10 ppm) and for high concentration (50 ppm), respectively. Main advantages of this treatment are simplicity and cost-effectiveness. By recovery of used Fe₃O₄@SiO_2/TiO₂ and could be at least 3 times. The efficiency of treatment by using the second used nanoparticle   was   90%    and    50%   at    low    and   high concentrations, respectively. These results show that the complete treatment at high concentration needs to more amount of nanocomposite. Using Fe₃O₄@SiO_2/TiO₂-NC nanocomposite in aqueous solution under UV irradiation is better than Fe₃O₄@TiO₂because insulation SiO₂ layer prevents photodissolution of iron. Efficiency of wastewater treatment for real sample spiked with neutral red and bromocresol green was determined. UV-Vis absorbance of the real wastewater before (curve a) and after treatment (curve b) are shown in Fig. 8.

ACKNOWLEDGMENTS

       The authors would like gratefully acknowledge the financial support of this project by research council of University of Isfahan.