Henry Journal of Nanoscience Nanomedicine & Nanobiology

In this study, the treatment of pollutants (CODdis, color and three polyphenols [4-methyl phenol (C₇H₈O) (4-MP), 4-hydroxyanisole (C7H8O2) (4-H), 2-methyl-4-hydroxyanisole (C8H10O2) (2-M-4-H)] from a textile industry wastewaters with the addition of nano-titani- um dioxide doped with nano-cerium dioxide (nano-CeO₂/TiO₂) were intended the different experimental conditions during sonication pro- cess. The effects of ambient conditions (25oC), increasing sonica- tion time (120 min and 150 min), increasing sonication temperatures (30°C and 60°C), increasing nano-titanium dioxide (nano-TiO ) (nano-titania) (0.5, 10 and 20 mg/L), increasing nano-cerium dioxide (nano-CeO₂) (nano-ceria) (10, 100 and 1000 mg/L) and increasing nano-TiO2 doped with nano-CeO2 (nano-CeO2/TiO2) (100, 500 and 2000 mg/L) (nano-CeO₂/TiO₂ ratios of 1/4, 1/1, 4/1, 9/1, w/w) con- centrations on the sonication of wastewater from textile industry wastewater (TI ww) treatment plant in Izmir, Turkey was investigated in a sonicator with a power of 640 W, a frequency of 35 kHz and a sonication time of 150 min for the treatments of Methylene Blue and Rhodamine B dyestuffs. CODdissolved, color and three polyphenols [4-methyl phenol (C7H8O) (4-MP), 4-hydroxyanisole (C7H8O2) (4-H), 2-methyl-4-hydroxyanisole (C8H10O2) (2-M-4-H)] removal efficien- cies were observed during sonication experiments. 99.37% CODdis, 98.07% color, 96% total phenol (PHE R), 93% 4-MP, 88% 4-H and 85% 2-M-4-H maximum removal efficiencies were found in the reac- tor containing nano-CeO2/TiO2=1000 mg/L ([nano-CeO2=500 mg/L / nano-TiO₂=500 mg/L]=1/1, w/w) after 150 min sonication time at 60°C. The addition of nano-CeO /TiO in TI ww was increased to removal efficiencies of pollutions (CODdis, color and polyphenols) higher than the each one of nano-CeO₂ and nano-TiO₂ catalysts ad- ditions in TI ww.

Nano-cerium dioxide; Nanoparticles; Nano-titanium dioxide; Polyphenols; Sonication; Textile industry wastewater

The use of nanoparticles (NPs) in water treatment has continuously increased in recent years [1-3]. The production and processing of nanomaterials (NMs) is a quick technology [1]. Metal-oxide nanoparticles (NPs) include nanoscale zinc oxide, titanium oxide, iron oxide, cerium oxide and zirconium oxide, as well as mixed-metal compounds such as indium-tin. In general, NMs are defined as materials of less than 100 nm in size. By the particle size reduction, the surface area of the NPs is increased. Surface activity is a key aspect of NMs. Agglomeration and aggregation blocks surface area from contact with other matter. Only well-dispersed NPs reduces the quantity of NMs [1].

The application of ultrasound (US) as an alternative to the removal of dyes in waters has become of increasing interest in recent years [4,5]. This technique is considered as an Advanced Oxidation Process (AOP) that generates hydroxyl radicals (OH^●) through acoustic cavitation, which can be defined as the cyclic formation, growth and collapse of microbubbles. Fast collapse of bubbles compresses adiabatically entrapped gas and vapours which leads to short and local hot spots [6]. In the final stage of the collapse, the temperature inside the residual bubble or in the surrounding liquid is thought to be above 5000^oC. The OH^● and hydroperoxyl radicals (O H^●) can be generated from H2O and O2 [7]. Cerium oxide (CeO2), the most reactive rare earth oxide, is studied and employed in various applications, including catalysts, water splitting in the treatment of pollutions, oxygen storage capacitors and ion conductors [8,9]. CeO2 are O2 vacancies and small polarons (electrons localized on cerium cations) because these two are located in the useful range of CeO2 . In the case of O2 defects, the increased diffusion rate of O₂ in the lattice causes increased catalytic activity as well as an increase in ionic conductivity. As the number of vacancies increases, the ease at which O2 can move around in the crystal increases, allowing the CeO₂ to reduce and oxidize molecules or co-catalysts on its surface. It has been shown that the catalytic activity of CeO2 is directly related to the number of O2 vacancies in the crystal. About 13% phenol and 93% Methylene Blue and 100% Congo Red photodegradation were observed in the case of Fe/Ce ratio of 1/1 ratio [10].

They are many reports of Fe doping on TiO2 to improve its photocatalytic activity. Amongst a variety of transitional metals, iron has been considered to be an appropriate material due to the fact that the radius of Fe⁺³ (0.79 A) is similar to that of Ti^{+ 4} (0.75 A), so that Fe⁺³ can be easily incorporated into the crystal lattice of TiO . Fe⁺³ has proved to be a successful doping element due to its half-filled electronic configuration [11-15]. Cerium oxides have attracted much attention due to the optical and catalytic properties associated with the redox pair of Ce⁺³/Ce^+4. Ce-doped TiO materials have been synthesized by the sol–gel and hydrothermal methods and used in the photocatalytic degradation applications. But there are very few reports on Ce doped catalysts and the beneficial effect of Ce doped TiO2 catalysts are known to depend on different factors, such as the synthesis method and the cerium content [16-18]. The photocatalytic performance of TiO2 catalysts depends strongly on the methods of metal ion doping and the amount of doping material, since they have a decisive influence on the properties of the catalysts. Therefore, it is necessary to investigate the effects of doping method and doping material content on the photocatalytic performance of TiO2 nanocatalysts.

There have been many reports of transition metals (Fe, Al, Ni, Cr, Co, W, V and Zr), metal oxides (Fe2O3, Cr2O3, CoO2, MgO + CaO and SiO2), transition metal ceramics (WO3, MoO3, Nb2O5, SnO2 and ZnO) and anionic compounds (C, N, and S) being used to dope TiO2 to improve its applicability [11,19-21]. Zeleska [22] has reviewed the preparation methods of doped TiO2 with metallic and nonmetallic species, including various types of dopants and doping methods. Rauf et al. [23] has given an overview on the photocatalytic degradation of azo dyes in the presence of TiO2 doped with selective transition metals.

Higher catalytic activity has been reported for the Ce and CeO₂ doped TiO₂ materials for photo-degradation of dyes and other pollutants [16-18]. Titanium dioxide nanopowders doped with visible responsive catalyst may shift the UV absorption threshold of TiO2 into visible spectrum range and photocatalytic activities can be higher than those of pure TiO2 and Degussa P25 [12-15,24]. Effect of silver, platinum and gold doping on the TiO₂ for photocatalytic reduction of CO₂ and sonophotocatalytic degradation of methyl orange and organic pollutant nonylphenol ethoxylate has been investigated [2528]. Also there are reports of tin, calcium, sulfur and zirconia doped TiO2 being used for photo-degradation of model pollutants [29-32]. Yu et al. [33] synthesized pure TiO₂ particles using ultrasonicallyinduced hydrolysis reaction and compared the photocatalytic activity of prepared samples with Degussa P 25 and samples prepared by conventional hydrolysis method. Neppolian et al. [34] also prepared nano TiO2 photocatalysts using sol–gel and ultrasonic-sol–gel methods using two different sources of ultrasonicator, i.e., a bath type and horn type. The effect of ultrasonic irradiation time, power density, the ultrasonic sources (bath-type and horn-type), magnetic stirring, initial temperatures and sizes of the reactors has been investigated. Li et al. [35] used the combination of ultrasonic and hydrothermal method for preparing Fedoped TiO₂ for photo-degradation of methyl orange. Zhou et al. [12] used ultrasonicaly-induced hydrolysis reaction for the preparation of Fe-doped TiO2 whereas Huang et al. [36] synthesized and characterized FexOy-TiO2 via the sonochemical method.

The synthesis of metal-loaded semiconductor oxide materials by conventional physical blending or chemical precipitation followed by surface adsorption usually yields insoluble materials for which the control over size, morphology and dispersion of the metal component remains inherently difficult. These methods often require a long time and are inherently multi-step procedures. Sonochemistry has been proven to be an excellent method for the preparation of mesoporous materials. The physical and chemical effects generated by acoustic cavitation can be expected to significantly influence the properties of doped materials [12,26]. Ultrasound has been very useful in the synthesis of a wide range of nanostructured materials, including high-surface area transition metals, alloys, carbides, oxides, and colloids. The collapse of cavitation bubbles generates localized hot spots with transient temperature of about 10000^oK (9726.850℃), pressures of about 1000 atm or more and cooling rates in excess of 109 K/s. Under such extreme conditions, various chemical reactions and physical changes occur and numerous nano-structured materials such as metals, alloys, oxides and biomaterials can be effectively synthesized with required particle size distribution [12,37-39]. In the past the sonochemical method has been applied to prepare various TiO₂ and doped nanomaterials and photocatalytic activity has been evaluated by different researchers [33-36,40,41].

The results of the photocatalytic degradation of various pollutants which can be present in industrial wastewater prove the purposefulness of CeO2 doping with metal and non-metal dopants [42]. The doping resulted in the following: (1) The formation of surface defects, which prevented electron-hole recombination or decreased recombination rates; (2) an increase in the surface area and a higher number of sites accessible for the adsorption of pollutants on CeO2 particles; (3) a decrease in the band gap energy, leading to visible light absorption; and (4) higher photocatalytic activity of pollutant degradation [42]. All results also showed that coupling TiO2 with CeO2 could produce special electrons and holes transfer from TiO₂ to CeO₂ which is able to facilitate the separation of the electron–hole pairs and therefore, improve photocatalytic activity of the hybrid photocatalyst [42].

Liu and Sun [43] investigated the degradation of an azo dye, Methyl Orange, in catalytic wet air oxidation process with Fe2O3CeO2-TiO2/γ-Al2O3 as catalyst at a room temperature in a synthetic wastewater containing 500 mg/L Methyl Orange. 98.09% of color and 96.08% of TOC was removed in 150 min [43]. Also, the influences of heat-treatment temperature (300, 500 and 700^oC) and heat-treatment time (20, 60, and 100 min) on the sonocatalytic activities of CeO2/TiO2, SnO2/TiO2 and ZrO2/TiO2 composites, and of irradiation time (20, 40, 60, 80 and 100 min) and solution acidity (pH=3-5-7-911) on the sonocatalytic degradation of Acid Red B was investigated. The Acid Red B decreases was in order: in CeO2/TiO2>67.41%, in SnO2/TiO2>65.26%, in TiO2>41.67%, in ZrO2/TiO2>28.34%, in SnO2>26.75%, in CeO2>23.33%, in ZrO2>16.67% with only US respectively, at pH=5 after 60 min sonication time. Methyl Violet color removals were observed as 40% in TiO2, 25% in ZrO2/TiO2, 55% in SnO2/TiO2, 50% in CeO2/TiO2, respectively. Chen and Liu [20] investigated the effects of CeO₂-ZnO composite nanofibers on the treatment of organic-polluted H₂O, Photocatalytic activity experiments showed that the Rhodamine B was almost completely decomposed when it was catalyzed by CeO2-ZnO NFs within 180 min, while only 17.4% and 82.3% of this dye was decomposed under catalysis by sole CeO2 and sole ZnO NFs, respectively [20].

TiO₂ is broadly used in environmental clean-up operations because of its non-toxic nature, photochemical stability and low cost [44,45]. TiO2 has a large band gap energy, more than 3.0 eV, and relatively long electron-hole pair recombination time [46-48]. To achieve rapid and efficient decomposition of organic pollutants and also easy manipulation of the catalyst in a total sonication process, it may be effective to load TiO2 NPs onto suitably fine adsorbents and thus concentrate pollutants around the NPs. Doping metal elements into TiO₂ may also strain the recombination of electron-hole pairs. The strong oxidative potential of the positive holes oxidizes H O to create OH^• radicals. In a study performed by Entezari and Petrier [49] 50-60% phenol removals was obtained after 45 min sonication time, at 423 kHz, at TiO =8 mg/L and at 60^oC. They reported that the OH^l produced by US can react with phenol resulting in a dephenolization process [49]. In a study performed by Khochwala and Gogate [50] it has been observed that 2000 mg/L TiO₂ is the optimum concentration for 78% phenol removal in olive mill industry wastewater (OMW). Wu et al. [51,52] have reported similar results for degradation of phenol and trichloroacetic acid in a combined operation involving two 9 W H-shaped ultraviolet (UV) lamps and an ultrasonic horn operating at 30 kHz and at 100 W in OMW. Kidak and Ince [53] and Shirgaonkar and Pandit [54] have also reported 78% and 79% degradation yields for phenol and 2, 4, 6-trichlorophenol, respectively, in the presence of 450 mg/L TiO₂ in the OMW. An anatase TiO₂ was used to remove the Aromatic Amines (AAs) namely, 4, 4-oxydianiline with a yield 85% from OMW after 120 min sonication time. 87% mineralization of AAs was achieved in a study performed by Cardoso et al. [55]. In a study performed by Mrowetz et al. [56] 36% COD removal was found in a TI ww containing 75 mg/L Acid Orange 8, at 20 kHz, at 250 W, in TiO =100 mg/L, after 100 min sonication time and at 35^oC. Wang et al. [57] obtained 60% color yield in a TI ww containing 100 mg/L Azo Funchsine solution at 40 kHz, at 50 W, after 60 min sonication time, at 30^oC with TiO =50 mg/L. In a study performed by Abbasi and Asl [58] 90% colour removal was achieved in a TI ww containing 15 mg/L Basic Blue 41, at 35 kHz, at 160 W, after 180 min sonication time, at 30^oC with TiO =100 mg/L. In a study performed by Wu and Yu [59] 63% Total Aromatic Amines (TAAs) removal was accomplished in a TI ww containing 20 mg/L C.I. Reactive Red 2 in TiO =2000 mg/L, at 40 kHz, at 400 W, at 10 W/cm², after 120 min sonication time, at pH=7.0 and at 60^oC.

The studies performed until now with real TI ww were not concern the removals of color originated from Methylene Blue and Rhodamine B and of polyphenols with nano-titanium dioxide doped with nano-cerium dioxide throughout sonication. The novelty of this study is the removals of s 4-methyl phenol, 4-hydorxyanisole and 2-methyl-4-hyroxyanisole, color and tdissolved COD with nano-CeO₂ doped TiO₂. Therefore, in the peresent study, the effects of ambient conditions (25^oC), increasing sonication time (120 and 150 min), sonication temperature (30^oC and 60^oC), nano-titanium dioxide (nano-TiO₂) (nano-titania) (0.5, 10 and 20 mg/L), nano-cerium dioxide (nano-CeO₂) (nano-ceria) (10, 100 and 1000 mg/L) and nano-TiO₂ doped with nano-CeO₂ at nano-CeO₂/TiO2 ratios of 1/4, 1/1, 4/1 and 9/1 on the sonication of the wastewater from TI ww treatment plant in Izmir, Turkey was investigated in a sonicator with a power of 640 W, a frequency of 35 kHz and a sonication time of 150 min. CODdis, color and three polyphenols [4-methyl phenol (C₇H₈O) (4-MP), 4-hy-droxyanisole (C7H8O2) (4-H), 2-methyl-4-hydroxyanisole (C8H10O2) (2-M-4-H)] removal efficiencies were observed during sonication experiments.

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