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Enhanced sonocatalytic performance of ZnTi nano-layered double hydroxide by substitution of Cu (II) cations
Ultrasonics - Sonochemistry 58 (2019) 104632
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Enhanced sonocatalytic performance of ZnTi nano-layered double hydroxide by substitution of Cu (II) cations
T
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H. Daneshvara, M.S. Seyed Dorrajia, , A.R. Amani-Ghadimb, M.H. Rasoulifarda a b
Applied Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, University of Zanjan, Zanjan, Iran Department of Chemistry, Faculty of Science, Azarbaijan Shahid Madani University, P.O. box 83714-161, Tabriz, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Sonocatalysis LDHs Active free radicals Water treatment
In this research, a series of CuZnTi-LDHs with different Cu2+/Zn2+ molar ratio were synthesized by co-precipitation method with the purpose of improving the sonocatalytic performance of ZnTi-LDH. All the LDH samples were synthesized by a facile co-precipitation process. The as-prepared LDHs were characterized by Powder X-ray diffraction (XRD), Field emission-scanning electron microscopy (FESEM), Transition electron microscopy (TEM), Brunauer-Emmelt-Teller (BET) analysis, and UV-visible diffuse reflectance spectroscopy (DRS) analysis. The results showed that Cu2+ substitution can significantly enhance the sonocatalytic properties of ZnTi-LDH. The Methylene blue degradation percentage over ZnTi-LDH reached 30% in 90 min, whilst this percentage reaches 71% over CuZnTi-LDH (1:1). The role of the Cu2+ incorporation on the observed enhancement in sonocatalytic performance was revealed by investigating the effect of radical scavengers on degradation efficiency and DRS spectra of ZnTi-LDH and CuZnTi-LDH (1:1). Benzoquinone (BQ), ammonium oxalate and tert-Bu lead to 22.5%, 53.5% and 74.6% decrease in degradation percentage by CuZnTi-LDH (1:1). However, the degradation efficiency showed 16.6%, 3.3% and 63.3% reduction in the presence of BQ, ammonium oxalate and tert-Bu respectively, in dye degradation by ZnTi-LDH. DRS spectra demonstrated that the band gap of the LDH decreases by Cu2+ substitution. The effect of operational parameters on sonodegradation was investigated as well. The kinetics of sonodegradation reaction obeyed the first order reaction kinetics with R2 of 0.95.
1. Introduction In recent years, sonocatalysis has received great interest as one of the advanced oxidation processes (AOPs) for water and wastewater treatment which is one major concerns that humankind face nowadays [1–3]. The presence of organic dyes in aquatic environments can lead to drastic damaging effects on such environments by constraining light penetration and photosynthesis. Because of the low biodegradability of organic dyes, conventional biological treatments are not efficient enough to degrade these dyes and treat colored wastewaters and the more effective strategies are in demand to treat these kind of wastewaters [4,5]. The origin of sonolysis is cavitation phenomenon, which consists of nucleation, growth and implosive collapse of gas microbubbles in the solution under the ultrasonic pressure waves [6]. This explosive collapse of bubbles generates localized hot spots with critical temperature and pressure (about 5000 K and 1800 atm) that results in thermal dissociation of water and formation of highly reactive radical species, i.e. H% and OH% which can initiate series of redox reaction that result in
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degradation of pollutants [7–9]. However, the practical application of sonolysis in water treatment has been hindered because of its high energy-demand and long treatment period [10,11]. It has been proved that the presence of solid particles in the reaction media provides additional nucleation sites which increase the number of cavitation events and produced H% and OH% consequently [12–14]. Additionally, semiconductor materials in sonocatalysis can themselves produce electron/ hole pairs, and accordingly, lead to further production of free active radicals leading to overall enhancement of degradation performance [15–17]. Thus sonocatalysis, using semiconductor materials is considered as one of the best solutions to ever-growing demand on wastewater treatment technologies. Among the new generation semiconductor catalysts, layered double hydroxides (LDHs) attract great interest as these materials pose several advantages: they are economic, environmentally-friendly, easy to prepare and reusable. LDHs are a class of hydrotalcite-like compounds whose structure is based on brucite-like layers (Mg(OH)2), in which a number of M2+ cations have been substituted isomorphically by M3+ or M4+ cations,
Corresponding author. E-mail address: [email protected] (M.S. Seyed Dorraji).
https://doi.org/10.1016/j.ultsonch.2019.104632 Received 2 April 2019; Received in revised form 2 June 2019; Accepted 7 June 2019 Available online 08 June 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.
Ultrasonics - Sonochemistry 58 (2019) 104632
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ZnCl2.
inducing positively charged layers and interlayer charge balancing anions. The composition of LDH materials can be represented by general formula [M1−xIIMxIII (OH)2]x+ (An−)x/n·mH2O, where MII and MIII are divalent and trivalent metal cations, respectively; x is defined as molar ratio of MIII/(MII + MIII), which lies between 0.2 and 0.33, An− is the interlayer anion, and m is the molar amount of hydrogen-bonded water molecules that exist in interlayer region [18–22]. Owing to their unique structure and tunable composition LDHs have been widely used in different fields such as photocatalysis [23], sensors [24], catalyst support [25] and adsorbents [26]. Nonetheless, most of the LDH based catalysts suffer from relatively high band gap energies and limited efficiency for pollutant degradation. Therefore, in addition to the mentioned structure, ternary LDHs involving a mixture of different divalent, trivalent or tetravalent metal ions have been synthesized with modified properties and have been extended to certain applications especially in the field of catalysis [27–29]. Parida & el. stated that incorporation of Co in the framework of CuCr-LDH to achieve Cu–Co/Cr ternary LDH, had a great impact on improving photocatalytic activity of the pristine LDH [30]. The results of a recent study show that compared to MgAlLDH, the cerium-containing LDH exhibited a relatively higher degradation capacity for methylene blue [27]. It has also been reported that the introduction of Cu, Zn or Ti into LDHs layers leads to the generation of active sites for catalytic reactions such as O–Cu–O, O–Zn–O, and O–Ti–O, and can significantly improves catalytic properties of these LDHs in AOPs [31]. Hence, these kinds of LDHs can be promising candidates to use in sonocatalytic processes. Recently, Khataee et al. have synthesized ZnFe-NLDH with Cl− anions in interlayer space as sonocatalyst for acid red 17 degradation, and reported that the synthesized catalyst has a high efficiency for dye degradation under ultrasonic irradiation (92% degradation after 120 min) [19]. However, to the best of our knowledge there is no research paper on the use of ternary LDHs in sonocatalytic processes and investigation of the effect of the different metal ion substitution on sonocatalytic properties. Therefore, in the present research with the purpose of enhancing the sonocatalytic performance of ZnTi-LDH, a series of CuZnTi-LDHs with different Cu2+/Zn2+ ratio were prepared by a simple co-precipitation method. The resulting LDHs were characterized by FESEM, TEM, XRD, BET and DRS analysis. Moreover, the quality and sonocatalytic activity of the synthesized CuZnTi-LDHs were studied on Methylene blue (MB) degradation and the role Cu2+ substitution on sonocatalytic performance improvement was discussed. The effects of various parameters including catalyst dosage, initial MB concentration, initial solution pH and radical scavengers on the sonocatalytic treatment of MB aqueous solutions were examined in details.
2.2. Materials characterization Powder X-ray diffraction (XRD) was recorded on Siemens X-ray diffraction ((D5000, Germany), with Cu Kα radiation (1.54065 Å and θ = 5–75°)). The surface morphology of the samples was investigated using field emission-scanning electron microscopy (FESEM) on Tescan (MIRA3 FEG-SEM) microscope. The transmission electron microscopy (TEM) images were obtained by a Jeol (JEM-2200 FS) microscope. N2 adsorption-desorption isotherms were recorded at 77 k (Bellsorp-min II) to measure the surface area of a sample by the Brunauer-Emmelt-Teller (BET) method. The degassing of the samples were accomplished at 120 ˚C in a vacuum (bellow 10–3 torr) for 15 h. 2.3. Sonocatalytic degradation The sonocatalytic activity of the prepared LDHs was evaluated by the degradation of MB in aqueous solution under ultrasonic irradiation. In a typical process, a certain amount of sonocatalyst was dispersed in 100 ml of the dye solution with a known initial concentration. The suspended solution was put in the ultrasonic bath for 90 min and the temperature of the bath kept constant at 25 °C by adding ice cubes. Every 15 min, 3 ml of the suspension was withdrawn and centrifuged to remove the sonocatalyst. The concentration of MB was measured by UV-Vis spectrophotometer in λ = 663 nm. Sonocatalytic activity experiments using different radical scavengers were performed similar to the above procedure but in the presence of 1 mM different radical scavenger. The effect of operational parameters including sonocatalyst dosage, pH and initial dye concentration was studied by altering the amount of the intended parameter in the specific range, while keeping the other parameters constant. 3. Results and discussion 3.1. Structural characterization The crystal structure of the synthesized LDHs is shown in Fig. 1. All synthesized samples display the characteristic reflection peaks of (0 0 3), (0 0 6), (0 0 9), (0 1 5), (0 1 2) and (1 1 0) planes centered at 2θ = 11.39°, 23.94°, 35.04°, 39.64°, 47.39°, and 60.64°. Sharp reflection peaks in XRD reveal high crystallinity of the synthesized samples [32,33]. It has also observed that the location of (0 0 3) peak, which relates to basal spacing, have been shifted to higher scattering angle in CuZnTi-LDHs (Table 1). The basal spacing decrease from 7.08 Å for ZnTi-LDH to 7.02 Å for CuZnTi-LDHs (1:1). This phenomenon can be explained by an increase in layer charge density, due to the larger electronegativity of copper (1.9) compared to that of Zinc (1.65) [34]. It has appeared that values for the lattice parameter “a” decrease with
2. Materials and methods 2.1. Materials and synthesis procedure TiCl4, ZnCl2, CuCl2, t-butyl alcohol, ammonium oxalate, benzoquinone, sodium carbonate, HCl and NaOH were purchased from Merck Co. Germany. All materials were used without further purification. To assess the effect of Cu2+/Zn2+ on the properties of CuZnTi-LDH, a series of LDH samples were prepared by varying the concentration ratio of the two metal ions (Cu2+ and Zn2+), with a constant (Cu2++Zn2+)/Ti4+ ratio of 4:1. The Cu2+/Zn2+ molar ratio were 0.33, 0.5 and 1, and the samples were named CuZnTi-LDH (1:3), CuZnTi-LDH (1:2) and CuZnTi-LDH (1:1), respectively. All the LDH samples were synthesized by well-known co-precipitation method. A solution of 1 M Na2CO3, was added dropwise to a homogenous solution of TiCl4, ZnCl2, and CuCl2 under vigorous stirring until the final pH = 10. The resulting suspension was stirred for one hour in room temperature and then aged at 70 °C for 24 h. Then the obtained material was filtered and washed with deionized water, finally, the filtrate dried in an oven at 40 °C overnight. Synthesis procedure of ZnTi-LDHs, was the same as described, but the metal salt solution contains only TiCl4,
Fig. 1. X-ray diffraction patterns of the synthesized LDH samples. 2
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structure of the prepared LDHs were more investigated by TEM images (Fig. 2c and d). As illustrated in Figure Fig. 2c and d, very thin hexagonal sheets can be observed for both synthesized samples indicated with the red dashed lines. The sonocatalytic processes requires the high specific surface areas as much as possible to facilitate catalytic reactions. The surface area of ZnTi-LDH and CuZnTi-LDHs (1:1) were investigated with N2 adsorption measurements (Fig. 3). The isotherms of both LDHs reveal type IV with H3 type hysteresis loop at P/P0 > 0.45 which is characteristic of mesoporous aggregated plate-like materials [36]. The BET analysis has revealed that the surface area of CuZnTi-LDH (1:1) (45.69 m2/g) is remarkably greater than ZnTi-LDH (26.66 m2/g) which is consistent with the FESEM images.
Table 1 Lattice parameter for the as-prepared LDHs. Sample
ZnTi-LDH CuZnTi-LDH (1:2) CuZnTi-LDH (1:1)
Unit cell parameters (A°) c
a
23.41 21.9 21.6
3.54 3.52 3.51
Basal spacing (A°)
7.08 7.04 7.02
increasing Cu2+ substitution in LDH layers. The cell parameter “a” indicates the average cation to cation distance, and the observed decrement in this distance can be defined by the size difference between the two cations. The larger ionic radius of Zn2+ (0.74 Å) with respect to Cu2+ (0.69 Å) in an octahedral environment, can result in higher “a” value [35]. The FESEM images of ZnTi-LDH and CuZnTi-LDH with Cu2+/Zn2+ molar ratio equal to 1 are exhibited in Fig. 2 (a and b), respectively. As seen in this figure, both LDHs consist of the irregular laminated 2D nano flakes. The mean lateral size of sheets is about 16–20 and 12–15 nm for ZnTi-LDH and CuZnTi-LDH (1:1), respectively. In comparison with pristine ZnTi-LDH, not only the thickness of layers is reduced in CuZnTi-LDH (1:1) but also the porosity is increased which will be more investigated by with N2 adsorption measurements. The
3.2. Sonocatalytic performance of synthesized LDHs In order to study the dye removal performance of the synthesized LDHs, the decolorization efficiency was tested by different processes including: surface adsorption, sonolysis and sonocatalysis and the results are shown in Fig. 4(a). To evaluate the adsorption ability of ZnTiLDH, an experiment was run under the dark condition and after 90 min of experiments, the decolorization percentage was negligible (10%), which is reasonable considering the electrostatic repulsion force between positively charged LDHs and MB molecules.
Fig. 2. FESEM (a and b) and TEM c and d) images for ZnTi-LDH and CuZnTi-LDH (1:1), respectively. 3
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Fig. 3. N2 adsorption/desorption isotherms of ZnTi-LDH (empty circles) and CuZnTi-LDH (1:1) (filled circles).
Fig. 4. (a) Comparison of different processes for MB degradation by ZnTi-LDH, (b) The effect of Cu2+ content on sonocatalytic efficiency of CuZnTi-LDHs; (catalyst dosage: 0.5 g/L, dye concentration: 10 ppm, in natural pH).
with increasing Cu2+/Zn2+ ratio. The decolorization percentages were 50%, 63% and 71% over CuZnTi-LDH (1:3), CuZnTi-LDH (1:2) and CuZnTi-LDH (1:1), respectively (Fig. 4b). To assure that the sonocatalytic degradation is responsible for high removal percentage of MB molecules, the adsorption efficiency over CuZnTi-LDH (1:1) was evaluated and in comparison with the 10% adsorption by ZnTi-LDH (Fig. 4(a)), the adsorption efficiency slightly improved and achieved to 15% (Fig. 4(b)). This significant enhancement in dye removal efficiency by substitution of Cu2+ can be explained by three main processes: (i) Heterogeneous nucleation, (ii) sonoluminescence and (iii) thermal catalytic mechanism [9,38,39]. In heterogeneous nucleation, the presence of semiconductor particles provides additional nuclei and increases the formation of cavitation bubbles, and hydroxyl radicals, subsequently. In sonoluminescence (SL), the emission of high wavelength light happens by recombination of the free radicals produced within the cavitation bubbles during collapse [40,41]. Therefore, in the presence of semiconductor material,
The dye degradation by sonolysis arises from acoustic cavitation which is the formation, growth and implosive collapse of bubbles in a solution. The collapse of bubbles produces a localized hot spot with very high temperature and pressure. Under such conditions, the dissolved oxygen and water molecules can undergo direct thermal dissociation to produce highly reactive radical species such as (%OH), (%H) and (%O), that can react with organic pollutants in water to oxidize them [37]. However in most cases, the efficiency of sonolysis is not enough to effectively reduce the concentration of pollutants in water, as in this research, the decolorization efficiency of MB by sonolysis was 10%. As it can be seen in Fig. 4(a) the MB degradation enhanced by the addition of ZnTi-LDH and reached 30% after 90 min of reaction time. The effect of Cu2+ substitution on MB removal percentage is presented in Fig. 4(b). As it clearly can be seen, incorporation of Cu2+ cation on LDH layers can significantly improve the degradation efficiency of resultant LDHs. It is also observed that under the same operational conditions, the higher amount of Cu2+ favors the decolorization efficiency of MB solution and degradation efficiency increase 4
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Fig. 5. (a) UV–vis diffuse reflectance spectra and (b) band gap energy of ZnTi-LDH and CuZnTi-LDH (1:1).
When the hydroxyl radical scavenger, tert-Butyl alcohol (t-BuOH), was added to the MB solution, 63.3% decrease in degradation efficiency was observed and by subjoining the ammonium oxalate, the degradation efficiency of MB after 90 min of sonocatalytic process decrease by 3.3% of its value. Hence, the main oxidative specie that produced by ZnTiLDH was hydroxyl radical. However, when CuZnTi-LDH (1:1) was used as catalyst in the system, different results were obtained. BQ, ammonium oxalate and tert-Bu leads to 22.5%, 53.5% and 76.4% decrease in decolorization efficiency, respectively. Thus, it can be concluded that the degradation of MB occurs mainly via hydroxyl radical and direct oxidation of MB molecules by h+. The cavitation phenomenon, which previously described, is the main cause of production OH% in both ZnTi-LDH and CuZnTi-LDH (1:1). It is observed that when CuZnTi-LDH is used, beside hydroxyl radical, a considerable amount of h+ also produced that can directly oxidize the pollutants in aqueous solution. Whereas h+ has no role in the degradation of MB molecules by ZnTi-LDH. Based on the results of radical scavenger experiments and DRS spectra, the role of Cu2+ substitution in CuZnTi-LDH sonocatalytic performance enhancement can be explained. As the DRS spectra reveal, substitution of Cu2+ in LDH layers can narrow down the band gap energy and red shift the light absorption edge. So, besides heterogeneous nucleation of microbubbles that happens in the presence of both ZnTi-LDH and CuZnTi-LDH (1:1), the emitted photons from SL generates electron-hole pairs in CuZnTiLDH (1:1). The results of photocatalytic testes in the presence of scavengers also confirm that mechanism, where no electron-hole pairs formed in ZnTi-LDH due to the higher band gap energy. In the case of CuZnTi-LDH, the 3d electrons of Cu2+ in MO6 octahedron field were excited from valence band to the conduction-band [30]. As a result of the produced holes reaction with adsorbed water molecules on the LDH surface, hydroxyl radicals (OH%) were formed, and the photoexcited electrons were captured by dissolved dioxygen molecules on the surface of the catalyst to form super oxide radicals (O2−%). The formation of O2−% on the LDH surface promotes the dye degradation, in addition to OH% radical and h+. Moreover, as discussed in front, since the heterogeneous nucleation is initiated on the surface of the catalyst, the sonocatalytic processes requires the high specific surface areas as much as possible to ease the degradation process [9]. Therefore, the higher surface area of CuZnTi-LDH (1:1) compared to ZnTi-LDH can also contribute in enhanced sonocatalytic properties of CuZnTi-LDH (1:1). The proposed mechanism is summarized in Scheme 1.
this emitted intense light can excite the electrons of solid catalyst from conduction band (CB) to the valence band (VB) to form electron-hole pairs, further producing O2 and OH%, respectively that is identical to the semiconductor photocatalytic mechanism [42]. In the last mechanism the local high temperature can possibly provoke the thermal excitation of the semiconductor, also leading to the generation of electron-hole pairs [9]. Since one of the main mechanisms that governs the sonocatalytic performance is SL, thus, the band gap of the synthesized samples plays an important role in the sonocatalytic performance of them [13,43]. The light absorption properties of the ZnTi-LDH and CuZnTi-LDH (1:1) were examined by UV-vis diffuse reflectance spectra and the results are shown in Fig. 5(a). The absorption band in the range of 200 to 300 nm which exist in both samples, can be attributed to ligand-to-metal charge transfer (LMCT) occurring in the MO6 octahedral of the layered structure. However, there is a clear red shift in the absorption edge of CuZnTi-LDH (1:1). This red shift can be attributed to metal-to-metal charge transfer (MMCT) induced by oxo-bridged binuclear TiIV–O–CuII (ZnII) linkages that enhance the light absorption properties of LDHs [30,44]. The band gap energy (Eg) of LDHs was calculated using the following equation [45]:
α hν = K (hν − Eg )1/2
(1)
where K, hν, α and Eg represent proportionally constant, photon energy, absorption coefficient and band gap energy, respectively. The band gap energy can be estimated by extrapolating the linear area of the (αhν)2 versus hν chart as illustrated in Fig. 5(b). The Eg values calculated to be 3.6 eV and 3.3 eV for ZnTi-LDH and CuZnTi-LDH (1:1), respectively. It can be seen that the presence of Cu2+ cation in LDH structure can effectively reduce the band gap energy of ZnTi-LDH. The decrease in the band gap energy suggests that less energy is needed in order to generate electron-hole pairs by SL. To reveal the responsible mechanism for dye degradation, and the role of Cu2+ in the sonocatalytic performance of LDHs as well, a series of experiments were conducted with the addition of adequate scavengers for active species. It is well known that super oxide anion radical (O2•-), hydroxyl radical (OH•) and the generated holes (h+) are the main active species responsible for pollutant degradation [46]. Hence, by addition of suitable quenchers for each active species, the responsible mechanism for dye degradation can be revealed. Fig. 6 (a and b) illustrates the effect of the presence of the scavengers on the soncatalytic performance of ZnTi-LDH and CuZnTi-LDH (1:1), respectively. As shown in Fig. 6(a), in the presence of benzoquinone (BQ), as O2%− scavenger, the degradation efficiency undergoes 16.6% of decrement. 5
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Fig. 6. Effect of different radical scavengers on decolorization efficiency of MB by (a) ZnTi-LDH and (b) CuZnTi-LDH (1:1) (catalyst dosage: 0.5 g/L, dye concentration: 10 ppm, in natural pH).
Basically, the pH value of the solution plays an important role in the surface charge of the sonocatalyst and the dye molecules, which affects the total active sites available on the surface of the catalyst. It is commonly known that the first step in catalytic degradation of organic pollutants is surface adsorption, so the generated active species can transfer to the target molecules and initiate a series of oxidative reactions [48]. The effect of solution pH on degradation efficiency is shown in Fig. 7(b). The degradation efficiency of 42%, 60%, 75% and 83% was yield for pH = 4, 6, 8 and 10 respectively. The results reveal that the dye removal percentage in alkaline solution is much higher than that of neutral and acidic solution. As mentioned before, LDHs and MB both have positive surface charge, thus, as the pH decreased, the surface of the sonocatalyst gets more positively charge due to the surface adsorption of H+. Hence, the adsorption of positively charged dye molecules will be hindered by electrostatic repulsion forces which results in a lower percentage of decolorization of MB. In contrast, an increase in pH value will increase the surface absorption, which itself increase
3.3. Effect of operational parameters on the sonocatalytic process Methylene blue degradation efficiency is conducted using different amounts of CuZnTi-LDH (1:1) ranging from 0.25 to 1 g/L, in the natural pH of dye solution (5.5) and the results have been shown in Fig. 7a. As seen in this figure, the removal efficiency is gradually increased with increasing catalyst dosage up to 0.75 g/L and after that it decreases, which is a general characteristic of heterogeneous catalysis. The increase in dye sonocatalytic removal efficiency by the increase in catalyst dosage (45%, 67% and 69% for 0.25 g/L, 0.5 g/L and 0.75 g/L, respectively) is probably due to the availability of more active sites on the surface of the catalyst. However further increment in catalyst dosage leads to decrease in degradation percentage (56% for 1 g/L). This decrement can be resulted from the aggregation of LDH nanoparticles and thus, reducing the number of active sites for the production of %OH. Besides, the collision of activated species with ground state LDH nanoparticles decreased the surficial holes and electrons density [11,47].
Scheme 1. The MB sonocatalytic degradation mechanism on the CuZnTi-LDH. 6
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Fig. 7. Effect of (a) sonocatalyst dosage, (b) solution pH and (c) initial dye concentration on sonocatalytic properties of CuZnTi-LDH (1:1).
degradation efficiency [49,50]. The impact of the initial concentration of dye solution on decolorization efficiency was studied in initial dye concentrations of 5, 10, 15 and 20 mg/L and degradation percentages of 87%, 80%, 52% and 33% were obtained respectively. The results in Fig. 7(c) indicate that the removal percentage decrement by increasing the initial dye concentration from 5 to 10 mg/L is negligible. Whilst, the further increase in initial dye concentration leads to a drastic decrease in degradation percentage. These observations can be attributed to the fact that at the certain catalyst dosage, the active sites for degradation of pollutant are constant, hence at high pollutant concentration these sites are covered by dye ions and the generation of oxidative spices on the surface of LDH is reduced. Furthermore, the removal efficiency decreases with an increase in the initial dye concentration [41,51]. The kinetics of MB solution sonocatalytic decolorization via CuZnTiLDH (1:1) has studied on the obtained optimum conditions. The obtained data of dye degradation, were analyzed by rate expression of the first order reaction kinetics, which can be presented by the following equation:
− ln(C/C0) = kt
Fig. 8. The first order reaction kinetic plot for sonocatalytic degradation of MB by CuZnTi-LDH (1:1).
4. Conclusion In summary, a series of CuZnTi-LDHs were synthesized and the effect of different Cu2+/Zn2+ molar ratio on characteristics and sonocatalytic properties were investigated. As-synthesized samples were used as sonocatalyst for the sonocatalytic decolorization of methylene blue in aqueous solution. The results showed that Cu2+ substitution can significantly increase the degradation percentage of MB molecule. The sonocatalytic degradation efficiency of MB has increased from 30% for
(2)
where C0 is the initial concentration of MB, C is the concentration at time t, and k is degradation rate constant. The linear relationship between −ln(C/C0) vs degradation time is presented in Fig. 8. The linearity of the curves (R2 of 0.95) indicates that the degradation rate fits the first order reaction kinetics with the rate constant of 0.0195 min−1. 7
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ZnTi-LDH to 71% for CuZnTi-LDH (1:1) under the same operational conditions. The scavenger experiments revealed that the main oxidative species in MB degradation by CuZnTi-LDH were hydroxyl radical and h+, whilst the active specie for ZnTi-LDH was only hydroxyl radical. On the other hand the DRS spectra showed that the band gap energy of CuZnTi-LDH has been narrowed down to 3.3 eV in comparison with ZnTi-LDH (3.6 eV). Therefore, it can be assumed that the band gap reduction which arises from MMCT induced by oxo-bridged binuclear TiIV–O–CuII (ZnII) linkages, is the main cause for sonocatalytic performance enhancement. Moreover, the effect of the operational parameter, namely sonocatalyst dosage, pH and initial dye concentration. The highest degradation efficiency (87%) achieved by using 0.75 g/L catalyst for initial concentration of 5 mg/L and pH = 10. The kinetics of sonodegradation reaction obeys the first order reaction kinetics with R2 of 0.95.
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Enhanced sonocatalytic performance of ZnTi nano-layered double hydroxide by substitution of Cu (II) cations
T
⁎
H. Daneshvara, M.S. Seyed Dorrajia, , A.R. Amani-Ghadimb, M.H. Rasoulifarda a b
Applied Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, University of Zanjan, Zanjan, Iran Department of Chemistry, Faculty of Science, Azarbaijan Shahid Madani University, P.O. box 83714-161, Tabriz, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Sonocatalysis LDHs Active free radicals Water treatment
In this research, a series of CuZnTi-LDHs with different Cu2+/Zn2+ molar ratio were synthesized by co-precipitation method with the purpose of improving the sonocatalytic performance of ZnTi-LDH. All the LDH samples were synthesized by a facile co-precipitation process. The as-prepared LDHs were characterized by Powder X-ray diffraction (XRD), Field emission-scanning electron microscopy (FESEM), Transition electron microscopy (TEM), Brunauer-Emmelt-Teller (BET) analysis, and UV-visible diffuse reflectance spectroscopy (DRS) analysis. The results showed that Cu2+ substitution can significantly enhance the sonocatalytic properties of ZnTi-LDH. The Methylene blue degradation percentage over ZnTi-LDH reached 30% in 90 min, whilst this percentage reaches 71% over CuZnTi-LDH (1:1). The role of the Cu2+ incorporation on the observed enhancement in sonocatalytic performance was revealed by investigating the effect of radical scavengers on degradation efficiency and DRS spectra of ZnTi-LDH and CuZnTi-LDH (1:1). Benzoquinone (BQ), ammonium oxalate and tert-Bu lead to 22.5%, 53.5% and 74.6% decrease in degradation percentage by CuZnTi-LDH (1:1). However, the degradation efficiency showed 16.6%, 3.3% and 63.3% reduction in the presence of BQ, ammonium oxalate and tert-Bu respectively, in dye degradation by ZnTi-LDH. DRS spectra demonstrated that the band gap of the LDH decreases by Cu2+ substitution. The effect of operational parameters on sonodegradation was investigated as well. The kinetics of sonodegradation reaction obeyed the first order reaction kinetics with R2 of 0.95.
1. Introduction In recent years, sonocatalysis has received great interest as one of the advanced oxidation processes (AOPs) for water and wastewater treatment which is one major concerns that humankind face nowadays [1–3]. The presence of organic dyes in aquatic environments can lead to drastic damaging effects on such environments by constraining light penetration and photosynthesis. Because of the low biodegradability of organic dyes, conventional biological treatments are not efficient enough to degrade these dyes and treat colored wastewaters and the more effective strategies are in demand to treat these kind of wastewaters [4,5]. The origin of sonolysis is cavitation phenomenon, which consists of nucleation, growth and implosive collapse of gas microbubbles in the solution under the ultrasonic pressure waves [6]. This explosive collapse of bubbles generates localized hot spots with critical temperature and pressure (about 5000 K and 1800 atm) that results in thermal dissociation of water and formation of highly reactive radical species, i.e. H% and OH% which can initiate series of redox reaction that result in
⁎
degradation of pollutants [7–9]. However, the practical application of sonolysis in water treatment has been hindered because of its high energy-demand and long treatment period [10,11]. It has been proved that the presence of solid particles in the reaction media provides additional nucleation sites which increase the number of cavitation events and produced H% and OH% consequently [12–14]. Additionally, semiconductor materials in sonocatalysis can themselves produce electron/ hole pairs, and accordingly, lead to further production of free active radicals leading to overall enhancement of degradation performance [15–17]. Thus sonocatalysis, using semiconductor materials is considered as one of the best solutions to ever-growing demand on wastewater treatment technologies. Among the new generation semiconductor catalysts, layered double hydroxides (LDHs) attract great interest as these materials pose several advantages: they are economic, environmentally-friendly, easy to prepare and reusable. LDHs are a class of hydrotalcite-like compounds whose structure is based on brucite-like layers (Mg(OH)2), in which a number of M2+ cations have been substituted isomorphically by M3+ or M4+ cations,
Corresponding author. E-mail address: [email protected] (M.S. Seyed Dorraji).
https://doi.org/10.1016/j.ultsonch.2019.104632 Received 2 April 2019; Received in revised form 2 June 2019; Accepted 7 June 2019 Available online 08 June 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.
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ZnCl2.
inducing positively charged layers and interlayer charge balancing anions. The composition of LDH materials can be represented by general formula [M1−xIIMxIII (OH)2]x+ (An−)x/n·mH2O, where MII and MIII are divalent and trivalent metal cations, respectively; x is defined as molar ratio of MIII/(MII + MIII), which lies between 0.2 and 0.33, An− is the interlayer anion, and m is the molar amount of hydrogen-bonded water molecules that exist in interlayer region [18–22]. Owing to their unique structure and tunable composition LDHs have been widely used in different fields such as photocatalysis [23], sensors [24], catalyst support [25] and adsorbents [26]. Nonetheless, most of the LDH based catalysts suffer from relatively high band gap energies and limited efficiency for pollutant degradation. Therefore, in addition to the mentioned structure, ternary LDHs involving a mixture of different divalent, trivalent or tetravalent metal ions have been synthesized with modified properties and have been extended to certain applications especially in the field of catalysis [27–29]. Parida & el. stated that incorporation of Co in the framework of CuCr-LDH to achieve Cu–Co/Cr ternary LDH, had a great impact on improving photocatalytic activity of the pristine LDH [30]. The results of a recent study show that compared to MgAlLDH, the cerium-containing LDH exhibited a relatively higher degradation capacity for methylene blue [27]. It has also been reported that the introduction of Cu, Zn or Ti into LDHs layers leads to the generation of active sites for catalytic reactions such as O–Cu–O, O–Zn–O, and O–Ti–O, and can significantly improves catalytic properties of these LDHs in AOPs [31]. Hence, these kinds of LDHs can be promising candidates to use in sonocatalytic processes. Recently, Khataee et al. have synthesized ZnFe-NLDH with Cl− anions in interlayer space as sonocatalyst for acid red 17 degradation, and reported that the synthesized catalyst has a high efficiency for dye degradation under ultrasonic irradiation (92% degradation after 120 min) [19]. However, to the best of our knowledge there is no research paper on the use of ternary LDHs in sonocatalytic processes and investigation of the effect of the different metal ion substitution on sonocatalytic properties. Therefore, in the present research with the purpose of enhancing the sonocatalytic performance of ZnTi-LDH, a series of CuZnTi-LDHs with different Cu2+/Zn2+ ratio were prepared by a simple co-precipitation method. The resulting LDHs were characterized by FESEM, TEM, XRD, BET and DRS analysis. Moreover, the quality and sonocatalytic activity of the synthesized CuZnTi-LDHs were studied on Methylene blue (MB) degradation and the role Cu2+ substitution on sonocatalytic performance improvement was discussed. The effects of various parameters including catalyst dosage, initial MB concentration, initial solution pH and radical scavengers on the sonocatalytic treatment of MB aqueous solutions were examined in details.
2.2. Materials characterization Powder X-ray diffraction (XRD) was recorded on Siemens X-ray diffraction ((D5000, Germany), with Cu Kα radiation (1.54065 Å and θ = 5–75°)). The surface morphology of the samples was investigated using field emission-scanning electron microscopy (FESEM) on Tescan (MIRA3 FEG-SEM) microscope. The transmission electron microscopy (TEM) images were obtained by a Jeol (JEM-2200 FS) microscope. N2 adsorption-desorption isotherms were recorded at 77 k (Bellsorp-min II) to measure the surface area of a sample by the Brunauer-Emmelt-Teller (BET) method. The degassing of the samples were accomplished at 120 ˚C in a vacuum (bellow 10–3 torr) for 15 h. 2.3. Sonocatalytic degradation The sonocatalytic activity of the prepared LDHs was evaluated by the degradation of MB in aqueous solution under ultrasonic irradiation. In a typical process, a certain amount of sonocatalyst was dispersed in 100 ml of the dye solution with a known initial concentration. The suspended solution was put in the ultrasonic bath for 90 min and the temperature of the bath kept constant at 25 °C by adding ice cubes. Every 15 min, 3 ml of the suspension was withdrawn and centrifuged to remove the sonocatalyst. The concentration of MB was measured by UV-Vis spectrophotometer in λ = 663 nm. Sonocatalytic activity experiments using different radical scavengers were performed similar to the above procedure but in the presence of 1 mM different radical scavenger. The effect of operational parameters including sonocatalyst dosage, pH and initial dye concentration was studied by altering the amount of the intended parameter in the specific range, while keeping the other parameters constant. 3. Results and discussion 3.1. Structural characterization The crystal structure of the synthesized LDHs is shown in Fig. 1. All synthesized samples display the characteristic reflection peaks of (0 0 3), (0 0 6), (0 0 9), (0 1 5), (0 1 2) and (1 1 0) planes centered at 2θ = 11.39°, 23.94°, 35.04°, 39.64°, 47.39°, and 60.64°. Sharp reflection peaks in XRD reveal high crystallinity of the synthesized samples [32,33]. It has also observed that the location of (0 0 3) peak, which relates to basal spacing, have been shifted to higher scattering angle in CuZnTi-LDHs (Table 1). The basal spacing decrease from 7.08 Å for ZnTi-LDH to 7.02 Å for CuZnTi-LDHs (1:1). This phenomenon can be explained by an increase in layer charge density, due to the larger electronegativity of copper (1.9) compared to that of Zinc (1.65) [34]. It has appeared that values for the lattice parameter “a” decrease with
2. Materials and methods 2.1. Materials and synthesis procedure TiCl4, ZnCl2, CuCl2, t-butyl alcohol, ammonium oxalate, benzoquinone, sodium carbonate, HCl and NaOH were purchased from Merck Co. Germany. All materials were used without further purification. To assess the effect of Cu2+/Zn2+ on the properties of CuZnTi-LDH, a series of LDH samples were prepared by varying the concentration ratio of the two metal ions (Cu2+ and Zn2+), with a constant (Cu2++Zn2+)/Ti4+ ratio of 4:1. The Cu2+/Zn2+ molar ratio were 0.33, 0.5 and 1, and the samples were named CuZnTi-LDH (1:3), CuZnTi-LDH (1:2) and CuZnTi-LDH (1:1), respectively. All the LDH samples were synthesized by well-known co-precipitation method. A solution of 1 M Na2CO3, was added dropwise to a homogenous solution of TiCl4, ZnCl2, and CuCl2 under vigorous stirring until the final pH = 10. The resulting suspension was stirred for one hour in room temperature and then aged at 70 °C for 24 h. Then the obtained material was filtered and washed with deionized water, finally, the filtrate dried in an oven at 40 °C overnight. Synthesis procedure of ZnTi-LDHs, was the same as described, but the metal salt solution contains only TiCl4,
Fig. 1. X-ray diffraction patterns of the synthesized LDH samples. 2
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structure of the prepared LDHs were more investigated by TEM images (Fig. 2c and d). As illustrated in Figure Fig. 2c and d, very thin hexagonal sheets can be observed for both synthesized samples indicated with the red dashed lines. The sonocatalytic processes requires the high specific surface areas as much as possible to facilitate catalytic reactions. The surface area of ZnTi-LDH and CuZnTi-LDHs (1:1) were investigated with N2 adsorption measurements (Fig. 3). The isotherms of both LDHs reveal type IV with H3 type hysteresis loop at P/P0 > 0.45 which is characteristic of mesoporous aggregated plate-like materials [36]. The BET analysis has revealed that the surface area of CuZnTi-LDH (1:1) (45.69 m2/g) is remarkably greater than ZnTi-LDH (26.66 m2/g) which is consistent with the FESEM images.
Table 1 Lattice parameter for the as-prepared LDHs. Sample
ZnTi-LDH CuZnTi-LDH (1:2) CuZnTi-LDH (1:1)
Unit cell parameters (A°) c
a
23.41 21.9 21.6
3.54 3.52 3.51
Basal spacing (A°)
7.08 7.04 7.02
increasing Cu2+ substitution in LDH layers. The cell parameter “a” indicates the average cation to cation distance, and the observed decrement in this distance can be defined by the size difference between the two cations. The larger ionic radius of Zn2+ (0.74 Å) with respect to Cu2+ (0.69 Å) in an octahedral environment, can result in higher “a” value [35]. The FESEM images of ZnTi-LDH and CuZnTi-LDH with Cu2+/Zn2+ molar ratio equal to 1 are exhibited in Fig. 2 (a and b), respectively. As seen in this figure, both LDHs consist of the irregular laminated 2D nano flakes. The mean lateral size of sheets is about 16–20 and 12–15 nm for ZnTi-LDH and CuZnTi-LDH (1:1), respectively. In comparison with pristine ZnTi-LDH, not only the thickness of layers is reduced in CuZnTi-LDH (1:1) but also the porosity is increased which will be more investigated by with N2 adsorption measurements. The
3.2. Sonocatalytic performance of synthesized LDHs In order to study the dye removal performance of the synthesized LDHs, the decolorization efficiency was tested by different processes including: surface adsorption, sonolysis and sonocatalysis and the results are shown in Fig. 4(a). To evaluate the adsorption ability of ZnTiLDH, an experiment was run under the dark condition and after 90 min of experiments, the decolorization percentage was negligible (10%), which is reasonable considering the electrostatic repulsion force between positively charged LDHs and MB molecules.
Fig. 2. FESEM (a and b) and TEM c and d) images for ZnTi-LDH and CuZnTi-LDH (1:1), respectively. 3
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Fig. 3. N2 adsorption/desorption isotherms of ZnTi-LDH (empty circles) and CuZnTi-LDH (1:1) (filled circles).
Fig. 4. (a) Comparison of different processes for MB degradation by ZnTi-LDH, (b) The effect of Cu2+ content on sonocatalytic efficiency of CuZnTi-LDHs; (catalyst dosage: 0.5 g/L, dye concentration: 10 ppm, in natural pH).
with increasing Cu2+/Zn2+ ratio. The decolorization percentages were 50%, 63% and 71% over CuZnTi-LDH (1:3), CuZnTi-LDH (1:2) and CuZnTi-LDH (1:1), respectively (Fig. 4b). To assure that the sonocatalytic degradation is responsible for high removal percentage of MB molecules, the adsorption efficiency over CuZnTi-LDH (1:1) was evaluated and in comparison with the 10% adsorption by ZnTi-LDH (Fig. 4(a)), the adsorption efficiency slightly improved and achieved to 15% (Fig. 4(b)). This significant enhancement in dye removal efficiency by substitution of Cu2+ can be explained by three main processes: (i) Heterogeneous nucleation, (ii) sonoluminescence and (iii) thermal catalytic mechanism [9,38,39]. In heterogeneous nucleation, the presence of semiconductor particles provides additional nuclei and increases the formation of cavitation bubbles, and hydroxyl radicals, subsequently. In sonoluminescence (SL), the emission of high wavelength light happens by recombination of the free radicals produced within the cavitation bubbles during collapse [40,41]. Therefore, in the presence of semiconductor material,
The dye degradation by sonolysis arises from acoustic cavitation which is the formation, growth and implosive collapse of bubbles in a solution. The collapse of bubbles produces a localized hot spot with very high temperature and pressure. Under such conditions, the dissolved oxygen and water molecules can undergo direct thermal dissociation to produce highly reactive radical species such as (%OH), (%H) and (%O), that can react with organic pollutants in water to oxidize them [37]. However in most cases, the efficiency of sonolysis is not enough to effectively reduce the concentration of pollutants in water, as in this research, the decolorization efficiency of MB by sonolysis was 10%. As it can be seen in Fig. 4(a) the MB degradation enhanced by the addition of ZnTi-LDH and reached 30% after 90 min of reaction time. The effect of Cu2+ substitution on MB removal percentage is presented in Fig. 4(b). As it clearly can be seen, incorporation of Cu2+ cation on LDH layers can significantly improve the degradation efficiency of resultant LDHs. It is also observed that under the same operational conditions, the higher amount of Cu2+ favors the decolorization efficiency of MB solution and degradation efficiency increase 4
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Fig. 5. (a) UV–vis diffuse reflectance spectra and (b) band gap energy of ZnTi-LDH and CuZnTi-LDH (1:1).
When the hydroxyl radical scavenger, tert-Butyl alcohol (t-BuOH), was added to the MB solution, 63.3% decrease in degradation efficiency was observed and by subjoining the ammonium oxalate, the degradation efficiency of MB after 90 min of sonocatalytic process decrease by 3.3% of its value. Hence, the main oxidative specie that produced by ZnTiLDH was hydroxyl radical. However, when CuZnTi-LDH (1:1) was used as catalyst in the system, different results were obtained. BQ, ammonium oxalate and tert-Bu leads to 22.5%, 53.5% and 76.4% decrease in decolorization efficiency, respectively. Thus, it can be concluded that the degradation of MB occurs mainly via hydroxyl radical and direct oxidation of MB molecules by h+. The cavitation phenomenon, which previously described, is the main cause of production OH% in both ZnTi-LDH and CuZnTi-LDH (1:1). It is observed that when CuZnTi-LDH is used, beside hydroxyl radical, a considerable amount of h+ also produced that can directly oxidize the pollutants in aqueous solution. Whereas h+ has no role in the degradation of MB molecules by ZnTi-LDH. Based on the results of radical scavenger experiments and DRS spectra, the role of Cu2+ substitution in CuZnTi-LDH sonocatalytic performance enhancement can be explained. As the DRS spectra reveal, substitution of Cu2+ in LDH layers can narrow down the band gap energy and red shift the light absorption edge. So, besides heterogeneous nucleation of microbubbles that happens in the presence of both ZnTi-LDH and CuZnTi-LDH (1:1), the emitted photons from SL generates electron-hole pairs in CuZnTiLDH (1:1). The results of photocatalytic testes in the presence of scavengers also confirm that mechanism, where no electron-hole pairs formed in ZnTi-LDH due to the higher band gap energy. In the case of CuZnTi-LDH, the 3d electrons of Cu2+ in MO6 octahedron field were excited from valence band to the conduction-band [30]. As a result of the produced holes reaction with adsorbed water molecules on the LDH surface, hydroxyl radicals (OH%) were formed, and the photoexcited electrons were captured by dissolved dioxygen molecules on the surface of the catalyst to form super oxide radicals (O2−%). The formation of O2−% on the LDH surface promotes the dye degradation, in addition to OH% radical and h+. Moreover, as discussed in front, since the heterogeneous nucleation is initiated on the surface of the catalyst, the sonocatalytic processes requires the high specific surface areas as much as possible to ease the degradation process [9]. Therefore, the higher surface area of CuZnTi-LDH (1:1) compared to ZnTi-LDH can also contribute in enhanced sonocatalytic properties of CuZnTi-LDH (1:1). The proposed mechanism is summarized in Scheme 1.
this emitted intense light can excite the electrons of solid catalyst from conduction band (CB) to the valence band (VB) to form electron-hole pairs, further producing O2 and OH%, respectively that is identical to the semiconductor photocatalytic mechanism [42]. In the last mechanism the local high temperature can possibly provoke the thermal excitation of the semiconductor, also leading to the generation of electron-hole pairs [9]. Since one of the main mechanisms that governs the sonocatalytic performance is SL, thus, the band gap of the synthesized samples plays an important role in the sonocatalytic performance of them [13,43]. The light absorption properties of the ZnTi-LDH and CuZnTi-LDH (1:1) were examined by UV-vis diffuse reflectance spectra and the results are shown in Fig. 5(a). The absorption band in the range of 200 to 300 nm which exist in both samples, can be attributed to ligand-to-metal charge transfer (LMCT) occurring in the MO6 octahedral of the layered structure. However, there is a clear red shift in the absorption edge of CuZnTi-LDH (1:1). This red shift can be attributed to metal-to-metal charge transfer (MMCT) induced by oxo-bridged binuclear TiIV–O–CuII (ZnII) linkages that enhance the light absorption properties of LDHs [30,44]. The band gap energy (Eg) of LDHs was calculated using the following equation [45]:
α hν = K (hν − Eg )1/2
(1)
where K, hν, α and Eg represent proportionally constant, photon energy, absorption coefficient and band gap energy, respectively. The band gap energy can be estimated by extrapolating the linear area of the (αhν)2 versus hν chart as illustrated in Fig. 5(b). The Eg values calculated to be 3.6 eV and 3.3 eV for ZnTi-LDH and CuZnTi-LDH (1:1), respectively. It can be seen that the presence of Cu2+ cation in LDH structure can effectively reduce the band gap energy of ZnTi-LDH. The decrease in the band gap energy suggests that less energy is needed in order to generate electron-hole pairs by SL. To reveal the responsible mechanism for dye degradation, and the role of Cu2+ in the sonocatalytic performance of LDHs as well, a series of experiments were conducted with the addition of adequate scavengers for active species. It is well known that super oxide anion radical (O2•-), hydroxyl radical (OH•) and the generated holes (h+) are the main active species responsible for pollutant degradation [46]. Hence, by addition of suitable quenchers for each active species, the responsible mechanism for dye degradation can be revealed. Fig. 6 (a and b) illustrates the effect of the presence of the scavengers on the soncatalytic performance of ZnTi-LDH and CuZnTi-LDH (1:1), respectively. As shown in Fig. 6(a), in the presence of benzoquinone (BQ), as O2%− scavenger, the degradation efficiency undergoes 16.6% of decrement. 5
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Fig. 6. Effect of different radical scavengers on decolorization efficiency of MB by (a) ZnTi-LDH and (b) CuZnTi-LDH (1:1) (catalyst dosage: 0.5 g/L, dye concentration: 10 ppm, in natural pH).
Basically, the pH value of the solution plays an important role in the surface charge of the sonocatalyst and the dye molecules, which affects the total active sites available on the surface of the catalyst. It is commonly known that the first step in catalytic degradation of organic pollutants is surface adsorption, so the generated active species can transfer to the target molecules and initiate a series of oxidative reactions [48]. The effect of solution pH on degradation efficiency is shown in Fig. 7(b). The degradation efficiency of 42%, 60%, 75% and 83% was yield for pH = 4, 6, 8 and 10 respectively. The results reveal that the dye removal percentage in alkaline solution is much higher than that of neutral and acidic solution. As mentioned before, LDHs and MB both have positive surface charge, thus, as the pH decreased, the surface of the sonocatalyst gets more positively charge due to the surface adsorption of H+. Hence, the adsorption of positively charged dye molecules will be hindered by electrostatic repulsion forces which results in a lower percentage of decolorization of MB. In contrast, an increase in pH value will increase the surface absorption, which itself increase
3.3. Effect of operational parameters on the sonocatalytic process Methylene blue degradation efficiency is conducted using different amounts of CuZnTi-LDH (1:1) ranging from 0.25 to 1 g/L, in the natural pH of dye solution (5.5) and the results have been shown in Fig. 7a. As seen in this figure, the removal efficiency is gradually increased with increasing catalyst dosage up to 0.75 g/L and after that it decreases, which is a general characteristic of heterogeneous catalysis. The increase in dye sonocatalytic removal efficiency by the increase in catalyst dosage (45%, 67% and 69% for 0.25 g/L, 0.5 g/L and 0.75 g/L, respectively) is probably due to the availability of more active sites on the surface of the catalyst. However further increment in catalyst dosage leads to decrease in degradation percentage (56% for 1 g/L). This decrement can be resulted from the aggregation of LDH nanoparticles and thus, reducing the number of active sites for the production of %OH. Besides, the collision of activated species with ground state LDH nanoparticles decreased the surficial holes and electrons density [11,47].
Scheme 1. The MB sonocatalytic degradation mechanism on the CuZnTi-LDH. 6
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Fig. 7. Effect of (a) sonocatalyst dosage, (b) solution pH and (c) initial dye concentration on sonocatalytic properties of CuZnTi-LDH (1:1).
degradation efficiency [49,50]. The impact of the initial concentration of dye solution on decolorization efficiency was studied in initial dye concentrations of 5, 10, 15 and 20 mg/L and degradation percentages of 87%, 80%, 52% and 33% were obtained respectively. The results in Fig. 7(c) indicate that the removal percentage decrement by increasing the initial dye concentration from 5 to 10 mg/L is negligible. Whilst, the further increase in initial dye concentration leads to a drastic decrease in degradation percentage. These observations can be attributed to the fact that at the certain catalyst dosage, the active sites for degradation of pollutant are constant, hence at high pollutant concentration these sites are covered by dye ions and the generation of oxidative spices on the surface of LDH is reduced. Furthermore, the removal efficiency decreases with an increase in the initial dye concentration [41,51]. The kinetics of MB solution sonocatalytic decolorization via CuZnTiLDH (1:1) has studied on the obtained optimum conditions. The obtained data of dye degradation, were analyzed by rate expression of the first order reaction kinetics, which can be presented by the following equation:
− ln(C/C0) = kt
Fig. 8. The first order reaction kinetic plot for sonocatalytic degradation of MB by CuZnTi-LDH (1:1).
4. Conclusion In summary, a series of CuZnTi-LDHs were synthesized and the effect of different Cu2+/Zn2+ molar ratio on characteristics and sonocatalytic properties were investigated. As-synthesized samples were used as sonocatalyst for the sonocatalytic decolorization of methylene blue in aqueous solution. The results showed that Cu2+ substitution can significantly increase the degradation percentage of MB molecule. The sonocatalytic degradation efficiency of MB has increased from 30% for
(2)
where C0 is the initial concentration of MB, C is the concentration at time t, and k is degradation rate constant. The linear relationship between −ln(C/C0) vs degradation time is presented in Fig. 8. The linearity of the curves (R2 of 0.95) indicates that the degradation rate fits the first order reaction kinetics with the rate constant of 0.0195 min−1. 7
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ZnTi-LDH to 71% for CuZnTi-LDH (1:1) under the same operational conditions. The scavenger experiments revealed that the main oxidative species in MB degradation by CuZnTi-LDH were hydroxyl radical and h+, whilst the active specie for ZnTi-LDH was only hydroxyl radical. On the other hand the DRS spectra showed that the band gap energy of CuZnTi-LDH has been narrowed down to 3.3 eV in comparison with ZnTi-LDH (3.6 eV). Therefore, it can be assumed that the band gap reduction which arises from MMCT induced by oxo-bridged binuclear TiIV–O–CuII (ZnII) linkages, is the main cause for sonocatalytic performance enhancement. Moreover, the effect of the operational parameter, namely sonocatalyst dosage, pH and initial dye concentration. The highest degradation efficiency (87%) achieved by using 0.75 g/L catalyst for initial concentration of 5 mg/L and pH = 10. The kinetics of sonodegradation reaction obeys the first order reaction kinetics with R2 of 0.95.
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