In this research, ZnO nanoparticles as a sonocatalyst for degradation of Eosin B dye under ultrasonic irradiation were synthesized. Various experimental (ultrasound irradiation power: 50-250 W, ultrasound irradiation time: 10-70 min, catalyst dosage: 1-3 g/L and initial dye concentration: 5-25 mg/L) using ZnO nanoparticles was investigated to find the optimal condition for the degradation of Eosin B. The experiments were analyzed via response surface methodology (RSM) based on central composite design (CCD). According to the results, the crystalline and grain size of samples were obtained using X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) of 15 nm and 30 nm, respectively. The nanoparticles structure was observed in the form of hexagonal. The band-gap of the prepared nanoparticles was measured as 2.9 eV which is appropriate for sonodegradation process under ultrasonic waves. The optimization of the process showed the maximum sonocatalyst degradation of 93.46% at irradiation power, irradiation time, catalyst dosage and dye concentration of 250 W, 70 min, 2.17 g/L and 5.08 mg/L, respectively.
Dye contaminants have complex molecular, toxic, non-degradable, and stable structure that by entering the environment create damaging effects . Eosin b with chemical formula of C20H8Br2N2O9 is an extensive and important chemical dye in textile and hygienic industries, which is very toxic due to its aromatic properties . Various methods are used to eliminate organic and colored materials, which included biological methods, coagulation, adsorption, etc [3,4]. In these methods, dye transfer from one phase to another is difficult, and further refinement is required due to secondary contaminants production. In these cases, the use of other complementary methods such as photocatalysts and sonocatalysts were suggested [5,6]. The main role in the degradation of organic materials in photocatalysts and sonocatalysts is the release of •OH radicals. It should be noted that each of these approaches have different mechanisms in the energy consumption of radicals formation .
In recent years, the use of ultrasonic waves for the refinement and control of pollution has found a wide usage . Among them, it can be mentioned to items such as degradation of organic and dye contaminants in water, remove of sediment pollution, deodorize and disinfection of water. Some of this operation done based on physical properties (cavitation and micro-jet) and the others chemical properties (production of free radicals) which caused by ultrasonic waves . Ultrasonic waves have a promising prospect in the degradation of natural contaminants due to their low cost, high efficiency, and energy saving . However, there has recently been much research on the degradation of contaminants using ultrasonic waves. The mechanism of this process is based on the cavitation, which causes very high local temperature and pressure (local temperature equal to 4500 °C and pressure of 200 bar) in the reaction medium . This process involves the formation, gradual growth and eventually explosion of a series of bubbles by applying ultrasound to the solution, which produces shock waves [12,13]. At these conditions, produced hot spots can convert water molecules into highly reactive species, such as hydroxyl free radicals, hydrogen, and hydrogen peroxide, and can destroy harmful contaminations . However, ultrasonic waves cannot eliminate contamination lonely and it can be possible in presence of a catalyst .
The sonocatalyst process involves ultrasonic waves along with active catalysts which are highly effective in removing organic and dye materials under various experiments . The optimization of operational conditions for the maximum removal efficiency can be also beneficial to reduce the operational costs. In this regard, response surface methodology (RSM) based on central composite design (CCD) has been widely employed as statistical approach in various studies [17-19]. The main purpose of RSM is utilizing the results of a series of experimental runs to achieve an optimum response. Moreover, in our prior works [20,21], we investigated the photocatalytic activity of the nanoparticles by degradation of methyl orange as a pollutant under radiation of ultraviolet (UV). In this study, ZnO sonocatalyst without any template and surfactant was synthesized by sol gel method. ZnO ability to absorb a wide range of electromagnetic waves and its efficiency under ultrasonic irradiation, are the main reason for use of ZnO in this research. The structure, morphology and absorption spectra of synthetized nanoparticles were analyzed using X-Ray diffraction, TEM images and UV-Vis spectrophotometer, respectively. The sonocatalytic ability of ZnO nanoparticles for the degradation of Eosin B was investigated using a sonochemical process. The aim of this research was to optimize the various experimental conditions (ultrasound power and time, catalyst dosage and dye concentration) for the degradation of Eosin B by central composite design (CCD). The experimental design, statistical analysis, response surface plots, and optimization were performed using the Design Expert software (version 10.0.7).
Zinc acetate (Zn(CH3COO)2. 2H2O) and methanol (CH3OH) were used as the source of Zinc and the solvents of synthesis, respectively. Sodium hydroxide (NaOH) and ethanol (C2H6O) were used as the motive in the gel formation and to wash the particles, respectively. Chromatic material of Eosin B (C20H8Br2N2O9) as pollutant was chose to investigation of sonocatalytic activity. Its chemical formula and molecular composition are depicted in Fig. 1. All materials were purchased from Merck chemical company.
ZnO nanoparticles were synthesized by sol gel method. Zinc oxide sol was prepared by adding zinc acetate at a concentration of 0.15 M into methanol. The solution was stirred for 120 min. The transparent solution changed to white color by adding of 1.5 molar NaOH where pH of solution was obtained equal to 10. Milky white suspension was stirred at room temperature for 60 min. In the next step, the sol was centrifuged for 15 min at 6000 rpm. The white color sediment was washed two times with ethanol and water mixture to completely remove organic materials on the surface of the synthetized sonocatalyst. The resultant sediment was dried for 2 h at 150 °C. Finally, white color powder was obtained.
The crystallographic structures of nanoparticles were evaluated using X-Ray diffraction with CUKα radiation (XRD-D8BRUKER). Morphology of the particles was analyzed by transmission electron spectroscopy (TEM-Philips CM120). Investigation of the absorption spectrum and sonocatalytic activity of the as-synthesized nanoparticles has been done using spectrophotometer UV-Vis (Varian- Cary 50 Bio).
Sonocatalytic performance experiments were carried out in ultrasonic homogenizer (Bandelin sonoplus hd-2200). In the experimental section, we used black covering to protect reaction from any light irradiation. The sonocatalytic activity of ZnO nanoparticles was evaluated using Eosin B as pollutant. The experimental conditions including total volume of 100 mL aqueous solution, initial concentration of 5-25 mg/L dye solution, addition amount of 1–3 g/l catalyst, temperature of room, irradiation power 50–250 W, irradiation time of 10-70 min and ultrasound of 20 kHz frequency were applied. Sonocatalytic degradation of Eosin B was measured by sampling of 5 ml at different time intervals with rapidly centrifuging after any experiment. The absorption changes were calculated by spectrophotometer UV-V. The degradation rate was determined using the Eqs. (1):  Degradation rate % = (C0-C) / (C0)×100= (A0-A)/(A0)×100 (1)
Where, C0, C and A0, A are initial and final concentrations and absorption of Eosin B, respectively.
In the present research, central composite design (CCD) was applied for the optimization of sonoocatalytic process by evaluating the effect of four main factors, i.e., ultrasound irradiation power, ultrasound irradiation time, Catalyst dosage and dye concentration. The total number of experiments for 4 variables (n = 4) was determined as 2n+2n+6=24 (16 factorial points) + 2 × 4 (8 axial points) + 6 (central points, replications) = 30 . Furthermore, the parameter of Alpha (α) is defined as distance from the center point in a central composite design . This value represents rotatability designing and depends on a number of parameters used in the experiment . The value of α equal to, less and greater than one, puts on the axial points, central points and outside of the cube, respectively . In this study four independent parameters have been selected, hence for rotatable design the value of alpha is 2. Rotatable designs provide the preferred property of constant prediction variance at all points which are equidistant from the design center, thus improves the quality of the prediction. The levels of the optional variables were designed according to the data processes via the Design Expert software (Table 1). A second order model was used to fit the quadratic equation (Eqs. (2)) : Y (%) = b0 + b1A + b2B + b3C + b4D + b12AB + b13AC + b14AD + b23BC + b24BD + b34CD + b11A2 + b22B2 + b33C2 + b44D2 (2)
Where Y represents the response variable (Eosin B degradation); A, B, C, and D are the independent variables, i.e., ultrasound irradiation power, ultrasound irradiation time, catalyst dosage and dye concentration, respectively. Moreover bi, bii, and bij are the regression coefficients for linear, quadratic effects and the coefficients of the interaction factors, respectively.
Fig. 2 shows the XRD patterns of prepared ZnO nanoparticles. The XRD results were compared to the Joint Committee on Powder Diffraction Standards (JCPDS) X-ray data file. Patterns peak of XRD shows a good agreement with hexagonal structure. As can be seen from Fig. 2, there is no other peaks except ZnO. Furthermore, the higher intensity and narrower width of the ZnO peaks in the spectrum affirmed that the synthetized nanoparticles have good crystallinity . Average crystal size of synthesized particles obtained from Scherrer equation (Eq. (3)) .
D= (180 K λ) / (π B Cosθ) (3) Where D, B, K, θ and λ are the average crystallite size, width of the maximum band in half of height, Scherrer constant equation (equal to 0.89), angle and wavelength of the X-ray, respectively. From Scherer equation, the crystallize size of ZnO nanoparticles is estimated to 15 nm.
The morphology, particle size and shape of nanoparticles were examined via high resolution transmission electron microscope (TEM). Fig. 3 shows that the nanoparticles with hexagonal polyhedral structure and average grain size of 30 nm were formed. Due to the polyhedral grains, crystalline structure of particles is observable. This observation confirms the high degree of crystallinity of produced powder .
The absorption spectra and band gap energy of the nanoparticles are shown in Fig. 4. According to the UV–vis absorption spectra, intense absorption peak is observed in 520 nm. The results revealed that the prepared ZnO particles are sensitive to light radiation. Quantities of the band gap of the particles were determined by following Eq. (4) . (αhυ)2 = B (hυ-Eg) (4)
Where α, B, hν, and Eg are the absorption coefficient, constant of the equation, photon energy, and band gap, respectively. The function of (αhν)2 versus photon energy (hν) depicted in Fig. 4. The band gap energy of particles was calculated to be 2.9 eV. The narrow band gap of particles is prone to being excited by light to produce •OH radicals in solution .
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