In this research, ZnFe2O4 nanoparticles stabilized on copper slag (CS) as an environmentally friendly catalyst were prepared simply and economically. ZnFe2O4 was stabilized on copper slag by the thermal process. Benzene is a pollutant that enters the water by various industries. ZnFe2O4 / CS catalyst was used in the UV + H2O2 process for the photocatalytic degradation of Benzene as a water contaminant. The full factorial design of the experiment at three levels was used to optimize and model the photocatalytic removal of Benzene. pH, initial Benzene concentration, and ozone flow rate were the process variables at three levels. Optimal conditions were pH = 5, initial concentration of Benzene equal to 10 ppm, and ozone flow rate equal to 2.18 mg / h. The removal efficiency of Benzene under optimal process conditions was 99.55%.
Keywords: Photocatalytic degradation, ZnFe2O4, copper slag, optimization, full factorial
Introduction
Volatile organic compounds (VOCs) are present in some of the various industrial production processes and their effluents. Benzene is also a component of VOCs. Chemical industry wastes such as polymer industries – production of resins and synthetic fibers – infiltration of oil or petroleum products, various organic wastes, and atmospheric pollutants – and coal industry wastes are the most important sources of Benzene pollution in water (van Afferden et al., 2011; Amin et al., 2013; Nourmoradi et al., 2012; Y.S. Son, 2017). The release of Benzene into running water and air and contact with them is associated with disorders of the nervous system – skin irritation – chromosomal aberrations, and leukemia. Benzene enters the body through absorption from the skin surface – inhalation of steam or through food or drink, and causes side effects (Smith, 2010). Therefore, it is necessary to control the concentration of this pollutant to acceptable levels.
There are several methods for removing this contaminant from the effluent, each of which has its own strengths and weaknesses. Advanced oxidation processes (AOPs) are one of the promising methods for the degradation of organic and aromatic pollutants (Berenjian et al., 2012). Among the AOPs methods proposed or under research and development, photocatalytic processes have been significantly useful (Singh et al., 2016). ZnFe2O4 ferrite is one of the semiconductor compounds that play a photocatalytic role in these processes. UV irradiation to ZnFe2O4 can effectively produce electron-hole pairs and ultimately cause strong oxidizing agents such as hydroxide and peroxide radicals in aqueous solutions. These radicals can convert VOCs into harmless molecules such as CO2 and H2O (Yadav et al., 2018; Tabari et al., 2017).
The main problem in photocatalytic processes is the separation of catalyst from the solution (Chong et al., 2010). One of the methods to solve this problem is to fix the photocatalyst on a suitable base (Chong et al., 2010; Bhatkhande, 2001). Copper slag (CS) is a cheap and very stable compound that has good mechanical and thermal properties, so it is suitable for use as a catalyst base. Ozone was used as an oxidant simultaneously to further increase the reaction rate of photocatalytic degradation.
Environmental engineers and chemists are trying to improve and develop existing processes to treat wastes containing hazardous organic compounds that are difficult to degrade by conventional methods. It seems that combining several methods can increase the speed and efficiency of degradation. For example, the degradation of an organic compound may be difficult to do by ozonation or photocatalytic reactions alone, and sometimes contaminants may become more hazardous due to ozonation and incomplete degradation. But a combination of different methods such as O3 / UV-UV / H2O2 – O3 / H2O2 / UV, etc. can improve the speed of removal and complete degradation of organic pollutants from the effluent.
Investigation of various advanced oxidation methods shows that the degradation of organic pollutant molecules by the photocatalytic / O3 method in the presence of ultraviolet radiation is very effective (Chong et al., 2010).
Ozone and intermediate compounds in the presence of ZnFe2O4 photocatalyst and UV radiation degrade harmful pollutants in the effluent through the formation of ozonide and hydroxyl radicals in the surface layers adsorbed on the catalyst.
An experimental design is one of the best tools for studying the effect of process operational parameters separately as well as their interaction effects simultaneously (Bhatkhande et al., 2001). An analysis in which more than one parameter can be evaluated is called the “full factorial method.” In a full multivariate factorial, all the main operational parameters and their interactions are compared. This method also provides a basic solution for concluding about variables and on the other hand makes it possible to determine the most effective variable in the process. This method can also provide a mathematical model for testing. Numerous studies have been carried out to investigate the application of design of experiments in different processes, including the application of the factorial method for the photocatalytic removal of paint from wastewater (Bhatkhande, et al., 2001), the optimization of the photocatalytic removal of copper (II) (Mecha & Chollom, 2020), and the optimization of the adsorption rate of Pb (II) on the walnut shell (Yeber et al., 2009). According to the related research conducted in this field as well as the design of experiments, no study has been reported on the application of the full factorial method to optimize and evaluate the effect of various operational parameters for the removal of Benzene through photocatalytic ozonation using the catalyst ZnFe2O4/ CS. Therefore, in this study, the experimental design was performed by the complete factorial method with respect to variables including pH, the initial concentration of the contaminant, and the concentration of ozone in the solution. The effect of each variable separately and their interaction effects were studied simultaneously through ANOVA, and the most effective parameter was determined. On the other hand, the optimal conditions were determined, and finally, the appropriate experimental-mathematical model for the removal of Benzene through the photocatalytic ozonation method was presented using ZnFe2O4 / catalyst.
Mechanism
When UV light is irradiated to ZnFe2O4, its electrons are transferred from the VB band to the transition band. The generated electrons (e-) and the created hole (h +) then migrate to other parts and enter the following reaction (Sakkas, 2010):
ZnFe2O4 + hν → ZnFe2O4 + (h+ + e-) )
O3 + e- → O3•-
O3•- + H+ → HO3-•
HO3•- → HO• + O2
h+ + H2O → OH• + H+
Moreover, O3 performs the following reactions when exposed to UV light:
O3 + H2O + hν → H2O2 + O2
H2O2 + hν →2OH•
In alkaline solutions, the following reaction also causes the formation of hydroxyl radical:
O3 + OH− → OH• + (O2• ↔ HO2•)
Through the reaction of hydroxyl radicals with organic matters, these substances are oxidized and degraded:
OH• + R• → Rox
Finally, the direct reaction of ozone with organic matter leads to the decomposition of the organic matter:
O3 +R → Rox
Materials and Methods
Materials
In this research, copper slag was purchased from the Iranian Copper Slag company. The rest of the materials used were purchased from the German Company Merck. The pH values of the medium were diluted with NaOH and H2SO4 solution and adjusted using the German Metrohm-M12 pH meter. Agilent 8453 spectrophotometer was used in the determination of COD.
- Preparation of ZnFe2O4 / CS
The previously presented method was used to prepare the required catalyst. (Malekhosseini et al., 2019) According to this method, 50 ml of 0.25 M solution of nitrate prepared from Zn (NO3)2.6H2O was added to 50 ml of 0.5 M iron nitrate solution (prepared from Fe (NO3)3.9H2O). Then, 50 ml of 1 M urea solution was added to it and heated in a boiling water bath for 8 hours (under reflux conditions). The precipitate obtained after separation from the solution by centrifugation was dried in an oven at 110 °C and ground in a porcelain mortar. The obtained powder was mixed with ethanol and added to the copper slag. Having dried at room temperature, the powder was placed in an oven at 550 °C for 6 hours. The product was a catalyst that was stabilized on copper slag (ZnFe2O4 / CS). - Photocatalytic degradation process
To perform the photocatalytic process, a reactor with a total volume of 1 liter with reverse-flow packed bed (RPBPR) was utilized. A UV lamp (15W, philips) protected by a quartz tube was placed inside the reactor and the surrounding wall was filled with a catalyst (ZnFe2O4 / CS).
The overall schematic of the process is shown in Figure 1. During the testing process, the ozone generator from Nab Zist Company (Iran) with a production capacity of 50 grams per hour was used. The oxygen pressure capsule (prepared by Saman Gas Company) was used as pure oxygen entering the ozone generator. To control the temperature at 25 °C in all experiments, a photoreactor with a water flow jacket connected to a Thermo Bath (model ALB64, Korean company FINEPCR) was used.
Figure 1. The Schematic of the experimental process
The full factorial experimental design was used by performing 8 experiments according to Table (2). Statistical data were analyzed based on the analysis of variance (ANOVA). The independent variables considered to evaluate the photocatalytic oxidation process were pH, initial Benzene concentration (CBenzene), and O3 concentration at three levels. The experimental range and levels of the variables are shown in Table 1. The basis for selecting the level values of each variable was determined using previous articles and performing initial experiments. The variables and their levels as well as the necessary experiments are given in Tables (1) and (2). The statistical software Minitab 17 was used to design and analyze the experiments.
Table 1. Parameters investigated in the removal of Benzene pollutant and measurable range
Variables Range and levels
-1 0 +1
pH 5 7 9
Initial Con. of Benzene (ppm) 10 15 20
O3 mg/h 0.34 1.26 2.18
Table (2). Experimental conditions for the photocatalytic process
Run pH Pollutant O3
1 -1 -1 1
2 1 -1 1
3 -1 1 1
4 -1 1 -1
5 1 1 1
6 1 -1 -1
7 1 1 -1
8 -1 -1 -1
Results and discussion
Analysis of variance (ANOVA) is a powerful test that divides total variability in data into meaningful and variable components. Each of these components is caused by a variable source, and with the help of variance, the contribution of each of these sources in the variability of the whole data and, in fact, the effect of each of these sources in the response changes, can be evaluated. The analysis of variance extends the way of comparing the means of two samples to the comparison between the means of multiple samples.
The quality of the polynomial model proportional to the R2 coefficient was determined and its statistical significance was evaluated by the F-Fisher test in the same program. Table 3 lists the predicted effects and R% coefficients. The square of the correlation coefficient for each response was calculated as the coefficient of determination (R2). The accuracy and variability of this model can be evaluated by R2. The value of R2 is always between 0 and 1. The closer the value of R2 is to 1, the better the model predicts the response (R%).
The value of R2 investigated in this paper was equal to 0.999. The proximity of the R-squared coefficient determination values, which is 0.9983, with the adjusted “R-squared” coefficient, which is 0.9996, confirms the good predictability of this model.
According to Table 3 and the significant effects of the variables in the reaction, the pH coefficients, initial concentration of Benzene, and the amount of ozone were determined to be 12.024, 7.814, and 13.241, respectively. Therefore, the effect of these parameters can be determined as follows:
Initial concentration of Benzene< pH solution< Soluble ozone concentration
However, it should be noted that in the presence of other variables, the initial concentration of Benzene has a negative effect on the response (-7.814). It means as the initial concentration of Benzene increases, a decrease of R% occurs, and vice versa. Similarly, the interaction effects between variables are reported in Table 3. From these results and the interaction of variables, it can be concluded that simultaneous interactions of the three variables as well as the interaction of initial concentration of Benzene with O3 concentration have no significant effect (Pvalue> 0.05). The interaction of the initial concentration of O3 with pH also has a positive effect (2.354) on the value of R%.
It should be noted that the P values are evaluated concerning α = 0.05 and the ANOVA results are shown in Table 4. Increasing the value of parameter F and decreasing the value of parameter P have incremental effects on the responses.
Table 3. The Effects of estimation and measurement as well as the percentage of conversion coefficients (x%)
Term Effect Coef SE Coef T value P value VIF
Constant – 61.546 0.146 423.00 0.000 1.00
pH 24.048 12.024 0.146 82.64 0.000 1.00
Initial Con. of Benzene -15.627 -7.814 0.146 -53.70 0.000 1.00
O3 26.482 13.241 0.146 91.01 0.000 1.00
Initial Con. of Benzene × pH -5.233 -2.616 0.146 -17.98 0.003 1.00
O3 × pH 4.708 2.354 0.146 16.18 0.004 1.00
R2=%99.99, Pred R2=%99.83, Adj R2=%99.96
R% = 61.546 + 12.024 pH – 7.814 Benzene Concentration + 13.241 O3
- 2.616 pH×Benzene Concentration + 2.354 pH×O3
Table 4. Results of ANOVA for photocatalytic degradation of Benzene
Source Degree of freedom Adj SS Adj MS F- value P-value
Model 5 3146.73 629.35 3715.97 0.000
Linear 3 3047.65 1015.88 5998.27 0.000
pH 1 1156.56 1156.56 6828.93 0.000
Initial Con. of Benzene 1 488.44 488.44 2883.98 0.000
O3 1 1402.65 1402.65 8281.91 0.000
2-Way Interactions 2 99.08 49.54 292.51 0.003
Initial Con. of Benzene × pH 1 54.76 54.76 323.32 0.003
pH × O3 1 44.32 44.32 261.69 0.004
Error 2 0.34 0.17
Total 7 3147.07
Table 5. Residual values
Exp. No. R% Fit Residual (x%–Fit) St Resid
1 65.32 65.6075 -0.2875 -1.3972
2 99.55 99.595 0.045 -0.21869
3 55.5 55.2125 0.2875 1.397202
4 33.15 33.4375 0.2875 -1.39702
5 78.78 78.735 0.045 0.218692
6 68.45 68.405 0.045 0.218692
7 47.5 47.545 -0.045 0.218692
8 44.12 43.8325 0.2875 1.397202
Figure 2, part (C) is the appropriate response to investigate the effect of a variable on the response (in terms of size), which is a standard plot of the variable effect on the response. In this figure, the variables whose effects on the response are negative (-) or positive (+) are identified. The results show that ozone concentration has a greater effect on R% value than other variables. The effect of this variable is positive, i.e., increasing ozone concentration leads to increasing R% value. A closer look at the residual values in each experiment is shown in Figure 2, part d. As it is shown, 4 points (residuals) are below the zero lines (negative) and 4 points are above the zero lines (positive). Given these points and comparing the distance of the points from zero, it can be said that the residual distribution is normal. Drawing a normal probability plot of the residuals is a very useful method. If the error distribution is normal, the figure will look like a straight line. Figure 2, part (a) is a natural probability plot that clearly shows that the redistribution is normal because the points (especially the central points) are close to the straight line. If the model and hypotheses are correct, the residuals must be categorized in a certain way and give a predicted answer to any other variable. A simple experiment is to draw the residuals against the fitted values. The mathematical model of Benzene photocatalytic degradation within the study area is shown by the following equation:
R% = 61.546 + 12.024 pH – 7.814 Benzene Concentration + 13.241 O3
- 2.616 pH × Benzene Concentration + 2.354 pH × O3
Figure 2. (a) normal probability plot, (b) residual value compared to fitted values, (c) Histogram of the effect of standardized factors (d) residual values compared to observation order
Figure 3 shows the effect of the three main variables. These figures show that the change in the initial concentration of Benzene has a negative effect on the value (x%) and the change in pH and concentration of O3 has a positive effect on the value (R%) (If the interaction of the variable is ignored). The slope of the line in the drawn plots shows the main effect of each indicator alone on the value (R%).
Figure .3. Main Effects plot for the variables
The results showed that:
1- Stabilization of ZnFe2O4 nanoparticles on the surface of the copper slag has increased its photocatalytic activity in removing Benzene contaminants.
2- 2- The results of statistical analysis showed that the model used in this research is significantly reliable and valid.
3- In the process of photocatalytic degradation by ZnFe2O4 / CS, three parameters pH, initial concentration of Benzene, and O3 concentration values affected R% values. Regardless of the interaction of the variables, the initial concentration of Benzene has a negative effect on x% while the ozone flow rate and pH have positive effects on x%.
4- The interaction of variables is very important and should be considered to optimize the research conditions because it significantly affects the value of x%. The optimum conditions for the Benzene degradation process by ZnFe2O4 / CS are pH = 5, CBenzene = 10 ppm, and O3= 2.18 mg/h where the maximum degradation is 99.595%.
Acknowledgments
The authors wish to thank the Islamic Azad Universities of Arak and North Tehran Branch.
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