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One of the most environmental polluting industries is industrial wastewaters. Textile and dyeing industries are very important for the development of countries. Furthermore, other industries such as cosmetics, paper and pharmaceutical also produce colored wastewater [1]. In dyeing processes, about 15% of the generated colors release to the sewage and then in this way painted wastewaters formed [2]. Different color-causing substances were used in industries and azo group dyes are the most common colors. One of the largest groups of synthetic colors which belongs to azo dyes contains one or more azo bond -N = N-. It is estimated that about 50 percent of annual worldwide production of colorants (700 thousand tons) are azo type. Colors and organic materials with complex structures are often toxic, carcinogenic, mutagenic, non-biodegradation and resistant in environment and if they release to it without any treatment may endanger the environment and human health [2, 3, 4, 5]. Colored wastewater discharging from industries into water streams can cause Eutrophication and interference in ecology and cause chemical changes in water streams. Also, dye molecules in wastewater are completely visible in water, even in very low concentrations, because of their strong bonds. Therefore, colors are one of the most obvious water contamination indicators. In previous decades, dye wastewaters treatment were highly regarded by Environmental engineers and has motivated some studies in recent decades done in the development of new processes for the removal of color and organic load from colored wastewater [6, 7, 8]. Diverse methods for wastewater treatment has been studied by many researchers including different physical chemical methods such as ultra-filtration, reverse osmosis, ion exchange and adsorption on different materials, such as activated carbon, coal, wood chips and Silica gel in order to remove dye and COD from wastewater. But since these methods can only transfer the pollutants from the aqueous phase to the solid phase and they aren't destructive or eliminating processes, they were not considered as universal techniques. Hence, in recent years, advanced oxidation processes based on the production of free radicals, which rely on the power of oxidation, especially OH have been fairly accepted scientifically [9]. In the process of Nanoscale Zero-Valent Iron and H₂O₂ than conventional Fenton process, hydroxyl radical production occurs in two stages. Thus, the efficiency increases. On the other hand, in Fenton process by ferrous ions formation, the efficiency of the process reduces and stops, While if Nanoscale Zero-Valent Iron is used, at first Ferro ions and then ferrous ions formed. So actually the efficiency of process would be more than the efficiency of conventional Fenton process [10]. The most important reactions in organic material removing by the hydrogen peroxide process in the presence of Nanoscale Zero-Valent Iron are as the following:
Fe⁰ + H₂O₂ →Fe⁺² + OH^- + OH⁰
Fe²⁺ + H₂O₂ →Fe⁺³ + OH^- + OH⁰
OH + Acid Red 18 → oxidized dye+ H₂O
Nanoscale Zero-Valent Iron (NZVI) is used as a regenerative for activation of hydrogen peroxide. Because Nanoscale Zero-Valent Iron ions release Fe (II) into solution under acidic conditions, these dissolved ions are the first step for oxidation of pollutants (such as dyes) in H₂O₂/Fe⁰ process. While the oxidation of dye occurs, Fe⁺² ions consumed and the pollutants oxidation depend on the solubility of Fe⁺² in solutions [11].
In addition to being a strong reduction in powder form  of NZVI, inexpensive, easy to use, non-toxic, quick reaction and high efficiency for pollutants decomposition and returning NZVI in to cycle by a magnetic are the characteristics of this nano particle [12, 13]. Also a wide variety of contaminants including chlorinated organic compounds, poly-chlorinated biphenyls, heavy metal ions, oxy anion, dimethyl phthalate and 2, 4-dichlorophenoxyacetic acid can be treated with the NZVI [13, 14, 15]. Colors are chemical materials that may be unsustainable by this process and recently they have been used for the removal of several azo dyes [12, 16, 17, 18].
Aims of the study: The main objective of this study is to evaluate the efficacy of Nanoscale Zero-Valent Iron in the presence of hydrogen peroxide (Fe⁰/H₂O₂) in order to remove the Acid Red 18 dye from aqueous solutions.
 

Materials & Methods

This is a fundamental- practical study which is done in the laboratory-scale and in a batch process. The variations in this study include contact time (5, 10, 30, 80, 140, 210) min, initial concentration Nanoscale Zero-Valent Iron (0.5, 2, 3, 4) g/l and pH (3, 5, 7, 9), hydrogen peroxide (24, 90, 140, 200, 300) mmol/l, initial concentration of Acid Red 18 dye (25, 50, 75, 100) mg/l. Mettler Toledo pH meter was used for measuring of pH. NaCl and HCL 1N were used for adjusting the pH. In this study, NZVI with an effective size of 50-35 nm, surface-to-mass ratio equal to 8-14 m²/g and American product (USNANO) was purchased from Nanosany Company. Figure 1 shows NZVI image which is taken by electron microscope (TEM)^[1]. Acid Red 18 dye was purchased from Rang Alvan Sabet Company in Iran. Chemical structure of Acid Red 18 is shown in Figure 2. Acid Red 18 dye specifications are shown in Table 1 [19]. To perform the experiments, 250 ml of dye with different concentrations were added to 500 ml glass beakers and the amount of pH was adjusted to the desired range. Then different concentrations of Nanoscale Zero-Valent Iron- hydrogen peroxide and hydrogen peroxide were added in to 250 ml of dye with different initial concentrations and after mixing by Jar Test at 250 rpm, Samples were taken at specified intervals. Maximum absorption of Acid Red 18 solutions at 506 nm wavelength were determined by using spectrophotometer (CECIL 7250) UV/VIS [21, 20]. Then, by knowing adsorption amount and using calibration curve, the remained concentration of dye was determined. The Calibration curve of spectrophotometer is shown in Figure 2. For example, at pH =3 and initial dye concentration of 50 mg/l, different doses of zero iron nanoparticles-Hydrogen peroxide and hydrogen peroxide were added and at specified contact times the sampling process were done and the remained dye concentration were determined.
 

Results

The results of the experiments are shown in Figures 4 to 8. In these figures the effects of contact time, initial dye concentration, pH, NZVI and hydrogen peroxide concentration on color removal efficiency in each stage is shown. The results show that by increasing the concentration of NZVI, hydrogen peroxide and contact time increase the removal efficiency. By the decrease of initial concentration and the
amount of pH to a certain value the removal efficiency also increases.

Figure1: zero valent iron nanoparticles image with transmission electron microscopy (TEM)
 

Figure 2: chemical structure of Acid Red 18
 

 
Table 1) Characteristics of Acid Red 18

Brand	Chemical formula	maximum wavelength (nm)	Number of azo bond	molecular weight
Acid Red 18	C20H11N2Na3O10S3	506	Mono azo	604.5