However, when the initial pH was 2, the decolorization rate decreased. According to Fenton's chemistry, if reaction is carried out in a high pH condition, the iron forms a precipitate, Fe(OH)3, which decomposes H2O2 to O2. Also, if pH is too low, Fe(III) is reconverted to Fe(II). Both cases lead to reduced reaction rate by decreasing the starting reagents [5]. The results of this study indicate that the efficiency of decolorization of the azo dye by nZVI increased with a decrease in the systempH. This findingwas consistent with the results reported by several researchers [11,14] using microsized zero-valent iron. During the reduction of Orange II with nZVI, the pH increased from 3 to 3.9, 5 to 5.5, and 7 to 7.3. After the reduction reactions were completed, pH became constant. The reduction of Orange II by nZVI consumed H+ ions, which resulted in an increase in the pH during the early stages of the reaction.