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The effect of iron addition in the treated solution on the decomposition of indigo carmine is reported. The effect of iron addition was evaluated for various Fe2+ concentrations ranging from 0 to 118 μmol/L (μM). The decolorization rate and efficiency with Fe2+ addition were higher than those without Fe2+ and increased with the rise in Fe2+ concentrations. The optimal Fe2+ concentrations for decolorization rate and efficiency were 95 and 83 μM, respectively. The pH effect was evaluated in the range of 2.3-11.3 values for the initial solution containing 71 μM Fe2+. The decolorization rate was approximately constant for pH ranging from 4.7 to 8.0. The increases in decolorization rate and efficiency with Fe2+ are attributed to a significant increase in hydroxyl radicals quantity, which is generated by Fenton's reaction of iron and hydrogen peroxide.