Promoted discoloration of methyl orange in H2O2/Fe(III) Fenton system: Effects of gallic acid on iron cycling

The promoting effect of gallic acid (GA) on the discoloration of methyl orange (MO) in H2O2/Fe(III) Fenton process was investigated in this study. The MO discoloration rate in H2O2/Fe(III) system with GA was significantly higher than that with sulfuric acid. Central composite design based on the response surface methodology (RSM) was adopted to evaluate the effects of initial Fe(III) concentration, initial pH and molar ratios of H2O2/Fe(III) and GA/Fe(III) on the discoloration rate constant kapp. The RSM model was derived and optimal molar ratios of H2O2/Fe(III) = 100:1 and GA/Fe(III) = 2:1 were obtained. The H2O2/Fe(III)/GA system also showed good adaptability with minor decreasing trend in alkaline circumstance. Reaction mechanisms for the promoted formation of hydroxyl radical (HO) by GA were proposed. GA could not only form complexes with Fe(III) to Fe(II) and prevent the precipitation of Fe(III) subsequently, but also accelerate the reduction of Fe(III) to enhance the formation of HO. Moreover, the readily oxidizable characteristic of GA makes it a good option with environmental benignity in Fenton system.