The present work consisted in evaluating the effect of a natural plant reducer, gallic acid (GA), on the discolouration/oxidation of two azo dyes by Fenton processes (Fe3+/H2O2 and Fe2+/H2O2). A kinetic study was performed to better interpret the discolouration data at different temperatures. The 1st-order kinetic model presented the best fit for the experimental data of methyl orange discolouration, while the 2nd-order was better for chromotrope 2R. Due to the addition of GA and the temperature rise, there were increases in discolouration and in the reaction rate constant values. As a highlight, it was possible to verify the reduction of the apparent activation energy (Ea) due to the presence of GA. For example, Ea for discolouring methyl orange corresponded to 81.5 and 53.6 kJ.mol-1 by Fe2+/H2O2 and Fe2+/H2O2/GA, respectively. Thus, it can be inferred that the GA reduces the energy barrier to increase the oxidation of dyes by Fenton processes.
Keywords: Fenton reaction; dyes; gallic acid; hydroxyl radical; kinetics.