Study of kinetic parameters related to dyes oxidation in ascorbic acid-mediated Fenton processes

Abstract

Ascorbic acid (AA) is a natural reducer that has been used as a prooxidant additive to improve dyes oxidation via Fenton processes (Fe2+/H2O2 and Fe3+/H2O2). In the present work, low concentration of AA (10 μmol L1) enhanced Bismarck Brown Y (BBY), Safranin T (ST), Rhodamine B (RB), and Reactive Black 5 (RB5) decolorizations in solution, mainly in reactions initially containing Fe3+ as a catalyst (Fe3+-reactions), e.g., Fe3+/H2O2 decolorization of RB5 was increased from 54 % to 73 % after 60 min due to added AA. Decolorization increased from 0 up to 60–90 μmol L−1 of added reducer. At higher concentrations, AA did not improve decolorization. The dyes were decolorized below 10 % when incubated with 1 % tert-butanol, indicating that HO• radical is the main oxidant involved in the reactions. The kinetic analysis showed that the 1st- and 2nd-order kinetic models fitted well to both Fe2+- and Fe3+-reaction data. The BMG kinetic model also fitted well to the Fe2+-reactions (with and without AA), along with Fe3+/H2O2/AA. Based on these kinetic models, we verified that reaction rate constants were increased due to added AA. There was a decrease in activation energy (Ea) in decolorizing ST from added AA by varying the reaction temperatures. For example, Ea decreased from 111.6 to 81.8 kJ mol−1 for Fe3+-reactions and from 72.6 to only 69.3 kJ mol−1 for Fe2+-reactions. The AA/H2O2 system was least effective in decolorizing ST since Ea was 130 kJ mol−1. In summary, ascorbic acid decreases the energy barrier to improve Fenton-based ST oxidation.