Effect of cysteine using Fenton processes on decolorizing different dyes: a kinetic study
Márcio Daniel Nicodemos Ramos, Larissa Aquino Sousa, André Aguiar
Abstract
Amino acid cysteine has been used as reducing mediator with the aim of improving dye degradation by homogeneous Fenton processes (Fe2+/H2O2 and Fe3+/H2O2). Through its known Fe3+-reducing activity, this amino acid can enhance the production of reactive oxygen species as HO• (hydroxyl radical) and its pro-oxidant properties have been verified while decolorizing diverse dyes in the present work. Its presence enhanced decolorization of Methyl Orange, Phenol Red, Safranin T, Rhodamine B, Reactive Black 5 and Reactive Yellow 2, mainly in reactions initially containing Fe3+ as a catalyst (Fe3+-reactions). E.g. Fe3+/H2O2 and Fe3+/H2O2/cysteine systems decolorized 27% and 44% of Phenol Red after 60 min, respectively. A kinetic modeling analysis has revealed that 1st-order and mainly 2nd-order kinetic models were well fitted to both Fe2+- and Fe3+-reactions data. Improvements in reaction rate constants have been observed by adding cysteine. In experiments performed at varied temperatures, it was found a decrease in activation energy (Ea) due to cysteine addition while decolorizing Safranin T: Ea decreased from 104.6 to 88.9 kJ mol−1 for Fe3+-reactions and from 81.0 to 52.2 kJ mol−1 for Fe2+-reactions. Therefore, it was found that cysteine decreases the energy barrier so as to improve Fenton-based decolorization reactions.