Effect of cysteine using Fenton processes on decolorizing different dyes: a kinetic study

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.