RESUMO
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 60â 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 88.9â kJâ mol-1 for Fe3+-reactions and from 81.0 to 52.2â kJâ mol-1 for Fe2+-reactions. Therefore, it was found that cysteine decreases the energy barrier so as to improve Fenton-based decolorization reactions.
Assuntos
Corantes , Peróxido de Hidrogênio , Cisteína , Ferro , Cinética , OxirreduçãoRESUMO
The fungal metabolite 3-hydroxyanthranilic acid (3-HAA) was used as a redox mediatorwith the aim of increasing dye degradation by Fenton oxidative processes (Fe2+/H2O2, Fe3+/H2O2). ItsFe3+-reducing activity can enhance the generation of reactive oxygen species as HOâ radicals.Initially, the influence of 3-HAA on decolorization kinetics of five dyes (methylene blue,chromotrope 2R, methyl orange, phenol red, and safranin T) was investigated using decolorizationdata from a previous work conducted by the present research group. Fe3+-containing reaction datawere well fitted with first-order and mainly second-order kinetic models, whereas the BMG(Behnajady, Modirshahla and Ghanbary) model obtained optimal fit to Fe2+. Improvements inkinetic parameters (i.e., apparent rate constants and maximum oxidation capacity) were observedwith the addition of 3-HAA. In another set of experiments, a decrease in apparent activation energywas observed due to introducing 3-HAA into reactions containing either Fe2+ or Fe3+ in order todecolorize phenol red at different temperatures. This indicates that the redox mediator decreasesthe energy barrier so as to allow reactions to occur. Thus, based on recent experiments and thereaction kinetics models evaluated herein, pro-oxidant properties have been observed for 3-HAAin Fenton processes.