Oxidative decolorization of triphenylmethane dyes by chloramine-T in alkaline medium catalyzed by Pd(II): A comparative spectrophotomeric kinetic and mechanistic approach

Abstract

Triphenylmethane dyes viz., p-rosaniline, crystal violet and ethyl violet (henceforth abbreviated as Dye) have been extensively used as colorants in textile industries. Effluents containing these dyes released by the industry causes significant environmental problems. Hence decolorization of dye effluents has acquired increasing attention. The oxidative decolorization of these dyes is of greater significance one understanding of their kinetic and mechanistic aspects in redox reactions is important. Consequently, Pd(II) catalyzed kinetics and oxidative decolorization of p-rosaniline, crystal violet and ethyl violet dyes by chloramine-T (CAT) in NaOH medium were studied spectrophotometrically. The oxidation reaction follows the identical kinetics for all the three dyes and obeys the rate law: rate = k[CAT] a [Dye] b [OH −] c [Pd(II)] d where a and b are unity, and c and d are less than unity. Addition of p-toluenesulfonamide, the reduction product of CAT, retards the rate and the order was found to be less than unity. The reaction rate increased with increasing ionic strength of the medium. Addition of chloride ions show negligible effect on the rate of reaction. Oxidation products were characterized by GC–MS analysis. Proton-inventory studies with ethyl violet as a probe have been made in H 2O–D 2O mixtures. Activation parameters and decomposition constants have been deduced. An isokinetic relation is observed with β = 423 K indicating enthalpy as a controlling factor. The rate of oxidation of triphenylmethane dyes follows the order: p-rosaniline > crystal violet > ethyl violet. This trend may be attributed to inductive effects of alkyl amino groups. Further, the kinetics of Pd(II)-catalyzed oxidation of these dyes have been compared with uncatalyzed reactions (in the absence of Pd(II) catalyst) and found that the catalyzed reactions are nearly four times faster. The catalytic constant ( K C) for Pd(II) was also calculated for each dye at different temperatures and the values of activation parameters with respect to the catalyst have been evaluated. Chlorite ion (OCl −) has been postulated as the reactive oxidizing species. The observed results have been explained by a general mechanism, and the related rate law has been obtained. Furthermore, the simple and elegant method developed for the decolorization of triphenylmethane dyes in the present work can be adopted for treating the aforesaid dyes present in industrial effluents to minimize the toxicity caused by these dyes.