Oxidative degradation of textile dyes with hypochlorite and chlorine dioxide.

Abstract

The oxidation reaction mechanisms of water soluble textile dyes amaranth (an azo dye), brilliant blue-R (a triaryl dye) and safranine-O (an azine dye) with oxidants- hypochlorite and chlorine dioxide, were investigated. The detailed kinetics of the reactions of the three dyes was studied under excess concentrations of the oxidant and other reagents. The depletion of concentration of the chosen dye, taken at low concentration was monitored using a Hi-Tech SF-61 DX2 double mixing micro volume stopped-flow apparatus. The hypochlorite initiated oxidations were investigated as function of varying concentration of oxidant and hydrogen ion, ionic strength and temperature. For the chosen dyes and reaction conditions, the depletion of dye followed pseudo first-order kinetics and the rate constants were estimated using KinetAsystTM software. All the three reactions had first-order dependence on the oxidant concentration, and the reaction rates increased by varied extent with increase in [H+]0. The role of acid in their reaction mechanisms was established. The kinetic data was analysed to evaluate the rate constants for the competitive pathways initiated by hypochlorite ion and hypochlorous acid. The overall second-order rate coefficients for the OCl- and HOCl initiated reactions were estimated for all the three reactions. Major oxidation products for the reactions were separated and characterized by 1H NMR and 13C NMR and GC-MS techniques and the stoichiometry was established. The energy parameters inclusive of Arrhenius factor, enthalpy, entropy and energy of activations for the oxidation of three dyes both by OCl- and HOCl species were estimated. Based on the experimental findings, the probable rate laws, mechanisms and reaction schemes were described. Simulations studies were conducted to validate the proposed mechanisms using SIMKINE2 computer programme. The rate of oxidation of safranine-O is greater than that of amaranth and brilliant blue-R with OCl- / HOCl reaction. Following similar protocol, the oxidations of the chosen dyes with chlorine dioxide were investigated by monitoring the depletion kinetics of dye as function of varying concentration of ClO2 and OH- ion, ionic strength and temperature. All the three dyes, exhibited pseudo first-order kinetics and the rate constants were estimated using KinetAsystTM software. All the three reactions had first-order dependence on the oxidant concentration at pH conditions 7.0, 8.0 and 9.0 suggesting that reaction mechanism remains unaltered with pH variation. The effect of hydroxide ion on the reaction rate revealed that it acts as catalyst. All the three reactions had first-order dependence on [OH-]0, when its concentration was low; but the order with respect to [OH-] decreased as [OH-] increased stoichiometry proportion to reactants. The catalytic constant for hydroxide catalysed reaction was estimated for all the three reactions. Kinetic salt effect experiments were conducted to identify the possible reaction species involved in the reactions. The major reaction products were characterized by 1H NMR, 13C NMR and GC-MS techniques. The stoichiometry ratios were established and energy parameters were estimated. The rate laws and probable reaction mechanisms were proposed and appropriate reaction schemes for all the reactions were described. The elucidated mechanisms were confirmed by simulation studies using SIMKINE2 software. At neutral pH the rate of oxidation of amaranth is greater than safranine-O and brilliant blue-R, and brilliant blue R being the slowest.