Kinetic processes in alkaline peroxide bleaching
Creators: name: Wright, P. J.
Publisher: name: University Of Tasmania
Year: 2023

description: The response of a commercial hardwood mechanical pulp, E. regnans stone groundwood, to alkaline peroxide bleaching has been studied by examining the kinetics of the bleaching process under conditions of constant reagent concentrations. Several kinetic models have been tested for their ability to describe the observed kinetic behaviour in terms of a minimum number of chemically meaningful parameters. A previously reported kinetic model, developed to describe the peroxide bleaching of softwoods, was found to be inadequate. As a consequence, two new kinetic models have been proposed, both of which are defined in terms of first order processes. The first model type assumes two distinct categories of chromophores which are eliminated to give colourless products. The chromophoric groups are divided into peroxide susceptible or peroxide resistant groups on the basis of their kinetic behaviour. The second model type assumes a single class of chromophores which are eliminated to give colourless products and potential chromophores. In this 'equilibrium' model, chromophore elimination is opposed by a chromophore creation route arising from conversion of potential chromophores to chromophores. An understanding of the main chemical processes occurring in peroxide bleaching has been achieved by analysing the behaviour of pseudo-first order rate constants for each model. Both of the proposed models are able to describe a number of kinetic features observed under constant reagent concentrations, such as a maximum in the rate of bleaching in the pH 11-12 range. However, only the model containing a reversible step can describe the full range of experimental behaviour observed when reagent concentrations are altered during bleaching, suggesting that colour formation is an important process during alkaline peroxide bleaching. The kinetics of reactions between alkaline peroxide and model lignin chromophores have been examined to lend further interpretation to the kinetic models developed for the bleaching of pulp. Cinnamaldehyde has been studied as a model for a,‚àövº~¬¨‚â†unsaturated aldehyde chromophores while the reactions of ortho quinone chromophores have been investigated using 4-tert-butylorthoquinone as a model. Reactions with cinnamaldehyde have shown that a,‚àövº-unsaturated aldehydes are readily attacked by perhydroxyl anions (H02-) to give colourless epoxide intermediates. The epoxides are further degraded to benzaldehyde derivatives leading to an irreversible removal of colour. Reactions between alkaline peroxide and 4-tertbutylorthoquinone indicate that ortho quinones undergo competing colour removing and colour creation reactions with perhydroxyl anions and hydroxide ions respectively. The colour creation processes result in the formation of peroxide resistant hydroxy para quinones which lead to incomplete removal of colour during bleaching. The kinetic behaviour of model a,‚àövº~-unsaturated aldehydes and ortho quinones can be related to the reaction steps in the two chromophore kinetic model described previously, and can explain certain kinetic features observed during the bleaching of pulp. However, the process of chromophore formation contained in the equilibrium kinetic model was not observed in studies on model a,‚àövº~unsaturated aldehydes and ortho quinones, and further work is required to identify possible colour causing reactions under peroxide bleaching conditions.