The role of radical species in peroxide bleaching processes

Analysis of expressions describing peroxide bleaching show that there are two distinct mechanisms involved in peroxide bleaching. These routes can be associated firstly with traditional bleaching by the perhydroxyl anion and the second mechanism appears to involve the action of radical species. This analysis has been used as the basis for experimental work to determine the existence and relative contribution of each of these mechanisms. Two-stage bleaching processes under acidic followed by alkaline conditions, without an intermediate washingstage, have been investigated in order to separate the contributions of anion and radical bleaching. The addition of chromium nitrate, which is known to catalyse peroxide decomposition under acidic conditions, to such a process enhances the brightness of the pulp compared with a single stage alkaline peroxide bleaching process. The addition of small quantities of aluminium nitrate to a two-stage bleaching process also enhances the brightness of the pulp, this process depending on the presence of both aluminium and manganese. The results are explained in terms of radical concentrations in the acidic stage and the proposed mechanisms and active bleaching species in peroxide bleaching. Many transition metal ions decompose peroxide, with some producing significant quantities of hydroxyl radicals. The effects, of addition and removal of these metals on peroxide bleaching have been investigated. Unusual negative effects are observed upon chelation and the addition of manganese, copper and iron does not appear to promote darkening reactions caused by the decomposition products of hydrogen peroxide. The effect of pulp consistency on brightness response has also been investigated. Work at constant conditions of peroxide and alkali, while varying either the total consistency or the ratio of bleachable to inert fibre shows that consistency is not a major factor in determining final brightness under alkaline conditions and indicates that the effect of radical species is small at low consistencies. A method of determining relative hydroxyl radical concentrations in solutions of hydrogen peroxide has been developed, using N,N-dimethy1-4-nitrosoaniline. Under alkaline conditions the effect of the hydroxyl radical appears to be negligible, whilst under acidic conditions the concentration of radicals can be corelated to the observed final brightness.