Peroxide Bleaching Reactions Under Alkaline and Acidic Conditions

Abstract

An examination of previously reported kinetic expressions describing peroxide bleaching of wood pulp under alkaline conditions reveals that the overall process can be considered as a combination of two parallel reaction routes. The first route corresponds to a reaction involving direct participation of the perhydroxyl anion in chromophore elimination. This mechanism can be identified with the classical explanation for peroxide bleaching. The second route can be associated with reactions in which chromophores are eliminated through the action of free radical intermediate species. New experimental evidence is presented to show that processes catalysed by transition metal ions can lead to enhancement of bleaching.A two stage peroxide bleaching sequence, initially under acidic conditions in the presence of chromium, followed by alkaline conditions produces an acceleration in bleaching rate, without significant additional consumption of peroxide.

Plant fibres: Botany, chemistry and processing for industrial use

“…Non-chlorinated bleaching agents are now very much in demand since they produce less-toxic effluent (Eriksson 1990). In recent years, less environmentally damaging methods have emerged (Shackford 1990;Strunk 1990;Hobbs and Abbot 1991). These involve oxygenated bleaching agents such as O,/OH-, H,O, and 0,.…”

FT-Raman, FT-infrared and NIR spectroscopic characterization of oxygen-delignified kraft pulp treated with hydrogen peroxide under acidic and alkaline conditions

“…While much has been done to elucidate the chemistry and technological effects of the two-stage hydrogen peroxide bleaching of mechanical pulps [6,7,[9][10][11][12], little attention has been given to chemical pulps [5]. There is a lack of information concerning the influence of acidic and alkaline conditions of the peroxide bleaching on the chemical pulp components, especially on lignin and its chromophores.…”

“…The enhancement of peroxide delignifying efficiency may be attained by the use of an acidic medium at the preliminary bleaching stage [5]. The two-stage hydrogen peroxide treatment under acidic and then alkaline conditions may be beneficial both as regards the peroxide consumption and the final pulp brightness [6][7][8][9][10][11]. Hydrogen peroxide is relatively stable under acidic conditions; therefore, transition metal catalysts are used for its activation [12].…”

“…Hydrogen peroxide is relatively stable under acidic conditions; therefore, transition metal catalysts are used for its activation [12]. The metal ions decompose peroxide into free-radical products, including the very reactive hydroxyl radical (OH • ) that exhibits poor selectivity toward pulp structural components (lignin and carbohydrates) [7,12]. Especially in chemical pulps, the degradation of cellulose by the hydroxyl radical may negatively affect its degree of polymerization (DP) and thus its mechanical strength properties.…”

Delignification of Lignocellulosic Biomass and Its Effect on Subsequent Enzymatic Hydrolysis

“…Since the first commercial installation over 40 years ago, the use of oxygen delignification processes has become more prominent in industry due to increased environmental concerns about chlorinated bleaching agents (Rowlandson 1971). Additionally, alkaline hydrogen peroxide has been used extensively for the bleaching of mechanical and chemical pulps (Hobbs and Abbot 1991). However, with increased degree of delignification, losses in fiber strength are expected to occur due to carbohydrate degradation (De Souza et al 2002).…”