Degradation of Cellulose

A study on the formation of hydrogen peroxide during oxygen bleaching of Eucalyptus

globulus confirmed the origin of cellulose degradation, as well as the effect

of metal ions on the degradation [143]. Hydrogen peroxide levels detected in the

effluent of the oxygen treatment of pulps were higher when lignin was present

(unbleached pulp), or in bleached pulp with the addition of phenolic lignin model

compound (vanillic alcohol). Moreover, the metal ions present also influenced the

content of H2O2 in the effluents of oxygen treatments [143].

Oxygen delignification became technically feasible when Roberts showed that

the addition of magnesium compounds retards the degradation of cellulose more

efficiently than that of lignin [145].

The protective effect of magnesium against hydroxyl radical formation was studied

by several groups [249–254]. The influence of combined magnesium and manganese

[156,255–259], transition metals [203,260–262], chelants [263–265], calcium

carbonate and silicate [266–268], sulfur compounds [155] and oxygen pressure

on hydroxyl radical formation has also been investigated.

Neither hydrogen peroxide [269] nor the superoxide anion radical [6] is capable

of degrading carbohydrates directly. The degradation is initiated by an attack of

the hydroxyl radical [6,269,270]. In the presence of metal ions, the superoxide

anion radical, which is formed during oxygen bleaching, can be oxidized to oxy-

668 7Pulp Bleaching

gen [see Eq. (17)] or reduced to the hydroperoxy anion [see Eq. (18)] [125]. The

reduction of Fe3+ by the superoxide anion can also accelerate the Fenton reaction,

producing a superoxide-driven Fenton reaction [Eq. (19)] [125]. In a carbohydrate

model study, it was found that at pH 10.9, degradation was strongly inhibited

[269], though this may have been due to the low solubility of Fe2+ and Fe3+ ions

under the conditions of oxygen bleaching. In contrast, manganese proved to be a

very effective catalyst for hydrogen peroxide decomposition during peroxide

bleaching [271] up to pH 9, but was inactive in acidic media. Copper was seen to

be the most effective transition metal to catalyze hydrogen peroxide decomposition.

The one-electron reduction of hydrogen peroxide is catalyzed primarily by

mononuclear transition metal ion species. At high pH, these species may only

arise when the concentration of the metal ion is very low. Copper appears to be

the most efficient Fenton catalyst under the conditions of alkali bleaching [145].

At higher concentrations, most metal ions aggregate or condense to form

hydroxo-bridged polynuclear species in alkaline solutions. Manganese (Mn2+) and

hydroxide ions (OH–) form aggregates that can be oxidized by oxygen to produce

MnO2 at a pH above 9. Colloidal MnO2 decomposes H2O2 efficiently by a two-electron

reduction to give oxygen and water directly, without generating any significant

amount of hydroxyl radicals [145]. Colloidal particles of metal hydroxides and

hydrated oxides may also catalyze the dismutation of superoxide [145].

The superoxide-driven Fenton [Eq. (19)] reaction can be written in a more common

form [Eq. (23)], starting with oxidation of the superoxide anion radical by a

metal ion. The second step – the reduction of hydrogen peroxide – is not an equilibrium

reaction, as the radical formed will immediately react with the substrate

due to the extreme reactivity of the hydroxyl radical. A maximum rate of hydroxyl

formation is expected in the pH range 11–11.5 [145]; thus, conditions of oxygen

delignification appear to be near-optimal.

Men__ _O_2 →Me_n_1___O2

Me_n_1___H2O2→Men___OH _ OH_

H2O2__O_2

Me

catalyst __ _OH _ OH__O2

_23_

How metal ion species may affect the four oxygen reduction steps is summarized

in Scheme 7.22.

Reitberger et al. [145] reported that the protective effect of magnesium compounds

might have different explanations, including:

_ Coprecipitation of transition metal ions with magnesium hydroxide,

which should stabilize hydrogen peroxide against decomposition

to give hydroxyl radicals and achieve redox stabilization of

Mn2+.

_ Formation of Mg–cellulose complexes which protect against

attack by hydroxyl radicals.

7.3 Oxygen Delignification 669

O2

(metal ion species)

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