Chemistry of hydrogen peroxide bleaching

Manfred Schwanninger

Although the major fraction of wood lignin can be removed by pulping, the

remainder of the lignin (residual lignin) is rather resistant under the pulping conditions.

In order to remove the residual lignin from the pulp, oxidative lignin degradation

with bleaching reagents such as dioxygen, H2O2, ozone, and chlorine

dioxide is required. Hydrogen peroxide is mainly used to brighten pulps (removal

of chromophores) during the final bleaching stages, and at the end of a conventional

bleaching sequence to prevent the pulp from losing brightness over time.

Carbonyl carbons or the vinylogous carbon atoms in intermediates of the enone

type (quinone methide intermediate; see Section 4.2.4, Chemistry of kraft pulping,

Scheme 3) are the locations where the nucleophile (the hydroperoxy anion)

begins the attack [11,12]. The hydroperoxy anion is incapable of degrading polymerized

lignin directly via an attack of the electron rich aromatic rings of the residual

lignin, but by cleaving the sidechain i.e. Dakin and Dakin-like reactions the

lignin can be depolymerised.

The parameters that influence bleachability, the composition of lignin and residual

lignin after cooking and their reactivity, as well as the composition of residual

lignin–carbohydrate complexes (RLCC) before and after oxygen bleaching, the

influence of inorganic substances and their role in the protection/degradation of

cellulose, have been described previously.

Hydrogen peroxide and the hydroperoxy anion respectively evolve in situ [13]

during oxygen bleaching. In contrast to dioxygen, which contains multiple bonds

between the O atoms, H2O2 has only one bond, and this can be easily broken.

7.6 Hydrogen Peroxide Bleaching 853

Under the conditions used in H2O2 bleaching, with the pH in the range of 10–12,

the standard redox potentials of the reactive species are substantially reduced

(Scheme 7.36) due to the lower potential of the ionized form. Hydrogen peroxide

(hydroperoxy anion) can either be oxidized by a one-electron step to the hydroperoxyl

radical (superoxide anion radical), or reduced to the hydroxyl radical (oxyl

anion radical) (Scheme 7.36).

O2

+e-, H+

HOOH H2O+ 2 H2O

p K a= 4.8 11.6 11.9

O2

-

H++ H++HOO- O- H+ +

+e-, H+

+e-, H+ +e-, H+

E 0 at pH 14 - 0.33 0.20 - 0.03 1.77

Dioxygen

Hydroperoxyl

radical

Hydrogen

peroxide

Hydroxyl

radical

Superoxide

anion radical

Hydroperoxy

anion

Oxyl anion

radical

Oxygen species

Anionic form

45.71

Hydroxide

ion

OH-

HOO HO Water

Scheme 7.36 Dioxygen reductions proceeding in four consecutive

one-electron steps (E0 standard reduction potential)

(1According to [14]).

The actual concentration of the hydroperoxy anion depends on the pH of the

solution (Scheme 7.36) and, of course, on the amount of H2O2 added. The pH value

is not the best measure to determine the effective hydroperoxy anion concentration,

however, because of the interaction of the OH– ion and H2O2, different

solutions where either component is in excess might have the same pH and yet

have a 10-fold difference in hydroperoxy anion concentration [15]. Conversely, two

solutions may give the same approximate concentration of hydroperoxy anions

and have different pH values [15]. Notably, in this very interesting study [15] it was

also found that, during H2O2 bleaching of cotton cellulose, the latter acted as a stabilizer

for the peroxide.

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