Effect of Transition Metal Ions

Ozone is decomposed by the catalytic action of transition metal ions present in

pulps and bleaching liquors, giving rise to hydroxyl, hydroperoxyl, and superoxide

anion radical intermediates [67]. Pan et al. [56] reported that transition metal ions

such as iron(II) and cobalt(II) ions enhance the decomposition of ozone, while

Chirat and Lachenal [68] demonstrated that iron and copper cations exert an

adverse effect on the selectivity of ozone bleaching. The detrimental effect on the

viscosity is observed already at low concentration in the range between 10 and

20 ppm on pulp. Chemiluminescence and electron spin resonance (ESR) measurements

confirmed the formation of hydroxyl radicals when iron cations – and,

to a lesser extent, copper ions – were added to the pulp suspension subjected to

ozone treatment. The presence of manganese cations, however, did not affect the

selectivity of ozone bleaching, which agrees well with the result that no additional

hydroxyl radicals were detected.

The scheme of ozone decomposition can be rationalized according to the

expressions in Eqs. (103) and (104):

O 3 _ Mn _ _ H _ _ M _ n _1___ HO _ _ O 2

HO _ _ O 3 _ HO _

2 _ O 2

HO _

2 _ O 2

__ _

H

_

O 2

__

_ M _ n _1___ Mn _ _ O 2

_103_

Net decomposition reaction:

2 O 3 _

Mn __ H 2 O 3 O 2 _104_

The influence of the presence of Fe2+ ions on the performance ofMCozone bleaching

of a beech acid sulfite dissolving pulp (B-AS) is illustrated in Fig. 7.94. The efficiency

of delignification is slightly deteriorated when Fe2+ ion concentration

814 7Pulp Bleaching

0 10 20 30 40 50

0.0

0.5

1.0

1.5

2.0

Viscosity [ml/g]

Kappa number after Z ΔKappa per chain scissions

Kappa number after Z or

ΔKappa per chain scissions

Fe2+ concentration [ppm on pulp]

Viscosity

Fig. 7.94 Effect of Fe2+ ion concentration on

kappa number after Z-stage, delignification

selectivity, Dkappa per number of chain scissions,

and on the course of viscosity during

ozonation (according to [28]). Pulp: E/O-pretreated

beech acid sulfite dissolving wood pulp

(B-AS), kappa number 2.1, viscosity 614 mL g–1

medium-consistency ozone bleaching: 10%

consistency, pH 2.5, 10 s mixing time; ozone

charge: 1.9 kg odt–1, no carry-over.

exceeds 10 ppm on pulp. The viscosity of the ozone-treated pulp remains unaffected

until an Fe2+ ion concentration of 20 ppm is reached. A further increase in

Fe2+ ion concentration finally leads to a slight decrease in viscosity, while the

extent of delignification further decreases; this is an indication of a shift to a higher

proportion of radical reactions initiated by the decomposition of ozone.

The degradation of a carbohydrate model compound, methyl 4- O -ethyl-b-d-glucopyranoside,

was enhanced when ozonation took place in an 0.5 mM FeSO4

aqueous solution compared to that in pure water (distilled water, pH 4) [53]. The

additional degradation of the model compound was attributed to radical reactions

initiated by the decomposition of ozone, catalyzed by the Fe2+ ions. Interestingly,

the additional degradation of the carbohydrate model compound initiated by the

presence of Fe2+ ions can be fully compensated for by acidifying the aqueous solution

to pH 2. The observation that the removal of Fe2+ ions is not necessary to

improve delignification selectivity if ozonation is carried out at low pH suggests

that the transition metal ion-catalyzed decomposition may be completely inhibited

by adjusting to low pH. Ragnar reported that the addition of various transition

metal ions such as Fe(II), Co(II), Cu(II) and Mn(II) to an aqueous solution of

vanillin, which served as a lignin model compound, did not affect the radical yield

during ozonation [57]. It was stated that hydroxyl radicals are rather formed in a

(Fenton-type) reaction with hydrogen peroxide which originates during ozonation

than in a direct reaction between ozone and transition metal ions. It can be sum-

7.5 Ozone Delignification 815

marized that the presence of transition metal ions (mainly Fe2+ ions) slightly

impairs the performance of ozone bleaching as soon as a certain concentration is

exceeded. Further investigations are required to elucidate the mechanism behind

the transition metal ion-initiated decomposition of ozone.

Дата добавления: 2015-10-21