Basic Considerations on the Selectivity of Ozone Bleaching

It is generally agreed that radical formation is a crucial factor that affects the selectivity

and performance of an ozone bleaching stage [75,102,103]. On the basis of

model compounds, the ratio of rate constants for delignification to that of carbohydrate

degradation (k L/ k C) is more than 105 in the case of molecular ozone,

whereas for hydroxyl radicals a value of only 5–6 has been determined [72,104].

However, there remains much debate about the pathways of radical formation

during the ozonation of pulp. It is well-documented that radicals are formed in

aqueous medium by self-decomposition of ozone [54,104]. However, the decomposition

of ozone is rather slow in acidic media (see Fig. 7.82). Therefore, additional

reasons have been suggested as being responsible for the unselectivity of ozone

treatment. Radicals are formed in the presence of transition metal ions

[56,103,105] and in a direct reaction between ozone and aromatic lignin structures

[57,58,106]. Recent trials using the TNM (tetranitromethane) method, however,

concluded that the addition of the transition metal ions Fe(II), Cu(II), Co(II) and

Mn(II) to a lignin model compound (e.g., vanillin) do not promote additional radical

formation. It has been argued that the loss in pulp viscosity in the presence of

transition metal ions reported in the literature may be attributed to radical formation

from the reaction with hydrogen peroxide formed during ozonation

7.5 Ozone Delignification 829

[57,103,107]. Instead, radical formation is caused by a direct reaction between lignin

model compounds and ozone [57,58]. In acidic solution, syringyl structures

yield more radicals at a given ozone charge as compared to the corresponding

guaiacyl compounds, mainly due to the lower oxidation potential of the former.

However, no radicals are formed in direct reaction between ozone and carbohydrate

model compounds. Following the finding that syringyl structures yield significantly

more radicals than do corresponding guaiacyl compounds, the viscosity

loss during ozone bleaching should be much more pronounced for hardwood

than for softwood pulps. However, in practice the opposite is true. Ragnar has

shown that the better selectivity of hardwood kraft pulps can be attributed to their

higher amount of hexenuronic acids (HexA), since ozone reactions with this component

do not yield radicals [108]. Magara et al. also reported that the presence of

lignin model compounds with free phenolic hydroxyl groups enhanced cellobiose

degradation during ozonation in water [78]. However, in the presence of nonphenolic

lignin model compounds, cellobiose degradation was retarded. Based on

these model compound studies, it can be concluded that the selectivity of ozone

bleaching gradually improves with decreasing incoming kappa number of a pulp.

In fact, this may explain the superior selectivity of oxygen-bleached pulps over

unbleached kraft pulps, since oxygen reduces both the total lignin content and the

phenolic structures in residual lignin [75,107]. The latter yields more radicals as

compared to nonphenolic structures [57]. Cellulose is also degraded by molecular

ozone according to an insertion mechanism (see Section 7.5.4.3) [53,109]. It is

however doubtful if this type of reaction is responsible for the degradation reaction

of pulps in the presence of residual lignin, because the reaction rate of ozone

towards intact carbohydrate structures is very low (0.21m –1 s–1), while the reaction

rate of hydroxyl radicals towards similar structures is several orders of magnitudes

higher [104].

During the course of final bleaching, when the residual lignin content

diminishes, it seems likely that the direct reaction between ozone and cellulose

gradually becomes the predominant reaction responsible for cellulose degradation

in ozone bleaching. Model compound studies using methyl 4- O -ethyl-b-d-glucopyranoside

were conducted to elucidate the participation of radical species in the

degradation of the polysaccharide during ozone treatment [53]. From the results

obtained it was concluded that both ozone itself and radical species participate in

the glycosidic bond cleavage of carbohydrates during ozonation in aqueous solutions.

The free radical-mediated reaction may lead to both direct cleavage and to

the conversion of hydroxyl to carbonyl groups. The contribution of radical species

was estimated to be about 40–70% during ozonation in distilled water acidified to

pH 2 (the ratio of ionic to radical reactions was calculated by the relative reactivities

at C1towards ozone in anhydrous dichloromethane as a reference for pure

ionic and toward Fenton’s reagent as a reference for radical reactions). Furthermore,

the model compound studies revealed that oxidation of hydroxyl groups at

C2, C3, and C6 positions to carbonyl groups is caused predominantly by radical

species.

830 7Pulp Bleaching

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