Effect of pH

The pH of the aqueous pulp suspension significantly affects the performance of

ozone bleaching. It is generally agreed that the best efficiency of ozone delignification

is achieved at a pH below 3 [50,51]. Therefore, industrial ozone bleaching is

carried out in the pH range between 2 and 3 by acidifying the pulp suspension,

predominantly with sulfuric acid. Lindholm found that acidification to pH 3 does

not affect viscosity development, while lignin removal was enhanced [44]. The

degree of delignification is increased if acidification is carried out as a pretreatment,

followed by dewatering. In this particular case, this may be explained by the

removal of transition metal ions during an acid prewash. The type of acid used

(among sulfuric, acetic, oxalic acid and SO2 water) to adjust the pH to about 3

prior to LC ozone treatment had no influence on the performance of delignification

[52]. Besides delignification efficiency, the alpha-cellulose content of a beech

acid sulfite dissolving pulp is also impaired by a high pH during ozonation

(Fig. 7.90). The degradation of the alpha-cellulose occurs parallel to a loss in viscosity,

which strengthens the hypothesis that pH determines the yield of reactive

species during ozonation [28]. This view is supported by the results obtained from

reactions between ozone and carbohydrate model compounds in aqueous media,

where the extent of radical reaction increases with increasing pH [53].

The beneficial effect of low pH has been ascribed to several factors. Ozone undergoes

self-decomposition in water by a radical chain mechanism, initiated by hydroxide

ions and propagated by the superoxide anion radical and the hydroxyl radical (see

Scheme 7.28 and equation (90) [54,55]). Pan et al. have shown that, for an ozonation

0 2 4 6 8 10

0.0

0.2

0.4

0.6

Alpha-Cellulose [%]

ΔKappa / O

-charge

Δ Kappa / kg O

-charge

pH-value

89.0

89.5

90.0

90.5

91.0

Alpha-Cellulose content

Fig. 7.91 Effect of pH on delignification efficiency,

Dj/O3-charge, and on alpha-cellulose

content during ozonation (according to [28]).

Pulp: E/O-pretreated beech acid sulfite dissolving

wood pulp (B-AS), kappa number 1.9,

viscosity 627 mL g–1; medium-consistency

Ozone bleaching: 10% consistency, temperature

55 °C, 10 s mixing time; ozone charge:

2.2–2.3kg odt–1, carry-over: 5 kg COD odt–1.

7.5 Ozone Delignification 811

time of 10 min, 95% of the original ozone charge will remain in water at pH 3,

and only 63% at pH 7 after 10 min [56]. The course of dissolved ozone concentration

in pure water as a function of time at both pH levels is shown in Fig. 7.82.

In MC ozone bleaching, the reaction time is, however, less than 1s. During this

short time, the decomposition of ozone is probably not sufficiently high (<1% of O3-

charge at pH7) to account for the difference in ozone reactivity observed between pH

3 and pH7. The higher efficiency of ozonation at low pHhas also been related to the

higher ozone solubility in water, as expressed in Eq. (87). The increase in reactivity is

thus explained by the higher rate of diffusion across the immobile water layer [18].

Furthermore, recent studies have suggested that the radical yield obtained in the

direct reaction between ozone and phenolic lignin model compounds starts to

increase rapidly at about pH 3, whereas nonphenolic compounds induce increased

radical formation at pH values beyond 6 [57,58]. The pH-dependence of radical yield

has been explained by the preference of reaction pathways involving homolytic

cleavage of the hydrotrioxide intermediate at higher pH levels. Results from

model compound studies support that the proportion of radical reactions involved

in the degradation of carbohydrate structures increases with rising pH.

The results outlined in Fig. 7.92 indicate that the extent of carbohydrate degradation

in acidic water (pH 2–3) is almost the same as in distilled water (pH 5.7),

partly because the pH of the reaction medium decreases during ozonation due to

the formation of acids (initial pH of 5.7 decreases to 3.6). However, in alkaline

aqueous solution (pH 10 and 11), the contribution of radical reactions increases,

as indicated by the lower relative reactivity at C1, which is shifted from 55% in

distilled water to 49% at pH 11 [53].

2 4 6 8 10 12

after 60 min after 120 min

Yield of model compound [%]

pH-value

Fig. 7.92 Effect of pH during ozonation on the yield of methyl

b-d-glucopyranoside (according to [53]). Conditions:

0.45 mmol of the model compound are ozonated at a rate of

0.125 mmol min–1 at room temperature.

812 7Pulp Bleaching

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