Side Reactions and the Role of Thiosulfate

The side reactions can be divided into two categories: (a) reactions involving lignin,

carbohydrate and their degradation products; and (b) reactions involving inorganic

sulfur compounds only. All side reactions (Scheme 4.54) have in common

the fact that they diminish the available sulfite concentration and hence destabilize

the cooking liquor. Hydrogen sulfite in aqueous solutions normally acts as a

reducing agent and antioxidant. However, under the conditions of the sulfite cook

a major part of hydrogen sulfite is consumed by the reducing end groups of sugar

monomers and other keto groups present in the liquor under formation of ahydroxysulfonates

and subsequent oxidation of the reducing end to the corre-

422 4 Chemical Pulping Processes

sponding the aldonic acids, according to Scheme 4.55. The hydrogen sulfite

bound as a-hydroxysulfonate is classified as “loosely bound sulfur dioxide”.

The tendency to form a-hydroxysulfonates and their stability depend on the

type of the parent carbonyl compound. Hexoses, pentoses, and lignin carbonyls

form less-stable adducts as compared to formaldehyde, furfural, or methyl glyoxal.

Formic acid is converted to carbon dioxide [48]by sulfite.

HSO3

S2O3

S4O6

OH

HO

HO

OH

OH

COOH

O

HO

HO

OH

OH

OH

H

+

S3O6 S5O6

HSO3 H

+

H O 2 SO S 4

2 -

SO4

Net reaction:

-

+

Disproportionation

Oxidation reduction

2- 2- 2-

Reaction with

Lignin

3 2 + +

-

+

2-

2-

Scheme 4.54 Side reactions in acidic sulfite cooking (modified from [51]).

O

HO

HO

OH

OH OH

OH

HO

HO

OH

OH

OH

SO3H

H

OH

HO

HO

OH

OH

COOH

HSO3

-

+

H O 2

-

S2O3

38 76 75

- 1/2

2-

Scheme 4.55 Formation of a-hydroxysulfonates (bisulfite

adducts), thiosulfate and aldonic acid.

The hydrogen sulfite oxidizes aldehyde groups to the corresponding acids,

which is the major process generating aldonic acids from cellulose and hemicellulose

degradation products (mainly xylonic acid, gluconic acid, some mannonic

and galactonic acid). Schoon [63]reviewed the kinetic studies in this field, and

analyzed the formation of thiosulfate under various conditions. For pH <4,

Schoon showed a faster conversion of xylose as compared to mannose and glucose,

the latter one reacting slightly faster than mannose. Other substances present

in the cooking liquor can be oxidized in the same manner (i.e., extractives),

and the oxidation of sugar alcohol has also been reported [49]. The hydrogen sulfite

is in turn reduced to thiosulfate, which retards delignification [50]. The thiosulfate

plays a key role in the side reactions, as it causes detrimental decompositions

of the cooking liquor that are thought to proceed autocatalytically, with thio-

4.3 Sulfite Chemical Pulping 423

sulfate being one of the catalysts [51]. High concentrations of thiosulfate may

finally result in a so-called “black cook” for calcium bisulfite operations. If sodium

is the base, the tolerable level of thiosulfate is considerably higher [52]. Disproportionation

of hydrogen sulfite leads to thiosulfate and sulfate ion formation, which

causes precipitates to occur when calcium ions are used as the base.

The reaction of thiosulfate with lignin was investigated by means of simple

model compounds (Scheme 4.56). Goliath and Lindgren demonstrated that thiosulfate

reacts in the same manner with the intermediate quinone methide as

hydrogen sulfite does. The thiosulfate hence competes with the sulfite for reactive

lignin positions. Upon prolonged reaction times or an increase in temperature,

condensation to sulfides occurs. The lignin-thiosulfate condensation products are

less hydrophilic, and thus have a lower solubility. Such organic excess-sulfur components

are increasingly formed towards the end of the cook [54,55].

OH

OMe

CH2OH

OH

SSO3

OMe

O

CH2

OMe

OH

S

OH

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