Effect of Metal Ion Concentration

The rather poor selectivity of oxygen delignification as compared to chlorine dioxide

is further impaired by the presence of transition metal ions. The wood used as

708 7Pulp Bleaching

©2006 WILEY-VCHVerlag GmbH&Co.

Handbook of Pulp

Edited by Herbert Sixta

raw material in kraft pulping is the primary source of the majority of non-process

elements (NPEs). The content of inorganic ions depends on the wood species and

the location of the growth place. Metal ions in wood are assumed to be bound to

carboxylate groups in hemicellulose, pectin, lignin, and extractives. Transition

metals may also be attached to lignin and extractives by complex formation, or as

metal salts of low solubility [62]. In oxygen delignification and peroxide bleaching,

cellulose degradation reactions are promoted by the presence of even trace

amounts of transition metal ions, such as copper, cobalt and iron [63].

The presence of cobalt (II) and iron (II) salts during oxygen bleaching of cotton

linters cause the highest rate and extent of cellulose degradation, while copper

has a less damaging behavior, and nickel has no visible effect. Manganese, on the

other hand, demonstrates both characteristics, being a degradation catalyst below

10 ppm and a protective agent above 60 ppm [64]. The transition to a cellulosepreserving

agent has also been observed for iron when present in sufficient

excess. At a concentration level above 0.1% on pulp, the precipitated ferric hydroxide

acts as an oxidation inhibitor, similarly to magnesium compounds [64]. Surprisingly,

the effect of transition metal ions on cellulose degradation during alkalioxygen

treatment show striking similarities to the catalytic processes occurring in

the aging of alkali cellulose [65,66]. The same metal ions that are found to accelerate

depolymerization in alkali cellulose cause increased viscosity reduction in the

course of alkali-oxygen treatment. Thus, it can be assumed that transition metal

ions such as cobalt, iron and copper promote free radical generation by catalyzing

the decomposition of the peroxides formed during oxygen delignification. Cobalt

is shown to be an even more effective catalyst than iron [67]. Moreover, it is observed

that the formation of both carbonyl and carboxyl groups is strongly favored

in the presence of cobalt ions (see Tab. 7.23).

Tab. 7.23 Influence of iron and cobalt ions on the degradation of

purified cotton linters during oxygen bleaching in the presence

and absence of magnesium carbonate (according to [67]).

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