Alternative Bleaching Methods

During the past few decades, there has been a constant search for environmentally

benign alternatives to pulp bleaching. The search continues today, and will

do so in the future. Besides the activation of peroxide stages, one such alternative

is offered through biotechnological means. Although hemicellulose-degrading

7.9 Alternative Bleaching Methods 885

enzymes (xylanases) were the first enzymes to be introduced on a large scale for

pulp bleaching [1], they function more as a bleaching aid than as a direct bleaching

agent. This is because they increase the efficiency of subsequent bleaching

steps by loosening the structure of reprecipitated xylans on the unbleached pulp

fibers, thereby saving on the amounts of bleaching chemicals required.

A direct approach might be to use lignin-degrading fungi (Basidiomycetes or

white rot fungi) or their enzyme systems (e.g., peroxidases, laccases), all of which

have long been recognized. These systems are able to selectively degrade lignin

not only in wood, but also in pulp. However, the time required for this process to

proceed to the desired extent is far too long for a modern pulp mill bleaching system.

This problem of extended reaction times was partly tackled by the application

of a so-called mediator. Discovered accidentally during the early 1990s by R. Bourbonnais

of Paprican during experiments with lignin model systems, the laccasemediator-

system (LMS) was found to consist of an enzyme (laccase) and a mediator

(ABTS) [2]. The mediator applied in this first LMS – a laccase substrate used

for an activity assay – was impracticable for large-scale applications, however. An

LMS suitable for pulp mill use was later patented by Call [3,4] which employed

different mediators (e.g., 1-Hydroxy-benzotriazole, HBT) [5], and initial large-scale

trials conducted with this material has shown promise.

The underlying working principle of the LMS can be summarized as follows.

The enzyme laccase, as a macromolecule, is unable to penetrate the pulp fiber,

despite such penetration being a prerequisite for lignin-degrading action. Moreover,

due to its oxidation potential, laccase on its own is only capable of oxidizing

phenolic lignin moieties, which react predominantly by dehydrogenative polymerization

rather than by lignin degradation. However, both of these difficulties were

overcome with the use of a low molecular-weight redox mediator. In this way, the

substrate range is extended to nonphenolic lignin units, as could be shown by

model compound studies, and the mediator can penetrate much more deeply into

the fibers. In the LMS redox cycle, the enzyme oxidizes the mediator to a more

reactive species, mainly of radical type, and these react in turn with the lignin

macromolecule, either via an electron transfer process or by hydrogen atom

abstraction, depending on the mediator used [6]. The reduced mediator is re-oxidized

by the enzyme, which utilizes dioxygen as a co-substrate and which, in turn,

is reduced to water.

Large-scale applications of the LMS remain inoperative, however, and some

major restraints for eventual mill usage have been identified:

_ The mediator should be a low-cost chemical which should exhibit

a minimum of side reactions. These undesired processes can

cause a reduction in enzyme activity or the production of harmful

degradation products, which in turn raises the issue of environmental

compatibility.

_ A sufficient gain in kappa number reduction usually requires several

LMS stages with additional extraction stages in between.

_ The increase in brightness is often limited, so that additional

bleaching is required.

886 7Pulp Bleaching

For further information on the LMS system, the reader is referred to some

excellent reviews [7,8].

Further developments include mediated electrochemical delignification systems

[9], and enzyme mimicking (porphyrin derivatives, manganese-based complexes,

metal–Schiff base-complexes; for a more detailed description, the reader is

referred to Ref. [10]). Mimicking the action of lignolytic enzymes is also the underlying

concept of bleaching system which was developed in the mid-1990s [11,12]

and today is receiving increased attention. A special class of metal clusters of the

Keggin-type – the polyoxometalates [13], often referred to as POMs – are utilized

as the catalyst. Polyoxometalates are metal-oxo anionic clusters with chemical

properties that can be largely controlled by transition metal substitution and the

countercation used. This, combined with their ability to donate and accept electrons

and their stability over a wide range of conditions, makes them attractive

targets for use as bleaching catalysis. To activate the cluster for delignification,

one or more structural metal atoms are donated by a first-row transition metal

atom (e.g., vanadium or manganese) [14]. Specific conditions (e.g., pH, type of

metal cluster) allow POMs to be selective towards lignin degradation and to be

thermodynamically stable in water [15]. The high-valent metal cluster anions oxidize

and thus degrade and solubilize the lignin, while themselves being converted

into the lower-valent reduced state. The re-oxidation with dioxygen is a process

that generates radicals, but this would result in undesired cellulose damage due to

highly unselective side reactions. Consequently, the POMs are reactivated in a separate

stage under conditions that effect the oxidation of dissolved lignin and other

dissolved organic matter to carbon dioxide and water. Actual delignification of the

pulp is carried out under anaerobic conditions. POMs must be applied in stoichiometric

quantities. Current types of POM are advanced products of development:

they are more easily synthesized than the original representatives of this compound

class, are not too costly, recyclable, and have the capability of self-buffering.

Recent progress has also shown that molybdovanadophosphate heteropolyanions

can be used under aerobic conditions in a single-stage process with either oxygen

or ozone as the reactivating agent [16].

Further research and development is required eventually to transfer novel

delignification principles to large-scale applications.

7.10

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