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Summary Using different independent methods, the kinetics of ozone consumption and the initial radical yield in reactions of ozone with lignin and carbohydrate model compounds were investigated. It was demonstrated that ozone reacts with phenolates several orders of magnitude more rapidly than with corresponding undissociated phenols. The pH dependence of the radical yield does not completely follow the pK a-value of the phenols. In fact, the radical yield starts to increase at pH 3 for all the phenolic model compounds investigated. Several indications suggest that superoxide rather than the hydroxyl radical is initially formed when ozone reacts with lignin model compounds. In contrast to lignin model compounds no radicals were detected in ozone reactions with carbohydrate model compounds or olefins. On the basis of this study, it may be concluded that ozone bleaching should preferably be performed at pH 3 and at a higher consistency. No significant effect of metal ions was observed.

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Summary A new mechanism including a homolytic cleavage of a trioxide intermediate forming superoxide is suggested to be the main course of radical formation in reactions of ozone and lignin like structures. The suggested mechanism is supported by quantum chemical and thermochemical methods.

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