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Several lignin-like model compounds (vanillin, syringaldehyde, guaiacol, syringol,  p -benzoquinone, naphthoquinone) and commercial softwood lignin were submitted to small charges of ozone, chlorine dioxide and hydrogen peroxide in aqueous solution under conditions simulating a final bleaching stage. In the case of ozone, the coloured quinone models were directly destroyed, whereas the phenolic models and lignin underwent a two-step decomposition mechanism: chromophores were formed at very low ozone charges, and then were destroyed with increasing ozone charge. Chlorine dioxide had hardly any effects on the quinone models, but formed coloured groups from the phenolic models and lignin. However, these were more intensely coloured and were only partially removed with higher ClO 2  charges. As for hydrogen peroxide, the colour of lignin and naphthoquinone were directly removed, at least partially, but high H 2 O 2  charges were necessary. Consequently, ozone seems to be the best reagent for final bleaching in which small chemical charges are applied. We suggest reaction mechanisms between ozone and the phenolic model compounds and  p -benzoquinone that can explain the observations.

Comparative effect of ozone, chlorine dioxide, and hydrogen peroxide on lignin: Reactions affecting pulp colour in the final bleaching stage