The effect of solvent and catalyst concentration, pH and ionic strength on ozone bleaching of commercial eucalyptus kraft pulp in acetone-water solution in the presence of Keggin-type heteropolyanion [PMo(7)V(5)O(40)](8-) (HPA-5) has been examined and compared with conventional (solvent/catalyst free) ozonation systems. The solvent content and medium acidity were found to be the principal factors affecting ozonation efficiency. Increase in acetone concentration from 6% to 60% (w/w) led to gain in pulp brightness by 20.6% and delignification degree by 28.4%. Within the operative pH 1-2 range required for ozonation, the increase in pulp brightness by 7.6% and delignification degree by 18.1% was observed with acidity change from pH 1 to 2. The HPA-5 concentration of 1mM was found to be optimal for effective pulp ozonation in acetone solution. The role of the Donnan effect was examined. Increase in ionic strength (cation concentration) up to 0.25 M caused significant bleaching improvement.
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