Why Acid pH Increases the Selectivity of the Ozone Bleaching Processes

The influence of pH on pulp properties was studied by the kinetic analysis of ozone bleaching of Eucalyptus Kraft pulp. The crystallinity of the pulps was also evaluated by XRD. The rate constants of delignification and chromophore removal decreased with the increase in pH, while the rate constant of cellulose degradation increased. The selectivity of the process was 6 times higher at pH 2.5 than at pH 10. There was a portion of lignin inaccessible that cannot be removed by ozone, even at high doses, promoting a strong cellulose degradation. At high ozone doses and acid pH, the ozone reacted primarily with the crystalline portions of the cellulose, while at alkaline pH it reacted indiscriminately with amorphous and crystalline cellulose.