Effect of hydroxyl radicals on cellulose and pulp and their occurrence during ozone bleaching

The effects of hydroxyl radicals on cellulose and pulp are studied using gamma irradiation of water. It is shown that hydroxyl radicals can depolymerize cellulose severely. For a given drop in DPV ozone leads to the formation of more carbonyl groups on cellulose than hydroxyl radicals. Furthermore, hydroxyl radicals do not delignify kraft pulps significantly at room temperature, contrary to ozone. It is also shown that the presence of metal ions, such as iron and copper, or a rise in temperature increases the formation of hydroxyl radicals during ozonation, which is reflected by lower viscosities. The addition of manganese (II) on the other hand does not lead to the formation of hydroxyl radicals with ozone. Increasing the pH of ozonation does not yield significantly more hydroxyl radicals and the cellulose is less degraded at high pH ozonation than al low pH, for a given ozone charge. It is concluded that molecular ozone is mainly responsible for cellulose degradation during ozonation. Ozonation of unbleached pulp leads to the formation of considerable amounts of hydrogen peroxide. An additional degradation is due to the hydroxyl radicals generated by the peroxide formed during ozonation. This explains the effect of the metal ions and the difference between high and low consistency ozonations.