ARE HYDROXYL RADICALS RESPONSIBLE FOR DEGRADATION OF CARBOHYDRATES DURING OZONE BLEACHING OF CHEMICAL PULP

Based on recent publications, there seems to be a consensus that in-si tu-generated hydroxyl radicals are responsible for carbohydrate degrad ation during ozone bleaching. This theory is further investigated in t his paper. The results showed that conditions which decrease the forma tion of hydroxyl radicals, such as metal removal from pulp by an acid washing or DTPA pretreatment, lead to only a relatively small reductio n in carbohydrate degradation during pulp ozonation. Conversely, addit ion of transition metals to fully bleached pulp results in only a slig htly higher carbohydrate degradation during ozone treatment. It was al so found that, with hydroxyl radicals effectively captured during ozon e bleaching of unbleached kraft pulp when impregnated with a 5% methan ol solution, the viscosity gain over that of pulp impregnated with aci dified water is only 2-4 mPa . s. Our experiments abo showed that the carbohydrate-lignin selectivity of ozone is similar when a mixture of lignin and carbohydrate model compounds is ozonated in water, 90% acet ic acid or 70% methanol. In light of the above evidence, the conclusio n is drawn that it is unlikely that the carbohydrate degradation occur ring during pulp ozonation is mainly caused by in-situ-generated hydro xyl radicals.