Changing the Molecular Structure of Kraft Lignins-Ozone Treatment at Alkaline Conditions

Homogeneous solutions of Kraft lignin can be prepared in alkali. In this environment, ozone-an effective and established bleaching agent-rapidly converts into a variety of reactive oxygen species, leading to complex oxidation processes. In this study, Kraft lignin was treated with ozone in alkaline solution, and the modified lignin was fractionated into a precipitable, an adsorbable, and a soluble fraction to trace and understand the effects of this treatment. Changing the starting pH showed a distinct effect on the proportion of the obtained fractions. During the oxidation process, several small organic compounds were formed, resulting in a severe drop in pH and a shift in the predominant reactive species. In addition, these fragments were linked quantitatively to specific reaction pathways, which allowed the study of occurring degradation mechanisms. Here, a clear predominance of ring-opening reactions over side chain cleavage was observed. Chemical characterization revealed an increase in carboxyl and a decrease in methoxyl groups in the lignin polymer, thus increasing its solubility at neutral pH.