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

Abstract

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.