New Insights into Lignin Modification During Chlorine Dioxide Bleaching Sequences (II): Modifications in Extraction (E) and Chlorine Dioxide Bleaching (D1)

Previously, it was shown that chlorine dioxide delignification (D0) modifies kraft residual lignin by oxidizing phenolic groups to both quinonoid and muconic structures in ∼2‐to‐1 ratio (1.7 mmol/g lignin vs. 0.8 mmol/g lignin), respectively. Alkali extraction (E), in addition to removing solubilized lignin and saponifing muconic acid methyl esters, re‐activates the residual D0 lignin towards ClO2 oxidation by converting quinones to polyphenols. This action of alkali on D0 oxidized lignin is analogous to that of hydrosulfite reduction (Y). Chlorine dioxide oxidation of D0E and D0Y treated pulps generally afforded ∼1.7 mmol quinones/g lignin onto the oxidized lignin, like that of the D0 stage. This study clearly indicates that quinones formed during the D0 stage can be easily re‐activated toward ClO2 oxidation in a D1 by alkali treatment, such as an E stage.