Production of chelating agents through the enzymatic oxidation of acetosolv sugarcane bagasse lignin

Abstract

Oxidation of lignin obtained from Acetosolv pulping of sugarcane bagasse was performed by polyphenoloxidase (PPO) using glycerol or polyethyleneglycol to increase the number of carbonyl and hydroxyl groups in lignin, and to improve its chelating capacity. Increase in the absorption in UV-spectrum related to α-carbonylphenolic and α,β-unsaturated structures was observed in all the experiments. The chelating properties of the original and oxidized lignins were compared by monitoring the amount of Cu²⁺ bound to lignin by gel permeation chromatography. The chelating capacity of original Acetosolv lignin was 354 mg Cu²⁺ /g lignin. On the other hand, lignin oxidized with PPO/O₂ showed an increase of 73% in chelating capacity in relation to the original lignin. The chelating capacity of lignin oxidized with PPO/O₂/glycerol was 110% higher than that of the original lignin. Glycerol stabilizes PPO, increasing its half-life. Average molecular weight $\left( {\overline {MW} } \right)$, measured by size-exclusion chromatography, was smaller for the oxidized lignins than for the original Acetosolv lignin. This result suggests that quinones can eventually be formed through the action of PPO, but are not polymerized. The chelating capacity of oxidized lignins increases with the incorporation of vicinal hydroxyl groups.