New Insights into the Carboxymethylation of Eucalyptus Kraft Lignin: Structural Features and Transition Metal Sorption Capacity

Lignin isolated from industrial kraft pulping of Eucalyptus globulus wood using the LignoBoost process was carboxymethylated to expand its applications as a biosorbent or biodispersant. Pilot-scale lignin modification was performed by reaction with sodium chloroacetate (SCA) under preoptimized conditions (60 °C, 2 h, 1.5 M NaOH, and SCA:lignin molar ratio 1:3). The resulting lignin was evaluated for the introduced carboxymethyl groups and its main structural characteristics by wet chemistry methods and infrared (FTIR-ATR), and 1D and 2D nuclear magnetic resonance (NMR) spectroscopy. Approximately 40% of the total phenolic hydroxyls and 18% of the total aliphatic hydroxyls of lignin were etherified, reaching a degree of substitution of 0.46 per aromatic ring. Among the aliphatic hydroxyl groups that reacted, methylol groups predominated over benzylic ones. The modified carboxymethylated lignin showed significantly increased solubility in water and sorption capacity toward transition metals (Cu(II), Cd(II), and Hg(II)), as revealed from batch experiments carried out under equilibrium conditions.