Characterization of lignins from wheat straw by alkaline peroxide treatment

Treatment of wheat straw with 2% H2O2, at 50 degrees C and pH 11.5 for 4-30 h or with 2% H2O2-0.05% anthraquinone (AQ) at 50 degrees C and pH 11.5 for 4.5 h resulted in solubilization of 79-86% of the original lignin and 77-91% of the original hemicelluloses, respectively. The seven isolated pure lignin samples were characterized by both destructive methods such as alkaline nitrobenzene oxidation and non-destructive techniques such as ultraviolet (UV), Fourier transform infrared (FT-IR), and carbon-13 magnetic resonance spectroscopy (C-13-NMR) as well as gel permeation chromatography (GPC). The results showed that addition of 0.05% AQ to the alkaline peroxide solution did not give any significant effect on the enhancement of delignification. Comparison of these pure lignin fractions with alkaline lignin preparation obtained from wheat straw in the absence of hydrogen peroxide provided evidence of similar chemical composition and structure. It was found that the alkaline peroxide treatment under the conditions used did not result in any significant change in the macromolecular structure of lignin. The only one minimal change of the lignin moiety was the formation of quinone structure. (C) 1999 Elsevier Science Ltd. All rights reserved.