Comparison of Structural Characteristics of Straw Lignins by Alkaline and Enzymatic Hydrolysis

Lignin was separated from wheat straw by NaOH cooking and enzymatic hydrolysis, obtaining yields of 86.2% and 78.6%, respectively. Fourier transform infrared spectrometry (FT-IR) and two-dimensional nuclear magnetic resonance (2D-HSQC) analyses indicated that the H units (p-hydroxyphenyl units) of the Enzymatic Hydrolysis Lignin (EHL) did not suffer much damage. The number-average molecular weight (Mn) of the EHL measured by gel permeation chromatography (GPC) was 3348 g/mol, which was higher than that (3047 g/mol) of Alkali Lignin (AL). The Cr(VI) adsorption experiments demonstrated that the maximum removal percentages and maximum adsorption amount of the EHL and AL were 31.5%, 88.9%, 135.1 mg/g, and 271.7 mg/g, respectively. In addition, the adsorption kinetics, adsorption isotherm and thermodynamics of Cr(VI) on these two lignins were reported in detail.