Comparative study of lignin fractionation by ultrafiltration and selective precipitation

The present work presents two different methods to fractionate the lignin resulting from the black liquor of the pulping process of Miscanthus sinensis (7.5% (w/w) NaOH, 90 min, 90 °C). The first method consists of selective precipitation, which is achieved by the gradual acidification of the black liquor, getting different precipitates according to the pH. The second employed technique is the ultrafiltration, which uses ceramic membranes of different cut-off (5, 10 and 15 kDa) to obtain different liquors containing lignins with specific molecular weight. Using both methods, different lignin fractions have been obtained and characterized by the following analysis techniques: FTIR, thermal analysis, GPC and NMR. Obtained results have shown that the fractionation process applied affects the properties of the obtained lignin. Ultrafiltrated fractions are less contaminated by lignin–carbohydrate complex than the fractions obtained by selective precipitation. Ultrafiltration process also allowed controlling the molecular weight of the obtained fractions.