Isolation of residual kraft lignin in high yield and purity

Abstract

In this paper, we report on a novel residual lignin isolation method. Initially we demonstrate that, in order to cellulolytically isolate residual lignin in high yield, the cellulosic structure of the pulp should be exposed as much as possible. Consequently, we developed a two-step method that first uses a mild enzymatic treatment to render most of the carbohydrates water-soluble and remove hemicelluloses. The solid residue obtained in this step contains about 94% of the lignin originally present in the pulp. A second mild acidolytic step is then applied (acid concentration 0.05 mol/L), which causes the quantitative cleavage of all lignin-carbohydrate linkages. Such linkages would otherwise survive an enzymatic treatment. Due to the relatively low enzyme/pulp ratio used in this step, only small amounts of proteins coexist with the lignin in the final preparation (2.3%), thus requiring no protease purification. At the same time the possibility of lignin structural changes are minimized by the low concentration of acid used throughout the developed method. The method offers a residual lignin preparation of high purity (95%) with an average yield of 70%.