Hardwood and softwood kraft lignins fractionation by simple sequential acid precipitation

Abstract

Abstract This study aimed at the fractionation of lignin by a simple acid precipitation from hardwood and softwood Kraft black liquors to propose new value-added applications. The precipitation was made sequentially by acidification at five different pHs (9, 7, 5, 3 and 1). The different lignin fractions were analyzed in terms of yield, ash content, elemental analysis, thermal analysis, FTIR, 1H NMR and GPC. Hardwood lignins showed distinct relative molecular mass distributions, while softwood lignins showed similar relative molecular mass distributions in the three fractions obtained at higher pHs (pH 9, 7 and 5). This simple acid precipitation was shown to be an effective method to obtain relatively high yields of lignin. Molecular characterization by FTIR (carbonyl band) indicated that hardwood lignin was most oxidized mainly in the low pH fractions (5, 3 and 1), while the softwood lignin only presented discrete oxidized products in the lowest pHs (3 and 1). Interestingly, precipitation at pH 3 produced ash-free hardwood and softwood lignin fractions, which is a valuable product for the development of better end uses.