Effects of precipitation pH on the physico-chemical properties of the lignins isolated from the black liquor of oil palm empty fruit bunch fibre pulping

Abstract

Five lignin fractions were obtained by precipitation at pH 4.8, 4.0, 3.0, 2.0, and 1.5, respectively, from the black liquor of oil palm empty fruit bunch (EFB) fibre pulping after isolation of the polysaccharide degradation products, and one lignin fraction was obtained by direct precipitation at pH 2.0 from the black liquor prior to isolation of the polysaccharide degradation products. A comparative study of their physico-chemical properties and structural features was made by using UV, FT-IR, 13C NMR spectroscopy and alkaline nitrobenzene oxidation. The results showed that the predominant component in the alkaline nitrobenzene oxidation mixtures was syringaldehyde, together with vanillin as the second major phenolic monomer and less p-hydroxybenzaldehyde, which corresponds to the results obtained by the non-destructive techniques such as 13C NMR spectroscopy. The molar ratio of S:V:H in the six lignin fractions was 18–21:10–12:1. The conditions for relatively higher yield and more purity of lignin depend on the precipitation pH. For example, the yield and purity of the lignins, isolated from the black liquor supernatants after isolation of the polysaccharide degradation products, increased with decreasing precipitation pH. Finally, the results indicated that the lignins obtained can be classified as SGH-lignin such as Gramineae lignins from straw and grass.