Organic acids as a greener alternative for the precipitation of hardwood kraft lignins from the industrial black liquor

Abstract

The use of three organic acids such as acetic, lactic and citric acid has been investigated as green precipitation agents for kraft lignin isolation from the industrial hardwood black liquor. Chemical composition, molecular structure characteristics in addition to thermal and antioxidant properties were evaluated and compared to kraft lignin precipitated with sulfuric acid. A clear influence of the organic acids was observed in the chemical composition and molecular properties. Organic acids generated lignins with high purity and low ash and carbohydrate contents. FT-IR and Py-GC-MS analyses revealed the ability of organic acids to produce lignins with greater content of methoxylated structures and carbonyl groups than sulfuric acid. Moreover, all evaluated kraft lignins exhibited a suitable radical scavenging activity, and higher than found for commercial antioxidant used as control. TG analysis showed that the employment of organic acids with higher ionic strength lead less thermally stable kraft lignin. In addition, the proximate analysis confirmed the high potential of kraft lignins as an energy source (21–24 MJ/Kg).