Method matters: the influence of the pH adjustment and liquid-liquid extraction on the analysis of kraft black liquors

Abstract

The increasing emphasis on sustainable biorefinery processes has brought black liquor, a complex byproduct of pulp mills, to the fore as a pivotal source for extracting valuable organic compounds, particularly lignin. The chemical composition of black liquor exhibits significant variations depending on the lignocellulosic source, the pulping method, and the conditions during processing. By examining pH adjustments and analytical methods such as HPLC and GC-MS, this study provides insights into the chemical behavior of black liquor and proposes strategies for its efficient characterization and utilization. The presence of different acids (H2SO4, HCl, and H3PO4) and pH adjustments (2, 5.5, 7, 9) in black liquor significantly influence its physical properties, such as density and viscosity, as well as its chemical composition. Softwood black liquor showed greater viscosity changes due to pH levels, while hardwood black liquor was primarily affected by the type of acid used. Syringyl-type compounds dominate in hardwood black liquor, whereas softwood black liquor is richer in guaiacyl-type compounds such as vanillin and catechol. HPLC analyses revealed higher phenolic yields at higher pH levels (7-9), with vanillin and protocatechuic acid being most abundant in softwood samples and syringaldehyde and syringic acid in hardwood samples. The optimal pH for extracting lignin-derived phenolics is 5.5-7, while pH 2 is preferred for extracting organic acids, highlighting the critical role of pH in maximizing extraction efficiency.