Evaluation of the effect of ultrasound on organosolv black liquor from olive tree pruning residues

Ultrasonic treatments (0, 15, 30, 60 and 120 min) were applied to black liquor resulting from organosolv fractionation of olive tree pruning residues (ethanol/water 60/40 v/v, 180 °C, 60 min) in order to determine their effect on black liquor components. HPLC analyses of ultrasound-treated liquid fractions demonstrated that ultrasonic irradiation promoted up to 20% degradation of monosaccharides for 15 min of sonication and an increase of monomeric sugars from 3% to 16% due lignin–carbohydrate complex rupture. The quality and purity of the lignin precipitated from sonicated liquors by adding acidified water were assessed. Attenuated-total reflectance infrared spectroscopy (ATR-IR), gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) confirmed that main lignin structure did not change due sonication, and thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and chemical composition and antioxidant behavior showed purification of lignin samples. These results established sonication as a suitable intensification technology in biorefinery processes.