Effect of ultrasound treatment on the physicochemical properties of alkaline lignin

Different sonication times (15, 30, 60 and 120 min) were applied in aqueous media to olive tree pruning residues lignin obtained by alkaline fractionation (7.5% w/w NaOH, solid to liquid ratio 1:10 w/w, 90 min, 85 °C). The observed sample solubilisation ratio due to ultrasonic irradiation was 24–28%. The resulted liquid fractions were analyzed by HPLC in order to determine their mono- and polysaccharides content (up to 0.404 and 3.393 g/L, respectively). The insoluble solid fractions were chemically characterized, and their chemical structure (ATR-IR, GPC), thermal behavior (TGA) and antioxidant activity (Folin–Ciocalteu and ABTS assays) were studied. The results showed that ultrasounds cleaved lignin–carbohydrate linkages and promoted hemicellulosic impurities removal from the alkaline sample (up to 32% of initial carbohydrates content), but also affected to insoluble lignin content (19% of Klason lignin solubilisation).

► Alkaline lignin samples were sonicated in aqueous media for different times.
► US affected hemicelluloses, inorganic and lignin content of the samples.
► Short sonication achieved mainly inorganics and small hemicelluloses removal.
► Longer sonication favored the removal of longer chain hemicelluloses.
► Condensation in lignin structure was observed by SEC for 60 min of sonication.