Organosolv extraction of lignin from hydrolyzed almond shells and application of the δ-value theory

The solubility of lignin from hydrolyzed almond (Prunus amygdalus) shells in different acetone, ethanol and dioxane–water mixtures and conditions (extraction time and temperature) was studied. The concept of the solubility parameter (δ-value) was applied to explain the effect of organic solvent concentration on lignin solubility. The organic solvent–water mixture that led to the highest lignin extraction was composed of a 75% vol. of organic solvent for all the solvent series investigated (acetone, ethanol and dioxane). Moreover, the best lignin extraction conditions were a temperature of 210 °C and an extraction time of 40 min for the acetone and ethanol series, and 25 min for the dioxane series. The δ-value of the hydrolyzed almond shell lignin [14.60 (cal/cm3)1/2] and that of the organic solvent–water mixtures was calculated. The experimental delignification capacity of the aqueous organic solvents clearly reflected the proximity of their δ-value to that of lignin. The hydrogen-bonding capacity of the solvent–water mixtures was also taken into account.