Environmentally friendly delignification of olive stones by microwave-assisted acidic deep eutectic solvents

Abstract

Olive stones (OS), a common form of agroindustrial waste, are rich in lignin and are typically burned for energy rather than being converted into value-added products. This study investigates the efficient extraction of lignin from OS using deep eutectic solvents (DESs) under microwave-assisted conditions. The DES mixtures employed included choline chloride (ChCl) combined with acetic acid (AA), formic acid (FA), or lactic acid (LA). The purity, Fourier transform infrared (FTIR) spectra, elemental composition, particle size, and color of DES-extracted lignin (DES-L) obtained from OS treated at 150 degrees C for 1 h were determined. In addition, FTIR, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) analyses were performed on the solid residue after DES treatment. Choline chloride/LA treatment achieved the highest delignification ratio (80.01%). Choline chloride/LA-lignin exhibited a smaller particle size (238.4 nm) than ChCl/AA-lignin and ChCl/FA-lignin. Moreover, ChCl/LA-treated OS showed a higher surface area (3.113 m2 g-1) and total pore volume (7.215 x 10-3 cm3 g-1). In contrast, ChCl/AA treatment produced lighter colored lignin (L* value: 44.29) than ChCl/FA or ChCl/LA treatments. All lignin samples exhibited purities greater than 87%. These results indicate that microwave-assisted DES pretreatment, particularly with ChCl/LA, is an effective and sustainable strategy for valorizing OS.