A sustainable valorization approach of Kraft black liquor from Robinia pseudoacacia L.: lignin precipitation followed by green solvents extraction of phenols from delignified liquor

Kraft black liquor is considered a renewable feedstock for value-added compounds such as lignin and phenols, which should be valorized rather than incinerated for energy production. Herein, Robinia pseudoacacia L. was subjected to Kraft pulping and the resulting black liquor was recovered for lignin and phenols extraction. The liquor was acidified to isolate lignin and the residual aqueous phase from lignin precipitation was subjected to liquid-liquid extraction. For latter, two bio-based solvents (2-methyltetrahydrofuran (2-MeTHF) and cyclopentyl methyl ether (CPME)) and ethyl acetate (EA) as a conventional organic solvent were used. The chemical composition of lignin was evaluated by standard method, whereas its structural characteristics were analyzed by Nuclear Magnetic Resonance (NMR) and Fourier-Transform Infrared (FTIR) spectroscopy, and Size Exclusion Chromatography (SEC). The phenols extraction efficiency of solvents was determined by UV-Vis spectrometry, using Folin-Ciocalteu assay, and subsequently, phenols were tentatively identified and quantified by HPLC-DAD chromatography. The antioxidant activity of lignin and phenolic extracts were characterized by DPPH scavenging tests. Recovered lignin (85 %) displayed a significant degradation of lateral chains and, therefore, a high phenolic content and low molecular weight. Based on these characteristics, a lignin with antioxidant activity was recovered. Concerning phenols extraction from delignified liquor, 2-MeTHF displayed the highest extraction capacity (78 %), followed by EA (52 %) and CPME (33 %). The organic extracts showed antioxidant activity, following a comparable tendency to that of the total phenolic content values (2-MeTHF > EA > CPME). Among the different phenols identified, highlights the flavonoids dihydrorobinetin and robinetin, with potential to be employed in different sectors.