Lignin Purification from Mild Alkaline Sugarcane Extract via Membrane Filtration

In this study, the separation of lignin from a mild alkaline sugarcane bagasse extract was studied, and the impacts of different parameters on the filtration performance were evaluated. The tested parameters included transmembrane pressure (0.5–3.0 bar), shear rates (2831–22,696 s−1), temperature (20 and 40 °C), membrane molecular weight cut-off (5 and 10 kDa), and membrane material (polyethersulfone and polysulfone). During the filtration process, the permeate flux and all the main components of the extract were analyzed, including lignins (acid insoluble lignin and acid soluble lignin), sugars (xylose, arabinose, glucose, and galactose), total phenolic compounds, and phenolic acids (p-coumaric acid, ferulic acid, vanillin, and 4-hydroxybenzaldehyde). It was proved that the tested conditions had a great impact on the permeate flux and molecule retention rate. Increasing the temperature from 20 to 40 °C resulted in a much higher permeate flux for the 5 kDa PES membrane, and the impact of shear rate was greater at 40 °C for this membrane. Although the 5 kDa PES membrane could retain slightly more large molecules, i.e., acid-insoluble lignin and xylose, the 10 kDa membrane afforded greater phenolic acid removal capacity, leading to higher purity. For the 10 kDa PS membrane, the polarization layer began to form at TMP below 0.5 bar. This membrane had a lower retention rate for all molecules than the 10 kDa PES membrane.