**4. Flocculation and membrane separation**

The use of flocculants increases the Zeta potential of black liquor and promotes lignin flocculation. Piazza and Garcia studied the application of bovine blood and poly(diallyldimethylammonium chloride) in the recovery of soda lignin derived from wheat straw, and both flocculants are effective (87–92%) in removing lignin from the supernatant [30].

Ultrafiltration is the most commonly used commercial process that recovers lignosulfonates during the sulfite pulping process, and is also used in the recovery of lignins from kraft pulping black liquor. Via ultrafiltration and nanofiltration with membranes of different cut-off, various lignin fractions can be obtained from black liquors. The applications of membrane filtration in lignin recovery has been reviewed by Humpert and Czermak [1]. After partial depolymerization (via pulping and/or other chemical processes), technical lignins can be fractionated via membrane separation and thus the low- and high-molecular-weight fractions can be utilized for different applications. For instance, Hulteburg et al. demonstrated the use of polymeric membrane to recover low molecular weight phenolics from softwood kraft lignins that have been partially depolymerized with aqueous NaOH [31]. The recovered phenolic compounds have a molecular weight of 250–450 g/mol and can be metabolized by some *Pseudomonas* strains [32, 33]. Mendes et al. reported the use of ultrafiltration to recover 83% of the lignins in the lignin-lean aqueous phase derived from the acidification of kraft black liquor [34]. Labidi et al. compared the performance of several ceramic ultrafiltration membranes with different cut-offs (5–15 kDa) in the fractions of soda pulping black liquor from Miscanthus, and discovered that other than the highest molecular weight fraction (15 kDa and higher; likely contaminated by lignincarbohydrate complexes), the other lignin fractions from different membranes have similar molecular structures (i.e., types of linkages among monolignols) despite their differences in molar mass [35]. Keyoumu et al. studied the ultrafiltration of hardwood (birch) and softwood (spruce-pine mixture) black liquor using ceramic membranes, and reported a negative correlation between membrane cut-offs and the phenolic group content in the obtained fractions [36].
