**2. pH-based precipitation**

Lignins obtained via kraft pulping processes have been commercially separated from black liquor via pH-based precipitation. As pH decreases, lignin molecules become protonated and have less intermolecular electrostatic repulsion, thus favoring the formation of lignin precipitates. Sequential pH adjustment has been studied as a process for producing kraft lignin fractions with different molecular weights, different chemical structures, and different ash content. Magalhães et al. studied a sequential acidification process and obtained low-ash, fully protonated lignins at pH 3 from kraft lignins derived from both hardwood and softwood feedstocks [6]. Helander and Theliander reported the positive correlation between precipitation pH and molecular weight in the recovered fractions, and the retention of most sulfur content in the low molecular weight lignin fraction [7].

Lignin precipitation is also affected by anionic strength, and the effects of different anions on lignin prepitation follow the same order as the Hofmeister series [8]. Sewring and Theliander compared the difference between chloride and sulfate anions regarding their effect on lignin precipitation, and discovered that sulfates not being an effective anion for salting out kraft lignins [9]. When carbon dioxide is used in lignin precipitation, lignin precipitates can be formed with low ash content (<1 wt%) [10]. This process has been commercialized (e.g., LignoBoost) and shows a potential as a sustainable technology given the availability of renewable CO2.
