Lignin-Based Water-Soluble Polymers Exhibiting Biodegradability and Activity as Flocculating Agents

In the present study, depolymerized lignins (originating from pine, poplar, and corn stover) were valorized into biodegradable, water-soluble polymers (WSPs) by grafting hydrophilic l-lysine to their aromatic framework via the Mannich reaction. Higher grafting efficiencies (90%) were observed for lignins less abundant in syringol monomer units. Light scattering methods indicated that the size of the WSPs in aqueous solutions was sensitive to pH as they would expand or contract depending on (un)favorable electrostatic interactions between ionizable chemical functionalities on their surface. Facilitated by the zwitterionic properties of the WSPs, highly acidic or basic conditions increased the hydrodynamic radii of the WSPs (ranging from 200 to 350 nm), while pH conditions near the isoelectric point diminished the hydrodynamic radii of the WSPs (ranging from 50 to 130 nm). A potential application of the lignin-based WSPs as flocculating agents was evaluated by investigating their efficacy for destabilizing turbid aqueous suspensions of clay minerals (e.g., kaolin). At a pH of 3, dosages of 10–20 mg/L of the lignin-based WSPs could reduce the turbidity of the kaolin suspensions by over 95%. The biodegradability of all three types of WSPs (0.3 < BOD5/COD < 0.4) indicated that they were eco-friendly, biodegradable, and would not persist in the environment.