All-Lignin Microspheres with Controllable Surface Charges Fabricated from Water-Soluble Quaternized Lignin

Technical lignin offers a renewable aromatic platform for functional materials. However, its direct transformation into well-defined particulate adsorbents remains challenging owing to its poor aqueous processability and uncontrolled interfacial properties. In this study, kraft lignin (KL) is quaternized by introducing permanent quaternary ammonium groups, yielding a water-processable quaternized lignin (QL) with tunable cationic characteristics. Polymer matrix-free lignin microspheres are then fabricated by water-in-oil emulsion crosslinking using KL and QL as lignin building blocks. Compared with KL-based microspheres (KLM), QL-containing microspheres (QLM) show improved structural stability, reduced lignin leakage, and enhanced affinity for anionic methyl orange (MO). QLM achieves a Langmuir maximum adsorption capacity of 651.4 mg g^-1, far exceeding that of KLM, primarily through the electrostatic attraction between quaternary ammonium groups and MO sulfonate groups, with additional contributions from π-π interactions, hydrogen bonding, and network-assisted diffusion. QLM retains high adsorption performance over multiple reuse cycles and enables continuous-flow MO removal in a packed-bed system. These results demonstrate a charge-engineering strategy for converting technical lignin into processable high-performance microsphere adsorbents for sustainable water purification.