Bio-derived cellulose adsorbent from licorice residues for high-efficiency and recyclable removal of phenothiazines.

The value enhancement of herbal residue is a sustainable strategy for resource recovery in Chinese medicine industry. In this study, licorice residues (LR) are valorized into microcrystalline cellulose (LRMCC) through a sequential alkali-acid process. The preparation route was optimized, and the obtained material was systematically characterized. The carboxylation in the preparation process introduced a high density of carboxyl groups, enabling the biosorbent to adsorb phenothiazine pollutants in water environment. The adsorption process conforms to the Freundlich isotherm and pseudo-second-order kinetic model according to linear fitting. Removal kinetics reached equilibrium after 20 min and the adsorption capacities for chlorpromazine and perphenazine were 9.17 and 26.87 mg/g, respectively. The adsorbent still exhibits favorable adsorption performance under flow conditions. Furthermore, the adsorbent can still achieve phenothiazines removal rates of over 91.73% even after ten cycles of reuse. X-ray photoelectron spectroscopy and density function theory calculation revealed that the satisfactory adsorption mainly attributed to the electrostatic interactions and hydrogen bonding interactions. As such, the plant-derived adsorbent eliminates the risk of causing secondary environmental pollution. This study established a practical and sustainable strategy to the issues of high-value utilization of herbal residues and remediation of pharmaceutical pollutants.