Rapid and Efficient Purification of Low-Concentration Fluoride-Containing Water Using Cationic Chitosan Fibers

In this study, a bio-based adsorbent, quaternary ammonium-modified cationic chitosan fibers (CCFs), were developed and systematically evaluated for fluoride removal from low-concentration aqueous medium, with particular emphasis on adsorption performance, regeneration behavior, practical applicability, and adsorption mechanisms. The results demonstrate that the fluoride adsorption capacity of CCFs is approximately 15.8 times higher than that of unmodified chitosan fibers (CFs). Furthermore, CCFs exhibit superior fluoride adsorption capacity and remarkably rapid kinetics relative to previously reported chitosan-based adsorbents in the literature. The adsorption process fits well with the pseudo-second-order kinetic model and reaches equilibrium within 10 min. Adsorption isotherm data are well described by both Langmuir and Freundlich models, with a maximum adsorption capacity of 28.5 mg/g. CCFs also show excellent regeneration performance, achieving efficient fluoride desorption within 3 min using a 0.02 mol/L NaCl solution, with no noticeable loss in adsorption capacity after five consecutive adsorption–desorption cycles. The adsorption performance remains effective in natural surface water containing competing ions. Mechanistic investigations reveal that fluoride adsorption is dominated by electrostatic attraction between quaternary ammonium groups (R₄N⁺) on the CCFs surface and fluoride ions, accompanied by ion exchange with chloride ions. Owing to their high efficiency, rapid kinetics, metal-free nature, and facile regeneration, the CCFs developed in this study represent a promising bio-based adsorbent for the advanced purification of low-concentration fluoride-containing water.