Fish scale waste-derived chitosan-mineral composite for fluoride removal: RSM-optimized synthesis and high-performance adsorption

This study introduces an innovative bio-based aluminum composite adsorbent (CS–Al) derived from fish scales, an underutilized byproduct of aquaculture, for efficient fluoride removal from water. Through response surface methodology (RSM) optimization, CS-Al achieved a remarkable fluoride adsorption capacity of 100.19 mg/g, representing a 173.5 % enhancement over commercial chitosan-based adsorbents. The adsorbent exhibited rapid chemisorption kinetics, with 85.79 % fluoride removal within 10 min, following a pseudo-second-order model (R² = 0.99) and spontaneous endothermic adsorption behavior (ΔG° = -1.41 kJ/mol, ΔH° = 7.93 kJ/mol, ΔS° = 31.36 J/mol at 25 °C). The unique mesoporous hierarchical structure of CS-Al, with a BET surface area of 12.84 m²/g and dual Ca/Al active sites, provides exceptional stability and anti-interference performance. It effectively removed over 93 % of fluoride across a broad pH range (3-12) and in the presence of competing anions like Cl⁻ and NO₃⁻. Mechanistic studies revealed a dual adsorption mechanism: Ca–F complexation facilitated by residual Ca(OH)₂ (90.2 %) and AlF₃ formation through ligand exchange with Al–O–C-anchored aluminum components (9.8 %), surpassing the efficiency of traditional single-component adsorbents. CS-Al not only excels in fluoride removal but also aligns with sustainable development goals by converting fish scale waste into a high-value adsorbent via a one-step coordination coprecipitation method. With its fluoride removal capability reducing concentrations from 10 mg/L to 0.47 mg/L (well below the WHO limit), CS-Al offers a viable solution for decentralized water treatment in high-fluoride regions. This work establishes a scalable framework for converting aquaculture byproducts into functional adsorbents, balancing adsorption performance with sustainable production for decentralized water treatment applications.