Eco-Engineered Al₂O₃–Chitosan Matrix for Sustainable Water Decontamination from Humic Acid

This study presents the synthesis and application of an alumina–chitosan (α-AO@CS) composite as a sustainable and cost-effective adsorbent for the removal of humic acid (HA) from water. The composite was prepared through in-situ dispersion and alkaline gelation, ensuring homogeneous nanoparticle incorporation within the polymeric matrix. Structural and surface characterizations (FTIR, XRD, SEM–EDX, and pHpzc) confirmed the successful integration of alumina into chitosan, yielding a material with enhanced surface heterogeneity and stability. Batch adsorption experiments demonstrated that AO@CS achieved maximum HA removal of 91.7% at near-neutral pH (pH ≈ 7), outperforming bare Al₂O₃ (49.2%) and pristine chitosan (74.9%). The point of zero charge of the composite (pHpzc = 7.3) enabled effective adsorption across a broad pH range, with optimum performance under conditions relevant to natural waters. The maximum adsorption capacity, derived from nonlinear Langmuir isotherm fitting, was 8.23 mg g⁻¹, while Freundlich modeling indicated multilayer adsorption on heterogeneous sites. Kinetic data conformed to the pseudo-second-order model (R² ≈ 1.0), confirming chemisorption as the dominant mechanism. Thermodynamic parameters revealed that adsorption onto α-AO@CS is spontaneous (ΔG° = − 45.14 to − 84.27 J mol⁻¹) and endothermic (ΔH° = 18.2 kJ mol⁻¹), with increased randomness at the solid–solution interface (ΔS° = 70.1 J mol⁻¹ K⁻¹). Importantly, the composite retained 83% of its initial capacity after five regeneration cycles, highlighting superior reusability compared with Al₂O₃ (47%) and chitosan (43%). These findings confirm the synergistic effect of integrating alumina’s hydroxyl-rich surfaces with chitosan’s amino functionalities, establishing AO@CS as a robust, scalable, and eco-friendly material for advanced water purification.