Sustainable α-AO@CS composite for effective humic acid elimination from water

This study demonstrates the synthesis and utilized of an environmentally friendly an alumina-chitosan (α-AO@CS) composite as a very effective and economical adsorbent for the removal of humic acid (HA) from water. The composite was prepared through in-situ dispersion and alkaline gelation, ensuring uniform incorporation of alumina nanoparticles 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 Al₂O₂ (47%) and chitosan (43%). These results demonstrate that combining alumina's hydroxyl-rich surfaces with chitosan's amino functionalities improves their performance, making AO@CS a strong, scalable, and eco-friendly material for enhanced water purification.