Facile Fabrication of Seashell-Derived Hydroxyapatite-Graphene Oxide (HAp/GO) Composites for High-Performance Dye Adsorption

This work aimed to determine the most effective and facile route for preparing hydroxyapatite-graphene oxide (HAp/GO) composites by comparing three simple methods: in situ (I-HAp/GO), hand grinding (G-HAp/GO), and sonication (S-HAp/GO). The use of seashell-derived HAp made all of the processes cost-effective, and the composites were evaluated through structural and adsorption studies toward Congo red (CR) dye removal performance from aqueous solution. Among the three composites, I-HAp/GO showed distinct advantages by possessing the highest BET surface area (114.65 m2/g), greater pore volume, and more homogeneous distribution of HAp nanoparticles (NPs) on GO sheets compared to G-HAp/GO and S-HAp/GO, which exhibited agglomeration and lower surface areas. I-HAp/GO also delivered the best adsorption capacity (68.49 mg/g) and fastest removal efficiency, achieving 94.07% removal of CR (50 mg/L, pH 7, and 50 mg adsorbent) within 10 min with adsorption following the Langmuir isotherm (R2 = 0.9806, linear fitting) and pseudo-second-order kinetics (R2 = 0.9998). Density functional theory calculations showed a reduced HOMO–LUMO gap (0.0089 eV) for HAp/GO composites, indicating enhanced electronic interactions between HAp and GO. Thermodynamic analysis indicated spontaneous adsorption, and regeneration study confirmed over 88% retention of efficiency using deionized water. These findings demonstrate that the in situ method produces a high-surface-area, fast-acting, and reusable HAp/GO adsorbent, making it the most superior among the tested approaches for dye removal applications.