Cost-effective and Eco-friendly Synthesis of Carbonated Hydroxyapatite Nanoparticles Based on Oyster Shells as Adsorbent for Brilliant Black Dye

Hydroxyapatite (HA) was successfully synthesized from oyster shell waste using a wet chemical precipitation method under magnetic stirring at varying durations (30 min, 1, 3, and 5 h). The synthesized HA samples were characterized to identify their crystalline structure, morphology, elemental composition, and functional groups. The results indicated that stirring time significantly influenced the phase purity and crystallinity of HA. Samples stirred for 30 min and 1 hour exhibited incomplete conversion, retaining traces of CaO, whereas those stirred for 3–5 h demonstrated complete transformation of CaO into hydroxyapatite with enhanced crystallinity and a well-defined apatite structure. The average crystallite and particle size of the prepared HAp at 3 h of stirring time were 18 and 34 ± 0.7nm, respectively. The synthesized HAp at 3-hour stirring time was then successfully applied for the adsorptive removal of Brilliant Black dye from aqueous solutions. To identify the optimal conditions, key parameters (initial concentration, pH, adsorbent dosage, and time) were tuned with a Box-Behnken design, which is part of the response surface methodology (RSM). In each experiment, a predetermined quantity of adsorbent was introduced into a brilliant black dye solution, where the pH was carefully adjusted using 0.1 M HCl or 0.1 M NaOH. The prepared mixtures were stirred for a predetermined time at room temperature, then centrifuged at 5000 rpm for 10 min. The results indicated that hydroxyapatite has a stronger adsorption capacity for brilliant black dye in aqueous solutions than calcite. The maximum adsorption of Brilliant black dye was obtained at the following optimized conditions of 62 min contact time, pH=5, 20 mg/L concentration, 0.045 g dosage and the maximum adsorption capacities obtained at these conditions is 93.52 %. Isotherm studies showed that the adsorption process followed the Languir model, as it is a strong fit for the experimental data.