Synthesis and application of modified waste acropora coral skeletons for Pb(II) adsorption: A bead adsorbent perspective

• A novel class of bead adsorbents derived from waste Acropora coral skeletons was synthesized and characterized. • The Ca-Alginate-GO/PVA/Chitosan/Pure SiO₂ composite demonstrated a high maximum adsorption capacity for targeted contaminants. • Kinetic modeling revealed that the adsorption data are best described by the pseudo-second-order model, with R² ≈ 0.990. • The Freundlich isotherm provided the best fit to the equilibrium adsorption data, indicating heterogeneous surface adsorption. • Positive ΔH values confirm that Pb(II) adsorption onto these bead adsorbents is an endothermic process. A novel class of bead adsorbents derived from waste Acropora coral skeletons, functionalized with various monomers including alginate, graphene oxide (GO), polyvinyl alcohol (PVA), chitosan, cellulose, natural silica, and pure silica, was synthesized, characterized, and evaluated for the adsorption of Pb(II) ions from aqueous solutions. The characterization techniques, including colorimetric analysis and FTIR spectroscopy, revealed characteristic absorption bands at 712.58, 1186, 825, 1470, 3562, 1660, and 1050 cm⁻¹, corresponding to functional groups introduced through modification of the coral skeleton using Ca²⁺ ions as a source. Among the synthesized beads, Ca-Alg-GO, Ca-Alg-PVA, Ca-Alg-Chitosan, and Ca-Alg-Pure SiO₂ exhibited a maximum adsorption capacity of 0.063 mg/g under optimized conditions namely, a contact time of 30 minutes and an initial Pb(II) concentration of 2 mg/L in batch systems. Kinetic analysis indicated that the adsorption data best fit the pseudo-second-order model (R² ≈ 0.99), suggesting that chemisorption via valence forces involving sharing or exchange of electrons dominates the process. Additionally, the Freundlich isotherm provided a superior fit compared to the Langmuir model for Ca-Alg-PVA, Ca-Alg-GO, Ca-Alg-Celulose, and Ca-Alg-Chitosan beads, with R² values of 0.991, 0.998, 0.997, and 0.984, respectively. These results imply that multilayer adsorption occurs on heterogeneous surfaces. The positive ΔH values confirm that Pb(II) adsorption on these bead adsorbents is endothermic, reflecting strong interactions between Pb(II) ions and negatively charged sites on the bead surfaces.