Adsorption of Cr(VI) and Pb(II) ions using an Fe3O4-decorated CaCO3 composite derived from waste eggshells

CaCO3-based nanomaterials have garnered extensive attention for their outstanding efficacy in separating toxic heavy metals (HMs) from aqueous solutions. However, a major challenge lies in retrieving adsorbents during the remediation process, which could lead to secondary contamination, as residual particles persist in the water. Embedding magnetic nanoparticles into CaCO3 frameworks offers a promising strategy to overcome the limitations of individual components. Herein, magnetite (Fe3O4) nanoparticles (∼30 nm) were uniformly integrated into a CaCO3 matrix derived from waste eggshells, forming a stable and magnetically separable adsorbent (CaCO3@Fe3O4) for the efficient removal of toxic Cr(VI) and Pb(II) ions from aqueous solutions. The CaCO3@Fe3O4 composite exhibits outstanding adsorption performance toward both anionic Cr(VI) and cationic Pb(II), achieving maximum capacities of 709.2 mg g−1 and 1278.6 mg g−1, respectively, across a wide pH range, signifying its versatility and stability under varying environmental conditions. The kinetic behavior of the adsorption process follows the pseudo-second-order kinetic model, while the equilibrium results adhere to the Langmuir isotherm. The uptake of Cr(VI) and Pb(II) ions is primarily facilitated by ion-exchange interactions, where Cr(VI) is exchanged with surface carbonate (CO32-) groups and Pb(II) replaces Ca2+ ions within the CaCO3 matrix. Furthermore, the material demonstrated good regeneration performance; the removal efficiency declined by only 4.89%, consistently remaining above 95% after six cycles. This study presents a low-cost, eco-friendly, and reusable material for heavy metal removal from contaminated water.