Mechanochemical synthesis of silver-integrated biogenic CaCO₃ nanocomposite for Cu(II) removal from aqueous solutions

Seashell-derived biogenic calcium carbonate was modified with silver through a simple and solvent-minimized mechanochemical ball-milling process to obtain a nanocomposite for Cu(II) removal from aqueous solutions. In contrast to conventional chemical reduction methods, metallic Ag was formed in situ during milling without the use of external reducing agents, emphasizing the simplicity and sustainability of the synthesis strategy. Structural and chemical characterization (SEM/EDX, XRD, XRF, FTIR, BET and pHₚzc) confirmed the successful incorporation of nanocrystalline Ag onto the CaCO₃ matrix. Batch adsorption experiments were performed to systematically investigate the effects of pH, adsorbent dosage, initial metal concentration, contact time, temperature, and stirring speed. Under optimal conditions (pH 5.5, 0.15 g/20 mL, 50 mg/L Cu(II), 60 min, 25 °C, 200 rpm), a removal efficiency of 97.7% was achieved. Adsorption equilibrium was best described by the Freundlich model (R2 = 0.956), while the Langmuir model predicted a maximum capacity of 22.32 mg/g. Kinetic data followed the pseudo-second-order model. Thermodynamic analysis indicated a spontaneous and exothermic process predominantly governed by physical interactions. The performance obtained is comparable to previously reported biogenic CaCO₃-based adsorbents and demonstrates that the developed nanocomposite has a wider range of applications in water treatment.