Eggshell-derived CaO₂@MgO–Al₂O₃ composite for phosphate removal through adsorption and precipitation

The increasing discharge of phosphate-containing wastewater and the accumulation of calcium-rich household waste are two significant environmental challenges that require sustainable remediation strategies. This study demonstrates the feasibility of converting eggshell waste into a value-added phosphate-removal material, with potential application as a low-cost adsorbent for phosphate reduction in wastewater treatment systems. Discarded eggshell waste was utilized as a sustainable calcium precursor for composite preparation, converting an abundant household waste material into a functional adsorbent for environmental applications. MgO and Al₂O₃ were incorporated as complementary components to enhance phosphate removal through synergistic adsorption and precipitation mechanisms. FTIR, XRD, and FESEM-EDS analyses confirmed the material characteristics. The composite demonstrated effective phosphate removal under different initial concentrations (5, 10, and 20 mg/L) and temperatures (30, 40, and 50 °C). Sorption experiments indicated that the adsorption behavior followed the Temkin isotherm model (R² = 0.9916) and pseudo-second-order kinetic model (R² = 0.94628), suggesting strong adsorbent–phosphate interactions. Thermodynamic analysis indicated a spontaneous adsorption process (ΔG < 0), with ΔH = + 24.07 kJ/mol and ΔS = + 60.39 J/mol·K. This study shows that eggshell-derived calcium-based composites can provide a sustainable pathway to simultaneously reduce phosphate pollution and valorize household waste into value-added environmental materials.