Efficient water defluoridation and phosphate removal using recycled alkali-activated glass-calcium composites: Mechanisms and optimization

• Recycled Alkali-Activated Glass-Calcium Composite is efficient F - and PO 4 –3 ions. • The removal efficiency of >96 % for F - and >62 % PO 4 –3 is achieved by AAC-P@Ca. • Thermodynamic analysis indicates endothermic & spontaneous F - and PO 4 –3 uptake. A sustainable and low-cost adsorbent composite, alkali-activated glass-calcium (AAC-P@Ca) was synthesized from waste glass, plantain stalk ash (alkaline activator) and eggshell-derived CaO for the efficient removal of fluoride and phosphate ions from aqueous solutions. The as-synthesized composite material was extensively characterized using, XRD, SEM-EDX, FTIR, XRF and TGA to evaluate its structure, morphology, functional groups and thermal stability. AAC-P@Ca exhibited a moderate specific surface area (82.249 m 2 /g), and a point of zero charge (pzc) of 12.6, which indicate a strongly positive surface favorable for anionic adsorption. Batch adsorption experiments were performed to study the influence of contact time, pH, temperature, initial ion concentration, and ionic strength on adsorption performance. The optimized composite material achieved fluoride and phosphate removal efficiencies of >96 % and 62 %, respectively. Adsorption kinetics followed the pseudo second order model for fluoride and a mixed mechanism for phosphate ions, which thermodynamic analysis confirmed exothermic (for phosphate ion), endothermic (for fluoride ion) and spontaneous uptake. Process optimization using Response Surface Methodology (RSM) based on Central Composite Design (CCD) validated the composite’s operational efficiency. The synergistic effects of Ca 2+ -rich active sites, porous morphology and multifunctional surface groups could contribute to the effective adsorption across the WHO-recommended pH range (6.5–9.5). These results highlight the AAC-P@Ca composite material as a promising circular-economy driven adsorbent for sustainable water purification and anion remediation applications.