High-efficiency fluoride capture in wastewater remediation using microbially mineralized HAP@Vaterite composites: Process optimization and environmental benefits

The presence of fluoride (F − ) in wastewater poses escalating threats to both the ecological systems and human health. This study developed an HAP@Vaterite composite through green biomineralization and phosphate modification for the efficient removal of F − from low-concentration wastewater, while maintaining promising engineering applications. The optimal formulation, Vat-5.9, was determined by single-factor experiments and the Box-Behnken response surface methodology. The predicted and experimental removal efficiencies differed by only 0.19%, confirming the model's reliability. Compared to chemically synthesized CaCO 3 mineralization material (Cal-5.9), Vat-5.9 exhibited a maximum F − adsorption capacity of 23.3 mg/g, achieving a removal efficiency exceeding 99%. The adsorption process is spontaneous and endothermic (ΔG < 0, ΔH > 0). Defluoridation proceeds via two synergistic pathways: (1) substitution of –OH by F − , leading to stable Ca–F bonds, and observed deposition of CaF 2 microcrystals on the material's surface, and (2) competition of F − with PO 4 3− for coordination sites, which drives the crystallization of amorphous calcium phosphate into Ca 5 (PO 4 ) 3 F. Notably, 0.1 mol/L NaOH solution was identified as optimal for sorbent regeneration. Importantly, Vat-5.9 exhibited superior performance in treating domestic sewage, reaching saturation breakthrough significantly later than in industrial wastewater, thereby underscoring its safety for domestic water supplies. Both techno-economic analysis and life cycle assessment indicated that HAP@Vaterite offers significant economic and health benefits, with a socio-economic value of 74,000 $/ adsorbent , alongside a net reduction in CO 2 emissions (1.80 tCO 2 -eq/t adsorbent ). This work establishes a practical framework for assessing the feasibility of green and cost-effective F − remediation, aligning with China's sustainable development objectives.