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

Abstract

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 CaCO₃ 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 CaF₂ microcrystals on the material's surface, and (2) competition of F⁻ with PO₄³⁻ for coordination sites, which drives the crystallization of amorphous calcium phosphate into Ca₅(PO₄)₃F. 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 CO₂ emissions (1.80 tCO₂-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.