Efficient Removal of Fluoride from Phosphogypsum Leachate: Insights into Microscopic Mechanism of Polyferric Sulfate-Assisted Precipitation via Molecular Simulation.

Fine calcium fluoride (CaF2) particles formed during fluoride removal processes often exhibit poor aggregation and separation behavior, especially in phosphogypsum (PG) leachate with high soluble fluoride concentrations. In this study, polyferric sulfate (PFS) was employed as a coagulation aid following CaO precipitation to enhance CaF2 aggregation and solid-liquid separation, with particular focus on the underlying molecular mechanisms. Experimental results showed that the sequential addition of CaO followed by PFS significantly improved fluoride removal compared with CaO precipitation alone. Under optimal conditions, the sequential CaO/PFS addition achieved a maximum fluoride removal efficiency of 94.1%, compared with 88.3% for the CaO-only system. Molecular dynamics simulations revealed that Fe-containing species promoted CaF2 aggregation through electrostatic attraction and bridging interactions involving OH-, Ca2+, and F-. Quantitative analyses based on radial distribution functions, coordination numbers, solubility parameters, and self-diffusion coefficients further demonstrated that pH, ion concentration, and dosing sequence strongly influenced aggregate formation, with sequential dosing showing superior performance over simultaneous addition. This work provides molecular-level insight into inorganic coagulation-assisted CaF2 aggregation and offers theoretical support and practical guidance for efficient fluoride removal and resource-oriented treatment of PG leachate.