<p>To overcome the challenges of advanced defluoridation, high chemical consumption, and substantial treatment costs associated with traditional processes for low-concentration fluoride-containing wastewater, this study developed a cost-effective defluoridation approach by utilizing inexpensive quartz sand as seed crystals to induce fluorapatite (FAP) crystallization. Thermodynamic modeling via PHREEQC demonstrated that the saturation index (SI) of FAP reached a high value 14.26 at pH 7.5, providing an abundant thermodynamic driving force for crystallization even at low concentrations. Under the optimized parameters (molar ratio&#xa0;<i>n</i>(<i>Ca</i>):<i>n</i>(<i>P</i>):<i>n</i>(<i>F</i>) = 10:6:1), seed dosage of 10 g/L, and reaction time of 60 min), the system demonstrated high defluoridation efficiency, achieving a 95% removal rate and stably reducing the effluent fluoride concentration to below 1.0 mg/L. Kinetic analysis indicated that the process was well-described by the Avrami crystallization kinetic model (<i>R</i>²&gt;0.97, <i>n</i>≈1.3), elucidating a diffusion-controlled growth mechanism characterized by instantaneous nucleation. Furthermore, microstructural characterizations (SEM–EDS and XRD) verified the heterogeneous nucleation and encapsulating growth of FAP crystals on the quartz sand surface. Ultimately, the incorporation of quartz sand seeds effectively overcame the nucleation induction barrier inherent in low-concentration systems, facilitating the rapid immobilization of fluoride into stable crystals, thereby successfully balancing high removal efficiency with economic feasibility.</p>