## 4 Results and Discussion
### Fluoride Leaching Characteristic  
As shown in Table 4, the fluoride concentrations in RCPB sample leachates exhibited significant reductions compared to raw phosphogypsum (PG) containing 167.5 mg/L F\({}^{-}\). Experimental results revealed that all four tested groups achieved 69-96% fluoride removal efficiency, with the highest residual concentration measuring only 0.6686 mg/L. Notably, all treated samples complied with the Class I groundwater quality standard [40] requiring fluoride levels \(\leq\) 1 mg/L [42].  
There were quite a few Ca ions present in all of the samples in this study, so it was inferred that the decrease in the F ion concentration could be caused by the CaF\({}_{2}\) solid produced by the reaction between Ca and F. The CaF\({}_{2}\) solid was produced by the reaction between Ca and F, and the CaF\({}_{2}\) solid was produced by the reaction between Ca and F [43]. However, although Ca ions can immobilize some F ions, the chemical precipitation reaction between Ca ions and F is not sufficient, and it is difficult to make the F ion concentration to such a low degree (the F content of all groups is under 1 mg/L) [44], and the generation of CaF\({}_{2}\) can be inferred to a certain extent through the determination of the Ca ion fugacity; the specific amount of Ca ions retained needs to be further detected by microscopic means, such as XRD, SEM, etc., so presumably, there are other factors; there may be other ions involved in the reaction and precipitation of F ions, and F may react with Al, Fe, Mg, and other cations to form complex precipitation, thus fixing the F ions, for example, a F and Na and Al(OH)\({}_{3}\) reaction to generate sodium hexafluoroaluminate flocculent precipitation and physical adsorption of F ions so as to achieve the effect of removal of F; the chemical reaction equation is given by. The change in the molecule in the chemical reaction formula is shown in Figure 9.  
\[6F^{-}+3 Na^{+}+Al(OH)_{3}+3H^{+}=Na_{3}AlF_{6}\downarrow+3H_{2}O \tag{7}\]  
A similar reaction is found in Fe ions, FeF\({}_{3}\) precipitation generated by the iron salt precipitation method or phosphorus salt precipitation method (the raw materials selected for this study contain P element), where F reacts with Ca(H\({}_{2}\)PO\({}_{4}\))\({}_{2}\) to form CaF\({}_{2}\) precipitation.  
Therefore, the F\({}^{-}\) removal mechanism is two parts: (1) chemical precipitation mechanism: The free Ca\({}^{2+}\) in the system combines with F\({}^{-}\) through ionic bonding to form insoluble CaF\({}_{2}\) precipitation, realizing the chemical immobilization of fluoride ions; (2) physical adsorption mechanism: Amorphous alumino-silicate gels generated by the hydration reaction and secondary minerals such as chalcogen, whose surfaces are rich in hydroxyl  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Group** & **Concentration of F\({}^{-1}\)** \\ \hline PR-63 & 0.5411 \\ PR-65 & 0.5784 \\ PR-67 & 0.6076 \\ PR-69 & 0.6686 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Fluoride leaching concentration.  
Figure 9: Schematic structure of the Na\({}_{3}\)AlF\({}_{6}\) complex precipitation reaction.  
functional groups, can be combined with F\({}^{-}\) through the role of ligand exchange, and at the same time, the developed porous structure can produce a parcel immobilization effect on F\({}^{-}\) in solution.