## 3 Analysis and Discussion
### Characterization of Changes in Phosphogypsum Leachate
#### 3.4.2 Characterization of TDS Changes  
Figure 7 illustrates the variation of TDS over time. In static dissolution experiments employing phosphogypsum leachate at pH = 2, TDS exhibited an initial rapid decline followed by stabilization, a pattern attributed to accelerated H\({}^{+}\)-carbonate mineral reactions causing ionic depletion during the early stage, succeeded by reaction rate decay and ion-exchange equilibrium. Contrastingly, systems at pH = 3-4 demonstrated V-shaped TDS trajectories: an initial decrease succeeded by a gradual rebound. Notably, the inflection point marking the transition from TDS decline to recovery occurred earlier under pH = 4 conditions compared to pH = 3, indicating accelerated geochemical transformations in higher-pH environments. Higher initial acid concentrations in closed systems led to faster depletion of reactive acids and lower subsequent reaction rates [47]. This TDS reversal mechanism suggests early-stage H\({}^{+}\) consumption and common-ion effects suppress Ca\({}^{2+}\) dissolution, while later CO\({}_{2}\) dissolution promotes HCO\({}_{3}\)\({}^{-}\) accumulation.  
Figure 6: pH vs. time. Static dissolution group (HY01–HY06): HY01–HY03 with different pH; HY04–HY06 with identical pH and different fracture apertures. Dynamic dissolution group (HY07–HY12): HY07–HY09 with different pH; HY10–HY12 with identical pH and different fracture apertures.  
Conversely, dynamic dissolution experiments showed opposing TDS evolution patterns characterized by an initial rapid increase followed by stabilization. Higher initial leachate pH enhanced TDS variation magnitudes, while smaller fracture apertures prolonged stabilization periods, reduced stabilized TDS values, and amplified cumulative TDS fluctuations compared to wider apertures.  
4.3 Characteristics of Changes in Ca\({}^{2+}\), SO\({}_{4}\)\({}^{2-}\), F\({}^{-}\), and Total P Concentrations  
Figure 8 delineates changes in the concentration of each ion. In static dissolution experiments, lower initial pH values of phosphogypsum leachate correlated with greater increases in Ca\({}^{2+}\) concentrations and more pronounced decreases in F\({}^{-}\) concentrations as the reaction progressed. Concurrently, SO\({}_{4}\)\({}^{2-}\) and total phosphorus concentrations stabilized but exhibited an overall declining trend, likely attributable to calcium sulfate precipitation (gypsum) from sulfuric acid-calcium carbonate reactions. The positively charged surfaces of calcium sulfate and calcium fluoride precipitates may electrostatically adsorb negatively charged ions, forming surface complexes characterized by P-O-Ca bonding. Additionally, partial incorporation of dihydrogen phosphate ions into the crystal lattice during gypsum growth could further contribute to phosphorus retention.  
Figure 8: Changes in the concentration of each ion (HY01\(-\)HY03 on static experiments, HY10\(-\)HY12 on dynamic experiments).  
Figure 7: TDS vs. time (HY01–HY06 on static experiments, HY07–HY12 on dynamic experiments).  
Conversely, dynamic dissolution experiments demonstrated inverted ion concentration trends relative to static conditions: Ca\({}^{2+}\) concentrations decreased while F\({}^{-}\) concentrations increased over time. Specimens with smaller fracture apertures required extended durations to reach concentration stabilization, and higher initial leachate acidity amplified concentration variations for both Ca\({}^{2+}\) and F\({}^{-}\). SO\({}_{4}\)\({}^{2-}\) and total P concentrations showed minimal fluctuations, gradually increasing throughout the experiments.