The surface exhibits a curved profile, highlighting the nonlinear relationship between these variables. Adsorbents with surface areas in the range of 1,000 -2,000 m 2 /g demonstrate increased Kd, when the pHpzc is in the range of 6 -10, indicating enhanced adsorption performance due to the greater availability of active binding sites. Similarly, pHpzc plays a significant role, where Kd increases with pHpzc until it reaches 10. When pHpzc was higher than 10, Kd values showed a decreasing pattern. This trend reflects the ability of adsorbents with higher pHpzc to maintain a positively charged surface, favoring interactions with negatively charged PFAS molecules. However, at very high surface areas or extreme pHpzc values, the adsorption performance plateaus suggest potential saturation effects. The plot underscores the importance of optimizing both adsorbent surface area and surface charge properties to maximize PFAS removal efficiency under various environmental conditions. The surface plot in Figure illustrates the relationship between the distribution coefficient (Kd, L/g) and two key predictor variables: Surface Area (m²/g) and pKa using the HOP-RSM XGB meta-model. The surface shows that Kd reaches its peak at moderate surface areas (~1000-1500 m²/g) and higher pKa values (~0 to 4). This suggests that adsorption efficiency is highest when the adsorbate has a moderate to high pKa, allowing better interaction with the adsorbent surface. However, beyond ~1500 m²/g, Kd declines, likely due to pore blockage, diffusion limitations, or reduced availability of active sites, indicating that excessive surface area does not always enhance adsorption.