The curvature of the surface plot demonstrates a strong interaction effect between pH and pHpzc. At low pH and high pHpzc, Kd reaches its maximum, confirming that PFAS adsorption is optimized under conditions where the adsorbent remains positively charged. However, at high pH and low pHpzc, Kd becomes highly negative, suggesting a repulsion-dominated desorption mechanism. The nonlinear adsorption response further highlights the complex interplay between electrostatic interactions and potential hydrogen bonding at intermediate pH values. These findings emphasize the critical role of pH control in PFAS adsorption and suggest that selecting adsorbents with a high pHpzc enhances removal efficiency, particularly under neutral to slightly acidic conditions. The significant p-values confirm that both pH and pHpzc should be considered key design parameters in optimizing adsorption materials for PFAS remediation.