The surface plot in Figure illustrates the relationship between the distribution coefficient (Kd, L/g) and two key chemical parameters: pKa and pH at the point of zero charge (pHpzc). Generated using a HOP-RSM XGB meta-model, the plot highlights the interactive effects of these parameters on adsorption efficiency. The surface exhibits a nonlinear curvature, indicating that Kd is maximized at moderate pKa (~0 to 2) and pHpzc values (~6 to 8), suggesting optimal adsorption conditions when the adsorbate has moderate ionization potential, and the adsorbent surface charge is near neutrality.