Activated carbon was produced by chemical activation of hydrothermally carbonized (HTC) banana empty fruit bunch (BEFB), using phosphoric acid (H3PO4) as the activating agent. The activation process was optimized using a Box-Behnken factorial design (BBD), with an outcome of 17 different experiments under the predefined conditions. Three different parameters (activation temperature (x1), activation time (x2), and the concentration of activating acid (x3)) were analyzed with respect to their influence on maximum adsorption percentage for divalent cations, Pb(II) (Y1) and Zn(II) (Y2), and carbon yield (Y3). All process parameters had strong positive effects on adsorption capacity up to a certain limit. The specific surface area of the hydrochar (HTC) was enhanced substantially after the activation process. Scanning electron microscopy (SEM) revealed that the morphology of the BEFB-based char changed noticeably after the acid impregnation and activation process. The Langmuir maximum monolayer adsorption capacity for Pb (II) and Zn (II) cations was 74.62 mg/g and 77.51 mg/g, respectively. Equilibrium isotherm data were in agreement with the Langmuir model. Thermodynamic characterization revealed that the equilibrium system was endothermic and spontaneous.