Activated carbon as a low-cost adsorbent prepared from almond shells using H₃PO₄ as a chemical activator and room vacuum pyrolysis as a physical activator, which is considered to be an eco-compatible preparation process. Experimental design methodology was used to study and optimize the effects of eight preparation parameters on I₂ adsorption expressed by the iodine index (mg g⁻¹). It was found that optimum activated carbon was obtained by chemical activation with H₃PO₄ at first, followed by physical treatment at 420 °C under a vacuum pressure of −0.8 bar. The obtained activated carbon was characterized by a thermogravimetric analyzer, scanning electron microscopy coupled to EDX, X-ray diffraction, and Fourier transform infrared absorption spectroscopy. The zero-charge pH and the characteristics of surface chemistry by Boehm titration were determined to predict the acid–base properties of the prepared material. An adsorption efficiency study of crystal violet dye on the optimally produced activated carbon was carried out. The obtained results of physicochemical characterization showed interesting properties of our activated carbon in comparison with those produced by other methods. Among these properties, an important porous surface, high thermal stability, and a disorganized graphitic crystalline structure were revealed. In addition to the carbon and oxygen elements, EDX analysis revealed the presence of phosphorus element, and the FTIR analysis indicated the existence of phosphonate groups and an acidic character, which resulted from chemical activation by H₃PO₄. An iodine index of 824.85 mg g⁻¹ was achieved for optimal preparation. Crystal violet adsorption studies show a pseudo-first-order kinetic process and fit well with the Freundlich isotherm model, and thus, the predicted adsorption capacity was 364.27 mg g⁻¹.