This work investigates the use of activated carbon in a batch system for the adsorption of cadmium from aqueous solutions. Two adsorbents were assessed: Commercially available activated carbon (CAC) and synthesized activated carbon (SAC) produced from walnut shell–derived agricultural waste. Adsorption efficiency increased with increasing pH, according to research on the impact of solution pH on cadmium removal. In the pH range of 7–12, maximum removal efficiencies of 81.9% for SAC and 80.5% for CAC were found at an initial cadmium concentration of 100 mg/L. Freundlich and Langmuir isotherm models were used to analyze the adsorption data. The Freundlich model provided a marginally better fit, suggesting heterogeneous adsorption sites on the activated carbon surfaces, while both models demonstrated good agreement with the experimental results. The highest adsorption capacities that were obtained from adsorption isotherms, under conditions of 4 h of contact time and 30 °C, were 69.5 mg/g for SAC and 81.2 mg/g for CAC.