A novel activation route for the preparation of carbon-based adsorbents functionalized with lanthanum and their application for arsenic adsorption was proposed. This procedure is based on carbon dioxide activation at room temperature, which could allow to reduce the energy consumption and adsorbent cost. A comparison of this novel activation approach with respect to the conventional thermal activation at 800 °C under N2 atmosphere was performed on biochar from avocado seeds and functionalized with lanthanum. Results showed that the arsenic adsorption properties of the lanthanum-functionalized adsorbent activated with CO2 at room temperature were higher up to 179% than those activated with N2 at 800 °C. The carbonation of lanthanum oxygenated functionalities on the adsorbent surface favored the ligand exchange and surface complexation interactions during the arsenic adsorption. Arsenic adsorption using this engineered lanthanum-based adsorbent was endothermic and transited from a multi-molecular process at 25 °C to a multi-anchorage process at 30 °C. The estimated arsenic adsorption capacities at saturation for these novel adsorbents were 15 – 20 mg/g at 25 – 40 °C, respectively.