This study focused on developing a stable, effective CO₂ adsorbent from Astragalus biomass for industrial gas treatment. First, the Astragalus root was carbonized at 600°C in a nitrogen and activated with phosphoric acid to produce porous activated carbon (with a specific surface area of 518.34 m2/g and an average pore diameter of 2.46 nm). Then, it was modified with magnesium oxide nanoparticles prepared by the co-precipitation method. For carbon dioxide absorption, cylindrical monoliths (4 x 5 cm) were fabricated using polyvinyl alcohol binder. Experiments in a pilot reactor with a mixed gas (10% CO2 and 90% N2) showed that the CO2 adsorption capacity at operating conditions (25 °C and 1 bar pressure) reached 1.355 mmol/g, which was improved due to the combination of physical (porosity) and chemical (magnesium carbonate formation) adsorption. The fractional order kinetic model (R2 > 0.999) and Freundlich isotherm (R2 > 0.998 at temperatures (25 to 85 °C) were the best fit to the data. This monolith was introduced as a promising option for large-scale industrial CO2 adsorption due to its low cost, structural stability, and high efficiency.