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Novel acid treated porous activated biocarbons (APABs) are synthesized from biomass waste, Arundo donax, through sulphuric acid polymerization followed by a single step solid state activation using KOH at a fixed carbonization temperature of 600 °C. The specific surface area and the pore volume of the prepared materials is controlled by varying the ratio of KOH to biomass from 1 to 4. The sample prepared with the KOH/biomass ratio of 3 is found to be the best as it exhibits the specific surface area of 2232 m2 g−1 and a pore volume of 1.01 cm3 g−1 which is much higher than those of other samples prepared in this study. Interestingly, XPS and FT-IR studies confirm the presence of oxygen rich functional groups on the surface of the samples, which play an important role in enhancing the CO2 capture performance of the materials. Among the samples studied, the sample with the highest specific BET surface area exhibits the highest CO2 adsorption capacity of 21.2 mmol g-1 at 0 °C and 30 bar and a moderately high value of 4.1 mmol g-1 at 0 °C and 1 bar. The high CO2 adsorption capacity is attributed to the presence of excellent textural parameters coupled with high micropore volume and the oxygen functional groups. The samples are highly stable and do not show any change in the adsorption capacity even after repeated adsorption experiments. The combination of high adsorption capacity, stability and low cost makes these materials as a potential alternative to other expensive commercially available CO2 adsorbents.