In this study, biochar was thermally and chemically (thermo-chemically) modified and compared to the unmodified parent char in carbon dioxide adsorption. The biochars were sourced from sawmill residues and produced via fast pyrolysis in an auger reactor. The biochar was chemically functionalized using two novel methods of amine functionalization: (i) nitration, followed by reduction, and (ii) condensation of aminopropyl triethoxysilane on the surface. The obtained outcomes indicated functionalization resulted in a reduction in the pore volume and surface area of the biochar. The biochars (unmodified and chemically modified) were thermally activated via air diluted with nitrogen at moderate 560 degrees C to determine if the adsorption properties could be enhanced. The thermally treated functionalized chars had a lower H:C ratio, higher surface area, micropore volume, and sufficient amount of nitrogen compared to the unmodified char. The thermally treated aminopropyl triethoxysilane char had the highest adsorption capacity of 3.7 mmol/g with 0.24 wt % nitrogen. Biochars sourced from residues demonstrated high efficiency of carbon dioxide removal, comparable to some synthesized adsorbents reported in the literature.