Walnut shell is a very potential biochar precursor because of its wide source, low cost, and easy structure modification. In this paper, the co-activation method of FeCl₃, ZnCl₂ and H₂O(g) was adopted to prepare walnut shell-based biochar with high microporosity and the effect of pore structure on CO₂ adsorption performance at different temperatures was investigated. The prepared biochar had a larger specific surface area (2647.8 m² g⁻¹), satisfactory micropore area (2008.7 m² g⁻¹) and high total pore volume (2.58 cm³ g⁻¹). At 273 K and 298 K, its CO₂ adsorption capacity was 4.79 mmol g⁻¹ and 3.20 mmol g⁻¹, respectively. Particularly, CO₂ adsorbed uptake on biochar was strongly sensitive to their narrow micropore volume, instead of the total specific surface area, total pore volume, and micropore specific surface area. The optimal pore size beneficial for CO₂ adsorption was 0.33–0.82 nm at 273 K, but the optimal pore size was 0.33–0.39 nm at 298 K. It provides theoretical guidance for future material preparation and selection, and FeCl₃, ZnCl₂ and H₂O(g) may be effective biochar activators.

Biochar-based carbonaceous adsorbents are gaining interest due to their high availability, ease of modification, and low cost; however, they show limited adsorption of CO₂ in flue gas due to common textural properties. In this study, TEPA-modified biochar was used to prepare a solid amine adsorbent for the efficient capture of CO₂ in flue gas. First, the porous biochar was prepared with FeCl₃, Mg(NO₃)₂, and H₂O (g) as activators and walnut shells as carbon sources. Next, the biochar was modified with TEPA to obtain a solid amine adsorbent. Porous texture properties and sample surface functional groups were characterized, and we measured the adsorption CO₂ of the amine-modified biochar in a breakthrough adsorption device. Results showed that biochar has a large specific surface area (744.38 m² g⁻¹), a total pore volume of 1.41 cm³ g⁻¹, and a high mesoporous volume ratio (82.7%). The high pore volume provided a more efficient support space for loading tetraethylenepentamine (TEPA). The adsorbent had an excellent CO₂ adsorption capacity, corresponding to 2.82 mmol g⁻¹, which increased to 3.31 mmol g⁻¹ and kept water resistance at 10% H₂O (g) simulated flue gas (SFG). The FTIR analysis showed that H₂O (g) inhibited urea production after cyclic adsorption. Therefore, solid amine adsorbent created by amine-modified biochar has potential advantages in its application for capturing CO₂ in SFG.