Role of defect-rich architectures in CO₂ capture ability of upcycled railway tie biochar

Spent hardwood railway ties are a high carbon(C) content biomass resource; however, their reuse has been limited due to polycyclic aromatic hydrocarbons (PAHs) contamination originating from creosote treatment. In this study, spent railway ties were upcycled into a high-performance CO₂ adsorbent via sequential microwave pyrolysis followed by K₂CO₃ activation. Microwave pyrolysis effectively removed PAHs (99.5% within 30 min) and produced stabilized biochar suitable for subsequent activation. Subsequent K₂CO₃ activation under N₂ at temperatures above 800 °C substantially increased the surface area and developed ultra-micropores (<1 nm), yielding a surface area of 772.79 m²/g. Increasing the K₂CO₃ activation temperature also intensified disordered graphitic carbon defect formation in the graphene-like carbon framework, as evidenced by an increase in the I_D/I_G ratio from 0.72 to 1.01 (200-800 °C) and XRD analysis. Accordingly, CO₂ uptake increased with activation temperature up to 800 °C and then approached a plateau, with a maximum capacity of 124.83 mg/g for the biochar activated at 800 °C. Mechanism study revealed that expanded surface area, crystallographic structural disorders and defect density contributed to CO₂ capture. CO₂ adsorption kinetics and thermodynamics of the 800 °C-activated biochar (A-800-RT) were investigated to elucidate the adsorption behavior. A-800-RT was successfully regenerated by N₂ treatment at 120 °C for 3 h without loss of CO₂ adsorption capacity or changes in chemical structure. Finally, a continuous CO₂ adsorption test using simulated exhaust gas demonstrated an excellent CO₂ uptake of 92.4 mg/g, indicating the high practical feasibility of the synthesized adsorbent.