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 ID/IG 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.