Enhanced CO2 adsorption of alkali-modified sludge biochar via hydrothermal activation: Mechanisms and kinetics

With the aggravation of global climate change, developing efficient CO 2 capture materials has become critical for mitigating climate issues. This study modified sludge biochar (SAC-KOH) via hydrothermal activation (HT) to enhance its CO 2 adsorption performance. Optimized by one-way and orthogonal experiments, the sample 3-SAC-KOH-HT prepared under conditions of 80 °C hydrothermal temperature, 24 h hydrothermal time, 1:1 raw material-alkali ratio, and 24 h aging time achieved a CO 2 adsorption capacity of 79 mg/g, 108% higher than the unmodified biochar, providing guidance for parameter optimization.Further analysis revealed that hydrothermal activation induced in-situ formation of nanoscale silica-aluminate clusters (characterized by XRD and TEM), which acted as "pore-expanding templates" to inhibit pore collapse, constructing a hierarchical structure dominated by micropores (2-5 nm, for CO₂ trapping) and mesopores (10-20 nm, for mass transfer acceleration). Simultaneously, it drove surface functional group reconstruction, introducing more hydroxyl (O-H) and carboxylate (COO - ) groups (forming chemisorption sites via dipole-quadrupole interactions) and alkali-metal active sites (K + , enhancing adsorption by polarizing CO 2 ), collectively boosting chemical adsorption capacity.Kinetic studies showed the adsorption process followed the Avrami model, involving sequential multilevel synergism: initial rapid physical adsorption (micropore filling) and rate-limiting chemical adsorption (functional group bonding). After 10 adsorption-desorption cycles, 3-SAC-KOH-HT retained 97% of its initial capacity (only 3% decay), attributing to silica-aluminate-anchored stable pores and chemically stable surface groups.This study elucidates the enhancement mechanism from "pore structure-surface chemistry-adsorption kinetics," highlighting the critical role of nanosilica-aluminate-mediated hierarchical pores and synergistic functional sites, providing mechanistic support for sustainable carbon capture materials and advancing sludge resource.