Dual-activation nanofluid with electrosterically stabilized lysine–CNT for enhanced CO₂ capture in K₂CO₃

Enhancing both the CO₂ absorption kinetics and colloidal stability of nanofluid-based carbonate capture systems remains a key challenge, particularly under industrially relevant high-alkaline conditions. In this study, we report a lysine-functionalized carbon nanotube nanofluid (Lys–CO–CNT) that simultaneously improves CO₂ mass transfer and dispersion stability in a 30 wt% potassium carbonate (K₂CO₃) solution. Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) revealed a brush-like lysine shell on the CNT surfaces, while X-ray photoelectron spectroscopy (XPS) confirmed a grafting density of 0.397 chains·nm⁻², approaching the dense brush regime. Fourier-transform infrared spectroscopy (FTIR) validated the formation of amide bonds and terminal –NH₂ groups, enabling carbamate-like CO₂ capture at the gas–liquid interface. The nanofluid exhibited excellent colloidal stability, with a sedimentation rate of 0.11 %·min⁻¹ over 120 min, which is nearly seven times lower than that of unmodified CNTs. In CO₂ absorption tests, the system achieved a peak CO₂ capture rate of 1.4 g CO₂ per 100 g solution per minute at 298.15 K, accompanied by a 34.7 % increase in the liquid film mass transfer coefficient and a surface renewal time reduction to 7.97 × 10⁻⁵ s. These enhancements result from a dual activation mechanism: (i) chemical activation via terminal amines and (ii) interfacial ion modulation by the –COOH/–COO⁻ shell, which enriches K⁺ and HCO₃⁻ at the gas–liquid boundary and promotes a dynamic bicarbonate-buffered interfacial layer.