Development of a hybrid material based on olive pomace biochar functionalized with 2-[2-hydroxyethyl (methyl) amino] ethanol chloride ionic liquid for enhanced CO2 capture

The development of efficient and low-cost sorbents for Direct Air Capture (DAC) of carbon dioxide (CO 2 ) is essential to mitigate anthropogenic greenhouse gas emissions. In this study, olive pomace-derived biochar was functionalized with the ionic liquid 2-[2-hydroxyethyl (methyl) amino] ethanol chloride (MDEACl) using an ultrasound-assisted impregnation method. The effects of ionic liquid loading, stirring time, ultrasound amplitude, duty cycle, and sonication time on adsorption performance were systemically investigated. Physicochemical characterization by elemental and proximate analysis, Brunauer–Emmett–Teller (BET), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) confirmed the successful incorporation of MDEACl onto the biochar surface while preserving the carbonaceous structure. Although functionalization and subsequent degassing procedures led to a reduction in the apparent specific surface area and pore volume due to partial pore occupation, the introduction of nitrogen-containing active sites significantly enhanced CO 2 adsorption. The optimum material was obtained with 1 wt% MDEACl and exhibited a CO 2 uptake of ∼ 53 mg CO 2 g −1 biochar under ambient air conditions (p CO2 = 0.00048 atm), reaching a maximum saturation capacity of ∼ 108.0 mg CO 2 g⁻ 1 biochar at higher equilibrium partial pressures (p CO2 = 0.001773 atm), compared with 17.7 mg CO 2 g⁻ 1 biochar for pristine biochar. Kinetic analyses showed that the adsorption process follows a hybrid mechanism involving rapid physisorption within the biochar pore network and reversible chemisorption through bicarbonate formation in the ionic liquid phase. The pseudo-second-order model provided the best fit to the experimental data (R 2 = 0.999). Regeneration studies demonstrated promising multicycle stability under the evaluated conditions, with substantial recovery of adsorption capacity at a moderate temperature of 80 °C, suggesting potential for low-energy thermal regeneration. X-ray photoelectron spectroscopy (XPS) analyses confirmed the reversible formation of bicarbonate-like species and the long-term stability of MDEACl on the biochar surface. These results highlight ultrasound-functionalized biochar/MDEACl as a promising and energy-efficient adsorbent for DAC applications.