Enhanced CO2 adsorption via one-step thermochemical activation of Alfa fiber.

In this work, Moroccan Alfa fibers were converted into porous biochars through a one-step thermochemical activation using KOH, ZnCl2, and melamine at impregnation ratios of 0.5, 1.0, and 1.5. The influence of the activating agents on the physicochemical properties and CO2 adsorption behavior of the resulting materials was systematically investigated using structural and surface characterization techniques. Chemical activation substantially enhanced pore development compared with the non-activated biochar, with KOH producing the most developed hierarchical micro-mesoporous structure. Among all samples, Af-K1 exhibited the highest CO2 adsorption capacity, reaching 3.70 mmol g-1 at 30 °C and 1 bar, which increased to 5.26 mmol g-1 at 0 °C. This superior performance was primarily attributed to its larger micropore volume and optimized pore-size distribution. Af-K1 also demonstrated excellent regeneration stability, retaining approximately 99% of its initial adsorption capacity after ten adsorption-desorption cycles. Furthermore, the material exhibited an IAST-predicted CO2/N2 selectivity of 58 for a simulated flue gas mixture (15% CO2), while model extrapolation suggested a selectivity of 24-41 under direct air capture (DAC) conditions (400 ppm CO2). These predicted DAC values should be regarded as indicative and require experimental validation under low-pressure and mixed-gas conditions. Thermodynamic analysis indicated that CO2 adsorption is spontaneous and exothermic, with ΔH° = -13.19 kJ mol-1 and ΔS° = -0.041 kJ mol-1 K-1, while ΔG° remained negative (-2.11 to -0.28 kJ mol-1) over the investigated temperature range, confirming the thermodynamic favorability of the adsorption process. These findings demonstrate that one-step KOH activation provides an efficient and sustainable strategy for producing high-performance biochar adsorbents for CO2 capture and separation.