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.