K-Fe based biochar granules prepared by one-step pyrolysis of herb-residue for stable CO2 capture: the importance of hydrophobic effect

Developing efficient, stable adsorbents for post-combustion CO 2 capture remains challenging, owing to competitive H 2 O adsorption, active component loss, and inactive byproduct formation. In this work, we report a one-step pyrolysis-activation strategy synthesizing herb-residue biochar co-anchored with K 2 CO 3 and K 2 FeO 4 (HPPF), enabling stable CO 2 capture under humid conditions via synergistic K Fe interactions. Compared with conventional post-impregnation methods, this approach enhances microporosity through molten-salt activation and reduces K vaporization by 56 % via Fe-O-K interfacial bonding, while disrupts K Ca interactions to prevent K 2 Ca(CO 3 ) 2 formation. The HPPF adsorbent exhibits a superior CO 2 capacity of 2.20 mmol g −1 at 60 °C under humid conditions (20 % CO 2 ), outperforming K 2 CO 3 -loaded biochar (HPC, 1.57 mmol g −1 ) and demonstrating a balanced H 2 O/CO 2 co-adsorption ratio (0.96, compared with 3.21 for HPC). In situ FTIR/XRD analyses reveal Fe accelerats the direct conversion from K 2 CO 3 to KHCO 3 , without formation of the hydrated intermediates. Furthermore, multi-model kinetic fitting confirmed Fe’s dual role in enhancing surface reactivity and modifying diffusion behavior. HPPF retained 99.1 % of its initial capacity after over nine adsorption-desorption cycles, demonstrating exceptional structural reversibility and cyclic durability. The estimated raw material cost (~$3.79 per kg CO 2 ) further supports its practical viability for scalable, low-cost carbon capture. This stable cycling capability of HPPF allows its incorporation into delayed feedback carbon cycles. In such systems, biochar accumulated during pyrolysis batch processes can progressively offset the CO 2 emissions generated by the biomass, offering a viable route for carbon-negative utilization.