Isotherms and kinetics of CO₂ adsorption on biochar-based activated carbon for sustainable climate solutions

Abstract

Increasing levels of atmospheric CO₂ are the main cause of human-induced climate change, leading to more frequent extreme weather occurrences, rising sea levels, and a decrease in biodiversity. In this light, creating effective and economical sorbents for CO₂ capture is critically important. This research involved a thorough assessment of CO₂ adsorption using KOH-activated biochars produced from pine and birch. The biomass feedstocks were carbonized at temperatures of 600 °C, 700 °C, or 800 °C and treated with KOH in a 3:1 impregnation ratio. Detailed characterization through Fourier-transform infrared spectroscopy (FTIR), N₂ physisorption (Brunauer–Emmett–Teller), Raman spectroscopy, Elemental analysis (CHNS), and scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX) indicated that higher carbonization temperatures enhance graphitic organization, preserve a substantial number of oxygen-containing functional groups, and create primarily microporous frameworks with specific surface areas. For pine-derived activated carbon, reached a maximum of 1416 m 2 /g at 800 °C, while birch-derived material achieved 1398 m 2 /g at the same temperature. The equilibrium CO₂ adsorption isotherms measured at 1 bar indicated maximum capacities of 3.5 mmol/g for pine biochar and 3.2 mmol/g for birch biochar that was carbonized at 800 °C. The isotherm data is well described by the Langmuir model, implying a high affinity for CO₂ and notable monolayer capacities. Time-resolved uptake experiments demonstrate that over 90 % of the equilibrium capacity is achieved within five minutes, with characteristic half-times decreasing from approximately 1.5 min for materials carbonized at 600 °C to less than 0.8 min for those treated at 800 °C. The adsorption kinetics conform to a pseudo-first-order model, corresponding with surface-controlled physisorption behavior. The results indicate that activating biochar via KOH at elevated temperatures produces affordable, high-surface-area biochar that can rapidly and efficiently adsorb CO₂. The differences between pine and birch, compared to the commercial coconut shell-based activated carbon, highlight the impact of precursor selection on the performance of sorbents. Collectively, these biochars have significant potential as effective, locally sourced from forestry residues, and therefore more sustainable options for carbon sequestration technologies.

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