In a recent study published in Energy & Fuels, José A. O. Chagas, Bianca P. Pinto, Ana Lúcia de Lima, and Claudio J. A. Mota explored the development of highly efficient chitosan-based biochars for CO₂ capture. Addressing the urgent global need to reduce carbon emissions, their research offers a promising alternative to traditional, energy-intensive CO₂ absorption technologies like organic amines, which often come with high operational costs and environmental hazards. Solid sorbents are gaining traction due to their lower energy requirements and prolonged stability.

The researchers synthesized chitosan biochars through two distinct pathways. The first involved direct calcination of chitosan with K₂CO₃, producing CHIT-CO3-XXX materials (where XXX denotes the calcination temperature). The second pathway began with hydrothermal carbonization (HTC) of chitosan, followed by calcination with K₂CO₃, yielding HTC48-CO3-XXX materials. Chitosan, a biopolymer derived from the seafood industry, is an abundant and sustainable precursor.

Characterization of the biochars revealed significant differences in properties based on the preparation method and calcination temperature. Generally, biochars obtained from direct chitosan carbonization had lower BET surface areas and nitrogen content compared to those derived from HTC chitosan at the same calcination temperature. However, the CHIT-CO3-700 sample, prepared by direct carbonization at 700°C, exhibited the highest CO₂ uptake of 5.3 mmol·g⁻¹ at 25°C and 1 bar. In contrast, HTC48-CO3-600, derived from HTC chitosan and calcined at 600°C, showed a CO₂ uptake of 4.7 mmol·g⁻¹ under the same conditions. This indicates that while CHIT-CO3-700 boasts a higher overall CO₂ adsorption capacity, HTC48-CO3-600 maintains high efficiency.

The study highlighted the importance of a balance between textural properties (like surface area and microporosity) and nitrogen content for optimal CO₂ adsorption. For instance, despite having a lower BET area than HTC48-CO3-700, HTC48-CO3-600 showed a slightly higher CO₂ uptake, which was attributed to its twofold higher nitrogen content (5.6 wt%). This suggests that nitrogen functional groups play a crucial role by providing basic sites for CO₂ chemisorption, especially at lower partial pressures. Kinetic analysis indicated that the CO₂ adsorption profiles were better fitted by the pseudo-second-order kinetic model, implying that the interaction of CO₂ molecules with adsorption sites is a key factor. Isotherm studies showed a better fit with the Freundlich isotherm, consistent with the heterogeneous nature of the biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More surfaces. Thermodynamic analysis further indicated that the CO₂ adsorption process was exothermic, meaning it releases heat, and became more effective at lower temperatures.

A significant finding was the selectivity for CO₂ over N₂. While both materials were more selective for CO₂ than N₂, HTC48-CO3-600 exhibited a selectivity factor of 10, significantly higher than CHIT-CO3-700’s 1.7. This superior selectivity of HTC48-CO3-600 is linked to its higher total basicity, suggesting more chemisorption sites available for CO₂ capture, which is particularly vital for direct air capture (DAC) applications where CO₂ concentrations are very low (420-430 ppm).In situ FTIR drift studies demonstrated that HTC48-CO3-600 could retain some adsorbed CO₂ even at 150°C. This thermal stability is essential for the long-term recyclability and commercial viability of these adsorbents in multi-cycle adsorption-desorption processes. The intermediate calcination step at 300°C during preparation also seemed crucial for preserving more nitrogen atoms on the HTC48-CO3-600 biochar, contributing to its enhanced properties for CO₂ chemisorption.

The study concludes that high-efficiency chitosan biochars can be effectively prepared through chemical activation and calcination of HTC chitosan. This approach, along with an intermediate calcination step, leads to superior basicity and preserved nitrogen content, critical for selective CO₂ chemisorption. These chitosan biochars present a sustainable, cost-effective, and highly efficient solution for global carbon capture efforts.

Source: Chagas, J. A. O., Pinto, B. P., Lima, A. L., & Mota, C. J. A. (2025). CO₂ Capture over Chitosan Biochars: Tailoring the Properties for Highly Efficient Adsorbents. Energy & Fuels.