Understanding how carbon dioxide interacts with porous materials is crucial for developing low-cost carbon capture technologies. Biochar, a carbon-rich material produced by heating organic waste in oxygen-limited environments, is widely recognized as a sustainable candidate for carbon removal. Traditionally, scientists believed that tiny micropores did almost all the work of trapping carbon dioxide molecules, while medium mesopores and large macropores merely served as passive highways to transport the gas deeper into the material. Research published in the journal Biochar by author Harn Wei Kua re-evaluates this assumption, revealing that larger pores actively contribute to trapping carbon dioxide by using their rough internal surfaces to slow down gas movement and promote surface binding.

To better understand these physical interactions, the study developed improved mathematical models to calculate the fractal dimensions, or structural roughness, of pores across different sizes. Conventional models often produced mathematically impossible or underestimated values because they relied on overly simplified geometric assumptions or ignored the complex surface behavior of gas adsorption. By incorporating dynamic contact angles for larger pores and molecular cross-sections based on classical adsorption theories for smaller pores, the new formulas yielded physically realistic fractal dimensions ranging between 2.81 and 3.00. These refined calculations allowed for a much more precise evaluation of how surface complexity influences gas retention.

The experiment evaluated teak wood sawdust alongside biochar produced across four distinct production temperatures: 300, 500, 700, and 1000 degrees Celsius. As the manufacturing temperature increased, the total carbon dioxide capture capacity grew significantly, rising from 0.20 millimoles per gram in raw sawdust to 3.82 millimoles per gram in the 1000-degree biochar. Higher production temperatures caused the smaller micropores to increase in volume and surface area, providing abundant binding sites for gas molecules. For larger mesopores and macropores, increasing heat caused neighboring pores to expand and merge together, increasing total volume while reducing total surface area through pore coalescence.

Interestingly, the study revealed that gas permeability through the larger pore network does not increase smoothly with higher production temperatures. When the material was heated to 700 degrees Celsius, structural flaking created internal wall projections, or in-foldings, that physically blocked passage through the pore network, causing permeability to drop sharply despite increases in overall volume and porosity. At 1000 degrees Celsius, further heating burned off these internal obstructions, restoring high permeability. Statistical regression analyses demonstrated that carbon dioxide capture correlates more strongly with total pore volume and overall porosity than with permeability alone, proving that structural roughness and internal volume govern gas retention far more than simple gas flow speed.

Source: Kua, H. W. (2026). Ascertaining the role of mesopores and macropores in capturing carbon dioxide in multi-hierarchical biochar sorbent: A theoretical and experimental approach. Biochar, 8, Article 33.