Biochar's bigger pores aren't just passageways - they actively help capture CO2

Biochar captures carbon dioxide. That much has been established. The material - produced by heating biomass like wood waste in a low-oxygen environment - locks carbon into a stable solid form and can adsorb additional CO2 from the atmosphere. But which part of biochar's intricate internal structure does the capturing has been a point of assumption rather than evidence. A new study challenges that assumption directly.

The micropore orthodoxy

Biochar is riddled with pores spanning several orders of magnitude in size. Micropores, less than about one nanometer in diameter, have long been considered the primary sites for carbon dioxide adsorption. Their tiny dimensions create strong physical forces that trap gas molecules. Larger pores - mesopores (2-50 nm) and macropores (above 50 nm) - have been treated as passive infrastructure: channels that allow CO2 molecules to diffuse inward toward the micropores where the real work happens.

The new research, published in the journal Biochar, revisits this hierarchy. Using improved mathematical models to describe the complex surfaces inside biochar pores, combined with experimental measurements on biochar produced at temperatures from 300 to 1,000 degrees Celsius, the study demonstrates that the geometry and surface roughness of larger pores directly influence how CO2 molecules interact with the material.

Temperature, pores, and capture capacity

The experiments used sawdust as feedstock, producing biochar at a range of pyrolysis temperatures. Carbon dioxide capture increased dramatically with temperature: biochar produced at 1,000 degrees Celsius captured up to 3.82 millimoles of CO2 per gram, compared to 1.26 millimoles per gram at 300 degrees Celsius - roughly a threefold increase.

Higher temperatures produce biochar with more developed pore structures. Micropore volume and surface area both increase, and the correlation between these properties and CO2 capture was strong. But the study also found significant correlations between capture performance and the fractal surface geometry of mesopores and macropores - a measure of surface complexity and roughness.

Microscopic imaging revealed that high-temperature biochar develops pore surfaces with folds, ridges, and irregular structures. These features are not merely decorative. They appear to slow the movement of CO2 molecules through the material, increasing the time molecules spend in contact with surfaces and raising the probability of physical adsorption.

Passageways that participate

The distinction matters for practical applications. If only micropores contribute to CO2 capture, then optimizing biochar means maximizing micropore volume - and nothing else. But if mesopores and macropores also participate, then the entire pore hierarchy becomes a design variable.

The study suggests that biochar engineered with the right combination of micropores for primary adsorption and roughened larger pores for molecular trapping and transport optimization could outperform biochar designed around micropores alone. The author describes this as optimizing the "full pore hierarchy" - a shift from a single-variable optimization to a multi-scale engineering problem.

Scaling and limitations

The study combines theoretical modeling with experimental validation, but the experiments used a single feedstock type (sawdust) and tested adsorption under controlled laboratory conditions. Real-world carbon capture applications involve variable gas mixtures, humidity, temperature fluctuations, and long-term cycling that can degrade sorbent performance. Whether the mesopore contribution observed in the lab persists under operational conditions is not addressed.

The theoretical models also involve assumptions about pore geometry that simplify a chaotic reality. Biochar's internal structure is heterogeneous and difficult to characterize fully. The fractal analysis provides a useful approximation but is not a complete description.

Still, the finding has practical value. As countries seek scalable, affordable approaches to atmospheric carbon removal, biomass-derived materials like biochar offer advantages: they are cheap, made from waste, and can serve multiple functions (soil amendment, water treatment, carbon sequestration). Understanding how their internal architecture affects performance is essential for engineering better versions. The larger pores, it turns out, are not just plumbing. They are part of the machine.