Solid materials that trap CO2 on their surfaces are attracting attention as potentially efficient, reusable tools for reducing emissions. Biochar, a carbon-rich material produced by heating biomass without oxygen, offers an alternative to more complex adsorbents. Marine algae provide an abundant feedstock with little competition for agricultural land. However, seaweed-derived biochars often have poorly developed pores and surface chemistry that limits CO2 uptake. Higher processing temperatures can open additional pores but also destroy chemical groups that help attract CO2, creating a challenge: how to improve the physical structure while preserving useful surface features.
A study (DOI: 10.48130/bchax-0026-0019) published in Biochar X on 27 July 2026 by Lina Liu's team, Taiyuan University of Science and Technology, demonstrates that potassium hydroxide treatment before pyrolysis at 400 °C combines extensive microporosity with abundant surface hydroxyl groups to improve CO2 adsorption capacity, capture speed, and reuse stability.
The researchers washed, dried, and ground Sargassum tenerrimum, then heated the powder under nitrogen at 400, 500, 600, or 700 °C for two hours. To investigate treatment sequence, they compared biomass mixed with potassium hydroxide before heating at 400 °C against biochar treated after an initial heating step. Electron microscopy and gas adsorption measurements examined morphology, surface area, and pore size, while infrared spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed CO2 desorption probed surface chemistry and adsorption interactions. Thermogravimetric experiments measured CO2 uptake at 40 °C and one bar using dry, pure CO2. Repeated adsorption and regeneration steps tested durability, with regeneration performed under nitrogen at 120 °C. The results showed that increasing the temperature alone improved surface area but did not consistently improve capture. Untreated biochar prepared at 400 °C adsorbed 24.1 milligrams of CO2 per gram, whereas samples prepared at 500, 600, and 700 °C captured 20.2, 10.9, and 16.5 milligrams per gram, respectively. Applying potassium hydroxide after initial carbonization increased uptake to 40.0 milligrams per gram.
Treatment before carbonization produced the best-performing material, Sar-KOH, with uptake reaching 120.5 milligrams per gram. Its specific surface area rose to 569.66 square meters per gram, compared with 1.14 for untreated biochar prepared at 400 °C. Pore analysis revealed abundant tiny cavities, including a prominent pore width near 0.6 nanometers. These confined spaces favor physical adsorption, while spectroscopy supported an additional contribution from hydroxyl groups interacting with CO2 through hydrogen bonding. Sar-KOH reached adsorption equilibrium in approximately 11 minutes, compared with 26-28 minutes for untreated samples. After nine cycles, its capacity declined by only 1.1%, indicating strong laboratory reuse performance. Together, the measurements link improved capture to the combination of accessible micropores and favorable surface chemistry, rather than surface area alone, and show why the order of chemical treatment matters for this seaweed feedstock.
The study establishes a practical design principle for algal biochar: develop pores while retaining chemical groups that assist capture. Its laboratory results support further investigation of Sargassum as a resource for reusable adsorbents. Before practical deployment, researchers will need to test performance under humid conditions and in gas mixtures, assess selectivity, and shape the powder into durable particles or structures suitable for reactors. These steps will clarify whether the material's promising capacity and regeneration stability translate into effective carbon capture beyond laboratory conditions.