Ultralight biomass-based carbon aerogel with hierarchical pore structure fabricated using unidirectional freeze casting and potassium hydroxide activation

• The scheme provides a new idea for high-quality transformation of straw waste. • The BCA-H has a good hierarchical pore structure. • Freeze casting is combined with KOH activation method to enrich pore structure. • Ultra-low cost of the BCA-H is good for commercial development. • The BCA-H has excellent cyclic adsorption stability. A low-cost biomass carbon aerogel with a hierarchical pore structure (BCA-H) was prepared from straw using unidirectional freeze casting and potassium hydroxide (KOH) activation. The morphology, microstructure, and properties of the material were investigated. The hierarchical pore structure is a three-dimensional structure consisting of micropores, mesopores, and macropores. The material exhibits an ultra-low density (0.038 g/cm 3 ), as well as a high porosity (98.2 %) and specific surface area (1531 m 2 /g). After hydrophobic modification, the contact angle of the sample with water was 169.5°, and its adsorption capacities for acetone and rapeseed oil were 29 and 33 times its mass, respectively. The material has promising prospects in adsorption technology, such as for organic solvent adsorption and oil–water separation.