Tofukasu-derived biochar with interconnected and hierarchical pores for high efficient removal of Cr (VI)

Abstract Herein, we report the synthesis of interconnected hierarchical pore biochar (HTB) via an ice-templating strategy using bio-waste (tofukasu). The abundance of N- and O-containing functional groups in tofukasu makes it easy to form hydrogen bonds with water molecules and water clusters, resulting in nano-micro structures like ice clusters and snow crystals during freezing process. More importantly, tofukasu will be squeezed by micron-scale snow crystals to form coiled sheet-like structures, and its surface and interior will be affected by needle-like ice nanocrystals from several nanometers to tens of nanometers to form transverse groove needles and mesopores. The ice crystals are then removed by sublimation with tofukasu, leaving the interconnected pore structure intact. Therefore, the ice template synthesis strategy endowed the interconnected hierarchical pore structure of HTB with a large specific surface area (S BET , 733 m 2 ⋅g −1 ) and hierarchical porosity (30.30% for mesopores/total pore volume ratio), which is significantly higher than the normal dry treated tofukasu biochar (TB), which had a S BET of 436 m 2 ⋅g −1 and contained 1.53% mesopores. In addition, the sheet-like structure with interconnected pores of HTB favors high exposure of active sites (N- and O-containing functional groups), and a fast electron transport rate. As a result, HTB had an excellent adsorption capacity of 159.65 mg⋅g −1 , which is 4.7 times that of typical block biochar of TB (33.89 mg⋅g −1 ) according to Langmuir model. Electrochemical characterization, FTIR and XPS analysis showed that the mechanism of Cr(VI) removal by HTB included electrostatic attraction, pore filling, reduction and surface complexation. Graphical Abstract