Three-dimensional hierarchical pore biochar prepared from soybean protein and its excellent Cr(VI) adsorption

Biochar is used as adsorption material for transition metal ions based on its special high surface area. However, the ion adsorption capacity is limited by the inability through the increasing of specific surface area. Three-dimensional hierarchical pore biochar (ASPB) is prepared by pre-carbonization and KOH activation using soybean protein with gelation properties to construct a three-dimensional framework structure of biomass. The saturation level of Cr(VI) adsorption in 250 mg·L−1 Cr(VI) solution is 489.66 mg·g−1, which is attributed to the ASPB's vast 2788 m2·g−1 surface area and three-dimensional hierarchical pore structure, with pore structure data indicating an empty volume of 1.65 cm3·g−1 and a 2.386 nm size average pore size. The Cr(VI) adsorption level is higher than the calculated saturated capacity according to Langmuir equation based on the specific surface area. The extra adsorption performance is contributed to the transport channels provided by the three-dimensional framework structure, the large and abundant ion riveting sites provided by micro-mesopores, and the redox and complexation of surface-active groups. The saturated adsorption capacity can keep 86.6 % after three adsorption–desorption cycles.