The BET model calculated that the specific surface areas of APBCs and BCs were 935 m2·g−1 and 443 m2·g−1 respectively, the total pore volumes were 0.74 cm3/g and 0.29 cm3/g respectively, and the average pore diameters were 3.15 nm and 2.65 nm respectively. The average pore diameters of micropores and mesopores of BCs are 0.97 nm and 5.76 nm, respectively. The average pore diameters of micropores and mesopores of APBCs are 1.29 nm and 15.54 nm, respectively. Evidently, the activated biochars had higher specific surface area, total pore volume and average pore size, which further exhibited that the co-activators KOH and Ca(OH)2 caused more defects in the carbon materials during the calcination process.(Bi et al., 2021) The pore size distribution (Fig. 2d) proved the hierarchical porous structure of APBCs. The micropore diameters of APBCs were concentrated at 0.7, 1.1 and 1.3 nm, and the mesopore diameters ranged from 8 nm to 10 nm and 20–50 nm. While, the micropores of BCs were mainly concentrated at 0.7 and 1 nm, and the mesopore diameter ranged from 2 nm to 16 nm. The generation of these microporous and mesoporous structures produced during the activation of APBCs should be attributed to surface damage of APBCs by co-activators under high temperature, which can provide more active sites and more storage space for the adsorbates (Alvin et al., 2020).