The preparation of AC with KOH was according to the method
reported for corn cob [23]. In fact, the AC was prepared in a multistep activation process with H3PO4 and KOH. In the first step, the
activation process was done with H3PO4 as described above. In the
second activation step, the resulting char was mixed for 1 h with
KOH solution, using KOH:C weight ratio of 3.5. After that, the slurry
was dried at 70 8C for 24 h. The dried sample was then pyrolyzed in
a ceramic tube furnace at temperature of 900 8C, holding time of
1 h and heating rate of 5 8C/min. The obtained AC was washed with
hot and cold distilled water until pH of 6–7 was attained. Finally, it
was dried at 70 8C and labeled HK