Activating cellulose with filtrate from washing active carbon of previous round of activation for maximizing utilization efficiency of activators and minimizing liquid waste

Activation of biomass with K 2 C 2 O 4 or KOH requires post-treatment of resulting activated carbon (AC) via acid (i.e. HCl) pickling to leach potassium derivatives (i.e. K 2 CO 3 and K 2 O) in form of KCl, which generates liquid waste. Water washing offers an alternative route to recover K 2 CO 3 and K 2 O, which can serve as activators in second-round activation, maximizing utilization efficiency of activator while minimizing formation of liquid waste. Such a hypothesis was verified herein by conducting activation of cellulose with K 2 C 2 O 4 or KOH, recovering potassium derivatives in filtrate via water washing and performing second-round activation of cellulose with the filtrate. The results showed that recycled K 2 CO 3 exhibited higher capability than K 2 C 2 O 4 for generating pores, forming AC with comparable specific surfacer area (1094.9 versus 1057.9 m 2 /g from K 2 C 2 O 4 activation), larger pore diameters (2.2 versus 1.9 nm) and special hollow honeycomblike surface structure. This was attributed to superior activity of the recycled K 2 CO 3 for gasification/cracking reactions. The recycled K 2 CO 3 from first-round KOH activation delivered AC with improved pore development (1168.8 m 2 /g) compared with AC directly prepared by fresh KOH activation (729.1 m 2 /g). Inserting K into the carbon skeleton structures during activation with KOH blocked partially pores of resulting AC, while the activation with recycled K 2 CO 3 did not. The life cycle assessment showed that the use of filtrate via water washing of AC for next-round activation could effectively diminish environmental impact.