Synergistic effects of calcium carbonate and alkali chlorides for activating biomasses for fabricating hierarchically pore structures for adsorbing phenol and methylene blue

This study develops an effective method for fabricating hierarchically porous activated carbons (ACs) from willow and poplar by employing KCl or NaCl modified CaCO 3 as activating agents. KCl or NaCl addition lowered energy barrier for decomposition of CaCO 3 by forming low-melting-point eutectics that disrupted dense CaO layer and facilitated CO 2 diffusion, maximizing utilization efficiency of CO 2 for etching carbon skeleton for creating micropores and CaO as hard template for generating mesoporosity. Such synergistic effects formed AC from activation of willow by CaCO 3 /KCl with more than doubled S BET (1352.2 versus 569.5 m 2  g -1 for AC-willow-CaCO 3 ) and also more mesopores (43.1% versus 14.7%) for enhanced adsorption of methylene blue. Such a result also applied to activation of poplar (1148.3 m 2  g -1 for AC-poplar- CaCO 3 /KCl versus 589.1 m 2  g −1 for AC-poplar- CaCO 3 ; mesopores: 12.6% versus 46.6%). CaCO 3 together with NaCl showed similar enhanced capability for creating more developed pore structures. Additionally, formation of low-melting eutectics between chloride salts and CaO resulted in enhanced salts permeability, volume expansion and localized stress, which together formed AC with wrapped ash and more fragmentated surface. In-situ IR characterization of activation processes confirmed activity of KCl or NaCl on cracking of aliphatic components and significantly intensified cracking/gasification together with CaCO 3 .