Preparation of activated carbon with recycled ZnCl2 for maximizing utilization efficiency of the activating agent and minimizing generation of liquid waste

[Display omitted] •Recycled ZnCl2 from pickling activated carbon (AC) is used for second-round activation.•Activation with recycled ZnCl2 increases AC yield via intensifying condensation reactions.•Recycled ZnCl2 catalyzes polymerization of intermediates bearing –OH, −C–H and CO.•Recycled ZnCl2 promotes formation of AC of mesoporous via merge of micropores.•Use of recycled ZnCl2 reduces environmental impact from minimized discharge of liquid waste. Post-treatment of activated carbon (AC), produced from activation with ZnCl2 via leaching AC with HCl, generates liquid waste (filtrate) containing ZnCl2 through the reaction (ZnO + 2HCl → ZnCl2 + H2O). The filtrate, in fact, might be reused for activating biomass again, as the recycled ZnCl2 is the same to fresh ones in composition. This minimizes discharge of liquid waste while enhances utilization efficiency of ZnCl2. Herein, the activation of lotus root and cotton fiber with recycled ZnCl2 from filtrate of first-round activation was conducted, aiming to assess feasibility of such a method for generating AC with minimized use of external ZnCl2. The results showed that activation with recycled ZnCl2 from filtrate was more effective for enhancing AC yield (48.7 % versus 33.5 % from activation with fresh ZnCl2) via promoting condensation of both sugar- and lignin-derivatives. The remarkable increase of AC yield was at expense of specific surface area (908.8 m2/g from activation with recycled ZnCl2 versus 1560 m2/g from fresh ZnCl2). Involvement of retained HCl and salts like KCl (originated from feedstock) from post-treatment changed reaction network in second-round activation, enhancing polymerization of aliphatic species with functionalities like –OH, −C–H and especially CO and merge of micropores to form more mesopores (19.0 % versus 8.5 %). Additionally, either fresh or recycled ZnCl2 could effectively induce structural re-construction of the ACs via generating flake-like or tangled fibrous structure via interparticle reactions. Nonetheless, the use of recycled ZnCl2 for activation exhibited lower environmental impact from least generation of liquid waste.