Preparation and characterization of a hierarchical porous char from sewage sludge with superior adsorption capacity for toluene by a new two-step pore-fabricating process

[Display omitted] •Sludge-derived char is prepared by pyrolysis at 500°C.•Hierarchical pores are fabricated by coupling citric acid and ZnCl2 activation.•Sludge-derived char performs hydrophobic surface.•Char has superior adsorption ability to toluene for pore and partition adsorption. A kind of hierarchical porous char (SCCA/Zn) was prepared from sewage sludge by a new two-step pore-fabricating process coupling citric acid (CA) with ZnCl2 in a pyrolysis process. The char was characterized by element analysis, N2-adsorption and mercury intrusion measurement etc. It is found that coupling CA and ZnCl2 can synergistically fabricate pores in the pyrolysis process, resulting in a hierarchical porous char, SCCA/Zn, with the largest SBET of 867.6m2 g−1 due to the fact that the former contributes to the fabrication of macro-pores, which provides more space for fabricating meso- and micro-pores by ZnCl2 activation. Although the SBET of SCCA/Zn was 15% less than that of activated carbon fiber (ACF, SBET=999.5m2 g−1), SCCA/Zn had a higher toluene adsorption capacity (0.83gg−1) than ACF. The inconsistence between their SBET and adsorption capacity can be ascribed to the strong hydrophobic property of SCCA/Zn.