In the present study, sewage sludge (SS) was applied for the preparation of activated carbons (ACs) by chemical activation with phosphoric acid (H3PO4) and acid (HCl, HF) and base (NaOH) washing as post-treatment steps were employed to eliminate the mineral matter and other impurity composition of sewage-sludge based ACs. The post-treatment methods under study for the AC include: (1) the as-prepared AC by chemical activation with H3PO4 (aAC) was soaked with sodium hydroxide solution (NaOH), (2) the as-carbonized SS (cAC) by chemical activation with H3PO4 was soaked with sodium hydroxide solution (NaOH), (3) the aAC refluxed with hydrochloric acid (HCl), washed with NaOH solution and then soaked with HCl, (4) the aAC refluxed with NaOH, washed with HCl and then soaked with NaOH, and (5) aAC refluxed with HCl and then autoclaved with hydrofluoric acid (HF). The resultants ACs were characterized by scanning electron microscopy (SEM), the standard Brunauer-Emmet-Teller method (BET), FT-IR spectroscopy and X-ray fluorescence (XRF). The results of FT-IR demonstrate that the properties of the post-treated final products are dependent on the method used and that it contains similar functional groups to those present in the untreated AC, but at a higher peak intensity. XRF results indicated that refluxing AC with HF removed the impurities and decreased the percentage composition of silicon dioxide. BET surface area and pore volume of the post-treated AC were about sixty times higher than that of aAC. The BET surface area rose from 5.58 to 511 m2 g−1 and pore volume increased around 13% when compared with that of the untreated AC sample. The five types of AC showed high Cr (VI) adsorption capacities, however, haAC adsorbed more Cr (VI) than AC.

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