Influence of activated carbon surface acidity on adsorption of heavy metal ions and aromatics from aqueous solution

Adsorption of toxic heavy metal ions and aromatic compounds onto activated carbons of various amount of surface C–O complexes were examined to study the optimum surface conditions for adsorption in aqueous phase. Cadmium(II) and zinc(II) were used as heavy metal ions, and phenol and nitrobenzene as aromatic compounds, respectively. Activated carbon was de-ashed followed by oxidation with nitric acid, and then it was stepwise out-gassed in helium flow up to 1273 K to gradually remove C–O complexes introduced by the oxidation. The oxidized activated carbon exhibited superior adsorption for heavy metal ions but poor performance for aromatic compounds. Both heavy metal ions and aromatics can be removed to much extent by the out-gassed activated carbon at 1273 K. Removing C–O complexes, the adsorption mechanisms would be switched from ion exchange to Cp-cation interaction for the heavy metals adsorption, and from some kind of oxygen-aromatics interaction to p–p dispersion for the aromatics.