Removal of heavy metals from aqueous solution by sewage sludge based sorbents: competitive effects

The removal of binary mixed metal ions [Hg(II), Pb(II), Cu(II) and Cr(III)] from water was investigated. Two different sewage sludge-based adsorbents were produced and used for this purpose: PS, obtained by pyrolysis of sewage sludge, and AS, by ZnCl2 chemical activation of sewage sludge followed by pyrolysis. The AS products showed a better development of surface and porosity (472 m2/g of BET surface, 0.05 and 0.22 cm3/g of micro- and mesopore volume, respectively) than the PS. Results on the adsorption of the above metal ions in competition were found and compared with single adsorption results. In competition, as with single systems, AS showed a higher adsorption capacity than PS. The amount of metal adsorbed, differed depending on the binary system. Both PS and AS equilibrium data were well suited to the Langmuir model and showed the capacity order: Hg(II)> Pb(II)> Cu(II)> Cr(III). Moreover, for both PS and AS, the Langmuir maximum adsorption capacity showed that each metal adsorption was diminished by the presence of a competitive metal ion. Once PS and AS were saturated in the binary systems, metal recovery by desorption showed that, except for Cu(II) desorption from AS, recuperation was quite low in all cases, especially for Pb(II).