ADSORPTION AND COPRECIPITATION OF MULTIPLE HEAVY-METAL IONS ONTO THE HYDRATED OXIDES OF IRON AND CHROMIUM

The removal of metal ions from solution using adsorption and coprecipitation techniques in the presence of more than one metal ion has been measured and modeled. The removal of all combinations of aqueous metal ions, Cr(III), Ni(H), and Zn(II), using amorphous iron(III) oxide has been measured and then modeled using the James-Healy method1-3 for metal ion adsorption. Similarly, the removal of aqueous Ni(II) and Zn(II), both singly and in combination, using amorphous chromium(III) oxide as the adsorbing or coprecipitating colloid has been measured and modeled. Simple competition between metal ions for surface sites can account for the results obtained in the simple systems involving Ni(II) and Zn(II) as adsorbing ions. The presence of Cr(III) in adsorption and coprecipitation experiments, however, enhanced the removal of both Ni(II) and Zn(II), and so simple competition for surface sites cannot explain the results obtained. Two different mechanisms are discussed which could account for this enhancement. Firstly, in the presence of Cr(III), the surface chemical characteristics of the adsorbent surface change as the various metal ions adsorb or coprecipitate, and this influences the removal characteristics of subsequently adsorbing ions. Secondly, a mixed hydroxide species forms in solution and this species is strongly adsorbed.