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 coprecipi tation 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 me tal 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 mod eled. Simple competition between metal ions for surface sites can acco unt for the results obtained in the simple systems involving Ni(II) an d 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 exp lain the results obtained. Two different mechanisms are discussed whic h could account for this enhancement. Firstly, in the presence of Cr(I II), the surface chemical characteristics of the adsorbent surface cha nge as the various metal ions adsorb or coprecipitate, and this influe nces the removal characteristics of subsequently adsorbing ions. Secon dly, a mixed hydroxide species forms in solution and this species is s trongly adsorbed.