Characterizing heavy-metal adsorption on oxides and oxyhydroxides

Adsorption of Cu(II), Zn(II), Co(II) and Cd(II) on three different substrates: goethite, alumina and silica, was investigated over the pH range from 3.5 to 10. The consistent variation in the pH at which 50% of the metal is adsorbed (PH50) indicates that the identity of the adsorbing ion is the principal factor determining the pH range over which adsorption occurs. However, the nature of the substrate does play a lesser role. The proton coefficient, chi, ranged from about 0.5 to 1.8, and was correlated with the difference between pH(50) and the pH for 50% precipitation. It is suggested that for most metal-substrate systems, for which chi is about 1, MOH(+) plays a dominant role in adsorption. Increasing the substrate concentration decreased PH50 for adsorption of metals on goethite, but there was little change in chi. The change in PH50 was inversely related to chi. While the effect of the nature and concentration of the background electrolyte on Cu(II), Zn(II) and Co(II) adsorption depended on the substrate, increased ionic strength was found to decrease Cd(II) adsorption on all substrates.