Surface ionization and complexation at the oxide/water interface II. Surface properties of amorphous iron oxyhydroxide and adsorption of metal ions

The site-binding model for the electrical double layer of hydrous oxides reported in a previous paper is applied to the adsorption of metal ions from dilute solution and to complex heterogeneous systems, i.e., amorphous iron oxyhydroxide. More than one stoichiometric surface reaction is usually needed to describe the adsorption behavior of dilute heavy metal ions. If mass law equations for surface reactions of metal ions are corrected for effects of the electrostatic field at the interface, the calculated adsorption density depends upon the type of surface species formed. It is shown that calculations with surface reactions involving hydrolytic complexes of metal ions, e.g., Pb(II), Cd(II), Cu(II), Ag(I), are more consistent with experimental adsorption data than complexation by bidentate surface sites. A table of intrinsic surface complexation constants for various metal ions and oxide substrates is presented. Similar to results reported earlier for major electrolyte ions, the stability constants of surface complexes of heavy metal ions with the silica surface are significantly less than for other oxide surfaces. Empirical surface parameters for model calculations with iron oxyhydroxide are derived and the results are compared with experimental adsorption data for Cu(II) and Ag(I).