Effects of Strong Binding of Anionic Adsorbates on Adsorption of Trace Metals on Amorphous Iron Oxyhydroxide

Abstract

Adsorption of three metal ions (Cd²⁺, Co^2+, Zn²⁺⁾ onto amorphous iron oxyhydroxide has been studied in the presence of several strongly binding anionic adsorbates (SeO ^2-₄ , SeO ^2-₃ , AsO ^3-₄ , AsO ^3-₃ , CrO ^2-₄ , PO ^-3₄ ). The anions either increase or have no effect on trace metal adsorption, indicating that competition between the anions and cations for surface sites is relatively minor. The results cannot be explained by electrostatic interactions unless the relative acidities of the anions are very different at the surface than in solution. The most likely cause of the enhanced metal adsorption in some systems is that a secondary surface phase forms, and the metals bind more strongly to the new phase than they do to the original substrate. The secondary phase appears to be an iron-anion solid rather than a trace metal-anion solid. In systems where a new surface does not form, electrostatic and competitive interactions are somehow suppressed, which may indicate that anion adsorption sites are physically and electrically isolated from cation adsorption sites.