Adsorption of EDTA and Metal–EDTA Complexes onto Goethite

The adsorption characteristics of a variety of divalent and trivalent metal-EDTA complexes onto goethite (alpha-FeOOH) were examined in aqueous solution. Uncomplexed EDTA is adsorbed as a binuclear complex at low pH and as a mononuclear complex at high pH. Adsorption is ligand-like with a high extent of adsorption at low pH. The process can be described by formation of inner-sphere complexes by the surface complexation model with constant capacitance. The EDTA complexes of the divalent metals Ca, Zn, Ni, Cu, Co(II), and Pb, which are quinquedentate in solution (free donor atoms bound to the metal ion), all showed the same ligand-like adsorption behavior. Their adsorption as a function of pH and concentration can be described by the formation of one type of ternary surface complex and can be fitted with the same equilibrium constant. Pd(II)EDTA, which is bidentate or quadridentate in solution, is adsorbed more strongly, but also in a ligand-like manner. The EDTA complexes of the trivalent metals LaEDTA and BiEDTA are adsorbed very strongly over the whole pH range. The sexidentate complex of Co(III) is weakly adsorbed at low pH outer-spherically, i.e., by electrostatic interaction only. Fe(III)EDTA is weakly adsorbed over the whole pH range with a predominant nonspecific surface complex at low pH and a specific complex at high pH.