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 aque ous solution. Uncomplexed EDTA is adsorbed as a binuclear complex at l ow pH and as a mononuclear complex at high pH. Adsorption is ligand-li ke with a high extent of adsorption at low pH. The process can be desc ribed by formation of inner-sphere complexes by the surface complexati on model with constant capacitance. The EDTA complexes of the divalent metals Ca, Zn, Ni, Cu, Co(II), and Pb, which are quinquedentate in so lution (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 ter nary surface complex and can be fitted with the same equilibrium const ant. Pd(II)EDTA, which is bidentate or quadridentate in solution, is a dsorbed more strongly, but also in a ligand-like manner. The EDTA comp lexes of the trivalent metals LaEDTA and BiEDTA are adsorbed very stro ngly over the whole pH range. The sexidentate complex of Co(III) is we akly adsorbed at low pH outer-spherically, i.e., by electrostatic inte raction only. Fe(III)EDTA is weakly adsorbed over the whole pH range w ith a predominant nonspecific surface complex at low pH and a specific complex at high pH. (C) 1996 Academic Press, Inc.