| Abstract: |
The adsorption of metal−EDTA complexes onto various oxides (aluminium oxides, crystalline and amorphous iron oxides) was described by the surface complexation model. Data from the literature and from our experiments with several oxides were analyzed by assuming only one type of ternary surface complex for Me2+−EDTA complexes. The surface complexes exhibit an anionic character with decreasing adsorption with increasing pH. This proposed model allows us to describe all available data sets very well. The adsorption of NiEDTA was interpreted by several models (constant capacitance, diffuse layer) onto different oxides (with different pKa values). Log K values within one model agree very well with each other. The pH, where 50% of the complex is adsorbed, increases in the series HFO < lepidocrocite < γ-Al2O3 < goethite < δ-Al2O3 from 6.5 to 8.35 (for 1 μM NiEDTA and 1 mM surface groups). Adsorption of Fe(III)EDTA onto several oxides was studied. Two types of surface complexes, anionic and cationic, were necessary to explain the observed adsorption edge. Fe(III)EDTA is adsorbed over a wide pH range and is the major adsorbed EDTA species at pH greater than 7. |
