RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The adsorption characteristics of a variety of metal-EDTA complexes onto hydrous oxides, principally aluminum oxide (γ-Al₂O₃), were examined in aqueous solution. Adsorption of these complexes increased with increasing proton concentration due to the formation of surface complexes between EDTA and the surface hydroxo groups, specifically the AlOH₂⁺ surface groups. The pH-dependent adsorptive behavior and the magnitude of adsorption of the “free” EDTA species were similar to those of the metal complexes. The results also showed that the adsorption of “free” EDTA was exothermic, while the adsorption of Ni(II)-EDTA complexes was endothermic in the lower pH region (∼3.5) and exothermic at higher pH values (∼6.0). This implied that the surface preferred the NiHEDTA⁻¹ species rather than the NiEDTA⁻² species. Specific adsorption of the metal complexes was evidenced by the charge reversal exhibited by the γ-Al₂O₃ particles at the highest surface loadings. A quantitative model was formulated based on the pH-dependent speciation of the oxide surface, speciation of the metal complexes in solution, and ζ potential measurements. This model proved valid over a wide range of pH (3–10) and for both high (>50% coverage) and low (<10% coverage) surface loadings.