Interactions of metal hydrous oxides with chelating agents: III. Adsorption on spherical colloidal hematite particles

Abstract

The adsorption of ethylenediaminetetraacetic acid (EDTA) on a well-characterized sample of α-Fe 2O 3 (hematite) at 25°C has been studied over a wide range of concentrations and pH. The plateau adsorption density vs pH indicates a maximum at low pH (2–3) and steadily decreasing uptake at pH values above 3. Significant adsorption at the electrokinetic isoelectric point (iep) of hematite (pH ∼7.0) and a shift to lower pH of the iep indicate specific interactions between hematite and the complexing solutes. Essentially the same curve is obtained when the adsorption density relative to its maximum value ( Γ T Γ max ) is plotted against the pH for EDTA and several other related chelating ligands (HEDTA, DTPA, CTPA, and NTA). A general electrical double-layer model which quantifies the change in chelate ligand solution speciation and the change in particle surface charge is used to model the normalized adsorption ( Γ T Γ max ) as a function of pH for the five chelating agents. Below the iep the experimental results can be reproduced by adding to the coulombic interaction a specific adsorption energy of approximately −5 RT. To fit the data above the iep it was necessary to assume a configurational change for the adsorbed chelating species.