Adsorption of Tetrammine-carbonato and -cis-diaqua and Triethylenetetramine-carbonato and -cis-diaqua Complexes of Cobalt(III) on Titanium Dioxide as Models for the Binding of Uranyl Ions from the Tricarbonatouranyl Complex Ion

Abstract

The adsorption of several cobalt(III) complex ions with carbonato or two aqua ligands in cis-position (and related complexes) was studied with the objective of a comparison with the adsorption of UO2 2+ ions from [UO2(CO3)3]4- solutions. One CO3 2- group per adsorbed UO2 2+ ion was found on the TiO2 surface after vacuum-drying at room temperature. Pre-adsorption of HPO4 2- ions resulted in an increased UO2 2+ uptake, whilst coverage of the TiO2 surface with adsorbed Al hydroxo complexes resulted in the opposite effect. The adsorption rate as well as the saturation uptake were higher for the cis-[Co(NH3)4(H2O)2]3+ complex than for [Co(NH3)4(CO3)]+. The diffuse reflectance spectra are similar for both adsorbates. Also with cis-[Co(trien)(H2O)2]3+ and [Co(trien)(CO3)]+ the diaqua is more rapidly adsorbed, whereas the saturation values are very similar. No carbonate is found in the adsorbates with both carbonato complexes, the ammine or trien ligands are retained on the adsorbed CoIII ions, however. Temperature-programmed decomposition spectra showed that the bonding of the ammine ligands is weakened in the adsorbed complexes compared to bulk salts of the complex ions. It is concluded that the metal complexes are anchored to the TiO2 surface by bonding to two surface oxygen ions.