Comparison of europium and neptunium adsorption to aluminum (hydr)oxide minerals

- Eu(III) adsorption edge is independent of aluminum (hydr)oxide mineral phase.
- Phase transformations alone do not explain Eu(III) adsorption behavior.
- Preferential adsorption of Np(V) follows the trend bayerite > corundum > γ-alumina.

Batch experiments and detailed solid-phase characterization (i.e., powder X-ray diffraction, scanning electron microscopy, and infrared spectroscopy) were used to determine the effect of secondary phase formation on the adsorption of europium and neptunium to a suite of aluminum (hydr)oxide minerals. Europium experiments were conducted as a function of gibbsite (γ-Al(OH)3), bayerite (α-Al(OH)3), corundum (α-Al2O3), and γ-alumina (γ-Al2O3) concentration (2.5-30 m2 L− 1), europium concentration (10− 8-10− 5 M), pH (3 − 12), and ionic strength (0.01-0.1 M NaCl). Neptunium experiments were conducted at constant neptunium and mineral concentrations. The composition of the mineral phase had no apparent effect on europium adsorption whereas preferential adsorption of neptunium followed the trend bayerite > corundum > γ-alumina. The data presented here suggest that there are at least two different mechanisms controlling the adsorption of europium and neptunium and highlight the need to study both aqueous-phase chemistry and the properties of mineral surfaces in order to understand the behavior of lanthanides and actinides at the mineral-water interface.