The influence of temperature and small organic ligands on the sorption of Eu(III) on Opalinus Clay

Abstract

The influence of temperature up to 50 ºC and small organic ligands (citrate, tartrate) on the sorption of Eu(III) on the natural clay rock Opalinus Clay (OPA) under aerobic (p_CO2 = 10^−3.5 atm) synthetic OPA pore water conditions (pH 7.6, I = 0.4 mol L⁻¹) was investigated. Batch sorption experiments and time-resolved laser-induced fluorescence spectroscopy (TRLFS) were used to study these influencing factors on the Eu(III) sorption.

Sorption isotherms and distribution coefficients R_d (15 ºC: log R_d = 4.50 ± 0.05...50 ºC: log R_d = 5.54 ± 0.06) at 2 × 10⁻⁹ mol L⁻¹ Eu(III) as a function of the solid-to-liquid ratio (up to 3 g L⁻¹) and temperature were determined. A significant temperature dependency of the Eu(III) sorption was observed. With rising temperature the Eu(III) sorption increases. The surface reaction is endothermic (Δ H_sorb ∼ 50 kJ mol⁻¹). Using TRLFS, a surface species with a luminescence emission lifetime of 201 ± 9 μs was identified.

In the presence of tartrate or citrate the Eu(III) sorption decreases with increasing ligand concentration due to a complex formation of Eu(III) in solution, with citrate having a more pronounced influence on the sorption than tartrate. With the batch sorption experiments it can be shown that at a citrate concentration larger than 10⁻⁵ mol L⁻¹ and at a tartrate concentration larger than 10⁻⁴ mol L⁻¹ an increasing Eu(III) desorption occurs. This result is supported by TRLFS measurements, which show the correlation between the complexation of Eu(III) by citrate or tartrate in solution and the Eu(III) desorption process. Possible Eu(III) citrate or Eu(III) tartrate surface species on OPA could not be detected using TRLFS.