Impact of competing anions on the sorption of trivalent actinides onto clay mineral surfaces

Abstract

Sorption processes on mineral phases are a key mechanism in the context of the safe final disposal of radioactive waste in deep geological formations. Especially clay mineral surfaces are known for their excellent adsorption properties of metal ions, like actinides and many fission products. Previous studies demonstrated their high retention abilities for diluted background electrolyte conditions (Imax= 0.7 M), compa-rable to pore water of bentonite and Opalinus clay (Switzerland), as well for saline conditions (I = 1 4 M) as described for Jurassic and lower Cretaceous clay rock lay-ers Nnorthern Germany). The application of a geochemical model (2SPNE/CE), de-veloped for diluted systems, was also demonstrated to be suitable for concentrated background electrolyte systems. The influence of competing ligands on the adsorption of trivalent lanthanides and ac-tinides onto the important natural clay minerals illite du Puy and montmorillonite was investigated within the present thesis. Sorption experiments were conducted under variation of the background electrolyte concentration, concentration of the competing ligand and the partial pressure of CO2. Representative competing ligands were chosen to study their impact on the An(III)/Ln(III) retention. The main focus was set on the influence of carbonate on the adsorption of trivalent lanthanides and actinides in NaCl electrolyte systems. Car-bonate is omnipresent in natural groundwater. Due to the ability to form strong com-plexes with metal ions, carbonate is highly relevant for the geochemical behaviour of actinides. Beside carbonate, also gluconate and citrate were studied in NaCl and CaCl2 background electrolyte solutions. Both ligands are model substances for ce-ment additives. Gluconate is also studied as an analogue for isosaccharinic acid, a degradation product of cellulose. Citrate is commonly used as a decontaminant. For this reason both are part of the inventory of a repository for low and intermediate level waste. In presence of carbonate, a somewhat increased retention of Eu(III) on clay minerals surfaces is observed in the slightly acidic and neutral pH range. Due to the formation of strong carbonate complexes, a strong decrease in retention is observed in the al-kaline pH range. A variation of the applied partial pressure of CO2 is significantly af-fecting the total concentration of dissolved carbonate / bicarbonate and by this the formation of stable aquatic species. During the batch sorption studies, no indication of a significant effect of ionic strength was observed neither for the Eu(III) adsorption onto illite du Puy, nor on montmorillonite. With a combined study of quantum chemi-cal calculations (DFT, AIMD) and spectroscopic methods (ATR-IR, TRLFS) it was possible to prove the adsorption of carbonate on the clay mineral surface. The ad-sorbed carbonate forms stable ternary surface complexes with Ln(III)/An(III), and, therefore, is responsible for enhancing the retention. At higher pH values a second surface sorbed complex containing an additional carbonate ligand is formed. These two carbonate stabilized surface complexes were implemented in a geochemical model (2SPNE/CE), resulting in a consistent model to accurately describe the ad-sorption of Eu(III) onto illite du Puy and montmorillonite for the whole pH range and under variation of ionic strength and the partial pressure of CO2. In presence of gluconate, the retention of Eu(III) onto illite du Puy and montmorillo-nite is significantly reduced between pH = 3 12.5 (up to three orders of magnitude). Using time resolved laser fluorescence spectroscopy, 3 pH regions of characteristic interactions of adsorbed and competing aquatic species were identified. The compe-tition of aquatic species with the cation exchange was observed in the acidic pH and with inner sphere surface complexation from neutral to high pH. In presence of Ca2+ the existence and influence of quaternary Ca-Cm-OH-GLU could be monitored under alkaline conditions. The presence of citrate is influencing the adsorption of Eu(III) onto clay minerals in 2 different ways. In NaCl background electrolyte solution a reduced retention is ob-served in the slightly acidic pH range, while the retention is increased in presence of Ca2+ in the same pH range. From neutral to high pH a nearly quantitative retention is observed in both systems.