Dissolution of LnYSiAlO glass (Ln=La or Ce) in aqueous media. I. Influence of the pH on hydrolysis mechanisms and kinetics

Static leaching tests have been performed on LnYSiAlO glasses (Ln=Lanthanide: La or Ce) that are considered as potential matrices for the specific immobilization of minor actinides. The influence of the nature of the rare earth on alteration mechanisms and kinetics is studied by synthesizing two analogous compositions with lanthanum or cerium. Four series of leaching tests have been carried out for different initial pH values ranged between 1 and 13 (HNO$_3$ and NaOH dilute media, $T=90^\circ$ and $S/V$=0.06 cm$⁻¹$). For acid pH around 1–2, Al and Y + Ln are released and the formation of a non-protective Si-rich layer (a few μm thick) at the glass/solution interface is observed. On the other hand, this family of glass presents a strong chemical resistance for slightly acid and basic pH media. This property is linked to the formation of a surface protective layer that includes Al or Ln + Y hydroxides (or oxides) depending on the Ln nature and the solubility of these compounds at the considered pH. In the case of LaYSiAlO glass where only La(III) is stable, the solubility of these secondary phases is the parameter probably controlling the dissolution through the formation of a very thin passivating surface layer. This property is quite interesting in the perspective of a storage in clay or granitic medium, where flowing natural waters present a slightly basic pH. In the case of CeY glass, the possible oxidation of Ce(III) to Ce(IV) during leaching leads to a thicker surface layer, probably CeO$_2$. At the same time, an increase of the initially near-neutral pH is observed, and a preferential release of Al and Si elements occurs.