Composition effects on synthetic glass alteration mechanisms: Part 1. Experiments

Abstract

Alteration of nuclear waste glasses and silicate minerals is governed by complex processes regulated by several coupled mechanisms. Among these processes is reactive mass transfer through the amorphous gel layer (known as the passivating reactive interphase (PRI) in case of a rate-limiting effect) located between the pristine glass and the bulk solution. In order to assess the influence of the glass composition and the pH on the properties of the PRI, and thus on the nuclear glass durability, an experimental leaching study was performed on borosilicate glass samples with or without Ca, Al, and Zr. Experiments were conducted to understand the influence of the pH and glass composition on the solvated cation diffusion coefficient within the PRI and to generate data for calibration of a PRI solubility model (not presented here). All the experiments were carried out at high S/ V ratios so that silicon rapidly reached apparent saturated conditions and the PRI could form: in such conditions glass alteration is controlled only by diffusion of water and dissolved species through the PRI and by precipitation of crystallized secondary phases. The constituents in the PRI and the crystallized secondary phases depend to a large extent on the glass composition and pH. Alkali metal (Na) or preferentially alkaline earth (Ca) elements are retained in the PRI for charge compensation of Al and Zr. The apparent diffusion coefficient calculated from the release of boron, a good tracer, varies with the pH from less than 4 × 10 −22 to 9 × 10 -18 m 2 s −1 in the studied glasses. These very low diffusion coefficients decrease as the pH increases. Concerning the PRI composition we show that Si, Al, Ca and Zr have strong interactions and thus major consequences on the glass durability. Our findings indicate that the SiO 2aq activity is relatively constant and independent of the pH below pH 9, followed by a drop at pH 10. In addition, the activity of SiO 2aq is affected by the glass composition, and especially by aluminum and zirconium. As soon as dissolved silicon reaches steady state in solution the aluminum and zirconium concentrations start to decrease, probably due to silicon, aluminum and zirconium interactions with retention in the PRI. The formation of crystallized secondary phases is observed at pH 10 for aluminum-free glasses, which diminishes the saturation state of amorphous silica in solution. In these glasses the saturation index indicates that the solution is oversaturated with respect to calcium silicate hydrates (ex: tobermorite, gyrolite). Moreover, the formation of crystallized secondary phases causes dissolution of the PRI and the glass, which sustains renewed alteration. This study leads to the conclusion that modeling nuclear glass dissolution kinetics over a wide pH range (typically from pH 7 to pH 10) must take into account (1) PRI composition variations and relations between the PRI composition and properties (solubility, diffusion coefficient); and (2) crystallized secondary phases that can consume elements from the PRI. Applying PRI modeling concepts to other kinds of natural glasses or even multi-oxide minerals might prove useful for enhancing our understanding of alteration mechanisms.