About the role of iron on the alteration of simplified nuclear glasses in deaerated solutions at 50°C

Abstract

The effect of Fe on the alteration of two SiO2-B2O3-Na2O based glasses (Fe2O3-free or containing 3.1 mol.% Fe2O3) was studied by measurement of initial alteration rates and long-term leaching experiments at 50°C in initially pure water or in a 10−2 mol.L−1 FeCl2 solution. The glass alteration kinetics were monitored by periodical leachate analyses; and morphology, structure and composition of solid alteration products were characterized at the end of long-term experiments (i.e. after 323 days).Iron(III) initially incorporated in the glass strengthens the glass network, leading to a decrease of the initial dissolution rate (from 0.15 to 0.026 g.m−2.d−1 at 50°C). The environment of Fe(III) incorporated into the gel depends on the pH of the leaching solution. A change of Fe(III) coordination from fourfold (i.e. charge-compensated by Na+ ions) to sixfold is expected when the pH decreases. Whatever its coordination, Fe3+ hinders the gel restructuring into a passivating barrier. Thus, the addition of Fe in the glass composition ultimately leads to higher alteration extent (≈1.5 times greater after 323 days of leaching in our experiments).In experiments where Fe cations are provided by the leaching medium (mainly at the +II oxidation state), precipitation of Fe-silicates is systematically observed, as well as an increase in glass alteration compared to experiments led in initially pure water (by a factor slightly less than 2). A drastic change in the average pH of experiments (from ≈9 in initially pure water to ≈6 in FeCl2 solution) is also observed. Among mechanisms proposed in the literature to explain the detrimental effect of Fe-silicates precipitation, it seems that the decrease in pH resulting from this precipitation is the main cause of increase in glass alteration observed in this study.