The Leaching Behavior of a Glass Waste Form – Part I: The Characteristics of Surface Layers

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Abstract
The Soxhlet-type leaching test was carried out on borosilicate glass that contained 14 wt% simulated high-level waste. The morphology, texture, composition, and crystallography of the surface layers that formed were examined using optical microscopy, scanning electron microscopy, electron probe microanalysis, and analytical electron microscopy. Four surface layers, made up of 100- to 1000-Å crystalline and noncrystalline particles, formed on the glass. The elements found were classified into three groups based on their behavior in the surface layers. Group I contained the alkali metals, such as sodium, potassium, and cesium, which were strongly depleted from the layers as a result of leaching. Group II contained elements such as manganese, iron, nickel, zirconium, lanthanum, cerium, and neodymium, which were more concentrated in the surface layers than in the unleached part of the specimen, probably because the layers had shrunk during the drying process. Group III contained the elements which behaved inconsistently as a group: Some, such as calcium, silicon, and aluminum, were poor in the layers; magnesium and barium were present, but had concentration profiles that differed from those of Group II. Only one crystalline phase, a sheet silicate, formed in the layers. It had the expected chemical form, (Ca, Ba, La, Ce, Nd)x(Mn, Fe, Zr, Mg, Ni, Al)y(Si, Al)z(O, OH)m; its formation probably influenced the leaching mechanisms.

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