BORON INCORPORATION AND SPECIATION IN SILICATE GLASS ALTERATION LAYERS: IMPLICATIONS FOR NUCLEAR WASTE IMMOBILIZATION

The dissolution behaviour of silicate glasses in aqueous environments is key to evaluating the long-term durability of materials for radioactive waste immobilization. The formation of an amorphous silicon-rich alteration layer limits ionic transport, enhancing radionuclide retention. In borosilicates, the thickness of this layer is often estimated by measuring boron release into solution, on the assumption that boron is not incorporated into the layer. Recent work has shown that this assumption may not be reliable; however, little is known about how boron is incorporated structurally into borosilicate alteration layers. In this work, layers formed on alkali boroaluminosilicate glasses subjected to MCC-1-type dissolution are analyzed by secondary-ion mass spectrometry (SIMS) and nuclear magnetic resonance (NMR) spectroscopy to obtain a clearer picture of the boron incorporation and network connectivity of the layers. Boron retention in the layers is 1.5 and 3 wt% for lithium and sodium glasses, respectively, with a measurable gradient across the altered region. ¹¹B NMR of layers harvested from the bulk glass reveals elevated trigonal boron fractions relative to the bulk sodium boroaluminosilicate glass, whereas the boron speciation in the analogous lithium glass layer differs only slightly from that of the unaltered bulk. ²⁹Si{¹H} cross-polarization NMR of the isolated layers shows a high fraction of Si-OH, consistent with current models of the altered glass structure. SEM measurements of the layer thickness confirm that using B as an alteration tracer is unreliable, leading to significant underestimation of the true alteration layer thickness.