PNNL

Abstract

Abstract: Selecting appropriate buffers is crucial for evaluating the chemical durability of glass under controlled conditions such as in the EPA 1313 test designed to measure elemental release as a function of pH. This study investigates the efficacy of two alkali-metal free buffers, TRIS (NH2C(CH2OH)3) and ammonium chloride - ammonia (NH3/NH4Cl), on a simulated Hanford low-activity waste borosilicate glass in the alkaline regime (pH 8.5 – 10.5) at varying temperatures (18 ± 1 °C, 40 °C, and 60 °C). The study found that while both buffers maintained the desired pH at room temperature, the effectiveness of TRIS decreased at elevated temperatures, particularly at pH 10.5. Although 11B NMR showed evidence of TRIS-B complexation, its effect on the rate of elemental release was found to be negligible under the test conditions. In the case of the ammonium buffer, release of alkali cations was slightly elevated when compared to the same conditions with TRIS and indicated that NH4+ may participate in ion exchange. For the EPA 1313 test, conducted with both buffer systems over 1-8 days, low calculated activation energies, incongruent dissolution and elevated sodium release rates indicate ion exchange as the dominant alteration mechanism over these short time scales.