Effective Buffers to Measure Elemental Release from Low Activity Waste Type Glasses as a Function of pH

Project GLAD (Glass Leaching Assessment for Disposability) investigates the US Environmental Protection Agency (EPA) 1313 Standard Method for assessing the durability of Low Activity Waste (LAW) glasses destined for disposal at the Integrated Disposal Facility (IDF). The EPA 1313 test measures elemental release as a function of pH over a pH range relevant to subsurface conditions in the IDF expected to be circumneutral to mildly alkaline. To fix the pH, and counteract the rapid increase in pH observed when LAW glass is initially exposed to water, the EPA 1313 test can be performed in buffered solution providing the buffer does not affect the glass dissolution rate. Alkali-metal-free buffers are desirable as the presence of alkali metals in solution can lead to ion exchange reactions with the glass surface. Two alkali-free buffers, TRIS (NH₂C(CH₂OH)₃) and ammonium chloride – ammonia (NH₃/NH₄Cl), were investigated to perform the EPA 1313 test in the alkaline regime (pH 8.5 – 10.5) at varying temperatures (RT, 40 °C, and 60 °C). Both buffers were shown to effectively maintain the desired pH at room temperature, and up to 40 ^oC, but the effectiveness of TRIS decreased at 60^oC. Previous studies have shown the presence of a TRIS-boron complex and raised concerns that it could lead to increased boron release. Here, 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. On the other hand, the ammonium buffered systems showed increased release of cations indicating that NH₄⁺ may participate in ion exchange.<br/>The TRIS buffer was therefore selected for further work and the EPA 1313 test was conducted on a set of LAW glasses for comparison to parallel tests using only acid addition to control the pH. The test response as a function of time (typical EPA 1313 tests are conducted over 48 hours) to ascertain how a range of glass compositions behave between 1- 8 days.