Hydration of Magnesium Oxide in the Waste Isolation Pilot Plant
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II8.3.1
Hydration of Magnesium Oxide in the Waste Isolation Pilot Plant Anna C. Snider Carlsbad Programs Group, Sandia National Laboratories Carlsbad, NM 88220, U.S.A. ABSTRACT Magnesium oxide (MgO) is the only engineered barrier being emplaced in the Waste Isolation Pilot Plant (WIPP), a U.S. Department of Energy repository for transuranic waste. MgO will lower dissolved concentrations of actinides by consuming CO2 from possible microbial activity, by buffering the pH between 8.5 and 9.5, and by reducing the amount of free water in the repository. This paper discusses results from experiments measuring the hydration of MgO. Results suggest that periclase (MgO) hydrates rapidly to brucite (Mg(OH)2) in deionized water and 4 M NaCl solution at 90°C; the hydration rate decreases as temperature decreases. In ERDA-6, a NaCl-rich WIPP brine, MgO hydrates directly to brucite; in GWB, a high-Mg brine, periclase hydrates to magnesium chloride hydroxide hydrate(s) until the dissolved Mg2+ concentration decreases, and brucite becomes the stable phase. Under humid conditions MgO fully hydrates at higher humidities(> 50%). All data are consistent with diffusion-limited hydration reactions. INTRODUCTION The Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy repository designed for the disposal of defense-related transuranic waste. The WIPP is located in southeast New Mexico at a depth of 655 m in the Salado Formation, a Permian bedded salt formation. Magnesium oxide (MgO) is currently the only engineered barrier certified by the U.S. Environmental Protection Agency. The MgO currently being emplaced is supplied by Premier Chemicals of Gabbs, Nevada. MgO emplaced in the WIPP will lower dissolved concentrations of actinides by consuming CO2 from possible microbial degradation of organic material (cellulose, plastic, and rubbers), and buffer the brine system between pH 8.5 and 9.5 and PCO2 at 10-5.5 atm. MgO will reduce the amount of water in the repository if subsequent carbonation produces hydrous magnesium carbonates, however formation of magnesite (MgCO3) would release water. Water (brine) within the WIPP is postulated to derive from the surrounding Salado Formation or from human drilling intrusion penetrating underlying brine pockets of the Castile formation. Investigations on the efficacy of MgO are underway at Sandia National Laboratories’ Carlsbad Programs Group. Experiments include possible repository conditions. The ambient temperature of the Salado Formation at the depth of the repository (655 m) is 28 °C [1]. Studies focus on hydration and carbonation rates, and characterization of the products formed under humid and inundated conditions. This paper describes chemical and mineralogical characterization of MgO and the hydration experiments. These experiments help determine hydration rates and pathways, and whether the reaction products passivate MgO.
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EXPERIMENTS Characterization of Premier Chemicals MgO by physical and chemical analyses determined mineralogy and quantity or impurities. Bulk MgO a
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