Strontium-90 Immobilization Via Coprecipitation with Respect to Calcite in Various Disposal Site Conditions

Abstract

90Sr is commonly present as a fission product of uranium and plutonium, and is released to the environments from nuclear power plants. The 90Sr nuclide requires special attention owing to its long half-life (28.9 years), high solubility, and acute toxicity. The main component of the concrete is calcium hydroxide (Ca(OH)2)(s) and Ca(OH)2 reacts with the carbon dioxide (CO2) in the air, resulting in calcite (CaCO3) precipitation, known as carbonation process. Additionally, in a high pH concrete environment, Sr tends to react better with CaCO3 compare with Ca(OH)2. Especially, Sr can react with CaCO3 such as ion exchange, adsorption, and coprecipitation etc. Hence, in this paper, the experiments were designed to prove whether coprecipitation could remove Sr more than adsorption under the same conditions. As a result, the Sr removal efficiency by coprecipitation was higher than that by simple adsorption on the calcite surface. Additionally, ionic strength (IS) increases, adsorption on to the calcite surface weakens. However, coprecipitation is not significantly affected even if the IS increases. These results prove that coprecipitation is better than adsorption in a disposal site environment with the presence of many external competing ions such as Ca, Na, Mg, and K etc. Next, a desorption experiment was performed using the same Sr molar mass for the adsorbed and coprecipitated samples. As a result of the released Sr total mass from the adsorbed and coprecipitated solid samples, the adsorbed Sr on calcite was released 1.7 times more than the Sr coprecipitated with respect to calcite. Next, a dissolution experiment was conducted to investigate the release of Sr from a sample in which Sr was coprecipitated in calcite with the same molar mass of Sr compared to Sr in pure strontianite. when Sr was coprecipitated within respect to calcite, the internal Sr was released predominantly upon the dissolution of the calcite's main structure. Conversely, with pure strontianite, dissolution occurred instantaneously, resulting in a higher release of Sr compared to the Sr-coprecipitated calcite, despite both possess the same molar mass of Sr. This finding unequivocally showed that coprecipitation within calcite effectively immobilized such as Sr, substantially minimizing its leakage into the surrounding environment.