Thermodynamics of AFm-(I₂, SO₄) solid solution and of its end-members in aqueous media

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• 作者：Laure Aimoz ; Dmitrii A. Kulik ; Erich Wiel ; Enzo Curti ; Barbara Lothenbach ; Urs M?der
• 刊名：Applied Geochemistry
• 出版年：2012
• 出版时间：October, 2012
• 年：2012
• 卷：27
• 期：10
• 页码：2117-2129
• 全文大小：827 K

文摘

Cementitious materials are foreseen as being used for the immobilization of radionuclides in deep-storage facilities for nuclear waste. The ¹²⁹I contained in the waste is of major concern due to its long half-life and its anionic speciation in groundwater (I^?), which hinders the retention by most minerals due to their negatively charged structural surfaces at near-neutral pH. AFm-SO₄ is one of the phases present in cementitious materials. Experimental data suggest that the formation of solid solution between AFm-SO₄ and AFm-I₂ has the potential to retard ¹²⁹I mobility. In order to improve the present geochemical models, thermodynamic properties of AFm-I₂ and of its solid solution with AFm-SO₄ were evaluated based on precipitation and dissolution experiments in aqueous solutions between 4 ¡ãC and 60 ¡ãC. The solubility product of AFm-I₂ was determined at 23 ¡ãC ¡À 2 ¡ãC to be (2¦Ò) for the dissolution reaction . The Gibbs energy of formation of AFm-I₂ at standard state (1 bar, 25 ¡ãC) was assessed to be ?3325.0 ¡À 2.4 (2¦Ò) kJ mol^?1. The experimental data for AFm-(I₂, SO₄) phases were consistent with the formation of a continuous solid solution without miscibility gaps. A good fit to these data was obtained using a sub-regular solid solution model with negative Margules interaction parameters W_SO4 = ?1.0 kJ mol^?1 and W_I = ?8.0 kJ mol^?1, and end-members (Ca₂Al(OH)₆I and Ca₄Al₂(OH)₁₂SO₄) defined according to the Vanselow convention for ion exchange. The results show the potential of AFm-SO₄ to act as a sink for ¹²⁹I, notably in carbonate-free systems. Small amounts of calcite can hamper the formation of AFm-(I₂, SO₄) solid solution. The study provides relevant thermodynamic data for geochemical modeling of ¹²⁹I in the cementitious near-field of a nuclear waste repository.