van Loon Luc R, Glaus Martin A
Waste Management Laboratory, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.
Environ Sci Technol. 2008 Mar 1;42(5):1600-4. doi: 10.1021/es702487m.
A key discussion in radioactive waste disposal is the question of whether adsorption behavior of radionuclides in dispersed and in highly compacted materials is similar. It is common practice to measure sorption data on dispersed systems and to use these data to predict the sorption in compacted systems. The latter represent the situation in a real, deep geological repository system. The discussions focus mainly on items such as the effect of compaction on the accessibility of sorption sites, that is, on sterical effects, and not on the effect of compaction on the thermodynamics of the sorption processes. Here we show that in the case of sorption of cesium on sodium bentonite, material compaction indeed affects the thermodynamics of the sorption process such that sorption increases. This increase is due to a reduction of the interlayer space, leading to a lower ability of the interlayer water for cation hydration. Cations with a low hydration tendency, such as cesium, therefore accumulate in the interlayer space, whereas highly hydrated cations such as sodium tends to accumulate in the bulk water where water is easily available for hydration. The fact that mechanical compaction affects the thermodynamics of ion exchange processes in clay is an important finding and brings in a new aspect in the discussion on the transferability of thermodynamic data from diluted to compacted systems. The common practice of applying chemical and thermodynamic concepts valid for diluted systems to compacted systems must be basically scrutinized.
放射性废物处置中的一个关键讨论是，放射性核素在分散材料和高度压实材料中的吸附行为是否相似。通常的做法是测量分散体系中的吸附数据，并使用这些数据来预测压实体系中的吸附情况。后者代表了真实的深部地质处置库系统中的情况。讨论主要集中在压实对吸附位点可及性的影响（即空间效应）等方面，而不是压实对吸附过程热力学的影响。在此我们表明，在铯在钠膨润土上的吸附情况中，材料压实确实会影响吸附过程的热力学，使得吸附增加。这种增加是由于层间空间减小，导致层间水阳离子水化能力降低。水化倾向低的阳离子，如铯，因此会在层间空间积累，而水化程度高的阳离子，如钠，则倾向于在大量易于水化的水中积累。机械压实会影响粘土中离子交换过程的热力学这一事实是一个重要发现，并为关于热力学数据从稀释体系到压实体系的可转移性的讨论带来了新的方面。将适用于稀释体系的化学和热力学概念应用于压实体系的常见做法必须从根本上进行审视。
