Actinide aqueous chemistry is a relevant, multifold and challenging research field of inorganic chemistry. In the context of nuclear waste disposal, actinides arise as potentially relevant dose contributors in the long term. Because of their specific electronic configuration, several oxidation states of actinides (+III to +VII) can exist in aqueous solution. This imposes a differential chemical behaviour as a function of the boundary redox conditions, which is of special concern for the safety case of repositories for nuclear waste disposal. In this framework, it is essential to continuously reduce experimental and systematic uncertainties, fill existing gaps in thermodynamic databases and improve knowledge and available data for more complex and relevant (geo)chemical systems.This presentation focuses on aqueous neptunium chemistry as a relevant case of actinide science and good example of the experimental and conceptual approaches employed at KIT-INE for investigating actinide solubility and thermodynamics. Based upon experimental studies characterizing neptunium redox transformation processes, separate series of solubility experiments were performed involving Np(V), Np(VI) and Np(VII) in dilute to concentrated saline systems. Investigations with other actinides (U, Pu) are discussed in order to support the interpretation of the Np studies. Solubility experiments were complemented with advanced spectroscopic techniques to gain insight on the chemical models governing the investigated systems. Examples will highlight studies related to:- Np(V) solubility in NaCl solutions: formation of ternary Na�Np(V)�OH solid phases- Redox chemistry of Np(V/VI) under near-neutral to hyperalkaline pH conditions- Np(VI) chemistry in alkaline NaCl media- Formation of Np(VII) under hyperalkaline pH conditionsAll studies included in this presentation aim at deriving comprehensive thermodynamic models of the investigated systems, which can be implemented in thermodynamic databases for geochemical model calculations.

Recent advances in aqueous chemistry and thermodynamics of neptunium

248th National Meeting (2014)

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