Oxyhydroxy Silicate Colloids: A New Type of Waterborne Actinide(IV) Colloids

Abstract

At the near‐neutral and reducing aquatic conditions expected in undisturbed ore deposits or in closed nuclear waste repositories, the actinides Th, U, Np, and Pu are primarily tetravalent. These tetravalent actinides (AnIV) are sparingly soluble in aquatic systems and, hence, are often assumed to be immobile. However, AnIV could become mobile if they occur as colloids. This review focuses on a new type of AnIV colloids, oxyhydroxy silicate colloids. We herein discuss the chemical characteristics of these colloids and the potential implication for their environmental behavior. The binary oxyhydroxy silicate colloids of AnIV could be potentially more mobile as a waterborne species than the well‐known mono‐component oxyhydroxide colloids.

Formation of CeSiO4 from cerium(iii) silicate precursors

“…The formation of actinide silicate colloids was observed at room temperature for thorium, uranium(IV) and neptunium(IV) and was proved to modify the mobility of the actinides in the environment. [10][11][12][13][14] The potential formation of Pu-based colloids in conditions representative of spent nuclear fuels repository settings deserves to be investigated. In this context, the study of cerium silicates appears as a compulsory first step to evaluate the behavior of plutonium in silicate-rich environments submitted to various redox conditions.…”

“…Data reduction and extraction of EXAFS oscillation was performed using the Athena and Artemis package. 33 The threshold energy, E 0 , was defined as the and CHNO 3 = 1.3 mol•L -1 (14). XRD lines of sample holder are pointed out by an asterisk.…”

The formation of PuSiO4under hydrothermal conditions

“…2 Among them, one can note natural actinide silicates ThSiO 4 (thorite), USiO 4 (coffinite) and associated uranothorite solid solutions. 4 The potential impact of actinide silicate species onto the actinide mobility [5][6][7][8][9][10][11][12][13] imply to take into account the chemical interactions between actinide elements and silica. Understanding their behavior appears as a crucial issue especially in the case of direct disposal of spent nuclear fuel (SNF) in deep geological repository conditions.…”

“…Due to the similar limitation encountered for Ce(IV) and Pu(IV) and the closeness between Ce(III) silicate and Pu(III) silicate reactivity when forming CeSiO 4 and PuSiO 4 , it is worth noting that cerium (IV) can be considered as the best surrogate element among those considered to mimic the behavior of plutonium (IV) in reductive and silicate rich media. Since CeSiO 4 was prepared from Ce(III) species at temperatures as low as 40°C, 76 it also raises crucial questions on the potential behavior of plutonium in the environment [5][6][7][8][9][10][11][12][13] in the presence of silicates. A particular impact could be found when storing Pu(III) based radioactive waste forms in reductive and silicate rich environments.…”

In situ study of the synthesis of thorite (ThSiO4) under environmental representative conditions

“…Due to the abundance of silicate species in the environment, identifying the actinide-silicate interactions is a key point to evaluate the actinides behaviour in environment. On the one hand, thorium, uranium and neptunium are known to form nanometric particles in silicate reactive media [1][2][3][4][5][6][7][8][9] . The structure, size and stability of these intrinsic actinideoxyhydroxy-silicate colloids have been investigated 1,3,5,9 .…”