Polyoxometalate-assisted crystallization for ultraselective Ln(III)/An(VI) separation under strongly acidic conditions

Abstract

The selective separation of actinides from lanthanides represents a longstanding challenge in advanced nuclear fuel cycle management. Here we present a fully inorganic crystallization strategy for Ln(III)/An(VI) separation based on the Preysslertype polyoxometalate [NaP₅W₃₀O₁₁₀]^14– (P₅W₃₀). Leveraging the strong affinity of Ln(III) ions for oxygen-donor environments, two crystalline phases, Ln-P₅W₃₀-1 and Ln-P₅W₃₀-2, are isolated, featuring peripheral coordination and cavity encapsulation modes, respectively. The coordination behaviour is governed by ionic radius and HNO₃ concentration, providing a controllable crystallization pathway under acidic conditions relevant to nuclear reprocessing streams. In contrast, U(VI), employed as a surrogate for Am(VI), does not crystallize with P₅W₃₀ under identical conditions, highlighting intrinsic differences in Lewis acidity and actinyl geometry. Binary separation experiments demonstrate highly selective incorporation of Ln(III) into the POM lattice while U(VI) remains in solution. Increasing the acidity to 3 M HNO₃ promotes exclusive formation of Ln-P₅W₃₀-1 and substantially enhances separation efficiency, affording separation factors up to 41,139 for Eu/U. This acid-tolerant, solid-liquid separation platform offers a chemically robust and potentially scalable route for lanthanideactinide partitioning in nuclear fuel reprocessing.