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Crystal Chemistry and Leaching Behavior of Sn-Based Hollandite Waste Forms for Cs Immobilization
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DOI：10.1111/jace.70960.png)
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Abstract
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Sn-containing hollandite has been highlighted by recent machine learning-guided design as a promising host for radioactive cesium (Cs), yet waste form validation remains limited. Here, a Cs-bearing Sn-based hollandite series, (Ba,Cs)1.66(Fe,Sn)8O16, was synthesized in air by solid-state reaction, with hollandite formation at 1125°C followed by pressureless sintering at 1200°C. x-Ray diffraction confirms hollandite as the major phase across the series, with minor Ba-bearing secondary phases only at low Cs contents. Rietveld refinement reveals anisotropic lattice expansion and a tetragonal-to-monoclinic symmetry change at the Cs-rich end member, accompanied by progressive tightening of tunnel descriptors, indicating a Sn-specific Cs accommodation response distinct from many Ti-based hollandites. Chemical durability was evaluated by monolithic leaching in deionized water at 90°C. At Cs loadings up to ∼17 wt%, 7-day Cs release is comparable to representative Ti-based hollandites tested under similar conditions. Sn exhibits ultralow normalized release, while Cs and Ba show higher release without detectable degradation of the hollandite matrix. Extended 28-day interval leaching on Ba0.8Cs0.86Fe2.46Sn5.54O16 (HS2) and Ba0.4Cs1.26Fe2.06Sn5.94O16 (HS3) separates rapid early wash-off from slower, diffusion-controlled release. By 14–28 days, Cs and Ba leaching rates converge to ∼2–4 × 10−2 g·m−2·day−1, with effective diffusion coefficients of ∼1–2 × 10−11 cm2/s and leaching indices of ∼11. Benchmarking against Ti-based hollandites places the Sn-based series within a competitive loading–durability window under a comparatively mild processing route, supporting Sn-based hollandite as a readily fabricated candidate for high-loading Cs immobilization.
Keywords:
Cs immobilization
leaching
Rietveld refinement
Sn-based hollandite
tunnel geometry
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