Reduction‐Assisted Immobilization of Uranium Into Rhabdophane/Monazite via Ascorbic Acid in Nitric Acid Waste Liquid

ABSTRACT

Rhabdophane (LnPO ₄ ·nH ₂ O) is a promising matrix for actinide immobilization, yet the chemical inertness of actinyl ions (AnO ₂ ⁿ⁺ ) in nitric acid waste streams significantly hinders their lattice incorporation. Here, we report a reduction‐assisted strategy integrating kinetically controlled L‐ascorbic acid (VC) reduction with hydrothermal precipitation for uranyl immobilization. Using in situ surface‐enhanced Raman spectroscopy (SERS) to circumvent colorimetric interferences, we identified 90°C and pH = 1.0 as the optimal regime for effective reduction and stabilization of U ⁴⁺ . Precipitation behavior confirmed that pure and stoichiometric Sr/U‐rhabdophane is stabilized by maintaining an initial Sr:U molar ratio ≥5. A gradient precipitation effect arises from the preferential enrichment of U ⁴⁺ during the initial nucleation stage, followed by Sr ²⁺ facilitated crystal growth. Simultaneously, Sr ²⁺ incorporation induces lattice distortion, resulting in elongated La–O/P bonds and accelerated lateral grain growth. Structural energetics reveal that Sr ²⁺ and U ⁴⁺ preferentially occupy [LaO ₈ ] sites, despite two‐thirds of the lattice sites being [LaO ₈ ·H ₂ O] units. Thermal treatment above 850°C converts rhabdophane into a stable monazite, forming [UO ₉ ] polyhedra that enhance the long‐term stability of U ⁴⁺ . Notably, this integrated strategy demonstrated high robustness in complex ²³⁵ UO ₂ ²⁺ ‐labeled liquid wastes, achieving quantitative removal of ²³⁵ UO ₂ ²⁺ (initial concentration: 1178 ppm) after 24 h despite interfering ions.