Hydrolysis and Coordination of Zr(IV) in Sulfuric Acid: Radiation Effects and Coassembly With Uranyl

Zirconium, a key component of nuclear fuel cladding materials, is commonly present in highly acidic aqueous environments associated with nuclear fuel processing and radioactive waste systems. Although Zr(IV) hydrolysis has been extensively studied, its behavior under radiolytic and actinide-containing acidic conditions remains poorly understood. Here, we investigate the hydrolysis and coordination chemistry of Zr(IV) in 2 M H₂SO₄ both in the absence and in the presence of γ-radiation and U(VI). In 2 M H₂SO₄, Zr(IV) forms amorphous Zr(IV)–SO₄²⁻–OH aggregate driven by sulfate coordination. γ-Irradiation reduces aggregate size without significantly altering the local Zr coordination environment, as confirmed by transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS) measurements. Under electron-beam irradiation during TEM imaging, these aggregates undergo decomposition followed by crystallization. In situ electron-beam mass spectrometry indicates that radiolysis of aqueous components promotes atomic rearrangement and generates a localized reducing environment, leading to ZrS₂ formation. The presence of U(VI) alters the Zr(IV) coordination environment and inhibits Zr(IV)–SO₄²⁻–OH aggregates formation. Upon γ-irradiation of mixed Zr(IV)/U(VI) solutions, cubic-shaped amorphous U(VI)–Zr(IV)–SO₄²⁻–OH assemblies are formed, likely through OH•-mediated coordination and aggregation processes. These findings provide new insights into the coordination chemistry and radiolytic aggregation behavior of Zr(IV) in highly acidic actinide-containing environments.