The long-term fate of uranium-contaminated sediments, especially downstream former mining areas, is a widespread environmental challenge. Essential for their management is the proper understanding of uranium (U) immobilization mechanisms in reducing environments. In particular, the long-term behavior of noncrystalline U(IV) species and their possible evolution to more stable phases in subsurface conditions is poorly documented, which limits our ability to predict U long-term geochemical reactivity. Here, we report direct evidence for the evolution of U speciation over 3,300 y in naturally highly U-enriched sediments (350–760 µg ⋅ g⁻¹ U) from Lake Nègre (Mercantour Massif, Mediterranean Alps, France) by combining U isotopic data (δ²³⁸U and (²³⁴U/²³⁸U)) with U L₃-edge X-ray absorption fine structure spectroscopy. Constant isotopic ratios over the entire sediment core indicate stable U sources and accumulation modes, allowing for determination of the impact of aging on U speciation. We demonstrate that, after sediment deposition, mononuclear U(IV) species associated with organic matter transformed into authigenic polymeric U(IV)–silica species that might have partially converted to a nanocrystalline coffinite (U^IVSiO₄·nH₂O)-like phase. This diagenetic transformation occurred in less than 700 y and is consistent with the high silica availability of sediments in which diatoms are abundant. It also yields consistency with laboratory studies that proposed the formation of colloidal polynuclear U(IV)–silica species, as precursors for coffinite formation. However, the incomplete transformation observed here only slightly reduces the potential lability of U, which could have important implications to evaluate the long-term management of U-contaminated sediments and, by extension, of U-bearing wastes in silica-rich subsurface environments.

铀污染沉积物的长期命运，特别是下游的前矿区，是一个普遍的环境挑战。对它们的管理至关重要的是正确理解还原环境中的铀 (U) 固定机制。特别是，非晶态 U(IV) 物种的长期行为及其在地下条件下向更稳定相的可能演变的记录很少，这限制了我们预测 U 长期地球化学反应性的能力。在这里，我们通过结合 U 同位素数据，报告了 Nègre 湖（Mercantour Massif，地中海阿尔卑斯山，法国）的天然高度富 U 沉积物（350–760 µg ⋅ g ^-1 U）中 U 物种形成超过 3,300 年演变的直接证据(δ ²³⁸ U 和 ( ²³⁴ U/²³⁸ U)) 具有U L ₃边缘X射线吸收精细结构光谱。整个沉积物核心的恒定同位素比率表明稳定的 U 来源和积累模式，允许确定老化对 U 物种形成的影响。我们证明，在沉积物沉积后，与有机质相关的单核 U(IV) 物种转化为自生聚合 U(IV)-二氧化硅物种，这些物种可能已部分转化为纳米晶栉 (U ^IV SiO ₄ · n H ₂O)-样相。这种成岩作用发生在不到 700 年的时间里，这与硅藻丰富的沉积物的高二氧化硅可用性一致。它还与提出胶体多核 U(IV)-二氧化硅物种形成的实验室研究一致，作为菱形石形成的前体。然而，此处观察到的不完全转变仅略微降低了 U 的潜在不稳定性，这可能对评估 U 污染沉积物的长期管理以及在富含二氧化硅的地下环境中的含 U 废物的长期管理具有重要意义。