Diagenetic formation of uranium-silica polymers in lake sediments over 3,300 years

Abstract

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 (UIVSiO4·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.

Climate-driven fluxes of organic-bound uranium to an alpine lake over the Holocene

“…As Lake Nègre was previously described to investigate U mineralogical transformations in sédiments over 3300 years, a full description of the study site is provided in the main text and SI appendix of Lefebvre et al (2021). Briefly, Lake Nègre is located in a remote, high-altitude glaciercarved watershed, at the border between France and Italy in the Mercantour-Argentera Massif, ~50 km north of the Mediterranean Sea (Figure 1).…”

“…A series of sediment cores was sampled in September 2018 at the deepest point of Lake Nègre using a hammer-equipped Uwitec gravity corer (63 mm internal diameter). This study focuses on cores NEG18-05 (187.5 cm long, sampled at N 44° 9.49926'; E 7°14.1516') and NEG18-06 (216 cm long, sampled 20 m southward at N 44° 9.48966'; E 7°14.1576'), taken in addition to cores NEG18-04 and NEG18-07 that were specifically stored in anoxic conditions for U speciation analysis and described in Lefebvre et al (2021). Both cores studied here (NEG18-05 and 06) were cut in two halves in the open air; one half was preserved for sedimentological description and XRF core-scanning, and 1 cm slices were sampled on the other half for chemical analysis.…”

“…They may indeed include dissolved or particulate transport/sedimentation, as well uranium trapping via its reduction in the water column or at the sediment-water interface (Anderson et al, 1989;Barnes and Cochran, 1990;Chappaz et al, 2010;McManus et al, 2005). To this end, the Lake Nègre catchment (Mediterranean Alps, South-East France) was chosen as case study since the lake sediments were previously shown to naturally contain exceptionally high U concentrations (Lefebvre et al, 2021).…”

“…Owing to its small catchment area and negligible anthropic activities, Lake Nègre offers the opportunity to study direct effects of climatic variations in the Mediterranean Alps on U fluxes to the sediments. In particular, its low sedimentation rate (Lefebvre et al, 2021) reflects its low sensitivity to physical erosion events (Arnaud et al, 2016), hence allowing to decipher the specific impact of chemical erosion and of the dynamics of land vegetation and lake productivity on U inputs. Exceptionally high U concentrations in Lake Nègre sediments enable the use of X-Ray Fluorescence (XRF) core-scanning to measure high-resolution (1-mm steps) sedimentary U profiles, together with other elements.…”

“…Exceptionally high U concentrations in Lake Nègre sediments enable the use of X-Ray Fluorescence (XRF) core-scanning to measure high-resolution (1-mm steps) sedimentary U profiles, together with other elements. In our previous study (Lefebvre et al, 2021), we investigated the solid-state speciation of U using X-ray absorption spectroscopy on a sediment core from Lake Nègre dating back to 3300 years BP and found that U initially deposits as noncrystalline organic-bound U before transforming to U-Si polymers through diagenetic aging. As U is thus thought to be deposited after binding on lake biomass or terrigenous organic matter (OM), variations in U sedimentary content are expected to be constrained both by (i) U inputs from the watershed to the lake water, and by (ii) the relative development of soil vegetation and lacustrine biomass.…”