Long-term stable sinks of uranium in soils and sediments [electronic resource]
Keywords: Environmental Earth System Science.

The toxicity and radioactivity of uranium contamination in groundwater, soils, and sediments make uranium a threat to human and ecosystem health. Anthropogenic uranium contamination stemming from mining, nuclear fuel production, nuclear weapons production, and waste disposal represent the major threats to both surface and subsurface environments. Managing and mitigating risk, in turn, requires an understanding of uranium fate and transport in near-surface environments. Uranium retention processes, including U(VI) adsorption, U(IV) precipitation, and uranium incorporation into host minerals play a crucial role in limiting the risks from uranium by immobilizing uranium and removing it from groundwater. Two of these retention processes, U(VI) adsorption and U(IV) precipitation, may be easily reversed by changes in groundwater chemistry, re-mobilizing the contamination. The incorporation of uranium into host phases such as iron (hydr)oxides (e.g., goethite, FeOOH) and silicates (e.g., opal, SiO2•nH2O) is less likely to lead to the release of uranium upon changes in groundwater chemistry. However, the mechanisms of and competitiveness of uranium incorporation processes versus other retention processes are still poorly understood. The goal of the research described herein is to elucidate factors controlling the incorporation of uranium into iron (hydr)oxides and opaline silica, two host phases which may sequester uranium for thousands to millions of years. Chapters two and three examine the process of uranium incorporation into goethite during the reaction of iron(II) with uranium and iron (hydr)oxides, in order to determine the competitiveness of the uranium incorporation pathway across a broad range of geochemical conditions. The second chapter focuses on the effect of groundwater conditions (e.g., iron(II) concentration, uranium concentration, pH) on the prevalence of each retention pathway. A molecular-scale mechanism of iron(II)-driven uranium incorporation is projected. Uranium(V) incorporation into goethite is a major retention pathway across a broad range of uranium and iron(II) concentrations at circumneutral pH, with U(IV) precipitation becoming more prevalent at higher iron(II) concentrations, higher uranium concentrations, and higher pH. Incorporated uranium is also resistant to release and re-mobilization, since dissolution of the goethite host phase is slow. The third chapter examines the effect of aluminum impurities in the iron (hydr)oxide on uranium incorporation; uranium incorporation decreased with increasing aluminum content in the iron (hydr)oxide. Chapter four focuses on uranium incorporation into opaline silica, which may occur as a result of silica precipitation in soils and sediments. When iron (hydr)oxides and silica are present, uranium is retained on the iron solid, but this iron-bound uranium is encapsulated by silica as well as coordinated by the iron phase. The short-range molecular order of uranium in synthetic iron-bearing silica appears nearly identical to uranium in natural opals that are millions of years old, suggesting that uranium incorporation into opaline silica can occur on timescales of days to weeks, and potentially stably sequester uranium for millions of years. This research suggests that uranium incorporation into iron (hydr)oxides may be a competitive retention pathway in a range of subsurface environments where iron(II) drives transformations of iron (hydr)oxides and uranium. Furthermore, uranium incorporation into iron (hydr)oxides and opaline silica host phases can occur rapidly (on the order of days to weeks), and potentially sequester uranium for millions of years. This enhanced understanding of long-term uranium retention processes may find applications in geochronology, and will prove useful for risk assessment, prediction of contaminant fate and transport, contamination management, and remediation of uranium contamination.