Uranium is a significant contaminant at shallow subsurface sites impacted by mining and nuclear fuel cycle operations and is also present in higher activity radioactive wastes destined for deep geological disposal. In both shallow and deep subsurface systems, iron (oxyhydr)oxides will be ubiquitous. In these environmental settings, sulfate-reducing bacteria will be present and may reduce sulfate in groundwaters to sulphide via microbial metabolism of electron donors (hydrogen and organics) present in the wastes. Here, we explore the evolution in uranium speciation during the abiotic sulfidation of model iron (oxyhydr)oxide phases (ferrihydrite and magnetite) with sorbed uranium. Geochemical aqueous analyses, X-ray Photoelectron Spectroscopy, X-ray Absorption Spectroscopy (XAS), and transmission electron microscopy have been used to gain a detailed mechanistic insight into the pathway of mineral transformation and uranium speciation during the sulfidation process. Experiments were performed under controlled and monitored conditions using a chemostat reactor. Experimental pH and sulfide addition were controlled, with sulfidation taking place over six hours. In systems with U(VI) adsorbed to ferrihydrite, sulfidation caused the ferrihydrite to transform to nanocrystaline FeS (mackinawite). During the early stages of this process, uranium was transiently released into solution at low but measurable levels. EXAFS analysis of samples suggested a complex pathway of U(VI) transition to U(IV) under these conditions with clear transition from U(VI)-adsorbed to ferrihydrite to uraninite via a transient and novel U(VI)-species.

Evolution of uranium speciation during sulfidation of iron (oxyhydr)oxides under conditions relevant to geological disposal of radioactive waste

255th National Meeting (2018)

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