Comarmond, and 7 co-authors, reported in 2016 that the ternary system containing aqueous U(VI), aqueous phosphate and solid SiO2 was comprehensively investigated using a batch sorption technique, in situ attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy, time-resolved luminescence spectroscopy (TRLS), and surface complexation modeling (SCM). The batch sorption studies on silica gel (10 g/L) in the pH range 2.5 to 5 showed no significant increase in U(VI) uptake in the presence of phosphate at equimolar concentration of 20 μM, but significant increase in U(VI) uptake was observed for higher phosphate concentrations.
In situ infrared and luminescence spectroscopic studies evidence the formation of two binary U(VI) surface species in the absence of phosphate, whereas after prolonged sorption in the presence of phosphate, the formation of a surface precipitate, most likely an autunite-like phase, is strongly suggested. From SCM, excellent fitting results were obtained exclusively considering two binary uranyl surface species and the formation of a solid uranyl phosphate phase. Ternary surface complexes were not needed to explain the data.
The authors indicated that the sorption of U(VI) on SiO2 in the presence of inorganic phosphate initially involves binary surface-sorption species and evolves toward surface precipitation. They also concluded that the fate and transport of uranium in subsurface environments is an important consideration, particularly at many legacy nuclear sites. For a comprehensive understanding of the migration behavior of U(VI) in aquifers, a detailed description of the molecular scenarios comprising sorption and precipitation processes at mineral surfaces is mandatory. In particular in the presence of strongly interacting ions, the environmentally relevant parameters under which either of these processes becomes decisive in geochemical settings have to be explored.
The focus of the authors work was that slightly acidic pH is very relevant to the environmental conditions where phosphate remediation strategies are being considered. Phosphate amendment to mitigate the transport of uranium has received considerable attention over recent years, whereby the precipitation of U(VI) by the addition of phosphate to contaminated groundwater can be used as an alternative remediation approach to bioreduction of U(VI). As SiO2 is a large component of soils and host rocks, this work provides important information to assess the formation and stability of uranyl phosphate precipitated at the SiO2 surface.
The information provided by the combination of macroscopic and spectroscopic experiments described by the authors suggested that the predominant formation of a surface precipitate is initiated by U(VI) binary surface complexes. The surface-mediated formation of sparingly soluble solid uranyl(VI) phosphate phases initiated by already sorbed uranyl(VI) species might represent a significant retention behavior of silica toward aqueous U(VI) under environmentally relevant conditions.
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