Important reactive phenomena that affect the transport and fate of radioactive material such as uranium (U) and strontium (Sr) in the environment occur at the mineral-water interface (MWI), including sorption and redox reactions.  Compared to ideal mineral-water systems, much less is understood about MWI in natural environments, which typically have nanometer to micron scale secondary mineral coatings on the surfaces of primary mineral grains and complex porosity properties  Mesoporous material are ubiquitous in surface and near-surface environments, and typically dominant the reactive surface area of geologic media.  Surface chemistry, ion sorption and related geochemical reactions can be significantly modified with these confined pore spaces, including the preferential enrichment of trace elements in mesopores.  Pore space confinement can also lead to kinetic restraints on thermodynamically favorable sorption/desorption, precipitation/dissolution, and redox reactions due to the slow migration of metals out of mesopores due to chemical gradients created by the pore space confinement or by steric constraints on larger reactants migrating in.  Using a combination of benchtop sorption experiments, TEM, and synchrotron-based x-ray absorption spectroscopy, U and Sr uptake on mesoporous silica (MCM-41) with a 4.67 nm pore diameter was measured as a function of time and solution composition under batch conditions.  Uptake of U was determined for U-hydrolysis, U-CO3, and U-CO3-Ca aqueous species.  This suite of techniques used here enabled determination of the rate of metal sorption and precipitation in the pore spaces, and identification of the reaction products.  Our results indicate that Sr(II) and U(VI) (at less than 10 uM total U) rapidly diffuse into MCM-41.  U at higher than 10 uM also rapidly diffuses in, but the higher pore volume U concentration eventually leads to polymerization and precipitation of nano-U-bearing phases.

U(VI) and Sr(II) sequestration in mesoporous materials: The importance of confined pore spaces

247th National Meeting (2014)

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