U(VI) and Sr(II) batch sorption and diffusion kinetics into mesoporous silica (MCM-41)

Abstract

Important reactive phenomena that affect the transport and fate of a radioactive material such as uranium (U) and strontium (Sr) in the environment occur at the mineral–water interface, particularly in mesoporous materials which are ubiquitous in surface and near-surface environments, and typically dominate the reactive surface area of geologic media. Ion sorption and physical bonding forces (including electrostatic forces) can be significantly modified within these confined pore spaces, leading to preferential enrichment of trace elements in mesopores. Pore space confinement may also lead to kinetic restraints on thermodynamically favorable sorption/desorption, precipitation/dissolution, and redox reactions, due to slow migration of metals out of mesopores, chemical gradients within the pore space, or steric constraints for inward migration of larger molecules. Using a combination of benchtop sorption experiments, TEM, and synchrotron-based X-ray absorption spectroscopy, U(VI) and Sr(II) uptake on mesoporous silica (MCM-41) with a 4.67nm pore diameter was measured in batch conditions at pH4.0 and 9.8 as a function of time and metal speciation. Uptake of U was determined for U-hydrolysis, U–CO3, and U–CO3–Ca aqueous species. This suite of techniques 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 and U (at less than 10μM total U) rapidly diffuse into MCM-41. U at a higher concentration than 10μM also rapidly diffuses in, but the higher pore volume U concentration eventually leads to polymerization and precipitation of nano-U-bearing phases. The steady state U sorption maximum after 48h of exposure to MCM-41 prior to precipitation was dependent on the size and charge of the dominant U species in solution, where the trend is: UO2(OH)3−>UO2+2>UO2(CO3)3−4≈CaUO2(CO3)3−2. Precipitation of a U-bearing phase within the silica pore spaces occurred only after a threshold time point and indicated that U uptake was both thermodynamically and kinetically controlled. Initial diffusion and adsorption were controlled by aqueous speciation and precipitation was controlled by the buildup of sorption species that subsequently created a bottleneck effect near pore openings. Acidic solutions wee more efficient at extracting U than carbonate solutions once the U has diffused into the mesopore region, and this may explain frequent observations of this behavior in extractions of natural sediments. A nitric acid wash was not completely effective at desorbing U and Sr from the mesoporous silica, where there was 5% (initial pH4.0) and 20% (pH9.8) remaining even after 500h of exposure to the nitric acid wash. The results show that U and Sr diffusion and sorption into mesopores result in a recalcitrant pool of ions that are sequestered in deep internal pore spaces, and this may have significant impacts on attempts to clean up contaminated soils and sediments.