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

摘要: Abstract Important reactive phenomena that affect the transport and fate of a radioactive material such as uranium (U) strontium (Sr) in environment occur at mineral–water interface, particularly mesoporous materials which are ubiquitous surface near-surface environments, typically dominate area geologic media. Ion sorption physical bonding forces (including electrostatic forces) can be significantly modified within these confined pore spaces, leading to preferential enrichment trace elements mesopores. Pore space confinement may also lead kinetic restraints on thermodynamically favorable sorption/desorption, precipitation/dissolution, redox reactions, due slow migration metals out mesopores, chemical gradients space, or steric constraints for inward larger molecules. Using combination benchtop experiments, TEM, synchrotron-based X-ray absorption spectroscopy, U(VI) Sr(II) uptake silica (MCM-41) with 4.67 nm diameter was measured batch conditions pH 4.0 9.8 function time metal speciation. Uptake U determined U-hydrolysis, U–CO 3 , –Ca aqueous species. This suite techniques enabled determination rate precipitation identification reaction products. Our results indicate Sr (at less than 10 μM total U) rapidly diffuse into MCM-41. higher concentration diffuses in, but volume eventually leads polymerization nano-U-bearing phases. The steady state maximum after 48 h exposure MCM-41 prior dependent size charge dominant species solution, where trend is: UO 2 (OH) −  > UO + 2 (CO ) − 4  ≈ CaUO − 2 . Precipitation U-bearing phase spaces occurred only threshold point indicated both kinetically controlled. Initial diffusion adsorption were controlled by speciation buildup subsequently created bottleneck effect near openings. Acidic solutions wee more efficient extracting carbonate once has diffused mesopore region, this explain frequent observations behavior extractions natural sediments. A nitric acid wash not completely effective desorbing from silica, there 5% (initial pH 4.0) 20% (pH 9.8) remaining even 500 h wash. show mesopores result recalcitrant pool ions sequestered deep internal have significant impacts attempts clean up contaminated soils