Microscopic characterization of porosity, diffusivity, and tortuosity in single particles of Hanford sediments using Nuclear Magnetic Resonance (NMR) Technique
EMSL Project ID
3481
Abstract
Our recent characterization of uranium(VI) speciation and physical location in 30-year contaminated Hanford Site sediments demonstrated that uranium(VI) primarily resides as U(VI)-silicate microprecipitates in small fractures, cleavages, or dead-end voids within sediment particle grains. X-ray microprobe (XRM) and a backscattered scanning electron microscope (SEM) analysis revealed that the uranium within intraparticle regions typically occurs as radiating crystal clusters and linear vein-filling masses predominantly within particle grain near-surface microfractures at a size of few microns or less. The U(VI) dissolved into undersaturated porewater and slowly diffused out of intraparticle regions. But intraparticle porosity, diffusivity, tortuosity, and diffusion characteristics are unknown. Although diffusion within particle grains has been frequently identified as one of major rate-limiting factors leading to the inefficiency of various groundwater remediation techniques, diffusion studies within single particle grains are lacking. Microscopic information of diffusive process is critical and presents an important scientific knowledge gap to understand the contaminant transport in the subsurface sediments and projection of future risks to the environments. In this project, we propose to fill this knowledge gap by studying diffusion process and measuring diffusion properties within single particle grains using nuclear magnetic resonance (NMR) technique.
The intraparticle porosity, and pore size and length scale distribution will be measured by saturating single particle grains with water (H2O) and then examined using NMR relaxation and pulsed-field-gradient (PFG) methods at the Environmental Molecular Sciences Laboratory (EMSL) at PNNL. The spatial heterogeneity of intraparticle porosity will be mapped by microscopic NMR imaging with 50-100 micron isotropic spatial resolution.
Intraparticle diffusivity or tortuosity will be measured by two experimental approaches. First, intraparticle H2O in the H2O saturated single particle will be displaced with deuterated water (D2O) and 1H NMR method will be used to trace H2O changes within intraparticle regions as a function of time. The H2O signals from the external particle can be suppressed via diffusion or flow weighting methods to leave only the intraparticle contribution to the signal ? alternatively the solid samples can be centrifuged to separate from aqueous phase before measurement to remove the water signal from external particles. Detailed distribution profiles of H2O relative to D2O displacement within intraparticle regions will be mapped using a rapid NMR materials-imaging technique to define the diffusion paths/channels. The second approach is to use NMR PFG methods to monitor the bounded diffusion of the individual H2O molecules (via their nuclear spins) thus there is no inherent measurement time limit. Diffusion can be measured with this method as a function of direction, potentially yielding diffusivity tensors. These experimental results and microscopic imaging from single particles will allow the development of a conceptual and mathematical diffusion model and estimation of diffusion coefficients.
Finally, the developed diffusion model from single particles will be mathematically upscaled to a field-relevant system with mixed mineral fragments using volume-averaging method. Both approaches using D2O displacement of H2O and PFG-NMR will be used to measure the apparent H2O diffusion in the mixed systems. The measured diffusion results from the mixed system will be used to test the upscaled diffusion model.
Project Details
Project type
Capability Research
Start Date
2003-10-01
End Date
2006-04-06
Status
Closed
Released Data Link
Team
Principal Investigator