Fracture Size Controls on Diffusion Rate Into Micro-Fractures
EMSL Project ID
40060
Abstract
Dissolution of uranium minerals within fractures has been found to be a diffusion rate limited process; thus, diffusive mass transfer in sediment micro-factures significantly affects the fate and transport of uranium in the subsurface. In 1-10 micro-m micro-fractures, the diffusion coefficient of uranium contaminated waste water, in natural sediment, into pore water has been measured by NMR as 2% of the measured unrestricted diffusion coefficient for uranyl in water. It is unclear, however, as the specific influence of fracture size on diffusion. Therefore the objective of this investigation is to quantify diffusion retardation as a function of micro-fracture size. To investigate this process a micro-fluidics model will be engineered by inductively coupled plasma-deep reactive ion etching system at EMSL. The model will have a linear main channel with micro-channels, 0.5 mm long and 5 micro-m - 30 micro-m wide, coming off the main channel. Water spiked with a uranium phosphorous complex will be pumped as a step input through the model, and uranium will be monitored while diffusing into the micro-channels. Diffusion will be calculated from image analyses obtained from using a Nikon Epiphot 200 epi-fluorescent microscope with 10x objective, and a RT Spot CCD digital camera. This equipment is available in the Subsurface Flow and Transport Laboratory at EMSL. Preliminary results demonstrate that diffusion of fluorescein dye into a 5 micro-m channel was reduced by 19% at 30 seconds and 30% at 7 minutes, compared to a 30 micro-m channel. While the time frames of these experiments are short, the results suggest the significance of microfractures in subsurface sedimentary material on the transport and fate of contaminants.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2010-10-01
End Date
2012-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members