Date of Completion
The ground water and surface water interface (GSI or Hyporheic zone) is a spatially- and temporally-fluctuating biogeochemical transition zone connecting these two distinct hydrological components. The mixing zone is characterized by a near neutral pH, ambient temperature and a sharp oxygen concentration which in turn has deep impact on the iron cycling and thus the fate of contaminants in the water column. We constructed an experimental micromodel that to observe key processes that affect biogeochemical iron cycling in the fresh water interface zones where the oxic surface water mixes with Fe (II) rich underlying ground water. The flow cell was constructed from Polydimethylsiloxane, (PDMS) a transparent, biologically inert silicone polymer which can be modified to represent the surface properties of natural porous media by oxygen plasma exposure. Our results demonstrated that with 25 seconds of high RF oxygen plasma exposure, the zeta potential of the PDMS surface is well within the reported range for other silicates and can be used as a surrogate for natural porous media. Using preformed iron oxide particles, a particle deposition rate of 415 min-1 and a single collector efficiency of 0.081 were obtained after PDMS “beads” had reached steady state coating. Iron oxide coating thicknesses on the PDMS beads were greatest at the inlet side of the flow cell and decreased in the flow direction toward the outlet. Reverse chemical gradients of dissolved Fe (II) and oxygen were created in a flow cell using a diffusive mixer to deliver source waters containing iron-only or oxygen-only solutions at circum-neutral pH. Subsequent monitoring of iron oxidation in the flow cell showed iron oxide particles to form and accumulate along the longitudinal center-line of the flow cell where the theoretical Fe (II) oxidation rate was calculated to be highest. Iron oxide particle accumulation in the flow cell was greatest around beads located at the inlet and decreased with distance along the flow path towards the outlet. These results indicate that PDMS micro-models may be used to simulate mixing zones in porous media, in particular the mixing of oxygen-rich water with anoxic iron- or metal-rich groundwater that represents an important aspect of biogeochemical iron cycling in the groundwater-surface water interface.
Ghaisas, Neha A., "Understanding Formation and Transport of Amorphous Iron Oxyhydroxides in Porous Media using Microfluidic Flow Cells- a Novel Method to Study Freshwater Iron Cycling" (2011). Master's Theses. Paper 170.