Upscaled Biologically Mediated Iron Reduction Reaction In Porous Media With FlowEPA Grant Number: F5A20133
Title: Upscaled Biologically Mediated Iron Reduction Reaction In Porous Media With Flow
Investigators: MacDonald, Luke H.
Institution: Princeton University
EPA Project Officer: Jones, Brandon
Project Period: September 1, 2005 through September 1, 2008
Project Amount: $111,172
RFA: STAR Graduate Fellowships (2005) RFA Text | Recipients Lists
Research Category: Academic Fellowships
The aim of this research is to understand the mechanisms of bacterial growth off a solid iron substrate via iron reduction in porous media, and answer the following questions:
- How do rate equations of biologically mediated iron reduction change from homogeneous systems without water flow, to more natural, heterogeneous systems with flow?
- Do upscaled equations describe experimental results and properly account for surface area loading effects of bacteria?
The proposed project will have two major modes of investigation: theory and experiment. Theoretically, the proposed research applies the method of volume averaging to pore scale reactive transport equations, assumed to be Monod type biokinetics, thereby upscaling them to the laboratory column scale. Monod kinetics describe microbial uptake and transport in macroscopic fluid volumes, but not in macroscopic porous media volumes without some modification that reflects the complexity of such a media. Volume averaging, a widely used mathematical technique in porous media fluid dynamics, integrates microscopic equations over macroscopic volumes. The mathematical rigor of volume averaging ensures that the upscaled reaction rates directly relate to processes that occur at the pore scale. Volume averaging preserves the underlying physics of microscopic mass and momentum conservation equations, allowing for accurate scaling. Prior studies have not used this approach to scale iron kinetics. Traditionally, most volume averaging studies focus on the fluid dynamics of a system, and researchers scale reactive and sorptive processes linearly. Several studies demonstrate the constraints on these traditional methods for transport and reaction within biofilms1, 2.
On the experimental side, the proposed research examines iron reduction in batch (no flow) and column (flow) experiments, with and without humics. Without humics, only biomass on the mineral surface participates in iron reduction, whereas humics act to shuttle electrons, thus enabling most of the bacterial biomass to reduce iron. I will conduct a series of experiments on quartz sand columns with heterogeneous, varying iron distributions, and seeding the columns with Geobacter Sulfurreducens (an iron and uranium reducing bacterium). For every porous iron distribution, I will setup a column with a humic substance and a column without a humic substance. I will also perform batch experiments for varying total iron concentrations.
To assess how well the upscaled equations match experimental results, I will use a numerical model. I will test to see if batch experiments yield the input parameters to the model by comparing the model output to large-scale, heterogeneous column experiments. Otherwise, I will use small column experiments with uniform media and iron distributions to input into the upscaling model.
Pore-scale volume averaged equations will most likely reflect processes at larger scales, provided they take into account surface loading effects of bacterial biomass and the presence or absence of electron shuttling humic substances.