The overall goal of the research program was to provide new fundamental data and numerical simulations to assess the impact of bioreductive remedial processes on rates and mechanisms of colloid generation and the subsequent impact on contaminant co-transport, as well as the propensity for irreversible changes in soil structure following bioremediation. Specific objectives included:
- Developing an improved understanding and predictive capability of the impacts of subsurface bioremediation on the kinetics and mechanisms of (a) media structural breakdown and loss in permeability as the result of aggregate dispersion and (b) the generation of Fe- and clay-rich colloids and secondary mineral precipitates.
- Quantifying the fate and transport of colloid particles in heterogeneous subsurface media as a function of changing hydrological, geochemical, and microbiological processes, and investigating the impacts of accelerated contaminant co-transport vs. medium pore clogging on post-bioremediation groundwater quality.
- Developing predictive models to establish bioremediation protocols and groundwater monitoring strategies that optimize contaminant destruction and immobilization while minimizing the disruption of the subsurface media structure and the formation of mobile colloids.
The objectives were resolved by a technical approach that included three well-integrated experimental tasks, as follows:
- Task A: Quantify the impact of time-dependent bioreduction reactions on (a) the formation of secondary iron-oxide mineral precipitates for different Fe(III)-oxide minerals, and (b) the changes in microbial community structure and activity during the reduction process, which in-turn will dictate contaminant destruction.
- Task B: Investigate media structural breakdown and mineral dispersion during bioremediation.
- Task C: Development of a modeling tool of coupled hydrological, geochemical, and microbial processes controlling Fe(III)-oxide bioreduction and the impact of spatially variable secondary mineral precipitates (Task A) and permeability (Tasks A and B).
Tasks A and C were largely addressed by the previous research team and results are summarized as follows:
- The research quantified coupled hydrological, geochemical, microbial, and mineralogical processes to assess the importance of Fe(III)-oxide reduction on contaminant destruction.
- These experiments showed that the dominant terminal electron accepting process (metal versus sulfate reduction) varied as a function of iron oxide type and crystallinity and solution chemistry.
- Significant secondary mineral formation occurred in all columns with primarily magnetite formed in the ferrihydrite columns and sulfur- and sulfide-bearing phases dominating goethite and hematite amended columns. Microbial community analyses were consistent with the secondary minerals formed. Namely, when highly crystalline Fe-oxides were the dominant mineral phase prior to bioreduction, Fe-reduction rates were slow; allowing sulfate reduction to out compete iron reduction resulting in communities largely dominated by sulfate-reducing bacteria.
- Small porosity changes were evident in all the columns and substantial colloidal transport was observed in the ferrihydrite columns only.
- Bioreduction under these conditions may therefore lead to iron oxides serving as a contaminant (e.g. As) vector in subsurface sediments.
- Predictive models were successfully developed from the initial column experiments as described above in Tasks A and C.
Task B was largely addressed by the substitute project team and results are summarized as follows:
- Experiments intended to investigate media structural breakdown with undisturbed subsurface media were unsuccessful and deemed intractable at the experimental scale investigated (See Jardine IPR May 2014).
- Thus, tracer transport studies before and after Fe(III)-bioreduction were executed in re-packed column experiments using water-stable soil aggregates to examine potential changes in pore-structure and hydraulic conductivity. The results showed that Fe(III) bioreduction resulted in aggregate breakdown and colloid dispersion depending on the extent of Fe(III) reduction and altered the pore structure and chemical reactivity of the porous media. However, under the experimental conditions, only minor changes in tracer transport were observed suggesting that medium structural breakdown poses a negligible threat to groundwater quality. However, it should be noted that the experimental design did not allow for assessment of the potential for down-gradient changes in the redox environment that may lead to precipitation of secondary mineral phases and clay colloids with potential pore-clogging and decreases in hydraulic conductivity.
For colloid generation and pore accessibility, the microbially mediated Fe(III)-oxide reduction was demonstrated in this project to have significant effect on the transport of molecular and colloidal tracers and colloid generation, depending on the duration of the bioreduction process. For bioreduction and re-oxidation, this project showed that electron-donor addition during biostimulation cannot continue indefinitely. Thus, upon termination of biostimulation, the treated area will ultimately return to its original redox status as oxygenated groundwater passes through the treatment zone.
Hansel, C.M., C.J. Lentini, Y. Tang, D.T. Johnston, S.D. Wankel, and P.M. Jardine. 2015. Dominance of Sulfur-Fueled Iron oxide Reduction in Low-Sulfate Freshwater Sediments. ISME J, 10.1038(20115.50):1-13.