A Thermodynamic Modeling Approach to Metal Fate and Bioavailability in Aqueous Microbial Systems
Bacterial adsorption of metals can have a great impact on environmental metal cycling. A comprehensive understanding of the extent of metal adsorption onto bacteria in aqueous systems is imperative in order to predict the fate of metals in natural and engineered geologic systems. This dissertation presents three studies that provide insight into metal-bacterial adsorption reactions. Chapter 2 describes the effects of metabolically inactive bacterial cells and bacterial exudates on HgO(solid) precipitation as a function of solution saturation state. The experimental results reveal that Hg removal due to the presence of bacteria can be explained through adsorption alone in systems that are undersaturated with respect to HgO(solid). The results also show that bacterial exudates can cause significant inhibition of HgO(solid) precipitation due to the formation of Hg-exudate aqueous complexes. Chapter 3 characterizes the adsorption behavior of divalent metal cations onto the sheathless variant of iron-oxidizing bacterium, Leptothrix cholodnii, and compares the adsorption behavior to that of the well-characterized Gram-positive bacterium, Bacillis subtilis. The results demonstrate that L. cholodnii and B. subtilis exhibit similar binding affinities for a range of metals and that any iron oxides produced during L. cholodnii growth are low in abundance and show no effect on the adsorption behavior. These findings were used to estimate the extent of divalent cation competition that occurs between an iron-oxidizing bacteria (FeOB) and the iron oxide byproducts of their metabolism, which are closely associated with the bacterial cell surface. The calculations demonstrate that precipitation of these iron oxides by FeOB leads to significant competition of nutrients with bacterial cells, but that the cells still can adsorb inorganic nutrients even with high concentrations of iron oxides present. Finally, Chapter 4 evaluates bacterial removal of Zn, Ni, Co, Mn, Pb, and Sr as a function of both pH and metal:bacteria ratio in order to derive metal-bacterial sulfhydryl binding constants. The results show no enhanced removal of both Pb and Sr by bacterial-sulfhydryl sites. In contrast, Zn, Ni, Co, and Mn exhibited high binding affinities to bacterial sulfhydryl sites relative to binding to the other bacterial reactive functional groups. Using these results, we derive a relationship to estimate metal-bacterial sulfhydryl binding constants that have not yet been measured, and apply our findings to calculate the importance of both bacterial adsorption in general and sulfhydryl binding in particular in two representative complex contaminated environmental systems. These calculations suggest that bacterial sulfhydryl sites dominate metal binding and speciation in environments when metal:bacteria ratios are low, and that these low loading conditions are likely to be the norm in even fairly highly contaminated systems.
History
Date Modified
2017-06-02Defense Date
2016-04-04Research Director(s)
Jeremy FeinDegree
- Doctor of Philosophy
Degree Level
- Doctoral Dissertation
Program Name
- Civil and Environmental Engineering and Earth Sciences