Metal Adsorption to Bacterial Cells and Their Products

Bacteria and their products have the ability to adsorb metals and can have a great impact on the global cycling of these metals. Detailed information about the quantity and reactivity of different components in the system is important to develop geochemical models that can predict the fate of these metals. This dissertation presents the work of three studies that provide critical insights into metal-bacterial adsorption reactions. Chapter 2 describes the adsorption and subsequent reduction of Au(III) both Gram-positive and Gram-negative bacteria, as well as reactions with bacterial exudates. The experimental results reveal that the speciation of the aqueous Au and the bacterial surface controls the rate of Au removal; under low pH conditions aqueous Au complexes adsorb readily and rapidly. With increasing pH, adsorption slows significantly. Bacterial exudates can reduce Au(III) at circumneutral pH, but on a slower time scale than the bacterial cell wall. Chapter 3 focuses on the extracellular polysaccharides (EPS) produced by bacteria and whether EPS is important in proton and Cd binding, relative to the cell wall. Our results suggest that EPS contains functional groups that are similar to those on the cell wall, and that EPS and bacterial cell walls exhibit similar site concentrations and affinities for adsorbing protons and metal cations from solution. In chapter 4, we applied potentiometric titrations and Cd adsorption experiments to determine binding capacities of acidophilic and alkaliphilc bacteria to better understand whether diverse and extreme environmental conditions have an effect on proton and metal uptake by the cells. While we found differences in the acidity constants calculated for the acidophiles and alkaliphiles, all the extremophiles exhibited similar extents of Cd adsorption above pH 5.5. This strongly suggests that bacteria that grow under extreme conditions exhibit similar proton and metal adsorption behaviour to that of previously studied neutrophilic species, and that a single set of proton and metal binding constants can be used to model the behaviour of bacterial adsorption under a wide range of environmental conditions.