Bacterial Adsorption of Aqueous Heavy Metals: Molecular Simulations and Surface Complexation Models

Bacterial cell walls can adsorb a wide range of metal cations, potentially altering the mobility of the metals in geologic systems. To constrain and mitigate contaminant transport it is essential that geochemical models be developed to measure and quantify adsorption of heavy metals onto bacteria. This dissertation presents the work of three studies that apply molecular simulations (Ch. 2) and surface complexation modeling (Ch. 3 & 4) to improve our understanding of metal-bacterial adsorption reactions. Simulation models (Ch. 2) enabled us to estimate the most stable configuration for bacterial surface complexes and to compare binding affinities and interatomic distances with experimental values to validate and predict metal adsorption behavior. We found that mechanics-based simulations adequately describe the interactions of Cd with the cell wall, defining metal ion coordinations and binding distances. However, this approach does not accurately describe Pb-cell wall interactions, possibly due to limitations in the simulation parameters, the propensity for Pb to form hydroxides at circumneutral pH, or other adsorption mechanisms. We studied the effect of bacterial metabolism (Ch. 3) on the extent of Cd adsorption to Gram-positive and Gram-negative bacteria. We found that while metabolically-active Gram-positive cells adsorb significantly less Cd than non-metabolizing cells, Gram-negative cells show little difference in Cd adsorption. The metabolic effect on adsorption for Gram-positive cells is likely due to the proton motive force. The lack of effect in Gram-negative cells suggests that Cd adsorption occurs in a region of the cell wall not affected by proton motive force. We use a thermodynamic modeling approach to estimate that the effect of the proton motive force lowers the pH at the cell wall from 7.0 to 5.7. We applied potentiometric titrations and metal adsorption experiments (Ch. 4) and found that changes in bacterial diversity do not impact proton and metal uptake of consortia grown from three locations and sampled throughout a year, strongly suggesting universal adsorption behavior for the species present. Applying an averaged-site surface complexation model we found a single set of averaged acidity constants, site concentrations, and stability constants for metal-bacterial surface complexes that can be used to model the adsorption behavior.