New insights into bacteria-related geochemical processes: Co-adsorption, temperature dependence of protonation, and effects on iron-hydroxide precipitation.

摘要

Microorganisms exist in a wide range of aqueous systems, from contaminated groundwater aquifers to mid-ocean ridge hydrothermal systems. Bacteria influence the mobility of mass in these systems because bacterial cell walls display a strong affinity for aqueous metal cations and organic molecules, and because bacteria can be involved in the dissolution and formation of mineral phases. This dissertation includes three distinct projects involving bacteria-related geochemical processes, as outlined below.; Chapter 2 deals with the effects of humic acid on Cd adsorption to the surface of Bacillus subtilis. Humic acid adsorption was measured with and without Cd, as a function of pH and humic:bacteria:Cd ratios. These experiments tested for the existence of ternary complexes in a bacteria-humic-metal system. We determine both the effects of humic acid on the bacterial adsorption of Cd, as well as the effects of Cd on the bacterial adsorption of humic acid. These experimental results constrain the relative importance of surface ternary and aqueous metal-humate complexes.; Chapter 3 reports results from the first potentiometric titrations of bacterial surfaces at elevated temperatures. We examine the protonation behavior of Bacillus subtilis at 30, 50, and 75°C. Using a surface complexation approach, we determine the functional group types present, their acidity constants, and their concentrations. Results indicate that the acidity constants for the surface functional groups do not change significantly over the temperature interval studied, and that the surface protonation at each temperature can be estimated using a single set of acidity constants and site densities. The lack of significant temperature dependence could greatly simplify the task of modeling bacteria-water-rock interactions at elevated temperature.; Chapter 4 deals with the adsorption of Fe(III) to bacterial surfaces and the effect of adsorption on the solubility of Fe-(oxy)hydroxide phases. We conduct a series of experiments that demonstrate that presence of bacteria cell walls enhance the extent of Fe removal from solution relative to the bacteria-free controls. We use the results of batch adsorption experiments in undersaturated conditions to explain the Fe removal in ‘oversaturated’ solutions. Some Fe removal by the bacteria was irreversible, indicating that the adsorption process is likely more complicated than that observed in previous metal-bacteria adsorption studies.