Search

Bentonite clays and Fe-reducing bacteria have been independently identified as important geochemical agents possessing the ability to influence metal(loid) mobility in aqueous environments. In the context of metal(loid) immobilization, however, the interactions occurring between Fe-reducing bacteria and bentonite clays remain largely undescribed. This thesis examines the immobilization of Cu2+, Cd2+, and As5+ from aqueous solution by natural bentonite clays, and compares sorption to sterile bentonite clays with sorption in the presence of Fe-reducing bacteria. The research questions under investigation in this thesis are: 1) What are the influences of potentially metal-tolerant Fe-reducing bacteria on metal(loid) uptake by bentonite clay sorbents of varying smectite content and on the stability of bentonite clay-sorbed metal(loid)s?; and 2) Are there differences in the influence of Fe-reducing bacteria on Cd2+ and Cu2+ sorption to bentonite clays when As5+, a reducible, metabolically-available metalloid, is present in solution? While the influence of bacterial augmentation on Cu2+ sorption was negligible in these experiments, Cd2+ sorption was enhanced in the presence of Fe-reducing bacteria, and most profoundly, As displayed time-dependent desorption during the experimental timeframe in the presence of Fe-reducing bacteria. These results highlight the existence of potential limitations to the use of bentonite clay sorbents for metal(loid)-contaminated wastewater reclamation, identifying the microbially-vulnerable nature of metal(loid) sorption reactions. Abundant facets concerning clay-bacterial-metal(loid) interactions exist which require further characterization and experimentation to generate deeper understanding of the potential utility of, as well as limitations to, the use of clay mineral-based sorbents for the sequestration of toxic metal(loid)s from aqueous solution. Master of Science (MSc)

Microorganisms inhabiting silica solute-rich environments often show various degrees of Si02 mineralization as a consequence of exposure to SiOrsaturated waters. As such, it has been thought that microorganisms exert a prominent role in the immobilization of amorphous silica phases. While this intimate spatial relationship of microorganisms and amorphous Si02 phases are almost always observed in hot springs, the exact mechanisms by which microbes affect Si02 secondary mineral precipitation is still poorly understood. Further, available laboratory investigations to date consistently showed that microbes do not significantly impact Si02 immobilization, suggesting that microbial silicification is a mere consequence of exposure to a largely abiogenically-driven Si02 precipitation. This study demonstrates that discernible microbially-mediated silicification can occur under conditions where the potential for microbial opportunity to biomineralize is promoted. Identification of the key geochemical requirements for biosilicification to occur include thermodynamically favorable, but sluggish silica reaction kinetics associated with acidic conditions, and the necessity for colloidal silica rather than dissolved silicic acid species. This work provides the first results to bridge the apparent literature discrepancy between widespread, in-situ observations of microbial silicification, and the inability to demonstrate a detectable microbial effect in this process under well-constrained laboratory conditions. Acid conditions promote microbial silicification by overriding the dominant repulsive forces arising from charge similarities between Si02 and cell surfaces, via neutralization of deprotonated surface silanol and carboxylic groups, respectively. Mechanistic consideration for Si02 coordination to cell surfaces suggests direct chemical bonding of silanol to carboxylic groups forming stable inner-sphere complexes largely insensitive to environmental perturbations. This result indicates that microbially immobilized Si02 are more tenaciously-coordinated on cell surfaces and not simply electrostatically-held. Surface-dependent silicification showed higher Si02 mineralization propensities for unmineralized microbial cells compared to silica-encrusted cell matrices. Moreover, the extent and style of microbial Si02 mineralization is impacted by cellular level of metabolic activity. These results suggest that a biological overlay may be discernible in microbially induced biosilicification. Thesis Doctor of Philosophy (PhD)