Layer-type manganese oxide (MnO₂) minerals produced by bacteria are ubiquitous environmental nanoparticles that play an important role in trace-metal scavenging. These minerals often occur as biomineral assemblages composed of bacterial cells, MnO₂ nanoparticles and a biofilm matrix. The goal of this dissertation is to provide a quantitative and molecular-scale understanding of metal attenuation by Pseudomonas putida-MnO ₂ assemblages (biogenic MnO₂). The sorption of Co, Ni, Cu and Zn by preformed and actively-growing biogenic MnO₂ was studied alongside metal inhibition of bacterial growth and enzymatic MnO₂ precipitation. This research highlights the geosymbiotic nature of metal-microbe-mineral interactions regulating the concentrations and distribution of metals in contaminated environments.

X-ray absorption spectroscopy showed that the reactivity of preformed biogenic MnO₂ was driven by the presence of vacancy sites [up to 20% of the total Mn(IV) sites]. Sorption mechanisms were dependent on solution pH, surface coverage, and metal properties such as coordination preference and redox potential. Triple-corner-sharing complexes at vacancy sites were formed by Ni, Zn and Cu, whereas Ni and Co became absorbed into the MnO ₂ sheets. Of the metals studied, only Cu—which formed Jahn-Teller distorted, octahedral complexes—showed possible adsorption at particle edges. Surface-catalyzed precipitation of Zn and Ni occurred at high surface coverage and low proton activity. Sorption by the biofilm matrix increased in the order Ni < Zn < Cu; metal complexation by organic functional groups increased as the binding sites on the mineral fraction became saturated and at lower pH values where protons and lower-valent Mn are favorably adsorbed by the oxide surface.

Metal attenuation was reduced significantly in the presence of actively growing biogenic MnO₂. Cobalt sorption depended primarily on MnO ₂ formation, while Cu and Zn sorption were influenced by the organic functional groups in the biofilm matrix and by the presence of metabolically active cells. Metal inhibition of MnO₂ formation led to the accumulation of Mn(II) in solution; competition from Mn(II) for organic and inorganic functional groups in the biomineral assemblages further reduced the sorption of Cu and Zn. Compared to the preformed biogenic MnO₂, which accumulated up to 10–13% (metal:Mn molar ratio) Ni and Zn, the metal-scavenging capacity of actively-growing biomineral assemblages decreased by up to a factor of four, ten, and twenty-five for Zn, Co, and Cu, respectively.

Concentration-response relationships showed that the metal tolerance of P. putida GB-1, as measured by bacterial growth, increased in the order Co << Ni < Cu ≈ Fe << Zn << Mn, whereas inhibition of enzymatic MnO₂ precipitation decreased in the order Ni >> Fe ≈ Co > Cu > Zn >> Mn. No direct correlation was found between metal inhibition of bacterial growth and metal inhibition of MnO₂ precipitation; Mn oxidation was disrupted at lower metal concentrations than bacterial growth, suggesting that the ability to oxidize Mn does not confer metal tolerance to P. putida GB-1. Lastly, metal toxicity was correlated with the metals' electronic structure and propensity for covalent interactions.