Contaminant metal immobilization by biogenic manganese oxide nanoparticles: Implications for natural attenuation and bioremediation

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.