Date of Award
Doctor of Philosophy (PhD)
Manganese (hydr)oxide (Mn (hydr)oxide) minerals are ubiquitous in aquatic and terrestrial environments. These minerals have high surface areas and are highly sorptive and redox active. From decades of accumulated knowledge about natural redox cycling, we have found that Mn (hydr)oxides play critical roles as electron donors and acceptors in elemental geochemical cycling and biological metabolisms in nature. Thus, Mn (hydr)oxides have garnered increasing interest to understand natural systems. Considering the variety of redox reactions with Mn (hydr)oxides in nature, it is also expected that there should be diverse pathways to form Mn (hydr)oxides through abiotic and biotic processes. Previous studies have focused on biotic oxidation and mineralization, and found that biotic processes are the dominant mechanism, with much faster oxidation of Mn2+ (aq) to Mn(IV) than observed in abiotic processes. Even in biotic processes, however, our understanding of the oxidation and formation mechanisms of Mn (hydr)oxides is limited. And not surprisingly, our understanding of the abiotic oxidation and formation of Mn (hydr)oxides is even more elusive. Therefore, further studies of oxidation of Mn2+ (aq) to Mn(IV) and consequent formation of Mn (hydr)oxides in both abiotic and biotic systems are necessary to reveal how nature forms current shapes of Mn (hydr)oxides, and uses them for natural redox reactions. Specifically, understanding the early nucleation mechanisms and kinetics of Mn (hydr)oxides is important because these nanoparticles can be starting points for bulk mineral formations of interest in both environmental and engineering applications. Therefore, in this dissertation, we have investigated the diverse oxidation and formation mechanisms of Mn (hydr)oxides in aqueous systems through both abiotic and biotic pathways. We have provided an advanced understanding of the Mn (hydr)oxide nucleation and phase transformation process by accomplishing three objectives.
First, the nucleation mechanism and kinetics of Mn(OH)2 (s) nanoparticles on quartz surfaces and in solutions were elucidated under varied ionic strength (IS) and pH conditions. Here, we found that varied IS and pH conditions influence the kinetics of Mn(OH)2 (s) formation on quartz, and change in the structural match between Mn(OH)2 (s) and quartz. From this objective, we provided a new quantitative and qualitative understanding of the influence of IS and pH on Mn (hydr)oxide formation, and of the mechanisms of heterogeneous nucleation of Mn (hydr)oxides on quartz substrates.
Second, the biomimetic oxidation of Mn2+ (aq) to Mn(III) and the subsequent biomineralization mechanism to α-Mn(III)OOH nanoparticles were studied using apoferritin, a protein produced by almost all living organisms, including bacteria, plants, and animals. The biotic processes of manganese oxidation by bacteria have been studied widely. However, in the literature, there are only limited explanations of how certain protein molecules mediate the formation of Mn (hydr)oxides inside the proteins’ cavity. Using the colorimetric method, we analyzed the kinetics of α-MnOOH core formation under varied experimental conditions, and found that Mn2+ and OH- are rate-determining agents, with orders of reaction of 2 and 4, respectively. This new information on the protein-mediated oxidation and formation of Mn (hydr)oxide gives important insights into the mechanisms of protein-mediated oxidation and the formation of Mn (hydr)oxides in natural systems.
Finally, we investigated the photochemically-assisted fast oxidation of Mn2+ (aq) and consequent formation of Mn(IV) oxides in inorganic abiotic systems. Even in the absence of organic matter and microorganisms, and contrary to prior reports, we found that Mn2+ (aq) is oxidized to Mn(IV) very rapidly under simulated sunlight. We observed Mn oxide nanoparticle formation and characterized the morphology and oxidation state change by reaction with a generated reactive oxygen species (ROS) source under simulated sunlight. The analyses of the oxidation state indicated the formation of Mn(IV) oxide, and in particular, of layered birnessite nanoparticles. We also found that in the presence of an inorganic ligand, pyrophosphate (PP), Mn (IV) oxide phase formation was enhanced and its structural change could be controlled by varying the concentration of PP. Elucidating the fast oxidation rate of Mn2+ (aq) into Mn(IV) oxides in inorganic abiotic systems, facilitated by ROS and a ligand, is a new and original contribution. Our results provide a novel, environmentally friendly, and facile synthetic pathway to make layered birnessite, which can be easily adapted by industry.
The advanced findings of this dissertation research provide crucial insights into the redox reactions and nucleation of Mn (hydr)oxides in dynamic natural and engineered environmental systems. They can also help to design environmentally benign nanoparticle synthesis methods and to control the phase identities of Mn (hydr)oxides, promoting their utility in engineered applications, such as Li-ion battery cathodes and heavy metal remediation.
John Fortner, Daniel Giammar, Srikanth Singamaneni, Elijah Thimsen,