Discovering Environmentally-Critical Nanomineralogy: Highly Reactive Mn-Oxyhydroxide Nanofiber Nucleation and Growth Catalyzed by Nanohematite

Manganese oxides typically form by the oxidation of aqueous Mn(II) catalyzed by mineral surfaces and are nonspecific but potent redox-active mineral components commonly found in the environment. They therefore participate in a wide array of reactions with organic and inorganic compounds. They often exhibit high sorptive reactivities and capacities exceeding those exhibited by iron oxides. When present, they can play a key role in the mobility and bioavailability of important aqueous ions. The present study investigates abiotic Mn(II) oxidation catalyzed by nanoparticulate hematite in the presence of molecular oxygen in batch reactors. The kinetics of the reaction is studied as a function of the hematite particle size and the presence of organic ligands. The effect of organic ligands on the morphology of Mn-oxides is also studied. Surface-area normalized rate constants suggest differences in reactivity of hematite depending on its particle size. The end product of Mn(II) oxidation, a higher valent manganese oxyhydroxide identified as the mineral hausmannite, has been characterized by employing a suite of analytical techniques including high-resolution TEM, EELS mapping, SAED and SEM. The resultant Mn-oxyhydroxides have a unique nanosized, fiber-like morphology. SEM analyses were used to describe the formation and growth of Mn-oxyhydroxides fibers over time. Mössbauer analysis on nanoparticulate hematite after complete Mn(II) oxidation indicate small detectable amounts of Fe(II) suggesting hematite reduction and resorption of ferrous iron. These findings suggest that the formation of Mn-oxides can be influenced by the bulk mineralogical and geochemical composition of the surrounding environment, as well as by the interfacial solute–solid nanochemistry of the solid-phase oxidant. This study further signifies the relevance of lowtemperature interfacial geochemistry in the formation and the transformation of environmentally pertinent nanominerals