Department of Earth & Environmental Sciences
Control of aqueous Mn(II) on the structure and reactivity of Mn(III, IV)-oxide minerals
Mn oxides are among the most reactive minerals found in the environment, exerting strong control on the aqueous and solid phase chemistry of soils and sediments through adsorption and redox reactions. Natural Mn oxides are predominantly of biological origin, representing either primary products of biological Mn(II) oxidation, or secondary products resulting from alteration of primary bio-oxides. Oxidation of Mn(II) by bacteria and fungi produces poorly crystalline and highly reactive birnessite-type Mn-oxides, which readily undergo abiotic transformation into secondary mineral products of higher crystallinity and stability. These secondary transformation reactions are, however, poorly characterized.
The work presented in this talk investigates the reaction between birnessite and aqueous Mn(II). Interfacial electron transfer between structural Mn(IV) atoms in the mineral lattice and adsorbed Mn(II) on the mineral surface produces extensive Mn(III), and ultimately leads to mineralogical conversion of birnessite into Mn(III)-oxide phases. The geochemical controls on the pathways of birnessite conversion, and the effects of Zn(II) impurities sorbed on the birnessite surface are discussed based on data from batch studies combined with X-ray diffraction, X-ray absorption, and infrared spectroscopic analyses. The results point to a potentially major role of aqueous Mn(II) as a control on the mineralogy and structure of environmental Mn-oxides, and as a moderator of the reductive arm of Mn-oxide redox cycling.