Soils, sediments and natural waters may become contaminated by toxic metals from anthropogenic sources, such as mining and industrial wastes, vehicle emissions, lead-acid batteries, paints, etc., threatening public health and ecosystem. Birnessite, the dominant type of wide-spread manganese (Mn) oxide mineral in the environment, is a metal scavenger and can hold several percentages of toxic metals of its body weight, hence acting as an effective detoxifying agent. The ability of birnessite to scavenge metals, as well as other properties, highly rely on its structure and chemical composition that are affected by unknown geochemical processes. The main objective of this funded research is to identify the geochemical processes and discover how they take place step by step, how rapidly they happen, as well as whether and how they are affected by common environmental conditions. Results will benefit not only the fundamental research on critical geochemical and mineralogical processes, but also industrial applications of Mn oxides, such as fabricating high-performance birnessite as sorbents and oxidizers for pollution control and as semiconductor for harvesting solar energy in the photovoltaic industry. The proposed study will train students at different levels. The PI and a high school teacher will work together to develop a curriculum in earth science for high school students.
The extraordinary properties of birnessite are determined by Mn vacancy and Mn(III) concentrations in its structure. There is, therefore, a critical need to identify those geochemical processes that control birnessite vacancy and Mn(III) concentrations. In the absence of such knowledge, assessing contaminant fate and transport particularly in Mn-rich environments will remain challenging. We hypothesize that the reactions of Mn(II) with birnessite control the vacancy and Mn(III) concentrations, and that the reactions are subject to influences of solution chemistry and birnessite formation rates. Both microbially mediated and chemically-synthetic reaction systems will be employed. X-ray absorption spectroscopy, diffraction and atomic pair distribution function analysis, and high resolution transmission electron microscopy will be used to characterize birnessite samples in addition to wet chemical analyses. Results are expected to advance the current understanding while generating new knowledge of Mn oxide mineralogy and geochemistry and their impact on biogeochemical cycles.
