Observations and assessment of iron oxide nanoparticles in metal-polluted mine drainage within a steep redox gradient, and a comparison to synthetic analogs

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Virginia Tech

The complex interactions at the interfaces of minerals, microbes, and metals drive the cycling of iron and the fate and transport of metal(loid)s in contaminated systems. The former uranium mine near Ronneburg, Germany is one such system, where slightly acidic mine drainage crossing a steep redox gradient (groundwater outflow into a stream) forms and transforms iron (oxy)hydroxide nanoparticles. These particles interact with toxic metal(loid)s in water and sediments. Iron oxidizing and reducing bacteria also play a role in these processes. Biogeochemical reactions are influenced by nanoscale properties, and thus it is critical to probe environmental samples with appropriate techniques such as analytical transmission electron microscopy (TEM). This dissertation presents two studies on the iron (oxy)hydroxide mineral nanoparticles found in the Ronneburg mine drainage system.

The first study uses TEM in conjunction with bulk analytical techniques to demonstrate the complexity of iron (oxy)hydroxide transformations at the steep redox gradient, and the partitioning of metal(loid)s within those mineral phases. An important result was the identification of Zn-bearing green rust platelets in the anoxic outflow water. Green rust minerals have only been identified in nature a handful of times, and we believe this work to be only the second to examine naturally occurring green rust using high resolution TEM (HR-TEM). Downstream of the outflow, aggregates of poorly crystalline iron oxide spheroids co-precipitated with amorphous silica formed and settled to the stream bed, where they aged to form nanoparticulate goethite and sequestered metals such as As and Zn. However, significant concentrations of Zn and Ni remained in the dissolved/nano (< 0.1 um) water fraction and continued downstream.

The second study demonstrates that natural green rust nanoparticles and their synthetic analogs can be complex polycrystalline phases composed of crystallites only a few nanometers in size, and often include nano-regions of amorphous material. In addition to the typical pseudo-hexagonal platelet morphology, green rust nanorods were synthesized, which has not previously been reported. This work has important implications for the reactivity of green rust with biogeochemical interfaces in natural, anthropogenic, and industrial systems.

A third study, presented in the appendix, characterizes the bacterial community at the Ronneburg mine drainage site and highlights iron oxidizers such as Gallionella sp., in particular those that form stalks of iron oxide nanoparticles. These biogenic stalks also contribute to the uptake of metal contaminants in water and sediments.

The science of iron cycling is complex. It requires field-based exploration to enrich the contributions made by experimental, laboratory and modeling studies. This dissertation adds another chapter in the search for filling in missing pieces of this interconnected system.

green rust, ferrihydrite, iron cycling, metal uptake, transmission electron microscopy, crystal growth