Abiotic reduction of contaminants by iron minerals: rates, redox potentials, and greenhouse gases

Iron (Fe) containing minerals are a major component of sediments and soils and participate in important environmental processes such as nitrogen and carbon cycling, water quality preservation, and microbial organism metabolism. Many Fe minerals act like batteries and can donate, accept, and store charge. Here, we explore these battery-like behaviors by studying the reduction and oxidation (redox) reactions of Fe minerals, specifically magnetite and hematite under conditions relevant to soils and sediments. Magnetite is a mixed valent Fe mineral that has two different Fe charge (i.e., oxidation) states in its crystal structure and has been described as a biogeobattery in soils and sediments. Magnetite can accept, store, and release electrons that can then be used to cycle nutrients, like nitrogen or carbon, or clean up water. Hematite is a fully oxidized Fe mineral (all Fe(III)) that, like magnetite, has several properties, including a low cost for industrial uses, that make it able to discharge and charge like a battery.

Here, I evaluated how these minerals affect nitrogen cycling in soils and sediments. Specifically, I explored how nitrite (NO_2^-) a water pollutant that can be formed from nitrate (NO_3^-), a common water pollutant in Iowa from nitrogen-based fertilizers, is reduced by a range of Fe minerals as well as a sediment we collected from a restored farmland. Nitrite reduction is a concern because nitrous oxide (N₂O), a potent greenhouse gas, is produced when NO_2^- reacts with Fe minerals through a process called chemodenitrification. Our results demonstrate that reduction rates were relatively similar across each Fe mineral I investigated, but that the native Iowa sediment had much slower NO_2^- reduction rates. My most compelling finding was that although reduction by the Iowa sediment was slow, it produced the most N₂O compared to the other Fe minerals. Since N₂O is a large factor in predicting global climate change, it will be important for global climate models to incorporate these findings and will need future work to more accurately determine the extent of N₂O emissions from agricultural soils.

One way to tell whether contaminants will be reduced by soils and sediments is to measure the oxidation-reduction potential (ORP) which tells us how likely something is to donate (more negative potentials) or accept (more positive) electrons. In the environment, charge can be used from the biogeochemical battery described before to reduce Fe minerals and contaminants. Interpreting redox potentials for environmental systems is extremely challenging because there are multiple minerals, chemicals, and microbes that can influence the measured potential. I measured the redox potentials for magnetite to figure out which redox couple to use when we interpret our results. My work provides clear evidence that the relevant redox couple to use is most likely either maghemite and Fe(II)_aq, or lepidocrocite and Fe(II)_aq. One feature of magnetite that makes it relevant in the environment is the mineral stoichiometry (x), which represents the ratio of reduced Fe(II) to oxidized Fe(III) in the mineral. I found that magnetite stoichiometry affects measured magnetite redox potentials, and that as stoichiometry increases, the measured potential decreases.

Taken together, my work provides a better understanding for how Fe minerals behave in the environment. We found that global climate models are likely not accurately accounting for N₂O releases from agricultural soils and sediments. Our results suggest that to best interpret magnetite redox potentials, we should use one of two Fe(III) oxide and Fe(II)_aq redox couples contrary to previous efforts to model magnetite redox potentials. We also found that magnetite stoichiometry affects magnetite redox potentials and when we interpreted our measurements, we needed to create a new way of looking at our results to make sure we were accurate.