Collaborative Research: Nitrate Reduction by Redox-modified Fe-bearing Clay Minerals

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Nitrate contamination in soils, sediments, and surface and ground water poses significant risks to human health and global environments. Even though much is known about nitrate degradation in soils, especially via microbiological pathways, a complete solution to this problem has been elusive. One missing piece to the puzzle which could be highly significant is the role played by iron-bearing soil clay minerals in promoting nitrate degradation. The long-term purpose of this project is to devise strategies for on-site or in-stream nitrate remediation using common clay minerals as the catalyst. Because the iron largely remains in the clay mineral it stays in place during the nitrate reaction and can, thus, be regenerated or cycled between its oxidized and reduced states to continue the nitrate reduction process.Clay minerals are everywhere in nature. They make up a significant fraction of soils and of lake, river, and ocean sediments. Virtually all of these minerals contain iron, the electrical charge of which can be decreased (called 'reduction') by bacteria growing in and around them or increased (called 'oxidation') by atmospheric oxygen under well drained and well aerated conditions. This susceptibility to changes in electrical charge, which is known as an oxidation-reduction (redox) reaction, allows iron in the clay minerals to react with other chemical species which have a similar susceptibility to charge alteration. Nitrogen is one such element which, like iron, is susceptible to reduction or oxidation. Nitrate is a very common form of nitrogen and can be degraded or removed by redox reaction, which transform it into nitrite, atmospheric nitrogen, or ammonium, among other forms. A redox reaction between iron in the clay minerals and nitrate should be possible, but has never been investigated to any great extent. The purpose of this project is to test the hypothesis that clay minerals containing reduced iron can degrade nitrate to convert it to less harmful forms, include ammonium and atmospheric nitrogen. In this study, the clay minerals with be prepared by reacting them first with chemicals and bacteria to reduce the iron; then they will be treated chemically to make them more attractive for nitrate adsorption and reaction with the iron. The nitrogen species produced will be measured by an accelerated diffusion method, then the amount of nitrate changed to other species will be compared with the amount of oxidized and reduced iron before and after the reaction with nitrate. Results will be presented at scientific conferences, published in scientific journals, and reported to NSF.