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

土壤、沉积物、地表水和地下水中的硝酸盐污染对人类健康和全球环境构成重大风险。尽管人们对土壤中硝酸盐的降解，特别是通过微生物途径的降解已经了解得很多，但对这个问题的完整解决方案一直是难以捉摸的。这个谜题的一个缺失部分可能非常重要，那就是含铁土壤粘土矿物在促进硝酸盐降解中所起的作用。该项目的长期目的是设计利用常见粘土矿物作为催化剂的现场或河流内硝酸盐修复策略。因为铁大部分留在粘土矿物中，所以它在硝酸盐反应中停留在原地，因此可以在氧化和还原状态之间再生或循环，以继续硝酸盐还原过程。粘土矿物在自然界中无处不在。它们构成了土壤、湖泊、河流和海洋沉积物的重要组成部分。几乎所有这些矿物质都含有铁，铁的电荷可以被生长在它们内部和周围的细菌减少（称为“还原”），或者在排水和通风良好的条件下被大气中的氧气增加（称为“氧化”）。这种对电荷变化的敏感性被称为氧化还原反应，它允许粘土矿物中的铁与其他对电荷变化具有类似敏感性的化学物质发生反应。氮就是这样一种元素，它和铁一样，容易被还原或氧化。硝酸盐是氮的一种非常常见的形式，可以通过氧化还原反应降解或去除，氧化还原反应将其转化为亚硝酸盐、大气氮或铵等形式。粘土矿物中的铁与硝酸盐之间的氧化还原反应应该是可能的，但从未深入研究过。该项目的目的是测试含有还原铁的粘土矿物可以降解硝酸盐，将其转化为危害较小的形式，包括铵和大气氮的假设。在本研究中，粘土矿物首先与化学物质和细菌反应以还原铁；然后对它们进行化学处理，使它们更容易吸附硝酸盐并与铁发生反应。用加速扩散法测定生成的氮种，然后将硝酸转变为其他种的量与硝酸反应前后氧化还原铁的量进行比较。研究结果将在科学会议上展示，发表在科学期刊上，并向美国国家科学基金会报告。

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.