Formation of Environmentally Persistent Free Radicals on Engineered Nanomaterials During Thermal Treatmen

环境持久性自由基是废物和有害物质在热处理过程中产生的一类新的环境污染物。在不同的环境中，如大气颗粒物中，环境持久自由基的存在水平足够高。与之前发现的寿命不到一秒的大气自由基不同，环境中持续存在的自由基存在几个小时到几个月，寿命足够长，足以使它们从源头传输到很长的距离。尽管对环境持久性自由基的开创性研究显著提高了我们对其对环境和人类健康影响的理解，但基本问题仍然没有得到回答。该项目将评估工程纳米材料在废物热处理过程中对环境持久性自由基形成的影响。结果将：1)提高对工程纳米材料在废物热处理过程中形成环境持久性自由基的相互作用的了解；2)提高对工程纳米材料和环境持久性自由基的行为、命运、变化以及人类和环境暴露的了解；以及3)通过为适合于工程纳米材料的处置策略的开发提供信息，支持纳米技术革命。结果将有助于减轻工程纳米材料和环境持久性自由基的不利影响，并将促进潜在更安全的工程纳米材料应用的开发，从而带来更多的经济机会。此外，代表人数不足的学生将通过参与研究项目接受科学和工程主题的培训。该项目还将通过整合项目的概念、方法和成果来促进教和学，以开发新的课程。以前的研究已经实验模拟了纳米过渡金属氧化物在大微米颗粒表面上形成环境持久自由基的过程。关于环境持久性自由基形成的主流观点认为，过渡金属是有机分子的关键成分，是有机分子的电子供体。这一观点解释了一些过渡金属氧化物上形成的环境持久自由基，但它与在氧化锌表面形成的自由基不一致，在氧化锌表面，电子转移到有机分子上。因此，目前的机制不能一概而论，环境中持久的自由基可能通过其他途径形成。这些问题的答案取决于这样一个假设，即由于工程纳米材料的带隙能量具有尺寸依赖性，而且热激发电子很容易穿过这个带隙，一些工程纳米材料可能会根据尺寸以及电子和化学性质，形成并稳定形成环境持久自由基的芳香有机物种。延伸这一假设，除了那些在焚烧中幸存下来的过渡金属组成的材料之外，工程纳米材料可能会形成环境中持久的自由基。实验研究将确定：(1)工程纳米材料和除过渡金属以外的工程纳米材料是否可以形成和稳定环境持久自由基；(2)工程纳米材料在热处理过程中的物理化学性质的变化是否会影响环境持久自由基的形成；或(3)工程纳米材料是否可以在低于燃烧过程中发生的温度下形成环境持久自由基。实验方法将使用这些分析技术：电子顺磁共振光谱来测量和表征环境中的持久自由基；质谱学来确定持久自由基重组时形成的化学物种的性质；以及电子显微镜和其他表面敏感技术来表征纳米材料的变化。这项研究的结果有望改变目前对环境持久性自由基的理解。首先，如果环境持久自由基能够在非金属工程纳米材料上形成，这将挑战现有的认为环境持久自由基只在过渡金属氧化物纳米颗粒上形成的想法。其次，如果环境持久性自由基可以在工程纳米材料上在较低的温度下形成，这表明环境持久性自由基很容易形成，因此更普遍。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Environmentally persistent free radicals, a new class of environmental pollutants, are formed during the thermal treatment of wastes and hazardous materials. Environmentally persistent free radicals are present at sufficiently high levels in diverse environments such as atmospheric particulate matter. Unlike previously identified atmospheric free radicals that exhibit lifetimes of less than a second, environmentally persistent free radicals exist for hours to months, lifetimes which are long enough for them to be transported over long distances from their source. Despite pioneering studies on environmentally persistent free radicals that have significantly improved our understanding of their environmental and human health effects, fundamental questions remain unanswered. This project will assess the impact of engineered nanomaterials on the formation of environmentally persistent free radicals during thermal treatment of wastes. Results will: 1) improve understanding of the interaction of engineered nanomaterial in forming environmentally persistent free radicals during thermal treatment of wastes; 2) improve understanding of the behavior, fate, transformation of and human and environmental exposures to engineered nanomaterials and environmentally persistent free radicals; and 3) support the nanotechnology revolution by informing the development of disposal strategies suitable for engineered nanomaterials. Results will assist in mitigating adverse impacts of engineered nanomaterials and environmentally persistent free radicals and will facilitate the development of potentially safer applications of engineered nanomaterials that can lead to enhanced economic opportunities. In addition, underrepresented students will be trained in science and engineering topics through their participation in the research project. The project will also involve the promotion of teaching and learning by integrating project concepts, methods, and results in the development of new curriculumPrevious studies have simulated experimentally the formation of environmentally persistent free radicals on nano-sized transition metal oxides that are supported on surfaces of large micron-sized particles. The prevailing thought on the formation of environmentally persistent free radical assumes that the transition metal is a key component, serving as the electron donor to an organic molecule. This view explains environmentally persistent free radicals that are formed over some transition metal oxides, but it is inconsistent with those formed over a zinc oxide surface in which an electron transfers to the organic molecule instead. Hence, the current mechanism cannot be generalized, and environmentally persistent free radicals may form via other pathways. Answers to these questions hinge on the hypothesis that since band gap energy for engineered nanomaterials exhibits size-dependence and thermally excited electrons may cross this band gap easily, some engineered nanomaterials, depending on size and electronic and chemical properties, may form and stabilize aromatic organic species forming environmentally persistent free radicals. Extending this hypothesis, engineered nanomaterials, other than those composed of transition metals that survive incineration, may form environmentally persistent free radicals. Experimental studies will determine if: (1) engineered nanomaterials and engineered nanomaterials, other than transition metals, can form and stabilize environmentally persistent free radicals; (2) changes in the physicochemical properties of engineered nanomaterials during thermal treatment impact environmentally persistent free radicals formation; or (3) environmentally persistent free radicals can form on engineered nanomaterials at temperatures lower than those that occur during combustion. Experimental methods will use these analytical techniques: electron paramagnetic resonance spectroscopy to measure and characterize environmentally persistent free radicals; mass spectroscopy to determine the nature of chemical species formed when persistent free radicals recombine, and electron microscopy and other surface-sensitive techniques to characterize changes in the nanomaterials. The results of this study are expected to transform the current understanding on environmentally persistent free radicals. First, if environmentally persistent free radicals can form on non-metallic engineered nanomaterials, it will challenge the existing thought that environmentally persistent free radicals form exclusively on transition metal oxide nanoparticles. Second, if environmentally persistent free radicals can form at a lower temperature on engineered nanomaterials, it suggests that environmentally persistent free radicals can form easily and are thus more prevalent.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.