The Solution to Pollution: Using Porous Materials to Clean the Air
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Academic Journey & Research Interests
The passion for my research was sparked long before I ever entered academia. As a young boy growing up on a farm in rural Nova Scotia, the idea of sustainability was taught at an early age. I recall cool springs spent planting crops followed by warm summers spent reaping the fruits of our labour. This was the first piece of tangible evidence that demonstrated to me that we could get our basic needs from the planet without completely destroying it in the process. This idea has influenced both my personal, and academic, journey to date.
Upon beginning my undergraduate degree at St. Francis Xavier University in 2012, I realized my passion for the environment could be further explored via a Bachelor of Science (BSc) in Chemistry. In my senior year at St. Francis Xavier University, I had the opportunity to gain some chemistry research experience in the MacLean Research Group (led by Dr. Brian MacLean). This work looked at the reduction of carbon dioxide (CO2), a harmful greenhouse gas, using hydrated transition metal-based complexes. More specifically, this work applied aquo (a complex containing water molecules) ruthenium (a transition metal) complexes for the reduction of CO2 to environmentally-friendly fuel sources. This work inspired me to continue doing research in environmental chemistry, which led me to apply to Memorial University of Newfoundland. In September 2016, I entered the Doctorate of Philosophy (PhD) in Chemistry program in the Department of Chemistry under the supervision of Dr. Michael Katz and Dr. Cora Young. Dr. Katz’s work focuses on synthesizing porous materials and using them for a wide variety of applications, while Dr. Young’s work is primarily environmental and atmospheric chemistry-based. The research project that sparked my interest most was one that combined both of their fields to explore the use of porous materials in capturing air pollutants and ultimately cleaning the air.
Solving The Pollution Problem Using High Surface Area Frameworks
As the population rises and human activity continues to produce greenhouse gases, researchers work tirelessly to not only remove the existing pollution but also plan for decades of pollution to come. The work I do at Memorial tackles the global environmental threat that is air pollution. More specifically, my project focuses on nitrogen oxides, which are released through the burning of fuels [1, 2]. As a whole, nitrogen oxides can directly degrade air quality which negatively impacts vegetation, visibility, and human health [3-5]. One of the other qualities that make nitrogen oxides even more dangerous is their ability to react and form dangerous secondary pollutants (pollutants that are formed in the atmosphere but not always directly emitted from a source) [6]. Of these dangerous secondary pollutants, my work has focused on one pollutant specifically – atmospheric nitrous acid [7]. Nitrous acid is an extremely reactive species that can break down in the presence of light to form hydroxyl radicals [8]. These radicals contain an unpaired electron that makes them highly reactive in the atmosphere. Hydroxyl radicals are able to enter the respiratory tract of humans and cause irreparable damage [9]. Unfortunately for humans, who spend over 80% of their life inside, levels of nitrous acid are over 10 times higher indoors [10]. The presence of nitrous acid inside the home can be a threat to human health due to its reactive nature. Nitrous acid can react with the chemicals present in both cooking and cigarette smoke (amines) to form carcinogenic nitrosamines (i.e., third hand smoke) [11]. Third hand smoke gets trapped in your home and, similarly to second hand smoke, can directly impact human health [12, 13].
This project began with first finding a way to capture nitrous acid from a gaseous stream at environmentally-relevant concentrations. By removing nitrous acid from the air, we are ultimately removing an important precursor to the formation of other dangerous species such as third hand smoke and ground-level ozone. In order to capture nitrous acid, we decided to focus on a class of highly porous materials called metal-organic frameworks (MOFs). To better understand how metal-organic frameworks work for nitrous acid removal, you can imagine their function to be similar to that of a sponge. A sponge has many channels in which water can be stored. These channels can be used to soak up dirty water which can then be transferred to a sink for proper disposal. Similarly, metal-organic frameworks can store air pollutants within their high surface area framework. The pollutants can then be converted to benign compounds (my work), or they can be released and disposed of in a manor in which there is minimal overall environmental impact. The main difference between a sponge and the metal-organic frameworks we utilize is that when the pollutants enter the sponge-like framework they undergo a reaction inside and the products squeezed out are environmentally benign. Much of the work completed in Dr. Katz research group deals with engineering these metal-organic frameworks and using them for a wide array of applications [14].
Figure 1: A schematic representation of the industrial production of pollutant species and the eventual conversion of nitrogen oxides to nitrous acid followed by using sponge-like metal-organic framework species to produce environmentally-friendly products.
From Lab-Scale to Large-Scale
One of the more interesting aspects of my work is the novel instruments that are used within both Dr. Katz and Dr. Young’s labs. In order to test if we are able to use specific metal-organic frameworks to selectively capture a pollutant(s), we first create a tunable source of the desired pollutant to simulate real-world conditions. Once produced, the nitrous acid is quantified with a gas analyzer designed to detect NOx (which is also capable of detecting nitrous acid). With a continuous stream of the nitrous acid from our source and a nitrogen oxides analyzer, we are able to place a metal-organic framework between the source and the analyzer to test which metal-organic frameworks are able to decontaminate nitrous acid.
Figure 2: A schematic of our tunable nitrous acid gas generator used for environmentally-relevant decontamination experiments. Grey tubing represents dry carrier gas (nitrogen), blue tubing represents humidified carrier gas (nitrogen containing water vapour), red tubing represents acidic gas (nitrogen gas containing gaseous HCl), purple tubing represents humidified acidic stream (nitrogen gas with water vapour and gaseous HCl), brown tubing represents gaseous nitrous acid. The production of nitrous acid is dependent on a humidified reaction between HCl and NaNO2.
Moving forward, this work will involve expanding on the applications of porous materials for the decontamination of other pollutants that threaten both the environment and human health. Next year, my project will focus on the creation of additional gas sources for the production of other gas-phase pollutants. Upon creation of new gas sources, metal-organic frameworks will be synthesized and tested for their ability to remove these harmful chemicals from the air. The goal of my work is to turn these frameworks into practical filters for use in households (e.g., air exchange systems) and in industry (e.g., smoke stack filtration systems). These frameworks have proven to have a massive capacity for pollutant removal and if they can be successfully implemented in the real-world, then they could dramatically reduce air pollution and ultimately slow the progression of climate change.
If you are interested in getting a detailed look at my work in removing gaseous pollutants using metal-organic frameworks you can read my most recent manuscript published in RSC Chemical Science entitled “Selective Decontamination of the Reactive Air Pollutant Nitrous Acid via Node-Linker Cooperativity in a Metal-Organic Framework“.
As well, if you would like to find out more about the other research going on in my environmental and inorganic materials research groups or contact my supervisors you can visit their websites at www.KatzResearchGroup.com and www.CJYgroup.com, respectively.
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References:
1. Boyle, E., Nitrogen pollution knows no bounds. Science, 2017. 356(6339): p. 700-701.
2. Chen, J.G., et al., Beyond fossil fuel-driven nitrogen transformations. Science, 2018. 360(6391): p. eaar6611.
3. Lelieveld, J., et al., The contribution of outdoor air pollution sources to premature mortality on a global scale. Nature, 2015. 525(7569): p. 367-371.
4. Chock, D.P., et al., Effect of nitrogen oxides (NOx) emission rates on smog formation in the California South Coast Air Basin. Environmental Science & Technology, 1981. 15(8): p. 933-939.
5. Ranjan, O., J. Menon, and S. Nagendra, Assessment of air quality impacts on human health and vegetation at an industrial area. Journal of Hazardous, Toxic, and Radioactive Waste, 2016. 20(4): a4016002.
6. Kinugawa, T., et al., Conversion of gaseous nitrogen dioxide to nitrate and nitrite on aqueous surfactants. Physical Chemistry Chemical Physics, 2011. 13(11): p. 5144-5149.
7. McGrath, D.T., et al., Selective decontamination of the reactive air pollutant nitrous acid via node-linker cooperativity in a metal-organic framework. Chemical Science, 2019. 10(21); p. 5576-5581.
8. Pusede, S.E., et al., An atmospheric constraint on the NO2 dependence of daytime near-surface nitrous acid (HONO). Environmental Science & Technology, 2015. 49(21): p. 12774-12781.
9. Datta, K., S. Sinha, and P. Chattopadhyay, Reactive oxygen species in health and disease. National Medical Journal of India, 2000. 13(6): p. 304-310.
10. Zhou, S., et al., Time-resolved measurements of nitric oxide, nitrogen dioxide, and nitrous acid in an occupied New York home. Environmental Science & Technology, 2018. 52(15): p. 8355-8364.
11. Pitts, J., et al., Photooxidation of aliphatic amines under simulated atmospheric conditions: formation of nitrosamines, nitramines, amides, and photochemical oxidant. Environmental Science & Technology, 1978. 12(8): p. 946-946.
12. Tuma, R., Thirdhand smoke: studies multiply, catchy name raises awareness. Journal of the National Cancer Institute, 2010. 102(14): p. 1004-1005.
13. Hang, B., et al., Thirdhand smoke causes DNA damage in human cells. Mutagenesis, 2013. 28(4): p. 381-391.
14. McGrath, D.T., V.A. Downing, and M.J. Katz, Investigating the crystal engineering of the pillared paddlewheel metal–organic framework Zn2(NH2BDC)2DABCO. CrystEngComm, 2018. 20(39): p. 6082-6087.