Polyaromatic Hydrocarbons (PAHs)
Polyaromatic hydrocarbons (PAHs) are a class of organic chemicals that occur naturally and as a result of combustion. They are composed of only Hydrogen and Carbon without any functional groups, which are arranged in two or more aromatic rings. These aromatic rings are most commonly, but not always benzene ( Cyclohexatriene). They are also known as Polycyclic Aromatic Hydrocarbons and Polynuclear Aromatic Hydrocarbons(1,2.)
PAHs occur naturally in coal, crude oil, and gasoline. They also are produced when almost anything is burned, such as: coal, oil, gas, wood, garbage, and tobacco. PAHs generated from these sources can bind to or form small particles in the air. (3.)
Etymology
Poly: Many or multiple. Cyclic: In Chemistry, a molecule in which at least some of the atoms form a ring. Nuclear: An atom (Nucleus). Aromatic: An Aromatic compound is a molecule that has a ring with with pi electrons delocalized all around the ring, often resulting in alternating double bonds in a ring. Hydrocarbon: A hydrocarbon is any type of organic molecule that is made up entirely of Hydrogens and Carbon(s).
The word Cyclic is redundant as all aromatic compounds are by definition Cyclic. Likewise Nuclear is redundant as all chemical compounds have more than one atom.
A chemical compound made of two or more aromatic rings composed entirely of Hydrogens and Carbons.
Examples
Image 1: Molecular structure of the 16 PAHs considered as priority pollutants by the American Environmnental Protection Agency (EPA)
Source: Harvey, P. J., Campanella, B. F., Castro, P. M., Harms, H., Lichtfouse, E., Schäffner, A. R., … & Werck-Reichhart, D. (2002). Phytoremediation of polyaromatic hydrocarbons, anilines and phenols. Environmental Science and Pollution Research, 9(1), 29-47.
Health
Studies have found a link between premature death and ambient air pollution, especially PAHs. Exposure to elevated PAH levels include but. are not limited to: atherosclerosis, cardiac hypertrophy, arrhythmias, contractile dysfunction and cancer. Over 90% of people live an area with air pollution levels above the WHO (World Health Organization) recommended safe levels, with it causing an estimated 4.2 million premature deaths per year.
At least some PAH’s have been shows to cause and exacerbate asthma as well.(7)
Remediation
The efficiency of chemical degradation of PAHs is limited because of their low vapour pressure and aqueous solubility. (4) Other factors impacting degradability is size of the PAH as well as soil qualities. (5)
Bioremediation
Rhizodegradation
Rhizodegredation has been the main type of bioremediation that has been recognized and researched in regards to PAHs. Tall Fescue (Festuca arundinacea) has been shown to have enhanced degradation of Pyrene and phenanthrene in its rhizosphere. (6)
Sources:
1. Harvey, P. J., Campanella, B. F., Castro, P. M., Harms, H., Lichtfouse, E., Schäffner, A. R., … & Werck-Reichhart, D. (2002). Phytoremediation of polyaromatic hydrocarbons, anilines and phenols. Environmental Science and Pollution Research, 9(1), 29-47.
2. Marris, C. R., Kompella, S. N., Miller, M. R., Incardona, J. P., Brette, F., Hancox, J. C., … & Shiels, H. A. (2020). Polyaromatic hydrocarbons in pollution: a heart‐breaking matter. The Journal of physiology, 598(2), 227-247.
3. CDC (Center for Disease Control) National Biomonitoring Program. (https://www.cdc.gov/biomonitoring/PAHs_FactSheet.html#:~:text=Polycyclic%20aromatic%20hydrocarbons%20(PAHs)%20are,small%20particles%20in%20the%20air.)
4. Gupte, A., Tripathi, A., Patel, H., Rudakiya, D., & Gupte, S. (2016). Bioremediation of polycyclic aromatic hydrocarbon (PAHs): a perspective. The Open Biotechnology Journal, 10(1).
5. Jonsson, S., Persson, Y., Frankki, S., van Bavel, B., Lundstedt, S., Haglund, P., & Tysklind, M. (2007). Degradation of polycyclic aromatic hydrocarbons (PAHs) in contaminated soils by Fenton’s reagent: a multivariate evaluation of the importance of soil characteristics and PAH properties. Journal of hazardous materials, 149(1), 86-96.
6. Cheema, S. A., Khan, M. I., Tang, X., Zhang, C., Shen, C., Malik, Z., … & Chen, Y. (2009). Enhancement of phenanthrene and pyrene degradation in rhizosphere of tall fescue (Festuca arundinacea). Journal of Hazardous Materials, 166(2-3), 1226-1231.
7. Karimi, P., Peters, K. O., Bidad, K., & Strickland, P. T. (2015). Polycyclic aromatic hydrocarbons and childhood asthma. European journal of epidemiology, 30(2), 91–101. https://doi.org/10.1007/s10654-015-9988-6