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Phenanthrene

Phenanthrene, is a potential carcinogenic compound that poses a large toxicity risk to exposed living organisms. Phenanthrene is a polycyclic aromatic hydrocarbon (PAH), PAHs are a large group of organic compounds occurring in groups of two or more. Phenanthrene occurs naturally and is also a man-made chemical. Commonly, humans are exposed to phenanthrene through inhalation from burning fossil fuels and cigarette smoke. Evidence, proven through animal studies, shows that phenanthrene is a potential carcinogen. The EPA has identified 16 PAHs of most concern in regards to environmental toxicity .

Phenanthrene is released during the incomplete combustion of "..coal, oil, gas, and garbage." Incomplete combustion occurs when oxygen or air supplied to the combustion process is small so that carbon monoxide and carbon gases are released instead of carbon dioxide.

Phenanthrene travels through the environment through water runoff, from rivers or other contaminated waterways, and through atmospheric transport.

Environmental Concerns

Toxicity

Phenanthrene has an octanol water partition coefficient value of 4.57 K_ow. This correlates to phenanthrene having the ability to bioaccumulate in an organism. Individual toxicity effects are emphasized over entire ecosystem effects.

Microplastics

A sample of microplastics in a test tube.

The tendency for microplastics to adsorb toxic contaminants poses a threat for the organisms that may come into contact with a microplastic release pathway. Sorption of contaminants, such as phenanthrene, to different kinds of plastics depends on the pi-pi bond interactions; they have a higher tendency to sorb aromatic compounds. Microplastics are of concern regarding the acute toxicity of phenanthrene and other PAHs because phenanthrene-containing microplastics may be released into an organism’s body following ingestion. A study published in the scientific journal, Ecotoxicology and Environmental Safety, found the distribution coefficients (log K_d) values of phenanthrene to be, “3.07-4.20 (log L/kg)” in relation to microplastic particles. These values are considered to be low and correlate to the sorption ability of a chemical. A low log K_d value correlates to a chemical having the ability to move freely within the particle; here, the particle being a microplastic. The study also found that different kinds of plastics had higher phenanthrene sorption capabilities. Polystyrene has the strongest sorption rate of phenanthrene in comparison to polyethylene, polypropylene and polyvinyl chloride. The log K_d values for phenanthrene decreased as the microplastic particle size decreased. However, when the microplastic decreased to the nanoparticle size, the K_d values increased for phenanthrene because less surface area was available for sorption.

The correlation between toxicity and low sorption values of phenanthrene means that the PAH will more easily detach from whatever substance it has adsorbed to. This can mean that the phenanthrene ingested through an adsorbed substance, such as a microplastic, can more easily interact within the organisms who ingested the microplastic, leading to acute toxicity. Therefore, it is important to understand phenanthrene's adsorption capacity to microplastics as it determines the fate of the PAH toxin within a given ecosystem. Microplastics are of large concern not only regarding their ability to adsorb toxic chemical compounds but due to the fact that they are so small, clean-up efforts are extremely difficult. Understanding the effects microplastics have on ecosystem health will improve efforts to prevent and treat the threats they pose to the health of an ecosystem.

Soil Toxicity

Phenanthrene is a commonly studied PAH due to it being found in high concentrations within PAH mixtures in the environment; it is used as a reference molecule for other PAHs. One of the uses of phenanthrene by bacteria organisms is as a mode of uptake of carbon and energy. The accumulation of phenanthrene in soil is an issue because it will lead to phenanthrene toxicity in the (soil) ecosystem. Phenanthrene toxicity produces effects on functional soil processes, including decreased reproduction of the soil organism, “Folsomia candida (Collembola)..at..concentration of..25 mg/kg soil..”. This organism’s ability to reproduce effects “..several molecular pathways". Other processes affected by phenanthrene toxicity include, “..oxidative stress.” and “..chitin metabolism and protein translation..down-regulated..”. It was found that as the toxicity of phenanthrene in a soil ecosystem increased, the effects become more specific on the molecular level. Overall, phenanthrene toxicity affects a soil ecosystem by decreasing reproduction, leading to decreased health of the soil itself.

Marine pH Acute Toxicity

In a study published in the journal, Scientific Reports, phenanthrene's toxicity was found to increase alongside a decreasing pH. The effect of toxicity on marine organisms is a stunted growth rate. The EC₅₀ value determines when the expected effects of a toxic compound are 50% of the total expected effect. Alongside lower pH values, from 9 to 6, phenanthrene's EC₅₀ value decreased from "..1.893 to 0.237 mgL^-1.." A lower EC₅₀ value means that a a lower concentration of phenanthrene under increasingly acidic marine ecosystem conditions will reach acute toxicity faster than at higher, more basic pH conditions. Ocean acidification is of high concern regarding decreasing ocean pH levels.

Removal From Environmental Systems

PAHs can be classified as high or low in molecular weight depending on how many aromatic rings they contain. High molecular weight PAHs contain 4 or more aromatic rings and low molecular weight PAHs contain 2 or 3 aromatic rings. As the number of aromatic rings increases, the PAH molecules hydrophobicity increases and the molecule will have a stronger resistance to microbial degradation. A higher molecular weight and increased hydrophobicity also means that increasingly, the compound is less soluble in water; phenanthrene's molecular weight is lower than most PAHs.

Removal in a Marine Ecosystem

PAHs are known for being difficult to remove from the environment due to their hydrophobic properties, they do not dissolve in water; making PAHs a persistent pollutant . Dissolution increases the ease by which a contaminant moves through an ecosystem. If a contaminant does not dissolve in water, then it stays in the system until toxin-specific processes dominate degradation. In regards to PAHs, their degradation pathway is not completely understood; it is known that bacteria play a large role in degradation processes. Specifically, the bacteria Cycloclasticus, degrades, “..naphthalene, phenanthrene, pyrene, and other aromatic hydrocarbons”, in marine environments. Cycloclasticus are found globally in many aquatic ecosystems including, “..estuaries, coastal areas, deep-sea sediments, and polar oceans.” The degradation pathway of PAHs separate from external factors begins with, “..dihydroxylation mediated by a ring-hydroxylating dioxygenase.” Little is known about the rest of the pathway and the difference in pathways of specific PAHs. Apart from being able to degrade phenanthrene, cycloclasticus can use phenanthrene as a carbon and/or energy source. The degradation pathway transforms each PAH into an intermediate and further, degradation through mineralization.

Human Mediated Removal

Sorption

One of the techniques identified to remove PAHs from aquatic ecosystems is adsorption. Toxins bind to solid materials present in a system such as “..activated carbon, biochar, and modified clay minerals..” These solid materials have the ability to remove up to 100% of the contaminate in aqueous solutions or immobilize them in a soil. In a review published in the journal, Chemosphere in 2015, the recorded sorption rates of activated carbon, biochar, and modified clay minerals from 1934 to 2015 were summarized. The study found that the sorption rate depends on, "..particle size, temperature, pH, contact time, salinity.." Some of the results for phenanthrene adsorption include, a wheat straw biochar material adsorbed, "71-88%" of phenanthrene present . Powder activated carbon (PAC) adsorbed "95%" of phenanthrene and a soybean stalk based carbon adsorbed, "99.89%."

Sepiolite

An extracted specimen of sepiolite.

Ground sepiolite was studied to determine its capability to adsorb phenanthrene in a study published in the journal of Ecological Engineering in 2014. Application was centered around the use of sepiolite to bind phenanthrene and another PAH, pyrene, in groundwater systems. Sepiolite was chosen as a sorbent material because it is a low-cost option; activated charcoal is the most effective material used to adsorb pollutants but its high-cost increases the need for the exploration of low-cost options. Sepiolite was examined due to its "..high sorption capacity and selectivity because of..high porosity and molecular sieving properties." The sorption capacity of a PAH is dependent upon their hydrophobic properties; if a PAH is less hydrophobic then, it will possess less of an ability to bind to an adsorbent materials, including sepiolite . Although, when the phenanthrene and pyrene were studied as a mixture, the less hydrophobic phenanthrene had a higher adsorption capacity . Overall, sepiolite was determined to be a low-cost and efficient material to adsorb PAHs such as phenanthrene and pyrene. Adsorption is the most common removal process for PAHs. The mechanism for adsorption is summarized as: first, a mass transfer. Second, a physical or chemical adsorption by the molecule to the adsorbent material’s surface, and then, molecule diffusion to an adsorption site by "..a pore diffusion process..or..a solid surface diffusion mechanism."