Polycyclic Aromatic Hydrocarbons in Water: A Review of the Sources, Properties, Exposure Pathways, Bionetwork and Strategies for Remediation ()
1. Introduction
Polycyclic aromatic hydrocarbons (PAHs) are hazardous organic chemicals consisting of two or more benzenoid groups (Marce & Borrull, 2000). They are ever-present pollutants in our surroundings, released during incomplete combustion or via industrial processes and are characterized by high mutagenic and carcinogenic potential (Abdulazeez, 2017; Ayoub & Ahmed, 2019).
They have been considered earlier to form only during high-temperature chemical decomposition of organic materials (700˚C), but the discovery of complex mixtures of wide molecular weight range polycyclic aromatic hydrocarbon in fossil fuels has concluded that pyrolysis of organic materials even at low-temperatures (100˚C - 150˚C) can lead to the production polycyclic aromatic hydrocarbons. Different types of polycyclic aromatic hydrocarbons are formed based on combustion temperature; high-temperature combustion creates a more complex polycyclic aromatic hydrocarbons and low-temperature combustion gives rise to simple polycyclic aromatic hydrocarbons (Tobiszewski & Namieśnik, 2012).
There are some compounds within the wide range of PAH compounds that are toxic, persistent, and regarded as pollutants therefore are being regulated. Sixteen PAHs have been designated as priority pollutants by the United State Environmental Protection Agency (US-EPA, 2007). The extensive occurrence, mutagenic, carcinogenic and teratogenic effects of PAHs in the environment have made them worthy to be given more attention (Freitag et al., 1985). The widespread occurrences of PAHs have been attributed to its formation and release in all processes of incomplete combustion of organic materials. The properties and environmental fate of PAHs are dependent on the number of benzene rings in their structure and the molecular weight. High molecular weight PAHs are compounds with four or more benzene rings fused together, while the low molecular weight compounds consist of two or more fused benzene rings (Law et al., 2000). The low molecular weight PAHs are less persistent, highly volatile, slightly soluble in water and less carcinogenic but are toxic to aquatic organisms since they accumulate in their tissues and affect humans adversely upon consumption (Law et al., 2000).
2. Chemistry of Polycyclic Aromatic Hydrocarbons
The most common PAHs are anthracene, benzo(a)pyrene, chrysene, fluorine and pyrene (EPD, 2011). Others include; tetracene, Benzo[k]fluoranthene, Benzo[c]fluorine, Phenanthrene (Figure 1). PAHs exist in the air, water, soil and
sediments at low concentrations with high thermal stability (Johnsen et al., 2005). They can undergo volatilisation, photolysis, oxidation, biodegradation in surface water and bind to suspended particles or sediments or accumulate in aquatic organisms (ATSDR, 1995). Although the solubility of PAHs in pure water is very low, they may be solubilised by such materials as detergents, or they may otherwise occur in aqueous solution associated with or adsorbed on to a variety of colloidal materials or biota and are thus transported through the water environment (Andelman & Suess, 1970). There are two major sources of PAHs in drinking water. Contamination of raw water supplies from natural and man-made sources and leachate from coal tar and asphalt linings in water storage tanks and distribution lines (EPA, 2002).
Apart from highly industrially polluted rivers, the concentrations of individual PAHs in surface and coastal waters are generally = 50 mg/l (WHO, 1997). Concentrations above this level indicate contamination by PAHs mainly through industrial point sources and shipyards, atmospheric deposition and urban runoff. Elevated concentrations of PAHs were observed in rainwater and especially in snow and fog (WHO, 1997). This may be as a result of the adsorption of the compounds to air particles, which diffuses into the water during rains. Two ringed PAHs and to some extent three-ringed PAHs as shown Figure 2 dissolve in water, making them more available for biological uptake and degradation (Mackay & Callcott, 1998; Johnsen et al., 2005; Choi et al., 2010).
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Figure 2. A Structure of three-ringed and two-ringed PAH.
3. Sources of Polycyclic Aromatic Hydrocarbons
Polycyclic aromatic hydrocarbons are formed and introduced into the environment through both natural and anthropogenic sources (Lau et al., 2010). PAHs can be produced geologically when organic sediments are chemically transformed into fossil fuels found primarily in natural sources (Ravindra et al., 2008). Natural sources include; air from volcanoes and forest fires. Anthropogenic PAHs arising from man’s activities in his environment some of which are shown in Figure 3 includes:
· Cigarette smoke.
· Asphalt roads, coke, crude oil, creosote, roofing tar.
· Residential wood burning (Lau et al., 2010).
· Emissions from automobiles and power generating sets.
· Forest fires (Lau et al., 2010).
· Charbroiled meat.
· Discharges from industrial plants and wastewater treatment plants.
· Hazardous waste sites, coal gasification sites, smokehouses, aluminium production plants (IDPH, 2020).
4. Bionetwork
Cancer is a primary human health risk associated with exposure to pollutants. Benzo(a)pyrene is the most common PAH to cause cancer in animals. Repeated skin contact to the PAH naphthalene can result in skin inflammation. Breathing or swallowing large amounts of PAHs can cause the breakdown of red blood cells (Shashi et al., 2014). Exposure to PAHs has also been linked with cardiovascular disease and poor foetal development (Korashy & El-Kadi, 2006; Suades-González et al., 2015).
Studies have shown that when people are exposed to benzo(a)pyrene, at levels above the maximum contaminant level for relatively short periods, the red blood cells are damaged leading to anaemia; suppressed immune system. Long-term exposure to Benzo(a)pyrene at levels above the maximum contaminant level has the potential to cause developmental and reproductive defects as well as cancer (EPA, 2002).
Like every other pollutant, the health effects of exposure to PAHs depend on how much PAHs has entered the body, duration of exposure to PAHs, and the way the body responds to exposure.
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Figure 3. (a) Construction using coal tar. https://www.pettypictures.com/asphaltlaying; (b) man smoking cigarette. https://www.discovery.com/mansmokingcigratte; (c) smoke from exhaust. https://www.scannernews.com/smokefromcarexhaust; (d) charbroiled/grilled foodstuff. https://www.goodhomekeeping.com/charbroiled/grilledfoodstuff.
5. Standards and Regulations for PAHs Exposure in Drinking Water
The United States government agencies have established standards that are relevant to PAHs exposures in drinking water. The maximum contaminant level goal for benzo(a)pyrene in drinking water is 0.2 parts per billion (ppb). However, EPA had in 1980 developed ambient water quality criteria to protect human health from the carcinogenic effects of PAH exposure. The recommendation was a goal of zero (non-detectable level for carcinogenic PAHs in ambient water). EPA, as a regulatory agency, sets a maximum contaminant level (MCL) for benzo(a)pyrene, the most carcinogenic PAH, and five other carcinogenic PAHs. The maximum contaminant level set by EPA is as shown in Table 1.
6. Remediation Strategies for PAHs in Water
As a result of their hydrophobic nature and high affinity to particulate matter, PAHs can be removed from water sources by adsoption. A study has revealed that phenanthene, fluoranthene and pyrene can effectively be removed from water by batch sorption processes using lightweight expanded clay aggregate (LECA) as sorbent (Nkansah et al., 2012).
http://www.epa.gov.
Remediation can also be achieved by chemical treatment using Fenton’s reagent, hydrogen peroxide, ozone and activated persulphate to generate reactive free radicals capable of attacking the aromatic structure of PAHs (Huling & Pivetz, 2006).
Biostimulation methods such as biosparging can be employed to deliver oxygen into groundwater, soil and sediments to stimulate aerobic degradation of PAHs in sites with limited supply of oxygen (Breedveld & Sparrevik, 2000).
7. Conclusion
Polycyclic aromatic hydrocarbons (PAHs) in the environment particularly in water bodies deserve attention as this appears to be their fastest route into the human body. This is due to the fact that water plays a major role in mans daily dietary needs. Investigations by various scholars across the globe have revealed the presence of PAHs in environmental samples. Their bioavailability, disposition and metabolism in organisms are therefore important for environmental monitoring purposes. To this effect therefore, government should sustain the barn on smoking in public places, wood burning. The use of concrete or interlocking stones should be encouraged in road construction as against the traditional surfacing of roads using coal tar. Cars running on fossil fuels can be made to run on compressed natural gas CNG or liquefied petroleum gas (LPG) which is a purer form of energy. Recent advances in car technology are already going green in terms of energy thus rechargeable and solar powered cars are already available. Effluents from industries should at all times be recycled.