Polycyclic Aromatic Hydrocarbon Pollutants: An Emerging Global Threat

Polycyclic aromatic hydrocarbons are ubiquitous soil contaminants originating from natural and anthropogenic activities.

Polycyclic Aromatic Hydrocarbon Pollutants: An Emerging Global Threat

Polycyclic aromatic hydrocarbons are ubiquitous soil contaminants originating from natural and anthropogenic activities.

Environmental pollution is a serious problem in many developing and developed countries, especially for those in the process of rapid industrialization and urbanization with increasing population. Air pollution is an emerging environmental issue in major cities of the globe.

Persistent organic pollutants (POPs) are toxic substances composed of carbon-based chemical compounds and mixtures; such as pesticides, aromatic compounds etc. These are the products and by-products of industrial processes.

POPs are chemicals that persist in the environment, bioaccumulate through the food web, and exhibit toxic effects that may threaten the health of humans as well as wildlife. Because of their physicochemical properties and persistence, POPs are distributed throughout the globe and are often detected in remote regions where they are not or have never been used.

Polycyclic aromatic hydrocarbons (PAHs) are an important group of POPs that consist only of hydrogen atoms and sp2 hybridized carbon atoms in two or more aromatic carbon rings, although some Polycyclic aromatic hydrocarbons can also be methylated.

The PAHs play an important role not only in many areas of scientific and industrial activity, but also in environmental toxicology. There are more than 100 PAH compounds but 16 have been identified as priority pollutants by the US-EPA, namely: naphthalene, acenaphthylene, acenaphthene, fluorine, phenanthrene, anthracene, fluoranthene, pyrene, benz(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, benzo(ghi)perylene dibenz(a,h)anthracene and indeno(1,2,3-cd)pyrene.

Polycyclic aromatic hydrocarbons are ubiquitous soil contaminants originating from natural and anthropogenic activities involving crude oils or petroleum products. These can also be formed during geochemical and biochemical processes or synthesized during volcanic eruptions and forest, bush and peat fires or burnings of city solid wastes.

Being ubiquitous in nature, the PAHs are phototoxic, interfere with normal plant development and reproduction, bioaccumulate in food chains and cause some harmful effects on humans like mutagenic/carcinogenic, tumour formation, neuro-behavioral impairment including learning disorders and changes in temperament, immune system changes, reproductive deficits and sex-linked disorders.

Polycyclic aromatic hydrocarbons are becoming increasingly prevalent in soil environments as a result of wastewater irrigation, sludge applications, solid waste disposal through burning, automobiles exhaust and industrial activities.

According to US-EPA, the typical environmental level of PAHs in fresh water, outdoor air and soil is10 ng dm-3, 100 ng m-3, 2000 µg kg-1, respectively. The partition of PAHs in natural water, air, soil and vegetation is < 0.01 %, 0.1 %, 94.4 %, 0.1 %, respectively.

The natural sources are forest fire, volcanic activities and bacterial decay of organic materials. The anthropogenic sources may be divided into the following four categories.

  • Industrial: Coke oven, aluminium production, iron and steel foundries, coal gasification and coke production are the main industrial sources of PAHs. Petroleum refining, thermal power plant, occupations where coaltars, pitch, asphalt (bitumen), shell-oil and creosotes are used, emit much of PAH to make air polluted.
  • Automobile: Motor vehicle emissions (especially diesel vehicles) make a considerable contribution to PAH concentration in air due to burning and incomplete combustion of diesel or gasoline. Aircraft engine as a source of PAH in the atmosphere has also been identified.

Diesel Exhaust Particles: The popularity of the diesel engine in heavy-duty applications in trucking, railroad, marine transport, DG sets and construction industry is due to both its fuel efficiency and long service life relative to the gasoline engine.

Compared with gasoline engine, diesel emissions are lower in carbon monoxide (CO), hydrocarbon (HC) and carbon dioxide (CO2), but higher in oxides of nitrogen (NOx) and particulate matter (PM). Diesel exhaust is a complex mixture of both particulate and gaseous phase.

  • Domestic: Cooking (fuel burning) and waste refuse incineration.
  • Human Habitats: Smoking cigarettes, cigar and tobacco.

Mobile sources are however likely to be the major PAH contributors in urban or suburban areas, where major stationary sources are not present.

Most of the PAHs entering the aquatic environment are localized in rivers, estuaries and coastal marine water. Since PAHs are less sensitive to photo-oxidation in water than in air, these are more persistent in the aquatic environment.

Polycyclic aromatic hydrocarbons entering water are quickly absorbed on particulate matter, and large amounts are deposited in bottom sediments. Relative concentrations of PAHs are generally the highest in sediments, intermediate in aquatic biota and lowest in the water column.

One of the most important properties of PAHs concerning the analytical determination appears to be water solubility. As a rule, PAHs water solubility decreases (and hydrophobic interaction increases) with an increase in the number of fused benzene rings and angularity.

Polycyclic aromatic hydrocarbons tend to accumulate in atmospheric aerosols and could pose a threat to agricultural production and health of human beings and animals. The term aerosol refers to the mixture, i.e., both the particles and the suspending gas.

Aerosols can be defined as solid or liquid particles suspended in air. Particle sizes range from about 2 nm to more than 100μm. At some level, they are unstable, i.e., their properties change with time. There are different types of aerosols based on their method of generation, the size of the particles, health effects and whether the particles are solid or liquid.

Soil acts as a repository of waste materials of all the kinds and a place for existence of life on this planet. Now-a-days, soils have been put under severe pressure to produce more in order to supply food, fiber and shelter to increasing number of heads.

An uncontrolled increase in population and industrialization has affected the environmental quality all over of the world. The concentration of PAH in soil is considered a useful indicator of air pollution levels because airborne particles finally settle on the ground and are incorporated into the surface dusts or transferred into the surface of the soil.

The work was conducted to determine whether PAHs present in contaminated soils are transferred to edible parts of crop plants. Food ingestion is the most important means of human exposure to PAHs.

Organic pollutants can enter vegetation from contaminated soils through plant roots, but pollutants can also be derived from the atmosphere by deposition on leaves or by uptake through the stomata. Uptake of gaseous pollutants is an important pathway for semi-volatile chemicals, including PAHs, to enter vegetables according to field and in laboratory studies.

Various controlled experiments permit of determining pathways for uptake of PAHs by plants. The root uptake of PAHs for various plant species, and found that plant accumulation of PAHs correlated with soil PAHs concentration and plant composition.

Elevated PAH levels occur in vegetables grown in PAH contaminated soil. Root uptake is probably the main pathway for high molecular weight PAHs accumulated in vegetables, based on a greenhouse study.

Although pot experiments are commonly used to study foliar and root uptake of organic pollutants, most experiments have failed to prevent volatile or semi-volatile organic compounds from evaporating from soil to air and subsequently entering subaerial parts of plants.

Systems have been developed to differentiate between the uptake by roots and migration from subaerial parts of plants. Airtight lids and silicon rubber were sued to separate roots from subaerial parts of plants for investigating sorption of phenanthrene (PHE), anthracene (ANT) and 4-nonylphenol in wheat and tomato roots.

It can be concluded that PAHs are imperative pollutants that have been identified in diverse environmental matrices worldwide. These are of environmental concern not only because of their toxicity and carcinogenicity, but also due to their resistance to decomposition.

These compounds are included in a group of derivatives termed POPs. The POPs, have harmful effects on soil organisms, plants, animals and humans. Uptake of PAHs by plants may occur through soil-to-plant and air-to-plant pathways, including root uptake and atmospheric deposition from gaseous or particulate forms.

Airborne constituents comprised of PAHs lastly stay on the soil and are integrated into the surface soil. Thus, soil can play a crucial role as accumulator of such particulate matter in the air.

This article is jointly published by Muhammad Mazhar Iqbal1*, Muhammad Nawaz Khan1, Tayyaba Naz2, Muhammad Anwar-ul-Haq2 from 1Soil and Water Testing Laboratory Chiniot, Ayub Agricultural Research Institute, Department of Agriculture, Government of Punjab; 2Saline Agriculture Research Centre, Institute of Soil and Environmental Sciences, University of Agriculture Faisalabad. Corresponding author can be reached at mazhar1621@gmail.com orWhatsapp No. +92-332-6806367.