Question

Describe the mechanisms for degradation of polycyclic aromatic hydrocarbons (PAHs) in the air.

Answer 1

The polycyclic aromatic hydrocarbons(PAHs) are potent environmental polutants that comprise fused aromatic rings.They are found almost everywhere in the environment and typically formed during incomplete organic materials such as wood, coal, oil, gasoline etc.Some PAHs in the environment originate from natural sources such as burning,natural losses or seepage of petroleum or coal deposits,and volcanic activities.PAHs are also formed by incomplete combustion of carbon containing fuels such as fat. PAHs are primarily found in soil,sediment and oily substrates.Their concentration in air is small compaired to soil.Natural crude oil and coal deposits contain significant amount of PAHs arising from chemical conversion of natural products molecules such as steroids.Atmospheric partitioning of PAH compounds between the particulate and the gaseous phases strongly influences their fate and transport in the atmosphere and they enter into the human body.

Many PAHs have toxic,mutagenic and carcinogenic properties.PAHs are highly lipid soluble and thus readily absorbed from the gastrointestinal track of mammals.They are rapidly distributed in wide variety of tissues with a marked tendency for localization in body fat.Metabolism of PAHs occurs via the cytichrome P450-mediated mixed function oxidase system with oxidation/hydroxylation as a first step.

Several different remediation technologies have been tested in efforts to remove these environmental contaminants.Among them,bioremediation is showing particular promise as a safe and cost-effective option.In spite of their xenobiotic properties,a variety of genera of gram positive and negative bacteria,fungi and algae have been isolated and characterized for their ability to utilize PAHs.

The effectiveness of bioremediation has been mainly investigated under ideal laboratory conditions, having a circum-neutral pH and ambient mesophilic temperature. However, in the real situation, bioremediation can be effective only at sites where environmental conditions permit microbial growth and express associated enzyme activity so that microorganisms can enzymatically attack the pollutants converting them to harmless products. Numerous abiotic and biotic factors (such as pH, nutrient availability and the bioavailability of the pollutants) can apparently differ from site to site, which in turn can influence the process of bioremediation in those environments either by inhibiting or accelerating the growth of the pollutant-degrading microorganisms.

There are a number of obstacles to the application of dispersion models for predicting ground level concentrations of PAH:-

• PAH emissions estimates (even for [BaP] alone) have a high degree of uncertainty. This level of uncertainty will be transferred to any PAH concentration predictions.

• Processes which may be important for long range transport modelling (e.g. on a regional or national scale) such as dry deposition and degradation are not well known for PAH.

• Emissions estimates in future years are uncertain.

• There are a paucity of high quality monitoring data against which validation studies can be carried out.

Quality Assurance and Control required for PAH determination in air :-

Member states and their laboratories may use different analytical methods for determination of PAH in air across the European Union. Consequently, quality assurance (QA) and quality control (QC) requirements differ. Quality Management is vitally important in insuring sufficient precision (and accuracy) of the measurements used for the legal aspects of a limit value of PAH in air.

Important stages to be covered by QA and QC are:

Sampling of PAH in air :-The management of the sampling filter during and after sampling is important. Weather conditions as well as mobile and stationary sources influence significantly the levels of PAH in air. Because of this, it is necessary to document why the sampling site was selected and describe it’s operation. Depending on geographical location, a large variation in ambient temperatures may be expected.

Transport and storage of PAH sample :-The transport and storage of samples needs to be validated, as it is important to know the “history” of samples.

Pre cleaning of PAH :-This step in the analytical procedure involves extraction of the collected sample followed by clean up of the extract to obtain a PAH fraction which is then separated and detected.

Separation and detection of PAH :- In this step, specificity, selectivity, the linear range as well as detection and quantification limits must be determined using reference materials. The robustness of the method must be evaluated and the robustness criteria stated.

The aim is of PAHs impact on the environmental and the magnitude of the human health risks posed by such substances. They also contain important information on concentrations, burdens and fate of polycyclic aromatic hydrocarbons (PAHs) in the atmosphere. The main anthropogenic sources of PAHs and their effect on the concentrations of these compounds in air are discussed. The fate of PAHs in the air, their persistence and the main mechanisms of their losses are presented. Health hazards associated with PAH air pollution are stressed.