Methods for Remediation of PAHs from Contaminated Soil

Polycyclic aromatic hydrocarbons (PAHs) are prevalent contaminants in our environment and originate from both anthropogenic and natural sources. Because of their genotoxicity, mutagenicity and carcinogenicity these contaminants have received a lot of attention in recent decades and their remediation from contaminated soils and water sources have become a high priority around the world. Substantial research efforts have been directed worldwide in order to work towards solving this problem. The adoption of sustainable technologies for the treatment and remediation of PAHs in soil has been at the forefront of these efforts.

PAHs soil testing is so necessary because PAHs pollution has become a widespread problem due to their low solubility and persistent longevity in soil. PAH contaminated soil is very dangerous and human sources such as leaking underground storage tanks, abandoned gas sites, and industrial sources are the primary culprit. Polycyclic Aromatic Hydrocarbons PAHs are primarily formed through pyrolytic processes. These mainly involve incomplete combustion of organic materials during industrial and other human activities, such as fossil fuels processing, natural gas combustion, burning of garbage, cooking and tobacco smoking. Natural processes such as forest fires and volcano eruptions also create PAHs through carbonization.

So if you have done PAHs soil testing that has resulted in high concentrations, what can be done to rid your contaminated soil of these dangerous toxins?

Most conventional methods for the remediation of PAHs contamination involve the excavation of soil to another site for:

• Incineration
• Thermal conduction
• Solvent extraction/soil washing.

All of these methods have proven effective in remediation over the decades. However, they are expensive propositions and in the case of incineration and soil washing have also been met with questions from the scientific community about the transfer of pollutants from one phase to another. Still they are effective options for remediation that will get the PAHs removed from your land.

Over the last few decades new research has developed at least five different approaches using biological techniques for the remediation of PAHs in soil.  These methods are:

1- Composting

Composting is a remediation technique in which nutrients are added to the contaminated soil along with moisture and oxygen control in a contained system. This is a technique that is most commonly used with municipal solid waste treatment and it was demonstrated to be effective in biodegrading polycyclic aromatic hydrocarbons (PAHs) as well. The addition of manures and earthworms in the composting matrix has shown further degradative effect. This kind of remediation would most likely be done at a remediation facility, but new innovations could make it possible on site as well.  

2- Landfarming

The idea behind landfarming involves the stimulation of indigenous microorganisms to degrade PAHs through the addition of nutrients and a carbon source, then mixing the soil to better distribute the nutrients and carbon.  Finally through the introduction of  oxygen deep into soil in order to increase the opportunity for microbial contacts with the contaminants and their ultimate breakdown. 

3- Biostimulation

The process of biostimulation greatly increases the activity of the indigenous population of microbes in contaminated soil to alter the composition of the pollutants. Biostimulation multiplies the microbes already present in the soil through the addition of nutrients and / or a terminal electron acceptor (TEA).  A wide variety of combinations and levels of macro- and micronutrients have been used to enhance the bioremediation process.  Linoleic acid and mushroom cultivation substrate have both shown in studies their potential as cost effective and green biostimulation agents.

4- Bioaugmentation

Bioaugmentation is biostimulation with the addition of the introduction of a specific potential microbe or group of microbes to enhance (augment) the metabolic capacity of present indigenous microbial populations. These augmenting agents will vary widely depending on the indigenous microbes of your soil and the contaminants they are remediating, but great success has been achieved in lowering levels of PAHs through a combination of processes and microbial agents.

5- Phytoremediation

One of the emerging on-site green remediation strategies showing real promise is Phytoremediation. Phytoremediation uses plants to reclaim contaminated soil and water   that has high levels of toxic pollutants. This remediation method increases microbial activity in the rhizosphere (rooted areas) by degradation of the PAHs in contaminated soils through metabolic processes. A long list of potential plants for phytoremediation is highlighted by rye grass and sunflowers.

These three chemical remediation options use natural chemical processes to degrade the PAHs in contaminated soils.

1- Photooxidation

Direct photooxidation uses sunlight to degrade PAHs in surface soils and atmospheric degradation. In this process PAHs are directly oxidized after they absorb sunlight radiation. The rate of chemical transformation that occurs during photooxidation varies depending upon sunlight intensity as well as overlapping spectral characteristics of solar radiation. 

Indirect photooxidation occurs when other substances like clay, organic matter, and inorganics absorb sunlight energy and then transmit that energy to the PAHs via electron orbital interactions leading to degradation of the PAHs.

2- Fenton system

Fenton’s reagent treatment involves the use of peroxide at different concentrations (from 3 to 35%) along with ferrous iron (Fe II). Because peroxide (H2O2) decomposes into highly reactive nonspecific hydroxyl radicals when combined with ferrous iron it chemically oxidizes the PAHs in the soil. This method is dependent on the composition of the soil but has proven effective.  

3- Ozonation

Ozonation or ozone treatment is the use of gaseous ozone for PAH remediation. It’s effectiveness rests in the molecular structures of the compounds; PAHs react very fast with ozone. In-situ ozonation has been shown to be an attractive option for PAH removal at numerous contaminated sites especially with more recent contaminations.  

Low level testing of PAHs in soil and water.

We offer 8270 and 8270 SIM testing for Polynuclear Aromatic Hydrocarbons (PAHs) in soil and water. The 8270 test method has reporting limits that are generally around 0.5 ppbv, which in most cases is sufficient for soil and water monitoring.

Testing Against ESLs

If your testing results must compare against Residential or Commercial environmental screening levels (ESLs), you may be required to test soil and water with a more sensitive method that can report the data at 0.005 ppbv level. Generally, we meet these low-level reporting limits by performing the test using the 8270 SIM technique.

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Although a complex task, the development of health based cleanup standards and remediation strategies for PAHs contaminated soils is a promising area for remediation. Both biological and chemical strategies offer new cost-effective solutions for PAHs remediation that are also environmentally friendly.

Although there are several drawbacks for all of the biological as well as chemical methods, new applications are being devised regularly including a bio-nano hybrid system using nanoparticles. So keep updated with the latest information with Torrent Laboratory.