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Bioremediation for Sustainable Environmental Cleanup

Table 5.4. Different remediation measures for PAH contaminated soils.

Sr.

No.

Remediation

Method

Details

1.

Solvent

extraction

method/Soil

washing

In the solvent extraction method, as the name suggests, solvents are used to separate out compounds

based on the solubility in the solvent. In this technique, PAHs are removed or washed from the soil

with the help of solvents or surfactants. As PAHs are hydrophobic, they get readily dissolved in

organic solvents. Individual or a mixture of different organic solvents, vegetable oils (sunflower oil,

vegetable oil) and cyclodextrins can be used for PAHs extraction. The solvent extraction method

is a two-step process, involving desorption and elution. The mixture of cyclohexane and ethanol,

(3:1), mixture of ethyl acetate, acetone and water, (5:4:1), mixture of 1-pentanol (5%), water

(10%) and ethanol (85%) can be used as solvents for PAHs extraction (Singh and Haritash 2019).

However, these solvents can be toxic, so selection of solvents is important before proceeding.

β-cyclodextrin (BCD), methyl β-cyclodextrin (MCD) and hydroxypropyl-β-cyclodextrin (HPCD)

are three cyclodextrins which are used for PAHs flushing from soil. They are non-toxic and

biodegradable. To enhance the efficiency of washing of PAHs surfactants can be used, known as

surfactant-aided soil washing. Non-ionic surfactants such as Tween 40, Tween 80, T-Maz 80, Brij

30, CA 620 are used in PAHs washing (Gan et al. 2009).

2.

Thermal

treatment

Heat is used in thermal treatment to destroy or volatilize the PAHs into gases which are then

collected for further ex-situ treatment. In thermal desorption, the maximum temperature of

450°C is applied to increase the vapor pressure of PAHs to convert them into a gaseous form. In

microwave frequency heating, microwave energy is converted into heat energy to remove PAHs

via volatilization. Vitrification is another thermal technique, in which extreme temperatures of the

range 1600–2000°C is used to melt the contaminants in soil. The temperature is provided through

an electric current via molybdenum electrodes.

3.

Chemical

treatment

Chemical treatment can be of different types viz; chemical oxidation, electrokinetic remediation

method or photocatalytic degradation. In the chemical oxidation method, there is an involvement

of electrons or redox reactions. Electrons are transferred from one chemical to another,

converting more toxic chemicals to fewer toxic ones. It involves the use of different oxidants

such as Ozone, Fenton’s reagent, hydrogen peroxide, persulfate, potassium permanganate and

peroxymonosulfate for PAHs remediation. In the electrokinetic method, direct low voltage electric

current is applied to remove soil contaminants. Contaminants get accumulated towards electrodes,

which are then collected for further treatment. The electrokinetic method can be combined with

different surfactants and solvents to increase the solubility and desorption of contaminants. In

the photocatalytic degradation method, photocatalysts are used for oxidizing reactions or photo

reactions for the degradation of PAHs in soil. The function of photocatalysts is to enhance the rate

of photodegradation. TiO2, and H2O2 are photocatalysts that help in the remediation of PAHs (De

Boer and Wagelmans 2016).

4.

Biological

treatment

Different types of biological remediation methods are available and are being developed.

Bioremediation, biostimulation, bioaugmentation, phytoremediation, landfarming, composting

and bioventing are biological techniques for PAHs remediation. In bioremediation, living

organisms mainly potential natural microorganisms that have the capability (enzymatic

machinery) of degradation, are used to convert toxic organic pollutants to CO2 and H2O.

Acinetobacter, Aeromonas, Alcaligenes, Alcanivorax, Arthrobacter, Bacillus, Corynebacterium,

Enterobacter,

Microbulbifer,

Micrococcus,

Mycobacteria,

Paenibacillus,

Pseudomonas,

Ralstonia Sphingomonas and Xanthomonas are the reported bacterial genera that are capable of

PAHs degradation. Bioremediation through fungi is known as mycoremediation. Aspergillus,

Bjerkandera adusta, Irpex lacteus, Phanerochaete chrysosporium, Pleurotus ostreatus are the

reported PAHs degrading fungi. In biostimulation, nutrients and oxygen (electron acceptor) are

added to contaminated soil to stimulate the degradative activity of microorganisms. Nutrients

can be carbon, organic biostimulants (phycocyanin) nitrogen (NH4CL), phosphorous (NaH2PO4)

(Singh and Haritash 2019). In bioventing, air or oxygen is supplied to contaminated soil through

wells to trigger the growth of PAHs degrading microorganisms. In the landfarming technique,

contaminated soil is excavated from the site and transported to the landfarming site, where by the

degradative action of the microorganisms’ PAHs are degraded slowly. In composting, mesophilic

and thermophilic microbes, degrade contaminants at high temperature, i.e., 55–66°C. Compost

beds (mushroom compost) are prepared for this. Phytoremediation is the process where plants and

associated microbes with some soil amendments and agronomic techniques are used to accumulate,

immobilize, adsorb, absorb, degrade or volatilize the harmful contaminants into less harmful ones.

Phytoremediation in combination with other techniques, increases the remediation efficiency. The

detailed account of phytoremediation method is discussed below.