**2. Remediation and biodegradation technologies**

Methods to remove PAHs have been classified as physicochemical, chemical and biological, and are briefly described in **Table 1**. Among biological techniques, bioremediation is consid‐ ered a viable technology, environmental friendly and inexpensive that uses the metabolic diversity of some microorganisms to degrade and decrease the concentration of toxic com‐ pounds.


**Table 1.** Technologies suitable to remove PAHs from contaminated soils.

Biological removal or (biodegradation) is a process carried out by aerobic organisms mainly indigenous microorganisms and commonly it reaches the mineralization of toxic compounds to inorganic forms (CO2 and H2O). However, anaerobically PAHs biodegradation under denitrifying and sulphate‐reducing conditions has been well recognized [1]. The aerobic biodegradation mechanism of PAHs begins with the initial oxidation step, either where two atoms of oxygen are incorporated into the aromatic ring to form *cis*‐dihydrodiol or where monooxygenases enzymes are involved in the first initial oxidation to form *trans*‐dihydrodiols. Otherwise, bioremediation can be conducted in two ways: (1) ex situ that is held off the contaminated site and requires excavation and site conditioning, and (2) in situ where the soil decontamination is performed without removing it from the area [2].

Three technological processes are well recognized for in situ bioremediation: (1) bioattenua‐ tion, which depends on the natural degradation processes to dissipate contaminants through biotransformation; (2) biostimulation, involving the addition of nutrients, water, electron donors or acceptors that stimulates microbial growth; and (3) bioaugmentation, which requires the inoculation of indigenous, allochthonous or genetically modified microorganisms with specific capabilities to degrade or biotransform the contaminant of concern. Bioaugmentation can follow two strategies: (1) isolation of microorganisms able to remove the contaminants from contaminated soils, culturing them in the laboratory and returning them to the original site (reinoculation of indigenous bacteria), or (2) inoculation of microorganisms obtained from different contaminated sites with proven abilities to degrade the contaminants of concern [2].
