## 6 Types and Mechanisms of Removing Xenobiotics from Water
### Types of Lacase- Assisted Removing Xenobiotics from Water
#### 6.1.2 PAHs (Polycyclic Aromatic Hydrocarbons)  
Polycyclic aromatic hydrocarbons (PAHs) are aromatic hydrocarbons with two or more fused benzene rings in different structural configurations that do not contain heteroatoms or substituents. The benzene ring systems in PAHs are responsible for their wide range of physical, chemical and toxicological properties [200]. Chemically, we can distinguish low molecular weight (LMW) and high molecular weight (HMW) PAHs. Both low molecular weight (LMW) and high molecular weight (HMW) PAHs can be microbially degraded [201]. LMW PAHs are more susceptible to degradation due to the rather higher volatility and solubility of the compounds [12]. Due to the stability, hydrophobicity and lipophilicity of PAH molecules, they are harmful, persistent, organic pollutants that are very difficult to biodegrade [12,200]. PAHs include a huge number of compounds. The PAH group contains substances such as anthracene, acenaphthene, acenaphthylene, benz(a)anthracene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, chrysene, fluoranthene, fluorine, naphthalene, phenanthrene, pyrene and many others [200,202]. These compounds are widely used in various industries, e.g., for the production of dyes, pharmaceuticals, plastics, fungicides and insecticides [12]. PAHs are widely distributed in various ecosystems. In aquatic environments, PAH concentrations range from trace amounts expressed in ng/L in the water of the Sea of Japan to values significantly exceeding the standards, as in the case of household wastewatertreatment plants in South Africa, where a record PAH concentration of over 8 mg/L was reported. Chryzene, due to its poor solubility in water and low vapor pressure, is one of the most persistent PAHs in the water column [203]. Moreover, PAHs can bioaccumulate in fish. More than 4 ug/g have been found in _Saurida undosquanmis_ in Egypt. Such a PAH is benz[a]anthracene (BaA), which tends to accumulate in lipid-rich tissues [12]. Aquatic ecosystems are exposed to PAHs mainly as a result of accidental oil spills, with huge health consequences for the fish, birds and mammals living in these ecosystems. The effects of PAH exposure in fish include immunotoxicity, gill deformity, increased mortality, reproductive disruption, liver tumors, uncontrolled cell growth, reduced growth, embryo malformations, osmoregulatory imbalance and endocrine disruption [201]. In birds, deformities and the death of embryos, low fecundity, slowed growth and metabolic disturbances have been reported [204]. More and more studies report that PAHs are organic compounds with a high degree of toxicity, carcinogenicity and teratogenicity [200]. Of the more than 100 known PAHs, 16 have been classified as priority environmental pollutants by the United States Environmental Protection Agency (US EPA). This group includes acenaphthenene, acenaphthylene, anthracene, benzo[ghi]perylene, benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, chrysene, dibenz[a,h]anthracene, fluoranthene, fluoresene, indene[1,2,3-cd]pyrene, naphthalene, phenanthrene and pyrene [201]. Among the various PAHs, benzo[a]pyrene (BaP) has the greatest carcinogenic potential. PAHs from the aquatic environment can be removed by physical, biological and chemical processes [200]. Physical and chemical methods for PAH removal are often inefficient, labor-intensive and expensive [205]; in addition, through the incomplete transformation of PAHs, they can lead to the formation of other stable intermediates that are more toxic than the starting compounds [12,201]. For the reasons mentioned above, interest in the development of biotechnology for the detoxification of PAH pollutants has increased [179]. Bioremediation is an effective strategy for the treatment of organic pollutants, including PAHs, based on the use of microorganisms to convert pollutants into substances inert to the environment.