## 6 Types and Mechanisms of Removing Xenobiotics from Water
### Challenges of Enzymatic Bioremediation
Many factors, e.g., biodegradability, sorption, photodegradation, volatilization and environmental conditions (e.g., temperature and solar radiation), affect the dynamics of xenobiotic transformations in water, which determine the degree of its contamination and the possibility of the bioaccumulation of toxic chemicals in aquatic organisms [262]. Currently, wastewater treatment methods mainly include physical, chemical and biological methods. Physicochemical methods, such as chemical oxidation, photocatalytic oxidation, flotation, coagulation, flocculation, distillation, membrane-based separation and adsorption techniques have been used for wastewater treatment [263; 264]. Wastewater treatment plants play a key role in removing organic and inorganic pollutants. Due to the huge amounts of newly formed chemicals delivered to the environment, traditional wastewater treatment is insufficient to completely remove many toxic pollutants that consequently pollute the biosphere. Moreover, climate change, the increasing intensity of precipitation and violent storms contribute to water pollution through wetland runoff [262]. The disadvantages of traditional pollution treatment methods include high costs, no specificity and the generation of secondary pollutants [265; 274; 275]. Biological methods are more environmentally friendly and can remove most pollutants from wastewater [263; 264]. The process that uses microorganisms, plants or their enzymes to restore an environment altered by pollution to its original state is named bioremediation [262; 263]. Bioremediation can also be described as \"a treatment technology that uses biological activity to reduce the concentration or toxicity of a pollutant\". The bioremediation process consists of detoxification and mineralization [266], during which waste is converted into the inorganic compounds CO2, H2O and CH4. Impure water areas where there has been an accidental or intentional release of pollutants or chemicals are locations where bioremediation is applied [276]. The bioremediation of contaminated groundwater or soil is currently the cheapest and least harmful method of removing xenobiotics from the environment [277]. Bioremediation processes can be ex situ or in situ. In ex situ bioremediation, wastewater treatment takes place at a different location than where the pollutants occur/are produced, while in situ bioremediation takes place where they occur [278]. The factors influencing the course of bioremediation processes are (i) the physicochemical bioavailability of pollutants (transport, adsorption, dispersion and volatility of pollutant compounds); (ii) microbial growth and development; (iii) the availability of alternative carbon sources; (iv) type and concentration of contaminants, their solubility, biodegradability, toxicity and chemical structure; (v) the environmental conditions, such as temperature, humidity, pH, oxygen availability, nutrient levels, redox potential, electron acceptor availability and nutrient sources; (vi) the chemistry and mechanics, hydrogeology and soil hydrology at the contaminated site; (vii) enzyme induction; and (viii) the occurrence of horizontal gene transfer and/or mutations in the microbial populations present in wastewater or bioremediation sites [264; 278; 279; 280]. Enzymatic conversion may be crucial for the removal of structurally diverse and physicochemical properties of xenobiotics from the environment [273]. Compared to conventional chemical processes, enzymatic processes have higher reaction kinetics and require less water and energy; additionally, enzymes not used in the reactions can be recycled [263]. In addition, enzymes are selective and can catalyze targeted conversion reactions. Low concentrations of xenobiotics, usually not exceeding a few ug/L, affect enzyme kinetics and conversion [273]. It should be noted that the use of an enzyme is possible if the enzyme reaction product is less toxic than the substrate [279].