## 6 Types and Mechanisms of Removing Xenobiotics from Water
### Challenges of Enzymatic Bioremediation
(2021), where lacase and ABTS were immobilized on chitosan magnetic nanoparticles modified with an ionic liquid with amino groups. Coimmobilization of these compounds increased the speed of the process 2-fold and the efficiency of removing indole, anthracene, BPA and 2,4-dichlorophenol [285]. Enzymes are macromolecules with a very complex structural conformation, and any physical and chemical changes result in a loss of enzyme activity [278]. Lower stability, productivity and activity are fundamental problems in the practical application of enzymes in bioremediation [278; 279]. Tools that can help solve these problems are techniques of genetic engineering and protein engineering, which can increase the productivity and activity of enzymes through DNA recombination, the heterologous expression of proteins or introducing modifications to the basic amino acid structure of the enzyme [38; 286; 287]. The large-scale production of enzymes, with increased stability and/or activity and at lower costs, is possible thanks to the use of recombinant DNA technology [279]. It is also possible to design enzymes to increase their stability and efficiency for special conditions or specific substrates. These issues are discussed in more detail in Section 7 of this paper. Omics technologies play a significant role in designing proteins with desired properties [265]. Metagenomic approaches can be useful to find potential gene-encoding enzymes responsible for the degradation and detoxification of a specific pollutant. The availability of whole genome sequences of environmental microorganisms has enabled the construction and screening of metagenomic libraries to identify genes involved in bioremediation [288; 289]. Examples include the University of Minnesota Biocatalysis/Biodegradation Database, EAWAG-BBD (biocatalysis/biodegradation database), ONDB (organointrogen degradation database), Aromadeg (aromatic hydrocarbon degradation database), OxDBase (biodegradation oxygenase database), RHODase (a database of ring-hydroxylating oxygenases) and MetaRouter [288; 290]. Advances in synthetic biology techniques and the exploration of expanding nucleotide databases will eventually enable the design of enzyme-based systems for the mineralization of organic compounds [291]. Another approach to improving the performance of enzymes in the detoxification of pollutants is their immobilization on natural and synthetic substrates of various natures and through various immobilization mechanisms [271; 292; 293]. The immobilization of lacases on a solid support increases their stability, half-life and resistance to proteolytic enzymes [278]. Immobilized enzymes tend to have long-term operational stability, being highly resistant to physical, chemical and biological denaturing agents. In addition, they can be reused and recovered at the end of the process [279; 294; 295; 296]. Immobilization is a method that limits the rotational movements of enzymes while maintaining their viability and catalytic function. Currently, the following immobilization techniques are known: adsorption, surface binding (electrostatic or covalent), flocculation (natural or artificial), entrapment and encapsulation. The use of a given immobilization method requires the use of carriers with specific properties [277]. The method of immobilization has a large impact on the properties of enzymes; it should not affect the conformation and activity of enzymes [278]. There are physical and chemical methods of immobilization. Chemical immobilization ensures good stability of the enzyme but reduces its activity. The covalent bonds that form between the enzyme and the carrier materials disrupt its native structure. Physical immobilization methods have less effect on the enzyme structure, but enzymes immobilized in this way generally show poor stability [263]. Appropriate immobilization allows for the stabilization of the enzyme and the preservation of high catalytic properties by biomolecules [297].