## 6 Types and Mechanisms of Removing Xenobiotics from Water
### Challenges of Enzymatic Bioremediation
The beneficial effect of enzyme immobilization on bioremediation consists of (i) increasing the efficiency of pollutant degradation; (ii) increasing the catalytic efficiency; (iii) easy regeneration and multiple uses of biocatalysts; (iv) ensuring a stable microenvironment for the enzymes; (v) ensuring resistance to forces occurring in bioreactors, e.g., pressure; (vi) increasing the resistance of biocatalysts to unfavorable environmental conditions, such as high temperatures, acidic or alkaline reactions, the presence of heavy metals and halides; (vii) increasing the stability of the biocatalyst during storage; and (viii) increasing tolerance to high concentrations of pollutants [159; 277]. Immobilization of lacase on a solid support increases the stability and resistance of lacase to proteases [278]. Finding the right immobilization methods and carriers to reduce the loss of enzyme activity and increase the reusability of the enzyme is fundamental [263]. A good carrier used in wastewater treatment processes should be characterized by insolubility in the reaction environment, high mechanical strength and nontoxicity both for the immobilized material and the environment. The carrier should be readily available, inexpensive, stable and easy to regenerate [277]. It should also provide significant affinity to the immobilized enzyme [273]. The modification and functionalization of the carrier surfaces, advanced research and the search for new support materials are ways to find appropriate carriers [263]. Nanotechnology offers tools to increase the stability of enzymes by reducing sensitivity to mechanical stress and maintaining the third structure of enzymes and protection against proteases [265]. Immobilization methods and carriers have a huge impact on the cost of enzymatic bioremediation, enzyme activity and the efficiency of the entire process [263]. [PERSON] et al. (2019) emphasize that to achieve high efficiency in the bioremediation of resistant pollutants present in wastewater, solutions based on immobilized oxidoreductase should be used, such as simultaneous adsorption and biodegradation and the use of bioreactors with immobilized enzymes [273]. For the simultaneous immobilization of enzymes and sorption of pollutants, compounds of inorganic origins (e.g., aluminum and iron oxides, silica gel and activated carbon) [298; 299], organic origins (e.g., cellulose, chitin, agar, starch, carrageenan, chitosan, pectins, polystyrene and polyvinyl alcohol) [300; 301] and hybrid origins (e.g., copper hydroxide nanocages and chitosan-halloysite hybrid porous microspheres) [302; 303] and composite origins (e.g., polymers and metal and ceramic composites) [304; 305] were used. They are characterized by different morphology, porous structures (pore diametersand surfaces) or the presence of many different functional groups on the surface of the material [263; 277; 267; 306]. Simultaneous enzymatic biodegradation with adsorption or separation increases the efficiency of pollutant removal, shortens the process time and prevents the accumulation of products in the reaction environment and, thus, its inhibition [273]. Simultaneous adsorption and covalent immobilization have been used to remove toxic compounds from wastewater. This technique reduces the diffusion of substrates and enzymes and provides better exposure of enzyme active sites to the substrates dissolved in solution [89; 154; 155; 273]. Appropriate exposure of the active center for laccases is important due to the fact that their active center requires an appropriate combination of enzyme-substrate and enzyme-carrier. The carrier can act as both an electron donor and an acceptor [273]. B bioreactors with immobilized enzymes enable tight process control. In these devices, parameters such as pH and temperature can be adjusted, the oxygen supply through mixing, the substrate flow rate and volume and the removal of products and by-products [273; 307; 308; 309; 310]. Mixing is primarily responsible for the heat transfer, substrate and oxygen in the bioremediation process.