## 6 Types and Mechanisms of Removing Xenobiotics from Water
### Challenges of Enzymatic Bioremediation
For example, many enzymatic transformation products of pesticides are toxic to the environment; therefore, combining many enzymes is a good way to obtain nontoxic reaction products [263]. However, in the case of most reactions carried out by lacases, the main products are dimers, trimers and oligomers formed as a result of the spontaneous reactions of phenoxy free radicals, which can be easily separated from the post-reaction mixture by simple precipitation and centrifugation or filtration [160]. Bioremediation is an easy, fast, environmentally friendly and socially acceptable approach used to bioremediate resistant xenobiotic compounds from the natural environment [263]. The key to the effective use of enzymes to remove micropollutants in aquatic ecosystems is the high efficiency and resistance of enzymatic reaction systems [273]. Unlike microbial bioremediation or phytoremediation, enzymatic bioremediation does not depend on the growth of a specific organism in a polluted environment but on the catalytic activity of the enzyme. The desirable features of enzymes used in bioremediation are their secretion outside the cell and their independence from cofactors [265]. The advantages of enzymatic bioremediation are also flexible operating conditions, easy control, speed of the process and its costs, specificity in relation to the substrate, the simplicity of processing and storage, standardized activity, mobility (small size) and no need to provide nutrients and biodegradability [265; 278; 280]. The disadvantages of xenobiotic removal by enzymes include the identification and selection of microbiological sources for the production of appropriate amounts of the enzyme, enzyme stability and activity in real process conditions (enzyme denaturation by substances present in wastewater), difficulties with increasing the scale of the process and maintaining its efficiency in subsequent cycles of bioremediation, high costs, time-consuming production and the purification of enzymes [266; 280]. Mixtures or complex combinations of many organic and inorganic substances are present in the polluted site, not individual pollutants. The complexity of the impurity can be associated with possible negative or positive synergistic effects on enzyme performance [279]. It should be noted that enzymes are catalysts with narrow (chemo-, region- and stereoselectivity) or wide specificity, so they can also be used for many different compounds in a mixture. Laccases produced by various groups of microorganisms are capable of oxidizing ortho- and paradiphenyl, aminophenols, polyphenols, polyamines, lignins, aryldiamines and some inorganic ions [65; 187; 243; 256]. In addition to oxidizing phenolic and methoxyphenolic acids, lacases also attack their carboxyl (decarboxylation) and methoxy (demethylation) groups. These enzymes are involved in the depolymerization of lignin, resulting in the formation of various phenols. The substrate specificity and affinity of the lacase can change with changes in pH. Halides (except iodides), azides, cyanides and hydroxides inhibit lacase catalytic activity, with different lacases having different tolerances to inhibition by these compounds [281]. In order to increase the efficiency of the removal of toxic compounds by lacases, various mediators can be used [273]. Different mediators can have different effects on the same reaction, leading to different toxicity of the resulting transformation products [259]. Unpurified enzymes may contain mediators [282] and unused nutrients, resulting in further contamination [259; 261; 263; 283]. In addition, crude enzymes usually show lower activity than their purified counterparts and can be partially inhibited by the components of the extract [261]. The problems with using this method are the costs of mediators, which are related not only to their production but also to high consumption and their toxicity [263; 284]. To prevent the release of the mediator into the environment, it is possible to co-immobilize the enzyme and the mediator, as in the case described by [PERSON] et al.