Water is the most important natural resource in the world, and many surface water resources are very vulnerable to pollution. Natural water sources are often contaminated by municipal wastewater, which contains xenobiotic pollution and sometimes contains more toxic degradation products. So far, various physical and chemical technologies have been developed to treat wastewater, such as coagulation, flocculation, and activated carbon adsorption. More advanced technologies, including reverse osmosis, nanofiltration, photolysis, ion exchange, and advanced oxidation, have also been successful in the laboratory. However, the cost of urban wastewater treatment is too high. In recent years, biological methods for wastewater treatment have received much attention. They are important because they can completely oxidize many impurities, including toxic impurities, while requiring comparatively low cost and simple equipment. Among them, the enzyme oxidoreductase which mainly catalyzes the oxidation-reduction reaction has broad substrate specificity, regioselectivity and enantioselectivity, and can effectively treat various waste liquids, especially dyes, phenols and related compounds.

Unlike many chemical oxidants, oxidative enzymes do not mineralize substrate, but instead form free radicals that can be broken down into parts of the transformation product or coupled to different molecules by non-enzymatic processes (oxidative coupling reactions) to form higher molecular weights compound. Moreover, the enzyme-catalyzed transformation product exhibits minimal toxicity or is more readily biodegradable than the parent compound, demonstrating the ability of the biological oxidative treatment to wastewater. Oxidative enzymes includes oxygenases, peroxidases, and polyphenol oxidases are mainly used in wastewater treatment, all of which cause decomposition of phenolic, aromatic, or inorganic contaminants through oxidation mechanisms.

Oxygenases

Oxygenases achieve region-, stereo- and enantioselectivity that introduce atomic or molecular oxygen into a range of substrates, thereby converting hydrophobic compounds into more water soluble and responsive forms. Oxygenases are primarily intracellular enzymes that play a role in biosynthesis and metabolism. In addition to its use in the fabrication of pharmaceuticals and the synthesis of specialty chemicals, oxygenases are also associated with the biodegradation of hydrocarbons and their analogues compounds: a process generally regarded as biotransformation. These compounds are considered environmental pollutants because of their improvident use, storage and disposal. There are three main reasons for biotransformation with oxygenases:

• The chemical counterparts of oxygenases do not exist or lack the requisite regio- and stereoselectivity.
• Molecular oxygen can be used as a cheap environmentally friendly oxidant compared to harsh chemical oxidants.
• They are involved in the modification of natural products or the synthesis of pharmaceutical intermediates and chiral building blocks.

Peroxidases

Peroxidases are sometimes referred to as heme-containing proteins, which are present in all life forms, from decomposers, producers to consumers. They prevent oxidative damage in plant leaves and act in the lignification process. They mediate the reduction of peroxides, especially hydrogen peroxide (H₂O₂) or any organic peroxide, accompanied by oxidation of chemically diverse compounds. Peroxidase activity precisely entails the transfer of electron to the substrates, causing them to be broken down into harmless components in the reaction. The peroxidase reaction mainly includes four kinds, oxidative dehydrogenation, oxidative halogenation, H₂O₂ disproportionation, and oxygen transfer reaction. These four reactions have peroxidase centrality in the treatment of recalcitrant heterogeneous wastewater contaminants such as dyes, phenols and aromatics from domestic or industries.

Polyphenol oxidases

Polyphenol oxidases are a type of binucleate cupriferous enzymes that participate in the suitability and objectionableness of certain commercial foods, from the browning of tea, coffee and cocoa to the poor browning of certain fruits, vegetables and processed wines and beverages. Polyphenol oxidase has been widely used in many applications, not only to enhance flavor, to determine food quality and to remove phenolic contaminants from wastewater. They achieve these applications by hydroxylation of aromatic rings, which are subsequently oxidized the mono- or diphenols to form highly reactive quinones and radicals, which can be further non-enzymatically polymerized or reacted with other materials to form highly insoluble molecular pigments. They are almost ubiquitous in nature, and their secretion is mainly related to the pathogenicity of fungi and the physiological response to stress and mechanical damage in plants. Due to the difference in phenolic compounds they oxidize, they fall into two main categories: tyrosinase and laccase.

• Tyrosinases

Tyrosinases are a type-3 cuprodinucleate metalloproteins that catalyzes the oxidation reaction similar to peroxidase and plays a key role in the synthesis of melanin from I-tyrosine in animal and human. They oxidize their substrates by withdraw a pair of electrons, and their activities are highlighted by two oxygen-contingent reactions: the ortho-hydroxylation of the monophenolic compound into o-diphenol, due to cresolase activity, followed by spontaneous o-deiphenols oxidation to ortho-adapted quniones is due to catecholase activity. The reactive hydrazine undergoes non-enzymatic self-polymerization or co-agglomeration with unsubstituted phenol to form insoluble agglomerates, which are less toxic than the parent phenol and highly separable. These mechanisms suggest that tyrosinase can be an option for treating phenol-containing wastewater or soil. However, their relatively low redox potential, their inactivation in the fluid matrix and the miscibility of the oxidized phenolic products make them less useful in the environment and industry.

• Laccases

Since 1883, laccases has been found in the resin of Japanese lacquer tree Rhus vernicifera (now Toxicodendron vernicifluum), it has been detected in various plants, fungi, insects and bacteria, which play an important biological role. As in bacteria, it may be involved in morphogenesis, bio-assimilation of recalcitrant aromatic substrates, pigmentation, sporulation, and formation of protective shield against oxidants and ultraviolet radiation. In addition, laccases also catalyze the oxidative cleavage of a variety of substrates, particularly phenolic compounds. Laccases have a substrate preference order, ortho > para > meta-substituted phenols, which can be extended to include a variety of aromatic compounds. These qualities of laccase suggest the ability to initiate and maintain the biodegradation of aromatic contaminants in wastewater when they are used to assist in the treatment of wastewater.

References

1. Unuofin J O. Aptitude of oxidative enzymes for treatment of wastewater pollutants: a laccase perspective. [J]. Molecules, 2019, 24(11).

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