Persistence and impact of steroidal estrogens on the environment and their laccase-assisted removal

Steroidal estrogens are widespread water contaminants with potential carcinogenic and endocrine-disrupting activities. The World Health Organization has listed estrogens as group 1 carcinogens. These contaminants are of substantial concern because of potential threats to human health, and aquatic organisms on long-term exposure. A range of methods, including oxidation, adsorption, electrochemical, and irradiation techniques have been employed for their remediation from aqueous systems. However, inadequate removal, toxic sludge generation, high operating costs, and the requisite for skilled operating and maintenance personnel commercially hampered the application of many methods. An interesting alternative treatment approach based on the use of oxidoreductases, particularly laccases, has recently gained amicability for the biotransformation of emerging pollutants. The use of immobilized enzymes is more cost-effective from an industrial perspective due to improved catalytic stability, reusability, reduction of product inhibition, and easier product separation. This review provides comprehensive knowledge on the use of laccases in the biodegradation of steroidal estrogens, including estrone, 17β-estradiol, and 17α-ethinylestradiol with endocrine-disrupting potency from the environment. After an overview of estrogens and catalytic properties of laccase, the use of free, as well as immobilized laccases with a particular emphasis on estrogens removal by laccase-based fed-batch, packed bed bioreactors, and membrane reactors, is discussed. A comparison of existing treatment technologies with enzyme technology for the removal of estrogens from different environmental matrices is made. Lastly, along with concluding remarks, future research direction aimed at bridging knowledge gaps for estrogenic compounds removal are also proposed in this very important research area.