Review
Laccases and peroxidases: The smart, greener and futuristic biocatalytic tools to mitigate recalcitrant emerging pollutants

https://doi.org/10.1016/j.scitotenv.2020.136572Get rights and content

Highlights

  • Emerging contaminants (ECs) are being detected globally in surface and drinking water.

  • Enzymatic remediation, using laccase and peroxidases, is a promising approach to degrade ECs.

  • Toxicity aspects of ECs and related hazardous contaminants are also summarized.

Abstract

Various organic pollutants so-called emerging pollutants (EPs), including active residues from pharmaceuticals, pesticides, surfactants, hormones, and personal care products, are increasingly being detected in numerous environmental matrices including water. The persistence of these EPs can cause adverse ecological and human health effects even at very small concentrations in the range of micrograms per liter or lower, hence called micropollutants (MPs). The existence of EPs/MPs tends to be challenging to mitigate from the environment effectively. Unfortunately, most of them are not removed during the present-day treatment plants. So far, a range of treatment processes and degradation methods have been introduced and deployed against various EPs and/or MPs, such as ultrafiltration, nanofiltration, advanced oxidation processes (AOPs) and enzyme-based treatments coupled with membrane filtrations. To further strengthen the treatment processes and to overcome the EPs/MPs effective removal dilemma, numerous studies have revealed the applicability and notable biocatalytic potentialities of laccases and peroxidases to degrade different classes of organic pollutants. Exquisite selectivity and unique catalytic properties make these enzymes powerful biocatalytic candidates for bio-transforming an array of toxic contaminants to harmless entities. This review focuses on the use of laccases and peroxidases, such as soybean peroxidase (SBP), horseradish peroxidase (HRP), lignin peroxidase (LiP), manganese peroxidase (MnP), and chloroperoxidase (CPO) as a greener oxidation route towards efficient and effective removal or degradation of EPs/MPs.

Introduction

Over recent decades, the world's population is growing rapidly, which has resulted in numerous ecological impacts with water being one among the foremost affected resources (Peña-Guzmán et al., 2019). The unprecedented rise in population has caused higher consumer demand and has subsequently led to increased ecological pollution. All different kinds of pollution have a profound impact on human health and aquatic organisms, either directly or indirectly. Human-made, industrial, and agricultural disposals play a significant role in causing wastewater pollution. Unfortunately, the numerous types of pollutants which are released in water bodies due to the different agricultural and industrial processes are not completely removed during wastewater treatment processes and therefore have the potential to directly affect humans (Deblonde et al., 2011).

A subclass of organic chemicals that are increasingly being detected in our water bodies have been classified as Emerging Pollutants (EPs), also known as “Contaminants of Emerging Concern” or Micropollutants (MPs) (Teodosiu et al., 2018). These emerging pollutants can be defined as human-made or manufactured synthetic chemicals or naturally occurring materials present in the natural environment without being monitored or regulated in most cases but can adversely affect human health and several other living organisms (Sauvé and Desrosiers, 2014; Bilal et al., 2019a).

They comprise an extensive array of various compounds and their transformation products: pharmaceuticals (e.g. nonsteroidal anti-inflammatory drugs (NSAIDs), analgesics, antibiotics, textile dyes, hormones, and personal care products pesticides) (Lapworth et al., 2012). They are mainly detected in wastewater treatment plants (WWTPs), pharmaceutical production plants, household products, hospitals, graveyards, household products, landfills, aquatic environment, industrial effluents and municipal sewage (Deegan et al., 2011; Ahmed et al., 2017; Bilal et al., 2019b). The concentration of EPs in our environment ranges from ng/L to a few hundred μg/L (Tran et al., 2013; Ahmed et al., 2017). These concentrations are suspected of causing serious ecological threats such as interfering with the endocrine system of high organisms, reproductive impairments, physical abnormalities and congenital disorder in some species, feminization of some fish species and many others (Belhaj et al., 2015). A study in 2011 concluded that the presence of perfluorinated compounds in serum could be correlated to breast cancer risk in Greenlandic Inuit women (Bonefeld-Jorgensen et al., 2011). Additionally, it has been reported that pollutants such as perfluorooctanoate and perfluorooctane sulfonate may be linked to decreased human reproductive abilities (Vélez et al., 2015).

Higher concentrations have been detected for some pollutants such as ciprofloxacin and per- and poly-fluoroalkyl substances (PFAS) as their concentrations reached mg/L and g/L, respectively, in our water supplies (Kelly and Brooks, 2018; Nakayama et al., 2019). Due to the ability of EPs to cause undesirable and deleterious effects on human health and to the ecosystem, they have become the main focus of many academic research groups. For example, a 2012 study found that N, N-Diethyl-meta-toluamide (DEET) can cause an inhibition in the activity of acetylcholinesterase, which is a central nervous system enzyme, in mammals and insects (Corbel et al., 2009). Dealing with EPs requires hard work, as there are many challenges a researcher can face. There is a lack of knowledge about the ecotoxicological information and a deficiency regarding the sampling and analytical techniques (Geissen et al., 2015). Moreover, the long-term effect of EPs on living beings and the environment is not available (Deblonde et al., 2011). Some notable adverse effects of these pollutants are shown in Fig. 1.

The term EPs covers three categories of compounds; the first category includes newly developed compounds that are introduced to the environment, the second category consists of compounds that are presented in the environment for a long time but are only being recognized newly, and the last category includes compounds that are detected since a long time but their significant impact on the environment and human health have been recognized recently such as hormones (Geissen et al., 2015; Bilal et al., 2019c; Rasheed et al., 2018a). More than 1000 EPs have been identified and categorized into different classes, which include pharmaceuticals, personal care products, pesticides, hormones, etc. Table 1 represents an interesting analysis conducted to document physiologically active concentrations of various hormones, antibiotics, and other emerging pollutants in the water bodies in several countries. As can be seen from the table, disturbingly high concentrations of various emerging pollutants are detected in various water bodies. Emerging pollutants can result from agricultural, industrial, household, and hospital discharges. As mentioned previously, there are many sources for the emerging contaminants, but the major source is the WWTP effluents. WWTPs are not designed to completely eliminate and degrade EPs and their metabolites, therefore, they can pass through WWTPs and enter our aquatic environments such as rivers and streams (Petrović et al., 2003; Bilal et al., 2019b). Fig. 2 shows the different sources of emerging pollutants and their transformation to our water supplies.

A wide range of approaches has been developed for the removal of these synthetic pollutants from water bodies, as well as wastewaters thus, reducing their impact on the environment. Various chemical and physical methods have been used for the treatment of contaminated wastewater such as membrane filtration precipitation, flocculation, irradiation, adsorption, and chemical oxidation such as Fenton's oxidation, and AOPs (Ikehata et al., 2006; Comninellis et al., 2008; Deegan et al., 2011; Ahmed et al., 2017; Alneyadi et al., 2018; Barrios-Estrada et al., 2018; Bilal et al., 2018a; Rasheed et al., 2018b; Teodosiu et al., 2018). Furthermore, hybrid systems in which two methods are combined and used have also been developed to enhance the removal efficiency of EPs. Another study demonstrated the photocatalytic degradation of buspirone, an anti-anxiety medicine, using TiO2 and xenon lamp (Calza et al., 2004). Diclofenac is an anti-inflammatory drug used to treat pain and inflammatory diseases. Recently, it has been realized as an ecological pollutant of concern due to its accumulation in the food chain, and identification in drinking water and aquatic systems. It has been detected in our water supplies in different concentrations up to 1.3 μg/L (Ternes et al., 2003). Many AOPs have been applied to diclofenac to evaluate their ability to degrade it effectively. Table 2 summarizes some of the AOPs that were used efficiently to remove various EPs. Other physical methods such as filtration and osmosis are efficient as well, but the cost of materials is expensive compared to the adsorption method (Ali, 2012). Table 3 shows some of the physical methods that have been used previously for the degradation of different emerging pollutants.

Although physical and chemical methods are widely used and these methods can work effectively, they have several potential limitations, such as overall high cost, inefficiency, production of high sludge, and formation of toxic side products. Hence, it is well accepted that there is a dire need to find better, novel, and more environmentally safe approaches for wastewater remediation. Biological (specifically microbial and enzyme-based) approaches for degrading various kinds of organic pollutants are a promising new area of research in water treatment (Al-Maqdi et al., 2017; Bilal et al., 2017a). Biodegradation or bioremediation has been successfully used for the removal of EPs from wastewaters. In this process, microorganisms such as bacteria, fungi or yeasts (or enzymes from these microorganisms) are used for the removal of organic chemicals from water bodies. In biodegradation, the microorganisms utilize the pollutant as a substrate and induce enzymes, then the pollutants are enzymatically converted into smaller molecules that are usually less toxic (Tran et al., 2013; Ahmed et al., 2017). Biodegradation processes have many advantages compared to the physiochemical techniques as they are safer, less disruptive, less expensive, require lower energy employment, considered as a green catalysis processes and can be used with pollutants having very low concentrations, which cannot be achievable using physiochemical techniques (Rauf and Ashraf, 2012; Al-Maqdi et al., 2017; Holanda et al., 2019). A major drawback of biological treatments is that they require a longer time, and the microorganisms may not be able to survive and grow under harsh and adverse environmental conditions (Rauf and Ashraf, 2012; Al-Maqdi et al., 2017).

In this review, following a brief overview of in-practice physical and chemical treatment methods, numerous oxidoreductases (laccases, SBP, HRP, LiP, MnP, and CPO) have been discussed with recent updates and suitable examples as a greener oxidation route towards efficient and effective removal or degradation of EPs/MPs. The later part of the work is focused on the identification of transformation products, eco-toxicity, and estrogenicity evaluation of EPs/MPs. Finally, practical challenges and perspectives for engineering robust biocatalysts are given that can pave the way for future studies.

Section snippets

Enzymatic biodegradation — towards greener oxidation route

The biological approach that uses oxidoreductase enzymes (such as peroxidases) for pollutant degradation is a relatively new and promising research area. Numerous enzyme systems have been employed for the efficient degradation of diverse organic pollutants and have shown to oxidize and degrade the pollutants into smaller intermediates. Using enzyme-based treatments offer many advantages such as the ability to operate at high and low concentrations of pollutants, reduced amount of sludge

Identification of transformation products and toxicity evaluation

Enzymes cause the degradation of various environmental pollutants by different pathways resulting in the generation of various metabolic intermediates and end products during the biocatalytic reaction. In the majority of the degradation studies, scientists and researchers principally focus on the parent compounds disappearance rather than the scrutinization of transformation pathways, intermediate metabolites, and evaluation of toxicity and estrogenicity of the transformed products (Becker et

Practical challenges and perspectives for engineering robust biocatalysts

In spite of the plentiful studies documenting biocatalytic proficiency and versatility for the transformation and degradation of an array of recalcitrant polluting compounds such as endocrine disruptors, xenobiotics, the application of oxidoreductases has not been endeavored at large scale. Ayala et al. (2008) appraised the current challenges that come across during the real-time application of peroxidases. Notable protein-engineering bottlenecks that have been realized include 1) augmentation

Concluding remarks

Various wastewater treatment technologies are discussed with a particular emphasis on laccases and peroxidases based enzymatic approach for the degradation and detoxification of emerging contaminants. Based on the extensive literature survey and our research struggles, this study accentuated that the biocatalytic approach has received unprecedented importance in mitigating an array of emerging pollutants typically present in wastewater effluents. So far, multiple controlled studies have been

Declaration of competing interest

We do not have any conflicting, competing, and financial interests in any capacity.

Acknowledgments

The authors are grateful to their representative universities for providing resources for this work. Partial funding for RM was provided by the College of Graduate Studies, UAE University.

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