Green Solutions for Degradation of Pollutants

Emerging Pollutants in Aquatic Environment: Critical Risk Assessment and Treatment Options

Neha Agarwal¹, *, Vijendra Singh Solanki², Sreekantha B. Jonnalagadda³, Keshav Lalit Ameta⁴, Neetu Singh⁵, Anupma Singh⁶, Vimala Bind⁷

¹ Department of Chemistry, Navyug Kanya Mahavidyalaya, University of Lucknow, Lucknow, India

² Department of Chemistry, Institute of Science and Research, IPS Academy, Indore, India

³ School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Durban, South Africa

⁴ Centre for Applied Chemistry, School of Applied Material Sciences, Central University of Gujrat, Gandhinagar, Gujrat, India

⁵ Department of Physics, Government Degree College, Kuchalai, Sitapur, Lucknow, India

⁶ Department of Chemistry, DDU Govt. P.G. College, Sitapur, Lucknow, India

⁷ Department of Zoology, Navyug Kanya Mahavidyalaya, University of Lucknow, Lucknow, India

Abstract

The chemical compounds that have been identified as dangerous to the environment, ecosystem and human health are classified as Emerging Pollutants (EPs). EPs include a variety of compounds such as dyes, pesticides, antibiotics, drugs, endocrine disruptors, hormones, industrial wastes and chemicals, and microplastics. These pollutants are malignant and non-biodegradable in nature, so they are responsible for the unhealthy and unsustainable environment. The occurrence of these pollutants has raised global concerns not only in various environmental matrices (air, water, and soil) but also in biological systems due to their toxic nature. These pollutants get accumulated in the environment and ecosystem and cause intensified environmental problems, global warming, deterioration of soil quality, the greenhouse effect, and ecological imbalance. Consequently, they affect the quality of life and the maintenance of the environment on a global level. Recent research indicates that if this trend is continued, situations will worsen in the near future. Sustainable solutions, such as bioremediation, nano-bioremediation, microbial degradation etc., are becoming increasingly important for the removal of these EPs as an efficient tool for sustainable development and pollution control. Therefore, the main aim of this chapter is to assess the current threats and future challenges associated with emerging pollutants so that focus can be drawn on sustainable green solutions for a greener and healthier environment.

Keywords: Bioremediation, Emerging contaminants, Ecosystem, Environment, Green solutions, Nano-bioremediation, Non-biodegradable, Pollutants, Pollution control, Sustainable.

* Corresponding author Neha Agarwal: Department of Chemistry, Navyug Kanya Mahavidyalaya, University of Lucknow, Lucknow, India; Tel: +91-9454784074; E-mail: [email protected]

INTRODUCTION

With the rapid technological advancements and industrialization, the environmental quality has deteriorated, which is an alarming sign for sustainability. Different categories of emerging contaminants (ECs), like heavy metals, pesticides, pharmaceuticals, endocrine disrupting agents, personal care products, dyes, detergents, plastics, etc., are causing menace at a global level as they adversely affect the environment, ecosystem and living beings [1-3]. Among different types of pollution, water pollution is an important subclass that severely affects global life. Water is a vital component of life; it has been contaminated due to high industrialization in recent decades and severely affects the quality of life [4]. For the last few decades, EPs have attracted worldwide attention, and many attempts have been made to mitigate the release and accumulation of EPs into the environment to prevent the dangerous impact on the environment. Many studies have been conducted to monitor progress in this field. For instance, in a study performed by Barbosa et al., various treatment techniques were reviewed with their removal efficacy of EPs that concluded the future research perspective for a risk-free environment. They also reviewed the interaction of microplastics with pollutants and concluded that marine microplastic debris may dangerously affect human health [5]. Another review done by Taheran et al. emphasized that if EPs are present in scarce concentrations, conventional sewage treatment processes are not capable of treating them efficiently [6]. In fact, chemical and physical methods that are used to treat effluents do not degrade these pollutants completely, but rather change their forms, which are more toxic to the environment and human health, even in low concentrations [7]. Literature studies also suggest that current information on mechanisms available for water remediation needs to be updated to avoid future risks to the ecosystem and environment [8, 9].

Due to high costs, difficult techniques and improper efficiency involved, the issue of emerging pollutants has become a challenge. Therefore, there is an urgent need to protect the environment and living beings by developing sustainable methods for the removal of these pollutants [10]. Bioremediation is the most promising technology over conventional methods of wastewater treatment because it is an eco-friendly and cost-effective technique with the possible recovery of elements and for solving environmental problems [11, 12]. Nanotechnology has also emerged as a promising technology, which has shown great potential in various fields along with the treatment of pollutants [13]. Currently, bionanotechnology is attracting great attention in the remediation of pollutants as green solutions, which are eco-friendly, cost-effective and easy-to-handle tools for the bioremediation of wastewater and other categories of environmental pollutants. This chapter presents a concept to assess the occurrence, fate and risk assessment of emerging pollutants and also provides an overview of sustainable solutions for water resource management.

EMERGING POLLUTANTS IN AQUATIC ENVIRONMENT: TRANSPORT, FATE AND BIOACCUMULATION

As a result of uncontrolled urbanization, industrial development, healthcare activities and other anthropogenic activities, there is a rapid increase in EPs on a global level [14]. The synthetic persistent organic chemicals that adversely affect the ecosystem and human health but are not monitored in the environment are known as EPs. Different routes and fate of EPs are shown in Fig. (1).

Fig. (1))

Different types of toxic EPs and their impacts.

Many studies have been conducted on the route and fate of EPs in aquatic environments [15]. However, EPs can enter into an aqueous environment through various direct and indirect routes and can get bioaccumulated through food chains and food webs, causing serious health hazards to living beings. Therefore, many studies have focused on their fates and bioaccumulation [16-18]. In aquatic environments, the concentrations of EPs can vary over a wide range from ng/L to g/L. Their toxicological effects on living organisms may result in acute and chronic toxicity, endocrine disruption, resistance to antibiotics and human health hazards [19]. According to a recent study, EPs, such as pharmaceuticals, pesticides, and phosphorus-based flame retardants, have been reported in marine bivalves in municipal wastewater and landfill leachate effluent discharges in Hong Kong [20]. For the first time, the presence of ninety-nine EPs was reported in the gonads of sea urchin Paracentroyus lividus by Rocha et al. (2018) [21]. Another study gave the first evidence of the presence of benzotriazoles (BTs) degradation products (BTTPs) in urban aquifers that may severely deteriorate the groundwater quality [22]. Another group of pollutants, known as contaminants of emerging concern, are also released into the environment, surface and groundwater resources [23, 24]. Although some EPs have existed in the environment for many years, which might be very harmful to our ecosystems, their occurrence has been analyzed only recently [25].

The most prominent classes of EPs are dyes, pesticides, pharmaceuticals, disinfection by-products, industrial chemicals, and plastics. For example, pharmaceuticals represent a subclass of emerging pollutants due to their uncontrolled use to treat a wide variety of diseases and their diverse physico-chemical and biological characteristics [26]. Pharmaceutical compounds, after excretion in the original form or as metabolites, can be found in different varieties such as urban wastewater, hospital sewers, and surface waters [27, 28].

The World Health Organization has also declared that resistance to antibiotics is the biggest threat to global health and the environment. Heavy metals also have the property of bioaccumulation and environmental persistence as EPs, which enter the aquatic systems through various routes and affect the ecosystem and human health negatively [29]. Plastic waste, after accumulation in the environment, is broken down into micro and nano plastics, gradually forming nano-plastics of less than 5 mm in size [30]. Once accumulated, microplastics migrate and diffuse into the environment and carry other environmental pollutants like antibiotics and heavy metals [31, 32]. Since these substances have a potential impact on aquatic life and human health, and there is a lack of knowledge regarding their environmental implications and analytical and sampling techniques, urgent action is required to tackle this problem at multiple levels.

CHALLENGES AND RISKS ASSOCIATED WITH EMERGING POLLUTANTS

Although EPs frequently occur in various environmental matrices on a global level, the knowledge of their hazards and ecological risks is not sufficient. EPs, even in low concentrations of ng/L, can have adverse effects on living beings, such as genotoxicity, carcinogenicity, hormonal interference in fishes, endocrine disruption, and immune toxicity [33]. Endocrine disruptors are highly toxic to wildlife, altering the reproductive behavior, and sexually dimorphic neuroendocrine system, and also to human beings by creating problems in the cardiovascular system and causing abnormal neural behaviors. They are also linked to diabetes and obesity. Similarly, perfluorinated compounds get bioaccumulated in fish and fishery products and have adverse effects on developmental and reproductive systems [34]. Engineered nanoparticles also have negative toxicological impacts and are very harmful to human health, resulting in cytotoxicity, oxidative stress, carcinogenic effects, inflammatory effects in the lungs, genotoxicity, and augmented intestinal collagen staining [35]. It is essentially important to understand that EPs are not isolated in the environment but in complex mixtures of contaminants [36, 37]. The mixture of ECs can have additive or multiplicative ecotoxicological effects [38]. The joint toxic effects may result in antagonistic interactions, which can lead to a cock tail effect [39].

Many studies had confirmed that many EPs are not dangerous for the environment if their concentrations detected in soil and water are very low [40]. However, relevant concentrations (ng/L or µg/L) can alter and have a negative impact on ecological interactions [41, 42]. According to several studies, it has been found that after entering the environment, these pollutants are transformed into metabolic by-products under different environmental conditions, such as degradation in the presence of light, oxidation and reduction, and microbial decomposition, but the risk analysis of these pollutants remains insufficient [43]. Many studies have demonstrated the effects of EPs on animal behavior and altered microbial communities and their function even in trace concentrations [44, 45]. EPs may also create resistance to antibiotics [46]. The effects of pesticides and pharmaceuticals on fluvial biofilms in a Mediterranean river were also studied, and it was observed that autotrophic biomass increased peptidase and decreased the photosynthetic efficiency when biofilms were shifted to highly polluted areas of EPs. In low concentrations also, heavy metals can affect and damage multiple organs such as kidneys, lungs, liver, esophagus, skin, and stomach, and can also cause neurodegenerative diseases and disorders [47, 48]. Heavy metals can also cause oxidative damage and endocrine disruption by accumulating in several organs in aquatic organisms, which can also affect their survival and growth [49]. Consequently, the potential ecological impacts of EPs require the development of efficient technologies that can easily remove them from water and other environmental matrices.

POSSIBLE SOLUTIONS FOR DEGRADATION AND REMOVAL OF EPs

As a result of the increasing risks due to the continuous occurrence and accumulation of EPs in the environment, their treatment and eradication have become necessary but cumbersome [50]. EPs that commonly occur in an aqueous environment are difficult to remove by applying conventional treatment technologies, such as physical and chemical methods, but these are not degraded completely and change their forms [51]. These modified forms are highly toxic and can cause damage even in trace concentration [52]. Over the conventional methods, bioremediation is considered the most promising technology for cleaning up the environment due to its eco-friendly and cost-effective nature. This technique can recover useful elements and can solve environmental problems [53]. Some of the commonly used strategies to mitigate the emission of EPs in different environmental matrices are given below.

Conventional Treatment Techniques and Advanced Oxidation Processes

Water pollution by EPs is a serious problem due to their continuous discharge and accumulation through various routes into the environment. Conventional treatment techniques such as membrane bioreactor and activated sludge have been used to remove biodegradable contaminants but failed to completely remove these EPs from wastewater [54]. Therefore, advanced oxidation processes (different photochemical and chemical processes, as mentioned in Fig. (2)) were used to treat wastewater. Both the traditional treatment methods to treat wastewater are effective to some extent and are still used today. However, the rising water scarcity requires the reuse of water after absolute filtration. The primary and secondary treatments are not very effective in meeting the standard of reusable water that can be used for domestic and industrial purposes [55]. Hence advanced treatment methods are required after the secondary treatment that helps in further removing the toxic materials [56].

Fig. (2))

Various advanced oxidation processes for the treatment of pollutants.

To solve this problem, activated sludge and conventional wastewater treatment processes can be used in combination with advanced oxidation processes such as ozonation, photodegradation, and biodegradation, which increase the efficiency of the treatment to a greater extent. However, the major drawback of this combination of processes is the high energy consumption and high costs involved. For the degradation of EPs, it is very important to know the oxidation potential of conventional and advanced processes in wastewater treatment plants [57, 58]. For instance, some EPs could be degraded by chlorine, such as methyl indole, chlorophene and nortriptyline, while benzotriazole and N, N-diethyl-m-toluamide were found to be recalcitrant and were not altered by chlorine [58, 59]. Another study reported that chlorine and ozone could degrade part of the EPs present in water and confirmed that EPs, which are easily oxidized by chlorine, are also oxidized by ozone with the same efficiency. Conventional and advanced oxidation processes such as chemical precipitation, ion exchange, and electrochemical removal, as discussed above, may remove some EPs from wastewater and can reduce their concentration in potable water but have many drawbacks, including high-energy consumption, incomplete removal, production of toxic sludge, and high operational and maintenance cost, which can result in improper and inadequate application of these technologies. Therefore, there is a need to develop effective and environmentally friendly solutions that include biological and nanotechnology approaches for the effective removal of these contaminants from the global environment [60, 61].

Advanced oxidation processes (AOPs) have shown a promising effect for treating contaminated water and also for the removal of naturally occurring toxins, impurities of emerging concern, pesticides, and other harmful contaminants, etc. AOPs include several methods for creating hydroxyl radicals and some other reactive oxygen species like superoxide anion-radical and hydrogen peroxide. However, hydroxyl radicals are still the most common species that enhance the effectivity of AOPs. Most of the organic compounds react with the hydroxyl radicals to form a carbon-centered radical. Further, this carbon-centered radical reacts with the oxygen molecule to generate a peroxyl radical, which undergoes further reactions and ultimately produces oxidation products such as ketones, aldehydes, and alcohols [62]. Hydroxyl radical is also able to detach an electron from the electron-rich substrates to create a radical cation, which is quickly hydrolyzed in an aqueous environment that leads to the formation of an oxidized product. It has been observed that the oxidation products are often less toxic and more receptive to bioremediation. Advanced oxidation processes involve UV/H2O2, UV/O3, Fenton, sonolysis, nonthermal plasmas, radiolysis, photocatalysis, and supercritical water oxidation processes. Sonolysis and radiolysis of aqueous media can form hydroxyl radicals when the chemical oxidants are not present in the water. On the other hand, photochemical methods like photo-Fenton-type processes require the presence of a catalyst or precursor to generate the hydroxyl radical [63]. Sonolysis produces the hydroxyl radicals at or near a gas−liquid interface, while the radiolysis process of aqueous media generates those hydroxyl radicals that are considered to be homogeneous for the timescales [64, 65].

The sonolysis method is not cost-effective as the operating cost is very high for large-scale water treatment, while the radiolysis treatments are low cost as the operation cost is quite low in comparison to sonolysis methods. Fenton and photo-Fenton-type processes have also grabbed significant attention for the treatment of water [66]. However, the consumption of Fe(II) and the requirement for the removal of generated iron sludge during Fenton-type advanced oxidation processes have restricted its application for the treatment of water. These restrictions can be controlled by photo-Fenton processes that effectively utilize solar irradiation to recreate the Fe (II) species that leads to hydroxyl radical production. The formation of the hydroxyl radical using various homogeneous and heterogeneous AOPs involves distinct reaction dynamics that consequently lead to different reaction pathways. A more comprehensive understanding of the structure-reactivity relationships for the groups of compounds for individual treatment processes is based on kinetic data for the identification of an effective AOP.

Bioremediation for Achieving Environmental Sustainability

It is essential to incorporate the ecological and biological components to attain the aims of environmental sustainability that have been lacking in conventional and advanced oxidation techniques. Environmentally friendly solutions are sometimes neglected in favor of technical solutions. As a result, for a sustainable ecosystem, biological treatment methods must be implemented. As one of the most favorable biotechnological applications, bioremediation uses microbial enzymes to break down harmful organic pollutants into less toxic compounds. The widespread use of genetically engineered microorganisms (GEMs) can also help to eliminate toxic organic pollutants such as naphthalene, benzene, petroleum, and other organic compounds [67]. Waste management can be done efficiently through bioremediation because persistent organic pollutants that are hard to break down can be successfully remediated through bioremediation. Bioremediation is the process of removing contaminated materials from the environment using bacteria, algae, fungi, plants and yeast [68]. Different enzymes produced by these microorganisms speed up biochemical reactions that break down pollutants through metabolic pathways [69, 70]. Enzymes play a very crucial role in the process of metabolism at every stage [71]. These enzymes must act on the pollutants for their bioremediation, and optimum environmental parameters are required for speedy microbial growth and degradation during biodegradation [72]. Several factors, such as soil type, physical, chemical, and biological factors, source of carbon and nitrogen, and type of microorganisms, affect the process of bioremediation [12, 72].

Different microorganisms can degrade EPs under aerobic and aerobic conditions. Different aerobic species of bacteria such as Rhodococcus, Mycobacterium, Bacillus, Pseudomonas, and Sphingomonas can degrade a variety of complex organic compounds such as pesticides, hydrocarbons, and polyaromatic compounds [73]. These microorganisms mineralize these contaminants and are used as a source of carbon and energy [74]. Bacterial species such as Pseudomonas, Aeromonas, and sulfate-reducing bacteria can be bioremediate EPs under anaerobic conditions. Microbial degradation of azo dyes occurs under anaerobic environmental conditions by the oxidation of organic substrates [75]. Bioremediation can be used in multiple ways; some of the commonly used methods are mentioned in Fig. (3).

Fig. (3))

Different bioremediation techniques.

Bioremediation can combat serious environmental issues in an environment-friendly and economical manner, and it has many advantages over traditional and physicochemical methods, such as cost and energy efficiency, specificity, selectivity, minimal requirement, etc. However, certain limitations are associated with bioremediation, such as the degradation of a toxic compound is time consuming. Moreover, its applications are restricted to severely contaminated sites with hazardous and toxic pollutants [76-79]. Therefore, given the benefits and drawbacks of every method and to tackle remediation problems, remediation methods can be integrated for better results. Nanobioremediation is one of the latest methods which have drawn a lot of attention from researchers in the past few decades. The benefits of nanotechnology and the advantages of bioremediation are successfully integrated into nanobioremediation.

Phytoremediation for Achieving Environmental Sustainability

Bioremediation with plants is known as phytoremediation. It is a natural biological process that degrades harmful EPs and recalcitrant xenobiotics that cause pollution in the environment. It is an eco-friendly method that can be successful. The plants absorb heavy metals and remove toxins from the water and soil in a cost-effective way. Contaminated soils can be cleaned up using plant extracts to remove pollutants and lower their bioavailability in soil [80]. Varieties of processes are employed in phytoremediation depending on the quantity and form of the pollutant [81]. Common methods of phytoremediation are extraction, sequestration, rhizofilteration, phytostabilization and transformation for removing heavy metals. Pollutants from the roots and shoots are removed as an important part of phytoremediation. Plants are better candidates for phytoremediation because they ideally absorb Cu, Zn, Cr and Ni. According to a study, microorganisms in the rhizosphere can increase the availability of heavy metals and their uptake by plants [82]. The accumulation of heavy metal in plants depends upon the metal, its solubility, translocation, and plant species [83]. Metals, pesticides, crude oils, explosives and other toxic pollutants have been lowered through phytoremediation processes around the globe.

Nanotechnology as a Sustainable Solution

Nanotechnology has numerous applications as it has high removal efficiency, less time period, and is economical in comparison to other technologies of environmental remediation. Nanotechnology has given a new perspective to wastewater treatment [84]. Depending on their shape, size, structure, and composition, different varieties of nanomaterials such as nanofibres, nanodots, nanotubes, nanoshells, nanocomposites and nanoclusters are used for eliminating contaminants from different environmental matrices [85]. Green synthesis of nanoparticles has multiple sources of synthesis, such as bacteria, fungi, algae and plant extract. The use of green synthesized nanoparticles for the treatment of EPs and other pollutants makes nanotechnology a promising alternative to the current forms of treatment [13].

In the green synthesis of nanoparticles, several biological factors, such as pH, reaction medium, and temperature, influence the properties of the formed nanoparticles. Different organisms can generate different compositions of metallic nanoparticles with different sizes, distributions and morphologies, such as spherical, triangular, cubic, and rod shape [86]. Large-scale production of nanoparticles is mainly governed by the choice of bacteria and methods of synthesis [87]. Biodegradable waste can also be incorporated into the process of synthesizing nanoparticles, which not only saves our environment but also prevents the exhaustion of any natural resource [88]. When bioremediation is combined with nanotechnology to achieve remediation, it is known as nano-bioremediation. Nano-bioremediation is more efficient, less time-consuming and environment friendly. Through an integrated approach, the disadvantages of individual technologies can be removed and provide better results. Recently, a group of workers combined both technologies for the removal of chlorinated aliphatic hydrocarbons and confirmed the integrated potential of nano-bioremediation by efficiently removing a wide range of chlorinated aliphatic hydrocarbons [89]. Polychlorinated biphenyls (PCBs) have been investigated by Le et al. (2015) by the integrated approach, and they found that the treatment of PCBs with Pd/Fe NPs followed by bioremediation with B. xenovorans could effectively transform PCBs into less toxic compounds [90]. Carbon nanotubes (CNTs) and carbon dots, along with bioremediation, are also successfully used for contaminant removal. Though highly efficient and frequently used, the toxicity of NPs for microorganisms is well seen in the literature. Therefore, efforts should be made to develop integrated approaches that are non-toxic and sustainable.

The sustainability of the environment is necessary for the survival of living beings. To sustain life, we must conserve our environment, ecosystem and habitats. The presence of toxic pollutants in the global environment in different forms cannot be denied. The long-term impacts of these pollutants seriously impact our environment, ecosystems, humans, and biota. Because of the limitations of available traditional and advanced remediation techniques, we should switch to green substitutes for the rehabilitation of the environment. Biological solutions, including plants and microorganisms, can facilitate the conservation and restoration of ecosystems in a sustainable manner. To relieve our planet from the undesirable anthropogenic concerns that generate huge amounts of pollutants, an integrated approach is the need of the hour. Though physiochemical and traditional treatments are effective, they have certain limitations and are unsustainable. Biological treatments are economical but not so effective and are non-consistent. However, to accomplish environmental sustainability goals, biological techniques, in combination with the latest techniques, must be used to their full potential to degrade pollutants in a green manner. Moreover, future technologies must be both effective and eco-friendly as well. Also, these technologies should be capable of removing the numerous types of emerging pollutants with low cost and energy consumption, as the existing system for testing emerging pollutants is a highly energy-consuming process. So, it is necessary to enhance energy potency to reduce the usage of energy. Furthermore, the efficacy of the treatment needs to be adjusted for the concentrations of emerging pollutants in the aquatic environment.

REFERENCES