Nanomaterials: An Approach Towards Environmental Remediation E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Fachliteratur
- Sprache: Englisch
This book explains various methods needed to overcome the challenges faced during environmental remediation with a focus on nanotechnology. The book comprises ten edited chapters that aim to inform and educate readers about recent technologies that are beneficial for pollution control.
Starting with an introduction to environmental remediation, the book covers innovative nanomaterials including spinel nanoferrites, carbonaceous quantum dots, carbon nanotubes and nanobioadsorbents. In addition to highlighting the environmental benefits of these materials, the book includes chapters on the potential of nanotechnology for harnessing the environment to generate energy through nanogenerators and piezoelectric energy harvesting devices.
Key features of the book include notes on fundamental issues and challenges regarding environmental remediation, easy to read content with pictorial illustrations and scholarly references for each chapter. The book is an informative resource for students and academicians in science, technology and environmental science discipline.
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Students and academicians in science, technology and environmental science discipline.
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Seitenzahl: 570
Veröffentlichungsjahr: 2024
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PREFACE
In the present era, environmental degradation has emerged as a major threat due to widespread urbanization and industrialization all over the world. Air, water, soil and all other natural resources are getting polluted by one or the other ways. The majority of industries use various inorganic and organic toxic chemicals and discharge them into nearby water streams as effluents without treatment. These pollutants severely influence the aquatic world as well as indirectly the human life. Global crisis due to depletion of natural resources that are not replenishable as well as degrade the environment leads to harmful consequences. Thus, environmental remediation has emerged as a significant field of research towards this direction. In recent times, nanomaterials have exhibited multifunctional properties in the fundamental arena of scientific activities because of their enormous applications especially their roles in environmental monitoring and remediation. The beauty of nanomaterials lies in their small size leading to real perfection, potency and wide range of applications. The inimitable properties of nanomaterials make them suitable for energy harvesting and removal of pollutants from the environment and ultimately cleaning up of the environment. They can be readily tailored for application in different environments and these properties make them unique for developing a new generation of efficient, cost-effective and environmentally friendly functional materials for energy harvesting and water treatment processes.
This book is an attempt to spread scientific awareness among the readers and discuss various methods needed to overcome the challenges faced during environmental remediation as well. The book comprises ten chapters. In this era of digitalization, the use of electronic devices has become an integral part of our everyday life. These devices used for sensing, analysing, and transmitting signals require a very small amount of energy. An alternative source to power these devices could be through harvesting the tiny mechanical motion associated with different motions. Chapter 1 presents a brief introduction to the need for nanomaterials for environmental remediation. Different nanoremediation pathways have been broadly catergorized into four categories: Adsorption, Photocatalysis, Nano-membrane, Nanosensors for different classes of nanomaterials. Nanomaterials for energy harvesting and storage applications have also been discussed in brief. Chapter 2 deals with the methods of preparation of spinel ferrites and their structural and magnetic characteristics. The importance of spinel ferrite in pollutant degradation for wastewater along with its recovery and reuse has been explored. The chapter also discusses the efficacy of adsorption and photocatalysis processes in conventional wastewater treatment techniques. Chapter 3 explains the significance of carbonaceous quantum dots in environmental remediation. In addition, the advantages of carbonaceous quantum dots over conventional quantum dots, methods for synthesizing carbonaceous quantum dots (top down and bottom up) their functionalization or doping to improve their selectivity and sensitivity, their applications in various fields such as sensing, photocatalysis, and bio-sensing have also been discussed. Chapter 4 explores the carbonaceous materials such as activated carbon, biochar, hydrochar, etc. for wastewater remediation. This chapter summarizes the role of carbonaceous materials, their importance and fabrication for their multidisciplinary applications. Chapter 5 deals with a systematic discussion of the role of pure and modified ferrites in the removal of various toxins and pollutants from the environment and their potential applications for environmental remediation. Chapter 6 deals with another material for wastewater remediation- Carbon Nanotubes. This chapter discusses functionalization or modification procedures, depending on the intended application and the chemical makeup of the target pollutants. Designer CNTs can significantly increase the effectiveness of contaminant removal and help with nanomaterial regeneration and recovery. Chapter 7 deals with cellulose-based nanomaterials in water remediation processes. In this chapter designing of various cellulose-based nano-materials has been depicted for the extraction of valuable metals from wastewater. Adsorption by various chemical transformations such as reduction, chelation and electrostatic interaction are discussed for the extraction of various metals. Lastly, composite systems consisting of cellulose and metal oxide nanoparticles have also been discussed for the extraction of rare earth metals from the mining industry. Chapter 8 discusses potential sources of energy harvesting by converting waste mechanical energies into useful electrical energy by nanogenerators. This chapter reviews the basic workings of different nanogenerators based on piezoelectricity, triboelectricity, pyroelectric, and flexoelectricity. This chapter attempts to present the energy management landscape of the country by developing cost-effective materials and devices that can harness both sunlight and vibrational energy and convert them into electricity is the need of the present day for a green and circular economy-driven future. Lead-free piezoelectric energy harvesting technology has been discussed in Chapter 9. The fundamental piezoelectric concept and several piezoelectric materials specifically KNN, BT, and BNT-based ceramics and their applications for energy harvesting are described and assessed in this chapter. Finally, based on their current developments, different challenges and future perspectives have also been encompassed. Chapter 10 aims to offer an overview of cold spray additive manufacturing including their advantages for sustainable manufacturing in terms of environmental concerns. Challenges associated with cold spray additive manufacturing have also been discussed. Chapter 11 aims to assess the ecotoxicity and risk associated with nanomaterials in environmental remediation. Overall, this chapter highlights the importance of careful consideration of the ecological impacts and risks of nanomaterials before implementing them in environmental remediation programs.
We believe this book has successfully targeted the fundamental issues and challenges regarding environmental remediation. Easy to read and pictorial illustrations have focussed on the theme and have justified its purpose. We anticipate that this book will be beneficial for students and academicians in broadening their horizons.
ABOUT THE EDITORS
Prof. Prianka Sharma, an alumnus of Himachal Pradesh University has obtained her Ph.D. in theoretical nuclear physics in 2008. She is a physicist by training and has expertise in theoretical nuclear physics, experimental condensed matter physics and material science. She has published more than 15 research papers in SCI and Web of Science and 01 book and has guided 03 research scholars successfully. Her passion for understanding new phenomena’s and technologies drives her interest in improving the properties of materials for environment-friendly green technology for water treatment and energy harvesting. An experimental physicist with strong analytical and logical skills makes her an expert trouble-shooter. With skills of high order interpersonal understanding, teamwork and communication, she envisions her commitment to the highest standards of professional endeavour.
Prof. Virat Khanna works as an associate professor at the UCRD, Chandigarh University, Punjab, India. He has more than 14 years of experience in academics and research. He has authored more than 30 national and international publications in SCI, Scopus, and Web of Science indexed journals. He is the editor of several books and conference proceedings with various international publishers like Springer, CRC Press, IGI Global and Bentham Science Books. He has delivered several invited seminars and keynote talks on international platforms and has been awarded with best research paper awards. He is also the academic editor of the “Journal of Nanotechnology”, Wiley and “Advanced Materials Science and Engineering, Wiley and is also the book series editor entitled ‘Recent Advances in Materials for Nanotechnology of CRC Press, Taylor & Francis-Publisher. He is also the guest editor of various special issues with various international journal publishers like Springer, De Gruyter etc. He is also a reviewer board member of several international journals.
List of Contributors
Nanomaterials for Environmental Remediation
Sarabjeet Kaur1,2,Madan Lal1,3,Prianka Sharma1,*Abstract
Environmental pollution has become biggest threat to mankind due to its adverse effects on human health and the ecosystem. Rapid industrialization, expansion of urbanization and adoption of latest technologies lead to the release of hazardous by-products and effluents that contaminate the environment. Nanotechnology has proved to be a potential technique for environmental remediation. It involves the most advanced processes that can be successfully utilized in overcoming the issues of environmental contamination due to their unique properties. Multifunctional characteristics of nanomaterials offer unparalleled opportunities in the elimination of pollutants in the nanoscale like volatile compounds, heavy metals, inorganic and organic ions, drugs, pesticides, aromatic heterocycles, biological toxins, pathogens, etc. Nanomaterials with smaller size, higher surface area, quantum confinement and low reduction potential bring versatility in their functionality. These nanomaterials can be utilized as chemical oxidants, catalysts, adsorbents, nanosensors, etc. Surface engineering of nanomaterials can be utilized to enhance their surface area and maximize their reactivity for adsorption of pollutants and promote catalytic reactions by oxidation or reduction of pollutants from contaminated medium. Besides surface area, the selectivity of specific nanoparticles also affects the remediation process. In this chapter, we have given a brief introduction to the nanoremediation pathways broadly categorized into four categories: adsorption, photocatalysis, nano-membrane, nanosensors for different classes of nanomaterials like carbon-based, metal and metal oxides, magnetic, two dimensional, etc. Nanomaterials can prove to be efficient in energy harvesting and storage applications due to the interplay between surface and interface. Hence, there has been continuous demand for nanomaterials with new architectures and physically controlled properties for the purpose of energy harvesting.
INTRODUCTION
In present times, the world is facing major environmental crisis that is not only costing our present but will definitely affect our future also. The year 2022-2023 has seen record-breaking environmental calamities like unprecedented heatwaves, rising pollution, devastating floods, extreme weather events, energy shortages and many more. The mean global temperature in 2022 has risen 1.15oC above the earlier times as stated in the annual report of the WMO’s State of Global Climate. This increase in temperatures has been followed by a series of natural disasters. In some regions, river levels have gone down affecting crops leading to, extreme drought conditions, affecting the international trade, while in other areas heavy rainfall and cloud bursts resulted in devastating floods. Unexpected heatwaves led to wildfires in Australia and Amazon forests. About 592,000 square kilometres of the ice sheet melted in the Arctic region in the middle of summer. The melting of the Greenland ice sheets has raised the sea level by one centimetre. Thus, rise in sea level by several metres has been caused due to extreme carbon emissions leading to global warming. Glaciers in the Alps and the Himalayas have lost one-third of their ice. This extensive melting of the ice sheets has affected the weather patterns and Amazon rainforests. Recently, the United Kingdom has recorded the hottest day in July 2022 alongwith Oklahoma and Texas. With these high temperatures and drought-like conditions, rivers Rhine in Germany and Po in Italy dried up, with severe consequences on hydropower and agriculture. In recent years, the air quality index (AQI) of New Delhi, the capital of India has deteriorated extremely with AQI levels between 350-400 leading to severe impacts on the environment and human health. Dr. Kshitiz Murdia stated “Pollution is not solely an environmental concern; it constitutes a severe threat to human health, including fertility and healthy reproduction also”. Though water is an abundantly available natural resource and covers 71% of the earth’s surface, only 1% of fresh water is available for drinking [1-3]. About 50% of the global population will face water scarcity and inadequacy in freshwater availability by 2025 and this figure will rise to 75% by 2075 [4]. Ever since industrialization, mishandling of industrial wastes has led to serious impact on mankind. In 1912, a painful disease called Itai-itai affecting bone and skin conditions outbroke in Japan as a result of the negligent dumping of toxic metal cadmium in the Jinzu River. In 1956 and 1965, Minamata disease caused by mercury was found in local water bodies. This disease leads to paralysis and sudden death. Pharmaceutical wastes released as effluents in water bodies or soil pose an extreme global threat to the environment and human health. Thirty-four different pharmaceutical drugs have been detected at a single site in the Kai Tak River of Hongkong. Hundred million barrels of toxic oil waste released into water bodies of the Amazon rainforest for many years have caused widespread health issues.
All these incidents and many more not quoted here have taken place due to the rise in global population and changes in human lifestyle. Rapid industrialization, expansion of urbanization and adoption of the latest technologies in agriculture led to the release of hazardous by-products and effluents that contaminates the environment and pose serious threats [5-7]. Moreover, fast-paced advancements in technology, and newer pollutants released have deteriorated the self-remediation capability of the environment [8]. These pollutants not only exhibit negative impact on the environment but have shown severe influence on human health and also on the entire ecosystem [9].
TYPES AND CAUSES OF ENVIRONMENTAL POLLUTION
The major causes of environmental contamination are related to air pollution, water pollution and land pollution due to different types of pollutants as shown in Fig. (1). Industries are the major contributors to environmental contamination. The effluents or by-products like plasticizers, aromatic hydrocarbons, and various xenobiotic compounds are dumped unauthorizedly putting severe warning to the terrestrial ecosystem [10]. These by-products and effluents hold carcinogenic and mutagenic properties that affect seed dormancy and decrease the efficiency of land for agricultural production. Sewage wastes from households and extensive use of fertilizers for crop production lead to the leaching of chemicals from fertilizers and pesticides, heavy metals like (cadmium, zinc, lead) and other micro-pollutants into the ground [11]. This affects the vulnerability of groundwater. Air pollution poses a major threat to the whole of mankind and is the leading cause of fatal deaths all over the world. Aerial pollutants such as carbon dioxide, methane, nitrous oxides, hydrocarbons, fuel soot particles, and ozone, etc. are contributing to global warming and pose a major environmental challenge [12]. Pollutants like sulfur and nitrogenous oxides lead to the deposition of acids and result in acid rain, smog, etc. Higher concentrations of particulates can bring about chemical and physical atmospheric changes. Volatile organic compounds emission can alter secondary organic aerosols [13]. Carbon monoxide a harmful compound present in the atmosphere when inhaled decreases the delivery of oxygen to organs in the body organs, affects vision, and excess levels are fatal. Similarly, exposure to sulfur oxides for longer time duration can cause carcinogenic effects [14]. Reactive species of oxygen are generated by a mixture of various air pollutants which create an imbalance in the concentration of free radicals causing oxidative stress. This leads to high risk of development of cancer [15]. Fine particulates and nitrogenous oxides are capable of crossing the barrier between blood and brain barrier triggering the inflammation of neurons [16]. The most essentially required substances for the existence of all life forms on the earth are water and the atmosphere, hence its contamination is major cause of concern [17]. With manifold increase in industrialization, different pollutants like organic dyes, toxic pesticides, and fertilizers, heavy metals, etc. are continuously entering the aquatic bodies which have high potential to affect human health adversely [18]. Contaminated water can lead to gastric inflammation, irritation in skin-eye, etc. in individuals. Due to high level of nitrogen present in water may risk infants of less than six months of age by decreasing the capacity of hemoglobin to carry oxygen causing Blue Baby syndrome [19]. The presence of heavy metals like mercury, lead, arsenic, etc. observed in groundwater of many regions disrupts the release of endocrine hormones causing reproductive and neurological disorders. Another pollutant Perand polyfluoroalkyl substances (PFAS) can lead to thyroid dysfunction, infertility, obesity, cancer, etc [20]. All these contaminants can contribute to many congenital problems also [21].
Fig. (1)) Different types of pollution and pollutants.Thus, it has been observed that the present era is dealing with major issue of environmental pollution which is posing severe threat to the mankind and the whole ecosystem. New technological advancements are being investigated for remediation of all these contaminants present in the atmosphere, land and water. Materials of different types employed with different approaches are exploited for the purpose of environmental remediation. Environmental remediation is a two-stage process- Firstly, the pollutant is sensed and captured and in the second stage the pollutant is degraded from the medium without leaving toxic compounds which may cause the second level of contamination. Complexity of the compounds of pollutants, low reactivity and high volatility are major challenges being faced by the researchers. Thus, extensive interest is being developed for designing efficient, reliable and cost-effective techniques for the degradation of pollutants present in the environment.
NANOTECHNOLOGY AND ENVIRONMENTAL REMEDIATION
The quest of this search lies in the most emerging technology: Nanotechnology a new domain that possesses ample potential for resolving the problem of environmental remediation. It involves the most advanced processes that can be successfully utilized in overcoming the issues of environmental contamination due to their unique properties. Nanotechnology involves manipulating matter at nanoscale which can fabricate nanoparticles with newer structures having improved properties for novel applications [23-28]. A variety of forms of nanomaterials like nanoparticles, nanowires, nanotubes, quantum dots, etc. can be developed. They are manipulated in size, shape, charge, surface area and solubility level which alter their physical and chemical properties at the nanoscale [29]. Elimination of pollutants from the environment, present in the nano dimension (1-100 nm) range is the most difficult. Nanomaterials with smaller size, higher surface area, quantum confinement and low reduction potential bring versatility in their functionality. These nanomaterials can be utilized as chemical oxidants, catalysts, adsorbents, nanosensors, etc. Surface engineering of nanomaterials can be utilized to enhance their surface area and maximize their reactivity for adsorption of pollutants from contaminated medium. Large surface area also promotes catalytic reactions by oxidation or reduction of pollutants at the surface of the nanomaterials. Besides surface area, selectivity of specific nanoparticle also affects the remediation process. This depends upon the surface charge density, chemical affinity, electron transfer ability, etc. Higher mobility of nanomaterials in solution catalyses the decontamination process. Hence, nanomaterials can be widely explored for various environmental issues like degradation of pollutants, transformation and detoxification of hazardous residues, etc [30-35]. Multifunctional characteristics of nanomaterials offer unparallel opportunities in the elimination of pollutants in nanoscale like volatile compounds, heavy metals, inorganic and organic ions, drugs, pesticides, aromatic heterocycles, biological toxins, pathogens, etc. Thus, nanomaterials combined with variety of techniques known as Nanoremediation prove to be the most efficient, cost-effective and eco-friendly way of treating wastewater from industries, municipal wastes or any other sources [36-40].
NANOREMEDIATION TECHNIQUES
In recent decades, interest has been renewed in the technology adopted and synthesis of nanomaterials with desirable properties for environment remediation. Different pathways for nanoremediation have been catergorized into four broad categories:- Adsorption, Photocatalysis, Nano-membrane, and Nanosensors (Fig. 2).
Fig. (2)) Various nanoremediation techniques.Adsorption
In recent decades, adsorption has emerged as a potential pathway for nanoremediation of the environment. Being an exothermic process, the surface phenomenon of adsorption includes phase transformation of an ion or molecule of the adsorbate (present in gaseous or liquid form) onto the solid or sometimes liquid surface of an adsorbent forming a layer on its surface via attractive forces i.e. chemical (covalent) or physiochemical (non-covalent) interactions under particular conditions [41-46] described in Fig. (3).
Fig. (3)) Schematic representation of adsorption.The process of adsorption depends upon two basic characteristics of the nano-adsorbents: external functionalization and their innate surface. It also depends upon the intrinsic composition and apparent size of the nano-adsorbent. A larger surface area and shorter intra-particle diffusion strongly affect the adsorption process. Higher surface binding energy and size-dependent nanoparticle structure create active sites for adsorption, increasing the chemical and adsorption activity of the nano-adsorbent. Easy surface functionalization of the adsorbent leads to selective adsorption of specific pollutants onto its surface. Besides the shape and size modulation of the nano-adsorbent, porosity is another factor influencing the adsorption process. Pore size tunability and its structure affect the adsorption kinetics. The process of adsorption depends upon the equilibrium conditions of the pollutants and the adsorption coefficient Kd. In the case of inorganic pollutants, redox reactions affect the toxicity of the contaminants [47]. After adsorption of the contaminant onto the surface of the nano-adsorbent, desorption of the pollutant is also a necessary requirement. This can be done by manipulating the conditions like pH of the solution, temperature, etc [48]. The process of desorption is also an important feature since it increases the efficiency of the nano-adsorbent by its reusability and reducing the cost of this nanoremediation process. Thus, the efficiency of nano-adsorbent depends upon its non-toxicity, higher capacity of adsorption, easy capability of desorption and easy reusability [49].
Nano-adsorbents can be broadly classified into several categories: metallic/non-metallic oxide-based nano-adsorbent, magnetic nano-adsorbent, carbonaceous nano-adsorbent, polymer-based nano adsorbent.
Metal- and Metal Oxide Based Nano Adsorbent
Metallic or non-metallic oxide-based nano-adsorbents are inorganic in nature and have been extensively utilized in the removal of hazardous contaminants from the environment [50]. Frequently used oxide based nano-adsorbents are TiO2 [51], ZnO [52], MnO2 [53], CdO [54], MgO [55], Fe2O3 [56]. These oxides-based nano-adsorbents have higher BET surface area, and lesser solubility, show the least impact on the environment and produce no secondary pollutants. The adsorption process depends upon the complex bonding between the oxygen ions of metal oxides and the dissolved metals or organic compounds present in the wastewater, soil, or air [57]. The higher adsorption capacity of manganese oxide (MnO) is due to its polymorphic structure and larger BET surface area. MnO is being widely utilized for the adsorption of arsenic [58]. The porous nanostructure of zinc oxide (ZnO) and larger BET surface area enhances the ability of ZnO as a good nano-adsorbent. Nanoparticle are modified into nanoplates, nanosheets, nanorods, and nano assemblies to enhance the adsorption efficiency for heavy metals like Hg2+, Cd2+, Ni2+, As3+, and Cu2+ from waste water [59]. Another class of oxide-based nanoadsorbent is magnesium oxide (MgO) [40].
Magnetic Nano-adsorbent
Iron is abundantly available in nature and its low synthesis cost have widen its applicability as nano-adsorbent for the removal of organic dyes or heavy metals from contaminated industrial effluents [60, 61]. Being an eco-friendly material, iron-based nano-adsorbents can be directly utilized for the contaminated water with a lesser probability of a secondary level of contamination. Properties like larger redox potential, surface charge and easy reusability make it a convenient nano-adsorbent. Other factors like temperature, pH, adsorbent dosage and time of incubation are important considerations for the efficient process of adsorption. Surface modifications exhibit a higher affinity of nano-adsorbents for the removal of heavy metals like Pb2+, Cd2+, Cr3+, Cu2+, Co2+,etc. from the industrial effluent. Magnetic nano adsorbents have been developed by Peralta et al. [62] for the degradation of organic contaminants like aliphatic and polyaromatic hydrocarbons. Though magnetic nano-adsorbents are highly reactive, cost-effective, easily separated for reuse and eco-friendly, however, they are easily corroded as a result of their high reactivity with water which shortens their life. Hence, magnetic nano adsorbents are coated with porous carbon, mesoporus silica, thiosalicylhydrazide, etc. to enhance their adsorption capacity.
Carbon-based Nano-adsorbent
Carbonaceous nanostructures are also promising adsorbent materials for environmental remediation [63]. Different allotropes of carbon like activated carbon, graphene, and carbon nanotubes (CNTs) find wide applications in this domain due to their low dimensionality and varied surface properties [64]. Carbon nanostructures are abundantly available, exhibit good thermal and chemical stabilities, provide large surface area for reactive edges, exhibit high adsorption capacity, are cost effective and most importantly eco friendly in nature [65]. Due to its large surface area and enhanced porosity, activated carbon has been the most commonly utilized carbonaceous adsorbent. However, its use has been limited due to its high cost. Thus, other functionalized carbon allotropes have been deeply investigated for their use as nanoadsorbent [66]. CNT is a widely utilized carbon nanostructure for adsorbing different classes of pollutants from the contaminated medium. The surface chemistry of CNTs can be tuned easily to modify their surface to enhance their functionality for various pollutants like heavy metals, and pharmaceutical residues [67, 68]. MWCNT nano-adsorbent made up of activated carbon doped with nickel ferrite (Ni-Fe) exhibits good adsorption capacity for antibiotics like levofloxacin and metronodazole [69]. Magnetic MWCNTs formed by combining magnetic NP with CNT act as effective adsorbents for tetracycline [70]. Polymer-functionalized CNT acts as good adsorbent for removing chemical oxygen demand (COD) from the effluent of electroplating industry. CNTs can be functionalized with different polymers like Polyhydroxybutyrate (PHB), amino polyethylene glycol (a@PEG), etc. This functionalization enhances the COD removal adsorption capacity of CNTs for the remediation of contaminated water. Another efficient allotrope of carbon widely used for water remediation is graphene. Graphene, a two-dimensional layered material with sp2 hybridization of carbon atoms has a hexagonal structure. It is a promising candidate for adsorption of pollutants. It possesses a large surface area thereby providing increased active sites for adsorption, excellent chemical stability, good thermal conductivity and exhibit room temperature quantum hall effects. All these properties can be tuned efficiently by modulating the stacking sequence and layer number of graphene sheets [71, 72]. Chemically oxidized graphene leads to the formation of two-dimensional nanosheets of graphene oxide (GO). Selected hydrophilic groups such as carboxylic acid and hydroxyl acid can be induced onto the GO nanosheet for enhancing the heavy metal adsorption capacity. No acid treatment is required further for this functionalized GO nanosheet for enhancement of adsorption. Graphene oxide functionalized with magnetic NPs has been explored widely for its adsorbent capacities due to its cost-effectiveness, simple design, low sensitivity, and higher affinity for toxic pollutants.
Polymer-based Nano-adsorbent
Over many years, several research groups have extensively investigated polymer nano adsorbents for environmental remediation techniques. Polymer-based nanocomposites provide a larger surface area, excellent stability, selectivity towards specific contaminants and are cost effective [73, 74]. Commonly used cost-effective polymer-based nanoadsorbents are polysaccharides, organic covalent polymers, magnetic polymers, cuclodextrin, etc [75, 76]. Nano-cellulose derived from cellulose are ubiquitous, non-toxic, biocompatible and biodegradable adsorbents. Easy surface modification of them makes them suitable candidates for decontamination of polluted water [77]. Dendrimers are also another class of nano-polymers investigated for the treatment of contaminated water.
Photocatalysis
In recent times, photocatalysis has been investigated extensively by many research groups as an efficient environmental nanoremediation technique. It has the potential for breaking down plethora of organic and inorganic pollutants from industrial effluent like dyes, estrogens, pharmaceutical wastes, pesticides, inorganic molecules, heavy metals, microbes, etc [78-81]. A photocatalyst (a solid material) absorbs photons to catalyse a chemical reaction. Hence, photocatalysis is defined as light-driven reaction supported by a catalyst that utilizes light energy (mostly UV) to break down the organic pollutants into biodegradable compounds. It is an Advanced Oxidation Process (AOP) that includes in-situ generation of potential chemical oxidants by UV light or a catalyst (Fig. 5). It is a photochemical process depending on the redox reaction of electron/hole pairs upon irradiation [82-84]. Being a surface phenomenon, the mechanism of photocatalysis undergoes the following basic steps: ● Absorption of light by the catalyst ● Separation of charges ● Interfacial charge transfer (CT) ● Residual products undergo desorption from the surface ● Finally, the residuals are removed from the interface Since all these processes occur on the surface of the catalyst, hence any modification in the structure or properties of the catalyst can bring changes in their surface which will overall affect the kinetics and mechanism of any photocatalytic reaction. With the absorption of visible light, catalyst particles get activated which creates electron-hole pairs. These electron-hole pairs then either recombine or undergo reduction and oxidation processes to decompose the organic pollutants in water. The schematic representation is presented in Fig. (4).
Semiconductor Photocatalyst
Surface modification of nanomaterials along with their excellent chemical, optical and mechanical properties and quantum confinement effects make the ideal choice as nano-photocatalyst [85, 86]. Among many, nanostructured metal oxide semiconductors like titanium oxide (TiO2), zinc oxide (ZnO), cadmium sulphide (CdS), tin oxide (SnO2) etc. have shown superior results due to their band gap properties upon irradiation with visible light. Besides band gap, other advantages of metal oxide semiconductors are easy modification of size, large surface area for reactive sites, easy doping, multifunctionality and cost-effectiveness. TiO2 is a widely used nano-photocatalyst. Band gap of 3.2 eV, large availability, high chemical stability and non-toxic nature and most importantly, its inexpensiveness make TiO2 the most effective photo-catalyst. Another semiconductor ZnO can also be utilized as photocatalytic material. The direct band gap of 3.37 eV, non-toxic nature, low cost, high exciton binding energy of 60 MeV, photosensitivity and high photo-stability make ZnO another interesting material for the process of photocatalysis. CdS exhibiting good optoelectronic properties and low toxicity is also limited due to its wide direct band gap (2.43 eV). However, the rapid tendency of recombination of electron/hole charge pairs due to the wide band gap of these materials reduces the overall effect of the photocatalysis mechanism. Different techniques can be adopted for enhancing the efficiency like dye sensitization, metal impurity doping, hybrid NPs or composites combining narrow band gap semiconducting materials, etc.
Fig. (4)) Schematic representation of the process of photocatalysis.Magnetic Nanoparticles as Photocatalyst
Among various materials, spinel ferrite has come on top because of its various advantages over others [87-89]. Spinel ferrite is a type of magnetic material that shows good adsorption and photocatalytic activity. In the nano range, spinel ferrite shows good magnetic properties making them more useful for the removal of pollutants from the environment. Spinel nano ferrites, in particular, are exceedingly stable, easily regenerable and reusable for a number of times without losing their characteristics, making them cost-efficient and useful for wastewater treatment techniques. Many scientists have used various spinel nano ferrites for the purpose of adsorption as well as photocatalysis. For photocatalysis, spinel ferrite nanoparticles may be a preferable choice. Spinel ferrites are economical and have good band gap energy (2.0-3.0 eV). In addition, they exhibit high recoverability due to their magnetic properties.
Fig. (5)) Schematic representation of nanofiltration using membrane.Two Dimensional Materials as Photocatalyst
For photocatalytic activity high intrinsic electron mobility and a large specific- surface area of graphene at room temperature make it accept electrons and create free radicals in graphene-based composite photocatalysts [90]. However, the zero band gap of graphene becomes a major disadvantage and limits its photocatalytic efficiency due to the fast recombination of electron-hole pairs formed under visible light irradiation [91]. Aiming at overcoming the shortcomings of carbon-based and semiconductor photocatalysis, recently many research groups have attempted to explore a completely new range of isostructural analogous of graphene, 2D Transition Metal Dichalcogenides (TMDCs) like MoS2, WS2, MoSe2,etc [92-94]. These transition metal dichalcogenides have tunable band gaps which can be utilized for practical applications as cost-effective and efficient visible light-induced photocatalysts. MoS2 gets self-excited under light irradiation. Tunability of its band gap, by manipulating the number of layers, extends the range of absorption of MoS2. Therefore, MoS2 as a photocatalyst makes maximum utilization of the solar spectrum. Change in band gap of MoS2 as its thickness decreases to the size of a monolayer shows high charge carrier separation and migration efficiencies [95, 96]. The large surface area of MoS2 provides more active sites on the surface which increases the adsorption capability of the substrates. Moreover, structure of MoS2 has proven to be effective in preventing agglomeration and controlling the size of particles in composites [97, 98]. Thus, the 2D planar structure of MoS2 has an appropriate compact degree, increasing the surface area, especially its insignificant thickness and high mobility of charge carriers which could be beneficial for reducing the recombination of photogenerated electrons and holes. MoS2 possesses the characteristics of an ideal photocatalytic material.
Nano-Membranes
Recently, membrane filtration has emerged as an advanced technique for environmental remediation technology [99]. Various nanomaterials are fabricated with multifunctionalities like high catalytic reactivity, high permeability and fouling resistance. The advantages of adopting this technology are its efficiency, low-cost establishment of treatment units, effectiveness in disinfecting contaminated water, etc. The membrane filtration technique can be utilized for the effective removal of heavy metals, organic dyes and other contaminants. The schematic representation of the membrane technology is presented in Fig. (5). Besides separating the contaminants, these nano-materials utilized as membrane help to decompose the organic foulants separated chemically [100-102]. These nano-materials for membrane technology are one-dimensional, nanotubes, nanofibers, nanoribbons, etc. Commonly used nano-material is a carbonaceous nanofibers (CNFs) membrane for selective filtration and removal of nanoparticles under high pressure. CNF membrane assembled with beta-cyclodextrine is efficiently utilized for the potential removal of fuchsin acid and phenolphthalein. Nano-membranes based on zeolite like MFI-type, sodalite, and Linda Type A can be employed for the process of osmotic separation. Nano-particle capturing potential can be enhanced by interconnecting negatively charged particles and nanoparticles on the macroscopic disc-shaped membrane of titanate-nanoribbon [103-105].
Carbon Nanotube Membrane
Among many carbonaceous nano-membranes, carbon nano-tube (CNT) membranes have gained major attention since they can be combined with polymers to enhance their performance by forming composite membranes having high tensile modulus, strength, flexibility, low mass density and large aspect ratio. Both single-walled and multi-walled CNTs can be used as membranes for filtration after modification with polymers [105].
Electrospun Nano-fiber Membrane
Electrospun nanofiber membranes (ENMs) are a novel way of treating a contaminated environment [106-110]. They are less expensive and require less energy in comparison to other conventional methods of water treatment. The electrospinning method of producing thin nanofibers is an advantage over the other traditional spinning techniques. With the thin diameter of the fiber, the surface-to-volume ratio and porosity of the ENMs are essential for enhancing their performance. The diameter of the nanofiber can be tuned by variations of the parameters like concentration of the solution, surface tension, applied voltage, spinning distance, etc. These electrospun nanofibers are made up of various synthetic and natural polymers like poly-vinyl chloride (PVC), polystyrene (PS), polybenzimidazole (PBI), polyurethanes (PUS), polycarbonates and many more. Electrospun nanofiber polyacrylonitrile/polyvinylidene fluoride doped with graphene exhibits improved electrical conductivity and high porosity. Polysulfone electrospun nano-fiber membrane has been utilized for the removal of contaminants like ammonia, COD, and suspended solids from bio-treated wastewater. Polyvinylidene fluoride electrospun nanofiber membrane exhibits efficiency for the separation of micro-contaminants. These membranes find potential use in the pretreatment of contaminated water before the process of ultrafiltration or reverse osmosis. Electrospun nano-fiber membranes find potential applications in the removal of toxic heavy metals like copper, cadmium, nickel, chromium, etc., desalination process and antimicrobial activities for viruses and bacteria [111-115].
Hybrid Nano-membranes
Additional functionalities can be incorporated into nano-membranes by fabricating hybrid membranes by tuning the pore size, porosity, charge density, mechanical stability and membrane’s hydrophilicity. Zeolite nano-membrane can be impregnated into polysulfone to remove toxic elements nickel and lead from contaminated water [116]. Though membrane filtration is an efficient technology, however, it has certain limitations due to membrane fouling. Thus, lower-cost membrane fouling is potentially required to improve the productivity of this technology. The interaction of organic contaminants present in water with the hydrophobic nature of the membrane is the major cause of membrane fouling. Contaminant particles get into the membrane pores or get deposited on the surface of the membrane. This reduces the water quality, nano-filtration flux due to low pressure and thus, membrane filtration’s reliability. For overcoming this limitation, nano-membranes are treated with chemicals or mechanically cleaned or maybe replaced or modified [117]. Modification of the membrane is an effective way of overcoming the limitation of membrane fouling.
Nanosensors
Sensors enabled with nanomaterials can prove to be an important technology for detecting environmental contaminants at the nano to sub-picomolar level [118-123]. Surface functionalization of nanomaterials and their highersurface-to-volume ratios can bring about change in their surface chemistry which enables the nanosensors to detect contaminants present in very low concentrations (Fig. 6). Sensitivity of nanosensors is enhanced due to the smaller size of the nanomaterial as compared to the analyte like pathogens, antibodies, metal ions. Portability, miniaturization and rapid response to signals have been possible due to the introduction of nanomaterials-enabled sensor design.
Fig. (6)) Schematic representation of nanosensors.Quantum Dots Nanosensors
Semiconductor nanocrystals Quantum Dots (QD) have shown a good response towards sensing environmental contaminants. Commonly employed QDs for sensing applications are CdSe/ZnS [124-126], CdSe [127], ZnS [128], CdTe/CdS [129], CdTe [130-133], and ZnSe/ZnS [134], etc. Fluorescence bands of emission of QDs are quite narrow, while the absorption bands are broad, which makes them excellent materials as optical transducers. Moreover, emission wavelengths of QD can be manipulated by changing the shape, size, or composition of the QD and can be utilized as ideal material for multiple detection of different types of analytes. Though QD of diverse size, shape and composition are excited by a single source of energy due to their broader absorption spectra.
Metal- and Metal Oxide-Based Nano Sensor
Noble metal nanoparticles (NP) exhibit facile functionalization of surface, with change in their size and high extinction coefficients (ε > 3 × 1011 M−1 cm−1) [135] which enables their capability in many sensor applications. For example, distinct color change from a smaller diameter (~ 5 to ~ 50 nm) sphere to a larger diameter (~ 100 nm) of AuNP can be significantly exploited for sensing contaminant analyte. Gold NPs have also proved to be biocompatible and stable for sensor applications [136-138]. Surface modifications by coatings can enhance their efficiency. On the other hand, nanostructured metal oxides (NMOs) like, zirconium oxides, titanium oxides, zinc oxides, cerium oxides, and tin oxides have also shown potential capabilities for sensor applications.
Carbon-based Nanosensors
Carbon-based nanomaterials like graphene and CNTs have exhibited excellent sensing properties due to their large surface area, high thermal conductivity, electrical conductivity, and mechanical strength [139]. Recently, glassy carbon electrodes (GCE) have been employed for electrochemical sensing [140, 141]. Fluorescence quenching with graphene can exploited well enough for the detection of contaminants.
ENERGY HARVESTING AND STORAGE
With the boom in population worldwide in the last three-four decades along with advancements in technology, energy consumption has increased manifold due to increasing dependence and usage of appliances. Energy resources of about 30 TW are predicted to be required by 2050 to maintain the pace of the growth in economy. Till now fossil fuels have remained to be the major option for meeting the energy requirement. Continuous depletion of the non-renewable sources from their limited reserve may lead to the energy crisis. On the other hand, the greenhouse gas emissions from the usage of these fossil fuels as sources of energy pose a serious threat to environmental pollution leading to global warming and its consequences. Hence, the environment and energy crises simultaneously created an alarming situation for sustainable development. Thus, energy harvesting from renewable sources of energy is the solution to this threat for the protection of the environment from further degradation. Nanomaterials can prove to be efficient in energy harvesting and storage applications due to their increased physical effects like ferro-, piezo- and thermos- electricity, photovoltaics, quantum effects, etc as presented in Fig. (7) [142-145]. This has been possible due to the interplay between surface and interface. Hence, there has been a continuous demand search of nanomaterials with new architectures and physically controlled properties for the purpose of energy harvesting.
Fig. (7)) Representation of different modalities of energy harvesting [146].Carbon-based Nanomaterials for Energy Harvesting
Electronic and intrinsic chemical properties of carbon and its allotropes in nanodimensions have been investigated for energy storage applications [147, 148]. The high surface area and excellent electrical conductivity of these nanomaterials play a significant role in the generation of electrochemical energy. Surface chemistry of carbon-based nanostructured carbon facilitates newer technologies for supercapacitor, fuel cell and battery applications. Doped nanostructure surface modification of carbon and its allotropes have been extensively investigated in hydrogen and anodic methanol oxidation reactions for enhancement of CO tolerance. Similarly, they have been investigated for Li-ion batteries. Hierarchical designing of carbon-based nanomaterials have been tailored for high porosity for electrochemical supercapacitor applications for cleaner energy. Recently carbon-based nanomaterials have been proposed for the technology of triboelectric energy harvesting due to their higher optical transparency, flexibility and electrical conductivity [149]. This gives an opportunity for the development of wearable and flexible electronics.
Metal and Metal Oxide-based Nanomaterials for Energy Harvesting
Nanostructured metal oxides show potential strength in energy conversion, storage and hydrogen generation applications for fuel cells, solar cells and other energy storage devices [150, 151]. These metal and metal soxide-based nanomaterials are utilized to generate eco-friendly, cost effective and non-toxic energy for a cleaner environment. Metal oxides like Fe2O3, NiO, ZnO, MnO2, Co3O4, RuO2, V2O5, NiCo2O4, etc. have been studied as electrode materials for supercapacitor applications due to their larger specific capacitance [152-154]. However, due to their poor cyclic stability their structure may collapse hence their capacitance performance, cyclic stability and energy density can be enhanced by combining them as coating for porous carbon electrodes. This type of composite electrode is bestowed with good electrochemical properties due to the synergistic effect between metal oxide and 3D porous carbon which arises as a result of increased surface area and availability of active reactive sites present in the composite material for enhancement of the electric charge storage
Two-dimensional Nanomaterials for Energy Harvesting
Recently, two-dimensional (2D) materials have shown vast scope for the purpose of energy harvesting and storage applications. Besides graphene, transition metal dichalcogenides, MXenes, borophene, tellurene, and black phosphorus have been excellent alternatives for this purpose due to their strong interatomic bonding, good mechanical flexibility, and higher surface-to-volume ratio [155-160]. They find promising applications in supercapacitors and batteries. These 2D materials possess higher electrical conductivity which permits faster charging-discharging rates, structural anisotropy, and quantum confinement leading to charge transport within bands resulting in tunable band gaps. All these features of 2D materials promote excellent electronic and optical properties to the 2D materials which can be exploited for the purpose of energy harvesting.
CONCLUSION
Environmental pollution has become the biggest threat to mankind due to its adverse effects on human health and the ecosystem. Hence, a detailed investigation is being done by many research groups all over the world to explore the techniques of remediation of environment contaminated by any source. In this search, nanotechnology has proved to be a potential technique for environmental remediation. In this introductory chapter, we have presented an overview of the general nanoremediation techniques along with discussion over some of the nanomaterials that are being used in these techniques. The chapters presented in this book will cover some of the major techniques and nanomaterials. Spinel ferrites have been discussed for both adsorption as well as photocatalytic applications for treating contaminated water. Carbon nanotubes have been discussed with their fabrication and functionalization for environmental remediation applications. Synthesis of quantum dots for sensing applications has also been exploited. Water remediation using the adsorption process by biochar has been discussed. Nanomembranes have also been investigated for various water treatment processes. Energy harvesting with piezoelectric materials has been discussed in detail. Types of nanogenerators with different nanomaterials have been investigated in detail for the purpose of energy harvesting and storing applications. A technique of thermal spray for protecting metals from environmental contaminations has been studied. Though these topics have a wide area to cover, every attempt has been made to give detailed information on a few of them. An important matter for consideration is the after-effect of the nanomaterials being continuously used for the purpose of remediation. These materials chosen for remediation of pollutants should not generate secondary stage pollution i.e. they should offer safer most preferably greener remediation strategies for pollutants. This important consideration has also been discussed in the end. Furthermore, nanoremediation strategies adopted are specific for particular contaminants, hence they possess lower efficiency for off-target pollutants. Hence, cost-effectiveness, facile synthesis of target-specific nanomaterials, their bio-degradability and non-toxicity, and recyclability are major key challenges that must be overcome before adopting the remediation technique. A deep understanding of the nanomaterial employed, the process of fabrication and optimization is necessary for developing a potential candidate that possesses the capability of addressing diverse environmental remediation issues.
