Nanotechnology for green materials and processes

C H A P T E R

7 Nanotechnology for green materials and processes Paolo Ciambellia,b,*, Giovanni La Guardiaa, Luca Vitalea a

Narrando, Fisciano, Italy; bUniversity of Salerno, Fisciano, Italy * Corresponding author. e-mail address: [email protected]

1. Introduction

the future problems (proactive applications). Some of the reactive applications include water treatment, environmental sensing, and remediation. Proactive applications primarily include green manufacturing and research of costeffective alternative energy sources like solar and fuel cells. One of the main objectives of the environmental nanotechnology is safe design and sustainable development of nanomaterials with potential environmental benefits.

Nanotechnology is the manipulation of matter at the nanoscale (1e100 nm) in diverse fields such as engineering, materials science, chemistry, physics, biology, and medicine [1]. The large surface area to volume ratio that characterizes the matter at that scale, compared to that of bulk components, confers unique properties, like specific optical, magnetic, interfacial, electrical, and chemical [2]. The application of nanomaterials and nanotechnology is very attractive in many industrial sectors, especially in biomedicine, energy, pharmacy, food, agriculture, and environment. A Business Wire report confirmed that the global nanotechnology market is poised to grow at a compound annual growth rate of 18.1% over the next years to a market size of $173.95 billion by 2025 [3]. In the specific case of the environment, the use of nanotechnology includes wastewater and contaminated soil treatment, remediation, sensors, and energy storage [4]. The applications of nanotechnology to the environmental field can be aimed to solve current environmental problems (reactive application) or to prevent

Catalysis, Green Chemistry and Sustainable Energy https://doi.org/10.1016/B978-0-444-64337-7.00007-0

2. Toward green nanotechnology Green nanotechnology is defined as the technology aiming at developing clean technologies to minimize human health and potential environmental risks. Green nanotechnology is for many aspects the natural evolution of nanotechnology, driven by the necessity of matching the large variety of possible and practical applications of nanotechnologies with the requirement of the so-called sustainable development. It is associated with the use of nanotechnology products and manufacturing process and encourages substitution of existing products to develop new

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nanoproducts. Extensive research in the area of nanotechnology has resulted in the establishment of green, reliable, efficient, and economical approaches with far-reaching impact in the domains of energy, industry, and environment. Environmental issues have taken precedence in gaining a great deal of attention from the political and scientific communities in the 21st century, also named the “Century of Environment” [5]. The term green nanotechnology can be interpreted in two ways. On one hand, it is possible to transform a number of general applications and processes to environmentally friendlier routes using nanomaterials via adopting resource/energy saving methods and substitution of toxic substances. A simple specific example is that related to Pt catalyst for fuel cells. Moving from the usual Pt catalyst to a Pt nanocatalyst has improved performances and reduced Pt amounts and, consequently, total costs. Although, the sustainability principles push to a different goal, i.e., to find environmentally friendly catalysts as substitutes for platinum. Alternatively, this term can be directly related to the application of nanomaterials in the domain of environmental technology [6]. Most of the applications of nanotechnology for the environment fall into three classes: 1. Environmental protection (eco-friendly and/ or sustainable products) 2. Environmental remediation using engineered nanomaterials (nanoremediation) 3. Environmental monitoring (nanotechnologyenabled sensors)

2.1 Environmental remediation Environmental remediation refers to the field of study that deals with clean-up methods capable of eliminating and/or degrading pollutants contained in soils, surface waters, and

groundwater as well as sediments. The increasing demand of freshwater resources due to expansion in population, urbanization, and industrialization is one of the most urgent concerns. In fact, according to the recent data provided by UNESCO (United Nations Educational, Scientific and Cultural Organization), shown in Fig. 7.1, a wave of increase in global water demand estimated to be 55% has been anticipated by 2050, mainly due to the ever-increasing needs from manufacturing industries, thermal electricity generation, and domestic usage. Nanotechnology development can give a really strong contribution to the improvement of environmental health. At present, substantial progress has been made in manufacturing functional nanomaterials that has created exciting new possibilities for environmental clean-up. Typically, the classification of conventional remediation techniques is equally applicable to the nanotechnology for environmental remediation, i.e., adsorptive versus reactive and in situ versus ex situ. Decontamination using adsorptive technologies involves removal of pollutants by sequestration, whereas reactive technologies mainly involve the complete degradation of contaminants into some harmless substances. In situ technologies are based on the treatment of contaminants on site, whereas ex situ technologies are based on moving the polluted sample to a different place, followed by its treatment [7]. The exceptionally high specific surface area as well as unique physical and chemical properties of nanomaterials, such as iron oxide, titanium dioxide, and alumina, make them highly useful and promising as adsorbents and enable highly efficient adsorptive removal of a wide variety of pollutants from wastewater. Different carbon nanostructures also have been effectively investigated as adsorbents for trapping or separation of organic compounds. For instance, carbon nanotubes (CNTs) have received special attention due to their exceptional prospects in wastewater remediation and treatment of chemical and

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2. Toward green nanotechnology

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FIGURE 7.1 Global water (fresh water, not rainfed agriculture) demand: baseline scenario, 2000 and 2050. BRIICS (Brazil, Russia, India, Indonesia, China, South Africa); OECD (Organisation of Economic Co-operation and Development); RoW (Rest of the World).

biologic pollutants. CNTs have been successfully applied as an efficient adsorbent material for the removal of a large number of contaminants such as heavy metals, arsenic compounds, organic compounds, atrazine, pharmaceuticals, personal care products, and endocrine-disrupting chemicals [8]. The use of nanocarbons in this field has aroused much interest, and many innovative solutions have been proposed. A recent example is the use of reduced graphene oxide (RGO) membranes for wastewater treatment (see Fig. 7.2). Liu et al. found that such membranes are capable of separating multiple types of surfactant-stabilized oil-in-water emulsions with oil droplets of nano/submicrometer size, which display high separation efficiency and excellent antifouling properties [9]. Growing interest is currently addressed to adsorbents for CO2 adsorption, such as the very numerous synthesized metal-organic framework nanostructures. Really, recently suggested materials [10] open the question: how really green are

they with respect to the organic ligand components? Perhaps, the most appealing green nanotechnology in the area of environmental remediation for purifying air and water is based on the heterogeneous photocatalysis, especially due to very impressive progress of innovative solutions, from catalyst formulation to performing photoreactors [11]. In fact, it can give real green solutions to the current environment degradation: the light-driven pollutant conversion (especially visible light), if compared to temperature-driven thermal processes, is the intrinsic sustainable characteristic of these processes. Other features are the use of environmentally friendly oxidant (O2) and highly efficient abatement of even low-concentration organics at room temperature and pressure, LED (lightemitting diode) illumination substituting old solutions based on Hg lamps, the superior performance of catalyst given by size and structure at the nano scale. Titanium dioxide (TiO2) is

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FIGURE 7.2 Schematic illustration of preparing the free-standing RGO membranes and the vacuum filtration process, showing the separation capacity of the oil-in-water emulsion [9].

the most used catalyst in advanced oxidation processes for water remediation, due to low level of toxicity, high photoconductivity, high photostability, easy production, and low cost, but other materials, such as zinc oxide (ZnO), iron (III) oxide (Fe2O3), and tungsten oxide (WO3) are also active photocatalysts. The synthesis and characterization of visible light-responsive TiO2- and ZnO-based nanophotocatalysts and suggested uses in the removal of pollutants from water, wastewater, and gas streams are discussed in Ref. [12]. Since the bandgap energy values of these semiconductors are 3.0e3.2 eV, less than 5% of the complete solar spectrum can activate them, so they need to be modified to enhance the absorption of visible light. The desired modification can be obtained in different ways: coupling with a smaller bandgap semiconductor (such as CdS, MoS2, or In2S3), sensitizing with dyes, or doping with metals or nonmetals (mainly by C, N, F, P, or S anions). Self-cleaning properties of surfaces functionalized with nanophotocatalysts active in the presence of visible light have been demonstrated in the case of ceramic tiles, using N-doped TiO2 films, produced by sol-gel method [13].

The degradation of organic compounds in water is one of the most studied applications of visible light heterogenous photocatalysis, since it can minimize the energy consumption and optimize the operating conditions for the mineralization of pollutants. An example of application for wastewater treatment is the photocatalytic degradation under visible light irradiation of organic dyes (OD) contained in the strongly colored effluents of the textile industry. In fact, their raw disposal causes extensive damage to aquatic life because of the low biodegradability, while conventional biologic treatment processes are much less effective [14]. Sacco et al. [15] investigated the performance of anatase NeTiO2 prepared by a modified sol-gel method, and the results of the experiments performed in a slurry photocatalytic reactor irradiated by white or blue light LEDs (emission ranges, respectively, 400e800 nm and 400e550 nm) showed a significant removal (up to 97%) of total organic carbon. Water treatment is also one of the various uses of polymer nanoparticles. Using a similar principle as surfactant micelles, polymeric nanoparticles have amphiphilic properties, where each molecule has hydrophobic and hydrophilic

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3. Renewable energy generation and storage applications

parts. In the application, polymeric nanoparticles offer an eco-friendly alternative for commonly used conventional surfactants able to enhance remediation of hydrophobic organic contaminants using a pump and treat system [16]. An interesting proposal is related to the concept of attributing a double function to the photocatalyst, expected to be able to detect and remediate contaminants. During laboratory experiments, a ZnO photocatalyst was successfully used to detect and eliminate 4-chlorocatechol [17].

2.2 Environmental monitoring Long-term exposure to particulate matter and heavy metal pollution is a recognized significant leading factor in causing several health problems, such as heart conditions and lung cancer. Rapid and precise sensors able to detect pollutants at the molecular level may enhance the human ability to protect the sustainability of human and environmental health. Nanowires or nanotubes offer great capabilities as materials for chemical and biologic sensors [18]. SWNTs (single-walled carbon nanotubes) have shown a faster response and higher sensitivity than the conventional probes that are currently used in the detection of gas molecules such as NO2 and NH3. In this case, gas molecules directly bond to the surface of SWNTs and influence the electrical resistance of the sensor. Another advantage of using SWNTs as sensors is their ability to achieve high sensitivity at room temperature, whereas, in general, conventional solid sensors are operated at temperatures of 200e600 C. Although SWNTs are highly promising alternatives to nanosensors, SWNTs also have some limitations. To detect various chemical and biologic species, the surface of nanotubes needs to be modified with specific functional chemical groups. Moreover, the flexibility of the chemical detection relies on the type of functional group

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doped on the nanotube surface. In contrast, some nano-semiconductors, such as Si nanowires (SiNWs), do not have this kind of limitation. Boron-doped SiNWs have been used for protein and antibody detection in real-time electrical detection. The small size and the ability of semiconductor nanowire to detect many types of analytes in real-time sensors can be used to develop detectors of chemical and biologic agents that are pathogenic in the air, water, and food [19]. Nanosensors were also produced using CuO nanoparticles obtained by stabilizing them in a water medium using carboxymethyl cellulose (CMC) at room temperature. These nanoparticles were then dispersed with multiwalled carbon nanotubes (MWCNTs) to get an electrode. The proposed electrode displayed high selectivity and reproducibility and has been successfully utilized for determination of nitrite in real samples [20].

3. Renewable energy generation and storage applications In the 21st century the environmental issues caused by nonrenewable energy sources have emerged as one of the most urgent challenges. The development of clean and renewable energy technologies is crucial to meet both environment regulations and to avoid dependence upon fossil fuels. The research of new energy sources is strongly linked to the necessity of developing devices capable of storing higher quantities of energy. In this context, nanomaterials and nanotechnologies represent a new frontier with promising perspectives. In Fig. 7.3 the progressive increase over the years in the interest toward the application of carbon nanotubes in different energy production/storage devices such as solar cells, supercapacitors, Li-ion batteries, and fuel cells is shown. An exponential growth of the number of publications has been evident in the past 15 years,

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FIGURE 7.3

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Number of publications over the years in the field of application of CNTs in different types of electrochemical

devices [21].

which shows how nanomaterials are gaining an increasingly important role in the development and innovation of these technologies. Among all the attempts to improve existing technologies or to develop new ones using nanomaterials, the most studied and promising innovations are in the fields of solar energy conversion, hydrogen production, and supercapacitors.

3.1 Solar energy conversion Solar cells are devices that use the photovoltaic effect to convert the energy of light directly into electricity, producing electrical charges that can move freely in semiconductors. The process was discovered as early as 1839, but the first solar cell was introduced by Bell executives in 1954. The first generation of solar cells was

produced on silicon wafers either using monocrystalline or polycrystalline silicon crystals. The most recent and promising generation of solar cells consists of concentrated solar cells, polymer-based solar cells, dye-sensitized solar cells, nanocrystal-based solar cells, and perovskite-based solar cells. Over the last decades, conducting polymers have revealed to be ideal candidates for the photovoltaics in solar cells, and the use of CNTs to improve their efficiency has been investigated. The addition of CNTs improves the charge conduction, the optoelectronic properties, and the thermal and chemical properties of the cells [22,23]. For the dye-sensitized solar cells (DSSC), dye-coated porous TiO2 nanostructures, assembled with CNTs, have been reported. CNTs also find application as heterojunctions in solar cells and as windows/back electrodes [24].

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3. Renewable energy generation and storage applications

3.2 Hydrogen production Hydrogen production and storage has gained a lot of interest due to its variety of applications in the energy sector. Producing chemical energy, as opposed to electrical energy, has its own advantages such as prolonged storage of energy and easy transport. Even though hydrogen is the most attractive and environmentally friendly source of energy, most of the processes and materials presently involved in hydrogen production are not. The hydrogen production via direct electrolysis of water combined with the use of solar energy is an attractive solution to the impending energy crisis since it is renewable, clean with no production of greenhouse gas emissions, potentially inexpensive, and sustainable. In recent years, significant advances have been made in the identification and design of novel nanomaterials with improved efficiencies for hydrogen production. In particular, to enhance hydrogen production, water hydrolysis is combined with the use of a so-called sacrificial agent, which is an organic substance able to combine with the valence band holes more effectively than water [25]. The result of this combination is equivalent to a reforming of the organic material, which is typically a high-temperature process. In the case of photocatalytic reforming where the sacrificial agent is a pollutant contained in the water, it is therefore possible to achieve a double resultdwater splitting, producing hydrogen, and water cleaningdthrough the decomposition and the mineralization of the pollutant [11]. In the literature are reported various cases where this process has been applied to wastewaters. For example, Speltini et al. [26] found that UV irradiation of olive mill wastewater over a TiO2-based photocatalyst led to a production of 80 mmol of H2 in 4 h of reaction with simultaneous reduction of chemical oxygen demand. Moreover, Iervolino et al. [27] treated wastewaters from cherry washing (see Fig. 7.4). The

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photocatalyst used in this process was based on metal-doped perovskites and deposited on nanoparticles with magnetic properties to avoid the drawback related to the separation of a catalyst in powder form after the reaction. As a result of this treatment, not only there was a production of about 12 mmol/L after 4 h of reaction under visible light irradiation, but the water obtained at the end of the process also had the characteristics necessary to be disposed of or for its possible reuse in irrigation. At the Gwangju Institute of Science and Technology (Republic of Korea), researchers successfully manufactured a Ni nano-pottery structure using an electrodeposition process into a nanoporous alumina template [28]. The nanostructure was then used as a cathode catalyst for hydrogen production via alkaline water electrolysis, and it was found to be both stable and effective. Carbon nanotubes have also been applied as a cathode-related material in electrolysis of water [29]. Their electrically conducting surface is very useful as a cathode for hydrogen production and absorption and to electrolyze water. Moreover, the hydrophobic carbon nanotubebased electrochemical cell is considered an alternative solution to hydrogen sorting due to the unique combination of hydrophobicity and conductivity of carbon nanotube forests [30].

3.3 Storage applications Like other capacitors, supercapacitors also consist of two solid/porous electrodes in contact with a separator and an electrolyte. Recently, development of supercapacitors with a high efficiency has been one of the most interesting and promising fields of research both in academia and in industry. At present, the use of carbon nanomaterials as electrodes in supercapacitors has gained a lot of interest. Usually, supercapacitors can be divided into two classes (see Fig. 7.5): pseudocapacitors and electrical double-layer capacitors (EDLC). The differences between these two types of

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FIGURE 7.4 Wastewater valorization and treatment plant based on AOPs. Credit: G. Iervolino, V. Vaiano, D. Sannino, L. Rizzo, A. Galluzzi, M. Polichetti, G. Pepe, P. Campiglia, Hydrogen production from glucose degradation in water and wastewater treated by Ru-LaFeO3/Fe2O3 magnetic particles photocatalysis and heterogeneous photo-Fenton, International Journal of Hydrogen Energy 43 (4) (2018) 2184e2196. Copyright: Elsevier, 2018.

FIGURE 7.5

Schematics of (A) an all-carbon EDLC and (B) a pseudocapacitor. Both devices have active materials (e.g., carbon and MnO2), a current collector, a separating membrane, and electrolyte [31].

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5. Nanotechnologies for agriculture

capacitors lie in their charge transfer and storage mechanisms, i.e., a pseudocapacitor follows faradaic charge transfer laws, whereas an EDLC obeys the nonfaradaic charge transfer mechanism. For a deeper and more accurate dissertation on this topic, we refer to another chapter in this book.

4. Reduced consumption/substitution of raw materials The use of engineered nanoparticles (ENPs) is being developed and is now fundamental for many industrial applications. Considering the toxicity of some ENPs and the potential occupational and environmental exposures, the question arises whether it is possible to substitute such particles for more toxic chemicals currently used in various applications, or whether the use of ENPs or nanotechnology (NT) in general will increase the risk associated with these products. It is likely that any practical use of ENPs as a replacement for a more hazardous material will encompass aspects of both material substitution and process change. Nanoparticles are rapidly being developed also for medical applications. In some of these new applications, ENPs are meant to replace other hazardous chemicals, e.g., the development of nanoparticle-based drug delivery systems for cancer therapy is receiving growing attention because of the very debilitating side effects of drugs now used for cancer treatment. Limited examples of ENPs being successfully used as substitutes for toxic chemicals in industrial applications are reported in published literature. One of the most promising areas is the development of substitutes for solvents. Since it is unlikely to directly substitute an ENP for a solvent, ENPs may be part of a water-based system that substitutes for a solvent-based one. It is the case of solvent of paint. For example, some paint manufacturers claim that nanoparticles are a component of a water-based paint system that

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can give similar properties to their solventbased paint products. The incorporation of ZnO or TiO2 nanoparticles into the formulation may make the paint surface more durable, allowing for thinner paint coatings and an overall reduction in chemical use [32]. The incorporation of nanoparticles into the paint may reduce adhesion through what is commonly called the lotus effect, since lotus leaves are known to readily shed water droplets. Cheng has shown that the presence of nanometer scale hair-like structures on the lotus leaf surface are responsible for the very high contact angle that leads to water shedding. In case of ships, if a marine paint incorporating similar structures could be developed, the use of other contaminating substances could be eliminated [33]. Finally, another goal is lead replacement inside wiring cables. Lead is inserted into the polymer cover to give high thermo-conductive properties. The idea was to replace lead with nanomaterials, in this case nanoclay, to have the same properties [34]. While nanoclay is a nonrenewable resource, it is widely abundant in surface deposits and thus can be used with relatively minor environmental impact, especially when compared to the processing required to produce lead salts. Schmidt found that a combination of nanoclay, nonlead heat stabilizer (Ca, Mg, and/or Zn based), and ELO (epoxidized linseed oil) were able to provide heat stability, fire protection, and flexibility in a single lead and phthalate-free additive package. This leads to the hope that a commercial product containing an appropriate combination of nanoclay, a relatively nontoxic metal salt, and ELO has the potential to replace lead and phthalates in wire and cable insulation [35].

5. Nanotechnologies for agriculture Over the next decades, the agricultural sector is expected to face a 30% rise in the world population: a more than one billion people increase in

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the next 15 years, reaching 9.7 billion by 2050 and 11.1 billion in 2100. Based on this projection, it would need to produce 60% additional food globally and 100% more in the developing countries to meet the demand at current levels of consumption by 2050 [36]. This required larger production will also suffer from competition for increasingly limited land, water, and energy resources as well as climate change. In this scenario, nanotechnology is expected to play a significant role. Due to an increase of research focused on its use in agriculture, nanotechnology has strongly contributed to transform the entire state of the agricultural and food industries by offering attractive innovations (Fig. 7.6): novel fertilizers and pesticides, use of new tools to treat various plant diseases, nano-based kits for rapid detection of pathogens, and improving the uptake of nutrients by plants are some examples. Nanocatalysts also can have a prospective role in increasing the efficacy of common pesticides and insecticides, thus reducing their dosage required by different crops, preventing in this way the excessive use of pesticides and their negative implications on our environment [37]. In the fertilization field, to date, urea (CO(NH2)2), is the principal fertilizer, due to nitrogen content, water solubility, and ready plant availability, but its fertilizing effect is rather low (about 25%) due to volatilization and leaching losses. This results in higher cost and negative effect on water contamination and air pollution by nitrous oxide. Despite a strong attention in the last decade to the parallel field of drug delivery, low attention has been addressed to slow nutrient release in the case of nanofertilizers. However, more recently the interest to this type of solution has increased. For example, a slow release nanofertilizer based on ureacoated hydroxyapatite nanoparticles has been produced. Higher crop yield with reduced urea amount and 12 times slower urea release is reported [39]. Application of various kinds of biosensors based on nanostructured materials and

their composites represents another exciting aspect of nanotechnology in the agricultural sector for remote sensing devices in precision farming. It has greatly helped in reducing agricultural wastage, hence keeping environmental pollution to a minimum [40].

6. Green synthesis In this section, attention will be focused on the green synthesis of nanomaterials, because not all nanomaterials are produced in an eco-friendly way.

6.1 Green synthesis of NPs There are several systems and methods for green synthesis of NPs, in particular, from enzymes, vitamins, by microwave, by bio-based methods, and from plants and phytochemicals. Some examples of green synthesis of the most common and used nanoparticles are presented subsequently. The synthesis of copper nanoparticles can use plants, due to their availability, costeffectiveness, environmentally friendly nature, and nonhazardous by-products. Various processes with different precursors such as Terminalia Arjuna bark [41], Artabotrys odoratissimus [42], and Nerium oleander [43] have been suggested. Zinc oxide (ZnO) nanoparticles can be produced in a green way by treating a solution of Zn(NO3) with Cassia auriculata blossom extract [44]. Green preparation of ZnO nanoparticles is a critical issue for their utilization for clinical antimicrobial wound-healing bandages [45]. CeO2 can be used in the pharmaceutical field for treatment against obesity [46]. Green synthesis in which the extract of Gloriosa superba leaf [47] or the aqueous extracts of ethereal parts of Prosopis farcta [48] are used as precursor has been reported. For the synthesis of cadmium sulfide nanoparticles, various green methods using both

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FIGURE 7.6

Application of nanotechnology in crop protection by utilizing various formulations of nanoscale products. Credit: K. Vishwakarma, N. Upadhyay, N. Kumar, D.K. Tripathi, D.K. Chauhan, S. Sharma, S. Sahi, Potential applications and avenues of nanotechnology in sustainable agriculture, in Nanomaterials in Plants, Algae, and Microorganisms, Academic Press, 2018, 473e500. Copyright: Elsevier, 2018.

plants and microorganisms have been proposed. In particular, they were synthesized by biomass of Fusarium oxysporum with size of 2e6 nm [49], while in a study on E.coli and Klebsiella pneumoniait, it was discovered that they are able to synthesize both CdS and other heavy metal nanoparticles [50]. Gold and silver nanoparticles are widely used in various sectors such as optoelectronics, catalysis, detection, and medicine. It has been discovered that green synthesis of such particles is possible using honey, as a rate of reduction accelerator. Furthermore, the nanoparticles formed

by the reduction with honey have anticorrosive, antimicrobial, and bio-sensitive properties [51]. Nanoparticles of Au and Ag were synthesized using Malacothrix glabrata leaves, and their metal salts and antimicrobial properties can be used for water purification [52]. Utilizing a low toxicity microemulsion and nanoemulsion system with castor oil as an oil phase, Brij 96V and 1,2-hexanediol as a surfactant and cosurfactant individually, a green combination with Ag NPs was obtained. In this synthesis the aqueous extract of geranium leaves was used as a reducing agent [53].

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6.2 Photocatalytic synthesis of organic compounds In the last century, the major development on photocatalytic studies has been mostly restricted to environmental catalysis studies. Partial photooxidation of paraffins to aldehydes and ketones with TiO2 was reported in 1972 [54], and 2 years later a reactivity scale of carbon in photooxidative reactions was determined, and surprisingly high selectivity for hydrocarbon partial oxidation to aldehydes and ketones was reported (n-alkanes 76%, i-alkanes 80%, neoalkanes 57%) when compared to the unselective total oxidation to CO2 of the relevant thermal reactions. A reaction mechanism of photocatalytic oxidation, responsible for such unexpected selectivity, was suggested, completely different from those usually involved in catalytic thermal oxidation carried out in the absence of irradiation [55]. Some more recent examples are reported subsequently. The effect of sulfate on the photocatalytic properties of MoOx/TiO2 catalysts for cyclohexane photocatalytic oxydehydrogenation has been investigated with a gasesolid continuous flow reactor. The presence of sulfate species on the surface of titania enhances selectivity and yield to benzene more as the sulfate content is higher. On the catalyst with the highest sulfate content, very high selectivity to benzene was obtained, very weakly dependent on cyclohexane conversion. The selectivity properties of MoOx/TiO2 catalysts are associated with the presence of both sulfate and polymolybdate species on the titania surface [56]. In a different work the conversion of ethanol to acetaldehyde using a photocatalyst based on VOx/TiO2 supported on phosphors particles is reported [57]. The analysis of thermodynamic data from the results of the various experiments has shown that the combined effect of the temperature increase, together with the irradiation, increased the conversion to acetaldehyde [58].

7. Enzymatic catalysis Enzymes are very efficient biocatalysts that are considered potential candidates for several applications in organic synthesis since they can catalyze numerous biochemical and chemical reactions under mild conditions, such as ambient temperature, physiologic pH, etc., with extraordinary selectivity. Nevertheless, their application in industrial processes is typically hindered by low operational stability and short lifetime. Different strategies have been proposed to solve these problems, and among them, the immobilization of enzymes on magnetic nanoparticles (MNPs). In a study of Sarno et al., the immobilization of lipases on Fe3O4 nanoparticles from Thermomyces lanuginosa (TL) is reported. The nanoparticles were prepared using a low-temperature thermal decomposition of the precursor in organic solvent. The particles were covered with surfactants such as oleic acid and oleylamine to help immobilization. With this method, 77% of immobilization was obtained, and the product can be easily recovered, as the nanoparticles can be magnetically separated [59]. Immobilized lipase on magnetite nanoparticles has been also tested for banana flavor synthesis. The immobilization phase took place by functionalizing the nanoparticles with citric acid (CA) followed by multiple physical interactions that bound the lipase on the nanoparticles from TL. The immobilized lipase showed much higher activity than the free one; furthermore, after 120 days, it still showed half of its bio-catalytic activity. The ester yield of this bio-catalytic system was 80% with a selectivity of 100% [60]. An optimized eco-friendly procedure for the preparation of a similar system has also been proposed and Fe3O4-CA nanoparticles achieved an immobilization efficiency of more than 96%, with a high level of protein loading of more than 19 mg lipase/g NPs. The produced enzymatic nanocatalyst was then used for the bio-

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8. Life cycle of nanomaterials: health and environmental risk

lubricant synthesis from waste cooking oils in a solvent-free system. High conversions of w45% and w68% after 3 and 6 h of synthesis, respectively, were observed, and it was possible to reach a maximum conversion of 94% after 24 h with the addition of molecular sieves [61]. In another work, lipase immobilized on Fe3O4\Au particles has been used for biodiesel production from tomato waste. The activity of the immobilized enzyme to transesterify the tomato seed oil was evaluated and very high yields were found. Moreover, thanks to the Au nanoparticles, the enzyme activity remained above 84% even after three cycles [62]. A novel biodiesel production process from spent coffee has also been proposed, as shown in Fig. 7.7. Lipase from TL was chosen as the enzyme, and it was bound on modified magnetite nanoparticles using acetic acid, which guarantees a high enzyme load. The enzyme activity was found to be very high even after 60 days. Moreover, the reaction rate was very high, and the yield was close to 100% after 24 h, with a high content of esters [63].

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In a recent work of Darwesh et al., peroxidase immobilized on magnetite nanoparticles was used for the decolorization of textile wastewater. The obtained results have shown that the immobilization of the lipase resists at pH and temperature variations, and the enzymatic catalyst is active for 90 days, even for 100 cycles. Furthermore, this process proved scalable to lab-scale bioreactor level and to be potentially employable in industrial bioremediation processes [64].

8. Life cycle of nanomaterials: health and environmental risk Many of the extraordinary properties possessed by nanomaterials create a new environmental, health, and safety paradigm. This paradigm could not be addressed by using the existing risk assessment and management concepts. In fact, according the classic toxicologic models that employ the concept of toxicity of materials, the risk can be quantified with the product of a measurable concentration and the exposure time. But with nanomaterials,

FIGURE 7.7 Schematic steps for preparation biocatalyst and biodiesel synthesis from spent coffee grounds. Credit: M. Sarno, M. Iuliano, Active biocatalyst for biodiesel production from spent coffee ground, Bioresource Technology 266 (2018) 431e438. Copyright: Elsevier, 2018.

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concentration and exposure time are not the only factors that determine the toxicity of a dose. Nanomaterial shape, size, morphology, composition, chemistry, crystallinity, and reactivity are some of the key factors that affect toxicity. Furthermore, the presence of functional groups or adsorbed contaminants on the nanomaterials’ surfaces could induce significantly greater toxicity effect than each pure component alone. All factors that contribute to nanomaterials’ toxicity could be organized in categories based on main nanotoxicity principles [65]: (1) Transport principle is associated with the nanomaterials’ routes of exposure. From an environmental, health, and safety perspective, when compared to the other three routes (ingestion, injection, and skin absorption), nanomaterial inhalation appears to be the most hazardous route of exposure. (2) Morphology principle incorporates all of the factors related to the morphology of a nanomaterial, including its size, shape,

porosity, and specific surface area. These factors are directly related to the reactivity, surface chemistry mechanisms (sorption/ desorption, radical formation, etc.), and other toxicity-related phenomena. (3) Materials principle is related to the toxicity originating from their chemical properties. Chemical structure, crystallinity, composition, and functionalization of nanomaterials are typically the main contributors to nanomaterials’ overall toxicity and environmental, health, and safety concerns. Life cycle analysis can provide a suitable framework for the estimation of the overall nanotechnology impacts, but so far, relatively little work has been done in this area. To determine whether or not nanotechnology is truly sustainable, it is necessary to consider all energy and material inputs over the entire life cycle, as well as any emissions and waste outputs (see Fig. 7.8).

FIGURE 7.8 Life cycle of nanomaterials (simplified) [66].

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9. The case of oil and gas industry

Life cycle analysis is a systematic approach for collecting data on material and energy flows for products and processes from extraction of raw materials, through refinement and manufacturing, use and disposal, or recycling at end of life [66]. In assessing the sustainability of nanotechnology, more efficient use of rare and strategic materials, substitution of hazardous materials, and reduction in the consumption of raw materials and energy are factors to be taken into account. Certain nanomaterials such as carbon nanotubes, while offering energy savings due to lightweight structure, are very energy intensive to produce, and this additional energy input during production needs to be correctly incorporated in the life cycle analysis. Estimation of exposure due to release into the environment represents a key challenge in determining nanomaterials safety. It is therefore necessary to know to what extent existing risk assessment approaches and tools can be applied or will require modification to account for the specific properties of nanomaterials. In the case of nanomaterials, exposure limits, based on doseeresponse relationships, and evaluation of the exposure cannot be properly quantified, and this leads to major uncertainties in carrying out a risk assessment. To carry out an accurate life cycle and risk assessment, there is also still insufficient information regarding the possible release routes for nanomaterials during production, use, and final disposal or recycling. For this reason, it is essential to gather more information regarding the release of nanomaterials during all phases of the product life cycle. A comprehensive survey of the experimental and theoretical knowledge on the release of engineered nanomaterials into the environment has been published [67]. It has been found that the majority of nanomaterials released from products are embedded in a matrix, but a significant fraction is released as individual nanoparticles. To arrive at a correct

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estimation of the exposure, it is, thus, essential to distinguish between inherently dispersive and nondispersive applications [68]. Another major problem in risk assessment of nanomaterials is the lack of accurate information about fate and behavior in the environment and the wide variability of the available data. For this reason, the life cycle assessment has to take into account the most likely and the worst case scenarios, considering the estimated nanoparticle release and the flows in the various environmental compartments. A probabilistic flow model has been developed to calculate the most likely concentrations in the absence of reliable data [69]. Comparing the predicted environmental concentrations (PEC) with the predicted no effect concentrations (PNEC), it is possible to have an estimation of the potential risk.

9. The case of oil and gas industry The increase of global energy consumption and the growing demand of fossil fuels as predominant energy resources have greatly improved the advancement of new technologies in hydrocarbon recovery processes. In the past few decades, rapid advancement of nanotechnology has led to the application of various nano-sized materials and nanoparticle-based devices and tools for oil and gas industry. In addition, the high global demand for energy and the remaining major challenges in the application of current conventional procedures have forced researchers to embark on the search for more economical, efficient, and environmentally sound techniques to extract more hydrocarbons. Recent advancement of nanotechnology enables researchers and engineers to potentially solve industrial problems. In this section, only the green approaches of nanotechnologies in this sector will be treated. In particular the contribution of

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green nanotechnology in this area is related to these steps: -

drilling production and stimulation refining and fuel production

9.1 Drilling Drilling is one of the most crucial processes in creating access to reservoir rock. Several types of nanoparticles have also been reported in drilling fluids formulation recently. The addition of nanosilica was able to significantly improve wellbore stability by reducing water invasion by 10e100 times, especially in the case of shale formation. The use of nontoxic nanoparticles such as nanosilica in drilling fluid may significantly reduce the drilling and disposal cost, so it offers great advantages to the environmental benefits [70].

of nanomaterials for enhanced oil recovery (EOR). In general, nanoparticles, such as SiO2, Al2O3, MgO, and Fe2O3, are usually added to enhance oil displacement efficiency and to improve pushing fluid stability. An application of nanofluids in EOR is in the alteration of wettability of reservoir rock, a property that plays a key role in the reservoir productivity and oil production. Recent studies have suggested that several types of nano-sized materials may also be used as wettability alteration agents, where several nanoparticles such as zirconium oxide (ZrO2), calcium carbonate (CaCO3), titanium dioxide (TiO2), silicon dioxide (SiO2), magnesium oxide (MgO), aluminum oxide (Al2O3), cerium oxide (CrO2), and carbon nanotubes have been compared; the most performing have been CaCO3 and SiO2 [71]. Nanoparticles also affect the surface tension of the fluid, the reduction of which helps the recovery of oil and replacing fluids that are more toxic. Fig. 7.9 shows the mechanism of recovery by nanofluid.

9.2 Production and stimulation In the current oil and gas productions the recovery from unconventional resources such as heavy and extra heavy oil, shale gas, and liquid, tight gas and oil, coal bed methane (CBM), and bitumen hydrocarbons is not very simple. Recently, the development of nanotechnology has enabled one to effectively and efficiently harvest hydrocarbon from unconventional resources. One of the most common applications of nanotechnology in this area is the application

FIGURE 7.9

9.3 Refining and fuel production The advancement in nanotechnology has contributed substantially to the development of more effective and efficient refining and processing steps to convert crude hydrocarbons into useful products. Nanotechnology has allowed researchers to develop catalysts that can improve the efficiency of hydrocarbon conversion and reduce or even eliminate catalyst-

Mechanism of oil recovery by nanofluid [72].

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References

poisoning issue. Research in clean fuels has become a trend to produce more environmentally friendly transportation fuels, which has reduced sulfur and aromatic content. It is found that the presence of NiO or PdO was able to show a better catalytic activity than fume silica support alone. Furthermore, it is also reported that bimetallic (both NiO and PdO) on silica nanoparticle supports showed the highest catalytic activity, confirming the ability of the catalyst to effectively lower the reaction activation energy at considerably low temperature [73].

10. Conclusion and future trends The disrupting role of nanomaterials and nanotechnology in many application fields will become more and more of a reality depending on the compatibility with the principles of global sustainability of the future society. Synthetic strategies based on wider use of natural resources and nonfossil energy sources and those that cause reduced environmental pollution will be the winner choices, if achieved. Moreover, the design and innovation stage of nanomaterials development should drive from the beginning the safe applicability of novel materials. Recovery/recycling/reusing should also be big drivers with respect the development of a circle economy. With this respect, green nanotechnology will have a predominant role in the future.

List of abbreviations and acronyms AOP BRIICS CA CBM CMC CNT DSSC EDLC

Advanced oxidation process Brazil, Russia, India, Indonesia, China, South Africa Calcium alginate Coal bed methane Carboxymethyl cellulose Carbon nanotubes Dye-sensitized solar cells Electrical double-layer capacitors

ELO ENP EOR LED MNP MWCNTs NP NT OD OECD PEC PNEC RGO RoW SiNW SWNT TL UNESCO

Epoxidized linseed oil Engineered nanoparticles Enhanced oil recovery Light-emitting diode Magnetic nanoparticles Multiwalled carbon nanotubes Nanoparticles Nanotechnology Organic dyes Organisation of Economic Co-operation and Development Predicted environmental concentrations Predicted no effect concentrations Graphene oxide Rest of the world Si nanowire Single-walled carbon nanotube Thermomyces Lanuginosus United Nations Educational, Scientific and Cultural Organization

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