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Nanomaterials and the Environment

Shivani Rastogi, Gaurav Sharma and Balasubramanian Kandasubramanian*

Nano Surface Texturing Lab, Department of Metallurgical and Materials Engineering, DIAT (DU), Ministry of Defence, Girinagar, Pune, India

Abstract

Nanomaterials (NM) possess unique mechanical, electrical, catalytic, anti-microbial and optical properties owing to their dimensional constraints and have been utilized since time immemorial. With advancement in technology, environmental opportunities like augmenting resource efficiency and controlling pollution and purification of natural resources like water and air have been pivotal in the development of NMs. Metallic and non-metallic NMs along with composite variations have been devised for ecological betterment, and their efficacy in the eradication of a plethora of air and water contaminants of organic as well as inorganic origins is inevitable. The NM engineered nano-dimension materials designed specifically for eco-conservational purposes comprise materials in membranous forms, embedded structures with NMs, etc., or composites that have adsorptive, oxidative or catalytic properties for degradation of pollutants which are deemed toxic when present in water bodies, soil contaminants and air foulers. The NMs generated in controlled environments are effectively utilized for several pollutant sequestering techniques but by-products of industrial usage and automobile expulsions are considered detrimental, posing a threat to the 55 billion Nanomaterial industry (estimated market share by 2022 by CAGR). Therefore, it is imperative to maintain an ecological balance and enhance the positive effects of nano-scalar materials on the environment.

Keywords: Eco-friendly, nanocomposites, purification, photocatalysis, oxidation, adsorption, nanoparticles

1.1 Introduction

1.1.1 Overview of Nanomaterials

Nanomaterials are the chemical substances having at least one dimension in the range of nanometers (10-9m). According to ISO/TS 8004 [1], nanomaterial is defined as a "material with any of its proportion in the nanoscale range or having internal surface area in the nanoscale," with nanoscale defined as the "length range approximately from 1 nm to 100 nm" [1]. Nanomaterials are not new to this world as there are many instances from medieval history in which nanomaterials were used during that period, i.e., Sabres from Damascus [2], now in Syria, before 900 AD, that were hard and sharp, made from a wootz type of steel. One more example of nanomaterials from history is their use by medieval stained-glass artisans. They were the first nanotechnologists and confined the nanoparticles inside a glass matrix to produce the ruby red color in the windows. Nanomaterials are designed to exhibit new and unique characteristics in comparison to the same material without nanoscale characteristics, such as enhanced strength, chemical reactivity or conductivity, etc. There are many naturally occurring nanomaterials (e.g., volcanic lava ash, ocean spray, or even biological matter like capsid, virus, etc.) and some are produced accidentally as the side products of various combustion processes (e.g., automobile engines, gas and carbon welding). Recently, researchers have been exploring various nanomaterial functionalized materials like nanofibers, beads, membranes, foams, etc., typically based on polymeric materials for environmental and water treatment applications [3-43]. On the other side, there is an extended range of nanoparticles that are engineered by physical and chemical processes for specific or targeted work and research, i.e., carbon black and titanium dioxide nanoparticles. Nanomaterials can be generally classified into two categories, as shown in Figure 1.1:

Natural Nanoparticles: This type of nanomaterial produced in nature is due to the bio-processes, as illustrated in Figure 1.2. Due to biological decaying and chemical weathering processes, a large number of environmental colloids can occur in the environment. These colloidal nanoparticles are basic oxides and oxyhydroxides of iron (Fe), manganese (Mn), aluminium and aluminosilicates. Some nanoparticles are produced accidentally in nature due to various human activities like burning of fire, running a motor car, washing clothes, rubbing papers, etc., but the use of these nanoparticles is not significant due to their uncontrolled generation.

Synthetic Nanoparticles: These types of nanoparticles, which are designed for a specific purpose with research by physical and chemical processes and by using different experimental techniques, are also known as man-made nanomaterials. These nanomaterials are being produced significantly by various physical and chemical methods in R&D laboratories for commercialization and at the educational level for learning purposes. The chemistry of synthetic nanomaterials can be controlled by various means and are being used in multiple industries to make technology more advanced, less harmful and available to all for a good lifestyle and health.

Figure 1.1 Types of nanomaterials [44].

Figure 1.2 Natural processes of nanomaterials formation [45].

Synthetic nanomaterials can be broadly classified into various categories, as shown in Table 1.1 with their types, synthesis, and examples.

Table 1.1 Types, synthesis, and examples of synthetic nanomaterials [46].

1.1.2 Overview of Environmental Health

The World Health Organization (WHO) states that environmental health denotes all the physical, chemical, as well as biological factors superficial to a person, and all the related components affecting behaviors. It circumscribes the study and ordered control of those environmental elements that are possibly harmful to health. It is focused on safeguarding against disease and creating health-supportive environments [47]. This definition excludes behavior not associated with the environment, as well as the action linked with social and cultural background and genetics. In simpler terms, environmental health concerns maintaining clean air and water, and keeping your food and the land around you safe, clean, and enjoyable, as illustrated in Figure 1.3.

The increasing effect of greenhouse gases has led to a severe issue of global warming due to which the environment is facing perils like seasonal shift, melting of glaciers, and a rise in overall air and ocean temperature. Due to global warming the lives of plants and animals are being highly impacted. To prevent the increasing effect of global warming, some human actions like burning of fossil fuels, deforestation, excessive vehicular uses, use of plastics, etc., need to be controlled.

Greenhouse gases confine heat and make the earth warmer. Various human activities are also responsible for this situation occurring. In the world, the United States is a monstrous source of greenhouse gas emission due to burning fossil fuels for heat, electricity, and transportation. In 2017 the levels of CO2 concentration in the US decreased as compared to 2016 due to the transfer of coal energy to natural gases, renewable sources of energy, etc. (see Figures 1.4 and 1.5).

1.1.2.1 Use of NMs in Environmental Health (Nanoremediation)

Nanoremediation is the application of nanomaterials for environmental remediation. The nanomaterials are used to treat groundwater, polluted air, sediments, soil, and other contaminated materials. For example, titanium dioxide (TiO2) is a good candidate for wastewater treatment. When TiO2 is exposed to sunlight, it produces hydroxyl radicals which are supremely reactive and can oxidize the environmental contaminants. Nanomaterials are widely used in various environmental applications (Figure 1.6) with a good success rate in cleaning up oil spills, water disinfectant, air pollution control, and much more. Thus, nanomaterials have given rise to a new technology for the purpose of creating a healthy environment, that is, green nanotechnology [51]. It is defined as the science and technology used to introduce nanoparticles into the environment with safe and pre-measured harmful effects. Its consideration for making nanoparticles is to create them without the use of toxic elements, at a narrow...