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Introduction

1.1 Global Challenges

Water, food, and energy security represent major challenges to the stability and continuity of human populations. However, rapid population growth and steadily improving living standards place enormous pressures on already stressed water resource and agricultural systems. Large amounts of energy are consumed to produce clean water and to treat wastewaters prior to their return to the environment, which inevitably leads to a considerable amount of carbon dioxide (CO2) emissions as well as releasing other environmental pollutants.

At the global scale, about 2600?km3 of water are withdrawn to supply food-driven irrigation needs every year. Viewed another way, agriculture consumes nearly 70% of total human freshwater withdrawals. This number is to increase to more than 83% by 2050 to meet the growing food demand by the rapidly growing population.

In the last 25?years, access to water with potable quality has gone up from 75% to 90% of the world population, and, nevertheless, 884 million people nowadays still lack access to adequate drinking water in many geographical regions [1]. Thus, ensuring a stable and sustainable water, food, and energy supply into the future is a priority for all nations.

Adding to an already dreadful situation, water pollution is becoming a major global challenge [2,3]. From the United Nations World Water Development Report in 2018, it is said that more than 2?billion people lack access to safe drinking water and more than double that number lack access to safe sanitation. With a rapidly growing global population, demand for water is expected to increase by nearly one-third by 2050 [4]. In addition, WHO estimated that 361?000 deaths in children under five years due to diarrhea, representing more than 5% of all deaths in this age group in low- and middle-income countries, could have been prevented through reduction of exposure to inadequate drinking water [5]. Thus, the ability to remove contaminants from these environments to a safe level and do it rapidly, efficiently, and with reasonable costs is important.

With the nonrenewable and pollutant-laden fossil fuels dominating the global energy supply, representing 78% of the world's primary energy, air pollution is worsening in many parts of the world especially where the economy is heavily dominated by low-tech manufacturing. Millions of people die every year from diseases caused by exposure to outdoor air pollution [6]. Ninety two percent of the global population, including billions of children, is exposed to hazardous effects of air pollution at which levels exceed WHO limits. Air pollution causes approximately 600?000 deaths in children under five years annually and increases the risk of respiratory infections, asthma, adverse neonatal conditions, and congenital anomalies.

1.2 Conventional Technologies in Environmental Science and Engineering

In the past century, the development in water and air treatment technologies by environmental engineers has significantly improved the quality of water and air. The research and relevant design of conventional water and air protection systems experienced its golden age in the first half of twentieth century and has gradually reached their steady states. At the same time, with the ever-growing population and ever-increasing life quality expectation, the demand for safe and clean water and air has never dwindled in the course of human existence and is gradually pushing existing technologies to their limits.

Conventional technologies, such as chemical coagulation [7], adsorptions [8,9], chemical treatment (e.g. advanced oxidation process [AOP]) [10-12], membrane-based separation [13-15], and biological treatment [16,17], are based on bulk water chemistry.

Coagulation, which involves adding chemical coagulants into bulk source water, is commonly used in drinking water plants. The particles in source water that cause turbidity (e.g. silt, clay) are generally negatively charged, while coagulant particles are positively charged. In coagulation and subsequent flocculation, the formed particles in the form of flocs are settled out or later removed by filtration. The effectiveness of the coagulation is controlled by bulk water chemistry, such as dose of the coagulants added and pH, among others.

AOP, as a type of chemical treatment, involves accelerated production of highly reactive hydroxyl free radical to degrade the organic pollutants [18-20]. The degradation rates can be affected by several factors from the bulk water chemistry [21]. Adsorption is a process in which pollutants are adsorbed on solid surfaces [9,22]. Adsorption is a proven and much used water purification technique due to its low energy consumption and maintenance cost, as well as its simplicity and reliability. However, its performance relies on the concentration of the to-be-removed substances, the presence of other competing species, temperature, and pH of the bulk water.

Biological treatments rely on bacteria, nematodes, or other small organisms to break down organic wastes using normal cellular processes [23]. Biological wastewater treatment is often a secondary treatment process, used to remove remaining biodegradable organics after primary treatment. These processes can be either anaerobic or aerobic. "Aerobic" refers to the condition where oxygen is present, while "anaerobic" describes a biological process in which oxygen is absent. To obtain an aerobic condition, huge amount of electricity is typically consumed to re-aerate the bulk wastewater, which can be completely oxygen-depleted.

Membrane separation is a technology in which membrane acts as a selective barrier allowing water flowing through while it catches suspended solids and other substances. Membrane separation technology is commonly used for the creation of process water from groundwater, surface water, or wastewater, and it works without the addition of chemicals, with a relatively low energy use and experiencing simple bulk water separation process [24].

Although these conventional technologies are crucial at providing quality water especially at heavily populated areas, conventional water treatment and its infrastructure systems allow little flexibility in response to the changing demand for water quality or quantity, leading to significant energy consumption, water loss, and secondary contamination. For instance, coagulation itself results in the formation of flocs, and thus additional treatment process is required to help the floc to further aggregate and settle. Biological treatment method is at the cost of a long time due to the slow biodegradation process [10]. On the other hand, impurities and pollutants build up on the surface and clog the filtration membranes over extended periods of use, and thus the flux of the wastewater across the filters decreases, leading to higher energy requirements.

From air quality point of view, many prevention measures have been taken in addressing air pollution problems: source control, development of clean energy, filtration technologies, etc. [25] Among them, air filtration technology is of great interest due to low equipment cost and low energy consumption. The conventional fibrous membrane (e.g. glass, polyethylene [PE], polypropylene [PP], polyester, and aramid fibers), as a kind of porous media, has been widely applied in different filtration scenes, including disposable respirators, industrial gas cleaning equipment, cleanroom air purification systems, automotive cabin air filters, and indoor air purifiers [26]. Such fibrous media still suffer from some structural and performance disadvantages, such as large fiber diameter, nonuniform fiber diameter and pore size, relatively low filtration efficiency, high basis weight, and poor high-temperature resistance [27].

While the conventional technologies are being pushed toward their capacity limits, innovations in nanomaterials and more broadly nanotechnology have been fueling advances in environmental science and engineering [28].

1.3 Nanotechnology

1.3.1 History of Nanotechnology Evolution

The term "nanotechnology" can be traced back in 1959 when it was first used by Richard Feynman in his famous lecture entitled "There's Plenty of Room at the Bottom," which is hailed by many as the herald of the era of nano [29]. Starting 1980s, two major breakthroughs sparked the growth of nanotechnology in the modern era. First, in 1981, the invention of the scanning tunneling microscope provided unprecedented visualization of individual atoms and bonds. Second, fullerenes were discovered in 1985 by Harry Kroto, Richard Smalley, and Robert Curl, who together won the 1996 Nobel Prize in Chemistry [30,31]. Initially, C60 was not described as nanotechnology while the term was used regarding subsequent work with related graphene tubes (called carbon nanotubes), which suggested potential applications for nanoscale electronics and devices.

In the beginning of 2000s, there were commercial applications of nanotechnology, although these were limited to the bulk application of nanomaterials and do not involve atomic control of matter, such as using silver nanoparticles as...