Nanomaterials in air filtration

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How nanomaterials are driving the new generation of air purification technologies.

Chemical and biological contaminants present in the air and water sources are a constant concern for human well-being. It is therefore desirable to eliminate these contaminants and at present conventional methods to remove pollutants suffer a number of drawbacks, regardless of their efficiency or convenience.

Ever-expanding cities worldwide are facing major problems with air quality and smog from pollutants such as particulate matter, NOx, soot, ozone, CO and Volatile organic compounds (VOCs). To meet stringent legislative requirements for air quality, new technologies incorporating nanomaterials are being developed to degrade air pollutants. Gaseous pollutants contain numerous irritants, toxic chemicals, and nitrogen oxides, which are ozone precursors, and these can have negative environmental impacts. The minute size and the abundance of these toxins give them a greater opportunity to enter our bodies via air. As a result, the filter industry is looking for new filter media that can create effective barriers for particles smaller than 3 μm and adsorb pollutant gases. Enhanced VOC controls will allow for increased health benefits for the health and productivity of billions of citizens and workers.

Indoor air quality is also becoming a serious concern for populations worldwide as people spend between 80-90% of their time in confined spaces and are exposed to a large range of VOCs from human activites, heating appliances, chemicals released by paints, photocopiers, office furnishings, plastics and building materials. Indoor air contaminants, such as the aforementioned VOCs, microorganisms, allergens, and other pollutants (e.g., tobacco smoke) pose serious health- and productivity-related problems for occupants of indoor spaces.

Currently, glass filters and activated charcoal are widely used in air filtration applications. However, the use of glass fibers can cause environmental and health problems both in mechanical recycling and end-of-life disposal through incineration (thermal recycling). Therefore environmentally friendly composite systems based on nanomaterials are being explored as an alternative to glass fibers. Nanoparticulate catalysts and coatings are displaying great promise for degrading indoor and outdoor ambient air pollutants. Technologies being developed by the industry utilizing nanomaterials include:

• Catalytic traps utilizing nanoscale metal oxides such as aluminium oxides for targeting NOx, soot and PM.

• Photocatalysts utilizing nanoscale TiO2 for targeting VOC, NOx.

• Nanostructured membranes for CO2 removal.

• Static filters utilizing Nanofibers, CNTs and Nanocellulose for targeting particulate matter.

Nanoparticles of, for example, MgO, TiO2, Al2O3 display unique abilities to decontaminate a wide variety of toxic gases such as chemical contaminants, biological contaminants (viruses, bacteria), pesticides, and many more.

Nanofibers

Nanofiber filter media has enabled new levels of filtration performance in several diverse applications with a broad range of environments and contaminants. Polymeric nanofibers have been used in a number of commercial air filtration applications over the last two decades, and hold promise for technical benefits in an expanding field of filtration applications. The ability of nanofiber membranes to selectively permeate moisture and thereby enhance breathability and block chemical vapours make them suitable for application in air filtration applications such as in protective clothing in the textiles industry for protection against chemical and biological warfare contaminants.

There are a number of companies developing nanofiber based air filtration products. eSpin Technologies, Inc. (www.espintechnologies.com) has developed and commercialized custom-made nonwoven membranes, whiskers, and 3-dimensional structures using nanofibers. The company’s nanofibers are 20-200 nm in diameter, have a very high surface area-to-mass ratio, and can be formed into sheet structures with very high porosity and tight pore size. The company are developing  advanced media filters that stop particulates smaller than 3 μm and adsorb VOC gaseous pollutants. Applications they are developing are in airports, aircraft cabins and healthcare facilities.

Donaldson (www.donaldson.com) are developing nanofiber products and nanofiber filter media, Spider-Web filters for gas turbine air filtration, Ultra-Web nanofiber technology for dust collection and Endurance air filters for heavy-duty engines.  DuPont™ HMT nanofiber (www2.dupont.com/Separation_Solutions/en_US/tech_info/hmt/hmt.html) sheets contain continuous polymeric filaments with a typical diameter between 100 nanometers and one micron, for application in air and liquid filters, energy storage devices and other applications.

Esfil Tehno (www.esfiltehno.ee) produces a nanofiber-based filter product for  for dust removal, bacteria-removal, and removal of micro flora from the air in public health and the food industry. Hollingsworth & Vose Company (www.hollingsworth-vose.com) has produced a NANOWEB®, an Advanced Nanofiber Technology with the company’s HVision technology platform, for application in air filtration media, liquid filtration media, and performance barrier nonwoven applications. Toray has developed nanofibers for air and fluid filters. A number of these products are used for industrial air filtration applications, but the companies are also developing their technologies for environmental air filtration.

Nanofibers are also being employed in HEPA filters for clean air applications such as hospitals (and other applications) wherein the contaminated air (bacteria and other pathogens) in a room can be filtered before entering into other rooms due to centralized air conditioning systems. United Air Specialists, Inc. (www.uasinc.com) is developing products in this market.

Nanocellulose

Highly porous nanocellulose materials are of great interest in the manufacturing of filtration media. Nanocellulose fibers are suitable for filter aids, in both primary rough filtration and precision filtration. They are being developed for application in automotive air filters. Nanocellulose has also been incorporated into water purification filters and membranes. The sorbent and filtering applications of nanoporous materials have strong potential for purification and desalinization of water.

Nanosilver

Nanosilver is used as an antibacterial/antifungal agent in applications including air sanitizer sprays, socks, pillows, slippers, face masks, wet wipes, detergent, soap, shampoo, toothpaste, air filters, refrigerator coatings, vacuum cleaners, washing machines, food storage containers and cellular phone coatings. Nanosilver is also used in the treatment of air from air-conditioning systems in meat processing plants. Filters incorporating nanosilver lead to elimination of microorganisms from the air.

Nano TiO2

Photocatalytic nanoscale titanium dioxide can breakdown numerous organic substances such as oil, grime and hydrocarbons from VOCs found in various building materials and furniture, and organic growth such as fungus and mildew.

Photocatalytic coatings are also being used to control Sick Building Syndrome by reducing the amount of VOCs and other toxic chemicals people are exposed to in hotels, restaurants, commercial business facilities, university laboratories, hospitals, and private residences. Photocatalytic coatings ae attractive for the green building concept as no energy is consumed, it is longer lasting, no maintenance is needed, and they are environmental friendly as the solution is a VOC-free, water-based solution.  Mostly, the technology is used in existing interior and exterior buildings or new construction building surfaces. The coating materials have also been integrated into paint, wallpaper, tiles, glass, siding, concrete, brick, roofing material etc. at the factory level for producing a product that has sterilizing, air purification, deodorization and self-cleaning properties and benefits.

Photocatalytic oxidation also reduces hydrocarbon waste produced from industrial factories burning fossil fuel or coal. A number of Asia-based companies, such as Green Earth Nano Science Inc. (www.gensnano.com) produce nanoscale TiO2 coatings in a liquid solution which breaks down organic pollutants for internal and external application.  Their surface coatings provide air and surface eliminate biocontamination caused by Listeria, Salmonella, E.coli, Swine Flu (H1N1), Bird Flu (H5N1), SARS and mould spores.

Nanostructured membranes

Approximately one third of the total global carbon dioxide (CO2) emissions are from energy production. In an effort to meet climate change objectives, the main way sought to reduce CO2 emissions from fossil-fired power plants is through CO2 capture. However, existing capture methods, such as adsorption and non-selective cooling, are expensive, require the use of chemicals and consume a great deal of energy – up to 25% of power plant output. CO2 emission reduction by gas separation using nanostructured membranes are being developed to meet this challenge. General Electric (www.ge.com) is developing nanostructured membranes which, with the correct adsorption and surface transport properties, could be used for precombustion CO2 capture. Precombustion capture is required in so-called ‘clean coal’ systems, in which coal is gasified and then has pollutants removed before it is used as a fuel.

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