Nanomaterials are a leading candidate for clean water tech
Nanomaterials have the clear potential to greatly benefit environmental quality and sustainability, through pollution prevention, water treatment and remediation. These benefits include improved detection and sensing of contaminants and pollution and their safe removal and filtration from air, water and soil, at a greatly reduced cost in comparison to existing technologies.
Nanomaterials are used in water treatment because of their unique physical, chemical and biological properties. The control and design of nanomaterials allows for increased affinity, capacity, and selectivity for pollutants. This is a result of the enhanced reactivity, surface area, sub-surface transport, and/or sequestration characteristics of nanomaterials.
Types of nanomaterials that are under development for water treatment and remediation include smart modified surfaces or membranes, reactive nanoparticles, molecularly imprinted polymers (MIPs), catalytic additives and nanoscale biopolymers.
Running on empty
A number of factors have increased the global demand for water such as ever growing populations, urbanisation, and economic growth. Water consumption is intrinsically linked to population growth: approximately 4000 km2 of water were consumed worldwide in 2000, and this is expected to rise to over 5000 km2 by 2025. Global consumption of water is doubling every 20 years, more than twice the rate of human population growth. According to the United Nations, more than one billion people globally do not have access to fresh drinking water.
With up to 60% of the world’s population expected to live in cities by 2030 (presently around 50%), there is pressure on water systems not only to deliver fresh water, but also to remove and process substantial quantities of wastewater safely. Water supplies are also becoming increasingly contaminated, driving the need for new technologies which are capable of large-scale water purification. Increasing energy production in developing countries is another driver of water usage.
To increase access to water, new supplies have to be enabled. One of the ways of doing this is to recycle a greater proportion of wastewater. New technologies are needed to provide more cost-effective means of large scale wastewater remediation and new approaches are continually sought for the removal of contaminants from water, either in-situ or in water purification systems. In the context of this critical challenge, nanomaterials are being investigated and engineered to treat contaminated groundwater through filtration, adsorption, chemical and catalytic methods and will play a huge role in the future of this technology area.
Diese mechanisch-stabilisierte nanoporöse Filtermembran weist eine regelmäßige Porenstruktur
auf. Zugleich ist die Porengrößenverteilung sehr eng und gleichmäßig.
Figure 1: Mechanically stabilized nanoporous filter membrane that exhibits a regular pore structure. (Courtesy: Fraunhofer Institute for Mechanics of Materials).
Filtration media
Nanomaterials are being developed as filtration materials with enhanced performance that arises from their high specific surface area and the abundance of surface-state electrons found in films and parts containing nanomaterials. Filtration membranes under development include nanostructured filters, with carbon nanotubes or nanocapillary arrays providing the basis for nanofiltration; and nanoporous membranes, where functionalized nanoparticles aid the filtration process for removal of arsenic, bacteria, organic material, nitrates, salinity and viruses from water.
Companies producing filtration media include Nano Ceram (www.nanoceram.com) and Argonide (www.argonide.com).
Figure 2: Nano Ceram water filtration devices (Courtesy: Nano Ceram)
Water remediation
Conventional technologies are often inadequate to reduce concentrations in groundwater to acceptable regulatory standards. Nanomaterials offer the potential for inexpensive, efficient and more cost-effective remediation via in-situ new collection and separation techniques, including:
• Dechlorination of chlorinated hydrocarbons via Nanoscale Zero Valent Iron (nZVI) and Reactive Nanoscale Iron; the surface area is 30 times larger than the regular ZVI and 1 to 1000 times higher reactivity has been reported. The high surface area and surface reactivity compared with granular forms enable the nanoparticles to remediate more material at a higher rate and with a lower generation of hazardous by-products.
• NanoTiO2 for photocatalytic degradation of chlorinated compounds;
• Dendrimers can serve as chelating agents for ultrafiltration of a variety of metal. They are impregnated in porous ceramic filters, resulting in hybrid organic/inorganic filter modules with enhanced mechanical strength and high surface area
• Silica-titania nanocomposites for elemental mercury removal from vapours such as those coming from combustion sources;
• Nanoporous ceramic materials to remove mercury or radionuclides from wastewater.
Nanoscale iron particles and their derivatives are the main area of investigation. The particle size of nanoscale iron allows for a greatly increased level of remedial versatility. The ability of the nanoparticles to act as strong reducers also enables the remediation of an extremely wide range of contaminants. PolyMetallix (www.polymetallix) manufactures nanoscale iron for in situ environmental remediation. Other producers of nanomaterials for remediation purposes include:
• LeHigh University (www.lehigh.edu/nano/environmental.html): Fe/B nanomaterials
• OnMaterials LLC (www.onmaterials.com): ZLoy
• PARS Environmental Inc. (www.parsenviro.com): NanoFe™ and NanoFe Plus™
• Toda Kogyo Corporation (www.toda.co.jp): RNIP
• VeruTEK Technologies, Inc. (www.verutek.com)
• Pacific Northwest National Laboratory (http://samms.pnl.gov): SAMMS™
Arsenic removal
Arsenic pollution in water is a major global problem, particularly in countries such as Bangladesh, where it is estimated that between 46 and 57 million people are exposed to arsenic levels above World Health Organization (WHO) guidelines of 0.01mg/l. South East Asia is a notable problem area where millions suffer from acute and chronic arsenic poisoning. This is also a problem in advanced industrialized nations. For example, there are numerous communities in the United States with acceptable levels of arsenic below the Safe Drinking Water guidelines of 50 to 10 μg/L.
There are no existing cost effective methods that effectively remove arsenic from groundwater and so there is a real need to develop new technologies. Nanomaterials demonstrate potential for cost-effective filtration media, either used with existing filter beds or in separate filter beds, to reduce arsenic to acceptable levels.
Nano-magnetite (Fe3O4, 12 nm) in combination with nano iron oxide has been proven effective at removal of arsenic (and lead) from water. This technology is now being used with the porous ceramic media to remove arsenic and lead from water. It has been demonstrated that fine sizes of nano-magnetite can remove 200-fold more arsenic than existing commercial sorbents. Researchers have shown that 15 g of nano-magnetite is equal to 1.4 kg of bulk iron oxide in terms of removing the same amount of arsenic (500 lg/l) from 50 l of water.
Phosphate removal
Phosphorus is an important element for agricultural and industrial development, and large quantities of phosphates are often present in domestic wastewater, groundwater, and industrial wastewaters. Phosphate solutions frequently include undesirable organic compounds. There is great demand for technology for phosphate recovery from various wastewaters and nanoscale zirconia and nano enhanced reactive media show potential as adsorbents. Meta Materia (www.metamateria.com) is developing nanomaterials for this purpose.
Disinfection
Nanoscale materials such as nanoparticulate oxides, titanium dioxide, thin coatings and mesoporous fibres can provide rapid killing of gram-negative bacteria, grampositive bacteria, and spores via photocatalytic oxidation. Photocatalytic processes involving nano-TiO2 for degradation of organic pollutants in water and air is the most investigated area. KACO new energy GmbH (www.kaco-newenergy.de/raywox) has developed Nanocrystalline photocatalytic technology for water purification under the RayWox brand name.
