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Posted: Feb 22, 2008
Nanotechnology and water treatment
(Nanowerk Spotlight) Only 30% of all freshwater on the planet is not locked up in ice caps or glaciers (not for much longer, though). Of that, some 20% is in areas too remote for humans to access and of the remaining 80% about three-quarters comes at the wrong time and place - in monsoons and floods - and is not always captured for use by people. The remainder is less than 0.08 of 1% of the total water on the planet (Source: World Water Council). Expressed another way, if all the earth's freshwater were stored in a 5-liter container, available fresh water would not quite fill a teaspoon. The problem is that we don't manage this teaspoon very well. Currently, 600 million people face water scarcity. Depending on future rates of population growth, between 2.7 billion and 3.2 billion people may be living in either water-scarce or water-stressed conditions by 2025:
The terms 'stress' and 'scarcity' do not take into account physical access to water sources, or the quality of the water, or the irregularity of availability due to droughts and storms, or seasonal change. Instead, the terms give an indication of the close relation between population dynamics and renewable freshwater availability. (Source: Environment Canada, Freshwater Website 2004)
We have written about this in a previous Spotlight (Water, nanotechnology's promises, and economic reality): Freshwater looks like it will become the oil of the 21st century - scarce, expensive and the reason for armed conflicts. While in our previous article we have only talked about nanotechnology and water in general terms, a new paper gives us the opportunity to look in more detail at the role that nanotechnology could play in resolving issues relating to water shortage and water quality. This review highlights the uses of nanotechnology in areas relevant to water purification, including separation and reactive media for water filtration, as well as nanomaterials and nanoparticles for use in water bioremediation and disinfection.
"The potential impact areas for nanotechnology in water applications are divided into three categories, i.e., treatment and remediation, sensing and detection, and pollution prevention" Prof. Eugene Cloete tells Nanowerk. "Within the category of treatment and remediation, nanotechnology has the potential to contribute to long-term water quality, availability, and viability of water resources, such as through the use of advanced filtration materials that enable greater water reuse, recycling, and desalinization. Within the category of sensing and detection, of particular interest is the development of new and enhanced sensors to detect biological and chemical contaminants at very low concentration levels in the environment, including water."
Cloete is Head of the Microbiology Department at the University of Pretoria in South Africa and Chairperson of the university's School of Biological Sciences. Together with Associate Professor Jacques Theron and J.A. Walker he published the review article, titled "Nanotechnology and Water Treatment: Applications and Emerging Opportunities", in the February 2008 issue of Critical Reviews in Microbiology.
Nanomaterials and water filtration
Membrane processes are considered key components of advanced water purification and desalination technologies and nanomaterials such as carbon nanotubes, nanoparticles, and dendrimers are contributing to the development of more efficient and cost-effective water filtration processes.
There are two types of nanotechnology membranes that could be effective: nanostructured filters, where either carbon nanotubes or nanocapillary arrays provide the basis for nanofiltration; and nanoreactive membranes, where functionalized nanoparticles aid the filtration process.
The researchers also note that advances in macromolecular chemistry such as the synthesis of dendritic polymers have provided opportunities to refine, as well as to develop effective filtration processes for purification of water contaminated by different organic solutes and inorganic anions.
Nanotechnologies for water remediation
Many areas, especially in developing countries, are seriously contaminated or damaged with consequent impoverishment of natural resources and serious effects on human health. Remediation of contaminated water – the process of removing, reducing or neutralizing water contaminants that threaten human health and/or ecosystem productivity and integrity – is a field of technology that has attracted much interest recently.
In general, remediation technologies can be grouped into categories using thermal, physico-chemical or biological methods. The various techniques usually work well when applied to a specific type of water pollution, though no readily available treatments were discovered that could clean all types of pollutants. Due to the complex nature of many polluted waters, it is frequently necessary to apply several techniques to soil from a particular location to reduce the concentrations of pollutants to acceptable levels.
Cloete and his co-authors write that most of the traditional technologies such as solvent extraction, activated carbon adsorption, and common chemical oxidation, whilst effective, very often are costly and time-consuming: "Biological degradation is environmentally friendly and cost-effective; but it is usually time-consuming. Thus, the ability to remove toxic contaminants from these environments to a safe level and doing so rapidly, efficiently, and within reasonable costs is important. Nanotechnology could play an important role in this regard. An active emerging area of research is the development of novel nanomaterials with increased affinity, capacity, and selectivity for heavy metals and other contaminants. The benefits from use of nanomaterials may derive from their enhanced reactivity, surface area and sequestration characteristics. A variety of nanomaterials are in various stages of research and development, each possessing unique functionalities that is potentially applicable to the remediation of industrial effluents, groundwater, surface water and drinking water."
The report provides detailed examples of various nanoparticles and nanomaterials that could be used in water remediation: zeolites, carbon nanotubes, self-assembled monolayer on mesoporous supports (SAMMS), biopolymers, single-enzyme nanoparticles, zero-valent iron nanoparticles, bimetallic iron nanoparticles, and nanoscale semiconductor photocatalysts.
Bioactive nanoparticles for water disinfections
There is a growing threat of water-borne infectious diseases, especially in the developing world. This threat is rapidly being exacerbated by demographic explosion, a global trend towards urbanization without adequate infrastructure to provide safe drinking water, increased water demand by agriculture that draws more and more of the potable water supply, and emerging pollutants and antibiotic-resistant pathogens that contaminate our water resources. No country is immune. Even in OECD countries, the number of outbreaks reported in the last decade demonstrates that transmission of pathogens by drinking water remains a significant problem. It is estimated that water-borne pathogens cause between 10 and 20 million deaths a year worldwide.
According to Cloete, nanotechnology may present a reasonable alternative for development of new chlorine-free biocides. Among the most promising antimicrobial nanomaterials are metallic and metal-oxide nanoparticles, especially silver, and titanium dioxide catalysts for photocatalytic disinfections.
What about toxicity?
As with any other nanotechnology application where there is a possibility that engineered nanoparticles could eventually appear in various environments, the potential human and ecological risk factors associated with this are largely unknown and subject to much debate. Cloete and co-authors discuss various toxicity studies of nanomaterials and also point out several recent studies of the toxicological impact of nanoparticles on different aquatic organisms.
The bottomline seems to be that it might be advisable to come to some definite conclusions regarding nanoparticle ecotoxicology before we embark an large-scale use of engineered nanoparticles in water applications. Nevertheless, there is a growing body of research and development that will lead to nanomaterials playing a key role in future water and wastewater treatment.