Nanotechnology for clean water: Facts and figures

Nanotechnology could help give millions clean drinking water. David J. Grimshaw outlines the potential, the progress and some of the risks.

Technology has long been important in providing clean drinking water and irrigation for food crops. Indeed, people have had water technology for thousands of years — the Romans were using aqueducts as conduits for drinking wateraround 300BC. But making modern technology accessible and affordable to the global poor is a daunting task. Can nanotechnology perform better than previous technologies?

Water is a scarce resource, and for many countries — particularly those in the Middle East — supplies already fall short of demand. With the pressures of climate change and population growth, water will become even scarcer, especially in developing regions. Moreover, in these regions, what water is available is often unsafe to drink (see Table 1).

 

The quest to ensure that all people have access to clean drinking water is now enshrined in the UN's Millennium Development Goals, which aim to halve the proportion of people without sustainable access to safe drinking water by 2015. According to the World Water Assessment Programme, that will mean improving water supplies for 1.5 billion people. [2]

But how to achieve this? Economics or technology have often driven approaches to providing water for poor communities. The economics route might typically centre on the importance of regulations, institutions and open markets. The technology approach might focus on designing a water pump, filter systems or novel applications, for example, of nanotechnology.

Nanotechnology's potential

Unlike other technologies, which have often sprung directly from a particular scientific discipline, nanotechnology spans a wide spectrum of science. Essentially, it is defined by the scale it operates at. Nanoscience and nanotechnology involve studying and working with matter on an ultrasmall scale. One nanometre is one-millionth of a millimetre and a single human hair is around 80,000 nanometres in width. [3] This kind of scale is difficult for us to visualise but if the distance between the Sun and the Earth were one metre then a nanometre would be the size of a football pitch.

The nanoscale deals with the smallest parts of matter that we can manipulate. Operating at the nanoscale makes assembling atoms and molecules to exact specifications easier. Rather like building a model from Lego bricks, we might envisage creating new materials or modifying existing ones. In applications like water filtration this means materials can be tailored, or tuned, to filter out heavy metals and biological toxins.

Materials at the nanoscale often have different optical or electrical properties from the same material at the micro or macroscale. For example, nano titanium oxide is a more effective catalyst than microscale titanium oxide. And it can be used in water treatment to degrade organic pollutants. But in other cases, manufactured nanoparticles' small size may make the material more toxic than normal.

The principal way nanotechnologies might help alleviate water problems is by solving the technical challenges that removing water contaminants including bacteria, viruses, arsenic, mercury, pesticides and salt pose.

Many researchers and engineers claim that nanotechnologies offer more affordable, effective, efficient and durable ways of achieving this — specifically because using nanoparticles for water treatment will allow manufacturing that is less polluting than traditional methods and requires less labour, capital, land and energy. [4]

New technologies in the past have made similar claims. Yet if we could develop new business models that let us use nanotechnologies sustainably to solve real problems, identified in participation with local communities, we might have cause for optimism. [5]

The story so far

A range of water treatment devices that incorporate nanotechnology are already on the market, with others either close to market launch or in the process of being developed.

Nanofiltration membranes are already widely applied to remove dissolved salts and micro-pollutants, soften water and treat wastewater. The membranes act as a physical barrier, capturing particles and microorganisms bigger than their pores, and selectively rejecting substances. Nanotechnology is expected to further improve membrane technology and also drive down the prohibitively high costs of desalination — getting fresh water from salty water.

Researchers are developing new classes of nanoporous materials that are more effective than conventional filters. For example, a study in South Africa has shown than nanofiltration membranes can produce safe drinking water from brackish groundwater. [6] And a team of Indian and US scientists have developed carbon nanotube filters that remove bacteria and viruses more effectively than conventional membrane filters. [7]

Naturally occurring attapulgite clays and zeolites are also used in nanofilters. These are locally available in many places around the world and have innate nanometer-size pores. A study using attapulgite clay membranes to filter wastewater from a milk factory in Algeria has shown they can economically and effectively reduce whey and other organic matter in wastewater, making it safe to drink. [8]

Zeolites can also be fabricated. They can be used to separate harmful organics from water and to remove heavy metal ions. Researchers at Australia's Commonwealth Scientific and Research Organization have created a low-cost synthetic clay, hydrotalcite, that attracts arsenic, removing it from water. [9] They have suggested a novel packaging for this product for low-income communities — a 'teabag' that can be dipped into household water supplies for about 15 minutes before drinking. And selling the used teabags back to the authorities might increase recycling and help with waste disposal of concentrated arsenic.

Nano catalysts, magnets and detectors

Nanocatalysts and magnetic nanoparticles are other examples of how nanotechnology could make heavily polluted water fit for drinking, sanitation and irrigation. Nanocatalysts owe their better catalytic properties to their nanosize or to being modified at the nanoscale. They can chemically degrade pollutants instead of simply moving them somewhere else, including pollutants for which existing technologies are inefficient or prohibitively expensive. Researchers at the Indian Institute of Science, in Bangalore, have used nano titanium dioxide for this very purpose (see 'Nanoscale water treatment needs innovative engineering').

Magnetic nanoparticles have large surface areas relative to their volume and can easily bind with chemicals. In water treatment applications, they can be used to bind with contaminants — such as arsenic or oil — and then be removed using a magnet. Several companies are commercialising such technologies and researchers are frequently publishing new discoveries in this area.

For example, scientists at Rice University in the United States are using magnetic "nanorust" to remove arsenic from drinking water. [10] Nanorust's large surface area means it can capture one hundred times more arsenic than larger counterparts. The team projects that 200–500 milligrams of nanorust could treat a litre of water. And it is developing a way of creating nanorust from inexpensive household items. This could significantly reduce production costs, making it a viable product for communities throughout the developing world.

As well as treating water, nanotechnology can also detect water-borne contaminants. Researchers are developing new sensor technologies that combine micro and nanofabrication to create small, portable and highly accurate sensors that can detect single cells of chemical and biochemical substances in water. [11] Several research consortia are field testing such devices and some expect to commercialise these soon. For example, a team at Pennsylvania State University in the United States has developed a way of detecting arsenic in water by using nanowires on a silicon chip. [12]

Nano research in the developing world

Research spending on nanotechnology in developed regions like Europe and the United States are very high as governments continue to prioritise technologies they think will underpin economic growth. And some intermediate countries, like China, are also investing heavily (see Figure 1).

South Africa has developed important capabilities in nanotechnology through its National Nanotechnology Strategy, launched in 2006. [15] It has, for example, set up innovation centres for nanoscience in two of the country's science councils. One of these includes a focus on nanoscience for water. The thrust of research here has very much been on solving local problems. The University of Stellenbosch, for example, is researching nanomembranes for water filtration.

India too has invested heavily in nanotechnology — although figures are difficult to verify, partly because investment is often a partnership between government and the private sector.

And other developing countries are increasingly seeing a need to support nanoscience, including research into how nanotechnology can help deliver clean water. Brazil, Cuba, Saudi Arabia and Sri Lanka all host nanoscience centres working on this issue. And the number of patents on nano-based inventions filed by developing country researchers is increasing rapidly.

Developments for the developing world

Some interesting products are now emerging from developing countries, and other products are being developed elsewhere that are highly relevant to the needs of the South (see Table 2).

 

Risks and opportunities

Any assessment of future markets for nanotechnology-based water treatments must take account of both the risks and opportunities.

Some researchers claim that investigations into the ethical, legal and social implications of nanotechnology are lagging behind the science. [20] They quote the low number of citations on such topics in the literature and the fact that, in the United States at least, not all available research funds are being used. For example, the US National Nanotechnology Initiative allocated US$16–28 million to research on nanotechnology's broader social implications — but spent less than half that amount.

And the generally lower scientific capacity in developing countries means it is likely that effective regulation of the ethics and risks of nanotechnologies will lag behind the developed world. Yet there are signs that the ethics of using nanotechnology for clean water are being discussed.

Some researchers have called for more research on the potential health and environmental risks of using nanotechnology for water treatment. [6] For example, there are concerns that the enhanced reactivity of nanoparticles makes them more toxic. Their small size also means they could be hard to contain, so could more easily escape into the environment and potentially damage aquatic life. The full effects of exposure to nanomaterials — from handling them at water treatment plants or drinking them in treated water — are as yet unknown.

But we can make a distinction, in terms of risk assessment, between active and passive nanoparticles. Passive particles, such as a coating, are likely to present no more or less a risk than other manufacturing processes. [21] But active nanoparticles that can move around the environment lead to risks associated with control and containment.

So can nanotechnologies really help solve water problems in developing countries? There are two positive signs that they will. First, water professionals and scientists are increasingly including local communities in dialogues to understand the problems with, and opportunities for, applying nanotechnology to water improvements.

Second, since the commercialisation of nanotechnology is at an early stage, we can hope that such discussions — between researchers, communities and industry — will encourage scientists and businesses to develop appropriate business models to exploit their inventions.

David J. Grimshaw is head of Practical Action's international programme in new technologies and new technologies consultant for SciDev.Net.