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Can nanosponges solve a continent’s water contamination problems? Munyaradzi Makoni investigates.
Some have likened them to honeycombs; others to unending teacups, each only a billionth of a metre wide. But when it came to naming them they just had to be called nanosponges.
The idea is that one could clamp them across a water source — whether it is a kitchen tap or the pipe taking water into a power station — and they would soak up the fluid, catching impurities in their myriad tiny cavities and letting water through only in its purest form.
South Africa is certainly hoping nanosponges will solve water purification problems where conventional treatments are not sufficient — from bringing clean water to all to decontaminating the coolant water that would otherwise rot a power plant’s turbines.
But whether nanosponges will live up to their promise, both technically and economically, is unproven. They are expensive to produce and still imperfect.
Nanosponges were invented at the Los Alamos National Laboratory in the United States by DeQuan Li and Min Ma over a decade ago. Within a couple of years Bheki Mamba and Rui Krause, of the University of Johannesburg in South Africa, began researching the sponges. They could see the potential for their own continent.
Krause, an organic chemist in the department of chemical technology, knows just how stubborn water pollutants are. Many are so minute that even detecting them is problematic.
South African water pollution mainly comes from mining which leads to acidic drainage that mixes with the ground water, says Nndanganeni Musekene, senior resource protection manager in the Ministry of Water Affairs and Forestry in Johannesburg.
Acid drainage from mines, such as this one in the United States, is a major source of water pollution
Agricultural runoff from phosphorous fertilisers and sewage adds to the problem.
Leonardo Manus, drinking water quality regulator in the ministry, points out that some 88 per cent of the country’s 48 million people do have access to clean water. The remainder — more than one in ten — tend to be the marginalised rural poor. And access to clean water will probably worsen with industrial growth and rising population. Contaminated water and food led to a cholera outbreak in Zimbabwe that spread to South Africa last October.
Better than filters
In their laboratory, Mamba and Krause oversee some 30 postgraduates from India, Swaziland and Zimbabwe. Their research is a partnership with two nanotechnology innovation centres launched in 2008 — one at Mintek, the country’s national mineral research organisation, based in Johannesburg, and the other at the Council for Scientific and Industrial Research in Pretoria.
The research is part of a broader nanotechnology programme that includes working with Rhodes University in Grahamstown on nanosensors and with the University of the Western Cape on bio-labelling, says Robert Tshikhudo, head of nanotechnology at Mintek.
To make a nanosponge, you begin with ordinary starch that, when fed to a particular natural enzyme, is broken down into rings of sugar molecules known as cyclodextrins. There are six to ten sugar molecules in a ring, or cup, and each one is just 1–2 nanometres (0.000000001 metres) in diameter.
You then connect each cyclodextrin to a molecule that links to the next cyclodextrin — and so on, forming a polymer, or long chain.
"These polymers have nano-cavities that can trap pollutants. They act as adsorbents, or sponges, for pollutants — so the nanosponges term just stuck," says Krause.
At the heart of the nanosponge magic is their ability to respond to a molecule’s charge, rather than just act as ordinary size-based filters.
Bheki Mamba: A leading nanosponge scientist
University of Johannesburg’s Audio and Visual Unit
The environment inside each cup repulses water while, outside the cups, the nanosponge attracts water. So water slips through the sponge easily, while pesticides and a host of other contaminants become trapped in the cups. Nanosponges are "smart filters", says Krause.
There’s more. The sponges can be ‘tagged’ with other substances that can target particular pollutants and even modify them into less toxic forms.
For example, a sponge with added iron nanoparticles will trap a toxic chlorinated pollutant like chloroform, but will also transform it into something less poisonous, like methane. Iron is already employed in the same way to clean up industrial spills, but this reaction is much faster because nano-iron has a much greater surface area.
"We do not hope to develop a one-solution fits all, but rather a set of polymer ‘building blocks’, that can be deployed in specific arrangements to suit the needs of the industry," says Krause.
The nanosponge’s final trick is that once saturated with the nasty stuff it can be washed with a safe solvent like ethanol and reused.
Some of the early research, published by the Los Alamos scientists, indicated that nanosponges might be able to reduce hazardous organic contaminants in water to "parts per trillion".
A study published by Mamba, Krause and colleagues last year (October 2008) shows this dream hasn’t yet materialised. They demonstrated that nanosponges could remove 84 per cent of dissolved organic material from water used as a power station coolant. This was about the same rate of success as the technologies already in use — resins and activated carbon.
But Krause remains optimistic. "Given that this is a new material, such an initial result in an industrial setting was very promising," he says.
The researchers are confident that nanosponges will eventually far outperform conventional technologies. The most common adsorbent water purifier used at the moment is activated carbon, made by heating organic material, such as bone or wood, until all that is left is carbon riddled with tiny holes that trap pollutants.
It is a common and cheap approach used everywhere from table-top water filter jugs to power stations.
"Existing technologies rely heavily on adsorption," says Krause. "So to make a big impact I believe we need to improve the wheel not reinvent it. If we can make a super adsorbent then we do not have to radically redesign all existing supporting technologies."
"It means there is no need to invent radically different [individual] solutions for power industries, rural drinking water and for urban drinking water treatment."
Power stations need plenty of clean coolant water
"We are not saying the polymers are perfect," says Mamba. "But they are better than activated carbon or other adsorbents. Nanosponges could take over where conventional treatment methods fail or could be used to augment these existing treatments."
Costs and risks
A major obstacle to the nanosponge programme is expense. The only real cost for activated carbon comes from heating the waste materials to produce it. But nanosponges are made using two monomers, both of which have to be manufactured (a multi-stage process) and imported.
But this will change.
"We have chosen urethane for the linking monomer as there is a very big polyurethane industry in South Africa, so this makes the linking monomer much cheaper," says Krause. The cyclodextrins could also be manufactured cheaply in South Africa.
Another potential problem comes from the nanosponges’ super-efficiency. While removing unwanted organic matter they might remove minerals that are useful to human health, acknowledges Krause. "In a rural community it may be important to remove biological pollution, such as [the bacteria] E.coli, while leaving some of the beneficial trace minerals if we can."
"Whether or not this is a real problem is open to debate, since we do not get a significant amount of minerals from the water we drink, and the polymers can be modified to remove things of specific polarity and size," he says.
Then there are the general fears about the potential health and environmental consequences of nanomaterials.
"Without knowing the properties of nanosponges we are being cautious," says Jo Burgess, manager of South Africa’s Water Research Commission, which funds nanotechnology for drinking water, wastewater and mine water treatment."
"Nanosponges may be wonder materials with only beneficial effects, or they may be another asbestos — an apparent wonder material that turned out to cause health problems. Or more likely they will be somewhere in between.
"More research needs to be done to know the efficacy of nanosponges, their fate and effects on environmental and public health before we can scale them up with confidence."
So pilot projects are the researchers’ next goal. "We are trying to set up a pool of engineers and other scientists so that we can work towards implementing our pilot project in two or three years," says Mamba.
Meanwhile, Mintek is designing rigs to test nanosponges’ ability to remove industrial pollutants in water. Tshikhudo hopes one will be ready by the end of this year. Krause and Mamba are also working with researchers in Brazil, India and the Netherlands to speed up the pilot tests.