Filtration+Separation July/August 2006
Nanotube membranes to reduce desalination costs? Their enormous strength and peculiar electronic properties have led to carbon nanotubes being named as the “saviour” of industries as diverse as clothing and the space industry. Now it appears that carbon nanotubes could revolutionise the desalination segment as well, with the development of a nanotube membrane on a silicon chip the size of a US ‘quarter’. Researchers at the University of California’s Lawrence Livermore National Laboratory say that a nanotube membrane used for desalination may offer a less expensive desalinisation due to the lower pressure required. “Our experiments have been conducted at relatively low pressure drops of just a few psi (just under atmospheric pressure),” postdoctoral researcher Jason Holt said. Low energy may not be the sole cost benefit, Holt argues. “The primary cost benefit, as we see it, lies in the lower energy (viz pressure) needed to achieve the same throughput as an RO membrane; alternatively, one can say that at the same energy cost, we can achieve a much higher throughput per unit surface area,” he says. The nanotubes are made of molecules made of carbon atoms in a unique arrangement. “The uniqueness of the arrangement lies with the ability of the membrane to mediate very high flow through very small holes, while at the same time offering a basic separation of molecules from solution,” Holt says. The tubes are hollow and more than 50,000 times thinner than a human hair, and are therefore suitable to act, in their billions, as the pores in the membrane. The laboratory researchers tested the carbon nanotubes by creating a membrane on a silicon chip with carbon
Artist’s rendering of methane molecules flowing through a carbon nanotube less than two nanometres in diameter.
nanotube pores making up the holes of the membrane. The membrane is created by filling the gaps between aligned carbon nanotubes with a ceramic matrix material.
Benefitting from a slippery surface The pores are so small that only six water molecules could fit across their diameter, the researchers say. However, these water molecules should slide through because of the “slipperiness” of the carbon nanotube surface, or the molecular ordering induced by spatial confinement – while the tiny pore size can block larger molecules. The experiments performed by the LLNL team demonstrated these predicted rapid flows of
gas and water through carbon nanotubes, but further research is needed to determine the exact transport mechanisms. “The gas and water flows that we measured are 100 to 10,000 times faster than what classical models predict,” said Olgica Bakajin, the Livermore scientist who led the research. “This is like having a garden hose that can deliver as much water in the same amount of time as fire hose that is ten times larger.” “Since water does not wet the outside surface of carbon nanotubes, we were skeptical that water would enter into them, let alone flow really fast,” Bakajin added. “But the molecular dynamics simulations in the literature
predicted fast flow, so we wanted to test the predictions.” “The first time we set up an experiment with water, we left it overnight thinking that the water level above the membrane would not budge,” said Hyung Gyu Park, a mechanical engineering graduate student and student employee at Livermore. “Instead, we came back in the morning and there was a little puddle on the floor under the membrane.” “The first thing that came to mind was that the membrane broke, but fortunately it didn’t,” added Holt. “The membrane
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Filtration+Separation July/August 2006
Continued from page 12... allowed water through and blocked gold nanoparticles that were just a bit larger than the nanotube pores.” According to the researchers, using these nanotube membranes, which are more permeable than those used in reverse osmosis, could reduce the energy costs of desalination by up to 75% compared to conventional membranes. Moreover, carbon nanotubes are a useful platform for studying molecular transport and nanofluidics. Their nanometre-size, atomically smooth surfaces and similarity to cellular water transport channels make them exceptionally suited for this purpose.
Better than Reverse Osmosis? Meanwhile, the high energy required by reverse osmosis has
been accused of not being particularly environmentallyfriendly. Is this new technology any better? “The materials used in fabricating these membranes are not inherently toxic (e.g. III-V semiconductor materials), and the nanotubes themselves are know to be stable in a variety of conditions,” Holt says. “Further, the filters have the potential to aid in cleaning polluted water/liquids (pesticides from ground water) and maybe even preventing harmful gases from escaping through industrial exhaust.” Considering the small size of the pores, are there any blockage problems? “We have preliminary data that suggests the membrane could get clogged,” admits Holt. “However, a relatively simply rinsing in deionised water
appears to regenerate the original permeability of the membrane. We have not tried backflushing on the membrane as yet, but see no reason why it couldn’t be done.” What about using the membrane for real-life desalination requirements? Would extra filtration processes, such as cleaning and ultrafiltration, be needed? “It would depend on the specifics of the situation – such as the source water,” Holt says. “In some cases, pre-treatment may be necessary, much the way it is for conventional membranes; however, we believe our filter may have inherent antifouling characteristics that would reduce the amount of pre-treatment necessary.” Gas filtration is not ruled out either. Simulations of gas and
water transport through carbon nanotubes reportedly predict that each should flow rapidly – because gas molecules should bounce off the atomically smooth surface of the nanotubes like billiard balls. Another potential application for the membranes is in gas separation. The high gas permeability and its affinity to hydrocarbons may allow for lower-energy, industrial-gas separations. “Though our membranes have an order of magnitude smaller pore size, the enhanced flow rate per pore and the high pore density makes them superior in both air and water permeability compared to conventional polycarbonate membranes,” Bakajin said. www.llnl.gov
ADC attains new low energy desal goal opportunities for practical operations and maintenance information. It has now achieved a new record for lowest energy use in reverse osmosis (RO) seawater desalination using FILMTEC SW30XLE-400i ‘low energy’ membranes from FilmTec Corporation, a subsidiary of The Dow Chemical Company.
More good news from the Affordable Desalination Collaboration (ADC) – a US, non-profit cooperation that aims to reduce desalination costs (see Desalination, a Filtration+Separation publication – page 12). The goal of the ADC demonstration facility is to demonstrate that SWRO can be a low energy-consuming and costeffective source of fresh water in California, and to provide water agencies and private companies studying the feasibility of seawater desalination plants with
This testing forms part of the organisation’s ADC II demonstration, which has a projected budget of US$2.2 million, and plans to test and demonstrate FILMTEC membranes along with other manufacturers’ products, including those supplied by Hydranautics, Toray, and Koch, along with Zenon’s ultrafiltration technology. ADC I, using exclusively FILMTEC membranes, attained energy levels between 1.5-2.0 kWh/m3 (5.7-7.6 kWh/kgal). In this second in a series of three tests, also using FILMTEC, the ADC Set II achieved a world
The ADC containerised demonstration system at the US Navy’s Seawater Desalination Test Facility in Port Hueneme, California.
record for low energy seawater desalination by RO at 6.00 kWh/kgal (1.58 kWh/m3), with operating conditions of 6 gfd (gallons per square foot per day) and 43% recovery. “SWRO technology has improved tremendously over the past few years, allowing for reduced energy consumption,” said Lance Johnson, manager of global desalination projects, at
the Dow Chemical Company. “As energy prices continue to increase, this will become more significant in lowering overall treatment costs. “Ultimately our goal is to achieve the highest water quality at the lowest possible cost of desalination, and we continue making progress towards this end,” he added. www.affordabledesal.com