Nanotechnology for Water Filter

These are carbon nanotubes, seen by scanning electron microscopy.

Nanotechnology has developed tremendously in the past decade and was able to create many new materials with a vast range of potential applications. Carbon nanotubes are an example of these new materials and consist of cylindrical molecules of carbon with diameters of a few nanometers -- one nanometer is one millionth of a millimeter. Carbon nanotubes possess exceptional electronic, mechanical and chemical properties, for example they can be used to clean polluted water.

Scientists of the University of Vienna had recently published to this new research field in the journal Environmental Science & Technology.

Among many potential applications, carbon nanotubes are great candidate materials for cleaning polluted water. Many water pollutants have very high affinity for carbon nanotubes and pollutants could be removed from contaminated water by filters made of this nanomaterial, for example water soluble drugs which can hardly be separated from water by activated carbon. Problems due to filters' saturation could be reduced as carbon nanotubes have a very large surface area (e.g. 500 m2 per gram of nanotube) and consequently a very high capacity to retain pollutants. "Maintenance and wastes related to water depollution could thus be reduced," says Thilo Hofmann, Vice Dean of the Faculty of Earth Sciences, Geography and Astronomy of the University of Vienna.
Assessing carbon nanotubes' environmental sustainability
A lot of research has focussed on carbon nanotubes in the past decade. However, the exceptional properties of carbon nanotubes make them difficult to study. Standard methods give limited results and the behaviour of carbon nanotubes in realistic conditions is still poorly understood. "Innovative technologies always come with benefits and drawbacks for human and environmental quality and a good understanding of the interactions between contaminants and carbon nanotubes as well as how carbon nanotubes behave in the environment is essential before they can be used in filters," explains Mélanie Kah, who does research on this project together with Xiaoran Zhang.
A team of researchers at the Department of Environmental Geosciences at the University of Vienna is currently carrying out research on the subject. They developed a method called "passive sampling." Data produced by this new method are much more reliable for realistic applications as they include concentrations likely to occur in the environment (generally very low). This was not possible with classical methods that can only deal with elevated concentrations.
The experiments published now in the internationally recognised journal "Environmental Science & Technology" took more than a year. First, the "passive sampling method" was developed which allows measuring the affinity of a category of carcinogenic contaminants -- i.e. Polycyclic Aromatic Hydrocarbons (PAHs) -- to carbon nanotubes. "Series of tests which use analytical chemistry and electron microscopy were performed with collaborators from the University of Utrecht in the Netherlands, to ensure that the method is suitable, reliable and optimised for carbon nanotubes," illustrates Thilo Hofmann. Once validated, the "passive sampling method" was used to measure the affinity (absorption and adsorption) of several contaminants (PAHs) to carbon nanotubes over a very wide range of concentrations.
Contaminants fight for a place on carbon nanotubes
Another aspect investigated by the scientists of the Department for Environmental Geosciences is the phenomenon of competition between contaminants. Many chemicals often co-exist in the environment, especially in polluted bodies of water. If competition occurs, it means that a contaminant may not attach to carbon nanotubes if better competitors co-exist. Competition is not acceptable for filter application as the efficacy of the filter will vary according to the quantity and type of contaminants present. Studying competition also provides information on the mechanisms of sorption.
Using classical techniques with relatively high concentrations showed that competition can be very strong when three PAHs co-exist with carbon nanotubes. Conversely, experiments with the "passive sampling method" at concentrations likely to occur in the environment showed that no competition occurs if 13 PAHs are considered together. This example highlights the importance of developing and using experimental methods to produce results relevant to environmental conditions. There are still many questions to answer to fully evaluate the potential of carbon nanotubes to clean polluted water. "We keep on working on the subject and the results of our last experiments will be soon presented at international conferences," concludes the environmental geoscientist, Thilo Hofmann.

New Test Measures Key Properties of Polymer Thin Films and Membranes

Nanomechanical measurements (model system and microimage of typical specimen). a) thin rigid film on elastic substrate b) initial strain induces surface wrinkles parallel to stress c) additional strain induces regular pattern of cracks in the film d) typical specimen imaged with optical profilometer (280 X 210 micrometers).

Researchers at the National Institute of Standards and Technology (NIST) have demonstrated a measurement technique that reliably determines three fundamental mechanical properties of near-nanoscale films. The technique, which highlights the challenge of making mechanical measurements on an object with at least one dimension comparable to the size of a virus, should enable better design and engineering for a variety of thin-film technologies, particularly reverse-osmosis membranes for water purification.

Reverse-osmosis membranes, explains NIST researcher Chris Stafford, are an interesting challenge for the materials scientist. The membranes are used in water purification systems -- a polyamide film no more than 200 nanometers thick backed by a thicker, porous support layer. Water holding dissolved salts or other contaminants is forced against one side of the membrane at substantial pressures up to about a thousand psi (roughly 7 megapascal), and comes out the other side leaving most of the impurities behind. The mechanical integrity of the membrane is obviously essential -- it can't tear or develop pinhole leaks under the pressure -- but engineers lacked a good way to measure the strength and breaking point, under stress, of these extremely thin films.

The NIST technique builds on earlier work by the team that demonstrated that you can reliably determine Young's modulus -- a measure of stiffness or elasticity -- for thin and ultrathin films by bonding it to a piece of silicon rubber, and then carefully stretching it in one direction. The film will develop a regularly spaced pattern of wrinkles (try it with a piece of plastic wrap), and the spacing of the wrinkles, the amount of stretch and some math gives you the modulus. In the new work, they basically pull harder until the film starts developing minute cracks crosswise to the tension. These too, it turns out, occur in regular patterns, and the spacing can be analyzed to determine both the fracture strength and the onset fracture strain, or the failure point, of the film.
Applying their technique to study the effect of chlorine on reverse-osmosis membranes, the team uncovered a puzzle. Chlorine in the water is known to cause a progressive deterioration in membrane performance, generally thought to be the result of prolonged chemical attack by the chlorine. Not so, according to the NIST team. "Chemically the chlorine attack is pretty quick," says Stafford. Spectroscopic chemical analysis showed that all the chemical damage from chlorine exposure happens in the first few hours. Tests using the wrinkle-crack method, however, show that the mechanical properties degrade continuously -- the material becoming more and more stiff, brittle and weak -- up to the longest duration tested, 10 days. "It may be an aging effect in polymers," says Stafford. "We're continuing to study that to figure out what's going on in there, because it's a real measurement challenge to get in on that length scale to follow the structure over time."
The project is part of a broader NIST program to study materials issues related to sustainable technologies like water purification, but the research team notes that the wrinkle-crack method itself would be broadly applicable to mechanical studies of almost any nanoscale thin film in fields as diverse as artificial skin, flexible electronics, thin-film sensors, fuel cells and photovoltaics

MIT researchers boost energy density of lithium-air batteries

MIT researchers have further improved the energy density of lithium-air batteries 

Last year MIT researchers reported improving the efficiency of lithium-air batteriesthrough the use of electrodes with gold or platinum catalysts. MIT News is now reporting that in a continuation of that work, researchers have been able to further increase the energy storage capacity of lithium-air batteries for a given weight by creating carbon-fiber-based electrodes.

The superior energy density offered by lithium-air batteries is due to the coupling of a lithium anode to atmospheric oxygen through a porous carbon-based air cathode, instead of the heavy conventional compounds found in lithium-ion batteries. During battery discharge, lithium ions flow from the anode through an electrolyte and combine with oxygen at the cathode to form lithium oxides, which are inserted into the cathode. During recharging, the lithium oxides separate again into lithium and oxygen and the process can begin again.

The carbon-based electrodes used in last year's research had only about 70 percent void space, but the new carbon-fiber-based electrodes are significantly more porous and boast more than 90 percent void space. This means the carbon-fiber-based electrodes can more efficiently store the lithium oxide that fills the pores as the battery discharges.

"We grow vertically aligned arrays of carbon nanofibers using a chemical vapor deposition process. These carpet-like arrays provide a highly conductive, low-density scaffold for energy storage," Robert Mitchell, a graduate student in MIT's Department of Materials Science and Engineering (DMSE), told MIT News.

The researchers claim the carbon-fiber-based electrode can store four times as much energy for its weight compared to current lithium-ion battery electrodes. However, Yang Shao-Horn, the Gail E. Kendall Professor of Mechanical Engineering and Materials Science and Engineering and senior author of the paper says further work still needs to be done before the advances make it from the lab and into a practical commercial product.

A paper describing the new findings appears in the journal Energy and Environmental Science.

More Proof That Wood Is The Greenest Building Material

Stair at Art Gallery of Ontario by Frank Gehry; Image credit Wood Works Ontario
Tim Wall at Discovery News adds another "R" to our growing list (we are up to ten):Reconsider our choices of building materials. He points us to a new study published in Carbon Management that looks at the full life-cycle analysis of using wood instead of other materials.

Image credit CORRIM
Lead Author Bruce Lippke of the University of Washington explains in a press release:
Every time you see a wood building, it's a storehouse of carbon from the forest. When you see steel or concrete, you're seeing the emissions of carbon dioxide that had to go into the atmosphere for those structures to go up.
We have previously shown a study that claimed that wood didn't really sequester all that much carbon, because so much was lost to slash, mill waste, processing and transport. I had my problems with that study, and am relieved that this one tells a different story.

A Northwest state or private forest, harvested regularly for 100 years, helps keep carbon dioxide out of the atmosphere year after year by storing carbon in long-term wood products (blue) and by substituting wood for fossil-fuel-intensive products like steel and cement, thus avoids carbon dioxide emissions during their manufacture (orange). Image credit university of Washington
The authors propose " growing wood as fast as possible, harvesting before tree growth begins to taper off and using the wood in place of products that are most fossil-fuel intensive." According to Lippke:

While the carbon in the wood stored in forests is substantial, like any garden, forests have limited capacity to absorb carbon from the atmosphere as they age. And there's always a chance a fire will sweep through a mature forest, immediately releasing the carbon dioxide in the trees back to the atmosphere.However, like harvesting a garden sustainably, we can use the wood grown in our forests for products and biofuels to displace the use of fossil-intensive products and fuels like steel, concrete, coal and oil.