EES Blog

Scientists in the US have developed a membrane-free, solvent extraction method to remove salt from seawater that works at low temperatures.

Access to clean, fresh water is a necessity. Unfortunately, supply is becoming over-stretched and there is a struggle to meet demand. As a result, the development of desalination technology (the conversion of salt water to fresh water) has become increasingly important.

Current desalination techniques require large amounts of energy or membranes that need to be changed constantly as they become blocked. Although significant advances have been made in these areas, Gang Chen and colleagues from the Massachusetts Institute of Technology, Cambridge, have gone a step further and removed the need for a membrane entirely.

The team used decanoic acid as a solvent to mix with the water. ‘Upon slight heating, our solvent dissolves the water out, leaving salts and impurities behind. Then, upon cooling, the mixture separates into two layers by gravity, releasing pure water. Unlike reverse osmosis, this method does not use expensive membranes and unlike evaporation processes, does not need heating to high temperatures,’ explains Chen. The process was shown to be effective at temperatures as low as 40 degrees Celsius and the recovered water met the salinity standards set by the World Health Organisation and the US Environmental Protection Agency.

Adel Sharif, an expert in water engineering and director of the Centre for Osmosis Research and Applications at the University of Surrey, UK, believes that further research is needed in areas such as scalability and practicality, but believes that the concept has promise. ‘The proposed desalination process has the potential for low environmental impact, since it uses low grade heat, and for low capital and operating costs,’ he says.

Chen believes that the work opens up a new field of research in desalination. ‘Being a simple, inexpensive process, directional solvent extraction also bears tremendous commercial potential in the desalination of seawater, clean-up of industrial waste water, treatment of water produced from oil and gas wells and other such uses,’ he concludes.

Rebecca Brodie

Read the Energy & Environmental Science article in full:

Very low temperature membrane-free desalination by directional solvent extraction
Anurag Bajpayee, Tengfei Luo, Andrew Muto and Gang Chen
Energy Environ. Sci., 2011, DOI: 10.1039/c1ee01027a

Scientists in China have made a reflective coating with a structure that mimics the underside of a poplar tree leaf. The coating could be used on the outside of buildings to counteract the heating effect of carbon dioxide emissions, reducing the energy needed to cool the building from the inside.

Yanlin Song and colleagues from the Chinese Academy of Sciences, Beijing, mimicked the structure of the leaf’s lower surface using polymers spun into reflective films consisting of long, hollow uniform fibres.

The underside of the poplar leaf is better at reflecting light than the top. This is because of the ‘cool roof’ effect, in which a layer of hairs on the underside reflects the light, so that less heat penetrates the leaf. The leaf turns over in strong sunlight to reveal the underside and as the light is being reflected rather than absorbed, the leaf appears white. ‘Normally, the poplar tree looks green, but sometimes in the summer, the tree shows a white cast,’ says Song.

The poplar leaf's hair structure (left) and the reflective coating with a magnified image showing one of the hollow fibres (right)

The team discovered that controlling the film thickness and making the cross section of the fibres as similar to the leaf hair as possible is the key to high reflectivity. They tested their films by coating them onto the compound diarylethene, which changes from red to colourless in the presence of visible light – the structure changes from a closed ring to an open ring. They found that the coating stopped the diarylethene changing colour, and had the additional benefit of being hydrophobic.

‘The reflectance and waterproof nature of the coatings make them ideal candidates for a number of building situations,’ says Robert Lamb, an expert on surface science. ‘Improving the durability of such delicate interfaces with the environment will be the major hurdle, but the alternative of sticking poplar leaves to our roofs to achieve the same effect is really not an option!’

Song says that his team will continue to develop highly reflective materials, widening the wavelength at which they function, to eventually be used to improve the efficiency of lighting.

Holly Sheahan

Read the journal article in full:

Highly reflective superhydrophobic white coating inspired by poplar leaf hairs toward an effective cool roof
Changqing Ye, Mingzhu Li, Junping Hu, Qunfeng Cheng, Lei Jiang and Yanlin Song
Energy Environ. Sci., 2011, DOI: 10.1039/c0ee00686f

‘HOT’ paper – Chemically Selective Gas Sweetening Without Thermal-Swing Regeneration

Scientists have reacted anhydrous alkanolamines with H₂S to produce switchable hydrosulfide-based ionic liquids, of which H₂S can be released without thermal heating. H2S removal from natural gas is commonly known as ‘‘gas sweetening,’’ a process that uses either physical or chemical sorbents.

Read the Energy & Environmental Science article today – hot off the press!

Chemically Selective Gas Sweetening Without Thermal-Swing Regeneration
P K Koech, J E Rainbolt, M D Bearden, F Zheng and D J Heldebrant
Energy Environ. Sci., 2011, DOI: 10.1039/ c0ee00839g