New water desalination method

Print edition : February 08, 2013

A prototype FTE-CD module, shown next to a glass of water for scale. Photo: BY SPECIAL ARRANGEMENT

A micrograph of carbon aerogel shows that the pores in the FTE-CD module are large enough to let water through. (Right) The walls of the big pores are themselves porous. Photo: by special arrangement

DESALINATION is hardly a new idea, but the current methods such as reverse osmosis (RO) and flash distillation are energy intensive and require extensive infrastructure, thus making them expensive. While investigating a new carbon aerogel for using it as capacitors for energy storage, Michael Stadermann and Ted Baumann at the Lawrence Livermore National Laboratory (LLNL), United States, realised that it was promising for capacitive water desalination.

Aerogels, a special class of ultralow-density materials, have a complicated, cross-linked internal structure that gives them the highest internal surface area per gram of any known material.

The new carbon aerogel has the electrical properties of traditional aerogels but has much larger pores with diameters of micrometres as against nanometres. It also is mechanically robust and can be machined and fabricated into different sizes and shapes.

A key feature of this aerogel is that the walls of the big pores are themselves porous, with pores just nanometres in size. Hierarchical porosity allows water to flow easily through the large pores at low pressure, while the small pores give the material an enormous surface area. One gram of the aerogel contains up to 3,000 square metres of surface area—equal to more than 10 tennis courts.

In capacitive desalination, saltwater flows through a capacitor with two electrically charged electrodes, and the positively and negatively charged sodium and chloride ions that make up the salt are captured on the surface of these electrodes. The aerogel is an ideal material for the electrodes because it is porous enough to let the water through and has a vast surface that can hold the ions.

Capacitive desalination using carbon aerogels itself is not a new concept. It was pioneered in the 1990s. Devices used aerogels with small pores and, therefore, the method required long desalination cycles. Because the new carbon aerogel has such large pores, water would flow through the entire capacitor instead of through a small channel between the electrodes. Thus, the capacitor could potentially desalinate more water in a shorter period.

The scientists have now built a prototype flow-through electrode capacitive desalination (FTE-CD) module with this new material. When a small potential (about one volt) is applied to the capacitor, the entire internal surface of the aerogel electrostatically attracts salt ions. Because the water now flows through the entirety of each electrode, all the desalinated water that was formerly trapped in the aerogel can be collected.

Experiments using FTE-CD modules showed that the device removed three times as much salt per charge and desalinated water 10 to 20 times faster than other capacitive devices.