Capacitive desalination, in which salt ions are electrically removed from saltwater, has been researched for decades as a potentially cheaper alternative to energy-intensive reverse osmosis (RO) and distillation, but so far its application has been limited to waters with very low salt concentrations. A new method, which may change that, is being developed by researchers at Lawrence Livermore National Laboratory (LLNL, Livermore, Calif.; www.llnl.gov). In capacitive desalination, saltwater is typically pumped through a channel between two capacitor-electrodes (diagram). The electrodes remove the Na+ and Cl– ions from the saltwater by capturing them on the surface of the electrodes. LLNL has built a flow-through electrode (FTE) module that has no channel, but consists of two highly porous blocks of carbon aerogel (with negative and positive charges), divided by a thin polymer membrane. Saltwater is pumped through the entire capacitor and the ions, which can pass through the membrane, are accumulated by the appropriate electrode. Earlier experiments (in the 1990s) used aerogels with small pores and had a channel between the electrodes. This limited ion-transport times from the channel into the electrodes, says Michael Stadermann, an LLNL staff scientist. The FTE uses a newly developed aerogel that combines large pores, of 1–3-μm dia. with small pores of 1–2-nm dia. The large pores allow easy flow at low pressure, while the small ones give the aerogel a vast surface area of up to 3,000 m2/g for capturing ions. The structure permits elimination of the channel, thereby reducing the transport distance from millimeters to micrometers, with a corresponding decrease in desalination time, he says. The aerogel is made by sol-gel polymerization of resorcinol with formaldehyde in water, using acetic acid as a catalyst. The water is removed by washing with acetone, which is then evaporated by heating. Finally, the material is pyrolized at 950°C and activated by a carbon dioxide etch. Pyrolysis stabilizes the pores and makes for a robust structure, says Stadermann. Laboratory tests with the FTE module achieved salt removal of 80 mmol/L in a single pass and indicate the process would require less than a hundredth of the pressure of conventional RO, he says, but the higher electricity use would even out the operating costs. However, he expects the capital cost would be lower and that the process would have an advantage over RO for processing brackish water.