Nanotube Membranes Desalinate Water

Arrays of densely packed, vertically aligned carbon nanotubes can serve as membranes to filter ions out of water while allowing the water to flow significantly faster than through conventional filters, according to a research team led by Francesco Fornasiero, Aleksandr Noy, and Olgica Bakajin of Lawrence Livermore National Laboratory (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0710437105). These nanotube membranes offer a promising technology for desalination, the researchers say. The team created the filtration membranes by embedding nanotubes in a silicon nitride matrix and then uncapping the ends of the tubes by an etching process that also introduced carboxylate groups around tube entrances. The carboxylate groups form a ring of negative charges through which ions must pass to enter the tubes. When the researchers filtered electrolyte solutions, the nanotube membranes rejected as much as 90% of K₃Fe(CN)₆ and 50% of KCl while maintaining a high flow per unit area. Additional experiments varying the pH of the solutions as well as filtering different salts demonstrated that ion exclusion stems primarily from electrostatic rather than steric effects. In addition to desalination applications, the nanotubes could be useful as models for biological membrane pores, the authors suggest.