Reverse osmosis technology is being tapped to meet the increasing need to augment freshwater supplies with desalinated seawater and brackish water sources. The performance of thin-film composite (TFC) membranes used to separate water from the ions in saline feed streams is constrained by permeability-selectivity trade-offs. Insufficient rejection of some ions and trace micropollutants requires additional purification stages that increase energy consumption and process cost. Carbon nanotubes with structures simulating biological water channels have been demonstrated by an international research team to offer a more efficient desalination alternative.

The nanotube porins are short segments of carbon nanotubes that self-insert into lipid bilayers and form artificial water channels. Narrow inner pores squeeze water down to a single-file configuration that enables Simulation showing the model system consisting of two water reservoirs, two graphene sheets, and a carbon nanotube that mimic aquaporin channel functionality and intrachannel single-file water arrangement. Source: Yuhao Li et al.extremely high water permeability, with transport rates exceeding 1 billion water molecules per second through each pore, which is just 0.8 nm in diameter.

The desalination efficiency was tested by measuring water and chloride ion transport through the carbon nanotube porins by use of fluorescence-based assays. Computer simulations and experiments using carbon nanotube pores in lipid membranes demonstrated the mechanism for enhanced flow and strong ion rejection through inner channels of carbon nanotubes. Water-salt selectivity values were comparable to those obtained with commercial TFC desalination membranes.

Scientists from U.S. Lawrence Livermore National Laboratory, Southeast University (China) and University of California Merced contributed to this research, which is published in Science Advances.