(123g) Carbon Nanotube-Polyamide Nanocomposite Membrane for Water Desalination

We have developed asymmetric carbon nanotube/polyamide membranes for water desalination applications. The carbon nanotubes are functionalized with zwitterionic groups to block the transport of ions while permitting a high flux of water. Both experiments and non-equilibrium molecular dynamics simulations were carried out to characterize the transport of fluids through functionalized carbon nanotubes, including entrance effects. The simulations utilized a model carbon nanotube membrane consisting of four carbon nanotubes embeded between two graphene sheets. The ends of each tube were functionalized with one or more zwitterion groups. The carbon nanotube membrane was embedded into a periodically repeated saltwater box in order to study the flux of water and salt ions through the memberane. A pressure drop of about 200 MPa across the memberane was maintained in order to generate a high water flux over a limited simulation time. The ion rejection ratio was found to be 100% when two zwitterions were affixed to the ends of (20,0) nanotubes. The simulations show that ions are rejected through a combination of electrostatic and steric effects. Experimental sorption studies also suggest that the zwitterionic groups are highly hydrolized and their presence selectively blocks the entry of molecules into the carbon nanotube pores. Water transport measurements show a salt rejection ratio of around 98% with water flux near 40 l/m²hr. Furthermore, both the water flux and salt rejection increase in proportion with the area density of the carbon nanotubes in the membrane.