(739i) Functionalized Carbon Nanotube Nanocomposite Membranes for Water Desalination: Experimental Study

We have fabricated a novel nanocomposite membrane for water desalination application by combining functionalized single-walled carbon nanotubes (SWNTs) with traditional thin film polyamide (PA). We then characterized this type of membrane using reverse osmosis technique with different salt/de-ionized water mixtures as the feed solutions in order to investigate the water permeation flux and salt rejection rate. The SWNTs were functionalized with zwitterionic functional groups (Z-SWNTs), which act as molecular gatekeepers at the entrance of SWNTs to enhance the blockage for salt ions. The nanocomposite membrane was synthesized with 26wt% of Z-SWNTs embedded in PA skin layer introduced by interfacial polymerization. Vacuum filtration was applied before the polymerization to align the SWNTs by flow-induced orientation. Cross-sectional image of the nanocomposite membrane taken by scanning electron microscopy (SEM) showed semi-aligned Z-SWNTs on top of a porous support covered by a thin PA film with an overall thickness of approximately 250nm. The nanocomposite membranes had significant improvement in permeation flux when the embedded Z-SWNTs content increased from 0 to 26wt%. There was almost three-fold increase in water flux and a slight increase in salt rejection for sodium ion. Experimental study also showed that Z-SWNTs/PA membrane had higher salt rejection rate when the cation concentration in feed solution increased from 50 to 2000ppm. This result demonstrated that separation mechanism is dominated by size exclusion over Donnan charge exclusion due to the absence of electrostatic screening effect from increasing solution ionic strength. Z-SWNTs/PA membrane also experienced higher salt rejection rate towards cations with larger outer hydration shell. For example, the rejection for potassium cation (K⁺), whose hydrated radius is 3.31 Å, was approximately 5% lower than the rejection rates for sodium, Na⁺, whose hydrated radius is 3.58 Å  and magnesium, Mg²⁺,with  a hydration radius is 4.38 Å . These results confirm that the separation mechanism of the functionalized carbon nanotubes is dominated by size exclusion rather than by charge exclusion. These results furthermore agree with our simulation study in which zwitterions offer steric hindrance at the tip of SWNTs to separate salt ions, rather than electrostatic repulsion.