Functionalized carbon nanotube thin-film nanocomposite membranes for water desalination applications
Cost-effective purification and desalination of water is a global challenge. Reverse osmosis (RO), the current method of choice, requires high pressure drops across the membranes in order to achieve acceptably high flow rates. Conventional polymer membranes are limited in their performance by a trade-off between water permeability and water/salt selectivity. Biofilm fouling is another critical problem in RO applications. Recent simulations and experiments suggest that properly functionalized carbon nanotubes (CNTs) can be used to construct RO membranes that have high permeation flux as well as complete ion rejection, and that are resistant to biofilm formation.
The objective of this research was to combine zwitterion-functionalized carbon nanotubes with traditional thin film polyamide (PA) to fabricate a novel desalination membrane which has both high permeability as well as selectivity. Zwitterion functional groups in CNTs act as molecular gatekeepers at the entrance of the nanotubes to enhance blockage for salt ions. Functionalized CNTs were oriented on a membrane support by high vacuum filtration. These oriented CNTs were sealed by a polyamide film via interfacial polymerization. Cross-sectional image of the nanocomposite membrane taken by scanning electron microscopy (SEM) showed semi-aligned zwitterion-CNTs on top of a porous support covered by a thin PA film with an overall thickness of approximately 250 nm.
When the concentration of zwitterion-CNTs in the membrane increased, the nanocomposite membranes experienced significant improvement in permeation flux while the ion rejection increases slightly or remains unchanged. This indicated that the increased water flux is not due to an increase in nonspecific pores in the membrane, but rather due to an additional transport mechanism resulting from the presence of the functionalized CNTs. Significant increase of flux was also observed in separating cations other than sodium. The separation of the PA skin layer dominated the ion rejection mechanism by size exclusion even when the carbon nanotubes were introduced into the polyamide coating.
The zwitterion functional groups exposed at the membrane surface also interacted with the feed water to form a strong hydration layer, which results in improved surface biofouling resistance. The adsorption rate of protein foulants on the nanocomposite membrane surface was significantly reduced compared to the control membrane without CNTs, and the adsorbed fouling layer could be easily removed by flushing with water. After washing, the nanocomposite membrane recovered 100% of the decreased water flux whereas the control membrane only recovered 10% of the decreased flux resulting in a permanent loss of 30% in water permeation. We have therefore demonstrated that advanced materials like CNTs can be synthesized with desired functional groups, and can be embedded into traditional RO membranes to simultaneously resolve the challenge of low flux and surface fouling in the current desalination process.