Zwitterion-Functionalized CNTs for Efficient Desalination

The availability of clean water for drinking and agriculture in many parts of the world is a growing crisis. Currently, almost one in nine people lack access to treated drinking water. Cost-effective and energy-efficient large-scale desalination of seawater is a key technology for addressing the water needs of many communities. While large-scale reverse osmosis membrane plants are being installed in developed countries, there is still a need for improved technologies that provide improved water flux and anti-fouling membranes. Functionalized carbon nanotubes (CNTs) offer the promise of more energy efficient desalination due to higher water flux resulting from the very fast, almost frictionless flow of water through the interior of CNTs. However, only very narrow carbon nanotubes afford ion rejection. We have investigated the ability of functional groups at the entrance of CNTs to act as gatekeeper moieties, allowing water to pass through the CNTs while providing high ion-rejection rates. In our study, we have performed non-equilibrium molecular dynamics simulations on multiple saltwater systems passing through a membrane involving four single-walled CNTs (~1.5 nm diameter), functionalized with varying numbers of zwitterion groups, embedded between two graphene sheets. When two zwitterions are used to functionalize the CNTs, a near 100% ion rejection rate is observed, regardless of any ion solution we tested, along with high water flux rates. A combination of steric hindrance and electrostatic repulsion coming from the zwitterion groups is responsible for the selectivity of the model membrane. Moreover, there is experimental evidence that zwitterions should inhibit membrane fouling by suppressing adhesion of biomolecules and bacteria. We conclude that zwitterion functionalized nanotubes are good candidates for improved desalination membranes.