(720f) Simulations and Experiments of Carbon Nanotube-Polyamide Nanocomposite Membranes for Water Desalination

We present a combined simulation and experimental study of zwitterion functionalized carbon nanotube (CNT) membranes for water desalination.  We constructed model CNT membranes for the simulation studies by embedding four CNTs between graphene sheets to prevent fluid flow around the outside of the CNTs. Each end of each CNT was functionalized with one or more zwitterion groups. The model membrane was placed a periodically repeated saltwater box in order to study the flux of water and salt ions through the membrane. A pressure drop of about 200 MPa was applied across the membrane in order to generate a high water flux over the course of the simulation. The ion rejection ratio was found to be 100% when two zwitterions were affixed to the ends of (20,0) nanotubes, which are 1.56 nm in diameter. We found that five zwitterions per tube end was required to completely block ion transport in a 2.03 nm diameter (26,0) CNT. In contrast, the rejection ratio for non-functionalized CNTs and functionalized CNTs with five COOH groups at each tube end was found to be 0% and 20%, respectively. We have synthesized asymmetric carbon nanotube/polyamide membranes with varying loadings of CNTs and assessed these membranes for water desalination in experiments.  The flux of water was found to be increased from 6.8 to 28.7 gallons per square foot per day as the weight fraction of zwitterion functionalized CNTs in the polyamide membrane was increased from 0 to 20 wt%. Importantly, the ion rejection ratio also increased from 97.6% to 98.6% with increasing weight fraction of functionalized CNTs. Additional simulation tests revealed the ion blocking mechanism is almost exclusively due to a steric hindrance, with a minor contribution due to electrostatic repulsion. Theoretical predictions indicate that an ideal CNT/polymer membrane having a loading of 20 wt% CNTs has a maximum flux of about 20000 gallons per square foot per day at the conditions of our experiments.