Designing Carbon Nanotube Membranes for Efficient Water Desalination
J. Phys. Chem. B, 2008, 112 (5), pp 1427?1434
School of Biomedical, Biomolecular and Chemical Sciences, The University of Western Australia, Crawley, Western Australia, 6009 Australia
This paper investigated the desalination potential of carbon nanotubes (CNTs) by using molecular dynamics. Both water flow and ion rejection were investigated to confirm whether CNTs could be used as membranes for seawater desalination.
The system simulated consisted of two water boxes, one of which contained Na and Cl ions. CNTs studied had different pore diameters ranging from 6-11 ?. Operating pressures were also varied at1000 to 3000 bar.
Results showed that CNTs can allow very high water flux. By investigating the structure of water inside the pores, the rates at which water molecules enter and exit the pores, the potential of mean force (PMF), and the amount of free energy barrier to water conduction, the reasons for high water flow through CNTs were revealed. Moreover, salt rejection of CNTs was also investigated by measuring the ion conductance through the nanotubes. It was found that there is a large energy barrier at the pore entrance of the CNTs which made it difficult for the ions to pass through at small diameters. However, for larger diameters, ions can pass through them because their hydration shells were intact (not stripped off water molecules surrounding it) which didn’t need to require much energy.
The effect of CNT length was also studied in the paper. The authors mentioned that doubling the length of CNTs didn’t affect the water and ion transport through the pores. In addition, a linear relationship between flux and applied pressure was observed. Extrapolation to 55 bar was conducted in order to project the performance of CNT membranes at typical reverse osmosis operating pressures.
In summary, the paper concluded that CNTs used as porous membranes have a high potential to be effective desalination membranes because of high water flow rate and ion rejection capability.
Contribution and application:
This paper gave various information about CNTs as desalination membranes. However, the authors missed to provide important details such as why it was valid to use the linear relationship with flux and pressure to extrapolate down to typical operating pressures. Moreover, it seemed unjustifiable to investigate the actual ion rejection because high pressure values were used. These observations could be used by our lab as starting points for further investigation.
By: Hannah Ebro