Paper title:
Water desalination across nanoporous graphene
Journal:
Nano Lett., 2012, 12 (7), pp 3602?3608
Author/s:
David Cohen-Tanugi and Jeffrey C. Grossman*
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
Summary:
This article looked into the feasibility of using nanoporous graphene for water desalination by evaluating water permeability and salt rejection. Desalination dynamics were characterized by observing the effects of varying pore size, pore chemistry and applied hydraulic pressure.
Pores on the graphene were obtained by passivation using hydroxyl groups and hydrogen atoms. Pore sizes were varied from 1.5?2 to 62?2. Water model used was TIP4P and interactions for atomic species were modeled using Lennard-Jones and Coulombic terms. Carbon atoms were fixed. The salt solution had a concentration of approximately 72 g/L. The ensemble used was NVT, with the Nose-Hoover thermostat fixing the temperature at 300K. Equilibration time was 100 ps while MD production runs varied from 5-10 ns with a time step of 1 fs. The properties reported were averaged from 5 separate runs with different initial conditions. All simulations were run using LAMMPS and visualized in VMD.
The performance of nanoporous graphene as a desalination membrane was evaluated using two criteria: water permeability and salt rejection. The effects of varying pore size, pore chemistry and applied pressure on these two criteria were determined.
1. Water permeability
· To determine water permeability, first, the flow curve was obtained by plotting water molecules filtered versus time. By assuming a membrane porosity of 10%, water permeability (L output/cm2 membrane/ day/unit applied pressure) was derived from the slope of the flow curve (flow per unit time).
· It was found that permeability scales linearly with pore area. This is in accordance with the principle of Hagen-Poiseuille equation for flow across a cylindrical pore.
· It was observed that water permeability is enhanced by hydroxylation because OH groups are hydrophilic. This hydrophilicity causes more hydrogen-bonding configurations inside the pore.
2. Salt rejection
· Salt rejection was obtained by measuring the salinity of permeate solution at t = t1/2.
· It was found that a larger pore area causes lower salt rejection.
· However, salt rejection doesn’t only depend on pore size but also in applied pressure. At higher applied pressures, salt rejection of a given pore decreases.
· It was observed that salt rejection is lower for hydroxylated pores because OH groups can form hydrogen-bonds with ions too, just like with water. This results to lower free energy barrier for ionic passage.
To support the observations obtained for water permeability and salt rejection, water structure and kinetic behavior were also investigated for this system.
1. Water structure
· The angular distribution function revealed that in a hydrogenated pore, there is a more ordered water structure because H-bonding configurations available to water molecules were restricted (due to hydrophobic character).
· For the hydroxylated pore, there is a smoother entropic landscape for the traveling of water molecules because the pore can form H bonds with water (due to hydrophilic character).
· Thus, faster overall water flow was observed in a hydroxylated pore.
2. Kinetics
· The Arrhenius model for both water and salt passage was used. It is a function of volume, pressure, free energy barrier and temperature.
· Although this is just a qualitative evaluation, it confirmed that salts have low passage rate when there is high volume of salts (occupancy relative to water molecules), high pressure, high free energy barrier, and low temperature.
Contribution and application:
The key finding of this paper is that it showed the possibility of using a nanoporous graphene to desalinate a salt solution. The mechanism for this process is convection, which is known to be faster compared to the solution-diffusion mechanism adapted in RO membranes. If graphene can be successfully fabricated to be a desalination membrane, it might be a good development for the desalination industry because it can allow for a faster water transport and low pressure requirement. In addition, graphene is the ultimate thin membrane and it has high mechanical strength which can allow for a wide range of operating conditions. But challenges for its complete application in desalination include making it mechanically stable under applied pressure (i.e., by adding a polysulfone support layer) and implementing an extremely narrow pore size distribution.
By: Hannah Ebro
hannah@gist.ac.kr