Designing Carbon Nanotube Membranes for Efficient Water Desalination

Abstract

The transport of water and ions through membranes formed from carbon nanotubes ranging in diameter from 6 to 11 Å is studied using molecular dynamics simulations under hydrostatic pressure and equilibrium conditions. Membranes incorporating carbon nanotubes are found to be promising candidates for water desalination using reverse osmosis, and the size and uniformity of tubes that is required to achieve a desired salt rejection is determined. By calculating the potential of mean force for ion and water translocation, we show that ions face a large energy barrier and will not pass through the narrower tubes studied ((5,5) and (6,6) “armchair” type tubes) but can pass through the wider (7,7) and (8,8) nanotubes. Water, however, faces no such impediment due to the formation of stable hydrogen bonds and crosses all of the tubes studied at very large rates. By measuring this conduction rate under a hydrostatic pressure difference, we show that membranes incorporating carbon nanotubes can, in principle, achieve a high degree of desalination at flow rates far in excess of existing membranes.

Tunable sieving of ions using graphene oxide membranes

“…As at such interlayer spacings there is a large energy barrier associated with the entering of the ions into the channel (Table S2), we do not observe ion permeation over the typical timescale of a simulation. Therefore to calculate the ion permeation rates, a pressure difference of 10 MPa was applied across the simulation cell by adding a constant force on all of the atoms in the simulation box along the direction of the channel, except on those belonging to the graphene sheets [24][25][26][27] . During the simulations the temperature was maintained constant at 298.15 K. The interaction parameters for ion, water and graphene atoms were taken the same as the previous simulations.…”

“…In contrast, graphene oxide (GO), a chemical derivative of graphene with oxygen functionalities 17 , has attracted wide-spread interest due to its exceptional water permeation and molecular sieving properties [18][19][20] as well as realistic prospects for industrial scale production 21,22 . Molecular permeation through GO membranes is believed to occur along a network of pristine graphene channels that develop between functionalized areas of GO sheets 18 (typically, an area of 40-60% remains free from functionalization 23,24 ), and their sieving properties are defined by the interlayer spacing, d, which depends on the humidity of the surroundings 18,19 . Immersing GO membranes in liquid water leads to intercalation of 2-3 layers of water molecules between individual GO sheets, which results in swelling and d ≈ 13.5 Å.…”

Characterization and evaluation of carbon nanotube Bucky-Paper membranes for direct contact membrane distillation

“…For Knüdsen numbers, K n > 1, the molecule -pore wall collisions dominate leading to Knüdsen type flow: For isoporous membranes with circular pores the permeance is given by [7][8][9][10][11][12][13][14][15][16][17]:…”

“…Thanks to the well dispersed CNT suspensions, the membrane surface and cross section were uniform and free from large CNT-bundles [8].…”

“…This has stimulated intense interest in CNT based membranes of various structures [8][9][10][11]. In this work we focus on CNT Bucky-Paper (BP) membranes, which is a paper-like structure of CNTs processed by vacuum filtration [12].…”