Abstract
Membranes made from nanomaterials such as nanotubes and graphene have been suggested to have a range of applications in water filtration and desalination, but determining their suitability for these purposes requires an accurate assessment of the properties of these novel materials. In this study, we use molecular dynamics simulations to determine the permeability and salt rejection capabilities for membranes incorporating carbon nanotubes (CNTs) at a range of pore sizes, pressures and concentrations. We include the influence of osmotic gradients and concentration build up and simulate at realistic pressures to improve the reliability of estimated membrane transport properties. We find that salt rejection is highly dependent on the applied hydrostatic pressure, meaning high rejection can be achieved with wider tubes than previously thought; while membrane permeability depends on salt concentration. The ideal size of the CNTs for desalination applications yielding high permeability and high salt rejection is found to be around 1.1 nm diameter. While there are limited energy gains to be achieved in using ultra-permeable CNT membranes in desalination by reverse osmosis, such membranes may allow for smaller plants to be built as is required when size or weight must be minimized. There are diminishing returns in further increasing membrane permeability, so efforts should focus on the fabrication of membranes containing narrow or functionalized CNTs that yield the desired rejection or selection properties rather than trying to optimize pore densities.
Highlights
An increasing number of regions around the world are water stressed as climatic changes and population increases are reducing the reliability of fresh water sources [1]
We are able to give an improved estimate from classical molecular dynamics simulations of the likely permeability of these narrow pores, with a membrane made from 1.1 nm diameter (8,8) carbon nanotubes (CNTs) at a realistic pore density of 2.5 × 1011 cm−2 having a permeability of 5.7 × 10−6 m3 m−2 Pa−1 s−1, more than 16 times greater than the leading polymeric thin-film composites (TFCs) reverse osmosis membrane
We have presented a systematic simulation study of factors affecting the permeation and salt rejection of CNT-based filtration membranes which we hope can guide the future development of these materials
Summary
An increasing number of regions around the world are water stressed as climatic changes and population increases are reducing the reliability of fresh water sources [1]. Improving our simulation conditions in this way enables concentrated ion layers on the membrane surface to be realistically modelled and their effects on water transport and ion rejection to be determined These differences from previous work allow us to improve the estimate of the likely permeability and rejection properties of CNT membranes coming from simulation, explore parameters not yet studied computationally, and from this to assess the potential of nanotube membranes in desalination and filtration technologies. We use molecular dynamics simulations to model a wide range of CNT-based filtration membranes, examining the influence of pore size, chemical functionality, pore density, pressure and salt concentration on permeability and salt rejection In this case, we focus on relatively narrow pores (less than or equal to 1.6 nm in diameter) which may be suitable for desalination using reverse osmosis. We can make estimates of the potential benefits to power consumption and membrane area associated with employing these membranes in desalination facilities
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