Abstract
Polyamide (PA) membranes possess properties that allow for selective water permeation and salt rejection, and these are widely used for reverse osmotic (RO) desalination of sea water to produce drinking water. In order to design high-performance RO membranes with high levels of water permeability and salt rejection, an understanding of microscopic PA membrane structures is indispensable, and this includes water transport and ion rejection mechanisms on a molecular scale. In this study, two types of virtual PA membranes with different structures and densities were constructed on a computer, and water molecular transport properties through PA membranes were examined on a molecular level via direct reverse/forward osmosis (RO/FO) filtration molecular dynamics (MD) simulations. A quasi-non-equilibrium MD simulation technique that uses applied (RO mode) or osmotic (FO mode) pressure differences of several MPa was conducted to estimate water permeability through PA membranes. A simple NVT (Number, Volume, and Temperature constant ensemble)-RO MD simulation method was presented and verified. The simulations of RO and FO water permeability for a dense PA membrane model without a support layer agreed with the experimental value in the RO mode. This PA membrane completely rejected Na+ and Cl− ions during a simulation time of several nano-seconds. The naturally dense PA structure showed excellent ion rejection. The effect that the void size of PA structure exerted on water permeability was also examined.
Highlights
Membrane separation processes that feature energy saving and ease of handling have been widely utilized to address the problems of clean water shortage and water pollution as global population and economic development both continue to grow
A channel with a suitable size is a promising candidate for forward osmosis (FO) membrane material, and, water and ion transport behavior in water channels composed of spiral protein molecules, such as Aquaporin, have been widely investigated since around 2001 [25,26,27,28]
0.3 nm, which approximated that of the water molecule
Summary
A channel with a suitable size is a promising candidate for FO membrane material, and, water and ion transport behavior in water channels composed of spiral protein molecules, such as Aquaporin, have been widely investigated since around 2001 [25,26,27,28] In those papers, water permeability was often calculated based on the diffusive flux across membranes or channels. When technical issues and calculation costs are considered, it is difficult to estimate reliable water permeability given a short amount of time and a small-scale simulation system when the water permeability of a membrane is low These are among the reasons that FO simulations have seldom been utilized to study important organic polymer membranes with relatively low levels of permeability. Correlations between water permeability and polyamide membrane structure were examined on a molecular scale
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