Pressure-Dependent Ion Rejection in Nanopores

It is generally considered that ion rejection of a desalination membrane is independent of the operation pressure drops (Delta Ps), which is typically not higher than 10 MPa. However, this may not be true for pressures as high as hundreds of megapascals usually used in simulations. Therefore, simulation results of high Delta Ps cannot be directly used to predict real-world ion rejections, which is often overlooked. Herein, we investigate the ion rejection of carbon nanotube membranes in a large scale of Delta Ps via nonequilibrium molecular dynamics simulations. With effective pressure drops (Delta P-e's) increased from 2.85 to 996 MPa, the ion rejection drops from 100% to nearly zero. Rather than directly investigating the rejection, the relationships of ion and water fluxes with Delta Ps are separately investigated. With rising Delta P(e)s, the water flux increases linearly, while the ion flux undergoes a two-stage increase: first, an exponential increase at Delta P-e <= 53.4 MPa and then a linear increase. An equation describing the Delta P-e-dependent ion rejection is then developed based on these observations. Moreover, the rejection mechanism is also discovered, which indicates that the enhanced input energy makes ions easier to overcome the energy barrier rather than the molecular-configurational reasons. These findings are expected to fill the big gaps between simulations and experiments and may also be helpful for the rational design of the next-generation desalination membranes.