Pressure-Dependent Ion Rejection in Nanopores

It is generally considered that ion rejection of a desalination membrane is independent of the operation pressure drops (ΔPs), 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 at high ΔPs 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 ΔPs via molecular dynamics simulations. With effective pressure drops (ΔP_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 ΔPs are separately investigated. With rising ΔP_e’s, the water flux increases linearly, while the ion flux undergoes a two-stage increase: firstly, an exponential increase at ΔP_e ≤ 53.4 MPa and then a linear increase. An equation of describing the ΔP_e-dependent ion rejection is then developed based on these observations. Moreover, the rejection mechanism is also discovered 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.