Boron nitride nanoslits for water desalination via forward osmosis: A molecular dynamics study

Abstract

The global shortage of freshwater resources has spurred significant interest among scientists in the development of cost-effective and highly efficient water desalination methods. The forward osmosis (FO) membrane has become well-known for its various advantages, such as its low energy usage, cost-effective performance, high efficiency in desalination, and minimal fouling. Herein, the desalination performance of an FO system containing a boron-nitride slit membrane (BNSM) was investigated using molecular dynamics (MD) simulations. The effects of parameters, including slit width, temperature, draw solution (DS) concentration, and its types (MgCl₂, CaCl₂, and KCl), on salt ion rejections and water flow rate were explored. The rejection percentages of Na⁺ and Mg²⁺ ions decreased from 100 % to 94 % and 96 %, respectively, as the slit width increased from 6 Å to 9 Å. Additionally, the water flow rate increased significantly, from 17.305 to 80.92 molecules/ns, with the same increase in slit width. The temperature elevation led to a decrease in ion rejection percentage and increased the water flow rate, since, according to the Stokes-Einstein equation, the diffusion coefficient of spherical particles increases with increasing temperature. The changes in DS concentration did not affect the ion rejection performance due to the small size of the slit width as well as the dominant effect of size exclusion. The increase in the DS concentration caused concentration polarization and a decrease in osmotic pressure resulting in a drop in the water flow rate. Due to the highest hydration radius of K⁺ ions, the rejection percentages of Mg²⁺ and Ca²⁺ ions were higher, while the Na ⁺ ion rejection percentages had the highest and lowest values in systems having DS of CaCl₂ and KCl, respectively, which was in accordance with the trend of water flow rate.