Molecular dynamics simulation of polyamide based reverse osmosis for boron removal

Abstract

The inadequate boron removal performance of Polyamide (PA) based reverse osmosis (RO) membrane is one of the bottlenecks restricting its use as drinking water. Therefore, it is of importance to investigate the trans-membrane mechanism of PA membranes at the atomic level. In this work, an all-atom solution-membrane-pure water sandwich RO model was built. Non-equilibrium molecular dynamics (NEMD) simulations were employed to explore the transmembrane behaviors of water, ions, H₃BO₃, H₄BO₄⁻, and D-gluconate borate complex (referred to Complex). The influences of transmembrane pressure difference, temperature, and chlorination of the PA membrane were investigated. The results indicate that an increase in transmembrane pressure difference and feed temperature facilitates the RO process by increasing the diffusion coefficients of all components. Both factors reduce the number of hydrogen bonds between boron-containing species and water while increasing those with the PA membrane, which benefits water molecule permeation. Pressure difference has little effect on salt rejection but impacts boron rejection ability, whereas higher temperatures decrease salt rejection. Chlorination enlarges PA membrane pores and increases component diffusivity, while weakening interactions with both water and boron species (H₃BO₃ and H₄BO₄⁻) and reducing associated hydrogen bonding. These changes enhance water permeability but affect boron rejection differently: H₃BO₃ rejection remains less impacted due to its weak membrane affinity, whereas H₄BO₄⁻ rejection declines markedly as chlorination disrupts its strong electrostatic/hydrogen-bonding interaction with the membrane. Among boron species, Complex exhibits the highest removal rate and is least affected by these factors, leading to the recommendation of adding polyhydroxylic substances to form hydroxyl borate complexes for optimal boron removal.