Rejection mechanisms for contaminants in polymeric reverse osmosis membranes

摘要

Despite the success of reverse osmosis (RO) for water purification, themolecular-level physico-chemical processes of contaminant rejection are notwell understood. Here we carry out NEMD simulations on a model polyamide ROmembrane to understand the mechanisms of transport and rejection of both ionicand neutral contaminants in water. We observe that the rejection changesnon-monotonously with ion sizes. In particular, the rejection of urea, 2.4 Aradius, is higher than ethanol, 2.6 A radius, and the rejections for organicsolutes, 2.2-2.8 A radius, are lower than Na+, 1.4 A radius, or Cl-, 2.3 Aradius. We show that this can be explained in terms of the solute accessibleintermolecular volume in the membrane and the solute-water pair interactionenergy. If the smallest open spaces in the membrane's molecular structure areall larger than the hydrated solute, then the solute-water pair interactionenergy does not matter. However, when the open spaces in the polymericstructure are such that solutes have to shed at least one water molecule topass through a portion of the membrane molecular structure, the pairinteraction energy governs solute rejection. The high pair interaction energyfor water molecules in the solvation shell for ions makes the water moleculesdifficult to shed, thus enhancing the rejection of ions. On the other hand, theorganic solute-water interaction energies are governed by the water moleculesthat are hydrogen bonded to the solute. Urea molecules have morehydrogen-bonding sites than alcohol molecules, leading to a higher rejection ofurea than occurs for ethanol, a molecule of similar size but with fewerhydrogen bonding sites. These findings underline the importance of the solute'ssolvation shell and solute-water-membrane chemistry in the context of reverseosmosis, thus providing new insights into solute transport and rejection in ROmembranes.