Richard LueptowNorthwestern University, Mechanical Engineering
The water permeability and rejection characteristics of ionic and organic solutes were studied for a polymeric reverse osmosis (RO) membrane using non-equilibrium molecular dynamics simulations, which permits molecular-level study of the mechanisms for water flux and contaminant rejection. The water flux increases with an increasing fraction of percolated free volume in the membrane polymer structure. Solute molecules display Brownian motion and hop from nanoscale pore to pore as they pass through the membrane. The solute rejection depends on both the size of the solute molecules and the chemical interaction of the solute molecules with water molecules and the molecular chains of the membrane. When the open spaces in the polymeric structure are such that solute molecules have to shed at least one water molecule from their solvation shell to pass through the membrane molecular structure, the water-solute pair interaction energy governs solute rejection. Organic solutes more easily shed water molecules than ions to more readily pass through the membrane. Hydrogen-bonding sites for solute molecules like urea also lead to a higher rejection. These findings underline the importance of the solute’s solvation shell and solute-water-membrane chemistry in solute transport and rejection in RO membranes.Funded by the Institute for Sustainability and Energy at Northwestern with computing resources from XSEDE, which is supported by NSF grant ACI-1053575.
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