Rejection mechanisms for contaminants in polymeric reverse osmosis membranes

📝 Abstract

Despite the success of reverse osmosis (RO) for water purification, the molecular-level physico-chemical processes of contaminant rejection are not well understood. Here we carry out NEMD simulations on a model polyamide RO membrane to understand the mechanisms of transport and rejection of both ionic and neutral contaminants in water. We observe that the rejection changes non-monotonously with ion sizes. In particular, the rejection of urea, 2.4 A radius, is higher than ethanol, 2.6 A radius, and the rejections for organic solutes, 2.2-2.8 A radius, are lower than Na+, 1.4 A radius, or Cl-, 2.3 A radius. We show that this can be explained in terms of the solute accessible intermolecular volume in the membrane and the solute-water pair interaction energy. If the smallest open spaces in the membrane’s molecular structure are all larger than the hydrated solute, then the solute-water pair interaction energy does not matter. However, when the open spaces in the polymeric structure are such that solutes have to shed at least one water molecule to pass through a portion of the membrane molecular structure, the pair interaction energy governs solute rejection. The high pair interaction energy for water molecules in the solvation shell for ions makes the water molecules difficult to shed, thus enhancing the rejection of ions. On the other hand, the organic solute-water interaction energies are governed by the water molecules that are hydrogen bonded to the solute. Urea molecules have more hydrogen-bonding sites than alcohol molecules, leading to a higher rejection of urea than occurs for ethanol, a molecule of similar size but with fewer hydrogen bonding sites. These findings underline the importance of the solute’s solvation shell and solute-water-membrane chemistry in the context of reverse osmosis, thus providing new insights into solute transport and rejection in RO membranes.

💡 Analysis

Despite the success of reverse osmosis (RO) for water purification, the molecular-level physico-chemical processes of contaminant rejection are not well understood. Here we carry out NEMD simulations on a model polyamide RO membrane to understand the mechanisms of transport and rejection of both ionic and neutral contaminants in water. We observe that the rejection changes non-monotonously with ion sizes. In particular, the rejection of urea, 2.4 A radius, is higher than ethanol, 2.6 A radius, and the rejections for organic solutes, 2.2-2.8 A radius, are lower than Na+, 1.4 A radius, or Cl-, 2.3 A radius. We show that this can be explained in terms of the solute accessible intermolecular volume in the membrane and the solute-water pair interaction energy. If the smallest open spaces in the membrane’s molecular structure are all larger than the hydrated solute, then the solute-water pair interaction energy does not matter. However, when the open spaces in the polymeric structure are such that solutes have to shed at least one water molecule to pass through a portion of the membrane molecular structure, the pair interaction energy governs solute rejection. The high pair interaction energy for water molecules in the solvation shell for ions makes the water molecules difficult to shed, thus enhancing the rejection of ions. On the other hand, the organic solute-water interaction energies are governed by the water molecules that are hydrogen bonded to the solute. Urea molecules have more hydrogen-bonding sites than alcohol molecules, leading to a higher rejection of urea than occurs for ethanol, a molecule of similar size but with fewer hydrogen bonding sites. These findings underline the importance of the solute’s solvation shell and solute-water-membrane chemistry in the context of reverse osmosis, thus providing new insights into solute transport and rejection in RO membranes.

📄 Content

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Rejection mechanisms for contaminants in polymeric reverse osmosis membranes (pre-print Aug. 10, 2015; see also J. Membrane Science 509:36-47, 2016 http://dx.doi.org/10.1016/j.memsci.2016.02.043 )

Meng Shen,1 Sinan Keten,1,2 Richard M. Lueptow1,3 1 Mechanical Engineering, Northwestern University, Evanston, IL, USA 2 Civil & Environmental Engineering, Northwestern University, Evanston, IL, USA 3 The Northwestern Institute on Complex Systems (NICO), Northwestern University, Evanston, Illinois, USA

Abstract Despite the success of reverse osmosis (RO) for water purification, the molecular-level physico- chemical processes of contaminant rejection are not well understood. Here we carry out non-equilibrium molecular dynamics (NEMD) simulations on a model RO membrane to understand the mechanisms of transport and rejection of both ionic and inorganic contaminants in water. While it is commonly presumed that the contaminant rejection rate is correlated with the dehydrated solute size, this approximation does not hold for the organic solutes and ions studied. In particular, the rejection of urea (2.4 Å radius) is higher than ethanol (2.6 Å radius), and the rejections for organic solutes (2.2-2.8 Å radius) are lower than Na+ (1.4 Å radius) or Cl- (2.3 Å radius). We show that this can be explained in terms of the solute accessible free volume in the membrane and the solute-water pair interaction energy. If the smallest open spaces in the membrane’s molecular structure are all larger than the solute size including its hydration shell, then the solute-water pair interaction energy does not matter. However, when the open spaces in the polymeric structure are such that solutes have to shed at least one water molecule to pass through a portion of the membrane molecular structure, as occurs in RO membranes, the pair interaction energy governs solute rejection. The high pair interaction energy for water molecules in the solvation shell for ions makes the water molecules difficult to shed, thus enhancing the rejection of ions. On the other hand, the organic solute-water interaction energies are governed by the water molecules that are hydrogen bonded to the solute. While these hydrogen bonds have pair interaction energies that are much larger than that of the non- hydrogen bonded water molecules in the solute solvation shell, they are significantly less the ion-water pair interaction energy. Thus, organic solutes more easily shed water molecules than ions to pass through the RO membrane. Since urea molecules have more hydrogen-bonding sites than alcohol molecules, urea sheds 2

more hydrogen-bonded water molecules and forms more hydrogen bonds with the membrane leading to a higher rejection of urea than occurs for ethanol, a molecule of similar size but with fewer hydrogen bonding sites. These findings underline the importance of the solute’s solvation shell and solute-water-membrane chemistry in the context of reverse osmosis, thus providing new insights into solute transport and rejection in RO membranes.

1. Introduction Reverse osmosis (RO) membrane separation is a widely used technology for drinking water purification [1] because it is energy efficient, requiring no high temperatures or phase transitions [2]. The performance of practical polymeric RO membranes can be predicted using continuum-level models such as the solution-diffusion model [3]. However, the molecular level physico-chemical transport and rejection mechanisms in RO membrane are still not clear. Molecular dynamics (MD) simulations with properly validated inter-atomic force fields offer the ability to study these mechanisms, because the movement of individual solute, solvent, and membrane molecules can be easily tracked, and chemical interactions such as hydrogen bonding can be isolated. In RO membrane separation, a high pressure is applied to the contaminated feed solution on one side of the membrane that opposes the osmotic pressure. In the ideal situation, water molecules pass through the membrane against the osmotic pressure, but contaminant solute ions and molecules do not pass. Although the rejection of monovalent ions by typical RO membranes is often greater than 98% [4], the rejection of small organic solutes, some of which have larger dehydrated molecular sizes than Na+ and Cl-, can be less than 80% [5]. Small organic compounds, which are potentially harmful to human health, can be found in produced water in the oil and natural gas industry as well as other situations [6]. Of course, it is also necessary to remove organic compounds from drinking water. However, it is difficult to experimentally investigate the underlying mechanisms that lead to high rejection of small charged ions but lower rejection of larger neutral organic solutes. Molecular dynamics simulations offer a means to directly consider the movement of individual ions and molecules as they approach and transit thro

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