Dynamics of water and solute transport in polymeric reverse osmosis membranes via molecular dynamics simulations

Abstract

The Ångström-scale transport characteristics of water and six different solutes, methanol, ethanol, 2propanol, urea, Na + , and Cl -, were studied for a polymeric reverse osmosis (RO) membrane, FT-30, using non-equilibrium molecular dynamics (NEMD) simulations. Results indicate that water transport increases with an increasing fraction of percolated free volume, or water-accessible open space, in the membrane polymer structure. The trajectories of solute molecules display Brownian motion and hop from pore to pore as they pass through the polymer chain structure of the membrane. The solute transport depends on both the Van der Waals size of the dehydrated solute and the electrostatic interaction of the solute with water and the membrane. For alcohol solutes, transport decreases for solutes with larger Van der Waals volume, which corresponds to less available percolated free volume, or solute-accessible space, within the membrane polymer structure. Urea has reduced transport compared ethanol, most likely due to more complex chemistry or polarity than the alcohol solutes, even though urea has a smaller Van der Waals volume than ethanol. Na + and Clexperience the lowest transport, likely due to strong ion-water and ionion electrostatic interactions. NEMD simulations provide a unique opportunity to understand molecular level mechanisms for water and solute transport in polymeric RO membranes for water purification.

Theory of Ion and Water Transport in Reverse-Osmosis Membranes

“…An average pore size of 0.76 nm [20,55] and film thickness of 100 nm were assumed. These values are within the range of values reported in literature [11,18,20,22,24,55,56]. Owing to the aqueous environment in the membrane, species were described with their solvation shell included.…”

“…In RO, the pores in the membrane refer to the percolated free volume that is present between the polymer chains in the PA toplayer. The number of pores and the pore size distribution depend on the polymerization process [11]. Relative to nanofiltration membranes, RO membranes have a more narrow size distibution, i.e., a more uniform pore size [20], typically in the range of an average diameter between 0.66 and 0.78 nm [20][21][22].…”

Rejection mechanisms for contaminants in polyamide reverse osmosis membranes

“…Like other recent studies [21,22], the simulated membranes under study are much thinner than actual polymeric membranes used commercially, which are typically about 0.2 µm thick [26]. Nevertheless, the water flux through the simulated membrane is in the range expected for commercial membranes [23]. More importantly, the atomic level structure of the simulated membrane is adequate to provide an opportunity to investigate the effects of local membrane structure on water transport and solute rejection.…”

“…Non-equilibrium conditions have been introduced in molecular dynamics to study water/ion transport in inorganic and biological membranes [18][19][20]. However, only very recently have non-equilibrium molecular dynamics simulations been used to investigate water transport and contaminant rejection in polymeric RO membranes [21][22][23]. In this approach the actual operating conditions of the RO process are simulated by applying a pressure difference across the membrane.…”

“…Progress has been achieved in understanding some aspects of water and ion transport, though two recent studies seem to consider the case of equal ion concentrations on both sides of the membrane [21,22], a situation that would not occur in practice. By contrast, we study the situation in which solutes are initially concentrated on the high-pressure feed side of the membrane with pure water on the low-pressure permeate side of the membrane, both in our recent work in which we considered the physical aspects of the transport including the impact of the percolated free volume and size of the solute [23] and the current study. Furthermore, in contrast to these recent non-equilibrium studies that only considered water permeability and ion rejection [21,22], we are able to more deeply probe the chemistry of solute transport and rejection by considering the transport of two ions, Na + and Cl -, as well as four organic solutes, methanol, ethanol, 2-propanol, and urea.…”