As freshwater scarcity increases with growing population, energy demand, and industrialization, membrane systems will play an increasingly important role in water purification. Nanofiltration membranes are ideal for selective contaminant removal and operate at significantly lower energy cost than reverse osmosis systems. However, poor understanding of electric charge interactions between water contaminants and the membrane at the molecular level makes application of nanofiltration membranes difficult to understand or predict in many situations. The scientific challenge lies in predicting how interactions between the charged membrane and multiple charged contaminants affect the membrane's ability to remove the contaminants while allowing water to pass freely. A computational approach called molecular dynamics simulation can be used to model the interactions of the membrane and the contaminants at the atomistic level. This makes possible better prediction of real membrane performance for a wide range of contaminants and paves the way computer-aided design of superior nanofiltration membranes tuned to remove particularly troublesome or difficult contaminants. The approaches developed here can be extended to other membrane systems, perhaps resulting in an entirely new paradigm for designing and predicting the performance of membrane molecular structures and compositions in a wide range of applications.
Non-equilibrium molecular dynamics simulations will be used to explore the molecular-scale physics and chemistry of nanofiltration membranes. The objective of the research is to understand the charge interactions between charged polymeric membranes and mixed ionic solutes that affect contaminant rejection and water flux. Piperazine and trimesoyl chloride monomers will be polymerized to create a virtual polyamide nanofiltration membrane in which the charge can be adjusted by assigning non-protonated functionality among the carboxylate groups in the membrane. The membrane will be challenged with a variety of single and multiple mono- and divalent solutes in a virtual environment using pressure-driven non-equilibrium molecular dynamics. This allows the exploration of multiple ion interactions with the membrane, examples of which include how cations interact with the negatively charged membrane to influence anion passage or how the presence of a dominant salt affects the passage of trace ions. Moreover, the simulations will provide a deep fundamental understanding of contaminant selectivity and rejection as well as water flux in charged membranes, leading to more rigorous approaches for prediction of membrane performance and the potential for developing superior "designer" membrane nanostructures and charge distributions. The approaches developed here can be extended to reverse osmosis membranes and a broad range of other membrane applications. Molecular dynamics simulations have the potential to become the first and decisive step in designing and improving membranes for water purification.
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