Efficient separation of small organic contaminants in water using functionalized nanoporous graphene membranes: Insights from molecular dynamics simulations
J Yang and ZQ Shen and JL He and Y Li, JOURNAL OF MEMBRANE SCIENCE, 630, 119331 (2021).
Small organic molecules, and specifically micropollutants, pose known hazards to human health, but their removal is particularly challenging in water treatment. Our work demonstrates promising single-layer nanoporous graphene (NPG) membranes with high water permeability and varying degrees of selectivity against common organic contaminants, using molecular dynamics (MD) simulations. Seven target organic molecules are considered-including methanol, urea, ethanol, 2-propanol, n-nitrosodimethylamine (NDMA), pyrrole, and phenol-to understand the molecular parameters that govern organic removal. We systematically study molecular transport dynamics and energetics through membranes having varying sizes of pores with hydrophobic (hydrogenated) and hydrophilic (hydroxylated) functionalizations. We find that NPG membranes with smaller, hydroxylated pores offer higher water/organic permselectivity compared to those with larger, hydrogenated pores, as they impede the transport of organics while facilitating the transport of water. Molecular size is the primary organic parameter that controls transport. Larger organic molecules have a greater affinity for the membrane and pore groups (interfacial-affinity sieving), but they are more hindered from entering the pore (pore-size sieving). There is a net decrease in transport with increased molecular size, which suggests that poresize sieving is the governing mechanism for most of the molecular separations. We further find that traditional measures of organic hydrophobicity are not strong predictors of transport, as they correlate poorly with interfacial affinity, and the contribution of dehydration to the energy barrier of permeation is limited, though nonnegligible. Finally, our simulations show that organic-pore interactions are not pressure independent: as flow rate is decreased, the effects of interfacial-affinity sieving become more pronounced. In short, this study establishes a comprehensive understanding of membrane and molecular parameters to guide the design of effective NPG membranes for organic contaminant removal, particularly in desalination and wastewater reuse.
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