Design of Graphene Membrane For Wastewater Treatment

Abstract

The growing global need for fresh water has driven the development of advanced membrane technologies with not only high-water permeability but also good rejection of contaminants and long-term operational stability. Graphene-based membranes have been considered as the promising candidate for next-generation water purification because of its atomic thickness, high mechanical strength and tunable nanoporous structure. We present here a study of the performance of various functionalized graphene nanopore membranes in water treatment via MD simulations. Two broad strategies for functionalization were explored: uniform surface functionalization, and pore-edge functionalization. Three different chemical groups, hydrogen (H), fluorine (F), and amine (NH₂) were chemically functionalized on graphene membranes at different levels of functionalization. Functionalization was implemented uniformly over the surface at a coverage of 20%, and pore-specific functionalization localized to regions around the edges of the nanopore at a coverage of 15%. The simulation system includes the pressure-driven water through graphene nanopores helping remove of contaminants such as PFBA⁻ anions and Pb²⁺ ions. Molecular dynamics simulations were carried out using the LAMMPS simulation package with explicit water models and realistic intermolecular interaction parameters. To accelerate transport processes, they were subjected to a piston-driven pressure of 20,000 bar and the resulting flux values were scaled to realistic operating pressures of 100 bar. These results show that pore chemistry is a key parameter in determining membrane transport behavior. Also, Fluorinated graphene membranes achieved the highest water flow rates since their reduced friction and hydrophobicity of channel structure favor almost frictionless transport of water in the nanopore. The permeability was a bit lower for hydrogen-functionalized membranes, but the transport properties remained stable. Fouling index analysis suggested that pore functionalization could generally reduce fouling relative to uniform functionalization owing to localization of chemical groups in proximity to the nanopore region which mitigates surface-wide contaminant adsorption. Electrostatic interactions occurring between the functional groups and charged species such as heavy metal ions and PFAS compounds are the primary driving mechanism for selectivity. Pore-functionalized membranes exhibited the best balance of desirable characteristics (high water flux, strong rejection, reduced foulant deposition tendency) among the studied systems.