Graphene Based Membranes for Osmosis Applications
Citation:Akca, Sevilay, Graphene Based Membranes for Osmosis Applications, Trinity College Dublin. School of Physics, 2020
Sevilay Akca PhD Thesis - Graphene Based Membranes for Osmosis Applications.pdf (Published (author's copy) - Peer Reviewed) 61.93Mb
With the discovery of two-dimensional hexagonal graphene (single layer of graphite), a tremendous interest has grown due its remarkable properties. Apart from all other properties, its superior thickness (one-atom thick) enabled to discover many opportunities in membrane science by being functionally active layers of separation membranes. Besides, graphene nanosheets provide fast transport of water molecules through its unhindered channels and offers high selectivity which can be controlled by highly ordered graphene laminates. The main goal of this research was to demonstrate the feasibility of fabricating thin-film composite membranes with stacked graphene nanosheets as selective layer for utilization in osmosis applications such as waste water treatment, desalination, power generation, food industry etc. Despite the fact that there are several ways to produce graphene from bulk material graphite, the widely used liquid phase exfoliation technique (LPE) was used in this research as it is mass-scalable and low-cost technique for industrial fabrication of graphene. Various solvents were studied to test the dispersibility of graphite via LPE since suitable solvent is essential to decrease the potential energy between adjacent layers in the bulk material in order to overcome the van der Waals interactions between layers. Seven reproducible dispersion methods have been established to fabricate multi-layer graphene from graphite with the help of high boiling point solvent NMP, low boiling point solvent ethanol and water-sodium cholate mixture, which provided a broad possibility of selection with respect to concentration, flake size and thickness, number of layers and type of solvent used in the dispersion. A high yield of up to 7.2% was achieved by revealing a highest concentration of 0.36 mg mL-1 in one of the NMP based dispersions. Furthermore, the concentration of graphene dispersion in ethanol-based systems was successfully increased up to 0.08 mg mL-1 with the assistance of potassium sodium tartrate salt (KNaC4H4O6). The characterization results provided graphene made of large number of multi-layers with lateral size less than a micron, mostly ≤500 nm. Except for residual solvents (NMP and sodium cholate) on silicon substrate, no oxygen related groups were found in the resultant dispersions which can be explained by the fabrication of pure graphene. By simply assembling millions of graphene flakes together onto a porous support membrane via pressure-assisted filtration technique, graphene-based membranes which included graphene composite membranes (assembled graphene onto support membrane) and graphene-polymer hybrid membranes (assembled graphene with or without cross linker in between two polyethyleneimine (PEI) layers onto support membrane) were fabricated. Here, the aim of using two PEI layers was to create a positively charged membrane surface to obtain a strong attachment of graphene layers and thereafter to improve selectivity further by 2nd layer of PEI after graphene assembly. Using a cross-linker (ethylene diamine, EDA) was proposed as a method to enhance the stability of nanosheets in aqueous systems. High-water permeability varied from 80 366 L/m2.h.bar and improved selectivity with 48% rejection for dextran 2000 kDa rejection was observed in some of the best graphene composite membranes. Ultrafiltration membrane made from polyethersulfone (PES) showed better performance compared to micro-filtration membranes from Nylon or polytetrafluoroethylene (PTFE) due to its smooth surface morphology. After a huge effort had been made in order to improve the quality of graphene-based membranes, significant achievement came from the graphene-polymer hybrid membranes fabricated via ethanol-based dispersion E2 and PES porous support membrane. Highly compact graphene layer assisted by the polymer provided a selective layer with a thickness of ~1μm. Filtration results for the graphene-polymer hybrid membrane without cross-linker provided that increasing graphene amount from 0.06 mg/cm2 to 0.2 mg/cm2 led to superior rejection of dextran 2000 kDa from 53% with 77 L/m2.h.bar water permeance up to 96% with 33 L/m2.h.bar water permeance. This meant that average pore sizes of the graphene-polymer/PES hybrid membranes with 0.2 mg/cm2 graphene were smaller than 50 nm which is defined as average molecular size of dextran 2000 kDa. Therefore, the filtration results from the graphene-polymer hybrid membrane with cross-linker provided that contribution of EDA on membrane separation performance was not so pronounced compared to graphene content, but cross-linker definitely enhanced membrane stability in aqueous medium. The highest dextran 2000 kDa rejection of 94% with 23 L/m2.h.bar water permeability was achieved from graphene-cross-linker modification. Additionally, by testing the resultant graphene-polymer hybrid membrane under forward osmosis experiments which included solution of polyacrylic acid sodium salt (PAA-Na) as draw and water as feed solutions, it has been also achieved 2.1 L/m2.h water flux with 0.6 g/m2.h reverse solute flux in active layer facing with draw solution mode while 1.4 L/m2.h water flux with 0.4 g/m2.h reverse solute flux were obtained in active layer facing with feed solution mode.
Trinity College Dublin (TCD)
German Academic Exchange Service (DAAD)
University of Duisburg-Essen
Author: Akca, Sevilay
Publisher:Trinity College Dublin. School of Physics. Discipline of Physics
Type of material:Thesis
Availability:Full text available
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